design manual for roads and bridges - vol 7 - pavement design and maintenance

DESIGN MANUAL FOR ROADS AND BRIDGES

VOLUM E 7 PAVEMENT DESIGN ANDMAINTENANCE

SECTION 1 PREAMBLE

PART 1

HD 23/99

GENERAL INFORM ATION

SUMMA RY

This Part is an introduction to the whole of Volume 7. Itupdates and replaces HD 23/94.

INSTRUCTIONS FOR USE

1. Remove HD 23/94 which is superseded byHD 23/99 and archive as appropriate.

2. Insert HD 23/99 into Volume 7 Section 1 Part 1.

3. Archive this sheet as appropriate.

Note: A quarterly index with a full set of VolumeContents Pages is available separately from TheStationery Office Ltd.

February 1999

HD 23/99

DESIGN MANUAL FORROADS AND BRIDGES

Volume 7 : Pavement Design andMaintenance

General In formation

Summary: This part supersedes HD 23/94

THE HIGH WAYS AGENCY

THE SCOTTISH OFFICE DEVELOPMENT DE PARTMENT

THE WELSH OFFICEY SWYDDFA GYMREIG

THE DEPARTMENT OF THE ENVIRONMENT FORNORTHERN IRELAND


Volume 7 Section 1Part 1 HD 23/99

February 1999

REGISTRATION OF AMENDMENTS

Amend Page No Signature & Date of Amend Page No Signature & Date ofNo incorporation of No incorporation of

amendments amendments

Registration of Amendments


SECTION 1 PREAMBLE

PART 1

HD 23/99

GENERAL INFORM ATION

Contents

Chapter

1. Introduction

2. Background

3. Use of Volume 7

4. Glossary of Terms

5. Principal Abbreviations

6. Enquiries


February 1999


Chapter 1Introduction

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1. INTRODUCTION

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General

1.1 This Part is an introduction to the whole Volume.

1.2 Volume 7 of the DMRB consists of a series oflinked documents. Table 3.1 in this Part gives a full listof the documents and a clear chart of the contents ofeach Part. Changes to Volume 7 are always in hand,Table 3.1 will not necessarily include all the latestamendments.

1.3 The Transport Research Laboratory (TRL),previously operated as the Transport and Road ResearchLaboratory (TRRL). Both abbreviations have been usedin Volume 7.

1.4 Paragraphs of Volume 7 which form part ofthe standards that the Overseeing Organisationexpects are highlighted by being contained in boxes.These are the sections with which the designer mustcomply. The remainder of the document containsadvice and enlargement which is commended todesigners for their consideration.

Implementation

1.5 This Part of Volume 7 of the Design Manualfor Roads and Bridges (DMRB) is provided forgeneral information. Each Part of Volume 7 shouldbe consulted for the relevant Implementation Clause

1.6 The use of Volume 7 is mandatory for trunk roadsincluding motorways. It is intended to provide morebackground, explanation and advice than the supersededocuments. The objective is, however, to provide aninstruction manual, not a full technical review ortextbook. With regard to pavement assessment andmaintenance, it should be read in conjunction with theOverseeing Organisation’s maintenance instructions.In England and Wales, these are contained within therespective Trunk Road Maintenance Manuals

February 1999

utual Recognition

.7 Where Parts of Volume 7 give the Overseeingrganisation’s requirements for products, they makerovision for the acceptance of equivalent products frther member states of the European Community.eference should be made to the statement in each Poncerned.

iscellaneous

.8 Volume 7 does not deal with seasonal or routineaintenance, safety aspects other than skiddingsistance, or specialist aspects such as the surfacin

ridge decks. For advice on these aspects referencehould be made to the Overseeing Organisation.

.9 Frequent references are made to documentsontained in the Manual of Contract Documents forighway Works (MCHW) as follows:

Specification for Highway Works (MCHW1).

Notes for Guidance on the Specification forHighway Works (MCHW2).

Highway Construction Details (MCHW3).

.10 References are made in the text to otherocuments by author and date except for Britishtandards, TRL/TRRL Reports and Overseeingrganisation Publications, which are referred to byumber (or name) and date. A full list of References icluded in each Part.

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Chapter 2Background

2. BACKGROUND

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2.1 Current UK practice in pavement design andmaintenance has developed from a combination ofpractical experience, laboratory research and full-scaleroad trials. Most of the research has been carried outover a number of years by the Transport ResearchLaboratory (TRL), some with the assistance of externaresearch contracts.

2.2 TRRL Report LR 1132 (1984) provides details ofUK research findings in connection with the design andperformance of flexible pavements. The report makesuse of the results of full-scale road experiments and usanalytical techniques to rationalise and extend the dataMany of the recommendations have since been adoptedby the Overseeing Organisations.

2.3 TRRL Report RR 87 (1987) provides comparablefindings in connection with rigid pavements and many othe recommendations have also been adopted by theOverseeing Organisations.

February 1999

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.4 Significant developments have also taken place ither countries, particularly in connection with thenalytical or mechanistic approach to design. This isased on the traditional structural design philosophy,hich requires an understanding of material behaviournder load and an appropriate theoretical analysis of thesign problem.

avement Components

.5 Figure 2.1 illustrates two typical cross-sections ooad pavements in the U.K. The terms used, togetherith others used in Volume 7 are defined in Chapter 4,

he Glossary of Terms.

.6 The underlying subgrade soil (cut or fill), cappingif used) and sub-base comprises the Foundation, thelatform upon which the more expensive andtructurally significant layers are placed. This platforms designed to be of a certain minimum standard qualityhatever the underlying soil condition. It is not a

drainage layer although it does itself require to beadequately drained since it is never totally impermeable

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Figure 2.1 Typical Pavement

FLEXIBLE, FLEXIBLE COMPOSITE& RIGID COMPOSITE

RIGID

WEARING COURSESURFACING

BASECOURSE

ROADBASE

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SUB-FORMATION

SUB-BASE

CAPPING

SUBGRADE

FOUNDATION

PAVEMENTQUALITY

CONCRETE

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Chapter 2Background

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2.7 The Roadbase is the main structural layer of thepavement, required to distribute the applied trafficloading so that the underlying materials are notoverstressed. It must be able to sustain the stresses anstrains generated within itself without excessive or rapiddeterioration of any kind.

2.8 The function of the Surfacing is to enable goodride quality to be combined with the appropriateresistance to skidding and to resist crack propagation.For this, texture and durability under traffic arerequired. In the case of concrete roads, the surfacing aroadbase are combined to form a single layer.

Pavement Types

2.9 Four different types of pavement are defined bythe Overseeing Organisation.

a) Flexible:

The surfacing and roadbase materials are boundwith bituminous binder.

b) Flexible Composite:

The surfacing and upper roadbase (if used) arebound with bituminous binder on a roadbase orlower roadbase of cement bound material.

c) Rigid:

Pavement quality concrete is used for thecombined surfacing and roadbase. The concretecan be:-

Jointed unreinforced (URC)Jointed reinforced (JRC)Continuously reinforced (CRCP)

d) Rigid Composite:

Continuously reinforced concrete roadbase(CRCR) with bituminous surfacing.

Pavement Performance

2.10 Pavements do not fail suddenly but graduallydeteriorate in serviceability to a terminal level whichmay be defined as failure. The rate of deterioration ofteaccelerates as failure is approached. This is represente

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.11 Some pavements, with thicker bound layersonstructed on a good foundation, maintain theirtrength or become stronger over time, rather thanradually weakening with trafficking. Such pavements,uilt above a threshold strength, will have a very longtructural service life, provided that distress, in the formf cracks and ruts appearing at the surface, is treatedefore it begins to affect the structural integrity of theoad. These are referred to as long-life pavements.

.12 It is good practice and the Overseeingrganisation’s intention to ensure that majoraintenance or strengthening of the pavement takeslace at a point such that the structural contribution of

he existing pavement layers is largely retained.

.13 To monitor the performance of a pavement, theverseeing Organisation requires the use of a number ossessment machines and methods. These include theigh speed Road Monitor, the Deflectograph, FWD,CRIM, visual condition surveys, etc. In this way theppropriate timing can be chosen for the variousecessary maintenance processes which all pavemenventually require.

aintenance

.14 Clearly any of the three main components of aavement (Foundation, Roadbase, Surfacing) caneteriorate, leading to a reduction in the quality oferformance of the overall structure. Depending upon

he mode of deterioration, deduced from the variousavement assessment processes, maintenance measanging from surface treatment through to totaleconstruction may be necessary.

.15 Whichever type of maintenance measure is to bedopted, it is always of the utmost importance that theorrect procedures are followed to render the repair/trengthening as effective and long-lasting as possible.

February 1999


3. USE OF VOLUME 7

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Chapter 3Use of Volume 7

3.1 The order of the section in Volume 7 is generallybased on the life cycle of a road pavement, ie. design,construction, assessment then maintenance (see Table3.1). This starts with consideration of the cumulativetraffic loading which the pavement is required to carry,followed by the design of the foundation, the roadbase omain structural component and the surfacing. There isthen a need to assess the behaviour in service and todesign and carry out appropriate maintenance. It must,however, be emphasised that there is interaction betweethe components and that they cannot always be dealtwith in isolation. The following is a brief outline of thecontents of each of the sections and parts.

Section 1: Preamble

3.2 Part 1 is this general introduction to Volume 7.

3.3 Part 2 gives technical information on theconservation and use of reclaimed materials.

Section 2 : Pavement Design and Construction

3.4 Part 1 describes the calculation of design trafficfor both new roads and maintenance. It also covers thecalculation of past traffic, needed in pavementassessment analysis.

3.5 Part 2 gives details of the design of a pavementfoundation and of tests used for the assessment ofexisting subgrade and sub-base materials.

3.6 Part 3 covers the design of the roadbase andsurfacing for new roads, including all the various typesof pavement allowed.

3.7 Part 4 covers particular aspects of theconstruction process that are relevant to this Volume butnot covered elsewhere, including pavement widening anrapid concrete construction.

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ection 3 : Pavement Maintenance Assessment

.8 Part 1 describes the use and interpretation ofCRIM for monitoring skidding potential.

.9 Part 2 lists the machines and methods used for tructural assessment of a pavement. It also describehe analysis and interpretation associated with each.

.10 Part 3 details the procedure to be used intructural assessment, the appropriate use of eachethod and the overall interpretation. The choice and

esign of strengthening measures is also included.

ection 4 : Pavement Maintenance Methods

.11 Part 1 gives details of the techniques foraintenance of roads containing bituminous materials

ncluding advice on recycling.

.12 Part 2 covers the maintenance of concrete roadrom surface treatments through to reconstruction.

ection 5: Surfacing and Surfacing Materials

.13 Part 1 covers the general suitability of materialsor different uses and other surfacing requirements.

.14 Part 2 details the various bituminous surfacingaterials and processes that are available.

.15 Part 3 details the various concrete surfacingaterials and processes that are available.

low Charts

.16 Figure 3.1 is a flowchart which is intended tohow the interrelations which exist between the sectiond parts in Volume 7.

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SECTION 1. Preamble SECTION 2. Pavement Design and Construction

PART 1 PART 2 PART 1 PART 2 PART 3 PART 4HD 23 HD 35 HD 24 HD 25 HD 26 HD 27General Technical Information Traffic Assessment Foundations Pavement Design Pavement

Information ConstructionMethods

1. Introduction 1. Introduction 1. Introduction 1. Introduction 1. Introduction 1. Introduction

2. Background 2. Conservation and 2. Standard Method 2. Subgrade 2. Design 2. Widening ofuse of Reclaimed Assessment Implementation Pavementsmaterials

3. Use of Volume 7 3. References 3. Structural 3. Capping and 3. Roadbase 3. RapidAssessment & Sub-base Materials ConstructionMaintenance and Repair of

Concrete Pavements

4. Glossary of 4. Enquiries 4. Lane Distribution 4. In-situ Testing 4. Surfacing 4. Not currentlyTerms Materials used

5. Principal 5. References and 5. References and 5. Design Criteria 5. Not currentlyAbbreviations Bibliography Bibliography used

6. Enquiries 6. Enquiries 6. Enquiries 6. The Analytical 6. References andApproach Bibliography

7. References and 7. EnquiriesBibliography

8. Enquiries

SECTION 3. Pavement Maintenance Assessment

PART 1 PART 2 PART 3 PART 4HD 28 HD 29 HD 30 HA 72

Skidding Resistance Structural Assessment Methods Structural Assessment Procedure Use and Limitation of GroundRadar for Pavement Assessment

1. Introduction 1. Introduction 1. Introduction 1. Introduction

2. Not currently used 2. High Speed Road Monitor 2. Routine Structural Assessment 2. Ground Radar forNon-destructive Testing

3. Use of SCRIM 3. Visual Condition Surveys 3. Collection and Review of 3. Use of Ground RadarExisting Data

4. References and Bibliography 4. Deflection Testing 4. Planning Detailed Investigation 4. Survey Procedure

5. Enquiries 5. Specialist Test Methods 5. Detailed Investigation 5. Reporting the Results of aGround Radar Survey

6. References and Bibliography 6. Interpretation 6. Enquiries

7. Enquiries 7. Strengthening Design

8. References and Bibliography

9. Enquiries

Annex Annex Annex Annex

1. Calibration 1. Deflection Beam A.Flexible Composite Pavements A.Technical Survey Briefand Pavement Condition Information

2. Operational Procedures 2. Deflectograph B. Quality Control Plan

3. Use of Different Types of Test 3. Deflectograph - Analysis and C.Determination of Layerin Accident Investigation Interpretation Thickness from a Ground

Radar Waveform

4. FWD Requirements forConsistency Check

February 19993/2



SECTION 4. Pavement Maintenance SECTION 5. Surfacing and Surfacing MaterialsMethods

PART 1 PART 2 PART 1 PART 2 PART 3HD 31 HD 32 HD 36 HD 37 HD 38

Maintenance of Maintenance of Surfacing for Bituminous Surfacing Concrete Surfacing andBituminous Roads Concrete Roads New and Maintenance Materials and Techniques Material

Construction

1. Introduction 1. Introduction 1. Introduction 1. Introduction 1. Introduction

2. Surface 2. Surface 2. Surfacing Options 2. Bituminous Surfacing 2. Transverse TexturedTreatments Treatments Materials Concrete Surface

3. Minor 3. Joint Repairs 3. Texture and Aggregate 3. Binders and Binder 3. Exposed AggregateMaintenance Properties Modifiers Concrete Surface

4. Major 4. Structural Repairs 4. Not currently 4. Hot Rolled Asphalt 4. Retexturing (Concrete)Maintenance used

5. Recycling 5. Strengthening 5. Tyre/Road 5. Porous Asphalt 5. Not currently usedSurface Noise

6. In-situ hot 6. References and 6. References and 6. Thin Surfacings 6. References andrecycling Bibliography Bibliography Bibliography

7. References and 7. Enquiries 7. Enquiries 7. Stone Mastic Asphalt 7. EnquiriesBibliography

8. Enquiries 8. Surface Dressing

9. High Friction Surfacing

10. Slurry Surfacing andMicro-surfacing

11. Retexturing (Bituminous)

12. Not currently used

13. Miscellaneous SurfacingMaterials

14. References andBibliography

15. Enquiries

Annex Annex

1. Maintenance and A.Method for DeterminationRepair Procedures of Loss of Chippings and

Proportion of BrokenChippings

TABLE 3.1 Layout of Volume 7 : Pavement Design and Maintenance

February 1999 3/3



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StructuralAssessment

Methods

Section 4, Parts 1 & 2

Maintenance

Section 2, Part 4

PavementConstruction

Methods

��Section 1, Parts 1 & 2

General and TechnicalInformation

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Surfacing andSurfacing Materials

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Surfacing andSurfacing Materials

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February 19993/4


4. GLOSSARY OF TERMS

Chapter 4Glossary of Terms

in

Capping : A subgrade improvement layer,protecting the subgrade fromdamage.

Chainage : Distance along a carriagewayfrom a reference point.

Crack and Seat : Process whereby a failed concretepavement is broken into relativelysmall slabs and compacted priorto overlaying.

Deflection : The recoverable movement of thesurface of a pavement under atransient load.

Deformation : The irreversible movement/compression of pavement layers,leading to rutting and settlement.

Design Period : The number of years for which apavement is designed.

Design Traffic : The predicted traffic occurringover the design period: usuallyexpressed in terms of millions ofstandard axles (msa).

Elastic Modulus : A measure of the materialstiffness properties.

Fatigue : The formation of cracks inpavement materials underrepeated loading.

Flexible : Bituminous roadbase andsurfacing.

Flexible Composite : Cement bound roadbase,bituminous surfacing.

Formation : Level upon which sub-base isplaced.

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oundation : All materials up to the top of sub-base.

rowth Factor : The proportional increase/decrease between the average‘whole life’ traffic flow on a roadand the flow at opening (orpresent flow).

rowth Rate : The annual percentage increase vehicle flow.

lay : The replacement of some of thelayers of an existing pavementwith new materials, ie. a form ofpartial reconstruction to existingor to different levels.

acro-Texture : The visible roughness of asurfacing material, enablingdrainage of water etc.

ega-Texture : The degree of smoothness of thesurface with wavelength between50 and 500mm.

ic ro-Texture : The microscopic properties of thesurface which enable it to developfriction and thus to provideskidding resistance.

odulus : The ratio, stress/strain.

verlay : The placement of new materialdirectly onto the surface of anexisting pavement.

avement : All layers above formation.

rofile : The variation of the longitudinallevel along the length of acarriageway.

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Chapter 4Glossary of Terms

Reconstruction : The replacement of some or alllayers of an existing pavementwith new (or recycled) materials.

Rigid : Concrete roadbase and surfacing.

Rigid Composite : Concrete roadbase, bituminoussurfacing.

Roadbase : Main structural layer ofpavement; placed above sub-base

Standard Axle : An axle with an 80kN total force.

Stiffness Modulus : The equivalent of elastic modulusbut for materials whose stiffnessvaries (eg. with temperature,stress state etc.)

Sub-base : A platform layer upon which themain structure of a pavement maybe laid.

Sub-Formation : Top of subgrade level if a cappingis used.

Subgrade : Soil underlying a pavement (maybe fill material).

Surface Dressing : A single/double/triple layer ofaggregate combined with one ormore layers of binder to form arunning surface.

Surfacing : Upper layers designed to carrytraffic directly.

Thin Surfacing : Thin surfacing systems aremachine-laid proprietary mixesthat have the capability toregulate and smooth surfaceprofile, restoring surface textureand skid resistance.

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ehicle Category : Description of vehicle by generalweight/damaging power, eg,‘Other goods vehicle category 2(OGV2)’.

ehicle Class : Description of vehicle by typeand number of axles eg. ‘3 axleArticulated’ or ‘Buses andCoaches’.

February 1999


5. PRINCI PAL ABBREVI ATIONS

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Chapter 5Principal Abbreviations

AADF Annual Average Daily Flow

AAV Aggregate Abrasion Value

ASTM American Society for Testing and Materials

BS British Standard

BSI British Standards Institution

CBM Cement Bound Material

CBR California Bearing Ratio

CHART Computerised Highway Assessment ofRatings and Treatments

COBA Cost Benefit Analysis

CRCP Continuously Reinforced ConcretePavement

CRCR Continuously Reinforced ConcreteRoadbase

Cu Concrete Cube Strength

cv/d Commercial Vehicles per Day

DBM Dense Bitumen Macadam

DBM50 Dense Bitumen Macadam- 50 Penetration Grade Binder

DCP Dynamic Cone Penetrometer

DMRB Design Manual for Roads and Bridges

DSR Dynamic Shear Rheometer

EVA Ethylene Vinyl Acetate

FWD Falling Weight Deflectometer

GGBS Ground Granulated Blast Furnace Slag

GPR Ground Probing Radar

HAPAS Highway Authorities Products ApprovalScheme

HAPMS Highways Agency Pavement ManagementSystem

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M Heavy Duty Macadam

B High Modulus Base

V Heavy Goods Vehicle

A Hot Rolled Asphalt

M High speed Road Monitor

TM High Speed Texture Meter

SM Indirect Tensile Stiffness Modulus

C Jointed Reinforced Concrete Pavemen

Liquid Limit

AMA Long LIfe Approach to MaintenanceAssessmemt

RCH Maintenance Assessment Rating andCosting for Highways

HW Manual of Contract documents forHighway Works

V Moisture Condition Value

a Millions of Standard Axles

SC Mean Summer SCRIM Coefficient

M Mini Texture Meter

MAS National Measurement AccreditationService

T Non Destructive Testing/Nuclear DensitTest

SA Network Evaluation from Surveys andAssignments (Scotland)

TF National Road Traffic Forecast

RS National Skidding Resistance Survey

V1 Other Goods Vehicle - Category 1

V2 Other Goods Vehicle - Category 2

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Chapter 5Principal Abbreviations

OSGR Ordnance Survey Grid Reference

Pa.s Pascal second (viscosity)

PA Porous Asphalt

PAV Pressurised Ageing Vessel

PANDEF Processing and Analysis of Deflections

PCV Proportional Change in Variance

PDPBT Portable Dynamic Plate Bearing Test

PFA Pulverised Fuel Ash

PI Plasticity Index

PQC Pavement Quality Concrete

PRD Percentage Refusal Density

PSV Public Service Vehicle

PSV Polished Stone Value

PVA Poly Vinyl Acetate

QUADRO Queues and Delays at Roadworks

RTFOT Rolling Thin Film Oven Test

SAMI Stress Absorbing Membrane Interlayer

SBR Styrene-Butadiene-Rubber

SBS Styrene-Butadiene-Styrene

SCRIM Sideway force Coefficient RoutineInvestigation Machine

SFC Sideway Force Coefficient

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SMA Stone Mastic Asphalt / Splitt MastixAsphalt (Germany) / Stone Matrix Asphal(USA)

SMTD Sensor Measured Texture Depth

SRV Skidding Resistance Value

STEAM Scottish Traffic and EnvironmentalAppraisal Manual

TAM Traffic Appraisal Manual

TRL Transport Research Laboratory

TRMM Trunk Road Maintenance Manual

TRRL Transport and Road Research Laborator

TFV Ten Percent Fines Value

URC Unreinforced Concrete Pavement

UKPMS United Kingdom Pavement ManagementSystem

VCS Visual Condition Survey (for concretesurfaced roads)

WLC Whole Life Cost

February 1999


February 1999 6/1

6. ENQUIRIES

Approval of this document for publication is given by the undersigned:

Quality Services DirectorThe Highways AgencySt Christopher HouseSouthwark Street J KermanLondon SE1 0TE Quality Services Director

The Deputy Chief EngineerThe Scottish Office Development DepartmentNational Roads DirectorateVictoria Quay N B MACKENZIEEdinburgh EH6 6QQ Deputy Chief Engineer

The Director of HighwaysWelsh OfficeHighways DirectorateCathays Park K J THOMASCardiff CF1 3NQ Director of Highways

The Technical DirectorDepartment of the Environment forNorthern IrelandRoads ServiceClarence Court10-18 Adelaide Street V CrawfordBelfast BT2 8GB Technical Director

Chapter 6Enquiries

All technical enquiries or comments on this document should be sent in writing as appropriate to the above.

November 2004


VOLUME 7 PAVEMENT DESIGN ANDMAINTENANCE

SECTION 1 PREAMBLE

PART 2

HD 35/04

CONSERVATION AND THE USE OFSECONDARY AND RECYCLEDMATERIALS

SUMMARY

This part, previously called ‘Technical Information’,gives advice on the conservation and use of reclaimedmaterials in road construction and maintenance. Thisrevised version has been updated and now includesadditional materials and the latest advice.


1. Remove Contents pages for Volume 7 and insertnew Contents pages for Volume 7, datedNovember 2004.

2. Remove HD 35/95 from Volume 7, Section 1which is superseded by this Standard and archiveas appropriate.

3. Insert HD 35/04 into Volume 7, Section 1.

4. Please archive this sheet as appropriate.


HD 35/04

Conservation and the Use ofSecondary and Recyclced

Materials

Summary: This part, previously called ‘Technical Information’, gives advice on theconservation and use of reclaimed materials in road construction andmaintenance. This revised version has been updated and now includesadditional materials and the latest advice.


THE HIGHWAYS AGENCY

SCOTTISH EXECUTIVE

WELSH ASSEMBLY GOVERNMENTLLYWODRAETH CYNULLIAD CYMRU

THE DEPARTMENT FOR REGIONAL DEVELOPMENTNORTHERN IRELAND


November 2004






SECTION 1 PREAMBLE

PART 2

HD 35/04

CONSERVATION AND THE USE OFSECONDARY AND RECYCLEDMATERIALS

Contents

Chapter

1. Introduction

2. Provisions for the Use of Secondary andRecycled Materials

3. References

4. Enquiries


November 2004


November 2004 1/1


1. INTRODUCTION

General

1.1 This part contains general items of a technicalnature that have relevance to the construction andmaintenance of all types of pavement. Advice is givenon the conservation and use of secondary and recycledmaterials in road construction and maintenance.Specific or general provision for the use of recycled andsecondary materials within the Specification(MCHW 1) does not automatically infer environmentalsuitability, and although the increased use of recycledand secondary materials can have a significant positiveimpact on the environment via reductions in landfilling,quarrying and transportation of aggregates, it isimportant to assess the acceptable application of thesematerials in line with current environmental guidelines.

Scope

1.2 This part gives guidance to design organisationson conservation techniques and the use of secondaryand recycled materials that are currently permitted inthe Specification (MCHW 1) and in the earthworks,drainage and pavement construction parts of the DesignManual for Roads and Bridges (DMRB 4.1, 4.2 and 7).

Implementation

1.3 This part may be used forthwith on all schemesfor construction, improvement and maintenance oftrunk roads including motorways being prepared,provided that in the opinion of the OverseeingOrganisation, this would not result in significantadditional expense or delay in progress. Designorganisations should confirm its application toparticular schemes with the Overseeing Organisation.For use in Northern Ireland, this part will be applicableto those roads designated by the OverseeingOrganisation.


Chapter 2Provisions for the Use of Secondary and Recycled Materials

SE OF SECONDARY AND
2. PROVISIONS FOR THE URECYCLED MATERIALS
General

2.1 The attention of those responsible for the design,specification, construction and maintenance of roads isdrawn to the opportunities to conserve and re-usematerials arising from roadworks, as well as thepotential uses for secondary or recycled materials fromother sources that may be proposed when cost effective.

2.2 It is Government policy to encourageconservation and facilitate the use of reclaimed andmarginal materials wherever possible, in order to obtainenvironmental benefits and reduce the pressure onnatural reserves of primary aggregates. The LandfillTax and Aggregate Levy are seen as powerful driverstowards sustainable construction with the main aimsbeing the increased use of secondary and recycledmaterials in construction, a reduction in constructionwaste going to landfill, and a reduction in primaryaggregates output per unit of construction value.

2.3 Suitable materials may be those reclaimed fromroads during reconstruction, from residues of industrialprocesses including mining, and from the demolition ofother construction projects. Such materials may providegood value for money, particularly if their use involvesless haulage. Where existing road foundations are ingood condition, conservation of the pavement structureby strengthening with an overlay or inlay and wideningwhere necessary, can also be an effective strategy forreducing the demand for primary aggregates.

2.4 The Joint Circular from the DoE (20/87), DoT(3/87) and Welsh Office (36/87) entitled ‘Use of WasteMaterial for Road Fill’ sets out administrativeprocedures to ensure that information about future roadschemes and their likely fill requirements are given tolocal Planning Authorities at the earliest possible stage.The circular states that, while it will be for tenderers tochoose their sources of fill on a commercial basis, theacceptability of sites for borrow pits is a matter for theland use planning system. It recommends that PlanningAuthorities should therefore treat planning applicationsfor borrow pits in the same way as applications forother mineral developments, taking into account theavailability of suitable material. In Scotland theNational Planning Policy Guideline NPPG4, Land forMineral Working, issued by the Scottish OfficeEnvironment Department, gives guidance to planning

November 2004

authorities on the recycling and re-use of constructionwaste and materials in waste tips where this isenvironmentally acceptable.

2.5 The Specification for Highway Works(MCHW 1) permits a wide range of reclaimed materialssuch as road planings, crushed concrete and mineralby-products such as slags, furnace bottom ash (FBA)and the like, to be used in place of primary aggregate.But where the Specification (MCHW 1) refers toindividual materials, this may be taken to imply thatother materials not specified by name might beinherently unsuitable in some way even if conformingto the appropriate standard. One aim of this advice noteis to highlight the existing provisions made in theSpecification (MCHW 1) for the reuse and recycling ofreclaimed materials and to encourage their widestapplication.

2.6 The variability of recycled and secondarymaterials may require more frequent test regimes toenable their re-use at high level within the roadpavement construction. Recycling of road planingsrequires considerable processing and control proceduresto ensure standards are maintained. Thus it may provemore efficient and cost effective to re-use such arisingslower down in the pavement or in the foundations, oralternatively to limit replacement levels in higher layersto percentages which will minimise additional testing.Similarly, the re-use of old pavement quality concrete,after crushing and grading, as recycled concreteaggregate for the lower layer of the monolithic slab oras a continuously reinforced concrete base (CRCR)slab, is likely to prove efficient and cost effective.

2.7 The conservation of existing pavements and theuse of secondary and recycled materials helps to reducethe impact on the environment by reducing theextraction of primary aggregates and at the same timereduces the amount of waste being generated.Environmental benefits can also arise from theconsequent reduction of construction traffic on localroads. Any additional costs of sorting and processing ofsecondary or recycled materials may be offset byreduced energy requirements compared with theextraction and transportation of primary aggregates.There may also be a reduction in the amount of freshbitumen required when recycling bituminous boundmaterials.

2/1



Application Pipe Embank- Capping Unbound Hydraulically Bitumen PQand Bedding ment and Mixtures Bound Bound Concrete

Series Fill for Mixtures LayersSub-base for Sub-base

and Base

Material 500 600 600 800 800 900 1000

Blast furnace Slag

Burnt Colliery Spoil x x x

China Clay Sand/Stent

Coal Fly Ash/PulverisedFuel Ash (CFA/PFA) x

Foundry Sand

Furnace Bottom Ash(FBA) x x x

Incinerator BottomAsh Aggregate (IBAA)

Phosphoric Slag

Recycled Aggregate

Recycled Asphalt

Recycled Concrete

Recycled Glass x

Slate Aggregate

Spent Oil Shale/Blaise x x x

Steel Slag x

Unburnt Colliery Spoil x x x x x

KEY:

Specific (permitted as a constituent if the material complies with the Specification (MCHW 1)) orGeneral Provision (permitted as a constituent if the material complies with the Specification (MCHW 1)requirements but not named within the Specification (MCHW 1)).

x Not permitted.

IMPORTANT NOTES:

1. Table 2.1 is for guidance only and reference must be made to the accompanying text and the Specification (MCHW 1).Materials indicated as complying with the Specification (MCHW 1) for a particular application may not necessarilycomply with all the requirements of the series listed, only particular clauses. For example in the 600 series, UnburntColliery Spoil can satisfy the Specification (MCHW 1) as a general fill, but is excluded as a selected fill; and in Series1000 recycled or secondary materials are not permitted within the running surface of PQ concrete. Reference should alsobe made to the Specification (MCHW 1) for any maximum constituent percentages of specific recycled or secondaryaggregates. For example in the 1000 Series, the maximum by mass constituent of Recycled Asphalt is given under thelimits for ‘other material’ (Table 10/2) within the Specification (MCHW 1).

2. There is no Specific or General Provision for the use of recycled glass as an aggregate in PQ concrete or HydraulicallyBound Mixtures due to the potential for deleterious alkali-silica reaction (ASR). However, its use may be permitted bythe Overseeing Organisation if sufficient provisions to minimise the risk of deleterious ASR are included in the mixturedesign.

3. There is no Specific or General Provision for the use of steel slag as an aggregate in PQ concrete or HydraulicallyBound Mixtures due to the potential for volume instability. However, its use may be permitted by the OverseeingOrganisation if sufficient assurance of volume stability is provided.

Table 2.1: Specification for Highway Works (MCHW 1):Application of Secondary And Recycled Aggregates

November 20042/2



The Provision for Secondary and RecycledMaterials Within the Specification for HighwayWorks (MCHW 1)

2.8 Table 2.1 illustrates that all the secondary andrecycled materials listed can be suitable in one form oranother for inclusion in drainage work or earthworks orfoundations or pavements, providing always that theycomply with the requirements of the Specification(MCHW 1). For definitions of terminology used in thissection refer to the Glossary of Terms (paragraph 2.37).

Health and Safety

2.9 Although Table 2.1 indicates that all thesecondary and recycled materials listed are permitted bythe Specification (MCHW 1), consideration must begiven to aspects of health and safety. Of particularconcern is dust arising from materials, which may affectboth workers and others, as well as reducing visibilityand hence safety. Leachate from reclaimed materialsshould be assessed from a health and safety point ofview, and appropriate protective equipment used andwashing facilities provided.

Series 500 Drainage

2.10 Recycled coarse aggregate and recycled concreteaggregate are specified by name. Maximum watersoluble sulphate levels may restrict the use of some ofthe secondary and recycled materials listed in Table 2.1.Other uses of secondary and recycled materials includerecycled tyres as drainage materials in the form ofbonded crumb rubber.

Series 600 Earthworks

2.11 The Specification (MCHW 1) permits a verywide range of bulk fill materials under the headings‘general granular fill’ and ‘general cohesive fill’.Unacceptable materials are listed and include materialshaving hazardous chemical or physical properties.Thus, the possibility of leachate from any material mustbe addressed.

Blast Furnace Slag can comply as selected granularfill and is specified by name, but this would be awasteful re-use of a premium material. Slag isspecifically excluded from granular fill overlyingburied steel structures.

Burnt Colliery Spoil can comply as bulk or selectedgranular fill and is specified by name for manyapplications.

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Slas

November 2004

hina Clay Sand and Stent, being natural aggregates,n both comply as bulk and selected granular fills.

oal Fly Ash/Pulverised Fuel Ash is specified byme as PFA in both general cohesive fill containingt more than 20% FBA, and as conditioned ash inlected cohesive fill to structures and for reinforcedrth.

urnace Bottom Ash (FBA) can comply as bulk filld as selected granular fill that is to be stabilised withment. However, it is not a named selected granular

ll since it would generally fail the strengthquirement.

oundry Sands can comply as a bulk fill and as alected granular fill.

cinerator Bottom Ash Aggregate (IBAA) canmply as bulk fill and as a selected granular fill.

hosphoric Slag can comply both as a bulk fill and asselected fill. However, as with blast furnace slag, thee of this premium material for granular fill would beasteful.

ecycled Aggregate, as crushed demolition debris, canmply as bulk fill provided it does not containacceptable materials as defined in this series.

ecycled Asphalt can comply as bulk fill, although thisould in general be a wasteful re-use of a premiumaterial. Accurate measurement of the moisture content bituminous planings can be difficult and should beven specific consideration.

ecycled Concrete can comply as a selected granularll and is specified by name as crushed concrete undernumber of applications.

ecycled Glass could comply as a bulk fill and selectedanular fill but would be unlikely to be used in thesercumstances.

ecycled Rubber in the form of shredded tyres or tyreles could potentially be used as a lightweight bulk fill back fill to structures. Rubber is not named in theecification (MCHW 1) and advice should be sought

om specialists, and the Overseeing Organisationnsulted with regard to its use. Of note is any potentialr pyrolysis or combustion of these materials.

ate Aggregate is a natural aggregate and can comply both bulk and selected granular fill.

2/3



Spent Oil Shale/Blaise can comply as bulk fill but isnot permitted as selected granular fill.

Steel Slags are excluded (except where mentionedbelow) due to the possible presence of free lime (CaO)and/or free magnesia and the consequent risk ofexpansion when hydration occurs. Weathering of steelslag over several months allows these hydrationreactions to occur and produces a more stable material.Weathered Steel Slag (as specified in Series 800) maybe permissible if adequately controlled to minimise riskof particle disintegration or expansion.

Unburnt Colliery Spoil can comply as bulk fill but isexcluded from selected granular fill.

Series 600 Capping

2.12 Table 2.1 shows that all secondary and recycledmaterials except steel slag (except where mentionedbelow) and unburnt colliery shale can meet theSpecification (MCHW 1) as capping on their own or aspart of a combination, as either unbound selectedgranular fill or bound with cement and/or other binders.Weathered steel slags can comply as selected granularfill. Selected conditioned PFA for stabilisation withcement to form capping is specified as a separate class.Accurate measurement of the moisture content ofasphalt planings, and the determination of optimummoisture content, can be difficult and should be givenspecific consideration.

Series 800 Unbound Mixtures for Subbases

2.13 Recycled concrete, asphalt (up to 50% by massas a constituent of recycled aggregate) and aggregate,blast furnace slag, and non-plastic shale (burnt collieryspoil and spent oil shale) can all comply with theSpecification (MCHW 1) and are named. Specificprovision is given for a sub-base comprised of asphaltplanings. Steel slag, well weathered and conforming tothe requirements of BS 4987-1 is also specified. Chinaclay Stent and slate aggregate are quarried rocks andcan comply when processed. Foundry sand, glass,IBAA (up to 15% by mass) and China clay sand canalso comply as components within the Specification(MCHW 1). Higher proportions of IBAA may beacceptable within unbound sub-bases followingadditional testing.

Series 800 Hydraulically Bound Mixtures forSubbase and Base

2.14 Slag Bound Material (SBM), bound withGranulated Blast Furnace Slag (GBS), and using

2/4

recycled coarse and/or concrete aggregate is namedwithin the Specification (MCHW 1). All the secondaryand recycled materials in Table 2.1 can meet theSpecification (MCHW 1) as the coarse aggregate orwithin blends for SBM. Alternative binders include FlyAshes (including PFA) to form Fly Ash Bound Mixtures(FABM), Flue gas desulphurised Gypsum and CementKiln Dust (CKD). The potential risk of expansivechemical reactions exists with the incorporation ofgypsum into a bound material and should be givenspecific consideration.

2.15 Cementitious binders in this series can include anumber of alternatives to Portland cement with wellestablished properties. These include Portland Fly AshCement, Portland Slag Cement and variouscombinations including GGBS and PFA. Wherecrushed concrete aggregate is used (within recycledconcrete/aggregate), it should only be sourced fromidentified structures with a known history. Appropriateadditional testing is required under the Specification(MCHW 1) to detect substances and chemicals harmfulto the durability of concrete.

2.16 All reclaimed materials listed in Table 2.1 can beused for Cement Bound Granular Mixtures (CBGM)formally called Cement Bound Materials (CBM) if theycomply with the Specification (MCHW 1). CementBound Granular Mixtures are now included in Series800 of the specification (MCHW 1) together with allother Hydraulically Bound Mixtures.

2.17 Any of the reclaimed materials listed in Table 2.1to comply with the requirements for Cement BoundGranular materials may require washing or processing.Strength requirements for the various Grades of CementBound Granular Mixtures are established on a scheme-specific basis.

2.18 Recycled aggregate, recycled concrete,Phosphoric slag, crushed air-cooled blast furnace slagand PFA are all capable of complying with theSpecification (MCHW 1). Stent and slate aggregate,crushed and graded, both have the potential to complywith the Specification (MCHW 1) being naturalaggregates. The maximum by mass percentage forrecycled asphalt is given under the limits for ‘othermaterials’ within the Specification (MCHW 1). Chinaclay sand, IBAA and foundry sand are also potentiallysuitable as fine aggregate. There are no Specific orGeneral Provisions for the use of steel slag or recycledglass as an aggregate. However, their use may bepermitted by the Overseeing Organisation.

November 2004



Series 900 Bitumen Bound Layers

2.19 Industrial By-products. Blast furnace slag, steelslag (well weathered), China clay sand and slateaggregate can meet the Specification (MCHW 1) andare named. Recycled glass, IBAA, foundry sand, PFA,recycled concrete and phosphoric slag can comply ascomponents of recycled aggregates within bitumenbound layers.

2.20 Reclaimed Materials. Reclaimed bituminousmaterials can be used for hot mix recycling in theproduction of bituminous base, binder course andsurface course; up to 10% of reclaimed bituminousmaterial being permitted without a requirement fortrials. The maximum content of reclaimed materials isrestricted to 10% for hot rolled asphalt surface courseand 50% in other layers. It should be noted that not allroad planings are suitable for hot mix recycling as olderlayers may contain tar. For further advice, refer toHD 31 DMRB 7.4.1.

2.21 In-situ Recycling. ‘Repave’ is a process thatconserves existing structurally sound pavements,restoring the surface by bonding a thin overlay or inlayto the pre-heated, scarified and reprofiled existing roadsurface. The ‘Remix’ process is an adaptation of the‘Repave’ system where the machine is fitted with asmall mixing unit. Material from the scarified surface isaugured into the pug mill mixer where it is blendedwith hot freshly mixed new material. The recycled mixis placed evenly on the heated surface to form thereplacement surface course which must conform to theappropriate standard.

2.22 Cold deep recycling of bitumen bound pavementsis covered within the Specification (MCHW 1) and formore detailed information refer to HD 31 (DMRB7.4.1). The primary aggregate source is from coldpulverisation of part, or all, of the existing roadstructure, with foamed bitumen or bitumen emulsionused as the primary binding agent. Trials are required todemonstrate that the existing pavement materials arecapable of being recycled to form the primary aggregatecomponent of cold recycled bitumen bound materialand to achieve the requirements of the Specification(MCHW 1).

Series 1000 PQ Concrete and Cold Recycled CementBound Material

2.23 PQ Concrete. Recycled concrete, crushed air-cooled blast furnace slag and PFA are capable ofcomplying with the Specification (MCHW 1), butexcluding exposed aggregate surfacing. The maximum

November 2004

by mass percentage for recycled asphalt is given underthe limits for ‘other materials’ for recycled concreteaggregate within the Specification (MCHW 1). Stentand slate aggregate, crushed and graded, both have thepotential to comply with the Specification (MCHW 1)being natural aggregates. IBAA, phosphoric slag andrecycled aggregate also have the potential to complywith the Specification (MCHW1). China clay sand, andfoundry sand are also potentially suitable as fineaggregate. There are no Specific or General Provisionsfor the use of steel slag or recycled glass as anaggregate. However, their use may be permitted by theOverseeing Organisation.

2.24 Cold Deep Recycling using a Cement Binder.Existing pavement layers can be recycled to form thefoundation or main structural layer of a new roadpavement. Hydraulic binder is used as the stabilisingagent with the pulverised material recycled for theaggregate. End product performance tests of therecycled cement bound material are required to judgethe expected performance of the recycled stabilisedmaterial against the performance of standard CBGMbase materials as detailed in the Specification(MCHW 1).

Conservation of Existing Pavements

2.25 Numerous opportunities exist for conservation,when maintaining or improving existing roads. By farthe most efficient approach is to conserve soundpavement layers or to stabilise and strengthen unsoundlayers, thus making the best use of materials by leavingthem in place. This will require careful attention to thetiming of maintenance, and decisions will be influencedby the type of road and the residual life of thepavement. For further advice refer to HD 30 (DMRB7.3.3). Traffic management alternatives and likelytraffic delay costs must be taken into account, to ensurethe most cost effective solution overall and inaccordance with good practice.

Concrete Pavements

2.26 Surface Restoration. Where concrete surfacesare still permitted, the skidding resistance of concretesurfaces that have polished, but are otherwise soundafter minor repair, can be restored by transversegrooving or by overlaying with an asphalt surfacing.For further advice refer to HD 32 (DMRB 7.4.2) andHD 38 (DMRB 7.5.3).

2.27 Strengthening. Using crack and seat techniques,concrete pavements can be conserved as the lower base

2/5



in flexible composite construction or as the subbase fora new pavement. Continuously reinforced concretepavements (CRCP) can be converted to become thebase (CRCR) with a strengthening flexible overlay.Jointed concrete pavements can be converted tofunction as a lower base with a strengthening flexibleoverlay of adequate thickness to inhibit the formation ofreflection cracks over joints. For further advice refer toHD 30 (DMRB 7.3.3). Concrete pavements which arestructurally sound can be strengthened by thin bondedconcrete or thick unbonded concrete overlays, using‘fast track’ concrete paving techniques where necessary,in accordance with HD 27 (DMRB 7.2.4).

Flexible, Flexible Composite and Rigid CompositePavements

2.28 Surface Restoration. The skidding resistance offlexible surfaces that have polished, but are otherwisesound, can be restored by application of a bituminousoverlay or inlay, or by retexturing. Apart from theretexturing option, the systems all help to reseal thepavement against deterioration. For further informationrefer to HD 31 (DMRB 7.4.1) and HD 37 (DMRB7.5.2).

2.29 Strengthening. Flexible and Compositepavements can be strengthened by the application of abituminous overlay, with or without the removal of theexisting surfacing layers. Where reflective cracking hasoccurred, restoration of Composite pavements(particularly with respect to Rigid CompositePavements) can be undertaken by removal of the upperasphalt layers, with subsequent conversion of theconcrete layer, using crack and seat techniques, to givea sub-base in a flexible pavement. Depending on designconstraints and traffic flows, continuously reinforcedconcrete pavements (CRCP) or bases (CRCR) can alsobe used to overlay or as an inlay to flexible pavements.These are probably the best options where significantdifferential settlement of the existing road has occurredand is likely to continue. For further advice refer toHD 30 (DMRB 7.3.3) and HD 31 (DMRB 7.4.1).

2.30 Road Widening. Advice on the widening ofroads and motorways is given in HD 27 (DMRB 7.2.4),which stresses that the maximum use should be made ofexisting construction wherever possible. There is littlerestriction on the type of material to be laid alongsidean existing pavement providing it conforms to theSpecification (MCHW 1). There can be positiveadvantages in having dissimilar construction, forexample to increase load carrying capacity of the newlane yet match the thickness of the new pavement withthe existing to ensure subsurface drainage is not

2/6

impeded. Where an existing pavement is to be partiallyor completely removed, the material arising can bere-cycled and re-used in or under the new construction,subject to compliance with the Specification(MCHW 1).

Protection of the Environment

2.31 Many reclaimed materials and industrialby-products that are available in sufficient quantities forroad construction cause no significant environmentalproblems. However, depending on their former usethese secondary and recycled materials may containvarying proportions of chemicals, such as organics andmetals. The potential for these chemicals to leach andmigrate, and their proximity to surface and groundwater, will affect their suitability for use. Mitigationmeasures at source or site, such as utilisation withinbound applications, can be employed to reduce anypotential risks.

2.32 The Environment Agency (EA) is responsible formaintaining the quality of surface and ground water inEngland and Wales. The Scottish EnvironmentProtection Agency (SEPA) is responsible for Scotland.The use of secondary and recycled materials shouldcomply with the relevant guidelines of the responsibleagency.

2.33 The handling, storage, processing and depositionof waste materials are subject to separate control by theWaste Management Regulations made under theEnvironmental Protection Act (EPA) 1990. These willapply to all surplus materials taken off site. Thecarriage and processing of waste will need to belicensed; however there will be a general exemptionfrom requirements for waste to be deposited at alicensed site, if instead it is to be incorporated into adevelopment which has planning consent. It ispresumed that such use of waste materials will beadequately controlled since they must be deposited inaccordance with the Specification (MCHW 1) for thedevelopment, including any special conditions requiredto prevent adverse effects on the environment.

2.34 The Waste Management Regulations permittemporary storage on site of restricted amounts of wastematerials approved for incorporation in the works, afterthe award of a contract, but prior to beginning work.Similarly, road planings may be stockpiled off site forreprocessing. Nevertheless, there are good reasons toutilise planings as they arise or as soon as possiblethereafter. Stockpiles can become quite difficult tobreak up if they are allowed to consolidate. On theother hand, if they become saturated with water, then

November 2004



compaction as fill or to form a capping layer may bedifficult and considerable amounts of energy are neededto drive off water if they are to be recycled to form anew base. There may also be concerns about thepossibility of both organic and mineral leachate if waterbegins to drain through a large stockpile. Certainmaterials are exempt from the regulations underspecific circumstances and consultation with the locallicensing officer should be undertaken to confirm siteand/or activity specific classifications. Exemptactivities are still required to be registered with the EAor SEPA.

2.35 Where reclaimed materials are being consideredfor use in landscaping and areas of planting, care shouldbe taken to ensure that they are compatible with theplants and trees to be grown. For further advice refer toHA 44 (DMRB 4.1.1).

Future Developments

2.36 It is expected that the quantities of materials thatcan be used will increase as the range of recycled andsecondary materials permitted in the Specification(MCHW 1) is extended. A number of research projectsare currently underway, including those on recycledplastic and non-ferrous slag from zinc production,which aim to develop suitable acceptance criteria toencourage the wider use of such materials when theyoffer best value for money. In addition, other materialssuch as quarry fines, recycled railway ballast andceramics may be considered by the OverseeingOrganisation on a site specific basis.

Glossary of Terms

2.37 The following terms are used in this Chapter:

BLAST FURNACE SLAG:A by-product from the production of iron, resultingfrom the fusion of fluxing stone (fluorspar) with coke,ash and the siliceous and aluminous residues remainingafter the reduction and separation of iron from the ore.Due to its potential variability and possible inclusion ofvolumetrically expansive components, ‘Old Bank’ slag,produced prior to modern high-level quality controlprocedures, may require additional processing prior touse. Various forms of solidified blast furnace slag areproduced dependent on the rate and technique used tocool the molten material. Granulated blast furnace slag(GBS) is formed by quenching the molten material withjets of water. This material is then processed to produceground granulated blast furnace slag (GGBS) for use asa hydraulic binder.

BAiol

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November 2004

URNT COLLIERY SPOIL:lso known as burnt minestone. The residue following

gnition of coal mine spoil heaps which results in partialr complete combustion of coal particles in the spoil,eaving calcined rocks.

EMENT KILN DUST (CKD):y-product from the calcining of raw materials during

he production of Portland Cement. CKD is extractedrom gases generated within the cement kilns during theanufacturing process and comprises a light fine

rained particulate material. If chemically suitable,KD is generally re-used within the cement works. Theomposition of the CKD will be dependent on the kilnrocess employed and the degree of separation in theust collection system.

OAL FLY ASH (CFA):ulverised fuel ash or Coal fly ash is extracted bylectrostatic precipitation from the flue gases of modernoal-burning power stations and is similar in fineness toement. It can be used both as a binder in Fly Ashound Mixtures (FABM) and as an aggregate.

HINA CLAY SAND AND STENT:he by-products from the extraction of china clay fromecomposed granite, consisting largely of two distinctaterials: ‘Stent’, which is waste rock, and ‘Tip Sand’.hina Clay Tip Sand is defined as the washed materialroduced as a by-product during the extraction of Chinalay (Kaolin) from Kaolinitic granite, predominantlyomprising quartz with some mica, and is relativelyonsistent from within each source of production.

OUNDRY SANDS:y-product of the castings industry, typicallyomprising uniformly sized sands with variousdditives and metals associated with the specific castingrocess.

URNACE BOTTOM ASH (FBA):urnace bottom ash is the coarser fraction of ashroduced in coal burning power stations resulting fromhe fusion of PFA particles which fall to the bottom ofhe furnace. It varies in size from fine sand to coarseravel and has a porous structure.

BS AND GGBS:ee Blast Furnace Slag.

YPSUM:y-product of power stations burning lignite or sulphurontaminated coal. Gas scrubbers remove the gypsumrom flue gasses in the desulphurisation process.

2/7



INCINERATOR BOTTOM ASH AGGREGATE(IBAA):IBAA is processed from the material discharged into theburning grate of Municipal Solid Waste (MSW)incinerators and comprises 80 to 90% of the total MSWash production. It is a heterogeneous material that maycontain varying proportions of glass, ceramics, brickand concrete in addition to clinker and ash.

NON-FERROUS SLAG:Non-ferrous slags include lead-zinc slag, tin slag andcopper slag, and are by-products produced during therecovery and processing of non-ferrous metal fromnatural ores. Slags of note for potential use assecondary aggregates are those associated with theproduction of phosphorous and zinc.

PFA:See Coal Fly Ash.

PHOSPHORIC SLAG:See non-ferrous slag.

QUARRY FINES:Processing of crushed rock for use as constructionaggregate consists of blasting, crushing, washing,screening and stockpiling operations. The production ofprimary aggregates can involve a number of processes,with quarry fines being the associatedby-product. The grading of the material is typicallyassociated with the parent rock and subsequentproduction process. Chemical and mineralogicalproperties are directly related to the parent rock.

RECYCLED AGGREGATE:An aggregate resulting from the processing of materialused in a construction process. Recycled Asphalt isnoted as an exception to the above and treated as aseparate material.

RECYCLED ASPHALT:Recycled asphalt can comprise millings, planings,return loads and offcuts from bituminous layer jointpreparation. Asphalt planings are defined as materialsderived from the layers of the pavement using a mobilemachine fitted with milling cutters. Granulated asphaltis defined as asphalt bound material recycled fromroads under reconstruction or surplus asphalt materialdestined for bound pavement layers, but unused, whichhas been granulated.

RECYCLED CERAMICS:IBAA processed from the MSW stream may contain acomponent of ceramics. The wide range of ceramicproducts is reflected within the variation of thematerials mechanical properties.

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2/8

ECYCLED GLASS:lass for subsequent recycling via crushing orranulation is typically separated at source, separated inecycling facilities, or collected from bottle banks. Therincipal raw materials are quartz sand and sodiumarbonate with varying proportions of otherompounds.

ECYCLED PLASTICS:lastics fall into two main categories, broadly defineds Thermoplastics, which can be reheated withoutignificant alterations to their structure, and Thermosetshich stiffen under application of heat. This

undamental division dictates applicable recyclingrocesses and subsequent possibilities for re-use.

ECYCLED RAILWAY BALLAST:ailway ballast is usually replaced due to inter-articulate attrition and filling of the aggregate voidsith fines. These fines can enter the railway ballast

rom a number of on-site sources, and treatmentrocesses such as screening and washing may beequired prior to subsequent re-use.

ECYCLED RUBBER:sed tyres comprise a combination of natural and

ynthetic rubber. They can either be recycled or used asecondary materials. Recycled products include crumbubber and granulate. Secondary uses include tyreales.

LATE AGGREGATE:he by-product of slate quarries primarily producing

oofing slates. The waste represents 70-90% of grossuarried volume.

PENT OIL SHALE:he residue of shale mined in the Lothian Region ofcotland after heating to drive off volatileydrocarbons. Similar in nature to burnt colliery shale.

TEEL SLAG:y-product of the manufacture of steel from pig iron.here are two types; basic oxygen slag (BOS) andlectric arc furnace (EAF) slag. EAF slag generally hasower free lime and magnesia contents than BOS as aesult of the manufacturing process, and is thus moreasily weathered.

NBURNT COLLIERY SPOIL:lso known as minestone, a by-product of coal mining.

t is derived from the rocks which lie above, below andometimes within the coal measures of Carboniferouseological age. These rocks comprise mainly siltstonesnd mudstones, and in some areas sandstones andimestones.

November 2004


Chapter 3References

3. REFERENCES

1. Design Manual for Roads and Bridges - TheStationery Office (TSO)

HA 44 (DMRB 4.1.1) ‘Earthworks: Design andContract Documents’ (includes amendment 1 April1995)

HD 27 (DMRB 7.2.4) ‘Pavement ConstructionMethods’

HD 31 (DMRB 7.4.1) ‘Maintenance of BituminousRoads’

HD 32 (DMRB 7.4.2) ‘Maintenance of ConcreteRoads’

HD 33 (DMRB 4.2.3) ‘Drainage: Surface and subsurface drainage systems for highways’

HD 38 (DMRB 7.5.1) ‘Concrete Surfacing andMaterials’ (includes amendment 1 dated Feb 1999)

HD 30 (DMRB 7.3.3) ‘Structural AssessmentProcedure’

HD 37 (DMRB 7.5.2) ‘Bituminous Surfacing Materialsand Techniques’

2. Manual of Contract Documents for HighwaysWorks - The Stationery Office (TSO)

Manual of Contract Documents for Highway Works,Volume 1 (MCHW 1): Specification for HighwayWorks.

3. British Standard Institution

BS4987 : ‘Coated macadam (Asphalt Concrete) forroads and other paved areas’; Part 1 : ‘Specification forconstituent materials and for mixtures’. BSI

November 2004

4. Others

1987

Use of waste material for road fill. DOE Circular 20/87,Department of Transport Circular 3/87, Welsh OfficeCircular 36/87; TSO

1990

Environmental Protection Act 1990; TSO

1994

Waste Management Licensing Regulations 1994. TSO

3/1


November 2004 4/1

4. ENQUIRIES

All technical enquiries or comments on this Standard should be sent in writing as appropriate to:

Chief Highway EngineerThe Highways Agency123 Buckingham Palace RoadLondon G CLARKESW1W 9HA Chief Highway Engineer

Chief Road EngineerScottish ExecutiveVictoria QuayEdinburgh J HOWISONEH6 6QQ Chief Road Engineer

Chief Highway EngineerTransport DirectorateWelsh Assembly GovernmentLlywodraeth Cynulliad CymruCrown Buildings M J A PARKERCardiff Chief Highway EngineerCF10 3NQ Transport Directorate

Director of EngineeringThe Department for Regional DevelopmentRoads ServiceClarence Court10-18 Adelaide Street G W ALLISTERBelfast BT2 8GB Director of Engineering

Chapter 4Enquiries

February 2006



SECTION 2 PAVEMENT DESIGN ANDCONSTRUCTION

PART 1

HD 24/06

TRAFFIC ASSESSMENT

SUMMARY

This Standard sets out the method for the estimationand calculation of traffic loading for the design of roadpavements. Design aids are provided for easydetermination of the number of standard axles for use inthe pavement design standard HD 26 (DMRB 7.2.3). Itsupersedes HD 24/96.


1. Remove Contents pages from Volume 7 andinsert new Contents pages for Volume 7 datedFebruary 2006.





CORRECTIONS WITHIN DESIGN MANUAL FOR ROADS AND BRIDGESNOVEMBER 2006

SUMMARY OF CORRECTION – HD 24/06 Volume 7, Section 2, Part 1TRAFFIC ASSESSMENT

Corrections have been made to figures in Tables 2.5 and 2.6 and also to equations. Paragraph 2.16 has also beenrevised.

We apologise for the inconvenience caused.

Highways AgencyNovember 2006

London: The Stationery Office

HD 24/06

Traffic Assessment

Summary: This Standard sets out the method for the estimation and calculation of trafficloading for the design of road pavements. Design aids are provided for easydetermination of the number of standard axles for use in the pavement designstandard HD 26 (DMRB 7.2.3). It supersedes HD 24/96.


THE HIGHWAYS AGENCY

SCOTTISH EXECUTIVE




February 2006







PART 1

HD 24/06

TRAFFIC ASSESSMENT

Contents

Chapter

1. Introduction

2. Calculation of Design Traffic


4. Enquiries


February 2006



1. INTRODUCTION

Mandatory Sections

1.1 Sections of this document which form partof the Standards of the Overseeing Organisationsare highlighted by being contained in boxes. Theseare the sections with which the DesignOrganisations must comply, or must have agreed asuitable departure from Standard with the relevantOverseeing Organisation. The remainder of thedocument contains advice and enlargement whichis commended to Design Organisations for theirconsideration.

General

1.2 This Part covers the estimation of design trafficfor new roads, and the estimation of past and futuredesign traffic for the maintenance of existing roads.

1.3 In the UK, road pavement structural wearresulting from traffic (i.e. fatigue cracking within thebound pavement layers and/or excessive subgradedeformation) and pavement designs (for particularmaterials) are intrinsically related. Pavement designsfor flexible and rigid pavements are presented in HD 26(DMRB 7.2.3).

1.4 Road pavement structural wear in the UK isestimated using wear factors based on vehicle axleloads. Wear factors have been produced using actualloads measured with Weigh-in-motion (WIM) sensorsinstalled on the highway network.

Implementation

1.5 This Part shall be used forthwith on all schemesfor the construction, improvement and maintenance oftrunk roads, including motorways, currently beingprepared provided that, in the opinion of the OverseeingOrganisation, this would not result in significantadditional expense or delay. Design organisations mustconfirm its application to particular schemes with theOverseeing Organisation.

February 2006

Use in Northern Ireland

1.6 For use in Northern Ireland, this Standardmust be applicable to those roads designated by theOverseeing Organisation.

1/1


Chapter 2Calculation of Design Traffic

N TRAFFIC
2. CALCULATION OF DESIG
2.1 This method shall be used to determine thepavement design traffic for new and existing roads.If it is considered inappropriate for any reason thenfurther advice must be sought from the OverseeingOrganisation.

2.2 The design traffic is the commercial vehicleloading over the design period expressed as the numberof equivalent standard (80kN) axles; it is calculatedusing the commercial vehicle flow, traffic growth andwear factors.

2.3 This method for calculating design trafficincorporates the latest research on wear and a trafficgrowth estimation based on the National Road TrafficForecast (NRTF, 1997). The background to the methodis reported in TRL Report PPR 066 (2006).

2.4 The factors used to calculate the Design Traffic(T) are as follows:

• Commercial Vehicle Flow at opening (F);

• Design Period (Y);

• Growth Factor (G);

• Wear Factor (W); and

• Percentage of vehicles in the heaviest loaded lane(P).

If reliable data for all these factors are not available, achart giving an acceptable design value is described inParagraph 2.16.

Commercial Vehicle Flow (F)

2.5 A key element in the design is the flow ofcommercial vehicles. This is expressed as AnnualAverage Daily Flow (AADF) and is the flow measuredin one direction (1-way flow). If the traffic is measuredin both directions (2-way flow) this is converted intoAADF assuming a 50:50 directional split, unless trafficcounts or studies show a significant directional bias.

23cv

2cC

November 2006

.6 Commercial vehicles are defined as those over

.5 tonnes gross vehicle weight. The structural wearaused by lighter traffic (i.e. bikes, cars and light goodsehicles) is considered to be negligible.

.7 Table 2.1 identifies the commercial vehicle (cv)lasses and categories to be used, as defined in theOBA manual (DMRB 13.1).

2.8 For new road schemes, the commercialvehicle class/category count data shall bedetermined from traffic studies using the principlesdescribed in the Traffic Appraisal Manual(DMRB 12.1.1).

2.9 For existing road schemes (i.e.maintenance design or re-alignment), a classifiedcount shall be carried out over a 12, 16 or 24 hourperiod. This must be converted to an AADF usingthe principles given in the COBA manual(DMRB 13.1.4). For Scotland, use NESA(DMRB 15.1.5).

2.10 To determine the design traffic, the AADFof commercial vehicles per day (cv/d) in onedirection, at scheme opening (or for existing roadschemes, the current flow) and the proportion inthe OGV2 category shall be used.

2/1



Table 2.1 Commercial Vehicle Classes andCategories

Commercial cv class* cvvehicle (cv) category

Buses and PSVCoaches

2-axlerigid

OGV13-axlerigid

3-axlearticulated

4-axlerigid

4-axlearticulated OGV2

5-axlearticulated

6 (or more)-axle

articulated

* Classed by axles in contact with the roadPSV = Public Service VehicleOGV = Other Goods Vehicle

2/2

Example

Count data converted to AADF using COBA 11classification.

Buses and Coaches 32 PSV2 axle Rigid 467 OGV13 axle Rigid 67 ”3 and 4 axle Articulated 274 OGV24 axle Rigid 49 ”5 axle Articulated 938 ”6 or more axle 530 ”_____________________________________________

Total Flow 2,357 cv/dTotal OGV2 Flow 1,791 cv/dPercentage OGV2 76%

2.11 Typical average commercial vehicle flowcompositions are given in Table 2.2 (Department forTransport, 2003). There is a wide variation in the valuesfor the proportion of commercial vehicles on the trunkroad network and the values in the table may beexceeded in many cases.

Table 2.2 Typical Commercial Vehicle FlowCompositions

Road Type Motorway Principalor Trunk

Percentage ofCommercial Vehicles 11 4(% cv) within AADF

% OGV2 65 38

2.12 For new road designs, the percentage ofOGV2 vehicles shall be obtained by calculation ormodelling but shall not be less than the percentagegiven in Figure 2.1.

February 2006



000 10000

low (cv/d)

Figure 2.1 Minimum Percentage of OG

Design Period (Y)

2.13 The number of years over which traffic is tobe assessed shall be selected. For past traffic, thiswill generally be the number of years sinceopening or last major structural maintenance. Forfuture design traffic this shall generally be 40years. Other design periods may be used if provento be economic and agreed with the OverseeingOrganisation.

2.14 Whole life cost considerations are important tothe selection of the design period. A 40-year designperiod without structural maintenance has generallyproven to be the most economic, particularly wheretraffic flow is high.

0

10

20

30

40

50

60

70

80

90

100

100 1

Daily F

Min

imum

perc

enta

ge o

f O

GV

2 v

ehic

les (

%)

November 2006

V2 Class Vehicles for New Construction

Standard Design Traffic Calculation for NewConstruction

2.15 For new carriageways, all lanes, includingthe hard shoulder, shall be designed to the samestandard as the heaviest loaded lane. The actualtraffic in other lanes is not considered.

2.16 Where additional information (seeParagraphs 2.17 onwards) is not available, Figure2.2 together with Figure 2.5 shall be used tocalculate the design traffic for the total traffic flow(i.e. for all lanes in one direction) for a designperiod of 40-years. In this case, the remainder ofChapter 2 can be omitted.

2/3


2


G

2ca1s

000 10000

low (cv/d)

-way) (cv/d)

rowth Factor (G)

2.17 The National Road Traffic Forecast (NRTF)is published in eight year intervals and predictsfuture traffic trends. The 1997 NRTF growth linesshown for OGV1+PSV and for OGV2 (the boldlines on Figure 2.3) shall be used unless specificalternative and more reliable local data areavailable.

.18 Past growth, where known from traffic counts,an also be used to give an indication of future trends in particular situation, but only where data over at least a0 year period are available, since averaging over ahorter period may give misleading results.

2.19 For each cv class or category, traffic growthcan be calculated which is dependent on theselected design period and the growth rate. Thegrowth factor represents the proportionaldifference between the average vehicle flow overthe entire design period and the present flow (orflow at opening). The growth factor for futuretraffic shall be found by using Figure 2.3.

1

10

100

1000

100 1

Daily F

De

sig

n T

raff

ic (

msa

)

100% OGV2

75% OGV2

50% OGV2

25% OGV2

Figure 2.2 Design Tra

Daily Flow (one

/4

2.20 If past traffic is being calculated, theapplicable growth factor is given in Figure 2.4.Bold lines are shown for OGV1+PSV and OGV2which represent national trends. These bold linesare to be used unless actual growth rates are knownfor a specific cv class or category.

2.21 If a series of past traffic counts is available itis preferable to use these to calculate mean vehicleflows for each class between count dates. Undersuch circumstances a growth factor for past trafficis unnecessary and therefore effectively becomes1.0.

2.22 The graphs for past traffic do not includeadjustments for historic changes in wear factors. As anapproximation, the current vehicle wear factors havebeen used to calculate growth factors.

ffic for 40 Year Life

November 2006



Wear Factor (W)

2.23 The structural wear to a road associated witheach vehicle that passes increases significantly withincreasing axle load. Although alternative methods areavailable, structural wear for pavement design purposesin the UK is taken as being proportional to the 4th powerof the axle load, i.e:

Wear/axle ∝ L4

(L = axle load)

Thus, a 50% increase in axle load results in a five-foldincrease in calculated structural wear.

2.24 A ‘standard axle’ is defined as an axle exerting orapplying a force of 80kN. The fourth power law is usedto equate the wear caused by each vehicle type to thenumber of equivalent standard axles, to give thestructural wear factor of that vehicle.

2.25 Sets of wear factors have been produced formaintenance and new design cases; the wear factors forthe new design case are higher than for the maintenancecase in order to allow for the additional risk that arisesfrom the additional uncertainty with traffic predictionsfor new designs.

2.26 The wear factors to be used for theMaintenance (WM), and New design (WN) cases,are shown in Table 2.3. The derivation of thesewear factors is given in TRL Report PPR 066(2006).

T

2falon

November 2006

able 2.3 Wear Factors for cv Classes andCategories

Wear Factors Maintenance NewWM WN

Buses and Coaches 2.6 3.9

2-axle rigid 0.4 0.6



3 and 4-axle articulated 1.7 2.5

5-axle articulated 2.9 4.4

6-axle articulated 3.7 5.6

OGV1 + PSV 0.6 1.0

OGV2 3.0 4.4

.27 In Table 2.3, the data used to calculate the wearctors were obtained from twelve core census sitescated throughout the Highways Agency’s trunk road

etwork and from traffic data collected in 2003.

2.28 The wear factors for the new road designcase, WN, shall be used to calculate design trafficfor all new road and pavement constructionprojects including road widening.

2/5



Extracted Growth Factor (G) values assuming 1997 NRTF growth:

Design Period (Years) 5 10 15 20 25 30 35 40

OGV1 + PSV 1.02 1.04 1.06 1.09 1.11 1.14 1.17 1.19

OGV2 1.05 1.12 1.19 1.27 1.36 1.45 1.56 1.67

Figure 2.3 Derivation of Growth Factor (G) for Future Traffic from the NRTF (1997)

Example

Considering OGV2Design Period 20 yearsGrowth Factor 1.27

1

1.5

2

2.5

3

0 5 10 15 20 25 30 35 40

Years

Gro

wth

Facto

r (G

)

4%

3%

OGV2

2%

1%

OGV1 + PSV

Source: National Road Traffic Forecasts 5%

OGV2 = 1.27

Gro

wth

per a

nnu

m

2/6 February 2006


20 25 30 35 40

0.86 0.83 0.80 0.78 0.75

0.66 0.60 0.54 0.50 0.46

th Factor (G) for Past Traffic


30 40

opening

6%

4%

0%

2%

OGV2

OGV1+PSV

Go

wth

pe

r an

nu

m

Years since opening 5 10 15

OGV1 + PSV 0.96 0.93 0.90

OGV2 0.89 0.80 0.72

Figure 2.4 Derivation of Grow

Example

Considering OGV2Time since Opening 19 yearsGrowth Factor 0.67

Percentage of Commercial Vehicles in HeaviestLoaded Lane (P)

2.29 As stated in Paragraph 2.15, all lanes aredesigned as for the heaviest loaded lane. For newand existing carriageways with 2 or more lanes inone direction, the proportion of vehicles in themost heavily loaded lane shall be estimated usingFigure 2.5.

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 10 20

Years since

Gro

wth

Fa

cto

r (P

ast

Tra

ffic

)

OGV2 = 0.67

February 2006

2.30 The heaviest loaded lane for carriageways with 2or 3 lanes is typically the nearside lane referred to asLane 1. The heaviest loaded lane for carriageways with4 or more lanes is not always Lane 1. Junctions withlane drops and lain gains will considerably influencethe flow of vehicles in each lane.

Percentage of Commercial Vehicles in Other Lanes

2.31 For maintenance purposes it is sometimesnecessary to estimate the traffic in the other lanesseparately.

2/7



2.32 The distribution of traffic between lanes can,under certain circumstances, vary considerably betweendifferent roads. The distribution can be influenced bytraffic flow, by the proximity to junctions and onapproaches to traffic signals and roundabouts.

2.33 For 2-lane roads, all traffic not in Lane 1will be in Lane 2. For 3-lane roads, it should beconsidered that all commercial vehicles not inLane 1 are in Lane 2 although commercial vehiclesup to 7.5 tonnes are permitted to use the right handlane. For roads with 4 or more lanes, data from acore census site or a traffic count shall benecessary to confirm the distribution of trafficacross each lane.

Design Traffic (T)

2.34 The future cumulative flow, in terms ofmillion standard axles (msa) for cv class Ti can bedetermined according to the following equation:

Ti = 365×F×Y×G×W×P×10-6msa

Design Traffic (T) = Σ Ti

Figure 2.5 Percentage of Commercial

50

60

70

80

90

100

100 1000

Total Daily Com

Per

cen

tage

of

com

mer

cial

veh

icle

s in

hea

vie

st t

raff

ic l

an

e

2/8

Where:

F = Flow of Traffic (AADF) for each traffic classat openingY = Design Period (Years)G = Growth Factor (from Figure 2.3)P = Percentage of vehicles in the heaviest loadedlane (Figure 2.5)W = Wear Factor for each traffic class (WM forMaintenance or WN for New Design Case) fromTable 2.3

For past traffic, Y = years since opening; G =Growth Factor according to Figure 2.4.

If the calculation of traffic in other lanes is formaintenance purposes, P shall be the percentage ofthe commercial vehicles determined to be in eachlane, refer to paragraph 2.33.

2.35 For new design cases, the calculation of designtraffic is typically made by category, e.g.: OGV1 andOGV2.

2.36 Design traffic calculations can be made using theform shown in Table 2.4 and Tables 2.5 and 2.6 presenttwo examples using Table 2.4 to calculate the designtraffic (for new design and for maintenance).

Vehicles in Heaviest Loaded Lane (P)

10000 100000

mercial Vehicle Flow

November 2006



Commercial vehicle AADF Growth Wear Factor Weighted Annualclass or category (F) Factor (W) Traffic

(G) (by class or by category)

Either by classBuses and Coaches(PSV)

OGV12 axle rigid3 axle rigid

OGV24 axle rigid3 and 4 axle artic5 axle artic6 axle artic

Or by category*

OGV1 + PSVOGV2

Total daily flow (cv/d) Total weighted annual traffic

Percentage of vehicles inheaviest traffic lane (P)

T = 365×F×Y×G×W×P×10-6msa Design Period (Y)

Design Traffic (T)

Weighted annual traffic = 365×F×G×W×10-6msa

Design Traffic (T) = Total weighted annual traffic ×Y×P

* See Paragraph 2.12

Table 2.4 Table for the Calculation of Design Traffic

2/9November 2006




(G) WM (by class or by category)

Either by classBuses and Coaches 77 1.09 2.6 0.08(PSV)

OGV12 axle rigid 914 1.09 0.4 0.153 axle rigid 59 1.09 2.3 0.05

OGV24 axle rigid 53 1.27 3.0 0.073 and 4 axle artic 302 1.27 1.7 0.245 axle artic 1,021 1.27 2.9 1.376 axle artic 574 1.27 3.7 0.98

Or by category*

OGV1 + PSVOGV2

Total daily flow (cv/d) 3,000 Total weighted annual traffic 2.95 msa

Percentage of vehicles in 86%heaviest traffic lane (P)

T = 365×F×Y×G×W×P×10-6msa Design Period (Y) 20 years

Design Traffic (T) 51 msa




• Maintenance calculation made by class• Commercial vehicle flow of 3,000 vehicles per day• The design period is 20 years• NRTF growth has been assumed

Table 2.5 Traffic Calculation Example for Maintenance

2/10 November 2006




(G) WN (by class or by category)

Either by classBuses and Coaches(PSV)

OGV12 axle rigid3 axle rigid

OGV24 axle rigid3 and 4 axle artic5 axle artic6 axle artic

Or by category*

OGV1 + PSV 744 1.19 1.0 0.323OGV2 456 1.67 4.4 1.223

Total daily flow (cv/d) 1,200 Total weighted annual traffic 1.55 msa

Percentage of vehicles in 94%heaviest traffic lane (P)

T = 365×F×Y×G×W×P×10-6msa Design Period (Y) 40 years

Design Traffic (T) 58 msa




• New design calculation made by category• Commercial vehicle flow of 1,200 vehicles per day• The design period is 40 years• NRTF growth has been assumed

Table 2.6 Traffic Calculation Example for New Design

2/11


February 2006

Chapter 3References and Bibliography

3/1

3. REFERENCES AND BIBLIOGRAPHY

Design Manual for Roads and Bridges (DMRB)

The Stationery Office Ltd

HD 26 (DMRB 7.2.3) Pavement Design.

Traffic Appraisal Manual (DMRB 12.1.1) Theapplication of traffic appraisal to trunk roads.

COBA (DMRB 13.1) Economic Assessment of RoadSchemes.

NESA (DMRB 15.1) Economic Assessment of RoadSchemes in Scotland.

Department for Transport and its predecessor

1997

NRTF. National Road Traffic Forecasts (Great Britain).

2003

Transport Statistics for Great Britain: 2003 edition.

Others

1984

LR1132; Powell W D, Potter J F, Mayhew H C andNunn M E, The Structural Design of Bituminous Roads,TRRL.

1987

RR87. Mayhew H C and Harding H M. Thicknessdesign of concrete roads. TRRL.

2006

TRL Report PPR 066. Atkinson V M, Merrill D andThom N. Pavement Wear Factors. TRL.


February 2006 4/1

4. ENQUIRIES




Chief Highway EngineerTransport WalesWelsh Assembly GovernmentCathays Parks M J A PARKERCardiff Chief Highway EngineerCF10 3NQ Transport Wales


Chapter 4Enquiries

IAN 73/06. Page 1 of 59 February 2009 Revision 1

INTERIM ADVICE NOTE 73/06 Revision 1 (2009) DESIGN GUIDANCE FOR ROAD PAVEMENT FOUNDATIONS (DRAFT HD25) SUMMARY This interim advice note provides design guidance for road pavement foundations. INSTRUCTIONS This IAN takes immediate effect. It supersedes HD 25/94 and also IAN 73/06 including the Specification Clauses and Notes for Guidance clauses that were included in IAN 73/06. It should be read in conjunction with HD 26/06 Contents Section 1 Background Section 2 Implementation Section 3 Departures from Standard Section 4 Draft Standard HD 25 Section 5 Draft Specification Clauses Section 6 Draft Notes for Guidance Clauses

Section 1 Interim Advice Note 73/06 Revision 1 (2009) Section 2 Design Guidance For Road Pavement Foundations Section 3 (Draft HD25)


Section 1. Background This Interim Advice Note provides design guidance for road pavement foundations. Road pavement upper layers are now subject to design methods and criteria that have been published in HD 26/06. The method is based on a classification for road pavement foundations that are separated into four Foundation Classes. The design guidance contained in this Interim Advice Note (presented as the draft HD 25) defines the four Foundation Classes and describes the methods to be used in their design and the testing regime associated with the design. It is published in this interim form to ensure that all road pavements may be designed in a coordinated manner using both HD 26/06 and the guidance in this IAN. The guidance has been produced in the form of a draft standard to replace HD 25/94, together with draft Specification and Notes for Guidance clauses that will be included in the MCHW. The Standard HD 25/94 (DMRB 7.2.2) and IAN 73/06 are now withdrawn. The new foundation classes are presented in two forms: ‘Performance Designs’ that allow a wide use of materials together with measures and testing to ensure design requirements are met and also ‘Restricted Designs’ that are included for schemes where performance testing may not be appropriate. The Guidance is included in this Interim Advice Note in 3 sections Section 4. Draft Standard HD 25 ‘Pavement Foundations’ Section 5. Draft Specification Clauses 890 to 896 Section 6. Draft Notes for Guidance Clauses NG890 to NG896 This revision to IAN 73/06 has been produced following comment received and a further review of the original text. The Chapters have been reorganised and the text updated. Section 2. Implementation This Interim Advice Note shall be used forthwith on all future schemes for the construction, improvement and maintenance of trunk roads. It shall apply also to all those schemes that are in preparation provided that, in the opinion of the Overseeing Organisation, this will not result in significant additional expense or delay progress. Design Organisations shall confirm its application to particular schemes with the Overseeing Organisation. Section 3. Departures from Standard The design guidance for pavement foundations included in this Interim Advice Note is presented into two separate chapters. The Restricted Designs included in Chapter 3 of Section 4 may be used by designers without reference to the Overseeing Organisation. The Performance Designs included in Chapter 4 of Section 4 should be referred to the Overseeing Organisation for approval under the Departure from Standards procedure. It is the intention that this will be required for an interim period until the guidance is published as a Standard in the DMRB and the Specification/Notes for Guidance clauses are published in the MCHW.

Section 4 Interim Advice Note 73/06 Revision 1 (2009) Design Guidance For Road Pavement Foundations (Draft HD25)


Section 4. Draft Design Standard HD 25 Pavement Foundations Replaces previous HD 25/94 and IAN 73/06 Contents Chapter 1. Introduction 2. Background 3. Restricted Foundation Designs 4. Performance Foundation Designs 5. Characterisation of Materials 6. Drainage and Frost 7. Testing 8. References Annex A: Equations of Thickness Design Examples – Performance Foundation Design Annex B: Procedure for Alternative Performance Foundation Designs Annex C: Performance Foundation Design procedure – Flowcharts and Examples

Section 4 Interim Advice Note 73/06 Revision 1 (2009) Chapter 1. Introduction Design Guidance For Road Pavement Foundations

(Draft HD25)


Chapter 1. INTRODUCTION General 1.1 The main purpose of the foundation is to distribute the applied vehicle loads to the

underlying subgrade, without causing distress in the foundation layers or in the overlying layers. This is required both during construction and during the service life of the pavement.

Scope 1.2 This Part covers the design of pavement foundations in order to achieve the

Foundation Classes called up in HD26. 1.3 The four Foundation Classes are defined by the Foundation Surface Modulus value

(see Paragraph 2.1 for modulus definitions) used for design purposes, as follows:

Class 1 ≥ 50MPa Class 2 ≥ 100MPa Class 3 ≥ 200MPa Class 4 ≥ 400MPa

1.4 The materials covered by this Part are the subgrade, either natural ground or

compacted fill, unbound capping materials and stabilised capping materials as defined in Series 600 of the Specification (MCHW1), and hydraulically bound subbase mixtures (including stabilised soils) or granular subbase mixtures as defined in Series 800 of the Specification. Further definitions can be found in HD23 (DMRB 7.1.1).

1.5 Two design approaches are presented. The first allows a limited number of Restricted

Foundation Designs to be applied for Foundation Classes 1, 2 and 3 and is particularly intended for use on schemes of limited extent. The designs are conservative, making allowances for uncertainty in material performance and in layer thickness.

1.6 The second approach is for Performance Foundation Designs. These cover all four

Foundation Classes and provide more flexibility to the designer. The main acceptance criterion for construction of a Performance Design is the in-situ Foundation Surface Modulus, measured immediately prior to the placement of the overlying pavement layers. A design method is provided with examples of how the four foundation classes might be achieved. Some duplication between the different design options has been included in order that each procedure can be read independently.

1.7 The choice as to which approach and which Foundation Class is selected is usually

made on economic grounds based on the materials that are available, the size of the scheme and relevant costing information. It is expected that designers will give full consideration to the use of local and secondary materials.

1.8 All Performance Designs will be subject to approval under a Departure from

Standards. Performance Designs must only be used in conjunction with the Performance Related Specification for Foundations, as given in Draft Clauses 890 onwards in Section 5 of this Interim Advice Note.

Section 4 Interim Advice Note 73/06 Revision 1 (2009) Chapter 1. Introduction Design Guidance For Road Pavement Foundations

(Draft HD25)


1.9 Performance Designs recognise that not all materials have equal engineering properties and permit designers to take advantage of improved foundation materials by reducing the thickness of overlying layers. The resulting designs are minimum thickness requirements to achieve the design assumptions, with no allowance for construction tolerance. (Also see Paragraph 4.54)

1.10 The important role of drainage in achieving good long-term pavement performance is

also highlighted and key requirements are given in Chapter 6 of this Section. 1.11 Issues with regard to frost penetration are also covered, with respect to their effect on

foundation durability. 1.12 Chapter 7 on test methods is included for general information. The particular tests

required by the Performance Related Specification for Foundations are detailed in Chapters 5 and 6.

Section 4 Interim Advice Note 73/06 Revision 1 (2009) Chapter 2. Background Design Guidance For Road Pavement Foundations

(Draft HD25)


Chapter 2. BACKGROUND DEFINITIONS 2.1 The following expressions used in this standard are defined below. Also see

Figure 2.1.

Stiffness Modulus: the ratio of applied stress to induced strain. Foundation Surface Modulus: a measure of ‘Stiffness Modulus’ based on the application of a known load at the top of the foundation; it is a composite value with contributions from all underlying layers. Subgrade Surface Modulus: an estimated value of ‘Stiffness Modulus’ based on subgrade CBR and used for foundation design. Layer Modulus: a measure of ‘Stiffness Modulus’ assigned to a given foundation layer; usually, this is a long term estimate that will take account of degradation due to factors such as cracking. Element Modulus: a measure of ‘Stiffness Modulus’ assigned to a discrete sample of material and usually characterised by a laboratory test; it does not normally take account of degradation due to factors such as cracking. Mean Foundation Surface Modulus: the value that must be equalled or exceeded by the moving mean of five consecutive in-situ Foundation Surface Modulus measurements carried out in accordance with Clauses 890, 891,892 and 895 of the Specification. Minimum Foundation Surface Modulus: the value that must be exceeded by all individual measurements of in-situ Foundation Surface Modulus, when measured in accordance with Clauses 890, 891, 892 and 895 of the Specification.

Figure 2.1 Modulus Definitions

Subbase

Capping (Optional)

Foundation Surface

Modulus

Layer Modulus

Element Modulus

Subgrade

Subgrade Surface

Modulus Estimated from CBR


(Draft HD25)


2.2 Great care should be taken not to confuse the Layer Modulus with the Foundation Surface Modulus, as values will not generally be similar. For example, a Class 2 foundation with 100MPa Foundation Surface Modulus may comprise an upper layer with a Layer Modulus of 150 MPa over a subgrade with a Subgrade Surface Modulus of 60 MPa.

2.3 The design procedure adopted in this standard establishes groups of materials.

These Mixture Groups are defined as follows:

Unbound: mixtures included in Specification Series 800 such as Clause 803 Type 1 Unbound Mixtures, Clause 804 Type 2 Unbound Mixtures, Clause 805 Type 3 (open graded) Unbound Mixtures, Clause 806 Category B (close graded) Unbound Mixtures and in Specification Series 600 including Clause 613 Capping Material Types 6F1, 6F2, 6F3, 6F4, 6F5 and 6S. Fast-setting: bound mixtures that achieve more than 50 per cent of their specified compressive strength class after 28 days curing at 20 degrees C. Slow-setting: bound mixtures that achieve 50 per cent or less of their specified strength class after 28 days curing at 20 degrees C.

ROLE OF FOUNDATION During Construction 2.4 The stresses in the foundation are relatively high during construction, although the

number of stress repetitions from construction traffic is relatively low and traffic is not as channelised as during the in-service life of the pavement.

2.5 During pavement construction, it is expected that loads will be applied to the

foundation by delivery vehicles, pavers and other construction plant. At any level where such loading is applied, the strength and material thickness have to be sufficient to withstand the load without damage occurring that might adversely influence, to any significant extent, the future performance of the pavement.

2.6 Foundation layers also have to be either protected from, or to be of sufficient

durability to withstand environmental effects from rain, frost, high temperature etc, without sustaining damage.

2.7 Damage may take the form of rutting or other uneven deformation, cracking in

hydraulically bound mixtures (including stabilised soils), or other forms of material-specific degradation.

2.8 The designs given in this Draft HD 25, in conjunction with the tests and material

restrictions given in the Specification, are intended to ensure that, under normal construction conditions, such damage is avoided.

2.9 The foundation also has to be of sufficient stiffness for the overlying pavement layers

to be placed and adequately compacted.


(Draft HD25)


In Service 2.10 During the life of a pavement, its foundation has to be able to withstand large

numbers of repeated loads from traffic. It is also likely to experience ingress of water, particularly if the upper pavement materials begin to deteriorate towards the end of their design lives.

2.11 It is essential that the Foundation Surface Modulus assumed in the design, and

relating to the choice of Foundation Class, is maintained throughout the life of the pavement. If this is not the case, deterioration of the upper pavement layers would typically occur more rapidly than assumed.

2.12 It is also essential that excessive deformation does not accumulate within the

foundation under repeated traffic loading, since this is a potential source of wheelpath rutting at the pavement surface.

2.13 The performance of the foundation will also depend on the design, construction and

maintenance of the earthworks and associated drainage system. HA 44 (DMRB 4.1.1) provides earthworks information and Chapter 6 of this Section provides further information on drainage. It is essential that the drainage system ensures that there is no accumulation of water in the pavement and foundation layers and that all excess moisture is allowed to disperse.

Section 4 Interim Advice Note 73/06 Revision 1 (2009) Chapter 3 Restricted Foundation Designs Design Guidance For Road Pavement Foundations

(Draft HD25)


Chapter 3. RESTRICTED FOUNDATION DESIGNS APPLICABILITY 3.1 The designs given in this chapter are intended for use in cases where it is

inappropriate to carry out the range of compliance testing required by the Performance Related Specification for Foundations. For this reason they are conservative and recognize the greater uncertainty present in material properties when subjected to more limited testing.

3.2 Designs are not included for Foundation Class 4 since it is considered essential to

measure the properties of such a foundation during construction to give adequate assurance that the appropriate long-term Foundation Surface Modulus is likely to be achieved.

3.3 Where bound subbase mixtures are permitted in Restricted Designs they have been

restricted to those using CEM1 (EN 197:1) as the primary binder, acknowledging the greater uncertainty and lesser experience at present in the UK with other hydraulic binders and the consequent need for testing to be carried out.

3.4 The information to be collected during construction of the pavement foundation and

covered by the Specification includes:

Strength measurement (CBR value) at the top of the exposed subgrade, immediately prior to placement of the overlying foundation layers, throughout the Works;

Material density and the actual thickness for each stage of foundation construction, throughout the Works;

Compliance with the relevant material specifications from Specification Series 600 and 800 at each stage of foundation construction, throughout the Works;

SUBGRADE REQUIREMENTS 3.5 For design purposes, the Subgrade Surface Modulus must be estimated from CBR

values using the procedure given in Chapter 5 (paragraphs 5.3 to 5.11). The Subgrade Surface Modulus used for design must be determined using the lowest value of the long term and short term CBR.

3.6 The Subgrade Surface Modulus for design and associated Design CBR must be

stated by the Designer in Appendix 7/1 for each foundation area. These values must not be increased after construction has started.

3.7 Other methods for estimation of Subgrade Surface Modulus will be permitted with a

Departure from Standards, provided that a satisfactory correlation with the reference method can be demonstrated.

3.8 The subgrade CBR value must be checked on site before foundation construction

starts, in accordance with Clause 893 of the Specification (also see Paragraphs 5.12 to 5.15) and must be equal to, or be greater than, the Design CBR.

3.9 If the in-situ CBR is found to be less than the Design CBR, then the subgrade must

either be improved to the Design CBR or the foundation redesigned.


(Draft HD25)


3.10 Where the in-situ subgrade has an estimated CBR value less than 2.5 per cent, it must be improved as described in Chapter 5 (paragraphs 5.16 to 5.21) and its Design CBR must be based on the statements in those paragraphs.

THICKNESS DESIGN 3.11 Required thicknesses for Restricted Designs are shown in Figures 3.1 and 3.2. 3.12 Restricted Designs are included for Foundation Class 1, but these are not permitted

for use on Trunk Roads including Motorways where pavements are designed for more than 20msa. This is because of the increased likelihood of damage during construction. Assurance against this would require Performance Design and use of the Performance Related Specification for Foundations.

3.13 Foundation Class 1 designs may make use of any of the capping options given in

Table 6/1 in Series 600 of the Specification. The finished surface of the foundation must meet the criteria for subbase in Series 700 of the Specification.

3.14 For Class 2 foundations, there are four different design options depending on whether

unbound or bound subbase is chosen and whether a capping is used. 3.15 Foundation Class 2 designs may make use of granular subbase mixtures to Clause

803, 805 and 806, Cement Bound Granular Mixtures to Clause 821 and 822 and Soil Cement to Clause 840. Cement Bound Granular Mixtures and Soil Cements must achieve compressive strength classes of at least C3/4. Granular subbase mixtures to Clause 804 (Subbase Type 2) may also be used for pavement with design traffic levels up to 5msa.

3.16 For Class 2, a capping may also be incorporated as part of the foundation (See

Figure 3.2). For all Foundation Classes, using a layer of capping material brings practical benefits by providing a working platform and a good base for compaction of the overlaying layers, which may be particularly appropriate for lower strength subgrades. A layer of suitable unbound material below a bound foundation layer also provides a drainage path, see Chapter 6.

3.17 Foundation Class 3 designs are restricted to those using Cement Bound Granular

Mixtures to Clause 821 and 822 achieving at least the compressive strength class C8/10.

3.18 Figures 3.1 and 3.2 are referenced to both Subgrade surface stiffness values (in

MPa) and to CBR values for consistency with previous standards and comparison with Performance Designs. The relationship between CBR and Stiffness Modulus is that given in Chapter 5.

3.19 Design thicknesses are to be rounded up to the nearest 10 mm. 3.20 Thicknesses derived for these Restricted Designs are subject to the normal

construction tolerances as given in Series 700 of the Specification (MCHW1).


(Draft HD25)


Design Example 1 Subgrade Surface Modulus for design estimated as 40MPa (approximately 3.5% CBR); the following Restricted Design options exist:

Foundation Class 1 (Specification Series 600 materials; Figure 3.1): - 465 rounded up to 470 mm Foundation Class 2 (CBGM A or B, C3/4 Figure 3.1): - 305 rounded up to 310 mm Foundation Class 2 (Types 1, 2, 3 or Category B subbase on capping; Figure 3.2): - 290mm subbase + 230mm capping Foundation Class 3 (CBGM A or B C8/10; Figure 3.1): - 305 rounded up to 310 mm

FIGURE 3.1 Restricted Design Options – Subbase or Capping only

0

100

200

300

400

500

600

0 50 100 150Subgrade Stiffness Modulus (MPa)

Laye

r Thi

ckne

ss (m

m)

Class 1 - Capping MCHW1 Series 600

Class 2 - Subbase MCHW1 803, 804, 805, 806 (804 mixture for not more than 5msa)

Class 2 - Subbase MCHW1 821, 822, or 840 soil cement: strength C3/4 or C5/6

Class 3 - Subbase MCHW1 821 or 822: strength C8/10

Subgrade CBR (%)

2 5 8 10 12 15 20

Restricted Design: Foundation Classes 1-3

{


(Draft HD25)


0

100

200

300

400

Sub

base

Thi

ckne

ss (m

m)

Subbase MCHW1 803, 804, 805, 806 (804 mixture for not more than 5msa)

Subbase MCHW1 821, 822, or 840 soil cement: strength C3/4 or C5/6

Subgrade CBR (%)

2 5 8 10 12 15 20

0

100

200

300

0 50 100 150

Subgrade Stiffness Modulus (MPa)

Cap

ping

Thi

ckne

ss (m

m)

Capping MCHW1 Series 600

Restricted Design: Foundation Class 2

FIGURE 3.2 Restricted Design Options – Class 2 Subbase on Capping

Section 4 Interim Advice Note 73/06 Revision 1 (2009) Chapter 5. Characterisation of Materials Design Guidance For Road Pavement Foundations

(Draft HD25)


Chapter 4. PERFORMANCE FOUNDATION DESIGNS APPLICABILITY 4.1 The main objectives for developing Performance Designs for foundations and the

Performance Related Specification for Foundations are:

to facilitate the efficient use of a wide range of resources, incorporating natural, secondary and recycled materials as both binders and aggregates;

to provide some assurance that the material performance assumptions made during the design process are being, or are likely to be, achieved;

to recognise the structural contribution of improved foundation performance and hence permit the adjustment in thickness of the pavement layers above.

4.2 The philosophy of Performance Design relies on performance testing to confirm the

physical properties that are critical to the design process. To ensure parity between different materials and minimise unnecessary exclusion, this evaluation is based on a common method of assessment.

4.3 The materials used in pavement foundations have a vast range of properties that

affect performance (e.g. particle size, strength, elastic stiffness, stress dependency, curing rates). However, it is not practical to carry out testing for all of these properties and a single foundation surface modulus performance test provides a pragmatic solution.

4.4 Pavement foundation design in the UK has been based on the principles of layered

linear elastic modelling since the 1980s (Powell et al, 1984). This approach requires the elastic stiffness of each foundation layer to be defined, enabling critical stresses and strains to be predicted. These are subsequently assessed against empirically derived limits, in order to reduce the risk of premature pavement failure to an acceptable level. The models have traditionally focused on a very restricted number of materials, with relatively well documented engineering properties.

4.5 Assessing the engineering properties of individual materials for both the construction

and in-service situations is a complicated and lengthy process. It is simpler and more cost-effective to develop a single proxy measure, which can be used in all situations with all types of material, to predict the likely overall performance of the foundation.

4.6 The use of a Performance Related Specification for assessing Foundation Surface

Modulus is compatible with the current UK methodology for pavement design, as described in HD26 (DMRB 7.2.3). This method requires a given level of Foundation Surface Modulus, referred to as a Foundation Class, to support various types of pavement construction and associated material thicknesses.

4.7 Performance Design is a method that can be used to predict the likely Foundation

Surface Modulus that will be achieved by certain combinations of foundation layers over different types of natural ground (the subgrade). The basis for the model is described in more detail in Annex B.

4.8 The process for designing, constructing and testing a Performance Related

Foundation is summarised in Figure 4.1. 4.9 Until publication of this standard in the Design Manual for Roads and Bridges, all

Performance Foundation Designs will be subject to approval under a Departure from Standards.


(Draft HD25)


Check for unacceptable levels of surface regularity

FIGURE 4.1 Summary Flowchart for Performance Related Foundations

Design:

Select Foundation Class

Design foundation

Demonstration Areas:

Conduct trafficking trial

Review design of foundation and/or choice of materials if inadequate performance encountered in any area.

Main Works:

Estimate Design Subgrade CBR andSubgrade Stiffness Modulus

Measure In-Situ Subgrade CBR(must be ≥ Design CBR)

Construct Demonstration Area

Check material compliance (MCHW1)(e.g. strength, thickness & density)

Check Foundation Surface Modulusagainst required value adjusted for In-Situ CBR

(See Para 4.38 – 4.93)

Measure In-Situ Subgrade CBR.Value ≥ Design CBR

Check material compliance (MCHW1)(e.g. strength, thickness & density)

Check Foundation Surface Modulusagainst UNADJUSTED values

Measure permanent deformation and remeasure Foundation Surface Modulus

for bound materials only – see Table 4.1

Construct Main Works

failure

failure

failure

failure


(Draft HD25)


4.10 Performance Foundation Design must only be used in conjunction with the

Performance Related Specification for Foundations (Clauses 890 to 896), see Section 5.

4.11 The Performance Related Specification for Foundations requires a range of

information to be collected during construction of the pavement foundation. The principal tests called for include those listed in Paragraph 3.4 and the measurement of the Foundation Surface Modulus at top of foundation level, throughout the Works (to be carried out immediately prior to construction of the overlying pavement layer).

SUBGRADE REQUIREMENTS 4.12 For design purposes, the Subgrade Surface Modulus must be estimated from CBR

values using the procedure given in Chapter 5 (paragraphs 5.3 to 5.11). The Subgrade Surface Modulus used for design must be estimated using the lower value of the long-term and short-term CBR.

4.13 The Subgrade Surface Modulus for design and associated Design CBR must be

stated by the Designer in Appendix 7/1 for each foundation area. These values must not be increased after construction has started.

4.14 Other methods for estimation of Design Subgrade Surface Modulus will be permitted

with a Departure from Standards, provided that a satisfactory correlation with the reference method can be demonstrated.

4.15 The subgrade CBR value must be checked on site before foundation construction

starts, in accordance with Clause 893 of the Specification (also see Paragraphs 5.12 to 5.15) and must be equal to, or be greater than, the Design CBR.

4.16 If the in-situ CBR is found to be less than the Design CBR, then the subgrade must

either be improved to the Design CBR or the foundation redesigned. 4.17 Where the in-situ subgrade has an estimated CBR value less than 2.5 per cent, it

must be improved as described in Chapter 5 (paragraphs 5.16 to 5.21) and its Design CBR must be based on the statements in those paragraphs

FOUNDATION SURFACE MODULUS REQUIREMENTS 4.18 Table 4.1 gives the unadjusted Mean Foundation Surface Modulus and Minimum

Foundation Surface Modulus values, for each Foundation Class, and for different categories of materials, to be achieved or exceeded at the top of foundation level immediately prior to the construction of the overlying pavement layers.

4.19 Foundations may also be constructed using layers of different materials making

composite foundations. Performance measures for these must be agreed as part of the Departure approval on a scheme specific basis.

4.20 Foundation Surface Modulus is measured using the Dynamic Plate Test (refer to

Chapter 7) in accordance with Clauses 890, 891, 892 and 895 of the Specification.


(Draft HD25)


Surface Modulus (MPa)

Class 1 Class 2 Class 3 Class 4

Long-Term In-service Surface Modulus ≥50 ≥100 ≥200 ≥400

Unbound Mixture Types: 40 80 #

Fast-setting Mixture Types: 50 100 300 600

Mean Foundation Surface Modulus

Slow-setting Mixture Types: 40 80 150 300

Unbound Mixture Types: 25 50 #

Fast-setting Mixture Types: 25 50 150 300

Minimum Foundation Surface Modulus Slow-setting

Mixture Types: 25 50 75 150

Table 4.1 Top of Foundation Surface Modulus Requirements

Note. Fast-setting and slow-setting mixtures are defined in Paragraph 2.3

Only permitted on trunk roads including motorways that are designed for not more than 20msa

# Not permitted for pavements designed for 80msa or above (HD26 requirement) Unbound materials are unlikely to achieve the requirements for Class 3 & 4

4.21 Dynamic Plate Test (DPT) devices for testing Surface Modulus, both Falling Weight

Deflectometers (FWD) and Lightweight Deflectometers (LWD) must conform with the Specification Clause 895 and are also described in Chapter 7.

4.22 Where a LWD is proposed, a correlation exercise must be carried out in the

Demonstration Area using the 25 measurement points (as specified in Clause 891 of the Specification). At each location, Foundation Surface Modulus must be measured using both the proposed LWD and the standard DPT device (an FWD) and the correlation procedure followed as defined in Specification Clause 895.

4.23 LWD devices may be used on a site specific basis to give an indication of likely

compliance with the Performance Related Specification for Foundations. However, the standard DPT device (FWD) will constitute the reference method other than when the requirements of paragraphs 4.21 and 4.22 are satisfied.

4.24 The Mean Foundation Surface Modulus values have been chosen to provide

assurance that the foundation is performing as expected in the short term, immediately before being covered by the overlying pavement layers. They are not expected to provide a direct method for predicting the long-term, in service, design Foundation Surface Modulus.

4.25 The Mean Foundation Surface Modulus is defined as the moving mean of five

consecutive in-situ Foundation Surface Modulus measurements. The results are expected to contain significant scatter due to the inherent variability of the subgrade and the inconsistency of subbase and capping materials.

4.26 The Minimum Foundation Surface Modulus is defined as the value that must be

exceeded by all individual measurements of in-situ Foundation Surface Modulus and it is intended to be the absolute threshold below which no part of an adequately constructed foundation is allowed to fall.


(Draft HD25)


4.27 The Mixture Type referred to in Table 4.1 (Unbound, Fast-setting or Slow-setting –

refer to Chapter 2, Paragraph 2.3) relates to the material used in the subbase layer and assumes that the same material is used throughout.

4.28 The Mean Foundation Surface Modulus requirement for Slow-setting Mixtures is

lower than that for Fast-setting Mixtures, acknowledging that they may require a longer curing period before achieving their full Layer Modulus potential.

4.29 The Mean Foundation Surface Modulus requirement for Unbound Mixtures is lower

than the expected long-term Modulus as it is measured in the partially confined condition (i.e. without the overlying pavement layers).

4.30 The Mean Foundation Surface Modulus requirement for Fast-setting Mixtures is

higher than the expected long-term Modulus because they gain strength quickly, but can be expected to deteriorate during the life of the pavement.

4.31 It is considered unlikely that Unbound Mixture Types will economically achieve

Foundation Classes 3 and 4, and Mean and Minimum Surface Modulus values have therefore not been included in Table 4.1. Approval may be sought if designers can justify specific material use for an individual scheme.

4.32 Experience of using Slow-setting Mixture Types is limited in the UK to date. Further

background information on this topic can be found in TRL Report 408 (Atkinson, Chaddock and Dawson, 1999).

4.33 As further experience is gained using the Performance Related Specification for

Foundations in the UK, it is anticipated that the values in Table 4.1 will be amended and the range of material specific values will be extended. The version in this Interim Advice Note reflects the generalised current state of knowledge.

4.34 A separate Departure from Standards may be sought for all Foundation Classes to

agree alternative Mean and Minimum Foundation Surface Modulus values where sufficient additional data is available to demonstrate satisfactory performance.

4.35 Materials complying with Clause 840 of the Specification (Treated Soils) are not

currently permitted in Foundation Class 4, without a separate Departure from Standards approval.

Demonstration Areas 4.36 Demonstration Areas are required to enable the adequacy of the performance of

each foundation design to be assessed. It also allows material production and laying procedures to be proved, prior to construction of the Main Works.

4.37 Generally, the Demonstration Area should be situated where the in-situ subgrade

CBR is equal to the Design CBR. However, where this is not possible (e.g. due to drying of clay subgrades in hot summers), it is only permitted for the in-situ subgrade CBR in the Demonstration Area to be greater than the Design CBR. Where the in-situ CBR is lower than the Design CBR, a new foundation design is required, as this demonstrates that the designer has not correctly estimated the lower value of the short-term and long-term CBR.


(Draft HD25)


4.38 The Mean and Minimum Foundation Surface Modulus values given in Table 4.1 must be adjusted (in the Demonstration Area only, see paragraph 4.39) if the in-situ CBR is greater than the Design CBR. High in-situ CBR values are likely to give unrealistically elevated Foundation Surface Modulus values, which will subsequently decrease when the subgrade weakens through moisture ingress.

4.39 For the demonstration area, the appropriate values selected from Table 4.1 to be

achieved or exceeded must be adjusted using the following equation:

Eadjusted = E x (1 + (0.28 × Ln (CBRin-situ/CBRdesign)))

Where: Eadjusted is the adjusted Foundation Surface Modulus value (either mean or minimum) E is the Mean or Minimum Foundation Surface Modulus value taken from Table 4.1 CBRin-situ is the actual in-situ CBR in the Demonstration Area CBRdesign is the Design CBR

Design Example 2 Design CBR estimated at 3% (Paragraph 5.3) Design carried out for Foundation Class 3 – 200MPa From Table 4/1:

Design Mean Surface Modulus (Fast-setting Mixture) = 300MPa Design Minimum Surface Modulus (Fast-setting Mixture) = 150MPa Adjusted Values for two possible In-situ CBR values are:

In-situ Adjusted Adjusted CBR Mean Minimum 5% 343 MPa 171 MPa 7% 371 MPa 185 MPa

4.40 It is recommended that after laying, the Surface Modulus testing is not carried out at

ages less than 24 hours in the case of Unbound Mixtures and 7 days in the case of Fast-setting Mixtures. For Slow-setting Mixtures, it is recommended that the testing be carried out after the same amount of time for which the foundation is likely to remain exposed during the main works. If site arrangements require that a foundation will remain uncovered for a longer or shorter time, then alternative arrangements for testing the demonstration area may be necessary.

4.41 It may be advisable to undertake Surface Modulus measurements at the surface of

intermediate foundation layers, as this may provide useful information in the event of inadequate performance at top of foundation level. The designer should provide details of the modulus values that should be achieved.


(Draft HD25)


4.42 A trafficking trial is required for each Demonstration Area at top of foundation level, as specified in Clause 891. This requires:

a limit on rut depth, depending on foundation thickness and Mixture Type,

and the Foundation Surface Modulus values continue to meet the Mean and

Minimum Surface Modulus requirements after trafficking (Fast-setting and Slow-setting Mixtures only).

Main Works

4.43 Performance testing is to be carried out on the main works as required by the Specification. The value of Foundation Surface Modulus set out in Table 4.1 for the main works is to be achieved not more than 24 hours before being covered by pavement layers. No adjustment for in-situ CBR is required, as the adequacy of the design has already been shown in the Demonstration Area.

4.44 For schemes where the foundation is to remain exposed for a long period, an earlier

check is advised to give assurance that the Foundation Surface Modulus requirements will be achieved. Further checks are likely to be necessary, where foundations are trafficked before being overlaid.

4.45 The Overseeing Organisation may permit the contractor to continue foundation

construction without a requirement to achieve the Foundation Surface Modulus values set out in Table 4.1 provided that other criteria were to be consistently achieved. Proposals for an alternative procedure that will still demonstrate that the constructed foundation meets the performance criteria should be included as part of the Departure application. Foundation Surface Modulus values would still need to be monitored for recording purposes only.

4.46 Material testing, density and thickness measurement in accordance with the relevant

clauses of Specification Series 600 and 800 (including Clauses 890 to 896 are required throughout the main works to demonstrate compliance.

THICKNESS DESIGN 4.47 The layer thicknesses derived using the Performance Foundation Design Method are

based on the consideration of three criteria (Chaddock and Roberts, 2006):

Protection of the subgrade during construction; Provision of adequate support stiffness to the overlying pavement layers; Practical minimum layer thicknesses for construction.

4.48 There are a large number of possible designs for the various combinations of

Subgrade Surface Modulus and foundation material, in order to achieve the desired Foundation Class. Some example design charts are shown in Figures 4.2 to 4.6. Interpolation between the lines can be used to develop designs with alternative Layer Modulus values. Equations for these example designs have been provided in Annex A. A method of generating alternative designs by modelling, not covered by the examples in Figures 4.2 to 4.6, is discussed in Annex B. The use of granular subbase to Clause 804 (Type 2) should not be used for any pavement design for more than 5msa.


(Draft HD25)


4.49 Experience has demonstrated that foundations constructed on subgrades with a Design CBR of less than 2.5 per cent may cause problems and designs for subgrades below these values are not given. See Paragraph 4.17

4.50 The practical minimum foundation thicknesses have been taken as 150mm for all

materials in a Class 1 or 2 foundation, 175mm for materials in a Class 3 Foundation and 200mm for materials in a Class 4 Foundation. The increase in minimum thicknesses for Classes 3 and 4 relates to their proportional sensitivity to variations in thickness. Thin layers of stiffer bound materials are also more susceptible to cracking and it is important that these materials do not crack beyond the levels assumed in the design.

4.51 Maximum permissible Layer Stiffness values have also been imposed for each

Foundation Class to minimise the risk of selecting very thin, very stiff foundation layers at lower subgrade CBR values. The maximum permissible Layer Stiffnesses to be used in design are:

Foundation Class 1 100 MPa Foundation Class 2 350 MPa Foundation Class 3 1000 MPa Foundation Class 4 3500 MPa

These maximum stiffness values apply whatever method is used to design the foundation.

4.52 Examples of designs with subbase on capping are only presented in this Chapter for

Foundation Class 2. The structural contribution of capping materials with low Layer Stiffness values is limited when compared with the stiffness of subbase materials required to achieve Foundation Classes 3 and 4. Their inclusion in the design model does not, therefore, demonstrate a significant reduction in the thickness of subbase required but designers should consider the practical advantages of including capping materials in the foundation design. More information is available in TRL Report PPR127 (Chaddock & Roberts 2006).

4.53 The inclusion of a capping layer however, should always be considered for the

practical benefits they afford, enabling construction plant to lay the subbase and providing a good base for the necessary compaction to be achieved. The provision of a capping layer may be particularly appropriate for lower strength subgrades and can, if the material is suitable, also provide a drainage path below a layer of bound material.

4.54 The thicknesses determined using the examples in this Chapter are the minimum

thickness requirements to achieve the design assumptions. Permitting construction using normal level tolerances (as specified in Clause 702 of the Specification) is likely to result in an as-built foundation that is significantly thinner than assumed in the design. This, in turn, will lead to a significant risk that the Mean and Minimum Foundation Surface Modulus Values will not be achieved. For this reason, it is essential that the designer either specifies a permitted negative tolerance of zero or increases the design thickness by an appropriate amount (possibly 15mm) if retaining the requirements of Clause 702 of the Specification.

4.55 It is recommended that design thicknesses are rounded upwards to the nearest

10mm.


(Draft HD25)


4.56 The completed surface of all foundations must meet all other relevant criteria in Series 700 of the Specification.

Design Example 3 Subgrade Surface Modulus for design estimated as 35MPa (approximately 3% CBR) Options covered by Figures 4.2-4.6 (minimum thicknesses, tolerances to be added and then rounded up):

Foundation Class 1 (Capping only; Figure 4.2): - 460mm of 50MPa material Foundation Class 2 (Subbase only; Figure 4.3): - 290mm of 200MPa material Foundation Class 2 (Subbase on 75MPa capping; Figure 4.6): - 150mm of 350MPa material + 240mm capping (75MPa) Foundation Class 3 (Subbase only; Figure 4.4): - 215mm of 1000MPa material Foundation Class 4 (Subbase only; Figure 4.5): - 435mm of 1000MPa material

0

100

200

300

400

500

600

0 50 100 150


Laye

r Thi

ckne

ss (m

m)

Layer stiffness = 50MPaLayer stiffness = 75MPaLayer stiffness = 100MPa

Subgrade CBR (%)

2 5 8 10 12 15 20

Performance Design: Foundation Class 1

FIGURE 4.2 Class 1 Designs – Single Foundation Layer


(Draft HD25)


0

100

200

300

400

500

600

0 50 100 150


Laye

r Thi

ckne

ss (m

m)

Layer stiffness = 150MPa

Layer stiffness = 200MPaLayer stiffness = 350MPa

Subgrade CBR (%)

2 5 8 10 12 15 20


0

100

200

300

400

500

600

0 50 100 150


Laye

r Thi

ckne

ss (m

m)


Subgrade CBR (%)

2 5 8 10 12 15 20





(Draft HD25)


0

100

200

300

400

500

600

0 50 100 150


Laye

r Thi

ckne

ss (m

m)


Subgrade CBR (%)

2 5 8 10 12 15 20


0

100

200

300

0 50 100 150


Cap

ping

Thi

ckne

ss (m

m)

Capping layer stiffness = 75MPa



FIGURE 4.6 Class 2 Designs – Subbase on Capping

0

100

200

300

Sub

base

Thi

ckne

ss (m

m)


Subgrade CBR (%)2 5 8 10 12 15 20


(Draft HD25)


Chapter 5. CHARACTERISATION OF MATERIALS SUBGRADE Design Phase 5.1 In the UK, the primary material performance characteristic used in foundation design

is Stiffness Modulus. For subgrades, this property is difficult to measure reliably and consistently, so historically California Bearing Ratio (CBR) has been used as an indirect measure.

5.2 Full access to the construction site is not always possible during the design phase so

it can be difficult to carry out in-situ testing. Where a Geotechnical Investigation is carried out, representative samples should be taken of the subgrade materials likely to be encountered on site.

5.3 Estimation of the likely long-term, short-term and hence Design CBR should be

derived using laboratory CBR tests in accordance with BS 1377 Pt 4 (1990). Further advice is given in HA44 (DMRB 4.1.1) and in Paragraph 7.6. The Design CBR is the lower of the long-term and short-term CBR.

5.4 Laboratory testing has the advantage that realistic conditions of moisture and

‘disturbance’ can be simulated. The tests should be carried out over a range of conditions to reproduce, as far as possible, the conditions of moisture content and density which are likely to be experienced during construction and in the completed pavement. Cohesive soils should be compacted to not less than 5% air voids to reproduce the likely conditions on site. Equilibrium moisture content can be deduced from measurements on a suction plate (Black and Lister, 1979).

5.5 Where it is not possible to collect material samples for assessment using the

laboratory CBR tests, the Suction Index Method should be used, as described in Appendix C of LR1132 (Powell, Potter, Mayhew and Nunn. 1984). Table 5.1 gives an extract from Table C1 in LR 1132 with estimated values for long-term CBR depending on soil type particularly for clay subgrades where moisture and plasticity index are significant issues. These CBR values assume a high water table and that the foundations may be wetted by ground water during their life. Further advice related to soils criteria are given in Figs C2 and C3 in LR 1132.


(Draft HD25)


Thin Thick SOIL PI % Estimated CBR %

Heavy Clay Silty Clay Sandy Clay

70 60 50 40 30 20 10

2 2 2

2.5 3 4 3

2 2

2.5 3 4 5 6

Silt* - 1 1 Sand (poorly graded) Sand (well graded) Sandy Gravel (well graded)

- - -

20

40

60 * estimated assuming some probability of material saturating Notes 1) A thick layered construction is a depth to subgrade of 1200mm 2) A thin layered construction is a depth to subgrade of 300mm

Table 5.1 Equilibrium Subgrade CBR Estimation

5.6 Where a scheme is large enough that it may include several different soil types or

moisture conditions, a more extensive survey will be necessary. The survey will consider the availability of materials and will ensure that all variations in subgrade are recorded and special emphasis can be made at transition zones and cut/fill interfaces.

5.7 In selecting the Design CBR value for the subgrade, consideration must be given to

the likely moisture conditions applying during construction, assuming that appropriate precautions are taken against excessive disturbance, as demanded by the Specification.

5.8 The designer must also consider the likely long-term equilibrium moisture condition,

making reasonable allowance for moisture ingress through the pavement, but assuming drainage is correctly installed as designed. Testing of moisture content beneath adjacent existing roads may provide useful information.

5.9 For imported fill soils, the stiffness of the subgrade must be assured in the long term,

which requires adequate compaction to ensure that it has a high density and low air voids content. The Moisture Condition Value (MCV – Matheson and Winter, 1997) is normally used to help ensure acceptable compaction and MCV testing is usually carried out at both the design stage, to evaluate the materials available, and at the construction stage, to ensure that soils are placed in an acceptable moisture condition. This process should ensure the long-term moisture equilibrium of the subgrade by minimising moisture ingress. In this manner, the subgrade should provide a stable platform for the pavement throughout its design life. The MCV can also be related to the ability of a subgrade to withstand construction traffic. There are a number of correlations between MCV and soil strength which, though to some extent material specific, may provide useful information for preliminary design purposes (Lindh and Winter, 2003; Winter, 2004). The MCV test is described in Clause 632 of the Specification (MCHW 1).


(Draft HD25)


5.10 Other methods for estimating the design CBR will be permitted with Departure from Standards approval, provided that evidence of both suitability and reliability can be demonstrated. For coarser materials, the plate bearing test may also be appropriate, more information is available in Section 7.

5.11 For Performance Design, the Design CBR must be converted to the Subgrade

Surface Modulus. The following equation has been derived from work on certain soils (Powell et al, 1984) and this must be used unless separate Departure from Standards approval has been given:

E = 17.6 (CBR)0.64 MPa

Where CBR is given as a % value.

Construction Phase 5.12 During construction, the in-situ CBR must be checked against the Design CBR, for

both Restricted and Performance Designs. 5.13 The Dynamic Cone Penetrometer (DCP) method must be used to measure the in-situ

subgrade strength, as described in Clause 893 of the Draft Specification. Not less than 5 tests are to be carried out in the Demonstration Area and at not more than at 60m intervals for the Main works as required by the Specification. Additional tests may be necessary to identify the location of different subgrade conditions.

5.14 The results from the DCP testing must be converted to CBR using the procedure

described in Clause 893 of the Draft Specification. 5.15 Other methods for measurement of in-situ CBR will be permitted with separate

Departure from Standards approval, provided that a satisfactory correlation with the Clause 893 reference method can be demonstrated.

Subgrade with low CBR (CBR < 2.5%) 5.16 The minimum permitted Design CBR is 2.5% CBR. Where a subgrade has a lower

CBR it is considered unsuitable support for a pavement foundation. It must therefore be permanently improved using one of the options given in the following paragraphs.

5.17 The material at the surface can be removed and replaced by a more suitable

material. If the depth of relatively soft material is small, it can be replaced in its entirety, although it may only be necessary to replace the top layer. The thickness removed will typically be between 0.5 and 1.0m.

5.18 Although the new material may be of better quality, the new Design CBR should be

assumed to be equivalent to 2.5%, in order to allow for effects of any softer underlying material and the potential reduction in the strength of the replacement material to its long-term CBR value.

5.19 If the soil is cohesive, a lime (or similar) treatment may be appropriate, subject to soil

suitability being demonstrated. Details of various soil treatments are given in HA44 (DMRB 4.1.1). The new Design CBR should again be assumed to be equivalent to 2.5% unless agreed otherwise under Departure from Standard approval. HA 74 (DMRB 4.1.6) contains further advice on stabilisation.


(Draft HD25)


5.20 For certain conditions, the incorporation of a geosynthetic material into the foundation design may be advantageous. Approval under a Departure from Standard to adopt an alternative Design CBR value will be necessary, based on testing or previous experience with the specific geosynthetic and the materials being used on the scheme.

5.21 If the soil is reasonably permeable, a deeper than normal drainage system may be

considered, together with a system of monitoring the improvement expected. Design of the main foundation may then be based on the conditions are achievable in the time available subject to consideration of the long-term equivalent CBR value.

CAPPING AND SUBBASE 5.22 In order to make use of the Performance Designs in Chapter 4 it is necessary to

estimate the long-term Layer Modulus values for the materials in each proposed foundation layer. Some of the techniques that may be suitable to assessing the Element Modulus of foundation materials are listed in Table 5.2.

Test Method Suitable for: Information in:

Dynamic Plate Testing of compacted trial layers

Unbound, Fast-setting and Slow-setting Specification Clause 895

Modulus of Elasticity testing in compression

Fast-setting and Slow-setting Mixtures BS EN 13286-43:2003

Triaxial testing Unbound Mixtures BS EN 13286-7: 2004

Springbox testing of laboratory specimens Unbound Mixtures Chapter 7

Table 5.2 Applicability of Stiffness Modulus Test Techniques

5.23 Estimates of Layer Modulus derived using the techniques given in Table 5.2 are not

necessarily directly comparable. This is due to variability in the test conditions (e.g. confining stress and sample size).

5.24 Use of the Dynamic Plate Test (DPT) on compacted trial layers has the advantage

that the same test is called up for compliance assessment by the Performance Related Specification for Foundations. The test can be carried out either on a small trial site or in a suitably sized (minimum 1m square and 0.5m deep) container in the laboratory. The advantage is that the material can be compacted in a realistic manner. However, the modulus will be a partially confined value, which for unbound materials can be approximately 60% of that expected when confined beneath a finished pavement. Furthermore, in the laboratory, the results will be affected by the substrate upon which the layer is compacted and due to the size of specimen, testing is usually restricted to an LWT.

5.25 The standard laboratory test for Modulus of Elasticity of HBM mixtures is the

Compression Test, one of three described in BS EN 13286-43. It is appropriate for those materials which have sufficient strength to remain intact during the test. However, the resulting Stiffness Modulus is that applying to a small intact and very well compacted specimen of material, whereas the condition in-situ may be less dense and is likely to include significant cracking due to shrinkage and temperature fluctuation. For these reasons, no more than 20% of the measured laboratory


(Draft HD25)


Stiffness Modulus may be taken for long-term design in the case of Fast-setting Mixtures. For Slow-setting Mixtures, no more than 10% of the laboratory value should be used. If well-documented evidence demonstrates that other values can be justified then, in such circumstances, Departure from Standards approval will be required to use the alternative. These values assume that no abnormal damage is caused to the material during construction.

5.26 Designers should be aware that because materials vary and also site practice and

conditions can alter, the long-term, in-situ design Layer Stiffness Modulus of an HBM, derived from laboratory stiffness testing using the factors described in the previous paragraph, may not always meet the Surface Modulus requirements of the Performance Related Specification (see Chapter 4) when site testing is carried out.

5.27 Springbox testing (See Chapter 7) has the advantage that it allows a small sample of

unbound material to be tested under approximately realistic stress conditions and under appropriate (normally soaked then drained) moisture conditions.

5.28 Stiffness Modulus varies according to the applied stress conditions and for unbound

materials also varies according to the moisture state of the material. 5.29 An unbound material which is confined by overlying pavement layers will often appear

stiffer than the same material when uncovered and therefore partially unconfined. This means that the Stiffness Modulus apparent during construction will tend to be lower than the Stiffness Modulus expected in service. This is reflected in Table 4.1 for the top of Foundation Surface Modulus requirements.

5.30 An unbound material at high moisture content can appear significantly less stiff than a

drier material. This means that the Stiffness Modulus apparent during construction can be significantly affected by the weather conditions applying at the time and the effectiveness of the drainage system, and often may not reflect the longer term in-service condition (at equilibrium moisture content).

5.31 Since a ‘cracked’ state is assumed for the in-service condition of hydraulically bound

mixtures (HBMs), the initial ‘uncracked’ or ‘less cracked’ material at the time of construction may have a higher Stiffness Modulus.

5.32 Some of the HBMs permitted by the Specification are relatively slow setting. This

means that the Stiffness Modulus apparent during construction will be significantly less than that achievable in the longer term. Also, such materials are susceptible to damage, particularly by trafficking, both during construction and early in their service life, which may result in their expected long-term Stiffness Modulus being reduced.

5.33 Dynamic Plate Tests on existing pavements can be used to derive Foundation

Surface Modulus values and are applicable in major reconstruction or widening schemes where an existing foundation layer is to be retained in the rehabilitated pavement. The technique known as back-analysis (see HD29, DMRB 7.3.2) will allow derivation of a Foundation Surface Modulus, and it will neither be practical nor necessary to distinguish the Stiffness Moduli of individual foundation layers. For Dynamic Plate Testing of existing pavement foundations refer to HD29 (DMRB 7.3.2).

Section 4 Interim Advice Note 73/06 Revision 1 (2009) Chapter 6 Drainage and Frost Design Guidance For Road Pavement Foundations

(Draft HD25)


Chapter 6. DRAINAGE AND FROST Drainage 6.1 It is of vital importance to keep water out of the subbase, capping and subgrade, both

during construction and during the service life of the pavement. 6.2 It is good practice and will reduce the opportunity for foundation deterioration if the

carriageway drainage is constructed and kept operational before foundations are constructed.

6.3 During construction every effort should be made to protect the subgrade by

constructing and protecting foundation layers before rain can soften it. The Performance Related Specification for Foundations provides a means of quantifying whether the actions, or omissions, of the contractor have contributed to the degradation of the foundation. Installing deep subgrade drains and sloping the formation to shed water could also prevent problems due to excess water not only during construction but also in the completed pavement.

6.4 In the long term, infiltration of water through the pavement should be minimised by

good design, construction and maintenance. An escape route for water that succeeds in entering the foundation should always be provided (Figure 6.1).

6.5 Wherever possible, the foundation drainage should be kept separate from pavement

run-off drainage in all new construction and in reconstruction work. There should always be a down-slope route from the subbase to the drain. Further details are given in HA44 (DMRB 4.1.1).

6.6 In reconstruction and widening projects it is necessary to maintain the continuity of

drainage from existing capping and subbase materials through adjacent new materials to a drain, using appropriate thicknesses and crossfalls. Where strict adherence to the designs in Chapters 3 and 4 would introduce a barrier to such drainage, a Departure from Standards should be sought and an alternative foundation design proposed. This may include use of permeable bound materials or weep-pipes, and may, exceptionally, involve departure from the pavement materials and thicknesses derived from HD26/06.

FIGURE 6.1 Foundation Drainage

6.7 A granular aggregate drainage blanket (MCHW 1 Series 600) of thickness at least 150mm and not more than 220mm may be used to drain water that infiltrates through the pavement. In order to stop pore clogging by fines from other adjacent layers, geosynthetic separators may be used when those layers are constructed of fine soil or fine capping. A drainage layer of this type may be particularly appropriate below a

Upper pavement Foundation Subgrade

Rain

to drain

Seepage

Section 4 Interim Advice Note 73/06 Revision 1 (2009) Chapter 6 Drainage and Frost Design Guidance For Road Pavement Foundations

(Draft HD25)


bound foundation layer. The drainage layers so formed may be treated as capping for structural design purposes.

6.8 When the water table is high and especially when the subgrade is moisture sensitive

with a Plasticity Index < 25, slot drains as detailed in the Highway Construction Details, can be beneficial. The drain is placed below the bottom of capping (or subbase if no capping is used), to drain any water that may permeate through these materials. Deeper drains can be beneficial in drying and strengthening these, and some other soil types.

6.9 It is useful to check the speed at which water can drain out of a granular subbase as

a result of ingress due, perhaps, to a cracked or damaged pavement or a surcharging drain. A procedure for calculating this is given in Jones and Jones (1989a) along with a means of estimating ingress through cracks in the bound layers. On this basis it may be possible to specify a permeability value. Care should be taken to ensure that the value required does not conflict with any limitations imposed by a specified grading, see Jones and Jones (1989b).

6.10 If it is necessary to determine the permeability of the subbase or capping material,

this must be done on the full grading, at the correct density under a low hydraulic head. A suitable permeameter and procedure is described in HA41 (DMRB 4.2.4)

6.11 Drainage of the subbase may be omitted only if the underlying materials (capping,

subgrade) are more permeable than the subbase, and the water table never approaches the underside of foundation closer than 300mm.

Frost Protection 6.12 For routine cases all material within 450mm of the road surface shall be non frost-

susceptible as required by the Specification Series 600 and tested according to BS812: Part 124 (1989).

6.13 This requirement can be over-severe in some places (e.g. coastal areas) and may be

reduced to 350mm if the Mean Annual Frost Index (MAFI) of the site is less than 50. Advice on the frost index for any particular area may be obtained from the Met Office and further information from TRL Report RR45 (1986).

6.14 The frost index is defined as the product of the number of days of continuous freezing

and the average amount of frost (in degrees Celsius) on those days.

Section 4 Interim Advice Note 73/06 Revision 1 (2009) Chapter 7. Testing Design Guidance For Road Pavement Foundations

(Draft HD25)


Chapter 7 TESTING 7.1 The two reasons for testing pavement foundation layers are to check compliance with

the Specification during construction and in pavement assessment. Also see HD30 (DMRB 7.3.3). This chapter introduces different test devices. It is for general information and advice only and does not comprise part of the Overseeing Organisation’s requirements although some tests are included in the Performance Related Specification for Foundations, (Draft Clauses 890 to 896).

Density Testing (Figure 7.1)

7.2 Tests on density can be carried out using a nuclear density gauge. A radiating source

is applied to the material. The amount of radiation detected decreases in proportion to the bulk density of the material between source and receiver. To determine the moisture content another source sends out radiation intercepted by hydrogen atoms in the test material. The dry density is calculated from the bulk density and the moisture content. If the material being tested is carbonaceous, care is required in interpreting the moisture content and dry density obtained. Testing is extremely rapid (less than 5 minutes) and a reading may be repeated readily. The machine is portable. Calibration is required for each soil or aggregate that is to be tested.

Figure 7.1 Density Testing Apparatus

7.3 It should be noted that two modes of nuclear density testing are possible. The

quickest and easiest is ‘backscatter’ mode, which is influenced only by the density of the top 100-150mm of material and is most heavily influenced by material very near the surface. ‘Transmission’ mode should generally be used for all testing of foundations. Regulations related to security as well as to health and safety must be followed.

7.4 The volumetric test involves excavating and weighing material removed from a small

hole and refilling with uniform sand. The test is time-consuming but can give a direct means of measuring density for comparison with laboratory values. Other non-nuclear means of measuring density using electronic methods are available but may not meet the depth requirement for measuring foundations.

California Bearing Ratio 7.5 The California Bearing Ratio (CBR) test involves the insertion of a 50mm diameter

plunger into the ground surface at a rate of 1mm per minute, whilst the load is recorded. Surcharge rings can be placed around the plunger to simulate an

Sand replacement

Nuclear DensityTransmission

modeBackscatter

mode


(Draft HD25)


overburden. A laboratory version of the same test is available in which the sample tested is constrained within a 152.5mm diameter mould. The load at penetrations of 2.5 and 5mm is compared with the result for a standard aggregate and the ratio given as a percentage. The test is not suitable for coarse aggregates because the plunger and aggregate particles will be of similar size. The test measures neither Stiffness Modulus nor Shear Strength directly – giving a somewhat combined measure of both. It takes around half an hour on site and between 1 and 2 hours in the laboratory and there is a large body of experience of its use.

7.6 There are several variants on the CBR test; laboratory, field, with surcharge,

saturated etc. In the context of this document the laboratory CBR with a surcharge to simulate the appropriate vertical overburden stress of the case being considered should be taken as the standard method used. The appropriate moisture content and wetting or drying condition is also important. Laboratory CBR results for granular soils are often higher than those in the field due to mould confinement effects. The test is penetration controlled and so does not model the stress level imposed by traffic. CBR is an empirical test and is best measured as initially intended although other test devices such as the cone penetrometer, the Dynamic Cone Penetrometer and the Plate Bearing Test can be used to determine approximate estimates of CBR.

Dynamic Cone Penetrometer 7.7 Various sizes of static field cone penetrometer for insertion into a test material exist

for the rapid approximate assessment of CBR. In general they only cover a fairly low CBR range and are therefore applicable to soft and medium fine grained subgrades.

7.8 The Dynamic Cone Penetrometer (DCP – Figure 7.2) is similar to other field cone

penetrometers except that it is driven into the ground under the action of a weight dropped onto an anvil. It is therefore suited to stronger and coarser materials than other penetrometers. The rate of penetration into the ground can then be related approximately to CBR. The standard equipment and its interpretation are discussed in more detail in HD 29 (DMRB 7.3.2)

7.9 The Dynamic Cone Penetrometer is a device incorporating an 8kg steel drop weight

that falls vertically through 575mm and makes contact with a relatively light steel anvil. The anvil is rigidly attached, via steel rods to a 20mm diameter 60o steel cone, which is thus driven vertically into the ground. Requirements are included in the Specification Clause 893.

FIGURE 7.2 The Dynamic Cone Penetrometer


(Draft HD25)


7.10 Exceptionally, other dynamic cone equipment may be used providing it has been

calibrated against equipment meeting the requirements of the Specification for the type of materials present.

Test Procedure 7.11 For subgrade assessment the result for each test are recorded as the distance in

millimetres per blow between 50mm and 550mm of penetration from top of subgrade level. The procedure for calculation of material strength is given in the Specification and is based on a relationship established by TRL for medium to fine grained soils.

Usage 7.12 The Dynamic Cone Penetrometer may also be used through many other materials,

particularly in a composite foundation, to measure both their CBRs and layer thicknesses. However, this strength measure will not normally be specified for materials overlying the subgrade since results are highly dependent on particle size and can therefore, without calibration to specific materials, be misleading. The DCP can be a useful additional measure for assessment of the Demonstration Areas, and is particularly valuable for evaluating the properties of an existing pavement foundation.

Plate Bearing Test (Figure 7.3) 7.13 For coarser materials the Plate Bearing Test may be found appropriate for

determination of subgrade CBR values. The test is described in detail in BS1377 (1990) and involves placing a circular plate on a foundation layer. Its use for testing is described in the MCHW 1 Series 600. For use on pavement foundation materials, there is no need for removal of surface material or for non-vibratory compaction.

FIGURE 7.2 Plate Bearing Test and DC

Figure 7.3 Plate Bearing Test

7.14 An approximate empirical relationship with CBR can be made as follows:

CBR = 6.1 × 10-8 × (k762)1.733 %

where k762 is the Modulus of Subgrade Reaction, defined as the applied pressure under the loading platen divided by the displacement (normally 1.25mm) with a plate of 762mm (30 inch) diameter. The Modulus of Subgrade Reaction for other plate sizes can be determined using the appropriate conversion factor from Fig 7.4.

Heavy Plant

Jack

Dial gauges Loading platenReference beam

Plate Bearing Test


(Draft HD25)


FIGURE 7.4 Conversion Factors for Smaller Plate Sizes

7.15 The test is laborious to set up and carry out and requires a heavy vehicle (typically a site truck or excavator) to provide the reaction force. The speed of loading is slow giving poor simulation of traffic loading.

Dynamic Plate Tests 7.16 These tests involve dropping a weight onto a platen and measuring the deflection.

Usually a damping mechanism (rubber buffers) is incorporated to control the magnitude and duration of the loading. The Specification sets out the requirements for the standard test and its interpretation. The Falling Weight Deflectometer (FWD) measures the stress applied and the resulting deflection of the foundation at several radial positions up to 2.5 metres from the loading plate. Interpretation is generally in terms of the Stiffness Modulus of each foundation layer but is not straightforward and should be carried out by an experienced pavement engineer. If only the central deflection is used to determine a Surface Modulus for the foundation, then interpretation can be carried out as for other Dynamic Plate tests. The Lightweight Dynamic Plate (LWD) apparatus (Figure 7.5) can be used for most foundation materials but care will be required for very stiff foundations, as it may be unable to deliver sufficient load to achieve a measurable deflection.

7.17 The Surface Modulus testing, required by the Specification, must be carried out using

a Dynamic Plate Test device, which has been properly calibrated to the manufacturer's specification; this includes the FWD as well as the LWD.

7.18 The requirements for the equipment including calibration are given in the

Specification. If an LWD is used, a correlation exercise with the FWD for the site and for the foundation materials being used will be required as set out in the Specification (MCHW1), Clause 895.

0 0.2

0.4

0.6

0.8

1 1.2

100 200 300 400 500 600 700 800Plate diameter (mm)

Factor = 0.079 + 0.001209 x Diameter

Con

vers

ion

fact

or

to o

btai

n k 7

62


(Draft HD25)


FIGURE 7.5 Lightweight Dynamic Plate Test

7.19 If any equipment is proposed which does not fully comply with these requirements, it may be permitted at the discretion of the Overseeing Organisation, provided that it is carefully calibrated against other compliant equipment, for the specific types of material and layer thickness encountered on the site. This calibration would normally be carried out as part of the testing of the Demonstration Area for Performance Designs.

Springbox 7.20 The Springbox equipment (Edwards et al, 2005) (Fig 7.6) is a suitable tool for testing

unbound granular and some weak hydraulically bound mixtures. It consists of a steel box containing a cubical sample of material, of edge dimension 170mm, to which a repeated load can be applied over the full upper surface. One pair of the box sides is fully restrained and the other is restrained through elastic springs, giving a wall stiffness of 10-20kN per mm.

7.21 The equipment enables a realistic level of compaction to be applied to the test

material, by means of a vibrating hammer and also includes a facility to introduce water to the sample or drain water from its underside.

7.25 Loading takes the form of repeated vertical load applications of controlled magnitude

at a frequency of at least 1Hz and no greater than 5Hz. The load capacity is equivalent to a vertical stress of at least 150kPa.

7.26 Measurements of both vertical and horizontal (spring restrained) deflection can be

made, with 2 measurement transducers for each measure. In the case of vertical deflection measurement, the equipment allows the transducers to make direct contact with the specimen, via holes in the loading platen.

7.27 The stiffness modulus of the material can be calculated from the averaged deflections

measured over a series of loading patterns.


(Draft HD25)


FIGURE 7.6 Springbox

Section 4 Interim Advice Note 73/06 Revision 1 (2009) Chapter 8. References Design Guidance For Road Pavement Foundations

(Draft HD25)


Chapter 8 REFERENCES 1976 Parsons A.W, “The rapid determination of moisture condition value of earthwork material”. Report LR 750, TRRL 1979 Black W.P.M and Lister N.W: “The strength of clay fill subgrades, its prediction in relation to road performance”. Report LR 889, TRRL 1984 Powell W.D, Potter J.F, Mayhew H.C and Nunn M.E, "The Structural Design of Bituminous Roads”. Report LR1132; TRRL 1989 Jones, H.A and Jones, R.H, 1989(a). Horizontal permeability of compacted aggregates, Proc. 3rd Int'l Symp. Unbound Aggregates in Roads (UNBAR3), Univ. of Nottingham, pp 70-77. Jones, R.H and Jones, H.A, 1989(b). Granular drainage layers in pavement foundations. Proc. 3rd Int'l Symp. Unbound Aggregates in Roads (UNBAR3), Univ. of Nottingham, pp 55-69. 1997 Mattheson GD and Winter MG, 1997. Use and application of the MCA with particular reference to glacial tills. TRL Report 273. Transport Research Laboratory, Wokingham 1999 Atkinson V, Chaddock B and Dawson. Enabling the use of secondary aggregates and binders in pavement foundations. TRL Report 408. Transport research Laboratory, Wokingham. 2000 Chaddock, B.C.J and Schoepe, B, 2000. The hydraulic and structural behaviour of unbound granular sub-base layers, Proc. 5th Int'l Symp. Unbound Aggregates in Road Construction (UNBAR5), Univ. of Nottingham, pp 297-305. 2003 Lindh, P. and Winter, M.G. “Sample preparation effects on the compaction properties of Swedish fine-grained tills”. Quarterly Journal of Engineering Geology and Hydrogeology, 36(4), pp 321-330. 2004 Winter, M.G. “Determination of the acceptability of glacial tills for earthworks”. Quarterly Journal of Engineering Geology and Hydrogeology, 37(3), pp 187-204. 2005 Edwards, J.P., Thom, N.H. Fleming, P.R. and Williams, J., 2005. “Accelerated Laboratory Based Mechanistic Testing of Unbound Materials within the Newly Developed NAT Springbox”. Transportation Research Record. Issue Number 1913. Journal of the Transportation Research Board. ISBN 0309093864 2006 Chaddock, B. and Roberts, C. Road foundation design for major UK Highways. Published Project Report PPR127. Transport Research Laboratory, Wokingham.

Section 4 Interim Advice Note 73/06 Revision 1 (2009) Annex A. Equations of Thickness Design Guidance For Road Pavement Foundations Design Examples (Draft HD25)


ANNEX A: EQUATIONS OF THICKNESS DESIGN EXAMPLES – PERFORMANCE FOUNDATION DESIGN In the following equations: Hcap (mm) is capping layer thickness,

Hsb (mm) is subbase layer thickness, Ecap is capping layer stiffness (MPa), Esb is subbase layer stiffness (MPa) and, CBR is the California bearing ratio of the subgrade (%). (S) and (D) denotes whether the thicknesses were determined using the subgrade strain criterion (S) or the deflection criterion (D).

Equations for Single-Layer Designs

Foundation class FC1 (Capping only):

For subgrade CBR ≥2.5% and ≤ 5%, the capping thickness (mm) is the greater of the thicknesses given by the following two equations:

))(.395.01(.10845.1)( 025.025.03 CBRLnEESH capcapcap−− −×=

)541.1)(.(10918.10)538.1)((.1000.2)( 32 −×−−×= CBRLnCBRLnEDH capcap

For subgrade CBR >5% and ≤ 15%, the capping thickness (mm) is given by:

))(.23.01(.10016.1)( 026.0214.03 CBRLnEESH capcapcap−− −×=

where, the minimum value of Hcap permissible is 150 mm and these relationships are valid for capping material with a layer stiffness of between 50 and 100 MPa.

Foundation class FC2 (Subbase only): For subgrade CBR ≥ 2.5% and ≤ 5%, subbase thickness (mm) is given by:

))(.316.01(.1085.2)( 021.0341.03 CBRLnEESH sbsbsb −×= − For subgrade CBR >5% and ≤ 30%, subbase thickness (mm) is given by:

)(.69.1025.9)( 202.02 CBRLnESH sbsb −×= −

where, the minimum value of Hsb permissible is 150 mm and these relationships are valid for subbase material with a layer stiffness of between 150 and 250 MPa. Note, these equations are not suitable for subbase materials with a layer stiffness between 250MPa and 350MPa.

Section 4 Interim Advice Note 73/06 Revision 1 (2009) Annex A. Equations of Thickness Design Guidance For Road Pavement Foundations Design Examples (Draft HD25)


Foundation class FC3 (Subbase only): For subgrade CBR ≥ 2.5% and ≤ 30%, subbase thickness (mm) is given by: ))(..261.00.1(.1044.8)( 008.048.03 CBRLnEEDH sbsbsb

−− −×= where, the minimum value of Hsb permissible is 175 mm and these relationships are valid for subbase material with a layer stiffness of between 500 and 1,000 MPa. Foundation class FC4 (Subbase only): For subgrade CBR ≥ 2.5% and ≤ 30% subbase thickness (mm) is given by: ))(.234.00.1(.1053.1)( 025.04833.04 CBRLnEEDH sbsbsb

−− −×= where, the minimum value of Hsb permissible is 200 mm and these relationships are valid for subbase material with a layer stiffness of between 1,000 and 3,500 MPa.

Equations for Subbase on Capping The following equations that describe the designs have been developed for subbase on capping foundation class FC2 and subgrade CBR in the range ≥2.5% and ≤ 15%:

The subbase thickness (mm) is given by:

)(..39.21)1)(4123.0.(1027.8)( ))(335.0271.0(745.1))(1933.02075.0(4 CBRLnEEEELnDH capcap ELnsbcap

ELnsbcapsb

−+− −−×= The capping thickness (mm) is given by:

)(.561001.3 2 CBRLnH cap −×= where, the minimum value of Hsb and Hcap permissible is 150 mm and these relationships are valid for capping material with a layer stiffness of between 50 and 100 MPa and subbase material with a layer stiffness of between 150 and 250 MPa.

Section 4 Interim Advice Note 73/06 Revision 1 (2009) Annex B: Procedure for Alternative Design Guidance For Road Pavement Foundations Performance Foundation Designs (Draft HD25)


ANNEX B: PROCEDURE FOR ALTERNATIVE PERFORMANCE FOUNDATION DESIGNS B.1 Alternative design options may be calculated analytically using a multi-layer linear

elastic analysis package. In such cases, the designer must show that all the design criteria given in the following paragraphs (subgrade strain, surface deflection and practical thickness limits) are met.

B.2 Protection of the subgrade during construction (short term) is based on the

calculation of the maximum vertical strain in the subgrade under the action of a standard 40kN wheel load travelling at the top of foundation level, as shown in Figure B.1. Trafficking at lower levels is permitted, but only so long as the deformation limits given in the Performance Specification are not exceeded.

Figure B.1 Input Parameters for Performance Foundation Design B.3 Limits on the maximum permitted subgrade strain vary according to the Stiffness

Modulus of the subgrade, as shown in Figure B.2. These limits are based primarily on the criteria used in Powell et al (1984) but adjusted for reasons given in Chaddock and Roberts (2006).

Figure B.2 Subgrade Strain Limits B.4 Adequate support for the pavement during its design life, is defined by calculating the

deflection of the foundation under the action of a wheel load (or Dynamic Plate load) at top of foundation level, also shown in Figure B.1. The deflection under a given load can be equated to a Surface Modulus for the foundation as a whole. The following are

40kN

Subbase

Subgrade

Surface deflection

Subgrade strain

10,000MPa layer assumed 1.5m below surface of subgrade

Circular contact area, radius

151mm

2000

2200

2400

2600

2800

3000

3200

0 50 100 150


Max

imum

Per

mitt

ed S

ubgr

ade

Stra

in (m

icro

stra

in)

CBR (%)1 2 5 10 15 20 25

Capping (if used)



the maximum deflections permitted for each Foundation Class under a standard wheel load (40kN load over a 151mm radius loaded area).

Class 1 – 2.96mm Class 2 – 1.48mm Class 3 – 0.74mm Class 4 – 0.37mm

B5 The maximum layer stiffnesses for each Foundation Class listed in Paragraph 4.51

must also be applied to alternative designs. It should also be noted that the values of Poissons ratio of 0.35 for subbase materials and 0.45 for capping, subgrade materials and those with stiffness >10,000MPa are normally adopted.

Design Example 4 Subgrade Stiffness Modulus for design estimated as 50MPa (approximately 5% CBR) Design a composite Class 4 foundation with 200mm of HBM upper subbase of design Stiffness Modulus 1500MPa over a HBM lower subbase of Stiffness Modulus 500MPa by calculating the thickness of the lower subbase. Limits applying:

1) Minimum lower subbase thickness = 150mm 2) Maximum surface deflection under a standard 40kN load = 0.37mm 3) Maximum vertical compressive strain in the subgrade = 3030με

Theoretical requirements: 1) 150mm minimum thickness 2) 193mm of lower subbase gives 0.37mm surface deflection 3) The upper subbase on its own satisfies the subgrade strain criteria, hence

0mm of lower subbase gives < 3030με vertical compressive strain in subgrade.

Resulting design: Take greatest figure = 193mm rounded up to 200 mm of HBM lower subbase (500MPa) overlaid with 200mm of upper subbase (1500MPa).



ANNEX C: PERFORMANCE FOUNDATION DESIGN PROCEDURE – FLOWCHARTS AND EXAMPLES

Design likely to achieve long-term foundation class

Is each individual EDA > EMin adjusted for Demo CBR value?

Is running mean of 5 EDA > E adjusted to Demo CBR value?

Go to MAIN WORKS “Start”

Select area for demonstration, test for Demo CBR and construct Demonstration Area

Go to A, Design Stage

Select Mean (E) and Minimum Foundation Surface Modulus (EMin) from Table 4/1 and adjust if Demo CBR

is higher than Design CBR. See Paragraph 4.38

DEMONSTRATION AREA

Traffic demonstration area

Test for trafficked foundation stiffness (EDA) and foundation deformation

Examination of the potential effect of site traffic

Is foundation deformation acceptable?

Prove compliance of foundation layers by measuring: • Thickness • Material properties (eg Strength when bound) • Density. Otherwise, reconstruct.

For cases where the potential of slow-setting material cannot be demonstrated by in-situ tests on the foundation due to the construction programme, evidence is to be provided of their adequate long-term performance and a departure from this standard is to be sought and the stiffness targets revised.

StartSelect long-term Foundation Class and associated

design stiffness

Carry out design process for required foundation class and design subgrade strength using selecting specific

materials and their thicknesses

Design carried out using charts, equations or by calculation

Decide subgrade Design CBR and therefore stiffness

For simplicity and possible conservatism, Design CBR is taken to be the lowest of the estimated long-term, equilibrium CBR and the estimated short-term, construction CBR

DESIGN

A

Test for “as-constructed” foundation stiffness at designated age

No

No

No

Yes

Yes

Yes

Section 4 Interim Advice Note 73/06 Revision 1 (2009) Annex C Performance Foundation Design Guidance For Road Pavement Foundations Design Procedure (Draft HD25)


Values of EMin and E for the Main Works from values in Table 4.1

Yes

Yes

No

Yes

Possible cause may be excess moisture in unbound

granular material

MAIN WORKS

Construct main earthworks

Is Works CBR > Design CBR

No

Reset Design CBR

Prove compliance of foundation layers by measuring: • Thickness • Material properties (eg Strength when bound) • Density. Otherwise, reconstruct

Measure subgrade Works CBR

Start

Improve subgrade CBR

Yes

Go to A, Design Stage

No

Local remedial work on foundation layers

Is non-compliance due to weak subgrade?

Construct foundation layers

Monitor until strength/stiffness

recovers

Is each individual EMW > EMin?

Is the mean of 5 adjacent individual values of EMW > E?

Yes Assess cause of non-compliance

Is cause temporary?

No

No

Yes

No

Test main works for foundation stiffness, EMW, just prior to pavement construction,

End

Yes

Is foundation deformation acceptable?

No

Remedial work to subgrade

Alternative arrangements for ensuring performance of

foundation may be agreed. See Paragraph 4.45



Performance Foundation Design Procedure – Example 5 Design A Designer estimates the short term CBR for a site as 4% and the long term CBR as 7%. Taking the lower of the two values, the design is based on 4% CBR or approximately 43MPa. The Designer wishes to use a Type 1 subbase complying with the Specification Series 800 over a locally won capping material to form a Class 2 Foundation. The Designer estimates that the Stiffness Modulus of the Type 1 is 150MPa and the Stiffness Modulus of the capping material is 75MPa. Using a layered linear elastic analysis, the Designer estimates that the design requires 211mm of capping and 123mm of subbase. As the minimum thickness permissible for Type 1 is 150mm, he adjusts the design to 200mm of capping and 150mm of Type 1 subbase (total 350mm). Construction thicknesses are selected taking into account permitted level tolerances. Demonstration Area The Demonstration Area is constructed on site in order that the design assumptions can be checked. Using Table 4.1 the requirements at the top of the foundation for a Class 2 unbound foundation are a Mean Surface Modulus of 80MPa and a Minimum Surface Modulus of 50MPa (assuming the design CBR is correct). The subgrade in the Demonstration area is checked and is found to be at the design CBR of 4%. Construction of the Demonstration Area proceeds to prove the production and placement process for the capping and subbase material. The density and material properties are checked against the requirements of the relevant clauses of Specification Series 600 and 800. The constructed layer thicknesses comply with the design requirements. After a specified time period Dynamic Plate Tests are carried out at top of foundation and the results compared to the Mean and Minimum Surface Modulus values. It is found that the Demonstration Area does not achieve the Target Value of 80MPa. The Designer asks for the locally won capping material to be tested in the laboratory. The results show that the capping has a stiffness of 50MPa rather than 75MPa that had been assumed. The design is recalculated to be 185mm of capping and 185mm of subbase (total 370mm). A second Demonstration Area is constructed and the Mean and Minimum Surface Modulus values are achieved. A trafficking trial is then undertaken to check deformation susceptibility. Main Works The values from the Dynamic Plate Tests are satisfactory and the design is taken forward for the Main Works. The Mean and Minimum Surface Modulus values are those given in Table 4.1 (i.e. 80MPa and 50MPa). During the Main Works, subgrade CBR must be assessed every 60m. If the CBR falls below the value assumed in the design (i.e. 4%) appropriate remedial action must be taken. Density and material properties of the subbase and capping must comply with the appropriate clauses of Specification Series 600 and 800.



Performance Foundation Design Procedure – Example 6 Design A Designer estimates the short term CBR for a site as 8% and the long term CBR as 5%. Taking the lower of the two values, the design is based on 5% CBR or approximately 50MPa. The Designer wishes to use a slag bound mixture (SBM) complying with the Specification Series 800 as subbase to form a Class 4 Foundation. A laboratory investigation has been carried out using the static stiffness modulus apparatus after curing the SBM for 28 days at 40oC (BS EN 13286-43). The results gave an average Modulus of Elasticity for the SBM of 10,000MPa. The Designer takes 10% of this value for his calculations (i.e. 1,000MPa) to take account of degradation and likely in-situ density (see Para 5.25). Using Figure 4.5, the Designer estimates that a layer thickness of 380mm of SBM is required. Construction thicknesses are selected taking into account permitted level tolerances. Demonstration Area The Demonstration Area is constructed on site in order that the design assumptions can be checked. Using Table 4.1 the requirements at the top of the foundation for a Class 4 slow curing foundation are a Mean Value of 300MPa and a Minimum Value of 150MPa (assuming the design CBR is correct). The subgrade in the Demonstration area is checked and is found to have a CBR of 7% following a recent dry spell. Therefore the top of foundation Target Value and Minimum Value must be adjusted, using Paragraph 4.39:

Adjusted Target Value = 300 × [1+0.28×ln(7/5)] = 328MPa

Adjusted Minimum Value = 150 x [1+0.28xln(7/5)] = 164MPa

Construction of the Demonstration Area proceeds to prove the production and placement process for the SBM. The constructed layer thicknesses comply with the design requirements. Testing is undertaken as required by the Specification Series 800 for density and compressive strength or tensile strength and stiffness. After a specified time period (generally 28 days for slow-curing mixtures) Dynamic Plate Tests are carried out at top of foundation. A trafficking trial is then undertaken to check deformation susceptibility and stiffness loss. Dynamic Plate Tests are repeated and the results compared to the adjusted Mean and Minimum values. The results are found to be satisfactory. Main Works The design is taken forward for the Main Works. Having proved the adequacy of the design assumptions in the Demonstration Area, the Mean and Minimum Values revert back to those given in Table 4.1 (i.e. 300MPa and 150MPa). During the Main Works, subgrade CBR must be assessed every 60m. If the CBR falls below the value assumed in the design (i.e. 5%) appropriate remedial action must be taken. Density and material properties of the SBM must comply with the appropriate clauses of Specification Series 800.

Section 5 Interim Advice Note 73/06 Revision 1 (2009) Draft Specification Clauses Design Guidance For Road Pavement Foundations

(Draft HD25)


Section 5 Draft Specification Clauses 890 Performance Related Specification for Foundations General 1 Performance Foundations as defined in HD 25 (DMRB7.2.2) must be constructed in

accordance with Clauses 890 to 896. 2 The foundations for a scheme must be divided into Foundation Areas defined in

Appendix 7/1. Each Area shall be defined by the Design subgrade strength or by different foundation materials used in the design.

3 The structure of each Foundation Area must be defined in Appendix 7/1 as either

Foundation Class 1, 2, 3 or 4 and foundation materials and minimum layer thicknesses to be constructed must be tabulated

4 The tests to measure performance in accordance with this specification must be

carried out for each Foundation stage at each of the following stages of construction: (i) Top of Subgrade (ii) Top of Foundation 5 The stiffness modulus performance requirements for the top of a foundation are set

out in the Chapter 4 of the draft HD25 for the following situations:

Long-term Foundation Class Short-term Mean Foundation Surface Modulus to be exceeded by the running

mean of five consecutive measurements Short-term Minimum Foundation Surface Modulus to be exceeded by all

individual measurements. Materials 6 All foundation materials must comply with Series 600 and Series 800 clauses except

that a layer thickness of up to 250mm may be used for layers other than the uppermost foundation layer.

7 The use of all materials used in foundations must be acceptable to the Environment

Agency and other bodies responsible for the local environment and for water quality and must not result in a deleterious reaction with other pavement or subgrade materials.

8 Where a Contractor’s proposed alternatives are permitted for unbound granular

materials, no such materials must have a plasticity index greater than 6% when tested according to BS1377: Part 2 on material passing the 425 micron sieve unless the fraction of such material is less than 10% of the whole.

Placement and Compaction 9 Class 9D or 9E stabilised materials must not be placed or constructed above Class

6F granular material or Class 6S granular filter layer material.


(Draft HD25)


10 The minimum material layer thicknesses to be constructed as defined in Appendix 7/1 must include an allowance for construction tolerances. The contractor must also make additional allowance for thickness or quality of material to ensure there is no damage if the foundation is to be used as a haul road.

11 The minimum compacted layer thickness must be the greater of the following: 2.5

times the maximum particle size or 150mm for bound layers, or 80mm for unbound layers.

12 Unbound foundation material may be compacted in a layer thickness up to 250mm

except for the uppermost foundation layer for which the thickness must not exceed 225mm.

13 Unless stated otherwise in Appendix 7/1, no restriction is placed on the method of

compaction of unbound materials so long as the dry density requirements given in 22Clause 894 are achieved and sub-clauses 12 to 16 of this Clause are satisfied.

14 For Performance Foundations, compaction of materials covered by Clauses 614, 615

and 643 must be carried out to Clause 894. 15 For cement and other hydraulically bound mixtures to Clauses 821, 822, 823, 830,

831, 832, 834 and 840, the compaction plant and method specified in Clause 814 must be used.

Subgrade Protection 16 The Contractor must limit any areas of completed formation to suit the output of plant

in use and the rate of deposition of sub-base. No prepared formation must remain continuously exposed to rain causing degradation or to be left uncovered overnight.

891 Demonstration Area for Performance Foundations General 1 For each Foundation Area, a Demonstration Area must be prepared using the same

methods, materials, thickness and compaction as proposed for the permanent works. Each Demonstration Area shall be not less than 400m² and not less than 60m long. For foundations constructed using HBM to Clause 810, the demonstration area shall comply with the requirements of Clause 817.

2 Records of the performance test results for each construction stage, referenced to the

following condition details must be presented to the Overseeing Organisation in an electronic spreadsheet format, prior to construction of the same foundation type for the Main Works:

(i) Subgrade CBR value immediately before foundation construction (ii) Date and Time of mixing (for stabilised and slow-setting materials)

(iii) Date and time of placing and compaction (iv) Date of performance testing (v) Values of Surface Modulus recorded (vi) Values of material properties including density and layer thickness (vii) Weather conditions including temperature (viii) Details of samples taken for testing.


(Draft HD25)


3 The materials placed in the Demonstration Area may form part of the Permanent

Works, provided that they meet the requirements of the Permanent Works. 4 Foundation layers containing at least 3% CEM1 cement by dry mass of mixture must

not be tested or trafficked until 7days after placing unless a strength criterion has been agreed with the Overseeing Organisation.

5 Where the foundation includes any HBM, then 5 laboratory specimens must be

manufactured from samples recovered at locations uniformly distributed across the Demonstration Area and tested in accordance with the requirements for that material.

6 Where the completed Demonstration Area meets all the requirements, the methods,

materials and thicknesses used must not be changed for the construction of the Main Works without further testing in a Demonstration Area.

Trafficking Trial 7 The Contractor must undertake controlled trafficking on the Top of Foundation Stage

of construction in the Demonstration Area. 8 Trafficking must be carried out using a heavy goods vehicle with axle configuration

and load as required by Clause 816.28. The number of passes should be equivalent to 1000 standard axles as given in Clause 802 or as agreed otherwise. The deformation must be measured in accordance with, and must not exceed the limits stated in Clause 896.

9 Foundation Surface Modulus performance tests at the Top of Foundation that

includes bound materials, must be carried out both before and after a Trafficking Trial. Each individual Surface Modulus measurement and the running mean of 5 consecutive measurements of the later series of tests must exceed the values in Clause 891 Sub-clause 16.

Top of Subgrade Performance Assessment 10 The short-term subgrade CBR strength within the Demonstration Area must be

determined in accordance with Clause 893 at not less than 5 locations, distributed uniformly over the Demonstration Area. The locations are to be identified to an accuracy of 0.5m. The measurement of strength must be taken at formation level or at sub-formation level if capping is part of the foundation design.

11 The Top of Subgrade within the Demonstration Area must be proved by not less than

5 in situ density measurements in accordance with the requirements of Clause 894, coinciding with the CBR test locations.

12 Where in-situ stabilisation of the subgrade is to be used as part of or as the first

foundation layer, then the subgrade CBR strength must be measured immediately below the depth of the stabilisation by means of a Dynamic Cone Penetrometer, to the requirements of Clause 893.

13 Where the subgrade CBR test values are less than the Design CBR, the area must

either be improved, and the improvement applied to the permanent works, or the Design CBR reset and another foundation designed, constructed and proved.


(Draft HD25)


Top of Foundation Stage Performance Assessment 14 Measurements of the short-term Surface Modulus must be carried out as detailed in

Clause 895. A minimum of 25 stiffness tests must be completed, distributed uniformly over the Demonstration Area, with at least five of these tests located above the Top of Subgrade CBR strength and density tests.

15 The foundation layer within the Demonstration Area shall be proved by not less than

5 in-situ density measurements, in accordance with the requirements of Clause 894. These 5 tests must be located above the five Top of Subgrade CBR strength and density test locations.

16 The short-term Surface Modulus performance requirements for each individual value

and for the running mean of five consecutive measurements must be equal to or greater than the requirements for the particular Foundation Class identified in Appendix 7/1 following adjustment in accordance with the procedure in the draft HD25 Chapter 4 to the median value of the five subgrade CBR values from the Demonstration Area.

17 Where the Surface Modulus performance measurements do not meet the

requirements detailed in this Clause, the foundation must be re-designed and another Demonstration Area constructed and the design proved.

892 Permanent Works for Performance Specified Foundations General 1. For each Foundation Area, records of the performance test results for each

construction stage, referenced to the following condition details must be presented to the Overseeing Organisation in an electronic spreadsheet format, prior to construction of the pavement layers above:

. (i) Subgrade CBR value immediately before foundation construction (ii) Date and Time of mixing (for stabilised and slow-setting materials)

(iii) Date and time of placing and compaction (iv) Date of performance testing (v) Values of Surface Modulus recorded (vi) Values of material properties including density and layer thickness (vii) Weather conditions including temperature (viii) Details of samples taken for testing.

2. Foundation layers containing at least 3% CEM1 cement by dry mass of mixture must not be tested or trafficked until 7days after placing unless a strength criterion has been agreed with the Overseeing Organisation.

Top of Subgrade Stage of Construction Performance Assessment 3 The short-term subgrade CBR strength must be determined according to Clause 893

at 60m intervals along each lane of prepared subgrade and staggered by 30mm between adjacent lanes. At least 10 tests shall be carried out for each prepared Foundation Area. The location of each test must be identified to the nearest 0.5m.


(Draft HD25)


The measurement of strength must be taken at formation level or at sub-formation level if capping is part of the foundation design.

4 The foundation must not be constructed in areas where the subgrade strength is less

than the Design CBR. Top of Foundation Performance Assessment 5 The top of the foundation must be tested for Surface Modulus in accordance with

Clause 895 immediately prior to construction of overlying pavement layers and at 20m intervals along each lane, staggered by 10m between adjacent lanes. Tests should coincide with subgrade CBR and Density tests where appropriate.

6 The short-term Surface Modulus performance for each individual measurement and

for the running mean of 5 consecutive measurements must be equal to or greater than the minimum and mean values set out in Chapter 4 of the draft HD25 for the Foundation Class and identified in Appendix 7/1.

7 A foundation containing unbound materials that fails to comply with the performance

requirements of this Clause when the recorded moisture content is in excess of that in the Demonstration Area, may be re-tested for compliance when the foundation moisture content has reduced to that of the Demonstration Area.

8 Where the Surface Modulus performance values do not meet the requirements

detailed in this Clause, the foundation must be re-designed and another Demonstration Area constructed and the design proved.

9 Density tests, to the requirements of Clause 894 are to be performed at a spacing of

200 metres staggered along each lane of the road when placement and compaction to Clauses 802 and 813 have been followed; otherwise the spacing of these tests must be every 60m, coinciding with Surface Modulus tests. Tests performed in adjacent lanes must be staggered by 30m.

10 Wheelpath deformation must be monitored and measured along all lengths of

prepared foundation in accordance with the requirements of Clause 896 and the measured values must not exceed those stipulated in that Clause.

893 CBR Strength Measurement 1 CBR strength measurements of the prepared subgrade must be carried out using a

Dynamic Cone Penetrometer (DCP) unless the type of soil is inappropriate for such testing when Dynamic Plate testing must be used. The DCP equipment must incorporate an 8kg steel drop weight that falls vertically through 575mm and makes contact with a steel anvil. This anvil must be rigidly attached, via steel rods (less than 20mm diameter), to a 20mm diameter 60o steel cone, which is driven vertically into the ground. Also see HD29 (DMRB 7.3.2).

2 The result for each test must be expressed as a 50th percentile penetration rate in

millimetres per blow between 50mm and 550mm of penetration from top of subgrade level. If the penetration rate is less than 2mm per blow, then the test should be aborted with one further test attempted nearby.


(Draft HD25)


3 Soil strength expressed as mm/blow must be converted to a CBR value using the following relationship:

Log10 (CBR) = 2.48 – 1.057 * Log10 (mm/blow)

where CBR is given as a percentage value and the penetration rate of the cone is given in units of mm/blow.

894 Density Measurement 1 Each stage of foundation construction must be tested for in-situ density by a nuclear

density gauge in transmission mode, calibrated for the material being tested, in accordance with BS1377: Part 9 for unbound materials or Clause 870 for cement and other hydraulically bound mixtures.

2 The unbound material used in each compacted foundation layer must achieve a

minimum in-situ dry density, when tested in accordance with BS1377: Part 9, of 95% of the maximum dry density, as determined from the method in BS EN 13286-4. Cement and other hydraulically bound mixtures must attain a minimum in-situ wet density, when tested in accordance with Clause 870, of 95% of the average wet density of at least five cubes manufactured to BS EN 13286-51.

3 Maximum dry density (for unbound materials) or maximum wet density (for cement

and other hydraulically bound mixtures) must be determined for every 1000 tonnes of material unless otherwise stated in Appendix 7/1.

4. Other non-nuclear density gauges will be permitted with approval if they can be

calibrated to the nuclear density gauge on the material being tested and can be shown to measure density over the thickness of the layer being tested.

895 Surface Modulus Measurement 1 Surface Modulus testing must be carried out using a Dynamic Plate Test device,

which has been calibrated to the manufacturer's specification. Regular checking and calibration of the load cell and deflection sensors must be carried out as recommended by the manufacturer. The equipment must be capable of producing a peak stress of 100kPa with a pulse rise time of between 8 to 12 milliseconds, applied through a rigid circular plate of 300mm diameter. Both the applied load and the transient deflection, measured directly on the tested surface, must be recorded. The deflection measurement transducer must be capable of measuring deflections in the range 40-1500 microns. The accuracy of the readings should be ±0.1 kN for the load and ±2 microns for deflection.

2 The peak stress applied during each test shall be selected to produce as high a

deflection as possible within the measurement range of the deflection sensor. 3 The following procedure is to be adopted for dynamic plate testing. Each test site

should be stable and flat and free from water, ice and snow. The temperature down to 100mm below the surface should be at least 4oC. For a lightweight test device, at least 10 drops are necessary at the beginning of each test session to warm up the rubber buffers. At each test point, 3 initial ‘seating’ drops shall be carried out to bed the plate into the surface. Three further drops shall then be carried out. The results from the last set of three drops shall be averaged to give the Surface Modulus applicable to that test point.


(Draft HD25)


4 The stiffness modulus shall be computed at each point tested, using the following

formula:

DP R )-2(1E

2 ××=

ν

where : E = Foundation Surface Modulus (in MN/m2 or MPa) ν = Poisson’s Ratio (ν, by default, = 0.35) R = Load Plate Radius (R, by default, = 150mm) P = Contact Pressure (in kPa) D = Deflection under the centre of the plate (in microns)

5. If a lightweight test device is used, it must be correlated to an FWD which will remain

the reference test method. The following procedure must be used to correlate a lightweight device: The FWD and the lightweight devices are to both to be used on the same material and at adjacent test positions in the demonstration area for the 25 measurements points. The Surface Modulus values obtained from the two devices are to be compared and the square of the correlation coefficient (r2) is to be calculated, if this value is more than 0.45 then there is considered to be sufficient correlation between the two devices. An adjustment factor should then be calculated as the mean of the ratios of each FWD value to lightweight value. The lightweight device readings are to be adjusted by this factor for all further readings on that material for that scheme.

896 Wheelpath Deformation Measurement

1 Ruts that develop under construction traffic, measured in accordance with this

Clause, shall nowhere exceed the following limits:

All stabilised/bound surfaces – 10mm < 250mm thick granular material – 30mm ≥ 250mm thick granular material – 40mm

2 At each point, the cumulative rut, calculated by summing the deformations from each

trafficked foundation layer shall not exceed 50mm. 3 Wheelpath Deformation measurement shall be carried out using a straight edge with

a length of at least 2m. The straight edge shall be placed transverse to the rut and raised clear from the rut by two identical blocks. The blocks shall be placed on undisturbed material outside of the wheel path. The amount of deformation shall be the difference between the deepest vertical measurement from the straight edge to the surface of the foundation (A) and the height of the blocks (B).

Deformation = A – B.

B A

Section 6 Interim Advice Note 73/06 Revision 1 (2009) Draft Notes for Guidance Design Guidance For Road Pavement Foundations

(Draft HD25)


Section 6 Draft Notes for Guidance Clauses NG 890 Performance Related Specification for Foundations General 1 Because of the variability of subgrades, the type and the design of foundations may

have to be varied during construction. It may also be necessary to adjust the number of Foundation Areas based on the various subgrade CBR strength values measured on site.

2 The use of capping will often provide an excellent platform for the construction and

compaction of the foundation. A capping layer enables construction plant to lay the subbase and can provide a good base for the necessary compaction to be achieved. The provision of a capping layer may be particularly appropriate for lower strength subgrades. The benefit of providing a capping layer can be taken into account as part of the Performance Foundation design process.

3 The requirements for minimum thickness layers for the various Foundation Classes

are explained and detailed in Chapter 4 of the draft HD 25. 4 The short-term, i.e. during construction, subgrade CBR strength would typically be

expected to differ from the long-term, i.e. under the completed pavement, equilibrium strength. Similarly, measurements of Foundation Surface Modulus during pavement construction are likely to differ from the long-term surface stiffness modulus assumed for design.

5 The measurement of Foundation Surface Modulus at the top of the foundation serves

several purposes:

To identify inadequate subgrade performance either not found during previous testing or where performance has reduced; eg. where water has been allowed to enter the foundation.

To identify inadequate upper foundation layers performance. To identify degradation of the foundation, possibly brought about by

construction traffic. To quantify the potential degradation which will be identified during a

Trafficking Trial to Clause 891 by measurement of the Foundation Surface Modulus both before and after trafficking, prior to construction of the next stage of the Permanent Works.

6 Testing for Surface Modulus at each intermediate foundation level where compaction

is carried out is recommended to identify any areas of concern as soon as possible and to ensure that the completed foundation will meet the requirements. Results for testing at intermediate layers will permit checks to be made against expectations as work proceeds.

Materials 7 Restrictions may be placed on some materials including unburnt colliery spoil and

certain other industrial by-products. Further information can be found in the Specification Series 600 and 800. Expert advice should be sought when proposed alternative materials are proposed.


(Draft HD25)


8 Demonstration Areas (Clause 891) afford an opportunity for gaining experience of the materials to be used as well as adjusting construction procedures and/or design thicknesses. They also permit Surface Modulus measurement with dynamic plate apparatus to be carried out.

9 For all materials, including Contractor’s proposed alternative materials, laboratory

testing forms an important step in characterising the mechanical properties prior to developing foundation designs. Advice is provided in the draft HD25, and the following should be noted:

The Triaxial test (BS EN 13286-7) provides stiffness modulus and shear

strength data for unbound materials, but may not be suitable for materials having large particle size. For cement and other hydraulically bound mixtures, BS EN 13286-43 describes two techniques for stiffness modulus measurement, whereas Parts 40, 41 and 42 describe different strength measurements.

Alternatively, the 170x170×170mm ‘Springbox’ which fits inside a NAT loading frame is suggested as a suitable tool for the measurement of mechanical properties of unbound granular and weak hydraulically bound mixtures. The Springbox allows the material to be tested in a realistic moisture state; soaking followed by a 24 hour drainage period is generally considered appropriate.

Characterisation of granular materials can be made using the 300×300×150mm Shear Box (see Transport & Road Research Laboratory Report RR64) where, if the ‘peak shear stress ratio (PSSR)’ is greater than 2.8, then the material is very likely to be suitable for direct trafficking by road vehicles. If it is between 1.9 and 2.8 there is some risk of rut development, and for PSSR less than 1.9 rutting is likely, such materials may still be suitable in the long-term as long as they are protected during the construction process.

A further practical alternative for in situ testing is the Dynamic Cone Penetrometer (DCP), see Clause 893, where experience suggests that materials with a penetration rate of less than 17mm per blow (>15% CBR) are likely to be suitable for direct trafficking (unless they are unsuited to such testing due to the presence of large particles) and for Class 1 foundations. The uppermost layer of a Class 2 foundation would usually achieve a DCP penetration rate of less than 9mm per blow (>30% CBR).

10 Crushed rock or sand filter layers of 50mm minimum thickness, made using Class 6S

granular filter layer material, can be used to prevent the ingress of cohesive particles from the top of the subgrade into an open graded foundation layer and can also provide a drainage path. A filter layer is not generally required if Class 6F granular material is used.

11 BS EN 13285 requires separate classes for Class 6F granular material from sources

other than the excavated parts of the same Site. It should be noted that part of the material characterisation relates to where the materials has been obtained.

Placement and Compaction 12 For thicker compacted foundation layers than those permitted in Clause 890.13, the

contractor must obtain separate departure approval, and describe fully the method by which the density requirements of Clause 894 are to be achieved and demonstrated throughout the full depth of material.


(Draft HD25)


NG 891 Demonstration Area for Performance Foundations 1 For slow curing HBMs, an extended curing period may be specified in Appendix 7/1

before testing and augmented by laboratory evidence showing that the expected 360 day performance will be met. A shorter period of time between laying and testing may be appropriate for a particularly slow curing HBM to ensure the stability of the mixture in the absence of bond in the short-term and confirm that the material is suitable for trafficking and construction of the next stage. A series of tests may also be appropriate to show the rate of gain of strength.

Trafficking Trial 2 In certain circumstances, a trafficking trial may not be necessary and it may be

omitted subject to a Departure from Standard. Trafficking at an intermediate stage will provide contractors with reassurance that materials will meet final performance requirements. Testing for stiffness modulus at different times will provide guidance on the possibility of damage to foundation layers by construction traffic.

3 With a foundation material for which an increase of water content may affect the

stiffness or the resistance to deformation, it is also recommended that a trafficking trial be carried out in a wetted condition. Wetting of the Demonstration Area and re-trafficking is intended to assess likely performance in wet weather. It is suggested that sufficient volume of water to cover the trial area to a depth of 10mm is spread as uniformly as possible and that a period of 1 hour is then allowed prior to re-trafficking. Failure to test in this way may result in rutting at a later date and foundation failure. The moisture content of the material should be recorded at the time of each trafficking trial.

4 If the measured deformation is in excess of the requirements of Clause 896, then

either the foundation should be improved and subsequently proved by another Trafficking Trial, or the planned works should be adjusted to reduce the construction traffic. If the foundation is to be trafficked by very heavy vehicles (e.g. to transport bridge segments), additional consideration should be given to proving the performance of the foundation under these vehicle loads. Whether a Trafficking Trial is performed or not, it will still be the responsibility of the Contractor to ensure that the foundation meets the requirements specified for the Permanent Works in Clause 896.

5 The purpose of trafficking a Demonstration Area is to understand the behaviour of the

foundation layers under construction traffic and to ensure that the subgrade is not overstressed. Based on a successful trial it may be concluded that the level concerned is able to withstand trafficking without any special precautions. However, often with marginal materials, special precautions in the form of a limit on traffic movements, a protection layer, or restricted movements in wet weather are necessary. The trial may help to make decisions about such restrictions.

6 Care may be necessary for a trafficking trial on a foundation including Class 6F3

material. This material may sometimes appear satisfactory in the short term but deform significantly later. A static test, for example a 12T axle parked for 24 hours, is likely to reveal if a deformation problem exists.


(Draft HD25)


Top of Subgrade Performance Assessment 7 For demonstration area and for permanent works where stabilisation or other ground

improvement is being used as part of formation preparation, the subgrade strength should be measured at formation level when these works are completed.

Top of Intermediate Foundation Layers Performance Assessment 8 It may be appropriate for the intermediate stage of the foundation in the

Demonstration Area to be tested to determine the surface modulus value. The value that should be achieved should be agreed beforehand with the designer and used to ensure that the full foundation will achieve the required stiffness.

Top of Foundation Performance Assessment 9 Where the designer considers that the foundation may be moisture susceptible,

wetting should be carried out to reflect the most pessimistic moisture condition anticipated on site. The foundation shall then be re-tested for stiffness modulus. The results of these tests shall also be reported to the Highways Agency (Overseeing Organisation) to assist in optimising future pavement design.

NG 892 Permanent Works for Performance Foundations General 1 For slow curing HBMs, an extended curing period may be specified in Appendix 7/1

before testing and augmented by laboratory evidence showing that the expected 360 day performance will be met. A shorter period of time between laying and testing may be appropriate for particularly slow curing HBM to ensure the stability of the mixture in the absence of bond in the short-term and confirm that the material is suitable for trafficking and the construction of the next ‘stage’.

Top of Subgrade Performance Assessment 2 It is important that the subgrade fully meets the design CBR value. Additional testing

may be necessary to identify the extent of weaker areas and their depth. Areas of lower value should be identified and suitably treated. Careful attention is necessary to ensure good drainage paths are available and water susceptible materials are handled responsibly. Trapped water must always have routes to drain. Drainage to the works should always be completed and connected before road foundations are constructed.

3 Remedial measures are also required to any area of subgrade whose CBR strength

falls below the Design CBR due to disturbance caused by inappropriate actions on the part of the Contractor,


(Draft HD25)


Top of Intermediate Foundation Layers Performance Assessment 4 Tests of performance at intermediate levels can be compared with those values

obtained from the Demonstration Area in order to give confidence that the performance requirement at the Top of the Foundation will be achieved.

Top of Foundation Performance Assessment 5 The minimum age of the bound material at which the Foundation Modulus should be

measured is typically 7 days for faster setting CEM 1 Cement bound materials but longer for slower setting HBMs dependent on strength development. It is advisable for the foundation not to be trafficked during this period.

6 It is useful to plot the running mean of five consecutive Surface Modulus results

against the site chainage as the trend in Foundation Surface Modulus may give notice of a possible future non-compliance.

7. Allowance should be made by the contractor for further subgrade CBR strength tests

at locations where either potential concern exists, or where evidence of poor subgrade condition, or soil weaker than expected, is observed.

8 It is only permissible for the Contractor to change material specifications and layer

thicknesses in order to increase (and not decrease) the foundation quality, as judged by foundation stiffness, in the Permanent Works relative to that approved in a Demonstration Area.

NG 893 CBR Strength Measurement General 1 For coarse-grained materials, there might be an appreciable difference between CBR

values obtained in-situ and in the laboratory, with the in-situ values being lower due to the effects of confinement in the laboratory test mould (although this should not be confused with low values resulting from exposure to rainfall or a high water table). This difference should be taken into consideration when specifying in-situ requirements in Appendix 7/1.

Dynamic Cone Penetrometer (DCP) 2 Further details of the calculation of subgrade CBR strength are provided in HD30

(DMRB 7.3.7) 3 Other dynamic cone equipment may only be permitted providing it has been

calibrated against equipment meeting the requirements of Clause 893, on the same type of materials.

4 The calculation of the 50th percentile penetration rate will not be normally influenced

by small stones in a generally cohesive material. 5 Where laboratory CBR tests have been carried out on the subgrade material, the

DCP values should be calibrated to those of the laboratory tests.


(Draft HD25)


NG 894 Density Measurement 1 Density testing of foundation layers is important to ensure that strength is provided

through the full depth of the foundation layers and that secondary compaction does not take place. Density values can be low if the material is too dry during compaction.

2 In interpreting density results, due account should be taken of the variation in

maximum dry or wet density with composition of the material; the grading envelope for foundation materials can be very wide. Where possible, information on the variation of density with gradation for the materials proposed should be used.

3 For coarse materials it may not be possible to assess density using the nuclear

density meter. Alternative standard, but time consuming, methods based on excavating a measured mass of material and determining the volume of the hole created are permitted by the Specification (subject to Overseeing Organisation approval) and may need to be adopted.

NG 895 Stiffness Modulus Measurement 1 When the device applies its maximum stress, especially on lower class foundations

and where intermediate stages are tested, the deflection of the structure tested can be over 1000 microns, whereas for the highest foundation class, maximum deflection of only about 50 microns will be produced. A peak stress of 100kPa should be targeted for Foundation Classes 1 and 2 and 200kPa for Foundation Classes 3 and 4, unless the deflection measurement typically falls outside the range 100-1000 microns.

2 For unbound materials, normally 3 drops are necessary to ensure satisfactory seating

before testing. For bound materials, one drop may confirm stability and satisfactory operation before testing. No more than 10 drops in total should be applied to unbound materials to ensure that there are no unrepresentative results.

3 If any equipment is proposed which does not fully comply with the Specification, it

may be permitted at the discretion of the Highways Agency (Overseeing Organisation), provided that it is calibrated against equipment complying with the Specification for the specific types of material and layer thickness encountered on the site. This calibration would normally be carried out as part of the Demonstration Area testing.

NG 896 Wheelpath Deformation Measurement 1 The limit on rutting is primarily intended to ensure that significant ruts (>20mm) at

subgrade level are avoided, to prevent accumulation of water and local subgrade softening. If the subgrade is sufficiently permeable, then this problem will not arise. It may also be possible for the Contractor to cut a trench and to prove that, notwithstanding the rut at the surface, no significant subgrade rut is present. Some sands and gravels may rut excessively during construction, however, following re-profiling and compaction, they may achieve satisfactory properties for placement of the upper layers and, once confined by the pavement, may perform satisfactorily in the long term.


(Draft HD25)


2 The more stringent rut limits applying to stabilised/bound surfaces recognises the fact that, in practice, if visible rutting occurs in such materials, then this rutting will be accompanied by significant loss of stiffness, which is likely to result in non-attainment of the desired Foundation Class.

3 Whilst the presence of shoulders to a rut is indicative of a deformable material, and

this may provide valuable information during a trafficking trial, the actual specified measurement of deformation is based on the change in level from an untrafficked datum to the bottom of the rut. This is because this measure is more closely related to deformation taking place in the subgrade.

4 It is the Contractor’s responsibility to ensure that the foundation does not suffer

excessive deformation. If a foundation needs to be re-profiled during foundation construction, then the implication is that the foundation has already failed to comply with the Specification. Re-profiling alone may not stop further deformation and may disguise problems for the future such as ponding of water in ruts in the subgrade.

February 2006




PART 3

HD 26/06

PAVEMENT DESIGN

SUMMARY

This Standard provides the details of permittedmaterials and of thickness for the construction ofpavements for new trunk roads. This revision updatesthe previous Standard and also introduces differentpermitted designs that relate to the strength of theavailable foundation.


1. Remove Contents pages from Volume 7 andinsert new Contents pages for Volume 7 datedFebruary 2006.





HD 26/06

Pavement Design

Summary: This Standard provides the details of permitted materials and of thickness forthe construction of pavements for new trunk roads. This revision updates theprevious Standard and also introduces different permitted designs that relate tothe strength of the available foundation.


THE HIGHWAYS AGENCY

SCOTTISH EXECUTIVE




February 2006







PART 3

HD 26/06

PAVEMENT DESIGN

Contents

Chapter

1. Introduction

2. Standard Designs

3. Materials

4. Alternative Design Procedures


6. Enquiries


February 2006



1. INTRODUCTION

Mandatory Sections

1.1 Sections of this document which form partof the Standards of the Overseeing Organisationsare highlighted by being contained in boxes. Theseare the sections with which the DesignOrganisations must comply, or must have agreed asuitable departure from Standard with the relevantOverseeing Organisation. The remainder of thedocument contains advice and enlargement whichis commended to Design Organisations for theirconsideration.

General

1.2 This part details various combinations ofmaterials and thicknesses that may be considered forpavement construction, whether for new build,widening of an existing carriageway, or fullreconstruction. The design guidance is also useful whendeveloping recommendations for partial reconstructionor strengthening overlays when used together with theinvestigation techniques described in HD 30 (DMRB7.3.3). It does not include the estimation of designtraffic (see HD 24, DMRB 7.2.1), nor does it cover thedesign of pavement foundations (see HD 25, DMRB7.2.2). Additional information on surfacing andpavement materials is given in HD 36 and HD 37(DMRB 7.5.1).

1.3 Chapter 2 sets down the philosophy behind theStandard Designs and summarises the alternatives in theform of nomographs or equations. Chapter 3 providesadditional information on material behaviour to assistthe designer. Chapter 4 discusses analytical proceduresand material properties, which may be used by thedesigner to produce Alternative Designs or developpartial reconstruction and strengthening overlayoptions.

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February 2006

lementation

.4 This Part must be used forthwith on allchemes for the construction, improvement andaintenance of trunk roads including motorways

urrently being prepared, provided that, in thepinion of the Overseeing Organisation this wouldot result in significant additional expense orelay. Design Organisations must confirm itspplication to particular schemes with theverseeing Organisation.

in Northern Ireland

.5 For use in Northern Ireland, this Standardill apply to those roads designated by theverseeing Organisation.

tual Recognition

.6 The construction and maintenance ofighway pavements will normally be carried outnder contracts incorporating the Overseeingrganisations’ Specification for Highway Works

SHW) which are contained in the Manual ofontract Documents for Highway Works Volume 1

MCHW 1). In such cases products conforming toquivalent standards and specification of otherember States (MS) of the European Economicrea (EEA) or a State which is party to a relevant

greement with the European Union and testsndertaken in other MS of the EEA or a Statehich is party to a relevant agreement with theuropean Union will be acceptable in accordanceith the terms of Clauses 104 and 105 (MCHW.100). Any contract not containing these Clausesust contain suitable clauses of mutual recognition

aving the same effect, regarding which advicehould be sought.

1/1


Chapter 2Standard Designs

2. STANDARD DESIGNS

DESIGN PHILISOPHY

2.1 The designs given in this Part are based on TRLReport 615 (2004) for flexible construction (includingpavements previously known as flexible composite);TRL Report 630 (2005) for rigid (continuous)construction; and TRL Report RR87 (1987) for rigid(jointed) construction.

2.2 The adoption of the material specific calibrationadjustment factors recommended in TRL Report 615(2004) will give pavement designs using traditionalmaterials that are in close agreement with the previousflexible and flexible composite designs, which werebased on TRL Report LR1132 (1984).

2.3 The standard designs for HMB35 have beenremoved, but designs for an Enrobe a Module Eleve(EME2) material based on French practice has beenadded. The design thickness lines for DBM50 andHDM50 have been combined, based on pastperformance of these two almost identical materials. InScotland, HMB 35 is also permitted using the samedesign thickness as shown for DBM50/HDM50 basematerials.

2.4 The CRC designs now include a wider choice ofconcrete strength and foundation classes thanpreviously. The design philosophy continues to bebased on TRL Report RR87 (1987).

2.5 Design thicknesses are based on four foundationstiffness classes, defined as the equivalent half-spacelong-term stiffness of the composite foundation underthe completed pavement, as follows:

• Foundation Class 1 ≥ 50 MPa;



• Foundation Class 4 ≥ 400 MPa.

2.6 It should be noted that the stiffness values givenin any performance related Specification will differfrom the above, since any in situ tests measure the earlyage and with the materials in different confinementconditions.

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February 2006

2.7 Foundation Class 1 is a capping only design,in accordance with HD 25 (DMRB 7.2.2) withouta subbase layer. This foundation must not be usedfor design traffic in excess of 20 million standardaxles (msa) without a ‘Departure from Standard’from the Overseeing Organisation.

2.8 Foundation Class 2 is either a subbase andcapping design, or a subbase only design, inaccordance with HD 25 (DMRB 7.2.2). Thisfoundation must not be used for design traffic inexcess of 80 msa, unless 150mm or more of abound subbase is used, without a ‘Departure fromStandard’ from the Overseeing Organisation.

.9 Foundation Classes 3 and 4 are designs typicallycorporating Cement or other Hydraulically Boundixtures (collectively referred to as HBM in this Part).efer to Chapter 3 for further details on HBM.

2.10 For new road design, all lanes, including thehardshoulder, must be constructed to carry thedesign traffic in the heaviest loaded lane,commonly the left hand lane, as calculated inHD 24 (DMRB 7.2.1).

.11 For maintenance design, each lane would, as ainimum, be strengthened/reconstructed to carry the

esign traffic for that particular lane. However, theesign must ensure continuity of drainage, both in andelow the pavement layers and across the carriagewayidth.

.12 For motorway widening the requirements of theverseeing Organisation will depend on the specificroject, and will take account of a range of constraints,cluding technical, operational and financial. Also seeD 27 (DMRB 7.2.4).

.13 The minimum design traffic for lightly traffickedunk roads should be 1msa as set out in HD 24 (DMRB.2.1).

lexible Pavements

.14. For trunk roads up to 30 msa, it may bevantageous to use cold recycled materials and a

esign guide is available as part of TRL Report 611004). These designs may also be suitable for non-

trunk roads including those with design traffic less than

2/1



1 msa. For roads carrying less than 2.5 msa, usingcement as the primary binder, the adjustments containedin Table 7.5 of TRL Report 611 (2004) have beenreviewed. Experience indicates that such adjustment isunnecessary and that HBM based on the use of cementwith strength classification H2 can be safely used forType 4 roads and H3 for Type 3 roads. Furtherinformation is available from the OverseeingOrganisation.

2.15 Monitoring the performance of all types offlexible pavements that are heavily trafficked hasindicated that deterioration, in the form of cracking ordeformation, is far more likely to be found in thesurfacing, rather than deeper in the structure. Generallyfor ‘long life’ it is not necessary to increase thepavement thickness beyond that required for 80 msa,provided that surface deterioration is treated before itbegins to affect the structural integrity of the road.

2.16 For ‘long-life’ flexible pavements with an HBMbase, a total 180mm thickness of asphalt overlay toHBM base is required to sufficiently delay the onset ofreflection cracking, provided transverse cracks havebeen induced in the HBM base at 3m intervals, whererequired by the Specification (MCHW1) Series 800.

Rigid Pavements

2.17 Rigid concrete construction is a permitted optionfor trunk roads including motorways but generally if ithas an asphalt surfacing, see HD 36 (DMRB 7.5.1) forpermitted surfacing options in each UK country. Therequirement for asphalt surfacing does not apply to lay-bys and hardstanding locations, which may have aconcrete surface, see Paragraph 2.46. This requirementgenerally makes jointed construction unsuitable forconsideration, as a result of reflection cracking of thesurfacing and the potential for future increasedmaintenance.

2.18 However, when widening to an existingjointed concrete pavement, there may beadvantages in using the same form of constructionas a base layer to receive an asphalt surfacing andprovide continuity across the carriageway width.Such construction must only be used in Englandwith the approval of the Overseeing Organisation.

2.19 Use of continuously reinforced concretepavements with a Thin Surface Course System (TSCS)can provide a ‘long life’ with all the advantages offeredby the noise reducing properties of the surfacing. Suchpavements are ideally suited to the application offurther asphalt overlays at stages during the futurepavement life.

2/2

Pavement Deterioration

2.20 Detailed information on pavement deteriorationmechanisms for all pavement types is given in HD 30(DMRB 7.3.3), and the ‘failure’ criteria for rigidpavements are described in TRL Report RR87 (1987)

Whole Life Value

2.21 Whole life costing examines the costs of aproject from inception to disposal, including the directcosts of constructing and maintaining a highway andthe indirect costs imposed on society and theenvironment by its use and operation (e.g. traffic delay,accidents at roadworks, skidding accidents, fuelconsumption and tyre wear).

2.22 The sustainability of a scheme shall beconsidered when considering different design options.The five principles of sustainable development set outin the UK Government’s strategy “Securing the Future”(2005) are:

• Living within Environmental Limits;

• Ensuring a Strong, Healthy and Just Society;

• Achieving a Sustainable Economy;

• Promoting Good Governance;

• Using Sound Science Responsibly.

Strategies applicable to the UK devolvedadministrations are also being produced.

2.23 Integrating these principles in highway designcan include:

• reusing in situ materials to minimise resourceconsumption, waste disposal and emissionsresulting from material haulage;

• using pavement designs that give good value inwhole life cost terms;

• using pre-formed components to maximisequality, minimise health and safety risksassociated with site works and minimise thedelays to road users;

• providing maintenance designs which maximisethe residual life of existing components;

• designing highways that allow recycling ofend-of-life materials to their maximum utility;

February 2006



• using alternative designs based on innovation andbest practice to minimise construction times andtraffic disruption.

2.24 The Highways Agency’s “Building Better Roads:Towards Sustainable Construction” explains how theHighways Agency is promoting sustainabilitythroughout its business and sets out its expectations forsuppliers.

DESIGN IMPLEMENTATION

Pavement Type

2.25 The options for types of pavement structurehave been reviewed and are described below undertwo main types. Options for permitted surfacingsystems are set out separately in HD 36 (DMRB7.5.1) and further details given in HD 37 andHD 38 (DMRB 7.5.2 and 7.5.3).

a. Flexible Pavements

Where the upper layers of the pavement are boundin bitumen and the lower (base) layers are eitherbound in bitumen or with a hydraulic binder.Permitted binder and base layers are as follows:

• Dense Bitumen Macadam (DBM50 or DBM125) or Heavy Duty Macadam (HDM50);

• Stone Mastic Asphalt (SMA), for use as abinder course only, not as a base, except inScotland where a ‘Departure from Standard’must be obtained from the OverseeingOrganisation;

• Hot Rolled Asphalt (HRA50), which (if itdoes not comply with Clause 943) must onlybe used in England, Wales and NorthernIreland with the approval of the OverseeingOrganisation;

• EME2, but only on a Class 3 or 4Foundation, subject to a Departure fromStandard being granted by the OverseeingOrganisation. (Also see Fig 2.1, Note 15);

• Permitted hydraulically bound materials(HBM) for use in the base layers. These mayinclude:

– CBGM, Cement Bound GranularMaterial;

February 2006

– FABM, Fly Ash Bound Material;

– SBM, Slag Bound Material.

Materials are further categorised in theSpecification (MCHW 1) Series 800. Somestandard materials are shown in the Table aspart of Fig 2.1.

Details of composition, manufacture and laying aregiven in the Series 800 and 900 (MCHW1) and inmaterial Standards.

b. Rigid Pavements

Preferred rigid pavement construction is either:

• Continuously Reinforced ConcretePavement (CRCP), (normally with anasphalt overlay of minimum thickness30mm); or

• Continuously Reinforced Concrete Base(CRCB) with an asphalt overlay of 100mm.

See HD 36 (DMRB 7.5.1) for details of permittedsurfacing in each UK Country.

Other forms of rigid construction, permitted forlimited use and in special circumstances but onlywith approval from the Overseeing Organisation,include:

• Unreinforced Jointed Concrete (URC);

• Jointed Reinforced Concrete (JRC).

2.26 Except where the pavement design is theresponsibility of the contractor, designs must becarried out for several options. These must coverthe range of base types permitted by theOverseeing Organisation, except where there aretechnical or environmental reasons why only onepavement type is suitable. Advice on surfacingtypes permitted by each Overseeing Organisationis available in HD 36 (DMRB 7.5.1).

Design Life

2.27 For trunk roads including motorways wheredesign traffic is heavy in relation to the capacity ofthe layout, and in all cases where Whole LifeValue is taken into account, 40 year designs must

2/3



be included as permitted options. 20 year designsmay be appropriate for less heavily traffickedschemes or for major maintenance where other siteconstraints apply. In England, a ‘Departure fromStandard’ must be obtained from the OverseeingOrganisation for use of 20 year designs. Thedesign traffic in msa should be obtained fromHD 24 (DMRB 7.2.1).

2.28 In addition to major maintenance, a surfacetreatment may be expected to be required at about 10and 20 year intervals dependent on the nature of thetraffic. The period until surface treatment is requiredwill also vary depending on the site’s requirement forskidding resistance.

Design Nomographs

2.29 The design thicknesses of the layers, fordifferent materials for Standard Designs, onFoundation Classes 1 to 4, can be derived using:

• Figure 2.1 Flexible Pavements;

• Figure 2.2 Rigid (Continuous) Pavements;

• Figure 2.3 Rigid (Jointed) Pavements.

Flexible Construction

2.30 For flexible pavements on an asphalt base, thetotal thickness of asphalt (comprising the surfacecourse, binder course and base) is obtained from theright hand portion of the Nomograph (Fig 2.1) anddepends on the base material type. A DBM125 base isthe least stiff material, and so requires the thickestconstruction. The stiffness of asphalt material thenincreases from HRA50, through DBM50/HDM50, toEME2. A reduced thickness of material will provide thesame structural equivalence, provided the stiffness isadequately increased.

2.31 For a flexible composite pavement, the left handportion of the Nomograph gives the thickness of HBMfor a given strength, with the thickness of overlyingasphalt in the central portion of the Nomograph. Betterperformance is expected for those mixtures made with acrushed rock coarse aggregate that has a coefficient ofthermal expansion less than 10 x 10-6 per oC (typicallylimestone).

2.32 Previous UK HBM base designations, are notdirectly comparable with the new HBM basedesignations now detailed in the Specification

(MagdwpeqFS

R

2/4

CHW1) Series 800. This is due to differences ingregate grading; compressive strength measured at

ifferent ages; and a wider range of HBM designationsith different strength properties. For standard designurposes some of the materials with nominally similaruivalence are identified in the supporting Table in

igure 2.1. Other materials are listed in thepecification (MCHW 1) Series 800.

2.33 Individual construction widths of HBM basemust not exceed 4.75m. This minimises the risk oflongitudinal cracking induced by combinedstresses in a ‘long life’ HBM base pavement. TheHighway Construction Details (MCHW3) givetypical joint layouts. Flexible roads with an HBMbase with thinner construction are expected todeteriorate by general cracking of the HBM suchthat restricting the individual construction widthwill not necessarily lead to improved performance.

igid Construction

2.34 For a rigid pavement, the total thickness(excluding any asphalt surfacing) is obtained fromthe right hand portion of the Nomograph (Fig 2.2)for CRCP; and the left hand portion for a CRCB.Thickness for a given design traffic depends on theflexural strength of concrete and the FoundationClass.

2.35 CRCP or CRCB options must be consideredwhere the design traffic loading exceeds 30 msa,especially where the advantages of lowermaintenance throughout the design life may beworthwhile.

2.36 To ensure that forces are not transmitted tostructures and adjacent forms of pavementconstruction by thermally induced movements ofthe slab, the ends of the CRCP and CRCB must beaddressed in the design. A site-specific/designedtermination will be considered by the OverseeingOrganisation. Transition slabs, are required whereshown in the Highway Construction Details(MCHW3).

2.37 Ground beam anchors must not be usedwhere the subgrade strength is poor, or on highembankments where consolidation may beinsufficient to restrain movement of the beamdownstands.

February 2006



2.38 As concrete strength increases, the spacing oftransverse shrinkage cracks in CRCP and CRCB wouldnaturally tend to increase. Therefore, the percentage oflongitudinal crack control steel is increased withstrength to maintain the appropriate crack spacing.Wider cracks increase the likelihood of corrosion in thesteel and should be avoided. The depth of steel in theconcrete slab has also been chosen to reduce the risk ofcorrosion caused by salts penetrating through theexpected fine cracks.

2.39 Transverse steel is required for ease andconsistency of construction, and to prevent longitudinalcracking and local deterioration. Transverse bars maybe incorporated into the support arrangement for thesteel, so long as the required quantities and position ofthe steel is maintained.

2.40 Figure 2.2 assumes the presence of anintegral minimum 1m edge strip or tiedhardshoulder adjacent to the most heavilytrafficked lane. Urban roads, and any other roadsthat do not have either of these adjacent to the lefthand lane, will require 30mm thicker slabs. Heavytrafficking of right hand lanes and hardshouldersduring future maintenance will be of relativelyshort duration and need not be considered in thedesign.

2.41 The use of a tied shoulder or 1m edge stripensures that the untied edge is remote from thewheelpaths, with a consequent reduction in stress atslab corners and edges. This load distribution occurswhether or not a longitudinal construction joint or wet-formed joint is included adjacent to the edge line. Edgetreatments and other construction drawings are given inthe Highway Construction Details (MCHW3). Forfurther advice on edge of pavement drainage, refer toHA 39 (DMRB 4.2.1).

2.42 The design equations for jointed concretepavements are given in Figure 2.3. Load inducedstresses at slab corners and edges are greater than in theslab centre, necessitating dowel bars to distribute loadsbetween slabs. Joint associated distress occursprincipally when dowels do not function properly.

Ground Subject to Movement

2.43 CRCP and CRCB pavements are the only typessuitable where large or significant differentialmovement or settlement is expected, because they canwithstand large strains while remaining substantiallyintact.

February 2006

2.44 Flexible with asphalt base and JRC pavementsare suitable where slight differential movements, orsettlement caused by compressible ground, orsubsidence are expected.

2.45 Flexible with HBM base and URC pavements arenot suitable where differential movement, subsidence orappreciable settlement are expected. This includes areaswhere mines are currently worked or may be worked inthe future.

Laybys and Hardstandings

2.46 To resist the problems caused by oil anddiesel spillage, laybys and hardstandings must besurfaced with either:

i) concrete, see Series 1000 (MCHW1);

ii) block paving, see Series 1100 (MCHW1);

iii) a deformation resistant surfacing made witha proprietary fuel resisting binder.

Central Reserves

2.47 Where there is a requirement for hardenedcentral reserves, the minimum standard forconstruction must be based on Paragraph 3.11 andFig 3.3 in HD 39 (DMRB 7.2.5). Other forms ofconstruction will be subject to approval by theOverseeing Organisation and must be based on aminimum of 70mm thickness of bound material toinhibit weed penetration and minimise futuremaintenance.

Alternative Designs

2.48 If any pavement design other than thosegiven in this Section is to be considered, approvalto proceed is required from the OverseeingOrganisation at the preliminary design stage.Submissions seeking approval for alternativedesigns must include a justification for the choiceof non standard materials and/or thicknesses,supporting calculations and an indication of anyadditional specification requirements or testingregime that may be necessary for their validation.Guidance is given in Chapter 4 of this Part.

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Volume 7 Section 2

Part 3 HD

26/06

C D

– C12/15 (or T4) CBGM B – C16/20 (or T5)– C12/16 (or T4) SBM B1 – C15/20 (or T5)– C12/16 (or T4) FABM1 – C15/20 (or T5)

– C16/20 (or T5)– C15/20 (or T5) -– C15/20 (or T5)

2/6

Chapter 2

Standard Designs

February 2006

Figure 2.1 Design T

hickness for Flexible Pavements

Examples of Hydraulic Bound Base Materials

HBM Category A B

Crushed Rock Coarse Aggregate: CBGM B – C8/10 (or T3) CBGM B(with coefficient of thermal - SBM B1 – C9/12 (or T3) SBM B1

expansion <10 × × × × × 10 –6 per 0C) FABM1 – C9/12 (or T3) FABM1

Gravel Coarse Aggregate: CBGM B – C8/10 (or T3) CBGM B – C12/15 (or T4) CBGM B(with coefficient of thermal SBM B1 – C9/12 (or T3) SBM B1 – C12/16 (or T4) SBM B1

expansion ≥ ≥ ≥ ≥ ≥10 × × × × × 10 –6 per 0C) FABM1 – C9/12 (or T3) FABM1 – C12/16 (or T4) FABM1



Notes on Figure 2.1:

Flexible Pavement Construction

1. Thickness to be rounded up to the next10mm.

2. Minimum allowable total asphalt materialthickness is 200mm for flexible construction withasphalt base. Minimum allowable HBM basethickness is 150mm for flexible construction withHBM base, except in Scotland where the minimumthickness of HBM is 175mm.

3. Asphalt surfacing thickness in mm (H) overHBM base is given by:

H = –16.05×(Log(N))2 + 101×Log(N) + 45.8

where:

N = Design traffic (msa), up to 400msa.

Calculated thickness (mm) to be rounded up to thenext 10mm; with a minimum thickness of 100mmfor <4msa, and a thickness of 180mm for >80msa.

4. Where the asphalt design thickness is300mm or less, the material is to be laid with nonegative tolerance.

5. Surface course and binder course must beone of the permitted materials presented in HD 36(DMRB 7.5.1). For further details refer to HD 37and HD 38 (DMRB 7.5).

6. For HRA surfacing, where permitted, referto either Clause 943 of the Specification(MCHW1), or Clause 911 of the Specification(MCHW1), with reference to BS 594-1: Annex B:Table B1 for stability and flow values related totraffic loading.

For Scotland, design values are given in theOverseeing Organisation’s special requirements inClause NG911S.SO (MCHW2).

In Northern Ireland recipe mixes to BS 594-1 maybe used where considered appropriate by theOverseeing Organisation.

7. If 50mm of Porous Asphalt (PA) surfacing isto be used, it must be modified with a polymer orfibre additive. Its contribution to the materialdesign thickness is only 20mm. A 60mm densebinder course is required beneath PA surfacing,compacted so that the air voids are less than themaximum in the Series 900 (MCHW1).

February 2006

8. A binder course, see the Series 900(MCHW1), must be provided beneath a TSCS, butis optional beneath other materials such as HRAwhere this is permitted. If used, the binder coursecan be of any permitted material (subject to Note7), and be at least 50mm thick (except for SMAbinder courses, which should be at least 30mmthick), and compacted so that the air voids are lessthan the maximum in the Specification (MCHW1)Series 900.

9. This Figure assumes that the binder courseis the same material as the base. However anypermitted material may be used as long as theoverall pavement thickness is adjusted to giveequivalent load spreading ability. Refer to Chapter4 of this Part for guidance.

10. DBM125 base and binder course mustcontain 100/150 penetration grade binder. HRA50,DBM50 and HDM50 base and binder course mustcontain 40/60 penetration grade binder. EME2base and binder course must target a penetration of15-20, which can be achieved using 10/20 or 15/25penetration grade binder. In Scotland, whereHMB35 might be used, the material must target apenetration of 30/45.

11. Where traffic exceeds 80 msa, binder courseand base materials must contain crushed rock, orslag coarse aggregate, unless local experienceexists of the successful use of gravel.

12. The thickness of asphalt layers for flexibleconstruction with HBM base is applicable to allpermitted base materials.

13. HBM designations shown in the table insertto Figure 2.1 are consistent with those detailed inthe Specification (MCHW1) Table NG 8/1.

14. All HBM layers that are expected to reach acompressive strength of 10MPa at 7 days musthave cracks induced in accordance with theSpecification (MCHW 1) Clause 818.

15. Where induced cracks are required in an HBM,these must be aligned (maximum 100mmtolerance) with any induced cracks in theunderlying construction.

16. EME2 must only be laid over a Class 3 or 4foundations or a Class 2 foundation that has asurface stiffness modulus of at least 120MPa attime of construction. Further details of EME2 aregiven in TRL Report 636 (2005).

2/7



Examples for Figure 2.1

1. Design traffic of 60 msa and Foundation Class 2:

Flexible Composite Option [A]:180mm Asphalt**, over180mm HBM Category C*

Note *: Refer to the table insert to Figure 2.1 forHBM Category C options, e.g. a CBGM B withlaboratory performance category C12/15 (or T4).Laboratory performance categories are detailedin the Specification (MCHW1 Series 800).

2. Design traffic of >80 msa (‘long life’ pavement)and Foundation Class 3:

Flexible Option [B]:320mm Asphalt**, comprising DBM50 bindercourse and DBM50 base

Note **: The total asphalt thickness from Figure2.1 comprises the surface course, binder courseand base.

February 20062/8



Foundation Class

Material Class 1 Class 2 Class 3 and 4

DBM125 240 mm 210 mm 200 mm

HRA50 230 mm 200 mm 200 mm

DBM50 and HDM50 200 mm 200 mm 200 mm

EME2 n/a n/a 200 mm

Table 2.1 Total thickness of asphalt for flexible construction with asphalt base for a design traffic of 1 msa

Foundation Class

Material Class 1 Class 2 Class 3 Class 4

DBM125 < 1msa < 1 msa 2 msa 6 msa

HRA50 < 1 msa 1 msa 2.5 msa 7 msa

DBM50 and HDM50 1 msa 2 msa 4 msa 10 msa

EME2 n/a n/a 10 msa 32 msa

Table 2.2 Traffic Design values where total asphalt thickness reaches minimum 200mm thickness

For clarification, Tables 2.1 and 2.2 provide detailsfor designs for low traffic levels

February 2006 2/9

Volume 7 Section 2

Part 3 HD

26/06

2/10

Chapter 2

Standard Designs

February 2006

Figure 2.2: Design T

hickness for Rigid (C

ontinuous) Pavements




Rigid Pavement Construction

1. Thicknesses are to be rounded up to next10mm.

2. ff (MPa) denotes mean concrete flexuralstrength at 28 days.

3. The concrete design thickness valueassumes the presence of a minimum 1m edge stripor tied shoulder, otherwise the concrete designthickness shall be increased by 30mm.

4. Notes 1, 4 and 5 for Figure 2.1 also apply toFigure 2.2. Notes 7-11 for Figure 2.1 apply toFigure 2.2 for CRCB but not CRCP construction.

5. Foundations below rigid pavements mustcomprise at least 150mm of bound subbasematerial, in order to ensure subbase materialdurability. A bound Foundation Class 2 must onlybe used with the approval of the OverseeingOrganisation.

6. Two options for CRCP are available:

• CRCP with minimum 30mm of TSCS, fornoise reduction; hence no binder course isrequired;

• CRCP with no surfacing - not a permittedoption in England.

7. Minimum allowable concrete materialthickness is 200mm for CRCP construction and150mm for CRCB construction. The concretethickness in Figure 2.2 does not include anyasphalt surfacing; minimum allowable asphaltmaterial thickness is 100mm for CRCBconstruction.

February 2006

8. If PA surface course is used over CRCB, itmust be modified with a polymer or fibre additive,and laid over a dense binder course that iscompacted so that the air voids are less than themaximum in the Specification (MCHW1) Series900. The PA shall be 50mm thick over a 90 mmbinder course or 50 mm thick over a 60 mm bindercourse with the CRCB increased by 10 mmthickness.

9. Longitudinal crack control steel in CRCPshall be 0.6% of the concrete slab cross-sectionalarea, comprising 16mm diameter deformed steelbars (T16 reinforcement). Transverse steel must be12mm diameter deformed bars at 600mm spacings.

10. Longitudinal crack control steel in CRCBshall be 0.4% of the concrete slab cross-sectionalarea, comprising 12mm diameter deformed steelbars (T12 reinforcement). Transverse steel must be12mm diameter deformed bars at 600mm spacings.

11. Concrete of flexural strength 5.5MPa orgreater must use aggregate that has a coefficient ofthermal expansion less than 10 x 10-6 per oC unlessa ‘Departure from Standard’ from the OverseeingOrganisation is obtained.

12. Exposed Aggregate Concrete Surfacing(EACS) must not be used without a ‘Departurefrom Standard’ from the Overseeing Organisation.For options and details refer to HD 36 and HD 38(DMRB 7.5.1 and 7.5.3).

2/11



Examples for Figure 2.2

1. Design traffic of 200 msa with a bound(‘approved’ – Ref. Notes on Figure 2.2, Note 4)Foundation Class 2:

CRCB Option [A]:

• 100mm Asphalt, over

• 220mm of 4.5MPa flexural strength concrete(with a tied shoulder or 1m edge strip)

• T12 longitudinal reinforcement bar spacing(i.e. maximum distance, centre to centre, betweenbars across the width of the slab)

100 × π × D2 100 × π × 122

4 × t × R 4 × 220 × 0.40

where:

t = concrete design thickness (mm)R = reinforcement (%)D = diameter of reinforcement bar (mm)

= = 129mm

2/12

2. Design traffic of 275 msa and FoundationClass 4:

CRCP Option [B]:

• 30mm Asphalt, over

• 210mm+30mm = 240mm of a 5.0MPa flexuralstrength concrete (without a tied shoulder or 1medge strip)

• T16 longitudinal reinforcement bar spacing

100 × π × D2 100 × π × 162

4 × t × R 4 × 240 × 0.55= = 152mm

February 2006


k contained in TRL Report RR 87 (1987) and are

} / 5.094

.150} / 4.786entententent

ab without a tied shoulder or 1m edge strip

ab with a tied shoulder or 1m edge strip

00msaN/mm2 or MPa) at 28 days

or


Designs for jointed concrete pavements are based on worrelated to the compressive strength of the concrete.

For unreinforced jointed concrete pavements (URC):Ln(H1) = {Ln(T) - 3.466 Ln(Rc) - 0.484 Ln(E) + 40.483

For reinforced jointed concrete pavements (JRC):Ln(H1) = {Ln(T) – R - 3.171 Ln(Rc) – 0.326 Ln(E) + 45

Where R = 8.812 for 500 mm2/m reinforcemR = 9.071 for 600 mm2/m reinforcemR = 9.289 for 700 mm2/m reinforcemR = 9.479 for 800 mm2/m reinforcem

H2 = 0.934 H1 - 12.5

Where: H1 is the thickness (mm) of the concrete sl= minimum 150mm

H2 is the thickness (mm) of the concrete slLn is the natural logarithmT is the design traffic (msa) = maximum 4Rc is the mean compressive cube strength (E is the Foundation Class Stiffness (MPa)

= 200MPa for Foundation Class 3= 400 MPa for Foundation Class 4

Figure 2.3 Design Thicknesses


1. Maximum transverse joint spacings for URCpavements:

a) for slab thickness up to 230mm- 4m for contraction joints;

b) for slab thickness 230mm and over- 5m for contraction joints.

2. The maximum transverse joint spacings forJRC pavements must be 25m (where the aggregatehas a coefficient of thermal expansion not less than10 x 10-6 per oC) except for slabs having<600mm²/m reinforcement, where the maximumjoint spacing depends on the slab thickness, asfollows:

<280mm slab thickness: maximum 25m290mm slab thickness: maximum 24m300mm slab thickness: maximum 23m310mm slab thickness: maximum 22m320mm slab thickness: maximum 21m>330mm slab thickness: maximum 20m

February 2006

for Rigid (Jointed) Pavements

3. For JRC pavements, the minimumlongitudinal reinforcement permitted is 500mm²/m.

4. If concrete is used with aggregate that has acoefficient of thermal expansion less than 10 x 10-6

per oC transverse joint spacings may be increasedby 20%.

5. For details of permissible concrete surfacingrefer to HD 36 and 38 (DMRB 7.5.1 and 7.5.3).

2/13



Example for Figure 2.3

Pavement Type JRC:

• Design Traffic 130 msa;

• Reinforcement 500mm²/m;

• Aggregate that has a coefficient of thermalexpansion less than 10 x 10-6 per oC;

• Mean compressive cube strength of 50N/mm2;

• Foundation Class 3.

Design Thickness =285mm without a tied shoulderTransverse Joint Spacing = 25m x 1.2 = 30m;

or

Design Thickness =255mm with a tied shoulderTransverse Joint Spacing = 25m x 1.2 = 30m.

February 20062/14


Chapter 3Materials

3. MATERIALS

FROST PROTECTION

3.1 All material within 450mm of the roadsurface, where the mean annual frost index(MAFI) of the site is ≥50 must be non frost-susceptible in the long-term. Where the MAFI is< 50 the thickness of non-frost susceptible materialmay be 350 mm. For slower curing HBMappropriate measures must be taken to preventfrost damage in the short term. Further guidance isprovided in HD 25.

BITUMEN BOUND MATERIALS

3.2 Most asphalt binder course and base materials arecharacterised by an aggregate skeleton, where theindividual particles are mechanically interlocked, boundwith penetration grade bitumen in the range 10/20 to100/150pen. The aggregate skeleton providesdeformation resistance (provided that in-situ air voidsare typically in the range 2-6%), as well as contributingto stiffness. Clause 929 of the Specification (MCHW1)sets out the requirements for the materials. The bindercontent should be sufficient to provide thick enoughbinder films on the aggregate to create fatigueresistance and achieve durability. Generally, the lowerpenetration binders are used to obtain increasedstiffness and deformation resistance.

Premature Rutting

3.3 Early age deformation (rutting) in surface andbinder course layers may be linked to trafficking byslow moving commercial vehicles (e.g. in acontraflow), especially on uphill lengths and whenpavement temperatures are high, relatively soon afterthe materials have been laid (e.g. after majormaintenance in the summer). Therefore, such situationsshould be avoided. Where Hot Rolled Asphalt (HRA)(if permitted) is used, Clause 943 of the Specification(MCHW1) sets out the requirements for performance-based surfacing.

Bond Between Layers

3.4 The designs contained in this part are based onthe principle that full adhesion is achieved between theindividual layers of asphalt materials, such that they actas a single monolithic layer. For this to be achieved in

pt

3aCrcs

3ligeppswssslpmsadb

3P

•

•

•

•

•

3oOo

February 2006

ractice and to ensure good long-life performance, aack or bond coat is required between all layers.

.5 Particular attention should be paid to specifyingnd achieving good bond between a Thin Surfaceourse System (TSCS) and the underlying flexible or

igid construction. This is because, under certainircumstances (e.g. braking vehicles), high sheartresses can be developed at these shallow interfaces.

.6 TSCSs normally have a higher void content (witharger individual air voids) than traditional HRA. Thiss often because they have been derived from gap-raded Stone Mastic Asphalt (SMA) type mixtures. It isssential that the chosen binder course under the TSCSrovides an effective barrier to water entering the loweravement layers. It is also vital that surface and sub-urface drainage arrangements are designed to avoidater being introduced into the pavement from the

ides. Particular care is required for resurfacingchemes, where the existing layer beneath the newurfacing has to provide the necessary impermeableayer or be replaced. Such durability issues arearticularly important where the base may beanufactured with a low binder content to provide high

tiffness and rut resistance. For SMA binder courses anir void content of between 2 and 4% is required forurability as well as the wheeltracking limits imposedy the Series 900 (MCHW1).

.7 When considering the costs and benefits of usingorous Asphalt (PA), it should be remembered that:

PA can be significantly more expensive thanother surfacings;

PA may have a shorter life than other surfacings;

PA can cost more to repair than other surfacings;

other noise reduction measures, or othersurfacings, may be more appropriate in WholeLife Value terms;

although spray may be reduced, there may be noreduction in accidents.

.8 A decision on whether to use PA should be takennly after consideration of all relevant factors. Theverseeing Organisation may be consulted for advicen the suitability of using PA in particular

3/1


Chapter 3Materials

circumstances. In Scotland, prior approval is requiredfrom the Overseeing Organisation for PA to be used.Further details on PA are contained in HD 37 (DMRB7.5.2).

PAVEMENT CONCRETE

3.9 The stress generated in a concrete slab partlydepends on the stiffness ratio between the slab and itsunderlying support. To maximise the pavement life, allrigid pavements require a bound (minimum 150mmthickness) subbase, since this would erode less readilythan an unbound subbase material and is less watersusceptible should joint sealants fail.

3.10 Concrete is inherently strong in compression, butweak in tension. Repeated stressing will eventually leadto crack initiation unless the stress is very low. Thickerslabs result in lower stresses being generated under thecombined influence of vehicular and temperatureloading.

Continuously Reinforced Concrete (CRCP andCRCB)

3.11 CRCP/CRCB pavements develop a finetransverse crack pattern soon after the concrete is laid.Initially the crack spacing is about 3 or 4m. Furthercracking is usual after the road has been in service for atime. The continuous longitudinal steel holds the crackstightly closed, ensuring load transfer by aggregateinterlock and minimising corrosion of the steel. Thecrack propagation in CRCP/CRCB pavements is closelyrelated to the sub-surface friction, the aggregate used,the strength of the concrete and the proportion of steel.

3.12 The separation membrane is omitted from CRCP/CRCB construction in order to give a higher level offriction between the concrete slab and the subbase thanfor jointed slabs. The restraint provided by the subbasereduces the amount of movement and is related to thedesired crack pattern. The use of a layer of materialunder the CRCP/CRCB with uniform surfaceproperties, such as may be provided by paver-laid wet-lean concrete or an asphalt material, is recommended.The thickness of any asphalt material may beconsidered as part of the bound subbase.

3.13 Discontinuities in the slab should be avoidedwherever possible as they encourage the formation ofclosely spaced cracks, with increased risk of spalling.Gullies and manholes should be located outside themain CRCP/CRCB slab for this reason. If this is notpossible, the slab around the gullies and manholes

3/2

should be heavily reinforced as shown in the HighwayConstruction Details (MCHW3).

3.14 Where a CRCP has a surface course, surfacenoise generation is reduced and water penetration (andthe potential for steel corrosion) is likely to be reduced.If the surfacing is 100mm thick (or more) it alsoprovides a degree of thermal protection from rapidtemperature changes for the concrete base. If the 30mmminimum TSCS is used, the bond coat required by theapproved system is important to ensure good adhesion.It will be necessary for the TSCS to comply with therequirements of the Specification (MCHW1) Clause942 and any additional BBA HAPAS requirements forthe TSCS being used.

Jointed Pavements

3.15 Temperature and, to a lesser extent, moisturechanges cause contraction/expansion of the slabwhich, if restrained, induce stresses in theconcrete. A separation membrane is requiredbetween slab and subbase for both URC and JRCpavements, in order to reduce this restraint andthus inhibit the formation of mid bay cracks. It alsohelps reduce the loss of water from the freshconcrete.

3.16 Three different types of joints are used inconcrete pavements. They are contraction, expansionand warping joints, typical details of which areillustrated in Highway Construction Details (MCHW3).

3.17 Contraction joints enable the slab to shortenwhen its temperature falls and allow the slab to expandsubsequently by approximately the same amount.Expansion joints cater for the expansion movementthat would naturally occur at temperatures higher thanthat of the concrete at the time the slab was constructedand allow the slab to shorten. Transverse joints areeither expansion or contraction types. However,longitudinal joints are of the warping type only. Thesetie the slabs together, and can be thought of as acting as‘hinges’ in the slab.

3.18 The permitted spacing of transverse joints is afunction of slab thickness, aggregate type, and, for JRC,the quantity of reinforcement. Joint spacing reflects thecapacity of the slab to distribute strain rather than allowdamaging strain concentrations.

February 2006


Chapter 3Materials

3.19 There is an advantage for concrete containingaggregate that has a lower coefficient of thermalexpansion than other aggregate types, resulting in lessexpansion/contraction of the slab, with greater jointspacings being allowed.

3.20 The effectiveness of reinforcement, as adistributor of strain, increases with the amount ofreinforcement used. Greater joint spacings can be usedwith larger areas of reinforcement, although this resultsin greater movement at each joint, necessitatingappropriate selection of sealants.

HYDRAULICALLY BOUND MIXTURES

3.21 HBMs comprise either cement, slag or flyash binder; or a factory blend of these binders.HBM must be produced, constructed and tested inaccordance with the Specification (MCHW1)Series 800.

3.22 The UK has traditionally characterised CementBound Material in terms of compressive strength atseven days for compliance purposes; and in terms ofboth dynamic stiffness modulus and flexural strengthfor design purposes.

3.23 The European Standard, adopted in the UK,characterises cement-bound HBM in terms ofcompressive strength at 28 days; and in terms of bothstatic stiffness modulus and direct tensile strength.

3.24 360-day values of HBM stiffness modulus andstrength are now used for UK design purposes, enablingslow curing HBMs to be included.

3.25 Hydraulic-bound base materials comprisingbinders other than cement typically have lower earlyage performance properties. This may have implicationson construction programme due to the need to avoidearly trafficking (although some HBMs can betrafficked satisfactorily due to the inherent stability ofthe aggregate skeleton) and a need for protection fromfrost. For design purposes the HBM 360-day strengthand stiffness are required. For materials with a historyof use these properties can be extracted fromperformance data on in-service roads, but for othermaterials it may be necessary to carry out laboratorytesting.

February 2006

RECYCLED AND SECONDARY AGGREGATES

3.26 Advice on recycled and secondary materials andtheir uses is given in HD 35 (DMRB 7.1.2). Materialsshould be produced in accordance with BS EN 13242and BS EN 13285 and Series 800 and 100 (MCHW1).Procedures for production are also described in theWaste and Resources Action Programme’s “The QualityProtocol for the Production of Aggregates from InertWaste” or “The Quality Protocol for the Production ofAggregates from Inert Waste in Scotland” asappropriate for each Overseeing Organisation. Thepurpose of the Quality Protocol is to provide a uniformcontrol process for producers from which they canreasonably state and demonstrate that their product hasbeen fully recovered and is no longer considered awaste.

3.27 Users of these materials often express concernsover the potential for leaching of contaminants andsubsequent pollution of the local environment,particularly the pollution of controlled waters. Researchdemonstrates that the perceived risk of leaching fromrecycled and secondary aggregates is often overstated,even when used in unbound engineering applications.Furthermore, the risk of deleterious leaching issignificantly reduced by binding aggregates in bitumen,hydraulically bound mixtures or concrete. The majorityof recycled and secondary aggregates pose nosignificant risk to controlled waters when used inproperly designed and constructed engineeringapplications that account for the sensitivity of the localenvironment. It is important that the use of all materialsof this type is agreed with all bodies responsible for thelocal environment and for water quality.

3.28 Further information and details of materialproperties for many of these materials can be found onthe websites for WRAP, Britpave and the UK QualityAsh Association.

3/3


Chapter 4Alternative Design Procedures

ROCEDURES
4. ALTERNATIVE DESIGN P
Alternative Pavement Designs

4.1 TRL Report 615 follows TRL Report LR1132and provides guidance that should be considered in thepreparation of alternative flexible pavement designs.TRL Report RR87 (1987) and TRL Report 630 (2005)provide guidance that should be considered in thepreparation of alternative rigid pavement designs.

4.2 Pavement design guidance comprising coldrecycled base material is presented in TRL Report611 (2004). These designs require a ‘Departurefrom Standard’ from the Overseeing Organisation,with the exception of an ex situ stabilized QuickVisco-Elastic (QVE) base material comprising aminimum 3% bituminous binder and a minimum1% cement, known as foamed asphalt, andclassified as Zone B2 material, for a maximumcumulative design traffic up to 30msa.

Alternative ‘Analytical’ Pavement Design

4.3 An analytical design approach provides a meansof customising a pavement to locally available materialsand/or construction methods, in an attempt to maximizethe whole life value. However, it is essential that thematerial properties assumed in the design are actuallyachieved in situ, and that due consideration is given tothe following:

• durability of the pavement structure (e.g.resistance of the materials to the deleteriouseffects of water, air, and other environmentalfactors);

• serviceability (e.g. skidding resistance, andpermanent deformation within the asphalt);

• maintainability (e.g. reflection cracking incomposite pavements, and surface initiatedfatigue cracking in thicker/long-life pavements);

• construction tolerances (allowable constructionthickness reductions to be added to the minimumanalytical design thickness).

February 2006

4.4 Where used appropriately, the OverseeingOrganisations will accept the use of an analyticalpavement design to justify alternatives. However,full supporting details must be submitted in orderfor a ‘Departure from Standard’ to be authorised.

4.5 The philosophy of analytical design is that thepavement should be treated in the same way as othercivil engineering structures, the procedure for whichmay be summarised as follows:

a) identify the pavement life requirement in termsof traffic loading (see HD 24, DMRB 7.2.1), interms of an equivalent number of ‘standard axle’loads (i.e. 40kN wheel loads);

b) consider available and permitted pavementmaterials (see HD 35, DMRB 7.1.2);

c) estimate the in situ dimensions and long-termperformance properties (stiffness and/or strength)of each individual layer of pavement material;

d) carry out a structural analysis, e.g. using asimplified multi-layer linear elastic model of thepavement structure;

e) compare critical stresses/strains and/ordeflections, with allowable values;

f) make adjustments to c) until the pavement liferequirement is achieved;

g) consider the whole life value of the resultantpavement design(s).

4.6 The following performance properties ofmaterials need to be considered when designingpavements:

• effective stiffness modulus, which governs loadspreading behaviour;

• deformation resistance of asphalt materials only,which governs rutting behaviour;

• fatigue resistance of asphalt materials, andstrength of HBM, which governs crackingbehaviour.

4/1



4.7 Critical stresses/strains considered in thestandard UK design approach include:

• excessive stress/strain (combination of magnitudeand number of load applications) causing fatiguecracking (typically at the bottom of the baselayer) of the asphalt, HBM or concrete material;

• excessive subgrade strain, resulting in structuraldeformation. [Note: This parameter becomesredundant in the TRL Report 615 (2004) forflexible designs but could still be important forthinner, lightly trafficked pavementconstructions, typical of those used in localauthority highways.]

4.8 Relationships between pavement life and thesecritical strains or stresses have been derived forhistorically standard UK materials from a combinationof laboratory testing and pavement performancemonitoring, see Chapter 5 of this Part for references andbibliography.

4.9 However, it is still necessary for the designer tomake appropriate judgements. For example, two verydifferent asphalt mixes, even if both nominally of thesame type (e.g. a HDM50) could yield differentpavement lives, depending on the aggregate structureand binder content. Similarly, the permanentdeformation behaviour of a sandy subgrade will differfrom a clay subgrade, even if they have the samestiffness.

4.10 For non-standard materials especially those fromEuropean standardisation, it is essential that thematerial properties are known. Properties shouldinclude stiffness modulus related to testing age, tocuring regime and related to degrees of confinement.Properties can be tested in various ways depending onthe nature of the material and the properties required inrelation to the needs of the design.

4.11 Where stiffness is to be measured as thedesign criteria for hydraulically bound materials,testing must be carried out in accordance withBS EN 13286-43.

4.12 To help inform the design process, the NAT‘Springbox’ can also be used for unbound and slightlybound materials and this can be used to identify bothshort-term and longer term stiffness moduli. Site testingas part of demonstration trials can also be used tomeasure on-site properties. Further details of testing forfoundation materials is given in HD 25 (DMRB 7.2.2).

4/2

4.13 For Highways Agency schemes, values oflong-term elastic stiffness modulus of standard UKasphalt materials for use in analytical design mustbe as follows, unless reliable data clearly reveals adivergence from these typical figures:

DBM125 2,500 MPaHRA50 3,100 MPaDBM50/HDM50 4,700 MPaEME2 8,000 MPa

Design stiffness moduli used for pavement designare values for the reference condition of 20oC and5 Hz They are not interchangeable with IndirectTensile Stiffness Modulus (ITSM) values, whichare measured for compliance testing at the lowerfrequency of 2.5 Hz.

4.14 Until further research is undertaken, amaximum reinforcement value of 900mm2/m widthof a CRCP (and CRCB) concrete slab shall be usedin calculations (despite more reinforcement thanthis being used in construction, according to theconcrete strength as detailed in Figure 2.2),consistent with TRL Report 630.

4.15 In other cases, in order to assist with theevaluation of the alternative, the following shouldalso be supplied to the Overseeing Organisation:

• comparisons with other published designs,especially from countries with similartrafficking levels, climatic conditions andmaterial properties to the UK;

• information on the analytical pavementdesign model adopted;

• material properties assumed and supportinginformation, e.g. from in situ or laboratorytesting, or published data;

• information on the failure mechanismsconsidered by the designer;

• experience of long term performance ofsimilar pavements, both in the UK andoverseas;

• sensitivity analysis to identify theparameters that have most influence on life;

February 2006



• procedures to be adopted on site to reducethe variability of pavement construction, inparticular the most influential parametersidentified from the sensitivity analysis;

• End Performance Test Procedures to ensurethat the mean and minimum properties ofmaterials assumed in the design, areachieved on site.

4.16 It should be noted that a specific analyticaldesign method has not been defined. The availablemethods referenced in Chapter 5 of this Part may differin their mathematical formulation, but each method isinternally consistent. It should be appreciated thatinadequate designs can result if elements from differentmethods are combined inappropriately.

February 2006 4/3



IOGRAPHY
5. REFERENCES AND BIBL
REFERENCES

Design Manual for Roads and Bridges (DMRB):

The Stationary Office Ltd.

HD 24 (DMRB 7.2.1) Traffic assessment

HD 25 (DMRB 7.2.2) Foundations

HD 27 (DMRB 7.2.4) Pavement construction methods

HD 30 (DMRB 7.3.3) Maintenance AssessmentProcedures

HD 35 (DMRB 7.1.2) Conservation and the use ofsecondary and recycled materials

HD 36 (DMRB 7.5.1) Surfacing materials for new andmaintenance construction

HD 37 (DMRB 7.5.2) Bituminous surfacing materialsand techniques

HD 38 (DMRB 7.5.3) Concrete surfacing and materials

HD 39 (DMRB 7.2.5) Footway design

HA 39 (DMRB 5.3) Edge of pavement details

Manual of Contract Documents for Highway Works(MCHW):

The Stationary Office Ltd.

Volume 1: Specification for Highway Works(MCHW1)

Volume 2: Notes for Guidance on theSpecification for Highway Works(MCHW2)

Volume 3: Highway Construction Details(MCHW3)

Transport Research Laboratory

1984

LR1132; Powell W D, Potter J F, Mayhew H C andNunn M E, “The Structural Design of Bituminous

February 2006

Roads”, TRRL.

1987

RR87; Mayhew, H.C. and Harding, H.M., “ThicknessDesign of Concrete Roads”, TRRL.

2004

TRL Report 611; Merrill, D., Nunn, M. andCarswell, I., “A guide to the use and specification ofcold recycled materials for the maintenance of roadpavements”.

TRL Report 615; Nunn, M., “Development of a moreVersatile Approach to Flexible and Flexible CompositePavement Design”.

2005

TRL Report 630; Hassan, K., Chandler, J., Harding, H.,Dudgeon, R., “New Continuously Reinforced ConcretePavement Designs”.

TRL Report 636: Sanders, P. G. and Nunn, M., “Theapplication of Enrobé à Module Elevé in flexiblepavements”.

Others

2005

BS 594-1, “Hot rolled asphalt for roads and other pavedareas. Specification for constituent materials andasphalt mixtures”. BSI.

BIBLIOGRAPH

British Standards

2003

BS 594-2 “Hot rolled asphalt for roads and other pavedareas, Specifcation for transport, laying and compactionof hot rolled asphalt”. BSI

BS 4987-2 “Coated Macadam asphalt concrete forRoads and Other Paved Areas, Specification fortransport, laying and compaction”. BSI

5/1



2005

BS 4987-1 “Coated Macadam asphalt concrete forRoads and Other Paved Areas, Specification forconstituent materials and for mixtures”. BSI

Transport Research Laboratory

1997

Nunn, M.E. and Smith, T., “Road Trials of HighModulus Base for Heavily Trafficked Roads”, Report231.

Nunn, M.E., Brown, A., Weston D. and Nicholls, J.C.,“The Design of Long-life Flexible Pavements forHeavily Trafficked Roads”, Report 250.

Ellis, S.J., Megan, M.A and Wilde, L.A., “Constructionof Full-Scale Trials to Evaluate the Performance ofInduced Cracked CBM Roadbases”, Report 289.

Nicholls, J.C., “Road Trials of Stone Mastic Asphaltand other Thin Surfacings”, Report 314.

2000

Weston, D., Nunn, M., Brown, A. and Lawrence, D.,“Development of a Performance Based Specificationfor High Performance Asphalt Pavement”, Report 456.

Others

1987

Brunton, J.M., Brown, S.F. and Pell, P.S.,“Developments to the Nottingham Analytical DesignMethod for Asphalt Pavements”, Proc 6th Int. Conf.Structural Design of Asphalt Pavements, Ann Arbor,Michigan, pp 366-377.

Dawson, A.R., Elliott, R.C., Rowe, G..M. andWilliams, J., “Assessment of Suitability of SomeIndustrial By-Products for Use in Pavement Bases inthe United Kingdom”, Transportation Research Record1486, pp 114-123, Transportation Research Board,Washington.

1996

Thom, N.H. and Shahid, M.A., “Controlled Cracking inCement Bound Bases”, Journal of the Institution ofHighways and Transportation, pp 20-23, October.

2

P“PC

2

HBo2

B“’B(

2

HS

2

WPWh

2

D“dh

W

Rw

Fw

Sw

Aw

C

5/2

000

otter, J.F., Dudgeon, R.P. and Langdale, P.C.,Implementation of Crack and Seat for Concreteavement Maintenance”, 4th International RILEMonference on Reflection Cracking, Ottowa.

002

akim, B., “The Importance of Good Bond Betweenituminous Layers”; Proc 9th International Conferencen Asphalt Pavements, Copenhagen, 17-22nd August002.

rown, S.F., Collop, A.C., Elliott, R.C., Williams, J.,The Use of High Stiffness Asphalt Mixtures in UKlong-life’ pavements”, 6th International Conference onearing Capacity of Roads Railways and Airfields

BCRA). Lisbon, 26th-27th June 2002.

003

ighways Agency, “Building Better Roads: Towardsustainable Construction”, December 2003.

004

aste and Resources Action Programme, “The Qualityrotocol for the Production of Aggregates from Inertaste”, November 2004. (Available from

ttp://www.wrap.org.uk)

005

epartment for Environment, Food and Rural Affairs,Securing the future – delivering UK sustainableevelopment strategy”, March 2005. (Available fromttp://www.sustainable-development.gov.uk)

ebsites with more information

ecycled and secondary aggregates (WRAP):ww.aggregain.org.uk

ly ash:ww.ukqaa.org.uk

ustainable development:ww.sustainable-development.org.uk

sphalt Materials:ww.asphaltindustryalliance.com

ement Bound Materials:www.britpave.org.uk

February 2006


February 2006 6/1

6. ENQUIRIES






Chapter 6Enquiries

February 2004




PART 4

HD 27/04

PAVEMENT CONSTRUCTIONMETHODS

SUMMARY

Chapter 2 of this Part has been revised to reflect currentpavement design issues. It offers broad guidance of theissues to be considered when widening existing roads.Chapter 3 that deals with rapid concrete constructionhas not been revised with this update of the Standardand although the advice continues to be valid,references may need to be checked.


1. Remove existing Contents pages for Volume 7.

2. Insert new Contents pages for Volume 7, datedFebruary 2004.

3. Remove HD 27/94 [Incorporating AmendmentNo. 1 dated March 1995 and Amendment No. 2dated February 1999] from Volume 7, Section 2,Part 4 and archive as necessary.

4. Insert HD 27/04 into Volume 7, Section 2, Part 4.



HD 27/04

Pavement Construction Methods


Summary: Chapter 2 of this Part has been revised to reflect current pavement designissues. It offers broad guidance of the issues to be considered when wideningexisting roads. Chapter 3 that deals with rapid concrete construction has notbeen revised with this update of the Standard and although the advicecontinues to be valid, references may need to be checked.

THE HIGHWAYS AGENCY

SCOTTISH EXECUTIVE




February 2004







PART 4

HD 27/04

PAVEMENT CONSTRUCTIONMETHODS

Contents

Chapter

1. Introduction

2. Widening of Pavements

3. Rapid Construction and Repair of ConcretePavements


5. Enquiries


February 2004


February 2004

General

1.1 This Part contains details of various constructionmethods and techniques which are particularly relevantto Volume 7. General requirements for pavementconstruction methods can be found in the Specification(MCHW1) and in the Highway Construction Details(MCHW3). This Part includes details for the wideningof existing pavements and covers the area of design,materials and construction. These techniques will beused more frequently as a high proportion of the trunkroad network needs to be widened or improved tohandle increased traffic capacity. Rapid constructionand repair of concrete pavements is also covered anddraws on experience gained from contracts both in theUK and in the United States.

Implementation

1.2 This Part shall be used forthwith on all schemesfor the construction, improvement and maintenance oftrunk roads including motorways, currently beingprepared provided that, in the opinion of the OverseeingOrganisation, this would not result in significantadditional expense or delay. Design organisationsshould confirm its application to particular schemeswith the Overseeing Organisation.

Mutual Recognition

1.3 The construction and maintenance of highwaypavements will normally be carried out under contractsincorporating the Overseeing Organisation’sSpecification for Highway Works (MCHW1). In suchcases, products conforming to equivalent standards andspecifications of other member states of the EuropeanCommunity and tests undertaken in other member stateswill be acceptable in accordance with the terms of the104 and 105 Series of Clauses of that Specification.Any contract not containing these Clauses must containsuitable clauses of mutual recognition, having the sameeffect regarding which advice should be sought.

1. INTRODUCTION


1/1


NTS

Chapter 2Widening of Pavements

2. WIDENING OF PAVEME

General

2.1 With the projected increases in traffic, elementsof the trunk road network, ranging from singlecarriageways to Dual 4-lane motorways, will continueto need to be widened and/or improved.

2.2 Each individual scheme will vary in scope sinceother improvements including changes in geometry,levels and superelevation and improvements to sightlines will often be included in addition to the provisionof extra lanes. Widening may be symmetrical on-line,asymmetrical on-line (see Figure 2.1) or off-line on newground, depending on the site and, in the case ofwidening from single to dual carriageway, will usuallybe a combination of all three in order to improve theline and gradients. Such changes will generally affectlevels and crossfalls and, therefore, also impact onpavement design.

February 2004

2.3 Widening, and associated improvement works,presents a valuable opportunity to upgrade the whole ofthe existing pavement, in line with current standards.All options for carrying out the works must be assessedon the basis of minimising whole life costs over a40-year analysis period. Consideration should also begiven to the principles of sustainable development,which will generally favour maximum use, or re-use, ofthe existing construction.

2.4 This chapter gives broad guidance on some of thepavement design and construction issues that should beconsidered where the widening of existing roads isproposed. It does not cover earthworks or generaldrainage issues associated with widening; advice onthese is given in HA 43 (DMRB 4.1.7).

EXTRA LANE EXTRA LANE

EXISTING CENTRAL RESERVE & DRAIN

SYMMETRICAL ON-LINE WIDENING

ASSYMMETRICAL ON-LINE WIDENING

Figure 2.1 Basic Approaches to Road Widening

EXISTING CENTRAL RESERVE AND CARRIAGEWAY RECONSTRUCTED NEW CONSTRUCTION

EXTRA LANES EXISTING CARRIAGEWAYS

2/1


works.


Evaluation of Existing Pavement

2.5 For on-line widening, where the newconstruction will abut the existing carriageway, a fullassessment of the condition of the existing pavementshould be obtained through surveys and detailedinvestigations on site in accordance with the advicegiven in HD 29 (DMRB 7.3.2) and HD 30(DMRB 7.3.3). As-constructed records may alsoprovide a useful source of information but it isimportant that the accuracy of such records, inparticular the depths and extent of the pavementconstruction, is verified through exploratoryexcavation.

2.6 Guidance on the analysis of pavement surveys isgiven in HD 29 (DMRB 7.3.2) while advice on theinterpretation of results and design of appropriatestrengthening measures is given in HD 30(DMRB 7.3.3).

2.7 In the case of flexible, and certain flexiblecomposite, pavements the categorisation of the existingpavement as ‘Long Life’, ‘Upgradeable to Long Life’ or‘Determinate Life’ should be determined, if not alreadyknown, in accordance with HD 29 (DMRB 7.3.2).Where the pavement is determined to be ‘Upgradeableto Long Life’ appropriate works to achieve the upgradeto the ‘Long Life’ classification should be considered,provided this is justified on the basis of the whole lifecost assessment.

2.8 In conjunction with new construction and anymaintenance or strengthening works to the existingpavement, consideration should be given to assessingthe skidding resistance and riding quality of the existingpavement with a view to improvement where necessary.It is likely that the existing pavement will require aquiet surface course and to match the surface of thewidened sections. Further information on the factorsthat affect skidding resistance and appropriate levels forvarying site conditions is given HD 28 (DMRB 7.3.1)while advice on surfacing options is provided in HD 36,HD 37 and HD 38 (DMRB 7.5).

Design

2.9 The design of widened areas of pavement shouldbe in accordance with HD 26 (DMRB 7.2.3). Proposalsfor on-standard designs must be submitted to theOverseeing Organisation for approval.

2.10 The ground conditions beneath an existingpavement, as determined during the investigation and

2/2

evaluation process, may be used as the basis forestimation of the long-term condition beneath anadjacent new pavement.

2.11 Where the carriageway is to be widened on-line itis essential that the greatest care is taken during designto ensure that the drainage paths under old and newpavements are maintained, particularly where falls andelevation are being altered. It is suggested thatlongitudinal and cross sections are produced at frequentintervals to check that the drainage is not hindered.Maintenance of drainage paths may result in a thickerbound construction than is required for the newpavement in accordance with HD 26 (DMRB 7.2.3),notably where an overlay is applied to strengthen orreprofile the existing pavement. Particular care must betaken in the use of bound sub-bases for widening toensure that this layer does not effectively form animpermeable barrier, trapping water in the foundationof the existing construction.

2.12 Where the proposed new design thickness for awidening strip is thicker than the adjacent, structurallysound, existing flexible pavement, special provisionmay have to be made for sub-base drainage or theexisting pavement may have to be overlaid to maintaindrainage paths.

2.13 As some surface water does percolate through thepavement surface it is essential not to impede the flowof water in the underlying pavement layers. Whenmatching new lanes against an existing carriageway thethickness of new bound layers will usually have to bethe same or less. The interfaces of the underlayersshould lead into each other or into positive drainagepaths.

2.14 However, where the new and existingconstruction cannot be adequately matched, due, forexample, to the use of different forms of construction ormaterials, the use of drainage layers may be required.Adjustment to the crossfall may also be required toensure water is not trapped under the pavement.

2.15 Advice on crossfalls and other surface drainageconsiderations for wide carriageways is given in TA 80(DMRB 4.2.2).

2.16 Where a new or modified drainage system isrequired for the widened pavement the detailing shouldtake into account the need to keep the existing systemoperative until the new system can be brought into use.

2.17 The construction sequence may need to beplanned to avoid the possibility of ponding during the

February 2004



2.18 The overall design should take account of theneed to keep disruption to a minimum. It is oftenbeneficial that sections of new carriageway are madeavailable to traffic as quickly as possible. Dueconsideration must also be given to the safety of thepublic and the Contractor’s workforce.

2.19 Consideration should be given to providing ahardened central reserve in conjunction with wideningworks, in order to reduce routine maintenance. Thesurface finish of a hardened central reserve shouldcontrast with that of the adjacent carriageway.

Pavement Materials

2.20 The only restriction on the type of material to belaid alongside the existing pavement, providing thematerials conform to the requirements of theSpecification (MCHW1), is that continuouslyreinforced concrete pavement construction should notbe laid alongside an existing jointed rigid pavement.This is because it is not possible to tie the twoconstructions to provide satisfactory edge and cornersupport while accommodating relative movement due tothermal effects. The choice of which constructionoptions to include in the tender documents is theresponsibility of the Overseeing Organisation at thecontract documentation stage and decisions should takeaccount of future maintenance costs so that the wholelife cost is kept to a minimum.

2.21 Consideration should be given to the use of highperformance bond coats to enhance adhesion and sealbetween adjoining constructions (see MCHW1,Clause 901).

2.22 In the assessment of whole life costs, on the basisof a 40-year analysis period, consideration should begiven to the future maintenance of both the existing andwidened elements of the pavement. This is likely tofavour options that ensure future maintenance can betimed to minimise interventions that cause disruptionand delay. However, where site constraints and/oroperational procedures indicate a preferred solutionother than that which would give the lowest whole lifecost, approval must be sought from the OverseeingOrganisation, supported by evidence to demonstratethat the proposed solution will provide acceptable valuefor money.

2.23 Where it is required to provide increased loadcarrying capacity for the additional lanes yet maintainthe same thickness as the existing pavement, the use ofHigh Modulus Base (HMB) asphalt, flexible composite

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February 2004

nstruction or Continuously Reinforced Concretevement (CRCP) should be considered.

4 Similarly, where overlay thickness is restricted,th HMB and CRCP should be considered as optionsr strengthening the existing carriageway.

nstruction

5 In matching new pavements to existing, the level the existing surface or any proposed overlay willve to be taken as datum for levels of the newrfacing irrespective of minor settlements. Any areasth major settlements affecting the riding qualityould be identified for regulating or reconstructionior to adding the new lanes. Alternatively, in the case a rigid concrete pavement, local settlements may bectified by slab jacking and vacuum grouting; see

32 (DMRB 7.4.2).

6 Asymmetrical widening should be designed soat longitudinal joints between old and new are mid-e or near lane divisions so avoiding wheel tracks.ort lengths crossing a lane diagonally, which may be

ctated by realignment, may be acceptable.

7 Widening may result in existing drainage runsing beneath the carriageway. It is normallyacceptable for manholes, gullies or other ironwork to placed within vehicle running lanes. If this cannot beoided then the use of higher specification chamberd gully tops should be considered. Advice onamber and gully tops is given in HA 104MRB 4.2.5), while guidance on general safetypects of road edge drainage is given in HA 83MRB 4.2.4). The use of combined drainage and kerbstems may also provide a viable option in somecumstances, and advice on these systems is given in 39 (DMRB 4.2.1).

8 Where widening asphalt pavements in similarnstruction, layers should be cut back and benched toy into the old construction (see Figure 2.2 forinimum widths). Joints into sub base or capping layerould be made where there is clean material. Ifcessary the 150 mm minimum should be increasedtil clean material is encountered. Special care isquired during compaction of any granular layers tosure a sound joint between the two materials.

2/3



Figure 2.2 Benching at Joint between Sections of Bituminous Paving

SURFACING

BASE

SUB-BASE

CAPPING

ExistingConstruction

ProposedConstruction

150 mm(min)

150 mm(min)

150 mm(min)

2.29 Where the pavement construction is on theprogramme critical path rapid construction methodsmay need to be considered.

2.30 Where sufficient space is available asphaltmaterials can be laid quickly with pavers in echelon toavoid longitudinal joints which need cutting back whencold. In winter, successive layers can be placed in rapidsuccession, but both this and echelon work will dependon sufficient compaction plant being available to ensureadequate compaction can be achieved. Information onlaying of asphalt surface course is given in HD 37(DMRB 7.5.2).

2.31 When ambient temperatures are high, it may benecessary to delay the opening of the pavement totraffic to ensure that the surface course and lower layersare sufficiently stiff to resist deformation under vehicleloading.

2.32 Where asphalt material is to be laid against anexisting pavement, drainage channels, kerbs, bridgeparapets etc, suitable preformed sealing strips should beconsidered for application to the vertical surface of theexisting pavement, after cutting back. This could avoidthe need to overband the longitudinal joint or to cut asealing groove and apply a poured sealant.

2.33 Details of rapid methods of concrete pavementconstruction are given in Chapter 3 of this Part. Theroad may be opened to normal traffic when the strengthof the concrete has been demonstrated to have reached25 N/mm2.

2/4

2.34 Where an extra jointed concrete lane is added toan existing jointed concrete carriageway, the transversejoints should be aligned and of the same type andwidth. At the longitudinal joint between the existingand new concrete, a groove should be sawn to anappropriate depth and filled with an approved sealingcompound.

2.35 To reduce the time and cost of drilling for tiebars, consideration may be given to the use of barswhich are deformed and so which may be shorter inlength and/or larger in diameter than the requirementsof the Specification (MCHW1).

2.36 Where existing concrete slabs have to bereplaced the slabs could be sawn into small sectionsduring one traffic closure and removed and replacedduring the next.

2.37 A technique for strengthening a concrete road inconjunction with the widening of the carriageway hasbeen developed in the USA. The strengthening consistsof an overlay, of minimum depth 50mm, which isbonded to the surface of the existing paving. Thewidening can be placed separately from the bondedoverlay or monolithically with the overlay in one pass(see Figure 2.3). Longitudinal joints are required in thenew concrete at positions which are dependent on thedimensions of the existing carriageway and thewidening strips. Tie bars are inserted into holes drilledin the sides of the existing slab.

February 2004



2.38 Prefabricated slabs have been used for newconcrete pavements in other countries. Construction isexpensive but fast and can significantly reduce trafficdelays. The slabs, which should be made to a highstandard of dimensional accuracy, can have self-levelling devices and special dowel fixings to connectwith neighbouring slabs. The voids between sub-baseand slabs are filled by pressure grouting. The processneeds special equipment.

Figure 2.3 Strengthening of Concrete Road in Conjunction with Widening

EXTRA WIDTH

EXISTING CONCRETE

EXISTING LONGITUDINAL JOINT

CONCRETE OVERLAY EXTRA WIDTH

LONGITUDINAL JOINTS WHERE REQUIRED

TIE BARS

February 2004 2/5


AND REPAIR OFS

Chapter 3Rapid Construction and Repair of Concrete Pavements

3. RAPID CONSTRUCTIONCONCRETE PAVEMENT

General

3.1 It has been traditional to wait long periods of 14to 21 days for concrete to cure before use by traffic.The degree of maturity was based on the requiredstrength at 28 days. This imposed restrictions on repairsand reconstruction in concrete when compared withbituminous surfaces. However, increases in traffic onmajor roads and the need to keep lane closures to aminimum has heightened the need for concrete tomature to sufficient strength in hours rather than days.

3.2 Extra rapid hardening cements were first used inthe UK in 1970 for repairs to the M4 motorway whichwere opened to traffic in 6 hours. However, longpossessions of carriageways for normal repairs orreconstruction gave no incentive for use of the moreexpensive rapid hardening cements. Other rapidconstruction methods have since been studied byBritish engineers and successful full scale trials havebeen carried out in the UK.

3.3 This Chapter gives the requirements for earlyopening of concrete pavements to traffic and includesadvice on materials and methods for rapid construction.It will be of particular importance in reconstruction,widening schemes, minor improvements and repairs forbenefits in traffic management.

Principles

3.4 Rapid construction in concrete depends on therate at which the concrete can develop sufficientstrength for opening the pavement to traffic.

3.5 Early strength development can be controlled bythe type of cement, cement content, the water-cementratio and by curing. Normal ingredients as forconventional pavement quality concrete should be usedtogether with admixtures or additives depending on thedegree of urgency. This may be under 6 hours or over 3days according to the type of construction or repairbeing considered.

3.6 Rates of gain in strength can be enhanced byusing rapid hardening types of cements, and increasingthe cement content to reduce the period before use bytraffic. Figure 3.1 illustrates this and the use of an early

February 2004

strength cement in preference to larger quantities ofOrdinary Portland Cement (OPC) will depend on theurgency and economics.

Figure 3.1 Increasing Cement Content toReduce Time before Traffic

3.7 Further increase in strength can be achieved byaccelerating curing methods.

Requirements

3.8 For designed mixes the concrete shall have toachieve the required 7 or 28 day strength forcompliance with the Specification (MCHW1) 1000Series.

3.9 Before use by any traffic, the following are thecharacteristic compressive strengths which eachpavement layer shall be expected to reach, subject tothe proviso given in paragraph 3.10.

Early strengthcement

OPC

Time toopeningroad totraffic

QUANTITY OF CEMENT INCREASING

6 hrs

12 hrs

18 hrs

24hrs

30hrs

3 days

3/1



Pavement quality concrete surface slabs 25N/mm2

Pavement quality concrete roadbases 20N/mm2

Roller compacted concrete, cement bound material orwet-lean concrete of the following grades:

CBM 4 or C20 concrete 15N/mm2



CBM 1 or C 7 concrete 4.5N/mm2

3.10 In the case of roller compacted concrete or CBMthe next pavement layer may be laid within 4 hours ofplacing the cement bound layer. Once the next layer islaid the pavement shall not be trafficked until thecement bound layer is expected to reach the requiredstrength given above.

3.11 Mortars for bedding iron work such as manholecover frames during repairs may be trafficked when thestrength is expected to be 20 N/mm2. For rapidconstruction, this strength should be achieved within 2hours.

MATERIALSCements

3.12 High early strength cements are now permitted inthe Specification (MCHW1) 1000 Series and should beused for rapid construction and repairs. Concretes withstandard rapid hardening Portland cements conformingto BS 12 (1991) have achieved 25 N/mm2 strength inless than 18 hours, with cement contents of 400 kg/m3

or more.

3.13 Examples of special cements or concrete mixeswhich can give high early strength include:

a) A calcium sulpho-aluminate based cement usedextensively in the mining industry (40N/mm2 in3 hours).

b) A high strength proprietary cement and fibrereinforced, non-shrink concrete (25N/mm2 in3 hours).

c) A mixture of early strength Portland Cement,microsilica and superplasticiser.

d) Proprietary polymer modified concrete mixes.

3/2

Pozzolanas

3.14 When Pulverized Fuel Ash (PFA) is added to themix it improves the workability and allows a lowerwater-cement ratio, which in turn increases the concretestrength. It can be as additional fine material or as apartial Portland cement replacement. With the latter,early strength will be reduced as the pozzolanic reactionis secondary but leads to higher ultimate strength anddenser concrete.

3.15 Microsilica can also be used to increase earlystrength and density, but it reduces the workability, soplasticisers or superplasticisers are necessary to keep alow water-cement ratio.

3.16 These materials tend to make cohesive mixeswhich require vibration for compaction and finishing.

Admixtures

3.17 As many of the high early strength cements havequick setting properties, the workability needs to bemaintained during transit to site. Plasticisers andsuperplasticisers should be used for this as well asreducing the water-cement ratio. Air entraining agentsshould be used for slab replacement wherever possiblebut need not be used for small batches of high strengthconcrete for thin bonded patches.

Aggregates

3.18 For high early strength concrete, angularaggregates from crushed rock or crushed gravel arepreferable. The addition of more fines in the form ofPFA and microsilica will produce a denser concretewith few voids.

3.19 As some cements may have high alkali contents,the requirements for prevention of Alkali SilicaReaction (ASR) will still apply, so non-reactiveaggregates may be needed.

MAIN CONSTRUCTION

3.20 Normal paving operations as specified in theSpecification (MCHW1) 1000 Series will apply.Concrete mixes should usually be designed mixes andtested for compliance with the Specification at theappropriate age, unless the use of prescribed mixes isapproved by the Engineer.

February 2004



3.21 Slip form pavers with their higher outputs havean advantage over fixed form plant for rapidconstruction, and setting out guide wire posts is quickerthan laying forms more common with the necessarybedding.

3.22 Auger pavers or truss type vibrating finishingbeams provide a good finish and are more versatile forsmall areas and are recommended for use with internalvibration using poker vibrators for compaction of thickslabs, as in the Specification (MCHW1) 1000 Series.

3.23 Where there is restricted access, eg. in single lanewidening, conveyors can help to spread the concretequickly.

3.24 Transverse joints other than in single bayreplacement, should be sawn but may be wet-formed inwinter. Cork seals can be used as formers and so avoidthe need for a separate sealing operation, for individualslabs or short lengths.

3.25 Higher slab temperatures may result from theheat of hydration of higher cement contents and causemore thermal stress on cool nights. It may beadvantageous to saw some joints early within 2 hours ofplacing the concrete, using a light saw which cuts agroove through a bearing plate which holds the concretein place and prevents coarse aggregate being pluckedout, instead of waiting about 4 hours or more beforebeing able to use a conventional saw. This may relieveearly stress and avoid random cracking before the jointscan be properly sawn. Any such joint grooves should bedeepened and widened for sealing using a conventionalsaw as required in the Specification (MCHW1) 1000Series if the joint is not already cracked.

3.26 Dowels can be inserted satisfactorily with slipform pavers, without creating surface irregularities andpoor riding quality, if the insertion mechanism does notstop the paver nor have a vertical reaction on anyconforming plate. Alternatively, dowels may be placedon cages. When dowels are inserted from the surface orif joints are to be wet-formed, additional compactionand regulation is necessary, by a longitudinal oscillatingand vibrating finisher.

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February 2004

uring

.27 The concrete should be cured, as required in thepecification (MCHW1) 1000 Series using resinompound or bituminous spray for the relevantavement layer.

.28 In addition thermal insulation blankets aspecified in Specification (MCHW1) 700 Series shoulde used to accelerate the strength development to meethe specified strength in the required time. Insulationlankets will be necessary if the road is to be traffickedn any period between 5 hours and 3 days after theoncrete is placed. They may also help the strengthevelopment of the special cements which can berafficked in under 5 hours.

.29 The purposes of the insulation blankets are to:

) Assist in the prevention of loss of moisture fromthe concrete so increasing the rate of hydrationand reducing shrinkage.

) Promote a uniform temperature gradient throughthe slab so reducing the risk of thermal cracking.

) Induce a higher temperature within the slab byretention of the heat of hydration and acceleratethe chemical reaction, leading to strengthdevelopment.

ONCRETE PAVEMENT REPAIRS

.30 Concrete for repairs may be designed mixes orrescribed mixes. Before prescribed mixes can be used,vidence of the strength development of such a mixhould be obtained. Proprietary prepacked mixes comeithin the term of prescribed mixes. All such mixes

hould be able to develop 40N/mm2 at 28 days and5N/mm2 at the time required for use by traffic.

.31 Once a prescribed mix has been approved, theroportions and ingredients should not be changed.uality will depend on correct proportioning.

3.32 Non-shrink concretes are preferred for thinbonded patch repairs. Some high strength concretesused for patching can be affected by high temperaturessoon after placing. The patches can be subjected towarping and tend to de-bond around the perimeter.Under traffic these can then crack.

3/3



3.33 Effective curing will reduce the risk oftemperature stress and warping. The traditional methodof curing patches is to spray with resin curingcompound and cover the patches with damp hessian andan impermeable sheet to retain moisture in the patch.Large variations of temperature in the patch can beavoided by using insulation blankets.

Sealing Joints

3.34 Most hot and cold poured sealants need to beplaced into dry mature concrete. They need to be placedinto grooves which have been grit blasted to ensure agood bond. Grit blasting may be carried out as soon asthe concrete is expected to reach 15N/mm2 and thejoints can be sealed as soon as possible after thatinstead of waiting 14 days as required in the BritishStandard. Otherwise the joints should be sealed inaccordance with the Specification (MCHW1) 1000Series.

3.35 As most joints will have to be sawn, the sealinggroove would be suitable for compression seals or pre-compressed neoprene impregnated expanding foamseal, or cork strips, which can be placed as soon as thegroove has been cut which would be earlier than sealingwith applied sealants. Pre-compressed neopreneimpregnated expanding foam sealant strips shouldcomply with The British Board of Agrément certificate88/2123C.

STRENGTH ASSESSMENT

3.36 Assessment of strength for trafficking willdepend on whether the mix is a designed mix or aprescribed mix. For designed mixes, cubes should betaken and tested at early ages commensurate with thetime required for opening to traffic, which should beallowed as soon as the concrete reaches 25N/mm2.

3.37 Cubes made, stored and tested in accordance withBS 1881 (1983) will not necessarily reflect the realstrength in the pavement. Without accelerated curing,the strength in the pavement will vary with temperatureand climatic conditions and is usually 5% less than thestrength of the cubes stored under laboratoryconditions. Temperature matched curing (TMC) ofspecimens for testing would give more realistic resultsif time allows.

3/4

3.38 In the absence of TMC, approximate values ofthe pavement strength can be obtained from cubes madeand stored alongside the pavement, insulated in such away as to simulate the conditions in the pavement.

3.39 Strength development curves for a particular mixcan be obtained by testing cubes at different ages ontrial mixes before work starts. If such curves are used toassess the time for opening to traffic or for gaugingwhen to test cubes on site, ambient temperatures at timeof placing should be compared with the temperature atwhich the strength development curve was obtained.For any significant drop in temperature (more than 5oC)additional time should be allowed, as strengthdevelopment slows in cooler weather. Insulationblankets will help to maintain an even temperature.

February 2004


February 2004


References

1983

BS 1881; Part 108; “Making Cubes”, BSI.BS 1881; Part 111; “Storing Cubes”, BSI.BS 1881; Part 116; “Testing Cubes”, BSI.Note: BS 1881 now replaced by BS EN 12390

1991

BS 12; “Specification for Portland Cements”, BSI.Now replaced with BS EN 197-1.

Design Manual for Roads and Bridges

HD26 (DMRB 7.2.3) Pavement Design.HD28 (DMRB 7.3.1) Skidding Resistance.HD29 (DMRB 7.3.2) Structural Assessment Methods.HD30 (DMRB 7.3.3) Structural Assessment Procedure.HD32 (DMRB 7.4.2) Maintenance of Concrete Roads.HD36 (DMRB 7.5.1) Surfacing materials for new andmaintenance construction.HD37 (DMRB 7.5.2) Bituminous surfacing materialsand techniques.HD38 (DMRB 7.5.3) Concrete surfacing and materials.HA39 (DMRB 4.2.1) Edge of pavement details.HA43 (DMRB 4.1.7) Geotechnical considerations andtechniques for widening highway earthworks.HA83 (DMRB 4.2.4) Safety aspects of road edgedrainage features.HA104 (DMRB 4.2.5) Chamber tops and gully tops forroad drainage and services: Installation andmaintenance.

Manual of Contract Documents for Highway Works


Notes for Guidance on the Specification for HighwayWorks (MCHW2).


4/1


February 2004 5/1

Chapter 5Enquiries

5. ENQUIRIES




Chief Highway EngineerTransport DirectorateWelsh Assembly GovernmentLlywodraeth Cynulliad CymruCrown Buildings J R REESCardiff Chief Highway EngineerCF10 3NQ Transport Directorate


May 2001




PART 5

HD 39/01

FOOTWAY DESIGN

SUMMARY

This part sets out the requirements and advice for newfootway construction. It covers footways constructedfrom common materials that are subject to a range ofpedestrian traffic and some overrun by vehicular traffic.


This is a new document to be incorporated into theManual.




HD 39/01

Footway Design

Summary: This part sets out the requirements and advice for new footway construction.It covers footways constructed from common materials that are subject to arange of pedestrian traffic and some overrun by vehicular traffic.


* A Government Department in Northern Ireland

THE HIGHWAYS AGENCY

SCOTTISH EXECUTIVE DEVELOPMENT DEPARTMENT

THE NATIONAL ASSEMBLY FOR WALESCYNULLIAD CENEDLAETHOL CYMRU

THE DEPARTMENT FOR REGIONAL DEVELOPMENT*


May 2001







PART 5

HD 39/01

FOOTWAY DESIGN

Contents

Chapter

1. Introduction

2. Design Considerations

3. Structural Design

4. Materials


6. Enquiries

Annex A Specification for 45% Type F Asphalt

B Specification of Bedding Sand andJointing Material for Small ElementPaving

C Compaction Specification

D Compaction by Method Specification

E Worked Examples


May 2001


May 2001 1/1


1. INTRODUCTION

General

1.1 This part provides guidance on new footwayconstruction. It covers footways constructed fromcommon materials and subject to a range of pedestriantraffic and some overrun by vehicular traffic. Thedesign of cycleways is subject to further research and isnot covered. Neither is the design of pedestrianisedareas since the number of delivery vehicles usuallymeans that a road pavement design will be required.

1.2 Guidance is provided on the construction offootways surfaced with bituminous material, concreteor clay pavers, precast concrete flags and in-situconcrete. Designs for paver and flag footways, in thesituation where there is overrun by heavy vehicles,remain to be validated. Recommendations on themaintenance of existing footways, is given in HD 40(DMRB 7.4.3).

Implementation

1.3 This Part shall be used forthwith on allschemes for the construction, improvement andmaintenance of trunk roads currently beingprepared, provided that, in the opinion of theOverseeing Organisation this would not resultin significant additional expense or delay.Design organisations should confirm itsapplication to particular schemes with theOverseeing Organisation.

Mutual Recognition

1.4 Where Parts of Volume 7 give the OverseeingOrganisation’s requirements for products, they makeprovision for the acceptance of equivalent productsfrom other member states of the European Community.Reference should be made to the statement in each Partconcerned.


Chapter 2Design Considerations

NS
2. DESIGN CONSIDERATIO
Introduction

2.1 Maintaining Agents and Local Authorities spenda significant amount of their highway maintenancebudgets on footways in order that all pedestrians,including those with mobility difficulties, can travel onthe footway in comfort. The footway surfacedeteriorates for a variety of reasons and it is importantthat the initial construction is such that subsequentdeterioration is minimised. Although it is expected thatthe upper layers will need attention because of generalwear, it is recommended that the foundations offootways should be sufficiently robust to give goodperformance over a design life of 40 years.

2.2 Research has been carried out at the TransportResearch Laboratory (TRL) to identify the causes offailure in footways and thus to recommend suitabledesigns to improve the surface condition of footwaysover their design life. Vehicle overrun and works byStatutory Undertakers have been identified as the mostcommon causes of failure in footways. It is hoped thatadherence to the HAUC Code of Practice Specification(DoT et al, 1992), or the NIRAUC Specification for theReinstatement of Openings in Roads (1995) in NorthernIreland, will ensure improvement in reinstatements afterutility works and consequently in the ensuing conditionof the footway surface. Growth of vegetation, naturalageing of bituminous material, and poor design andconstruction have also been identified as significantcauses of deterioration.

Selection of Footway Category

2.3 To choose the appropriate footway design it isnecessary to consider the pedestrian and vehiculartraffic which the footway may have to support and thecharacteristics of the ground on which the footway is tobe constructed. Designs are given for three constructioncategories, the appropriate category being chosenaccording to the risk and type of vehicle overrun and onthe amount of pedestrian usage.

The category required is selected by following theflowchart in Figure 2.1.

May 2001

Pedestrian-only : Footways designed for pedestrian use only.Light-vehicle : Footways which will support overrun by light vehiclesHeavy-vehicle : Footways which will support overrun by heavy

vehicles.

Figure 2.1 : Flowchart for Selection ofFootway Category

Notes on Figure 2.1

1. The footway is considered to be physically separated fromthe carriageway if there is a verge of width 3m or more,closely spaced trees or other physical obstructions suchthat vehicular traffic cannot mount the footway.

2. Any footway in a residential area is likely to be used forparking private cars. However, if the footway is in a ruralarea it may be sensible to adopt the pedestrian-onlydesign, even if vehicle overrun is not physicallyprevented. If there is little pedestrian traffic the risk ofany damage by overrun causing inconvenience topedestrians is small.

Start

Is footwayphysically

separated fromcarriageway

?

Is footwayin rural area

?

Is heavyvehicleoverrun

likely?

Pedestrian-onlycategory

Light-vehiclecategory

Heavy-vehiclecategory

Yes

Yes

Yes

No

No

No

2/1



3. There are many situations where light vehicle overrun iscommon, but overrun by heavy vehicles would not beexpected to occur more than very occasionally. This mayapply to domestic crossings (access to private driveways);situations where cars may park between obstructions thatwould prevent heavy vehicles parking; and footwaysadjacent to roads on housing estates. Some heavy vehicleoverrun is to be expected when footways are adjacent toroads in areas where deliveries take place, such as outsidelocal shops. Obstructions, such as broken down vehicles,will cause traffic to overrun the footway occasionally.

4. ‘Pedestrian only’ footways are not designed to supportany type of vehicle use, not even small cleaning andmaintenance vehicles, except those that are pedestriancontrolled.

Site Investigation

2.4 To perform satisfactorily, a footway must beconstructed on an adequate foundation. A soft subgradeprovides insufficient support for compaction of thelayers above, which may subsequently deterioraterapidly. For road pavement construction the subgrade isconventionally assessed in terms of its CaliforniaBearing Ratio (CBR) and as footways are associatedwith road pavements it is convenient to use the samemeasure.

2.5 Methods for measuring CBR are described inHD 25 (DMRB 7.2.2.4). If, because of siteinvestigations prior to constructing structures, theModulus of Subgrade Reaction (k) of the soil is known,this can be related to the CBR by the relationship givenin HD 25 (DMRB 7.2.2).

2.6 Tests should not be carried out on the soil nearthe surface as the moisture content will not berepresentative of the equilibrium condition at depth. Itis very important to remember that Design CBRs relateto equilibrium conditions. A prolonged dry spell maydistort the results and lead to failure in wetterconditions. If a cone penetrometer is used to assessCBR, care must be taken in case services are present;the use of a Cable Avoidance Tool (CAT) isrecommended.

2.7 The CBR chosen for design purposes should bethe minimum measured value, not the average,otherwise local failure will occur at soft spots.Alternatively, soft spots can be removed and replacedwith better material to improve the subgrade CBRvalue. If it is not possible to estimate the CBR becausethe condition of the subgrade is extremely variable then

2/2

a default of 2 per cent should be assumed, unless thematerial is granular in which case the CBR can beassumed to be over 5 per cent.

2.8 For a footway subject to vehicle loading theestimate of CBR should normally relate to the moisturecontent which is expected to be present in the subgradeunder the completed footway, when any change in thewater table due to construction and the installation ofdrainage has taken place. However, if the in situ CBR atthe time of measurement is less than the expectedequilibrium CBR, then the in situ value should be usedfor design, otherwise failure may occur beforeequilibrium is reached.

2.9 Tables 2.1 and 2.2 provide guidance onestimating Design CBRs if testing is notpossible. They are more conservative than theinformation presented for road pavements inHD 25 (DMRB 7.2.2.2), and should only be usedin conjunction with the designs presented in thisPart.

Soil Type Plasticity DesignIndex CBR%

Plastic Clay 50 or greater 2+

Silty Clay 40 2

Silty Clay 30 3

Sandy Clay 20 3

Sandy Clay 10 2+

Silt - Less than 2

Sand (poorly graded) - 7*

Sand (well graded) - 10*

Sandy Gravel (well graded) - 15*

Notes: + CBR may be less than 2 if constructionconditions are poor.

* Indicates estimated values assuming someprobability of the material saturating in service.

Table 2.1 Equilibrium CBR Values

May 2001



Soil Condition CBR

Very soft, exudes between fingers when Less than 1%squeezed

Can be moulded by light finger Between 1 and 2%pressure

Can be moulded by strong finger Between 2 and 3%pressure

Can be indented by a thumbnail but More than 6%not by a thumb

Table 2.2 Rough Guide to CBR

Geometry

2.10 When assigning geometric parameters tofootways the comfort of the user is taken into account,together with the necessity for providing adequatesurface drainage. Steep gradients or crossfalls make itdifficult for elderly or encumbered pedestrians to walkon the footway, while insufficient gradients would notfacilitate the removal of surface water. Where possiblethe footway width should be sufficient to allow twowheelchairs or double buggies to pass. The basicgeometrical parameters are set out in Table 2.3.

Parameter Recommended ExtremeLimits Limits

Longitudinal gradient 1.25% to 5% 8% maximum*(normally the same asadjacent highway)

Width 2m minimum 1.3m minimum

Crossfall 2% to 3.3% 1.5% minimumto 7% maximum

at crossings

Note: *In some cases it may be necessary to construct afootway with a gradient of more than 8 per cent.Provision of a handrail is recommended if siteconstraints necessitate a gradient steeper than 10per cent.

Table 2.3 Geometric Parameters for Footways

2.11 Crossfall should be limited to that absolutelynecessary to dispose of surface water. Crossfalls steeperthan about 3 per cent are uncomfortable to walk on andif the slope runs towards a road it can be dangerous, aswheeled users will tend to edge down the crossfall.

May 2001

2.12 The direction of the crossfall should take surfacewater away from buildings. However, if backfalltowards buildings is unavoidable then covered drainagechannels can be used to remove the surface water.

Drainage

2.13 The strength of the construction and of thesubgrade can vary considerably with moisture content,so it is extremely important to keep the structure welldrained during its service life. Issues of drainage relateto ensuring adequate longitudinal falls, grips/gullies andlevel tolerances to prevent surface water ponding and toaid its disposal. The drainage system should bedesigned to last the life of the footway and it should beeasy to maintain. Drainage should be kept away fromthe centre of the footway because of the likelihood ofworks by Statutory Undertakers. Any potentialproblems which may be caused by tree and hedge rootsshould be considered.

2.14 Generally footway drainage will be the same asthat of the adjacent highway, which should ensure a lowwater table and efficient disposal of surface water.Where a footway is separated from the highway themain consideration should be to ensure that surfacewater drains away from the footway into the highway. Ifa separate drainage system is required it should besimple and robust; it should keep the water table belowformation level and deal satisfactorily with storm water.Regular maintenance of the drainage system will benecessary for long-term performance. Drainage isespecially important if there is any risk of vehicleoverrun, as a sub-base can lose a considerableproportion of its load spreading properties if it becomeswaterlogged.

Statutory Undertakers

2.15 Where possible, footways should be designed sothat services can run in the verge rather than under thefootway. If this is not possible the provision of serviceducts minimises any disruption during maintenancework. On new footways all Statutory Undertakers’equipment must be placed before the footway isformed. (Recommendations on positioning new servicesare contained in NJUG Publication Number 7, 1997).Any trenches excavated before construction of thefootway should be backfilled with suitable material andfully compacted. Compaction of the backfill to anytrenches should be carried out according to AppendixA8 in the HAUC Code of Practice Specification (DoTet al, 1992), or the NIRAUC Specification for theReinstatement of Openings in Roads (1995) for

2/3



Northern Ireland, to ensure that the reinstatement willhave a bearing capacity at least as good as the naturalsubgrade.

Environmental Considerations

2.16 The position of the footway and the choice ofsurfacing will be affected by environmental factors. Thefootway should complement the surroundingenvironment and natural features should be disturbed aslittle as possible. The roots of large trees can causeproblems and ideally footways should not be positionedclose to such trees. If footways must be built near toexisting trees then material around the existing rootsshould be excavated carefully by hand, any sub-base orother granular material should be placed by hand andnon-toxic sand should be placed around the roots. Ifnecessary the vertical alignment of the footway can belifted to facilitate this.

2.17 In a new development, trees should be carefullychosen to have deep rather than spreading roots andsufficient space must be provided for root growth.Planting verges with low shrubs discourages parkingand removes the need for grass cutting, whilemaintaining sight distances. For ease of maintenancetrees can also be underplanted with such shrubs.

2.18 The footway surface should be even and haveadequate slip resistance and abrasion resistance. Thetype of surfacing chosen will depend on the requiredappearance, the usage of the footway, the availablebudget and the loading to which the footway issubjected. The recommended and most commonly usedtypes of surfacing are bituminous materials (bitumenmacadam or hot rolled asphalt), concrete or clay paversand precast concrete (PCC) flags. It is notrecommended that in situ concrete is used due to thedifficulties caused when works by StatutoryUndertakers are carried out, although concrete may beused locally for corner reinforcement against overrunand for vehicle crossovers. Natural stone flags, cobblesand setts are not commonly used for new footways asthey tend to be both expensive and difficult to lay,though they are used in conservation areas.

2.19 If the pedestrian usage is very high, masticasphalt can provide a very tough surface and it alsoprovides a useful thin waterproof surface over cellars.In some areas, tactile surfacing may be required toassist those pedestrians who have impaired sight. (Referto DETR publication “Guidance on the use of TactilePaving Surfaces”, also available on the internet:www.mobility_unit.detr.gov.uk/guide/ tactile/index.htm.

Iawbu

2Aa

2/4

n Northern Ireland refer to “Roads Service’s Policynd Procedures Guide, RSPPG_E010"). Pavers, evenhen well laid, may not provide ride quality as good asituminous or flag surfaces, which is a disadvantage forsers of small wheeled buggies and the like.

.20 A series of worked examples are provided innnex E of this Part to demonstrate the design methods

nd the use of the Tables.

May 2001


Chapter 3Structural Design

3. STRUCTURAL DESIGN

Introduction

3.1 For most soils, other than silt, the CBR of thesubgrade at the time of construction will be at least2 per cent, which should provide an adequate surfacefor compaction of the sub-base. If the CBR of thesubgrade is lower than this, the soil can be stabilised,usually with lime (which should achieve a CBR of atleast 5%), or some soil can be removed and replacedwith extra granular sub-base/capping material.

3.2 Alternatively, if construction is being carried outin poor conditions, on a clay soil which is expected tohave an equilibrium CBR of at least 2 per cent, theremay be a case for using a geosynthetic separating layeras a construction expedient. The geosynthetic does notimprove the CBR, but prevents soft groundcontaminating the sub-base, which would weaken thestructure and lead to inadequate support for compactionof the upper layers. Construction layers above the sub-base would consequently not be properly compactedand would deteriorate faster than would otherwise bethe case, with subsequent increase in maintenancecosts. The subgrade should be levelled and compactedbefore placing the geosynthetic or the sub-base.

3.3 If the sub-base is required to supportconstruction traffic, then the sub-base thickness willneed to be designed accordingly, even if the footway isa pedestrian-only design.

3.4 For prevention of frost damage all materialwithin 450mm of the surface should be non frost-susceptible, unless the mean annual frost index is lessthan 50, in which case the requirement can be reducedto 350mm. Advice on the frost index for any particulararea can be obtained from the Meteorological AdvisoryServices. The frost index is defined as the product ofthe number of days of continuous freezing and theaverage amount of frost, in degrees Celsius, on thosedays. If the subgrade is frost susceptible then it shouldbe protected by a blanket of suitable non-frostsusceptible materials.

3.5 Soils which are most likely to be frostsusceptible are low plasticity clays, silts, and clayey andsilty sands and gravels. Medium and high plasticityclays are normally insufficiently permeable to besusceptible to frost heave, and clean sands and gravels

icwduwbtosc

P

33CSm1fufsd

3athdHqMths

May 2001

cannot generate sufficient suction to draw water to the

e front. Well drained soils where the water table isell below the construction are less likely to beamaged by frost in any situation, because footways aresually alongside road pavements and the road drainageould tend to ensure a low water table. It is unlikely toe cost effective to build the footway deep enough totally prevent frost damage. Modular footways are less

usceptible to frost damage as they can move withoutracking.

edestrian-only Design

.6 Construction thicknesses are as shown in Table

.1, and construction materials are discussed further inhapter 5 of this Part. The sub-base is a Type 1, seepecification Series 800 (MCHW 1) or equivalentaterial with a minimum compacted thickness of

00mm to ensure a smooth and regular laying surfaceor the upper footway layers. The wearing course issually bituminous, concrete or clay pavers or PCClags. If pavers are used the sub-base surface must beufficiently dense to prevent bedding sand leakingown into it.

.7 Using a single 60mm thick combined basecoursend wearing course avoids the problems associated withe compaction of very thin layers. The material cools

own more slowly, allowing more time for compaction.owever, it can be more difficult to achieve a good rideuality when a separate regulating layer is omitted.ore care needs to be taken with the surface finish ofe sub-base if bituminous material is to be placed in a

ingle layer.

3/1



SurfaceOptions

Bituminous Pavers Flags

Layer

≥50mm clay ≥60mm concrete ≥50mm20mm wearing course pavers blocks

Surfacing(2) 40mm basecourse(1)

30mm bedding sand (compacted) 25mm beddingsand (compacted)or mortar

Sub-base(2) 100mm

Subgrade(3) -

Table 3.1 Pedestrian-only Footways

SurfaceOptions

Bituminous Pavers Flags Concrete Layer

300mm x 300mm x 60mm20mm wearing ≥50mm ≥60mm concrete blocks or 400mm x 400mm x 65mm 150mm

Surfacing(2) course clay pavers or 450mm x 450mm x 70mm unreinforced,40mm basecourse (1) Grade C30P

30mm bedding sand (compacted) 25mm bedding sand (compacted) ormortar

Sub-base(2) 150mm 250mm 200mm 150mm 250mm 200mm 150mm 75mm

Subgrade(3) - CBR 2% ≤ CBR CBR CBR 2% ≤ CBR CBR -≤2% ≤5% >5% ≤2% ≤5% >5%

Table 3.2 Light-vehicle Footways

Notes for Tables 3.1 and 3.2:

1. 60mm combined wearing/basecourse is an alternative, but may require a slurry surfacing if a close textured surfacecannot be achieved.

2. Refer to Chapter 5 for material options.

3. It may be necessary to stabilise subgrade or replace with granular capping, if CBR <2%.

May 20013/2



3.8 Slurry surfacing can be used to provide a finetextured surface if it is considered that the bituminousmaterial used in a single 60mm layer would give asurface that is too open textured. However, this thinsurfacing has a short life span and will probably needreplacing every 4 years.

Light-vehicle Design

3.9 This design is used for cross-overs to privatedriveways and wherever light vehicle overrun is likely.Construction thicknesses are shown in Table 3.2. Thedesigns for segmental footways are thicker than theequivalent designs for bituminous footways sinceexperimental work has shown that segmentalconstructions do not exhibit equivalent load spreadingability to bituminous constructions. 50mm concretepavers are now available and there is no reason toassume that their performance will be inferior to 50mmclay pavers. However, 50mm concrete pavers are notincluded in current British Standards and so cannot berecommended at present.

3.10 In situ concrete can be used for vehiclecrossovers. Kent County Council (1988) recommendusing an unreinforced concrete, class C30P, laid 150mmthick over 75mm of sub-base, as illustrated in Table 3.2.

3.11 If there is a possibility of very occasional heavyvehicle overrun, such as might occur two or three timesa year with occasional delivery vehicles to privatehouses, and if the footway is being constructed on aweak subgrade, it is recommended that theconstructions are strengthened to those illustrated inTable 3.3. Under bituminous surfacing, if the subgradeCBR is less than 2 per cent, the sub-base thicknessshould be increased to 225mm. If the wearing course isflags or pavers, a 70mm Dense Bitumen Macadam(DBM) or Cement Bound Material (CBM) layer(CBM1 or stronger) should be incorporated between thesub-base and the bedding sand where the subgrade CBRis 5 per cent or less. Where the subgrade CBR is above5 per cent a sub-base thickness of 200mm should besufficient to permit very occasional heavy vehicleoverrun. No strengthening is necessary if the wearingcourse is of concrete construction.

Heavy-vehicle Design

3.12 If there is uncertainty about the type of overrun,or if the footway is adjacent to a busy road and overrunis not prevented by some physical means, then thefootway should be designed to sustain heavy vehicleoverrun.

May 2001

This does not include pedestrian areas that generallysee a significant amount of delivery or maintenancevehicles. For such areas a road pavement design, asgiven in HD 26 (DMRB 7.2.3.2) is more appropriate.

3.13 For this category of footway the design traffic isassumed to be 50,000 standard axles. This allows forapproximately one vehicle per working day over adesign life of 40 years, equal to the design life ofassociated road pavements (assuming that one heavyvehicle is, on average, equivalent to one standard axle).The number of standard axles has been multiplied by 3to take channelisation into account and some allowancehas been made for dynamic loading due to the vehiclemounting the footway.

3.14 Recommended design thicknesses are given inTable 3.4, and are based on the performance of lightlytrafficked roads (Road Note 29, Road ResearchLaboratory, 1970), with a minimum sub-base thicknessof 150mm. This does not allow for the sub-base to beused as a platform for construction traffic. If the sub-base is to be trafficked the thickness must be increasedto the values in HD 25 (DMRB 7.2.2.2). It is advisablethat all footways subject to possible trafficking byheavy vehicles include a bound roadbase.

3/3



Surface Options

Bituminous Pavers or Flags Concrete Layer

Surfacing(2) 20mm wearing course As Table 3.2 As Table 3.2 50mm basecourse(1)

Base(2) - 70mm DBM (or stiffer)orCBM1 (or stronger) -

Sub-base(2) 225mm 150mm 200mmSubgrade(3) CBR ≤2% CBR ≤5% CBR >5%

Table 3.3 Light-vehicle Footways with Very Occasional Vehicle Overrun

SurfaceOptions

Bituminous Pavers or Flags Layer

Surfacing(2) 25mm wearing course As Table 3.2

Base(2) 90mm DBM (or stiffer) 90mm DBM 100mm CBM1(or stiffer) (or stronger)

Sub-base(2) 365mm 270mm 210mm 165mm 150mm

Subgrade(3) CBR ≤2% CBR ≤3% CBR ≤4% CBR ≤5% CBR >5%

Note : Foundation requirements apply to all surface/base combinations

Table 3.4. Heavy-vehicle Footways

Notes for Tables 3.3 and 3.4 :

1. 70mm combined wearing/basecourse is an alternative, but may require a slurry surfacing if a close textured surfacecannot be achieved.

2. Refer to Chapter 4 for material options.3. It may be necessary to stabilise subgrade or replace with granular capping, if CBR <2%.

May 20013/4



Edge Restraints

3.15 A requirement of all footway constructions isedge restraint, which is provided at the front by a kerband at the back, unless the footway abuts a wall orbuilding, by an edging (Figure 3.1). Good edge restraintis essential to prevent the footway spreading causingwide gaps in a segmental footway or longitudinalcracks in a bituminous footway which might allowwater ingress. Wherever footways do not abut a kerb orexisting boundary wall, precast concrete edging, laid onand backed with concrete to grade C7.5P, should beused. The sub-base should be extended beyond andbeneath the edging. There should be a minimum of100mm of sub-base under the concrete bed of the edgerestraint. Common practice is to extend the carriagewayformation and overlying sub-base under the kerb bed,which would normally give more than the minimumrequirement of sub-base.

��

��

2.0m

Wearing CourseBasecourse

Type 1 Sub-base

150mmConcrete Bedand Haunch

100mmConcrete Bedand Haunch

Type 1 Sub-base

125mm x 255mmKerb

50mm x 150mmEdging

May 2001 3/5

Figure 3.1 : Typical Pedestrian-only Footway Cross-section


Chapter 4Materials

4. MATERIALS

Introduction

4.1 Table 4.1 lists typical materials for use infootways, together with a reference for the materialspecification and for any sources of guidance.Background information on proprietary products suchas slurry surfacing is contained within HD 37 (DMRB7.5.2).

Materials Specification Guidance

Sub-base, Type 1 or CBM MCHW Vol 1, Series MCHW Vol 2800 or 1000

50/14 hot rolled asphalt basecourse; 0/3F or 15/10F hot BS594: Part 1 BS594: Part 2rolled asphalt wearing course

45/6F or 45/10F hot rolled asphalt wearing course Annex A of this Part

28mm dense bitumen macadam roadbase BS 4987: Part 1 BS4987: Part 2

20mm dense bitumen macadam basecourse;

14mm close graded bitumen macadam wearing course;

6mm dense bitumen macadam wearing course;

6mm medium graded bitumen macadam wearing course;

3mm fine graded bitumen macadam wearing course

Laying course sand; BS 7533: Part 3 (Note 1) BS 7533: Parts 1–3

Jointing sand

Clay pavers BS 6677: Part 1 BS 6677: Part 3

Concrete blocks BS 6717: Part 1 BS 6717: Part 3

Flags; Kerbs; Edging BS 7263: Part 1 BS 7263: Part 2

Concrete, C30P BS 5328: Parts 2,3 & 4

Note: MCHW = Manual of Contract Documents for Highway WorksMCHW1 = Volume 1, SpecificationMCHW2 = Volume 2, Notes for Guidance

Table 4.1 Materials

May 2001 4/1


Chapter 4Materials

Sub-base

4.2 Type 1, see Specification Series 800 (MCHW 1)is the most commonly used sub-base material andgenerally originates from a primary aggregate source.For good compaction it should have a moisture contentof +1% to -2% of optimum. If Type 1 is delivered toodry it can be prone to segregation. Although allmaterials should be protected from the weather whenstored on site, providing Type 1 is not close to the finelimit of grading it will not retain water significantlyabove its optimum moisture content and any excess willrapidly drain from stockpiled material.

4.3 Type 1 was designed to be placed using largeplant, in relatively thick layers and rolled using 8-10tonne dead-weight rollers. Footways are constructedusing small plant and are often laid in thin layers(100mm) for which the nominal size of Type 1 is toolarge. Alternative locally available materials may besuitable which fall into the category of secondary orrecycled aggregates; for example:-

• Initial sweepings from 10mm and 14mm surfacedressing

• Bituminous planings

• 20mm and 28mm nominal single sizedaggregates

• Spent railway ballast screened to remove the20mm down material (and thus anycontaminants)

• China clay sand

• Crushed kerbstones

• Slate waste

The single sized nature of some of these sources maymake them difficult to compact, so blending ofaggregates may be beneficial. Bituminous planingsexhibit considerable resistance to compaction due tofriction of the bitumen coated aggregate. They must becompacted at optimum moisture content, to a maximumcompacted layer thickness of 150mm.

4.4 If secondary aggregates are to be used, therequirements for durability set down in theSpecification (MCHW1) for sub-bases should stillapply. For example, materials used in the constructionof the footway must be resistant to frost heave. Somematerials, such as chalks, oolitic limestones, slate

4/2

wastes, incinerator and furnace bottom ashes andcolliery shales, both burnt and unburnt, may be subjectto degradation due to freeze/thaw cycling. If it cannotbe guaranteed that the sub-base will remain welldrained throughout its design life then the materialshould be stabilised. Local experience can be a usefulguide particularly where materials have a long historyof satisfactory performance. Secondary aggregates maybe suitable for use after stabilisation, for example bycement, foamed bitumen, or other binders. If it cannotbe guaranteed that the sub-base will remain welldrained throughout its design life then the materialshould be stabilised.

4.5 A permeable sub-base may be useful undermodular surfacing, which is, to some extent, porous. Itmay therefore be better to provide the drainage at alower level and have a more “free-draining” sub-base.This material is often referred to as Type 1X and agrading is available in TRL Report PA/SCR243, “RoadHaunches: A guide to maintenance practice” (1994).Use of a permeable sub-base will only be possiblewhere there are no services in the footway so that awelded felt type geosynthetic can be used to preventflow of bedding sand into the voids.

Bituminous Materials

4.6 The decision on which bituminous materials touse will depend on appearance, durability, initial cost,maintenance requirements, total thickness, ease oflaying and likelihood of disturbance by StatutoryUndertakers. Where bituminous materials abut kerbs,manholes, small areas of tactile paving flags etc, thevertical faces of these should be cleaned and paintedwith a uniform coating of hot bitumen, before thebituminous material is laid. Advice can be found inBS 4987: Part 2.

4.7 Three wearing course materials that are suitablefor footways are compared and contrasted in Table 4.2,assuming that all materials are fully compacted; thedense materials comply with the specification in AnnexC of this Part, and 200 pen or harder bitumen is used.The good durability of the 45/6 HRA wearing coursecompared to the 6mm medium graded bitumenmacadam, and its comparatively small extra cost, meansthat it is probable that the HRA will have the lowestcost over the life of the footway. However, a limestoneaggregate is specified, so it may not meet skidresistance recommendations.

4.8 Permanent cold-lay surfacing materials (PCSMs)are currently being developed and this will allow hours

May 2001


Chapter 4Materials

rather than minutes for handling and compaction. Earlyresults suggest that these materials are as durable asequivalent hot materials when laid on footways (TRLReport 134) but their long term performance remains tobe established. At the moment, unless NJUGaccreditation has been gained, their use should berestricted to the pedestrian-only category of footways.

4.9 The choice of basecourse may be limitedbecause of the thickness specified in the design.Pedestrian-only footways require 40mm of basecourse,which is below the minimum thickness for basecourse

May 2001

complying with BS 4987. The solution is to either use a14mm close-graded wearing course as a basecourse or a50/14 hot rolled asphalt basecourse, the latter beingmore expensive but more durable. Alternatively, thebasecourse and wearing course can be combined into asingle layer, which could be a 45/10 hot rolled asphaltwearing course, as shown in Annex A of this Part.

4.10 The binder penetration should be specified as100 pen or less where vehicle overrun occurs, otherwise200 pen can be specified.

Material6mm medium graded 6mm dense bitumen 45/6 hot rolledbitumen macadam macadam asphalt

Property

Appearance open textured close textured smooth

Durability poor to fair fairly good good

Workability good fair good

Maintenance highest intermediate lowestneeds

Permeability permeable slightly permeable effectively impermeable

Resistance to fairly poor poor when new, fairly goodstiletto heels otherwise goodand horses

Table 4.2 Comparison of Bituminous Wearing Courses

4.11 The pavers or flags chosen will depend on thefootway category and on aesthetic considerations. Largeflags can be difficult to lay, requiring mechanicalhandling, and will be damaged by any vehicle overrun.Flags for footways other than strictly pedestrian only,should therefore be restricted to those of plandimension 450 x 450 mm or less – types E, F or G toBS 7263: Part 1. Natural stone flags, where used,should conform to the above maximum plan dimensionsif they are to support any vehicular traffic, butconsideration should also be given to durability andabrasion resistance.

4.12 400 x 400 x 65mm flags are compatible with thestandard 200 x 100 x 65mm rectangular concrete blocks

so that the two types of surfacing can be combined toform attractive layouts. However, if these products areto be used together the joint spacings for the flags needto be relaxed to allow wider joints. Tactile flags areavailable in compatible sizes and are of the correctcolour, and should be used at crossings.

4.13 It is possible to include clay pavers in schemesthat also use flags and/or concrete blocks, but thetolerances on sizes of clay are different from and lessonerous than the tolerances on concrete pavers andflags, and therefore laying problems may occur. It ispossible to obtain clay pavers to the tighter tolerancesof concrete, but they are likely to be more expensive.

4/3


Chapter 4Materials

4.14 Difficulties may be caused if advantage is takenof the larger variety of shapes, sizes and colours ofconcrete blocks available. Stocks of each type andcolour would need to be kept for maintenance purposes.It is therefore suggested that blocks are specified asrectangular, with plan dimension 200 x 100mm.Installation should follow the current British Standardand further guidance is available from the appropriatetrade association (Interpave for concrete blocks andBrick Development Association for clay pavers).

Bedding Sand

4.15 The bedding sand and jointing material shouldbe in accordance with Annex B of this Part; Class IIbedding sand being used for footways designed tosupport heavy-vehicle overrun, and Class III elsewhere.The moisture content of the bedding sand should bewithin +/-1% of optimum determined in accordancewith BS 1377: Part 4: 1990, Method 3.3. The quality ofthe bedding sand is critical to long life. The sand mustbe free of deleterious salts or contaminants. Sand fromquaternary geological series and sea dredged sands arepreferred. Sands from crushed rock sources and triassicgeological series in the north-west are not advisedparticularly for Heavy-vehicle Footways. Gradingsshould be checked by wet sieving.

4.16 Jointing material should not be of a type whichcould stain the surface, and should be supplied andinstalled kiln dry and free flowing. The need to guardagainst risk of failure due to removal of sand by suctionsweepers during early life should be recognised. Thereare various proprietary sealing products available toprevent the loss of jointing sand, but they have yet to beproven satisfactory in the long term.

Compaction

4.17 The choice of compaction plant is limited by thesmall scale of the works, and obstructions such as streetfurniture, but the use of appropriate plant is essential,together with an established testing scheme whenrequired. The subgrade must be levelled and compactedadequately if the sub-base, when placed and compacted,is to achieve the required density. The performance ofboth unbound and bound materials dependssubstantially on the degree of compaction achieved.

4.18 Compaction of asphalt materials can be carriedout by a method specification (refer to Annex D of thisPart), but it is important that the work is adequatelysupervised to ensure that the requirements are beingachieved. However, as dense bitumen macadams are

4/4

more difficult to compact it is recommended that theyare compacted to satisfy an end-product specification,in terms of air voids (refer to Annex C of this Part).

4.19 The durability of dense bituminous materials ofall types is heavily dependent on reducing thepermeability of the material to a level which willrestrict weather and oxygen attack to the top surface.The level of compaction is best characterised by airvoid content which is generally recommended to be inthe region of 2-8%. Void contents less than 2% are notrecommended for heavy-vehicle footways (Table 3.4),else deformation may occur under trafficking.

Skidding Resistance

4.20 The footway wearing course must not becomeslippery and difficult for pedestrians to walk on whenwet. It is unlikely that this will be a problem withbituminous or concrete materials, but care should betaken when specifying clay pavers or natural stone. Theskidding resistance of concrete products is alwaysadequate for use in footways, as the BS 6717requirement for acid soluble content precludes the useof limestone for both coarse and fine aggregates. Someclay pavers in areas of heavy pedestrian use can becomeslippery when wet. The resistance to polishing of claypavers is expressed as a polished paver value (PPV). Asa first approximation PPV is equivalent to polishedstone value (PSV) for aggregates. A minimum polishedpaver value (PPV) of 45 should be specified for generaluse. Further guidance on skid resistance is available inCSS publication ENG1-96. “The assessment of slipresistance in paved areas for use by pedestrians andhorse riders”.

Tolerances

4.21 The tolerances given in Table 4.3 are intended toapply generally and also take into account that afootway is usually hand laid. They are more stringentthan those in Table 7/1, Clause 702, of the Specification(MCHW1). This is because the tolerances for roadpavements would allow too great a reduction in layerthicknesses for thin footway construction layers. Ifkerbs and edging strips are properly laid it should bepossible to achieve a high degree of compliance withdesign levels. Surface regularity is given in terms ofmaximum deviation under a 1m straightedge, as the 3mstraightedge used in road pavement measurement is toolarge for footway use.

May 2001


Chapter 4Materials

Testing

4.22 In order to reduce the need for testing incomingmaterials, all materials should be supplied under aquality assurance scheme, preferably a productcertification type, which gives the most assurance of theproduct conforming to the required standards. Noscheme absolves the purchaser from checking thequality of supply. If products are kitemarked, testingshould only be necessary when the visual appearancediffers from normal. The frequency of testing certifiedproducts would normally be between 10 per cent and 20per cent of the frequency for uncertified ones. Wherenone of the above schemes apply then a much morefrequent regime of testing will be required.

4.23 Testing should be carried out in accordance withTable NG 1/1, Notes for Guidance (MCHW2). Wherepossible the testing should be carried out by alaboratory which is NAMAS accredited for theparticular test. Sampling is a very important part of any

May 2001

test and should be undertaken by a laboratoryaccredited for sampling. All sampling and testingshould be carried out in accordance with theappropriate standards. The materials related tests mostrelevant to footway construction are given in Table 4.4.

4.24 The monitoring and testing of workmanship atall stages of construction is very important. Bituminousmaterials are more likely to fail early because of poorcompaction than because the material was delivered outof specification, particularly on footways where it ismore difficult to carry out complete compaction whilethe material is still hot. A guide to the type andfrequency of testing for workmanship is given in Table4.5.

4.25 As the quality of workmanship will depend onthe operatives and their supervisor it is advisable thatthe chosen contractor has a QA scheme which spellsout training for operatives. Each job should have aqualified supervisor and the checks that he will makeshould be listed.

Parameter Tolerances

Horizontal alignment Horizontal alignments shall be correct to within 25mm, except for kerbs, channels andaccuracy edge strips which shall be correct to within ±13mm.

Formation level After completion of any drainage and immediately before laying sub-base thesubgrade surface shall be within + 10mm and –30mm of its design level.

Sub-base level If the footway is surfaced in bituminous material the compacted sub-base surface shallbe within +10mm and –20mm of its design level. If segmental surfacing is used thesub-base must be within ± 10mm of its design level.

Sub-base thickness The thickness shall not be more than 10mm less than specified.

Bituminous basecourse The compacted basecourse level shall be within ± 10mm of the design level.

Wearing course The wearing course level shall be within + 5mm and –0mm of the adjacent kerb,edging strip or any ironwork.

Bituminous thickness The total thickness of bituminous material shall not be more than 5mm less thanspecified.

Bedding sand The compacted bedding sand level shall be within ± 5mm of the design level and notless than 25mm thick.

Kerbs and edging strips The surface level shall be within ±6mm of the design level.

Joints between flags Joints should be not less than 2mm and not more than 5mm wide. For pedestrian-onlyand pavers footways flags can be laid with wide (6-10mm) joints filled with compacted mortar.

Surface regularity The maximum deviation of the footway surface under a 1m straightedge shall notexceed 3mm.

Table 4.3 Tolerances

4/5


Chapter 4Materials

Material Test Frequency of testing

Drainage All drainage pipes will be checked for satisfactory operation.

Geosynthetics Tensile load 1 per 400 square metres.PermeabilityPore size

Type 1 sub-base Grading 1 per 400 tonnesPlasticity Index 1 per 400 tonnes10% fines value 1 per sourceSoundness 1 per source

Bituminous mixtures Grading 1 per sourceBinder content 1 per source

Compacted Percentage refusal As required in BS 4987: Part 2bituminous material density

Precast concrete Transverse strength Minimum of 3 per 1000 units of each product (BS 7263: Part 1)flags

Water absorption No frequency recommended

Concrete block Compressive strength 16 per 5000 blocks (BS 6717: Part 1)paving

Clay pavers Transverse breaking Minimum of 10 per 10000 pavers (BS 6677: Part 3)loadSkid resistance Minimum of 5 per 10000 pavers (BS 6677: Part 3)

Table 4.4 Materials Related Testing

May 20014/6


Chapter 4Materials

Parameter Test Frequency

Foundation CBR Visual, Mexe penetrometer for Visual over whole site, Mexevehicle overrun penetrometer on soft spots or every

50m.

Formation level Levelling or dipping Every 25 linear m and where doubtexists

Sub-base moisture content Field identification according to Each deliveryHAUC Code of PracticeSpecification

Sub-base thickness Dips before and after laying Every 25 linear m(trial holes in case of dispute only)

Sub-base compaction Density. Compare with trial area Average of 3 per 500 linear m or partthereof

Bituminous materials Temperature (°C) On delivery and throughout laying

All compacted bituminous Air voids or layer thickness Average of 3 per 500 linear m ormaterials except medium (from cores or dips) part thereofgraded macadam

Medium graded macadam Layer thickness (from cores or Average of 3 per 500 linear m or partdips) thereof

Ride quality Rolling straightedge Whole length

Surface regularity 1m straightedge and wedge Every 25 linear m and where doubtexists

Table 4.5 Frequency of Testing for Workmanship

May 2001 4/7




References

1. Design Manual for Roads and Bridges

1994

HD 26 (DMRB 7.2.3) Pavement Design

1996

HD 25 (DMRB 7.2.2) Foundations

2001

HD 40 (DMRB 7.4.3) Footway Maintenance

2. Manual of Contract Documents for HighwayWorks (MCHW)



3. Transport Research Laboratory (TRL)

1967

Craney, D. and Jacobs, J.C., “The Frost Susceptibilityof Soils and Road Materials”, LR90.

1970

“A Guide to the Structural Design of Pavements forNew Roads”. Road Note 29, 3rd edition, HMSO,London.

1979

Black, W.P.M. and Lister, N.W. “The Strength of ClayFill Subgrades: its Prediction in Relation to RoadPerformance”, LR889.

1984

Powell, W.D., Potter, J.F., Mayhew, H.C., and Nunn,M.E., “The Structural Design of Bituminous Roads”,LR1132.

1994

“Road Haunches: A Guide to Maintenance Practice”,Contractor Report PA/SCR 243.

May 2001

IOGRAPHY

1995

Burtwell, M(Ed), “A Study of Footway Maintenance”.Report 134. (Research Project Funded by TRL/CSS/HA).

1997

“Footways: Design and Maintenance Guide”,Application Guide 26.

4. British Standards Institution

1986

BS 6677: Part 1: Clay and calcium silicate pavers forflexible pavements: Part 1: Specification for pavers.

1990

BS 7263: Precast concrete flags, kerbs, channels,edgings and quadrants. Part 2: Code of practice forlaying.

1992

BS 882: Specification for Aggregates from naturalsources for concrete.

BS 594: Hot rolled asphalt for roads and other pavedareas. Part 1: Specification for constituent materials andasphalt mixtures. Part 2: Specification for the transport,laying and compaction of rolled asphalt.

BS 7533: Guide for the structural design of pavementsconstructed with clay or concrete block pavers.

1993

BS 6717: Part 1: Precast concrete paving blocks: Part 1:Specification for paving blocks.

BS 4987: Coated macadams for roads and other pavedareas: Part 1: Specification for constituent materials andfor mixtures. Part 2: Specification for transport, layingand compaction.

1994

BS 7263: Precast concrete flags, kerbs, channels,edgings and quadrants. Part 1: Specification.

5/1



1997

BS 7533: Pavements constructed with clay, naturalstone or concrete pavers. Part 3: Code of practice forlaying precast concrete paving blocks and clay paversfor flexible pavements.

5. Others

“Joint Study of Highway Maintenance: A Code ofGood Practice”. Association of County Councils,Associations of District Councils, Association ofMetropolitan Authorities and Convention of ScottishLocal Authorities, London, 1989.

Street Works (Northern Ireland) Order 1995. NorthernIreland Road Authority and Utilities Committee(NIRAUC). “Specification for the Reinstatement ofOpenings in Roads”. The Stationery Office, Belfast.

Department of Transport Traffic Advisory Leaflet 4/90.“Tactile markings for segregated shared use by cyclistsand pedestrians”. HMSO, London. December 1990.

Department of Transport, The Welsh Office and TheScottish Office. “Code of Practice Specification for thereinstatement of openings in highways”. HMSO,London, 1992.

“The assessment of slip resistance in paved areas foruse by pedestrians and horse riders”. ENG/1-96. CountySurveyors Society, Engineering Committee. January1996 (Draft).

“Guidance on the use of Tactile Paving Surfaces”.Department of Environment, Transport and TheRegions, Mobility Unit 1/11, Marsham Street, London.

“Recommended Positioning of Utilities, Apparatus forNew Works on New Developments and in ExistingStreets”. National Joint Utilities Group, NJUG, 1997.

Bibliography

Bull, J.W., and Al Khalid, H., “An Analytical Solutionto the Design of Footway Paving Flags”. Computersand Geotechnics, Vol 4, No. 2, pp85-96, 1987.

Lilley A.A, “A Handbook of Segmental Paving”, Spon,London, 1991.

Kent Design Guide. Kent County Council, 1998.

“Guide for the design of footways”, National Pavingand Kerbing Association, Leicester, 1991.

5/2

“Construction and surfacing of car parking areasincluding private drives”. Quarry Products Association,Asphalt Information Service, 1997.

“Construction and surfacing of parking areas formedium and heavy weight vehicles”. Quarry ProductsAssociation, Asphalt Information Service, 1997.

“Precast Concrete Paving - a design handbook”.Interpave, The Precast Concrete Paving and KerbAssociation, Leicester, 1999.

May 2001


May 2001 6/1

6. ENQUIRIES


Chief Highway EngineerThe Highways AgencySt Christopher HouseSouthwark Street J KERMANLondon SE1 0TE Chief Highway Engineer

Chief Road EngineerScottish Executive Development DepartmentVictoria QuayEdinburgh J HOWISONEH6 6QQ Chief Road Engineer

Chief Highway EngineerThe National Assembly for WalesCynulliad Cenedlaethol CymruCrown BuildingsCathays Park J R REESCardiff CF10 3NQ Chief Highway Engineer

Director of EngineeringDepartment for Regional DevelopmentRoads ServiceClarence Court10-18 Adelaide Street G W ALLISTERBelfast BT2 8GB Director of Engineering

Chapter 6Enquiries


May 2001

ANNEX A SPECIFICATION FOR 45% TYPE FASPHALT

Extract from the County Surveyors’ Society Pavement Design Manual, ENG/6-94 (unpublished).

BS Sieve Percentage by mass of totalaggregate passing BS Test Sieve

45/6F 45/10F

14mm 100

10mm 100 90-100

6.3mm 85-100 45-80

2.36mm 45-57 45-57

600µm 34-57 34-57

212µm 8-35 8-35

75µm 4-8 4-8

Table A.1 - Type A : Grading for 45% Stone Asphalt

Notes:

Maximum % of aggregate passing 2.36mm sieve and retained on 600µm sieve is 11.0%.

Binder content percentage by mass is 6.6%.

The coarse aggregate shall be crushed rock with a polished stone value as described in the contract. For 45/6,which is for footway use only, the aggregate shall be limestone.

The binder shall be 200 pen bitumen and the maximum temperature at any time for this binder shall be 160°C.

Annex ASpecification for 45% Type F Asphalt

A/1


May 2001

ANNEX B SPECIFICATION OF BEDDING SANDAND JOINTING MATERIAL FOR SMALL ELEMENTPAVING

Sieve Size Percentage Passing

Class II Class III

5mm 90-100 90-100

2.36mm 75-100 75-100

600µm 35-60 35-70

300µm 8-35 8-35

150µm 0-10 0-10

75µm 0-1 0-3

Table B.1 : Grading for Bedding Sand

Sieve Size Percentage Passing

2.36mm 100

1.18mm 95-100

600µm 55-100

300µm 15-50

150µm 0-15

75µm 0-3

Table B.2 : Grading for Jointing Material

Annex BSpecification of Bedding Sand and Jointing Material for Small Element Paving

B/1


May 2001

ANNEX C COMPACTION SPECIFICATION

Material Mean of 6 Cores Mean of Pairs

Min Max Min Max

Dense macadam wearing course 2% 6% 1% 8%

Dense macadam basecourse (BS4987 2% 7% 1% 9%cl 6.5) and 14mm close gradedwearing course used as a basecourse

All asphalt materials, ie roadbase, 2% 6% 1% 8%basecourse and wearing course

Table C.1 : Air Void Limits

C.1 The adequacy of compaction of bituminous materials will be judged against the air void levels given in TableC.1. The following procedure should be used.

C.2 Compliance should be judged from the determination of air voids for areas of 1000m² or from the area laidin one day where the area is less than this. Where a number of small areas are laid in a day the client shoulddetermine whether these are to be grouped into one site for testing purposes. Three 100mm nominal diametercore pairs should be taken from each area in a random manner.

C.3 The density corresponding to zero air voids should be determined using ASTM method D2041-91. One corefrom each set of six should be taken for the determination of the maximum density; where only one set istaken then the determination should be carried on 2 cores from the set.

C.4 When the material contains applied chippings the void content should be calculated from the whole layerincluding chippings.

C.5 The air void content should be calculated as 100(1-Dm/Do) per cent, where Dm is the measured density and Dois the ASTM D2041-91 density, and should be calculated for the whole layer including chippings.

C.6 The Contractor should inform the Engineer of the sources of the constituents of the mixes at the start of thecontract and of any changes during the contract.

C.7 Where these requirements for the air voids are not met the Contractor should determine the full extent of thearea of the defective material to the satisfaction of the Engineer. The full depth and width, minimum 15mlong and the full width of the footway, of the defective material should be removed and replaced with freshmaterial laid and compacted to this Specification.

C.8 The nuclear density gauge (NDG) may be used to reduce the amount of coring required but in case of disputethe core density method should be used.

Annex CCompaction Specification

C/1


May 2001

ANNEX D COMPACTION BY METHODSPECIFICATION

Granular Bituminous

Compaction passes required for layers ofcompacted thickness up to:

100mm 150mm 200mm 50mm 75mm 100mm

Vibrotamper 50Kg minimum 6 12 u/s 6 9 12

Vibrating Roller Over 600 - 1000Kg/m twin 6 12 u/s 6 9 12vibrating drum

Over 600-1000 Kg/m single 12 u/s u/s 12 u/s u/svibrating drum

Over 1000-2000Kg/m twin 3 6 12 4 6 8vibrating drum

Over 1000-2000Kg/m single 6 12 u/s 8 12 u/svibrating drum

Over 2000Kg/m twin 2 3 4 3 4 5vibrating drum

Over 2000Kg/m single 3 5 7 6 8 10vibrating drum

Vibrating Plate Over 1400-1800Kg/m² 5 9 u/s 8 12 u/s

Over 1800Kg/m² 3 5 7 4 6 8

Table D.1 : Specified Passes for Various Compaction Plant

u/s - compaction plant is unsuitable for that material/layer thickness.

Vibrotampers are not the recommended plant for bituminous wearing courses.

Annex DCompaction by Method Specification

D/2


May 2001

ANNEX E WORKED EXAMPLES

EXAMPLE 1

A footway is to serve terraced dwellings on a new estate, linking them to a small garage court. Barriers preventchildren running straight out into the garage court, which is surfaced in concrete pavers. From the groundinvestigation, prior to building the estate, the CBR is 2%. There are no services under the footway.

Stage 1: Decide appropriate footway category Chapter 2, Figure 2.1

Footway is physically separated from the carriageway. Footway is not a cycleway, therefore categoryis pedestrian-only.

Stage 2: Check design considerations Chapter 2, Table 2.3

Width and gradients should be within the limits of Table 2.3. Crossfall should run away from houses.Adequate drainage should be provided. The plan shows that there are no services under the footway.The footway surfacing should enhance the environment; concrete pavers would be chosen to matchthe garage court.

Stage 3: Structural design Chapter 3, Table 3.1

There are no services in the footway so a geosynthetic can be used to separate the subgrade from thegranular sub-base. Design CBR is 2 per cent.

Annex EWorked Examples

E/1


May 2001

The structural design (pedestrian-only) is:

Note: Pavers can be chosen to contrast with the paved parking area to signify a change of use.

Stage 4: Construction Chapters 3 and 4

As the footway is not adjacent to a road or wall, edge restraint should be provided on both sides of thefootway. The cross section of the footway is as follows:


E/2

Pavers ≥ 50mm

100mm sub-base

30mm bedding sand

Geosynthetic


May 2001

EXAMPLE 2

A new school has been built on the outskirts of a rural village. A footway is required to connect the school to theneighbouring village so that pupils can cycle or walk to school safely. The road running past the school is a rurallane, but there is ample parking and delivery space in the school grounds. At present the lane has a 3m wide grassverge bounded by a hedge. The soil along the verge is a soft clay. The length for which a footway is required isapproximately 1 km.


Footway is not physically separated from the carriageway and may be considered to be lightly used.However, footway is also to be used as a cycleway, which may require the use of clearing andmaintenance vehicles, so it would be wise to design for light-vehicle category.

Stage 2: Estimate subgrade CBR Chapter 2, Table 2.2

Soil is very soft clay. A quick test shows that it “exudes from the fingers when squeezed”. Accordingto Table 2.2, this indicates a CBR of less than 1%.


Width and gradients should be within the limits of Table 2.3. A minimum of 2m width is requiredwhich could usefully be increased to 2.5m to provide space for cyclists to overtake pedestrians. Thegradient will be as for the road. The crossfall will be towards the road utilising existing drainage. Thelength and situation of the footway make a bituminous construction the best option. If there areservices in the verge they may need to be moved or made deeper.


CBR < 1%. Assume lime stabilisation is used to raise the design subgrade CBR to over 5 per cent.

Very occasional heavy vehicle overrun may occur, but the treated subgrade now has a CBR > 5% sothe designs in Table 3.3 can be used. The structural design is 70mm of bituminous material on 150mmof sub-base.


E/3


May 2001


The kerb provides the lateral restraint on one side of the footway, edging is required on the other side.The cross section of the footway is as follows:

EXAMPLE 3

A footway is to be built alongside an existing road, which now receives some pedestrian traffic due to constructionof an out-of-town shopping area. There will be no verge between the footway and the carriageway, as the existingverge, on which the footway is to be constructed, is only 1.8m wide. There is a brick wall along the whole length ofverge. It has been decided that concrete blocks will be used as the footway surfacing. The soil is sandy clay.Delivery vehicles may overrun the footway occasionally if, for example, the road is partly blocked by a brokendown vehicle.


Footway is not physically separated from the carriageway, and is not very lightly used. Heavy vehicleoverrun is likely to occur. Therefore the footway category is heavy-vehicle.

Stage 2: Estimate subgrade CBR Chapter 2, Table 2.1

Soil is a sandy clay. From Table 2.1 the subgrade CBR is likely to be approximately 3 per cent. Testingis required to confirm this.


Width and gradients should be within the limits of Table 2.3. The gradient will be as for the road. Thecrossfall will run towards the road utilising existing drainage.


E/4


May 2001


The subgrade CBR = 3%, so no treatment is necessary.

The required sub-base thickness for heavy-vehicle design is 270mm. Concrete blocks should be atleast 60mm thick and should be placed on 30mm compacted thickness of bedding sand. A roadbase of90mm DBM or 100mm CBM1 or stronger, is required between the sub-base and the bedding sand.


The brick wall provides lateral restraint on one side of the footway and the kerb provides lateralrestraint.

E/5


August 2004



SECTION 3 PAVEMENTMAINTENANCEASSESSMENT

PART 1

HD 28/04

SKID RESISTANCE

SUMMARY

This Standard describes how the provision ofappropriate levels of skid resistance for trunk roads willbe managed. It details how measurements of skidresistance are to be made and interpreted and iscomplemented by HD 36 (DMRB 7.5.1), which sets outadvice on surfacing material characteristics. This latestrevision has changed requirements for settinginvestigatory levels, for annual SCRIM surveys, fordetermining the characteristic SCRIM coefficient andhas further updates in line with current policy.


This revised Standard is to be incorporated in theManual.

1. This document supersedes HD 28/94, which isnow withdrawn.

2. Remove existing Contents page for Volume 7 andinsert new Contents page for Volume 7 datedAugust 2004.

3. Remove HD 28/94, which is superseded byHD 28/04, and archive as appropriate

4. Insert HD 28/04 in Volume 7, Section 3, Part 1.



HD 28/04

Skid Resistance

Summary: This Standard describes how the provision of appropriate levels of skidresistance for trunk roads will be managed. It details how measurements of skidresistance are to be made and interpreted and is complemented by HD 36(DMRB 7.5.1), which sets out advice on surfacing material characteristics. Thislatest revision has changed requirements for setting investigatory levels, forannual SCRIM surveys, for determining the characteristic SCRIM coefficientand has further updates in line with current policy.


THE HIGHWAYS AGENCY

SCOTTISH EXECUTIVE




August 2004






SECTION 3 PAVEMENTMAINTENANCEASSESSMENT

PART 1

HD 28/04

SKID RESISTANCE

Contents

Chapter

1. Introduction

2. Operation

3. Measurement of Skid Resistance

4. Setting the Investigatory Level

5. Site Investigation

6. Prioritisation of Treatment

7. Use of Warning Signs

8. References

9. Enquiries

Annex 1 Background Information on theMeasurement and Interpretation of SkidResistance

Annex 2 SCRIM Survey of Operational Procedures

Annex 3 Processing and Computation ofCharacteristic SCRIM Coefficient

Annex 4 Site Investigation

Annex 5 Assessment of Accident Data

Annex 6 Use of Different Types of Test in AccidentInvestigation


August 2004



1. INTRODUCTION

General

1.1 The purpose of this document is to describe howthe provision of appropriate levels of skid resistance onin-service UK Trunk Roads, i.e. motorways and all-purpose trunk roads, will be managed. This documentdescribes how measurements of skid resistance are tobe made and interpreted and is complemented byHD 36 (DMRB 7.5.1), which sets out advice onsurfacing material characteristics necessary to deliverthe required skid resistance properties.

1.2 In this document, the term “skid resistance”refers to the frictional properties of the road surfacemeasured using a specified device under standardisedconditions. The term always refers to measurementsmade on wet roads, unless specifically stated otherwise.These measurements are used to characterise theroad surface and assess the need for maintenance,but cannot be related directly to the frictionavailable to a road user making a particularmanoeuvre at a particular time.

1.3 The skid resistance of a wet or damp road surfacecan be substantially lower than the same surface whendry, and is more dependent on the condition of thesurfacing material. The objective of this Standard is tomanage the risk of skidding accidents in wet conditionsso that this risk is broadly equalised across the trunkroad network. This is achieved by providing a level ofskid resistance that is appropriate to the nature of theroad environment at each location on the network. Theappropriate level of skid resistance is determined from anetwork accident analysis plus local judgement of site-specific factors.

1.4 In this Standard, the provision of appropriatelevels of skid resistance is treated primarily as an assetmanagement issue rather than one of road safetyengineering, although the accident risk is assessed inorder to determine an appropriate level of skidresistance for each site. Specifically, this Standard doesnot address the identification of locations or routeswhere road safety engineering measures could bebeneficial to reduce accidents.

1.5 This Standard provides advice and guidance toassist the engineer in determining an appropriate levelof skid resistance for each site. It lays down theprocedure to be used for measuring the skid resistance

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and, for cases where the measured skid resistance is ator below a predetermined level, it provides amethodology to assist the engineer in assessing therequirement and priority for remedial works. Remedialworks will be subject to an economic assessment of thecosts and benefits before proceeding, to promote thebest use of maintenance budgets.

Structure

1.6 Chapter 2 summarises the operation of the skidresistance Standard. Chapters 3 to 7 describe keycomponents of the Standard: the measurement of skidresistance, the process of setting Investigatory Levels,site investigation, the prioritisation of treatments anduse of warning signs. These chapters are supported by anumber of Annexes that give more detailed instructionsor advice.

1.7 Annex 1 provides background informationrelevant to the measurement and interpretation of skidresistance. Annex 2 gives operational details formeasuring skid resistance and Annex 3 gives themethods for processing the raw survey data to derivevalues that characterise the skid resistance.

1.8 Annex 4 and Annex 5 describe methodologies forsite investigation and for assessing accident datarespectively.

1.9 Annex 6 discusses the different test methods usedin accident investigation. Police collision investigatorsuse physical evidence (e.g. marks on the road surface)and skid tests at accident sites to estimate the approachspeed of vehicle(s) involved and its relevance to roadsurface condition. In giving evidence in court, policeofficers have, on occasion, used the results of skid tests,often made in dry conditions, to comment on theadequacy or otherwise of the skid resistance of the roadsurface. This has, on occasion, led to claimants usingpolice evidence to try to demonstrate that a road surfacewas unsatisfactory, even though the purpose of theirtests are quite separate from skid resistance tests toassess the surface condition, as explained in Annex 6.

Implementation

1.10 This Standard shall be used forthwith on allschemes for the construction, improvement and

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maintenance of trunk roads including motorways,currently being prepared provided that, in the opinionof the Overseeing Organisation this would not result insignificant additional expense or delay. Designorganisations should confirm its application toparticular schemes with the Overseeing Organisation.

Mutual Recognition

1.11 The construction and maintenance of highwaypavements will normally be carried out under contractsincorporating the Overseeing Organisation’sSpecification for Highway Works (MCHW1). In suchcases products conforming to equivalent standards andspecifications of other member states of the EuropeanUnion and tests undertaken in other member states willbe acceptable in accordance with the terms of the 104and 105 Series of Clauses of that Specification. Anycontract not containing these Clauses must containsuitable clauses of mutual recognition having the sameeffect regarding which advice should be sought.

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Chapter 2Operation

2. OPERATION

2.1 This Chapter summarises the procedures formaking and interpreting skid resistance measurementson UK trunk roads.

2.2 Routine measurements of skid resistanceshall be made and processed to deriveCharacteristic SCRIM Coefficient (CSC) valuesin accordance with Chapter 3, supplemented byspecific instructions issued by the OverseeingOrganisation.

2.3 The CSC is an estimate of the underlying skidresistance once the effect of seasonal variation has beentaken into account. This value will be taken to representthe state of polish of the road surface. These terms areexplained in Annex 1.

2.4 On receipt of processed survey data, theCSC values shall be compared with thepredetermined Investigatory Levels, to identifylengths of road where the skid resistance is at orbelow the Investigatory Level.

2.5 Investigatory Levels represent a limit, abovewhich the skid resistance is assumed to be satisfactorybut at or below which the road is subject to a moredetailed investigation of the skid resistancerequirements. Investigatory Levels are assigned basedon broad features of the road type and geometry (thesite category) plus specific features of the individualsite. Investigatory Levels will be reviewed on a rollingprogramme, to ensure that changes in the network areidentified, local experience is applied and consistencyis achieved. The process for setting Investigatory Levelsand the normal range of Investigatory Levels for eachsite category is described in Chapter 4.

2.6 Wherever the CSC is at or below theassigned Investigatory Level a site investigationshall be carried out, to determine whethertreatment to improve the skid resistance isrequired or whether some other action isrequired.

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2.7 A site investigation shall also be carriedout if, in the normal course of accidentinvestigation processes separate from thisStandard, sites are identified where increasedwet or skidding accident levels have beenobserved.

2.8 The process of site investigation is described inChapter 5. The decision of whether treatment isnecessary is unlikely to be clear-cut, but requiresprofessional engineering judgement taking into accountlocal experience, the nature of the site, the condition ofthe road surfacing and the recent accident history. Ifsuccessive site investigations show that treatment is notwarranted at the current level of skid resistance thenconsideration should be given to lowering theInvestigatory Level.

2.9 The processes of setting Investigatory Levels andundertaking site investigations are complementary,since local knowledge and experience gained throughconducting detailed site investigations can be used torefine the criteria for setting Investigatory Levels forsimilar types of site.

2.10 The process of site investigation will result in anumber of lengths being recommended as needingtreatment to improve the skid resistance. The priorityfor treatment will be established taking into account theobserved accident history, the need for othermaintenance works in the vicinity, the cost and thebudget available for the works. This process isdescribed in Chapter 6.

2.11 Signs warning road users that the roadcould be slippery shall be erected, as describedin Chapter 7.

2.12 The decision-making process leading to theidentification and prioritisation of sites for treatment issummarised in Figure 2.1.

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Chapter 2Operation

Figure 2.1: Procedure for Indentification and Prioritisation of Sites

Categorise s ite s (Table 4.1) and assign initial Investigatory Levels

Carry out site investigation in prioritised order

R eview /revise Investigatory

Level

Define/review local criteria for setting I nvestigatory L evel s

Identify and cost suitable treatment strategy

Prioritise and treat sites, taking account of budget and programme

cons iderations

Yes

No

CSC at or

below Investigatory

Level?

Yes

No

Treatment needed ?

Consider revising Investigatory Level

Erect warning signs if required

No

Network changes or >3

years since Investigatory

Level review?

Yes

Site treated ?

Yes

No

Add to n ext year’s programme

Remove warning signs

Other indication of increased skidding

accident risk

Carry out SCRIM survey(s) and calculate

CSC for site

No further action until next CSC meas urement

No further action until next CSC measurement

No further action until next CSC meas urement

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Chapter 3Measurement of Skid Resistance

D RESISTANCE
3. MEASUREMENT OF SKI
Measurement equipment

3.1 Various types of equipment are available formeasuring skid resistance. In different ways, allmeasure the force developed on a rubber tyre or sliderpassing over a wetted road surface and derive a valuethat is related to the coefficient of friction and the stateof polish of the road surface.

3.2 However, the results from the different devicesare not directly interchangeable. For this reason onlyone device is to be used for monitoring the in-serviceskid resistance of UK trunk roads for the purposes ofthis Standard.

3.3 Measurements for monitoring the in-service skid resistance of UK Trunk Roads, inline with this Standard, shall be made with aSideway-force Coefficient Routine InvestigationMachine (SCRIM).

3.4 SCRIM (see Figure 3.1) was introduced in the1970s to provide a method for routinely measuring theskid resistance of the road network. This machine usesthe sideway force principle to measure skid resistance.A freely rotating wheel fitted with a smooth rubber tyre,mounted mid-machine in line with the nearside wheeltrack and angled at 20° to the direction of travel of thevehicle, is applied to the road surface under a knownvertical load. A controlled flow of water wets the roadsurface immediately in front of the test wheel so that,when the vehicle moves forward, the test wheel slidesin the forward direction along the surface.

3.5 The force generated by the resistance to sliding isrelated to the wet road skid resistance of the roadsurface. Measurement of this sideways componentallows the Sideway-Force Coefficient (SFC) to becalculated. SFC is the sideway force divided by thevertical load.

3.6 Depending on the machine, the vertical load iseither assumed to be constant or it is measureddynamically. It has been shown that more reliableresults are obtained when calculating the SFC using themeasured vertical load than when a fixed constant loadis assumed.

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Figure 3.1 SCRIM, test wheel and principle

3.7 Equipment with a vertical loadmeasurement capability shall be used ifspecified by the Overseeing Organisation.

3.8 Measurements are recorded as SCRIM Readings(SR). A SCRIM Reading is the average SFC multipliedby 100 for a pre-determined length of road, normally10m, recorded as an integer.

3.9 Each SCRIM used on Trunk Roads shallhave been accepted by the OverseeingOrganisation. Acceptance will be based uponsatisfactory performance at group correlationexercises arranged through the OverseeingOrganisation.

3.10 A national correlation exercise will usually beheld annually before the start of the testing season, withadditional trials as necessary. Machines that do notperform satisfactorily at the main exercise will berequired to attend and achieve acceptance at anadditional exercise(s). Machines that are unable toattend the main trial will also be required to attend asupplementary correlation exercise.

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Chapter 3Measurement of Skid Resistance

Use of SCRIM

3.11 Skid resistance is not a constant, but isinfluenced by various factors, including temperature,test speed and weather conditions, plus longer-termeffects such as seasonal variation and changes in trafficflow. Further information is given in Annex 2. For thepurpose of this Standard, SCRIM measurements will bemade under standardised conditions to control theseeffects as far as possible, including:

• limiting the testing season to a specific time ofyear;

• specifying a standard test speed;

• specifying the test line to be followed;

• specifying the ambient conditions under whichacceptable measurements may be made.

Further details are given in Annex 2.

3.12 SCRIM operators providingmeasurements under this Standard mustdevelop appropriate procedures to ensure thatmeasurements are carried out safely and to astandard of quality agreed with the OverseeingOrganisation. This must include adhering to theprocedures given in British Standard BS7941-1(1999) for making skid resistance measurementswith a SCRIM and for calibrating and makingregular checks on the equipment, and thefurther instructions given in this Chapter and inAnnex 2.

Survey Strategy

3.13 The surveying strategy is planned so that theeffects of seasonal variation, both within a singleseason and/or between successive years, can be takeninto account in the determination of the CSC for anyparticular length of road.

3.14 Because of the different nature of traffic patternsand road networks in the different UK regions, the wayin which surveys are planned and seasonal variation isaccounted for may be different for the individualOverseeing Organisations. The alternative monitoringstrategies to be followed are explained as part ofAnnex 3.

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3.15 The whole trunk road network will normally betested once during each testing season. However, therate at which skid resistance changes may be slow onsome roads, particularly where traffic volumes are low.In such circumstances it may not be necessary tomonitor annually.

3.16 The Overseeing Organisation will specifythe network to be surveyed, the test lane, thesurvey strategy and the method and/or theaccuracy of location referencing required.

Processing of survey data

3.17 After collection, survey data will be validatedand subject to processing to determine the CSC valuesthat will be used for further analysis. Validation andprocessing will be carried out as specified by theOverseeing Organisation.

3.18 Typically, processing will include:

• application of correction factors, e.g. incircumstances where it was not possible tomaintain the specified standard test speed;

• multiplication by the Index of SFC applicable tothe SCRIM at the time it was making themeasurement;

• calculation of the CSC;

• aggregation of raw data to longer averaginglengths, typically 100m or 50m, for furtheranalysis (see paragraph 4.8).

3.19 Further details of the processing are set out inAnnex 3.

3.20 On receipt of processed survey data, theOverseeing Organisation (through itsmaintaining organisation) shall check that thewhole of the specified network has beensurveyed. Appropriate action must be taken toensure that, as far as possible, valid data isobtained in the following planned survey forlocations where there is missing or invalid datain the current survey.

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Chapter 4Setting the Investigatory Level

ATORY LEVEL
4. SETTING THE INVESTIG
Objective

4.1 The objective of setting the Investigatory Level isto assess the nature of the site and assign an appropriatelevel of skid resistance, at or below which a moredetailed site investigation must be undertaken. It isimportant that the Investigatory Level is not set too low.If, during the site investigation, the Investigatory Levelis found to be too high then it can be lowered. However,if the Investigatory Level is initially set at too low alevel then the need to improve the skid resistance maynot be detected until it has already fallen further than isdesirable.

4.2 The Overseeing Organisation (through itsmaintaining organisation) shall assign a SiteCategory and Investigatory Level to each partof the network, so that the Investigatory Levelcan be compared with the CSC. Thisinformation must be recorded in a formatagreed with the Overseeing Organisation,together with the date of assessment.

Procedure

4.3 Site categories and associated InvestigatoryLevels are defined in Table 4.1. These categories andranges have been developed for trunk roads and maynot be applicable to local authority roads, which aremore diverse in nature.

4.4 The site category most appropriate to the layoutof the site will be selected from the list in Table 4.1. Ifmore than one site category is appropriate then the sitecategory with the higher Investigatory Level will beselected or where the highest Investigatory Levels arethe same then the category highest up the Table will beselected.

4.5 Slip roads will be allocated to categories B, Q, Gor S as appropriate to their length and layout.

4.6 A single site category can vary in length from afew tens of metres to several kilometres, depending onthe nature of the site. The length of site categories Qand K will normally be the 50m approach to the feature,but this shall be extended where justified by site

cat

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August 2004

haracteristics, e.g. if queuing traffic creates andditional risk, to allow for traffic joining the back ofhe queue.

.7 After selecting a site category, the appropriatenvestigatory Level is assigned from the range availableor that site category, following the process described inaragraphs 4.10 to 4.13. The range of Investigatoryevels for each site category has been developed as a

esult of UK research studies on trunk roads andeflects the variation in accident risk within a siteategory. Further details are given in Annex 1. Thelexibility to set different Investigatory Levels forifferent sites within the same category allows for siteshere the risk of skidding accidents is potentiallyigher to have a higher Investigatory Level and possiblye treated to maintain a higher level of skid resistance.

.8 Investigatory Levels are applied to the mean CSCithin 100m averaging lengths (50m lengths for someverseeing Organisations), except that the Investigatoryevels for site category R are based on 10m averaging

engths. Shorter lengths will be necessary where the siteategory is less than 100m long or at the end of a siteategory longer than 100m. Residual lengths less than0% of a complete averaging length may be appendedo the penultimate length, providing the site category ishe same.

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Investigatory Level at 50km/h

Site category and definition0.30 0.35 0.40 0.45 0.50 0.55 0.60 0.65

A Motorway

B Dual carriageway non-event

C Single carriageway non-event

Q Approaches to and across minor and majorjunctions, approaches to roundabouts

K Approaches to pedestrian crossings and otherhigh risk situations

R Roundabout

G1 Gradient 5-10% longer than 50m

G2 Gradient >10% longer than 50m

S1 Bend radius <500m – dual carriageway

S2 Bend radius <500m – single carriageway

Notes:

1. Investigatory Levels are for the mean skidding resistance within the appropriate averaging length

2. Investigatory Levels for site categories A, B, and C are based on 100m averaging lengths (50m lengthsfor some Overseeing Organisations) or the length of the feature if it is shorter.

3. Investigatory Levels and averaging lengths for site categories Q, K, G and S are based on the 50mapproach to the feature but this shall be extended when justified by local site characteristics.

4. Investigatory Levels for site category R are based on 10m lengths.

5. Residual lengths less than 50% of a complete averaging length may be attached to the penultimate fullaveraging length, providing the site category is the same.

6. As part of site investigation, individual values within each averaging length should be examined and thesignificance of any values which are substantially lower than the mean value assessed.

Table 4.1 Site categories and Investigatory Levels

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4.9 Dark shading in Table 4.1 indicates the range ofInvestigatory Levels that will generally be used fortrunk roads carrying significant traffic levels. Lightshading indicates a lower Investigatory Level that willbe appropriate in low risk situations, such as low trafficlevels or where the risks present are well mitigated anda low incidence of accidents has been observed.Exceptionally, a higher or lower Investigatory Levelmay be assigned if justified by the observed accidentrecord and local risk assessment.

4.10 On first implementation of this Standard, thelowest Investigatory Level with dark shading in eachsite category, or the middle Investigatory Level forcategory Q, may be assigned. On a rolling programme(see paragraph 4.17), this initial assignment shall besuperseded by the more detailed risk assessmentdescribed below.

4.11 The accident risk for each individual site must beevaluated in comparison with other sites in the samesite category, in order to set an appropriateInvestigatory Level. Criteria for making this assessmentwill be developed at a local level, taking into accountthe features of the local network. Guidelines of thefactors to consider in developing these criteria are givenbelow.

4.12 Roads within the site category with noexceptional risk of skidding accidents will be assignedthe lowest Investigatory Level. It is envisaged that thiswill apply to the majority of sites. The following factorscould influence the choice of a higher InvestigatoryLevel. However, a higher Investigatory Level will notbe appropriate if the overall level of risk is low(e.g. because of low traffic flow or low traffic speed) orif the risk has been mitigated by other means(e.g. through clear signing of a hazard or reduced speedlimit.)

• Notable potential for conflict between road usersat the site, particularly where the outcome islikely to have severe consequences, e.g. ahead-on or side impact at speed.

• Road geometry departing substantially fromcurrent Standards.

• Known incidence of queuing where the trafficspeed is otherwise high.

• Accesses onto the main carriageway, if they arebusy (e.g. for a service station) or have pooradvance visibility, or if the speed of leaving orjoining traffic creates conflict with other trafficon the main carriageway.

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• Two or more events in proximity, e.g. a junctionon a downhill gradient.

• Low texture depth (less than 0.8mm measured asSensor Measured Texture Depth (SMTD)),except for High Friction Surfacing materials.

• Bends where the traffic speed and/or geometry isjudged to give rise to added risk. This couldapply to some sites in non-event categories aswell as sites in the bend categories.

• Sharp left-hand bends, such as on the approach toor exit from a roundabout, where greaterpolishing action by traffic could lead to lowerskid resistance occurring in the right-hand wheelpath than is measured in the left-hand wheel path.

• For non-event categories B and C: approach toslip road leaving main carriageway or mergingarea downstream of slip road joining maincarriageway, except if low traffic flow means thatthe slip road gives rise to little added conflictbetween road users, compared with the mainline.

• For approaches to minor junctions: poor advancevisibility; high risk of head-on collisions; highapproach speed, except if this is mitigated(e.g. by adequate taper length for traffic leavingand joining the main carriageway and/or aseparate lane for right turning traffic).

• For pedestrian crossings: poor advance visibility;high approach speed.

• For roundabouts: high speed of circulatingtraffic; high incidence of cyclists ormotorcyclists; absence of signalised control onroundabouts at grade separated interchanges.

• For all site categories on all-purpose roads,significant numbers of pedestrians crossing at agiven location.

• Known history of accident occurrence beingmore frequent than normal, particularly in wetconditions or where skidding is reported.

4.13 The Overseeing Organisation (through itsmaintaining organisation) shall develop and documentcriteria for assigning Investigatory Levels within eachsite category. This could consist of local examples ofsites assigned different Investigatory Levels. Thesecriteria will be refined on an on-going basis to takeaccount of local experience of detailed site

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investigations.

4.14 The network evolves both through substantialchanges such as the addition or improvement ofjunctions, or the addition of traffic calming measuresand through more subtle changes such as increasingtraffic levels or change of land use. The criteria bywhich Investigatory Levels are assigned may alsoevolve as experience of local conditions is gained, e.g.where treatment to improve skid resistance in somelocations is found to be more effective in reducingaccident rates than in others.

4.15 The Investigatory Levels throughout the networkthat is surveyed for skid resistance must be reviewed ona rolling programme, so that:

• changes in the network are identified and takeninto account;

• local experience is applied;

• consistency is maintained.

4.16 A review of the Investigatory Level shallbe carried out when a significant change to thenetwork is made.

4.17 Notwithstanding the above, a procedureshall be put in place to ensure that theInvestigatory Levels are reviewed at least everythree years unless agreed otherwise with theOverseeing Organisation.

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Chapter 5Site Investigation

5. SITE INVESTIGATION

Objectives and outcomes

5.1 Sites where the CSC is at or below theInvestigatory Level require a detailed investigation. Theobjective is to determine whether a surface treatment isjustified to reduce the risk of accidents, specificallyaccidents in wet conditions or involving skidding,whether some other form of action is required, orwhether the site should be kept under review. Thisinvestigation is an important part of the operation of theskid resistance Standard. In conjunction with theprocess of setting Investigatory Levels, the objective isto promote effective targeting of treatments.

5.2 Treatment will normally be a surface treatment toimprove the skid resistance. However, if the siteinvestigation identifies any characteristic of the site orroad user behaviour that suggests other road safetyengineering measures could be appropriate, then theappropriate specialist dealing with safety schemes mustbe consulted before deciding upon the best course ofaction. If it is found that there is a need for other typesof routine maintenance, for example re-application ofroad markings or additional road sweeping, then thismust also be addressed.

5.3 Some form of treatment will be justified if:

• based on an accident analysis, the number ofaccidents observed is higher than average for thetype of site being considered;

• based on an accident analysis, the site has ahigher than average proportion of accidents inwet conditions or involving skidding for the typeof site being considered;

• the nature of the individual site and the demandsof road users mean that a higher accident risk(compared with other sites in the same category)might be expected with the skid resistance at itscurrent value or if it were to fall further beforethe next measurement. In this case, preventivetreatment is justified to pre-empt a potentialincrease in accident risk.

5.4 If none of the above are true then there iscurrently no justification for treatment to increase theskid resistance. If the site remains below theInvestigatory Level at the next measurement, then it

August 2004

will automatically be subject to a further investigation.That is, sites with skid resistance remaining below theInvestigatory Level are automatically kept underreview.

5.5 Further details of making these assessments aregiven in Annex 4 on site investigation and Annex 5 onassessment of accident data.

5.6 If the skid resistance and accident pattern remainstable for an extended period, for example, more than 3years, then lowering the Investigatory Level should beconsidered. However, it is important that stability isobserved before reducing the Investigatory Level,because, unless the skid resistance falls further, regularinvestigation that would detect an increase in accidentswould no longer be prompted by the skid resistanceStandard.

Procedure

5.7 Sites requiring investigation shall beidentified as soon as practicable on receipt ofthe CSC values.

5.8 CSC values should be used in preference tosingle run survey values that have not been adjusted forseasonal effects, as the latter are less reliable. However,if single run survey data indicate that the skid resistanceis significantly below the Investigatory Level (e.g. atleast 0.05 units SCRIM coefficient below) then it islikely that the CSC will also be found to be below theInvestigatory Level. Single run data can therefore beused to give early warning of sites requiringinvestigation, but any decision on the need fortreatment should normally be based on the CSC.

5.9 Site investigations shall be carried out ina prioritised order, by personnel experienced inpavement engineering.

5.10 Site investigations will be prioritised initially onthe basis of the amount by which the skid resistance isbelow the Investigatory Level. This order may berefined to take into account the efficiency of conducting

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Chapter 5Site Investigation

investigations and as a result of other informationgathered during the early part of the investigation, suchas the recent accident history.

5.11 Persons with relevant local experience must beidentified, and should be consulted if appropriate,during the site investigation process. These will includethe person locally responsible for accident investigationand prevention.

5.12 The results of the investigation, includingwhether further action is required, shall bedocumented and retained together with theidentity of the assessor and other partiesconsulted.

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EATMENT

Chapter 6Prioritisation of Treatment

6. PRIORITISATION OF TR

6.1 Site investigation results in the identification oflengths of pavement where treatment is recommendedto improve the skid resistance. This chapter addressesthe prioritisation of these treatments on the assumptionthat budget resources are limited. If other actions areidentified as a result of the site investigation then thesemust be prioritised as appropriate.

6.2 HD 36, HD 37 and HD 38 (DMRB 7.5.1-3) giveadvice about the choice of surfacing materials toprovide the appropriate level of skid resistance andabout the use of re-texturing treatments to provideshort-term improvements to skid resistance and/ortexture depth. Other aspects of pavement conditionmust also be taken into account in selecting the mostappropriate form of treatment.

6.3 The most appropriate form of treatmentshall be identified for each treatment lengthtaking account of current advice.

6.4 Priority must be given to completing treatmentswhere the skid resistance is substantially below theInvestigatory Level (e.g. at least 0.05 units CSC below),or low skid resistance is combined with a low texturedepth, or the accident history shows there to be aclearly increased risk of wet or skidding accidents. Inother cases, treatment will be programmed as a longer-term measure taking into consideration othermaintenance requirements.

6.5 A comparison of the estimated accident savingand the cost-effectiveness of treatments can be used toassist in establishing the relative priority of treatmentsat different locations. The Overseeing Organisation willprovide information that can be used to make thisassessment. Other estimates of accident savings may beused if backed up by local experience or otherinformation.

6.6 The cost effectiveness of treatments at differentlocations can be calculated by dividing the estimatedaccident saving by the anticipated cost of treatment.

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6.7 The priority for treatment should be establishedfor all new treatment lengths and for those lengthspreviously recommended for treatment to improve theskid resistance, but where treatment has not yet beencarried out or definitely programmed. If more than ayear has elapsed since the site investigation was carriedout then the accident history and priority for treatmentmust be re-examined using the most recent dataavailable. This programme should be reviewed andprogress recorded at appropriate intervals.

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August 2004

7. USE OF WARNING SIGNS

7.1 Warning signs shall be erected at siteswhere the need for treatment to improve skidresistance has been identified following a siteinvestigation. In Scotland, sites identified in thisway shall be referred on an individual basis tothe Overseeing Organisation for a decision onthe provision of warning signs.

7.2 This strategy provides a targeted use of signs andis designed to avoid a proliferation of signs that wouldundermine their effectiveness and would not make bestuse of resources.

7.3 Since warning signs are erected (if required) aftera site investigation, it is particularly important tocomplete site investigations in a prioritised order andwithin a reasonable time period, so that warning signscan be placed where they are needed without unduedelay.

7.4 The slippery roads warning sign(Diagram 557) is to be used in accordance withthe instructions contained in The Traffic SignsRegulations and General Directions (2002).

7.5 A visual inspection of the site shall bemade after the signs are erected to confirm thatthey have been erected and correctly placed anda record of this observation shall be made andretained.

7.6 Depending on the nature of the surface treatment,it may be necessary to leave slippery road warningsigns in place for a period after a new surface is openedto traffic. Additionally, at sites where the risk ofskidding may temporarily increase after surfacetreatment, it may be necessary to erect warning slipperyroad signs if not already in place.

7.7 Warning signs shall be removed as soonas they are no longer required.

Chapter 7Use of Warning Signs

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August 2004

Chapter 8References

8/1

8. REFERENCES

1976

LR738; Hosking JR and Woodford GC, “Measurementof Skidding Resistance Part ii: Factors Affecting theSlipperiness of a Road Surface”; TRRL.

1999

BS7941; Part1; “Methods for Measuring the SkidResistance of Pavement Surfaces. Side-ways ForceCoefficient Routine Investigation Machine”, BSI.

DMRB

HD 29 (DMRB 7.3.2) Structural Assessment Methods.

HD 36 (DMRB 7.5.1) Surfacing Materials for New andMaintenance Construction.

HD 37 (DMRB 7.5.2) Bituminous Surfacing Materialsand Techniques.

HD 38 (DMRB 7.5.3) Concrete Surfacing andMaterials.


August 2004 9/1

9. ENQUIRIES




Chief Highway EngineerTransport DirectorateWelsh Assembly GovernmentLlywodraeth Cynulliad CymruCrown Buildings M J A PARKERCardiff Chief Highway EngineerCF10 3NQ Transport Directorate


Chapter 9Enquiries


FORMATION ON THERPRETATION OF SKID

Annex 1Background Information on the Measurement and Interpretation of Skid Resistance

ANNEX 1: BACKGROUND INMEASUREMENT AND INTERESISTANCEGeneral

A1.1 When a vehicle travels over a road, each part ofthe tyre in contact with the road surface is momentarilyat rest. The frictional forces generated at thesestationary contact areas between the tyre and the roadsurface can allow vehicles to be manoeuvred. However,a vehicle will start to skid whenever the availablefriction between the road surface and the tyre isinsufficient to meet the demands of the driver inwhatever manoeuvre (including braking) they areattempting to make.

A1.2 The friction available to a driver attempting aparticular manoeuvre depends on many differentfactors. The influence of road surface characteristics isdescribed below. Other factors include the vehicle’styres and braking system, the dynamic interaction of thevehicle suspension with the road geometry andenvironmental factors, such as the temperature and thepresence of water or other contaminants. The objectiveof measurements carried out under the operation of thisStandard is to characterise the influence of the roadsurface skid resistance and hence define the skidresistance available to road users.

Road Surface Properties

A1.3 The contribution of the road surface to theoverall friction is known as skid resistance. In practice,it is found that the skid resistance measured on dry, in-service road surfaces is generally high, but that lowerand more variable measurements are obtained when thesame road surfaces are wet or damp. For this reason,measurements of skid resistance for the purpose ofroutine condition monitoring are made on wetted roadsurfaces.

A1.4 The level of (wet road) skid resistance isdependent on two key properties of the surface, themicrotexture and the texture depth. The fine scalemicrotexture, on the surface of aggregate particles andprovided by the fines in the mixture, is the maincontributor to skid resistance at low speeds and themain property measured in wet skid resistance tests.Greater texture depth generates friction by physically

August 2004

deforming the tyre surface and also provides rapiddrainage routes between the tyre and road surface.

A1.5 The effects of microtexture and texture depthcombine to influence the skid resistance at higherspeeds. The standard SCRIM measurement is carriedout at a slip speed less than 20km/h, much lower thanthe slip speed in locked-wheel braking from normaltraffic speed. The typical reduction of skid resistancefrom the 20km/h value at higher speeds, and theinfluence of texture depth, is illustrated in Table A1.1.The effect of texture depth becomes apparent at speedsas low as 50 km/h, but is increasingly significant athigher speeds.

Speed Texture depth (mm SMTD)

Below 0.5 0.5 – 0.8 Above 0.8

50 km/h 40% 30% 25%

120 km/h 70% 60% 50%

Table A1.1 Typical Reduction in Skid ResistanceCompared with 20km/h Value

Effect of Traffic

A1.6 Under the action of traffic, the microtexturebecomes “polished”, leading to a reduction in skidresistance. HD 36 (DMRB 7.5.1) requires thecomponents of the surfacing mixture to satisfy certaincriteria in relation to their resistance to polishing, sothat surfacing materials generally provide adequate skidresistance during their service lifetimes.

A1.7 In combination with the specification ofsurfacing materials, the skid resistance of roads ismonitored to identify areas where the microtexture hasbeen lost as the surface has been polished by traffic andtreatment might, therefore, be needed to improve theskid resistance. This is necessary because theperformance in service cannot be predicted preciselyfrom the properties of the surfacing components andtraffic levels, and the effects of manoeuvring vehicles at

A1/1



the location might be greater than was anticipated at thetime the surfacing was designed.

A1.8 Similarly, the texture depth of road surfacingscan reduce with time under the combined influences oftraffic flow, temperature and the nature of the surfaceand is also monitored. The measurement of texturedepth is described in HD 29 (DMRB 7.3.2).

Early Life Skid Resistance of Asphalt Surfacings

A1.9 Asphalt surfacings exhibit different skidresistance properties in the initial period after laying,compared with the same surfacings that have beenexposed to traffic for a period of time. Thisphenomenon is not within the scope of thisdocument, which is concerned with in-servicecondition rather than the properties in the initialperiod.

A1.10 Sections of road that exhibit these different skidresistance properties during this the initial period, mustbe identified, so that they can be excluded from certaintypes of analyses, as described in Chapter 3 andAnnex 4 of this Standard. The duration of this initialphase will depend on local conditions but, for thepurpose of interpreting skid resistance measurements, itis assumed that the surface has reached an equilibriumstate one year after opening to traffic on trunk roads inthe UK.

Seasonal Variation of Skid Resistance

A1.11 After the initial period of wearing in, roadsurfaces reach an equilibrium state of polishing. Forroads where the traffic level is constant, the skidresistance will then fluctuate through seasonalweathering and polishing cycles but will remain atabout a constant level for many years. If the traffic levelsubsequently increases or decreases, the position of theequilibrium will shift so that a lower or higher overalllevel of skid resistance is observed, but with the sameseasonal fluctuation superimposed.

A1.12 An example of long-term variation in skidresistance is shown in Figure A1.1. A suggestedexplanation for the annual variation is that in the winter(October to March) when the roads are wet for much ofthe time, the detritus is mainly gritty so that the roadsurface becomes harsh and the skid resistance rises. Thelowest skid resistance is generally observed in thesummer period, when the roads are wet for a relativelyshort time, the detritus on them is mainly dusty so thatthe road surface becomes polished and the skid

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esistance falls. In practice, the minimum skidesistance varies from year to year and occurs duringifferent periods depending on the prevailing weatheronditions.

Figure A1.1 Example of Long-Term Variation inSkid Resistance (from LR 738)

1.13 Because the skid resistance varies continuously,arious strategies have been developed to provide aeasurement that characterises the state of polish of theicrotexture. Survey strategy and processing

rocedures are designed to reduce the effect of theariation within a year and/or between successive years,o the sites with low skid resistance can be identifiedore accurately. Typically, measurements are made

uring the summer period, when the lowest measuredalues are observed.

1.14 The survey and analysis methods to be used forhe purposes of this Standard are described inhapter 3, Annex 2 and Annex 3 of this Standard.

tandardised Measurements

1.15 To characterise the condition of theicrotexture, measurements of skid resistance are made

nder standardised conditions that restrict the influencef other factors on the measurement as far as possible.or example, measurements are made at a specifiedpeed, using a specified tyre and a controlled amount ofater. Corrections may be applied to the measuredalue where the specified standard conditions were notchieved, e.g. where it was not safe to maintain thepecified speed. Further details of test procedures areiven in Chapter 3.

1.16 The measurements made and interpretedccording to this Standard provide a guide to theeneral condition of the road to assist inaintenance planning. Because they indicate the

eneral level of skid resistance under standardised

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conditions, the values do not relate directly tospecific accident situations, where other factors suchas the tyre condition, vehicle speed and manoeuvreattempted all influence the level of frictiongenerated at that time.

A1.17 In contrast, skid tests carried out by the Policefor the purpose of reconstructing the situation leadingto an accident are intended to recreate the specificconditions of the accident. The results of these differenttypes of test cannot be compared precisely. Furtherdetails of Police skid tests are given in Annex 6.

Relationship to Accident Risk

A1.18 Within normal ranges, low skid resistance doesnot cause accidents on its own although, depending onthe particular circumstances, it may be a significantcontributory factor. The level of skid resistance, evenon a polished surface, will generally be adequate toachieve normal acceleration, deceleration and corneringmanoeuvres on sound surfaces that are wet but freefrom other contamination. However, higher skidresistance can allow manoeuvres that demand higherfriction to be completed, e.g. to stop quickly or cornersharply. Higher skid resistance can therefore reduceaccidents in cases where drivers need to complete amore demanding manoeuvre in order to avoid anaccident. A key part of this Standard is the judgement oflocations where this is more likely to occur, so that theprovision of higher levels of skid resistance can betargeted at these locations.

A1.19 Accident analyses have shown that there arerelationships between measured skid resistance andaccident risk. These relationships are not precise, in thatdifferences in skid resistance may account for only arelatively small part of the difference in accident riskbetween individual sites because of all the other factorsinvolved. Nevertheless, they have allowed generalobservations to be drawn that make it possible toprovide guidance for managing the provision of skidresistance on the network.

A1.20 The influence of skid resistance on accident riskis markedly different for roads with differentcharacteristics. For this reason, site categories havebeen defined to group roads with similar characteristics.

A1.21 For some site categories, no statisticallysignificant relationship, or only a weak relationship, isobserved between skid resistance and accident risk. Agood example of this is motorways, where the roaddesign has effectively reduced the potential for conflict

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etween road users. Although the skid resistance is stillmportant, because of the need to provide uniform roadharacteristics, the level of skid resistance can be lowerhan other categories.

1.22 For other site categories, progressively moreccidents are observed, on average, as the skidesistance falls. For these categories, there are benefitsn maintaining a higher level of skid resistance toontribute to reducing the number of accidents at theseites.

1.23 However, not all sites within a single categoryre equivalent in terms of their accident risk. Figure1.2 illustrates the range in accident risk present for

ndividual sites within a single site category. This ranges not surprising when the range of characteristicsresent within a single nominal site category isonsidered, e.g. in road design and traffic flow. Ithould also be noted that there is no boundary at whichhe skid resistance passes from being “safe” to beingdangerous”.

1.24 Judgement of the relative accident risk andppropriate level of skid resistance for different sitesithin the same category forms a key part of the

ffective operation of this Standard.

Figure A1.2 Accident Risk and Skid Resistance -Variation Within Site Category

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F OPERATIONAL

Annex 2SCRIM Survey of Operational Procedures

ANNEX 2: SCRIM SURVEY OPROCEDURES

A2.1 This Annex lists the standard testing proceduresthat are required to limit the variability of skidresistance measurements resulting from factors otherthan the road surface condition, e.g., the test speed.

Testing Season

A2.2 For standardised tests, measurementsshall be made during the testing season, definedas the summer period 1 May – 30 September.

A2.3 In exceptional circumstances the testing seasonmay be extended but only with the prior agreement ofthe Overseeing Organisation.

Testing Speed

A2.4 On Motorways and Dual Carriageway AllPurpose Trunk Roads where the posted speedlimit is greater than 50mph, the target testvehicle speed is 80km/h. On all other roads, orwhere a SCRIM is being used without dynamicvertical load measurement, the target testvehicle speed is 50km/h.

A2.5 The SCRIM driver shall maintain avehicle speed as close to the target test speed aspossible. This is achievable for most parts of thenetwork.

A2.6 If it is not safe or practical to maintain the targetspeed then, in exceptional circumstances, a differentspeed may be used at the discretion of the SCRIMdriver. The safety of the SCRIM and other roadusers has priority at all times.

A2.7 The Investigatory Levels for the CSC valuesdefined in Chapter 4 have been set in terms of the50km/h standard testing speed. The method forapplying speed corrections is given in Annex 3 of thisStandard.

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Testing Lane and Line

A2.8 For most roads, the leftmost lane will be tested inboth directions of travel. This lane usually carries themost heavy traffic and can, therefore, be expected toshow the lowest skid resistance. In areas where this isnot the case (for example, approaching points whereroutes diverge and a greater proportion of heavyvehicles uses the offside lane) then a different lane, ormore than one lane will be tested.

A2.9 The test lane shall be as specified by theOverseeing Organisation.

A2.10 Measurements shall be carried out withthe test wheel in the nearside (left) wheel pathof the running lane unless an alternative testline has been agreed with the OverseeingOrganisation.

A2.11 If it is necessary for the SCRIM todeviate from the test line (e.g. to avoid aphysical obstruction or surface contamination)the data shall be marked as invalid andeliminated from the standard analysisprocedure.

A2.12 On urban roads where roadside parking inunmarked positions is commonplace, there may be twopairs of wheel paths – one followed at times of daywhen parked cars are mostly absent and another whenthey are largely present. In such circumstances the linethat normally carries the most commercial vehicletraffic must be followed.

A2.13 In situations where the test line is prone tophysical obstruction, for example by parked cars, thenan alternative test line must be agreed with theOverseeing Organisation to avoid recording invalid datafor the same length of road in successive years. Testingthe offside wheel path might be an appropriatealternative in these circumstances, with the aim ofachieving a consistent test path, year on year.

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Annex 2SCRIM Survey of Operational Procedures

Testing on Bends

A2.14 There are no special requirements for testing onbends. At locations where a sharp bend is combinedwith traffic braking or accelerating, the wheel path onthe outside of the bend can become more polished thanthe inside wheel path. This is taken into account insetting the Investigatory Level (See Chapter 4) andduring the site investigation (See Paragraph A4.15).

Testing on Roundabouts

A2.15 Roundabouts can present practical problemsregarding potential traffic conflicts and testing speed.They range from small, mini-roundabouts to largegrade-separated interchanges. Larger roundabouts mayhave free-flowing traffic or traffic light controls atcertain times of day.

A2.16 Mini-roundabouts or small island roundaboutsshould be treated as part of the main carriageway testline and do not need to be tested separately. (Thisapplies to the testing procedure – the roundaboutsection may be assigned to a different Site Category tothat of the main line.)

A2.17 On most roundabouts or interchanges, a lineequivalent to the outermost lane should be tested.

A2.18 On roundabouts with lane markings for specificroutes, a representative line should be chosen broadlyfollowing the most polished path. Usually this will bethat followed by most heavy vehicles, which, becauseof their size, may not be able to keep to the markedlanes.

A2.19 The test line(s) to be followed atroundabouts shall be as agreed by the SCRIMoperator and the Overseeing Organisation.They will take into account the need forconsistency in representative measurements insuccessive surveys and possible variations inSite Category through the intersection.

A2.20 On some smaller roundabouts where the distancebetween the arms is short, it may be appropriate torecord data at a shorter interval than the standard 10mlength. Changes to the recording interval will bespecified by the Overseeing Organisation.

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mbient Conditions During Testing

2.21 The ambient conditions can have an effect both the skid resistance of the road and on theeasurements. The SCRIM operator shall record theeather conditions at the time of the survey as required BS7941-1 (1999).

2.22 Testing in extremely strong side winds must beoided because these can affect the measurements byeating turbulence under the vehicle that causes theater jet to be diverted from the correct line.

2.23 Testing must be avoided in heavy rainfall orhere there is standing water on the road surface.xcess water on the surface can affect the drag forces ate tyre/road interface and influence the measurements.

A2.24 Measurements shall not be undertakenwhere the air temperature is below 5°C.

2.25 The maintaining organisation shall maintain acord of general conditions throughout the testingason. Both the SCRIM operator and the maintainingganisation shall endeavour to record any roadnditions that could affect the results.

2.26 Contamination of the road surface by mud, oil,it, or other contaminants is to be noted and thefected measurements identified (in the same way asr out-of-line testing) so that results are eliminatedom the standard analysis procedure. If thentamination is severe, emergency action may bequired to remove the contamination. In this case theoblem must be reported without delay to theverseeing Organisation, together with the relevantsults if possible. The Overseeing Organisation willen notify the managing organisation for appropriatetion to be carried out.

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COMPUTATION OFOEFFICIENT

Annex 3Processing and Computation of Characteristic SCRIM Coefficient

ANNEX 3: PROCESSING ANDCHARACTERISTIC SCRIM C

A3.1 This Chapter lists the processing options thatmay be required by the Overseeing Organisation.Options that are required must be applied in the orderlisted below.

A3.2 On completion of the survey, thecorrected SCRIM Coefficient (SC) shall bedetermined for each 10m section for which avalid SCRIM Reading is available. Thecorrected SC shall be used to determine theCSC using one of the methods listed below.

Speed Correction

A3.3 The test speed has a significant effect on themeasurements of skid resistance.

A3.4 For the purpose of this Standard,measurements collected within the speed range25 to 85 km/h will be corrected to a speed of50km/h, using the following equation:

SC(50) = SC(s) + (s*2.18*10-3 - 0.109)

Where:

SC(50) is the SC corrected to 50km/h

SC(s) is the SC measured at the test speed, s.

Temperature Correction

A3.5 The temperature of the air or road can have asmall effect on the tyre rubber and the measurementsmade. It has been found that under normal UKconditions, the influence of temperature is not ofpractical significance in comparison with other factorsaffecting the measurements. Temperature correction isnot necessary for surveys carried out under theconditions set out in this Standard.

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ex of SFC

.6 The Index of SFC was originally introduced as ator to relate the values given by SCRIM to the SFCtained from the equipment at TRL during the period63-1972 used to derive information on which to baseposals for specification. From this revision of thendard, it serves as a general correction factor that

ll allow the Overseeing Organisation to maintain the at a consistent general level as future developments made to the equipment or monitoring techniques.

.7 The Index of SFC is defined by the Overseeingganisation. The value currently in force is 78 per cent78) and is applicable to all SCRIMs in current uset it may be amended in future, either for the wholeet or for individual SCRIMs.

A3.8 The SC value shall be multiplied by theIndex of SFC currently in force.

lculation of Characteristic SCRIM Coefficientlues

.9 As noted in Annex 1, the skid resistance of roadfaces can fluctuate within a year and betweencessive years, while maintaining a similar generalel over a long period of time. The basis of thisndard is that skid resistance will be assessed on theis of the overall (summer) level of skid resistance

her than an instantaneous measurement.

.10 By removing the effect of seasonal variation as as possible (both variation within a single year andween successive years) sites exhibiting a lower skidistance can be identified more accurately.

.11 The following paragraphs list different methodsproviding an estimate of the summer skid resistance,erred to as the CSC from the corrected SC values.e choice of which method to apply determines thevey strategy that will be necessary to obtain theuired data.

A3/1



Single Annual Survey Method

A3.12 This approach is based upon a single annualsurvey of the network. The method uses measurementsfrom the preceding 3 years to characterise the long-termskid resistance of the network. This value is used withthe mean network skid resistance in the current year, tocalculate a correction factor, which is applied to thecurrent year’s data to make current values consistentwith the long-term average.

A3.13 As the effect of seasonal variation will vary indifferent geographical areas (e.g. due to differentamounts of rainfall), larger networks will be split intosmaller localities and the correction factor will bedetermined and applied separately within each locality.

A3.14 The Single Annual Survey Method isimplemented as follows:

A3.15 The whole network shall be surveyedonce during the Testing Season in each year.Surveys must be planned such that in successiveyears each road length is tested in the early,middle and late parts of the season.

A3.16 The early middle and late parts of the season aredefined, respectively, as: May to mid-June, mid-June tomid-August and mid-August to the end of September.For example, a route tested in the early part of theseason in year 1 could be tested in the late part of theseason in year 2 and in the middle part of the season inyear 3. In year four, it must be tested in the early part ofthe season again, etc.

A3.17 Each site on the network shall beallocated to a locality by the OverseeingOrganisation.

A3.18 A locality is a collection of road sections orroutes for which a Local Equilibrium Correction Factorwill be determined. A locality must be small enough sothat similar weather conditions will normally beexperienced within it, and large enough so that a stablevalue can be calculated to represent the long-term skidresistance. This approach is based on the assumptionthat the climatic effects leading to seasonal variationinfluence all the roads in a local area in a similar way.

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3.19 The Local Equilibrium Correction FactorLECF) is the correction factor determined within eachocality to bring the current year data to a levelonsistent with the long-term average.

3.20 By surveying all road sections within a localityt the same time, this method can remove a componentf the within-year seasonal variation as well as theariation between years.

A3.21 All the road sections within each localityshall be surveyed within the same part of thetest season.

3.22 The LECF is calculated in three stages:

i) The Local Equilibrium SC (LESC) isdetermined to represent the average skidresistance level for the locality over recent years.The LESC is the average SC, calculated for allvalid 10m sub-section measurements in thedefined locality over the 3 years that precede thecurrent testing season. This must contain surveysfrom each of the three parts of the test season.Valid measurements are those that were made inthe required part of the test season, on therequired test line, on road surfaces that were atleast 12 months old at the time of testing.

ii) The Local Mean SC (LMSC) is determined forthe current survey. The LMSC is the average ofall valid 10m sub-sections in the locality in thecurrent year survey.

iii) The LECF is determined by dividing the LESCby the LMSC, i.e.:

LECF = LESC/LMSC

A3.23 The CSC for each 10m sub-section shallbe determined by multiplying the corrected SCby the LECF.

nnual Survey with Benchmark Sites Method

3.24 This alternative approach to the determination ofSC is not to be used in England but has been used in

he past and may be adopted by other Overseeingrganisations.

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A3.25 Historically, when the Investigatory Levels werebased upon MSSC measurements, this approachallowed the survey frequency to be reduced on lightlyused parts of the regional networks. It is based onmonitoring all of a selected network every year andusing the average of the Mean Summer SCRIMCoefficient (MSSC) for selected sites known as“Benchmark sites” spread around the area to indicateseasonal variation.

A3.26 The Annual Survey with Benchmark SitesMethod is implemented as follows:

A3.27 The Overseeing Organisation will agree anumber of Benchmark Sites to cover a relevantgeographical area.

A3.28 The Benchmark Sites shall all be testedthree times with surveys spread through eachtesting season to provide MSSC values for eachBenchmark Site and an overall average MSSCvalue for the area.

A3.29 The whole of the selected network shallbe tested once in each year. It is acceptable tosurvey different parts of the network indifferent parts of the testing season.

A3.30 Whenever a part of the network issurveyed, all the Benchmark Sites shall betested at the same time.

A3.31 The LECF is the correction factor determinedfor the network area to bring the current year data to alevel consistent with the long-term average.

A3.32 With this method it is assumed that the averagebehaviour of the Benchmark Sites is representative ofthe area and that the climatic effects leading to seasonalvariation between years will have influenced all of theBenchmark Sites in an area in a similar way. Bysurveying the benchmark sites three times each season,some account can be taken of the within-year variation.Comparing the sites in successive years allows theeffects of between-year variation to be reduced.

A3.33 The LECF is calculated in five stages:

(i) The Mean Summer Correction Factor (MSCF)is determined to take account of variation in skidresistance between the time of a particular surveyand the average during the testing season. TheMSCF is the overall average of all of the

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Benchmark Sites for the testing season, dividedby the average of all of the Benchmark Sites atthe time of the relevant survey.

i) The MSSC for each 10m section in the survey isestimated by multiplying the SC for each valid10m sub-section by the MSCF.

ii) The LESC is determined to represent the averageskid resistance level in the area over recent years.The LESC is the overall average MSSC for all ofthe Benchmark Sites over the three years thatprecede the current testing season.

v) The LMSC is determined to represent theaverage skid resistance level in the area for thecurrent testing season. The LMSC is the averageMSSC of all Benchmark Sites in the area for thecurrent testing season.

) The LECF is determined by dividing the LESCby the LMSC, i.e.:

LECF = LESC/LMSC

A3.34 The CSC for each 10m sub-section shallbe determined by multiplying the MSSC foreach 10m sub-section by the LECF.

3.35 Because MSSC is used to calculate therrection factors, determination of CSC using thisternative approach will not be possible until after thed of the testing season when the final Benchmark Sitervey has been completed.

ean Summer SCRIM Coefficient Method

3.36 This method uses the MSSC to represent theuilibrium summer level of skid resistance and thiskes the place of the CSC used in the Single Annualurvey method.

3.37 Although the MSSC method takes some accountf within-season variation, it has been found fromperience that the approach is potentially vulnerable to

ifferences between particular years or, with longerpeat cycles, to changes in skid resistance in thetervening period between surveys. Particularly hot oret summers, for example, could give rise to relativelyw or high MSSCs compared with the underlyinguilibrium value. In a “low-MSSC” year, smallanges could give rise to significant lengths of the

etwork requiring investigation and subsequent

A3/3



treatment that may not be necessary. Conversely, in a“high-MSSC” year, sites that should be investigatedmay be missed and not reviewed for another threeyears.

A3.38 Thus, relatively small changes in MSSC betweensurvey years have given rise to large fluctuations in thelengths of road being identified for treatment, withconsequent difficulties for maintenance planning. Also,with a longer repeat cycle, on some sites importantchanges to the skid resistance occurring in theintervening years between tests might pass undetected,with increased accident risk for a time.

A3.39 Using the MSSC method, the networkshall be surveyed three times in the same year,in the early, middle and late parts of the testingseason.

A3.40 The MSSC is determined for each 10m sectionby taking the average of the three SC values from thethree surveys. The MSSC averaged over the relevantsite should be used as the CSC value for comparisonwith Investigatory Levels.

A3.41 In areas where the MSSC method is used,dividing the network into two or three parts and testingthe parts over successive years can reduce theproportion of the network to be surveyed in any year.Thus, half the network is surveyed in alternate years orone-third of the network may be surveyed each year sothat the whole network is covered over a three-yearcycle.

A3.42 This method takes no account of variationbetween years.

Measurements Outside the Normal Testing Season

A3.43 Occasionally, SCRIM measurements may bemade outside the normal testing season. Although datafrom such measurements can be used for comparativepurposes by experienced personnel, such measurementsare subject to the full uncertainty of seasonal variationand do not form part of this Standard.

A3.44 Survey planning should allow for the possibilityof delays, for example, due to a machine breakdown orsevere wet weather, and allow for recovery within thedefined testing season. This also applies to tests madein the early or middle parts of the testing season usingthe main Single Annual Survey method to determineCSC.

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.45 Exceptionally, surveys may be completed up to end of the first week of October with the agreementthe Overseeing Organisation, provided that theeral weather conditions in the area remainparable to those experienced in September and that

frosts or treatments to the road such as gritting haveurred.

.46 Where the Annual Survey with Benchmark Sitesroach is being used, then if measurements are to bede in early October, the relevant Benchmark Sitesuld also be included. The SCs obtained should thenadjusted to a late-season equivalent value byltiplying them by a factor obtained by dividing therage for the benchmark sites in the late part of the

ndard season by the average for the benchmark sitearly October. The corrected results would then bed in the determination of MSSC as described inagraph A3.34.

her Types of SCRIM Survey

.47 Surveys with SCRIM are occasionally carried for special purposes such as research or for localestigations and may not be following one of thenitoring methods set out above.

.48 Such measurements are subject to the fullertainty of the factors affecting test procedures anduire careful interpretation. The data do not form partthis Standard.

.49 Measurements that are made outside the testingson are subject to the full uncertainty of seasonaliation. They do not form part of this Standard.vertheless, a site with low resistance to skiddingasured outside the testing season is likely to have aer resistance to skidding within the testing season.

.50 Although non-standard testing can be of some to experienced personnel who are fully aware of theitations, its main use is only to test roads on aparative basis.

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ION

Annex 4Site Investigation

ANNEX 4: SITE INVESTIGAT

A4.1 This Chapter describes the process of siteinvestigation and should be read in conjunction withAnnex 5 on the assessment of accident data.

Format of the Site Investigation

A4.2 Paragraphs A4.14 to A4.22 provide a list ofheadings under which a structured site investigationmay be carried out. Each heading contains a number ofitems for consideration. Answers are not required toindividual points, but a written assessment of sitecharacteristics must be made under appropriateheadings, taking into account relevant factors from theitems listed plus any other relevant points. Reference tosupporting documents or data should be made asappropriate.

A4.3 The site investigation must include a review ofthe Investigatory Level(s) assigned. If a change isrequired then it should be reported as required by theOverseeing Organisation.

A4.4 The review of the Investigatory Level shouldinclude a check of whether the current criteria forsetting the Investigatory Level have been followed (seeChapter 4). In the event of treatment being requiredthen the Investigatory Level will be needed todetermine the appropriate specification for treatment. If,additionally, issues are identified during the siteinvestigation that have implications for reviewing thecriteria for setting the Investigatory Levels, then thesemust be noted so that they can be acted upon at anappropriate stage.

A4.5 The level of detail appropriate for theinvestigation will depend on the nature of the site andthe time since a detailed investigation was last carriedout. Many of the points listed are only relevant to morecomplex sites. If the site has been investigated recentlythen it will only be necessary to identify the changesthat have occurred since the last investigation.

A4.6 The level of detail used should also beappropriate to the likely cost of the treatment and theamount of disruption likely to be caused to road users.A relatively cheap treatment, that has potential toreduce accident risk and where the works cause littledisruption or added risk to road users, will not warrantan extensive investigation.

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.7 Reviewing the nature of the site and condition of surfacing is an important component of the siteestigation. This can be achieved through a

mbination of the following methods, but it isommended that a physical visit to the site is alwaysde unless the site is a motorway or dual carriageway

on-event” category:

On foot (allows the condition of the road to beobserved in detail but has associated safety risksthat must be controlled).

From a parked or moving vehicle (allows patternof traffic movement and speed to be observedduring the visit).

From recent local knowledge of the site (mayprovide a more general knowledge of the roadusage under a wider range of traffic, weather andlighting conditions).

From video records and maps.

commendation

.8 As a result of the investigation, a clearommendation must be recorded of the actions to been (including if no immediate action is required).

.9 Treatment should be recommended if, taking intocount the nature of the site and the observed accidenttory, it appears that improving the skid resistance of surfacing, or improving the surface condition iner respects is likely to reduce the risk of skidding

cidents.

.10 If the site investigation identified anyaracteristic of the site or road user behaviour thatggests other road safety engineering measures could appropriate, then the appropriate people must betified so that the site is considered for safetyprovements.

.11 If the site investigation identifies requirementsr additional routine highway maintenance, such aseeping, re-application of markings etc thenpropriate action must be taken.

.12 If there is no justification for treatment then norther action is required other than to review the site

after a period of time. If the site remains below the

A4/1



Investigatory Level after the next measurement of skidresistance then it will automatically become subject to afurther investigation.

A4.13 If the skid resistance and accident pattern havebeen stable for more than 3 years then lowering theInvestigatory Level should be considered (see alsoparagraph A4.3).

Content of Site Investigation

A4.14 Site location and use:

• What is the location and nature of the site?

• Are there any features that could be expected torequire road users to be able to stop ormanoeuvre to avoid an accident? For example,junctions, lay-bys, other accesses, crossings,bends or steep gradients.

• What are the site category and the currentInvestigatory Level? Has there been anysubstantial change in the amount or type oftraffic using the road that would influence therequirement for skid resistance and could requirethe Investigatory Level to be changed?

A4.15 Pavement condition data:

• What is the CSC, by how much is it below theInvestigatory Level and over what length? Is theskid resistance uniform along the site or are thereareas of lower skid resistance or large changes inskid resistance? Is the lowest skid resistance inlocations where road users have a specific needto stop or manoeuvre? (The risk of accidentsgenerally increases as the skid resistance falls,but the increase in risk will be greater for siteswhere the road user is likely to need to stopquickly or manoeuvre.)

• Are there any individual 10m lengths that fallsignificantly below the mean for an averaginglength, and is the location of such lengthssignificant, e.g. a short length of low skidresistance within a sharp curve.

• Does the site contain a sharp bend to the left incombination with traffic braking or accelerating,e.g. a sharply curved roundabout approach orexit? In these circumstances the offside wheelpath can become more polished than the nearsidewheel path and the skid resistance in the offsidewheel path can be up to 0.05 units CSC lower

•

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than that measured in the nearside wheel path.However, this does not mean the skid resistanceis more than 0.05 units CSC below theInvestigatory Level, because the InvestigatoryLevel will have been raised in the vicinity of thecurve to compensate for this effect (Chapter 4).

What is the texture depth and do areas of lowtexture depth (below 0.8mm SMTD) coincide withareas of low skid resistance?

Are there any extreme values of rut depth orlongitudinal profile variance that could affectvehicle handling or drainage of water from thecarriageway?

4.16 Accident history:

A methodology for analysing the accident historyis given in Annex 5.

4.17 Site inspection:

Has a visit to the site been carried out? If so,then what range of weather and traffic conditionshas been observed and over what period? If not,then what other information has been drawnupon?

4.18 Visual assessment:

Is a visual inspection of surface conditionconsistent with the available survey data?

Skid resistance and texture depth are generallymeasured in the nearside wheel track in lane one.Is the rest of the area of the maintained pavementsurface visually consistent with the measuredpath, or are there any localised areas of polishedsurfacing, low texture depth, patching or areasotherwise likely to give rise to uneven skidresistance? If it is likely that the skid resistanceof other lanes could be lower than the lane testedthen additional surveys may need to be carriedout to investigate this. This could occur, eg. If thesurface in other lanes (including the hardshoulder) is different to the lane tested, and theselanes carry a similar volume of heavy traffic tothe lane tested.

If so, is the location such that the lack ofuniformity is likely to increase the risk ofaccidents occurring?

August 2004



• Is the area of the maintained pavement surfacefree from debris and other sources ofcontamination? Is water known to drainadequately from the carriageway during heavyrain? Is the pavement free of other defects suchas potholes?

A4.19 Road users:

• What is the volume and type of traffic, includingvulnerable road users? Are observed trafficspeeds appropriate to the nature of the site? Ifthere is significant variation in the speed, type orvolume of traffic during the day, haveobservations been made in an appropriate rangeof traffic conditions? What types of manoeuvresare made and what are the consequences if notcompleted successfully, e.g. head-on or sideimpact at speed are likely to have severeconsequences? Is there any evidence that roadusers consistently fail to negotiate the sitesuccessfully, such as tyre tracks into the verge?

A4.20 Road layout:

• Is the road design still appropriate for the speedand volume of traffic? Is the layout unusual orlikely to be confusing to road users?

• Is the carriageway particularly narrow and is ahard shoulder or 1 metre strip provided? Is theroad layout appropriate for the number and typeof vulnerable road users (pedestrians, cyclists,motorcyclists, equestrians, bus and tram users)?

• Are junction sizes appropriate for all vehiclemovements? Are right turning vehiclesadequately catered for? Are priorities atjunctions clearly defined? Are traffic signalsoperating correctly and are they clearly visible toapproaching motorists?

A4.21 Markings, signs and visibility:

• Are all pavement markings, warning anddirection signs appropriate and effective in allconditions (e.g. day, night, fog, rain, on colouredpavement surface)? Have old pavement markingsbeen removed properly? Are there any redundantsigns that could cause confusion? Are signs orother roadside objects on high-speed roadsadequately protected from vehicle impact?

August 2004

• Is visibility adequate for drivers to perceive thecorrect path? Do sight lines appear to beadequate at and through junctions and fromminor roads or other accesses? Is the end oflikely vehicle queues visible to motorists? Doeslandscaping, taking into account future growth ofvegetation and the effects of wind and rain,reduce the visibility, including visibility of signs?

A4.22 Additional information:

• Are any other sources of information available,such as reports or visual evidence of damageonly accidents, incidental damage to streetfurniture or reports from the Police? Suchreports are likely to be subjective but are relevantif the reliability of the information is borne out byobservations of the site.

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ACCIDENT DATA

Annex 5Assessment of Accident Data

ANNEX 5: ASSESSMENT OF

A5.1 The most recent available records of personalinjury accidents recorded on the STATS19 databaseover at least a three-year period should be used in theanalysis. The location of accidents should be plotted,identifying accidents that occurred in the wet, or thatinvolved skidding.

A5.2 The following aspects should also be considered:

• Is the location of accidents significant in relationto the observed pattern of skid resistance?

• Is there any other pattern apparent in the locationor type of accidents that would warrant moredetailed analysis or consultation with the personresponsible for accident investigation?

• Have there been any significant changes to thesite or the traffic using it in the analysis period,that could have affected the number of accidents?

A5.3 The number of accidents occurring within aparticular site will be related to the length of the siteand the amount of traffic as well as the length of timeconsidered. This means it is necessary to consider boththe number of accidents and the accident rates, whichcan be calculated given the site length and an estimateof the annual average daily two-way flow.

A5.4 The number of accidents, the average number peryear and the accident rates and severity ratio should betabulated:

Accidents/100km = (Average number ofaccidents per year / site length in km) * 100km

Accidents/108veh-km = Average number ofaccidents per year / (site length in km * vehiclesper day * 365 / 108)

Accident severity ratio = Accidents where acasualty was killed or seriously injured / totalinjury accidents

A5.5 The sites most likely to benefit from treatmentare those where the accident rates observed are greaterthan normal for the type of site. However, if the totalnumber of accidents is rather small, then the sameresource may be more effectively used at other sites.

August 2004

A5.6 If a greater than normal proportion of accidentsoccur in wet conditions or involve skidding, thisprovides an indication that increasing the skidresistance may reduce the accident risk. The followingpercentages should be calculated and tabulated:

%Wet = (Number of accidents that occurred inwet conditions / total accidents) * 100

%Skid = (Number of accidents where skiddingwas reported / total accidents) * 100

%Wet skid = (Number of accidents that occurredin wet conditions where skidding was reported /number of accidents that occurred in wetconditions) * 100

A5.7 Here, “wet” is where the road surface wasrecorded as wet/damp or flood in the STATS19 accidentrecord and “skid” is where any of the skidded, jack-knifed and overturned options was recorded in thevehicle record for any of the vehicles involved in theaccident.

A5.8 However, these percentages do not provide agood indication of the influence of the surfacing incases where the total number of accidents is rathersmall, when a small change in the number of wet orskidding accidents results in a substantial change in thepercentages calculated.

A5.9 Control data are necessary to be able to assesswhat the “normal” accident risk is. For trunk roads inEngland, control data for the whole trunk road networkand for the route that the site is a part of can beobtained from the appropriate HA Route Manager.These will provide values for the parameters above,broken down into Motorways, built-up A-roads andnon-built-up A-roads. Non-built-up A-roads are furtherdivided into dual carriageways and single carriageways.The most appropriate category should be selected andboth the national and the route values should becompared with the values for the site underinvestigation, for example, by using Table A5.1.

A5/1


Annex 5Assessment of Accident Data

A5.10 The control data will help to indicate whetherthe accident risk at the site is greater than the route orthe national average. This should only be taken as aguide because the route and, particularly, the nationaldata will include different types of site, with differentrisk factors to the site being investigated. If available,accident data from other, similar sites (e.g. with thesame site category as defined in Table 4.1) would alsoprovide a useful comparison. However, there will berelatively few accidents recorded at individual sites,including the site being investigated, and this numberwill be subject to random fluctuation. If there have beenchanges during the analysis period that could haveinfluenced the past accident history, for example, amarked change in traffic, road works or major events,these should be listed with the dates and accidentsaffected and taken into account in the analysis.

Site data Control data

Route National Similar sites

Number of accidents in analysis period

Accidents/year

Accidents/100km

Accidents/108vehicle-km

%Wet

%Skid

%Wet skid

Table A5.1 Example Table for Comparison of Site Data and Control Data

August 2004A5/2


T TYPES OF TEST IN

Annex 6Use of Different Types of Test in Accident Investigation

ANNEX 6: USE OF DIFFERENACCIDENT INVESTIGATION

General

A6.1 Road accidents usually result from a combinationof several factors and these can be difficult to isolateand identify.

A6.2 When investigating accidents, the police areusually attempting to determine the causes of theaccident and to obtain evidence that will assist them inaccident reconstruction and any proceedings againstindividuals that are subsequently deemed necessary. Inthis regard they are particularly interested in thebehaviour of the vehicles involved in the accident. Thespeed of a vehicle, the condition of its brakes,suspension and tyres and the driver’s response willaffect its ability to avoid a collision. The highwayauthority’s objectives will be different, as they will beseeking to determine the extent to which the roadlayout, construction and general surface condition,including its skid resistance, are contributing toaccident risk and whether some form of remedial actionshould be considered.

Friction and Skid Resistance

A6.3 For a vehicle to complete a particular manoeuvresafely there must be adequate friction available betweenthe tyre and the road surface at the particular time. Thelevel of friction required will be influenced by thenature of the manoeuvre and the site layout.

A6.4 The coefficient of friction is the ratio of the forceresisting motion to the vertical force, and strictly refersto a particular tyre on a particular surface at a particulartime. For most vehicles, an average coefficient offriction is normally assumed for all tyres on a vehicleunless there is particular evidence to do otherwise.

A6.5 Skid resistance is a measure of the contributionthat the road provides to tyre/road friction made understandardised conditions. It provides an indication of thegeneral condition of the road but does not relatespecifically to the coefficient of friction that a road userwill experience with particular tyres and the road at aparticular time.

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August 2004

6.6 The friction available to a manoeuvring vehicleill depend upon the underlying skid resistance of the

oad surface, which should be appropriate for the typef site, but it will also be affected by seasonal factorsnd specific conditions that apply at the material time,uch as the weather e.g. dry, wet or icy, and theresence of other contaminants on the surface such asud or oil.

he Vehicle and the Police Skid Tests

6.7 Before attempting to reconstruct an accident,olice collision investigators usually measure the tyre/oad coefficient of friction in conditions that are aslose as possible to those prevailing at the materialime.

6.8 When a vehicle has been involved in an accidentnd has left skid marks which indicate that one or moref its wheels were locked, an estimate of its likelypeed can be made from the length of the skid marksnd the coefficient of friction, which is directly relatedo the deceleration rate.

6.9 In the police tests, the “locked wheel brakingoefficient” of a vehicle at the accident site is derivedy applying the brakes of a test vehicle quickly, so thathe wheels lock rapidly, and measuring either theistance or the average rate of deceleration that it takeso stop from a known speed. The initial test speed isormally 50 km/h. The vehicle used in the test shoulddeally be the one involved in the accident (or a similarne in terms of model etc) but may often be a policeehicle.

6.10 Stopping distances are indicated using a chalkun that fires at the road when the brakes are firstpplied. Deceleration is measured using specialroprietary equipment that incorporates accelerometersnd is mounted in the test vehicle.

6.11 In dry tests, the average coefficient of friction isssumed to apply at any speed (it is not normally speedependent) and so can be used in calculating vehicles’peeds from stopping distances measured from the skid

marks left at the accident site.

A6/1



A6.12 In wet tests the average value recordedrepresents a range of values of friction which willincrease as the skidding vehicle slows down. In thiscontext the results are unsuitable for estimating speedsbut the average value may be used to provide acomparison with wet roads in general to assist ininterpreting other evidence obtained at an accidentscene.

The Road Surface and Skid Resistance Tests

A6.13 A number of factors affect the skid resistance ofthe road surface and these are fully described inAnnex 1. In summary, research has shown that in dryconditions all clean surfaced roads have a high skidresistance but their performance varies greatly in thewet.

A6.14 On wet surfaces ‘slipperiness’ is caused by thelubricating effect of the water film which becomesmore effective as vehicle speeds increase, so that skidresistance generally falls as vehicle travel faster. Therisk of an accident involving skidding increasessignificantly when roads are wet.

A6.15 On some newly laid asphalt surfaces, thebitumen layer coating the aggregate at the surface canaffect both wet and dry skid resistance. More details aregiven in HD 36 (DMRB 7.5.1).

A6.16 The wet skid resistance of a road surface isroutinely measured on trunk roads using SCRIM.SCRIM has a test wheel fitted with a smooth tyre whichis mounted mid-machine in line with the nearside wheeltrack and angled at 20 degrees to the direction of travel.A controlled jet of water wets the road surfaceimmediately in front of the test wheel. As the vehiclemoves forward the test wheel, whilst freely rotating inits own plane, scuffs in a forward direction on the wetroad surface generating a sideway-force. The ratio ofthis force to the vertical force between the test wheeland the road gives a measure of the skid resistance thatcan then be recorded continuously along the road.

A6.17 Because skid resistance varies with speed,SCRIM tests are made at standardised operating speeds,normally about 50km/h or 80km/h depending on thetype of road. All test results are corrected to anequivalent standard speed of 50km/h before comparingwith Investigatory Levels.

A6.18 Because the test wheel is set at an angle to thedirection of travel, the effective slip speed of the test

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tyre is much slower than the vehicle operating speed.

A6/2

measurement is therefore one of low-speed skidstance: at a 50km/h standard test speed, the sliped is approximately 17km/h.

.19 Extensive research has established relationshipsween wet skid resistance as measured by SCRIM accident risk for different types of site. Table 4.1s investigatory skid resistance bands for differentgories of site. Roads are routinely monitored with

RIM to detect any sections where skid resistancey have fallen to a level where skidding accident riskthat type of site may increase; such sites areestigated and if necessary remedial works are put ind.

.20 Some UK Highway Authorities may use other devices to measure wet skid resistance in particularumstances. As with SCRIM tests, these tests arended to investigate the general condition of the road

face and not specific friction conditions. The devicesrate on different principles to SCRIM, such as usingine locked-wheel friction for spot checks or an in- fixed-slip technique in which the test wheel ised to rotate at a slower speed than the vehicle speed so skids over the surface. The different principles slip speeds mean that different values are obtained they cannot necessarily be compared with SCRIMfficient values.

.21 Another device used to assess skid resistance ast of an accident investigation is the pendulum tester, EN 13036-4:2003. Like the other tests, results from pendulum tester do not relate directly to the actualfficient of friction experienced by a particular tyre particular situation. This device measures skidstance on a small length of road (approximatelymm) and therefore positioning and frequency ofpling are critical. It was originally designed toulate a patterned tyre skidding at 50km/h but withnges to tyre compounds, construction and treadterns, this can no longer be safely assumed. SCRIMasures skidding resistance using a smooth tyre andrefore the surface texture will influence thetionship between SCRIM measurements anddulum Test Values (PTVs). The portable tester is therefore recommended for use on fine texturedfaces and results on coarse textured surfaces can beleading because of operational difficulties.

evance of the Tests

.22 When considering these two general types of, it is important to understand that:

August 2004



• Police skid tests are carried out in differingconditions and are used at accident sites to assistin accident reconstruction. They are frequentlymade in dry conditions. The measurements arenot suitable for assessing whether a roadsurface is substandard or in need of remedialtreatment. However, if a dry skid test indicates alower than expected dry road skid resistance, thisshould be drawn to the attention of the highwayauthority so that the cause can be investigated;

• the SCRIM test is a standardised wet road testand is used for routine monitoring of the skidresistance of the nearside wheel track of the roadsurface to assess if maintenance is required. It isnot specifically carried out after an accident. Theresults of SCRIM tests may be used to assesswhether the road surface might have been acontributory factor where it is known that avehicle skidded on a wet road, but even if thiswere considered likely, it would not necessarilyimply that the road condition was sub-standard;

• results given by the SCRIM test and the policetests can appear similar. SCRIM Coefficients,which represent friction, have a typical range of0.30 to 0.60. The Police skid test produces valuesthat may look numerically similar but becausethey are from quite different measurements theyare not related.

A6.23 Both groups of test are valid only for theparticular purposes for which they were derived.

August 2004 A6/3

volume 7 pavement design and maintenance section 3 pavement maintenance assessment

part 2

Hd 29/08

data for pavement assessment

summarY

This Standard describes the data required for pavement assessment and the data collection methods that are currently approved by the Overseeing Organisations. These methods cover measurement of the construction and condition of different types of pavements, except skidding resistance which is covered in HD 28 (DMRB 7.3.1). This revision has been updated to reflect current practice and includes requirements previously issued in IAN 42/05.

instructions for use

1. Remove Contents pages from Volume 7 and insert new Contents pages for Volume 7 dated May 2008.

2. Remove HD 29/99 from Volume 7, Section 3 which is superseded by this Standard and archive as appropriate.



Note: A quarterly index with a full set of Volume Contents Pages is available separately from The Stationery Office Ltd.

design manual for roads and Bridges

may 2008

design manual for roads and Bridges Hd 29/08 volume 7, section 3, part 2

tHe HigHwaYs agencY

scottisH government

welsH assemBlY government llYwodraetH cYnulliad cYmru

tHe department for regional development nortHern ireland

Data for Pavement Assessment

Summary: This Standard describes the data required for pavement assessment and the data collection methods that are currently approved by the Overseeing Organisations. These methods cover measurement of the construction and condition of different types of pavements, except skidding resistance which is coveredinHD28(DMRB7.3.1).Thisrevisionhasbeenupdatedtoreflectcurrent practice and includes requirements previously issued in IAN 42/05.

may 2008

volume 7 section 3 part 2 Hd 29/08

registration of amendments

amend no

page no signature & date of incorporation of

amendments

amend no page no signature & date of incorporation of

amendments



part 2

Hd 29/08

data for pavement assessment

contents

Chapter

1. Introduction

2. Traffic Speed Condition Surveys

3. Visual Condition Surveys

4. Deflectograph Testing

5. Falling Weight Deflectometer

6. Ground-Penetrating Radar (GPR)

7. Coring and Test Pits


9. Enquiries

Annexes


2. TRACS

A. Assessment Criteria B. Detailed Description of TRACS Condition Data


4. Deflectograph

A. Calibration B. Data Processing using PANDEF

5. Falling Weight Deflectometer (FWD)

A. Requirements for Consistency Checks B. FWD Surface Modulus and Reporting Template

6. Ground-Penetrating Radar (GPR)

A. Introduction to GPR B. Calibration of GPR for Determination of Layer Thickness C. Reporting the Results of a GPR Survey


may 2008


chapter 1 introduction

1. introduction

mandatory sections

1.1 Sections of this document which form part of the Standards of the Overseeing Organisations are highlighted by being contained in boxes. These are the sections with which the Design Organisations must comply, or must have agreed a suitable alternative approach through a departure from Standard with the relevant Overseeing Organisation. The remainder of the document contains advice and enlargement which is commended to Design Organisations for their consideration.

scope

1.2 This Part describes the data required for pavement assessment and the data collection methods that are currently approved by the Overseeing Organisations. These methods cover measurement of the construction and condition of different types of pavements, except skidding resistance which is covered in HD 28 (DMRB 7.3.1). Guidance is also given on the processing of data obtained by the methods (where appropriate). Interpretation of the results is generally covered in HD 30 (DMRB 7.3.3) which also describes the use of each assessment method in the context of the overall pavement monitoring and assessment process. Advice on the interpretation of TRACS data (see Chapter 2) is given in this Part. The list of methods is not exhaustive and is not intended to exclude the use of other machines and methods. However, those which are presently part of the Overseeing Organisations’ standard assessment procedure are all included.

implementation

1.3 This Part must be used forthwith on all schemes for the improvement and maintenance of trunk roads including motorways currently being prepared, provided that, in the opinion of the Overseeing Organisation this would not result in significant additional expense or delay. Design organisations must confirm its application to particular schemes with the Overseeing Organisation.

u

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se in northern ireland

1.4 For use in Northern Ireland, this Standard will apply to those roads designated by the Overseeing Organisation.

utual recognition

1.5 The construction and maintenance of highway pavements will normally be carried out under contracts incorporating the Overseeing Organisations’ Specification for Highway Works (SHW) which is contained in the Manual of Contract Documents for Highway Works Volume 1 (MCHW 1). In such cases products conforming to equivalent standards and specification of other Member States (MS) of the European Economic Area (EEA) or a State which is party to a relevant agreement with the European Union and tests undertaken in other MS of the EEA or a State which is party to a relevant agreement with the European Union will be acceptable in accordance with the terms of Clauses 104 and 105 (MCHW 1.100). Any contract not containing these Clauses must contain suitable clauses of mutual recognition having the same effect, regarding which advice must be sought from the Overseeing Organisation.

ealth and safety

1.6 All surveys and data collection on or in the vicinity of highway pavements must be carried out in accordance with:

• Health and Safety at Work Act (1974);

• Management of Health and Safety at Work Regulations (1999);

• Construction (Design and Management) Regulations (2007) (CDM Regulations);

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• Traffic Signs Manual Chapter 8 (2006); and

• Safety at Street Works and Road Works – A Code of Practice.

1.7 In Northern Ireland, the relevant Health and Safety documents are:

• Construction (Design and Management) Regulations (Northern Ireland) 2007;

• Health and Safety at Work (Northern Ireland) Order 1978;




1.8 For the Highways Agency network, further information on Health and Safety is given in Part 1 of the Network Management Manual.

1.9 Where data collection involves excavating or driving probes into the subgrade at locations where there may be buried services, the public utility organisations must be contacted for details of the locations of their equipment. The exact location of buried services should be established prior to carrying out the work using cable locating equipment.

glossary

1.10 A glossary and list of principal abbeviations is given in HD 23 (DMRB 7.1.1).

1.11 The term “asphalt” replaces “bituminous material” as the generic term for pavement material consising of mineral aggregate combined with a bitumen binder and which is normally laid by a paver. “Asphalt” includes all bitumen bound base, binder course and surface course materials, except surface dressing. There are some exceptions to this as indicatedbelow, where “bituminous material” continues in use:

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• Deflectograph processing uses the technical terms “Equivalent Thickness of Sound Bituminous Material (ESBM)”, “Total Thickness of Bituminous Material (TTBM)” and PANDEF Base Type Classification “BITS”. Use of the term “asphalt” would require changes to these acronyms and to text in the associated software.

• In some instances the term “bituminous materials” may also refer to surface dressings as well as paver laid material.

1.12 The term “Hydraulically Bound Mixture” or “HBM” is used as the generic term for pavement material consisting of mineral aggregate bound with cement, lime, slag or fly ash binder, or a combination thereof. The terms “lean concrete”, “cement bound material” or “CBM” are no longer used except in connection with PANDEF Base Type Classification “CEMT”.

may 2008


ion surveYs

chapter 2 Traffic Speed Condition Surveys

2. traffic speed condit

types of surveys

2.1 On the trunk road network in England, surveys carried out under the Highways Agency (HA) TRAffic-speed Condition Survey (TRACS) contract replaced the previous High-speed Road Monitor (HRM) surveys in the summer of 2000. TRACS data is collected under a central HA contract which includes the loading of data to HAPMS for subsequent use by Agents and others.

2.2 On the National Network in Scotland, designated roads in Northern Ireland and on the Local Authority Road Network in England, traffic-speed surveys are carried out using the Surface Condition Assessment of the National NEtwork of Roads (SCANNER) system. On the National Network in Wales a method similar to SCANNER is used.

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figure 2.1: tra

2.5 TRACS surveys are carried out annually usually over:

a) both lanes of single carriageways;

b) lanes 1 and 2 of the main carriageway on dual carriageways; and

c) lane 1 of slip roads.

Roundabouts are excluded from TRACS surveys. For precise details of the coverage and frequency of these surveys the HA should be consulted.

2sQi

2

•

•

may 2008

.3 The SCANNER survey details are given in the ve-volume User Guide and Specifications published by

he UK Roads Board (2007). A summary of the features f SCANNER surveys is given below, following the ext relating to TRACS surveys.

raffic-Speed Condition Survey (TRACS) (Relevant o the HA in England)

.4 TRACS surveys are carried out using survey ehicles equipped with lasers, video image collection nd inertia measurement apparatus to enable surveys f the road surface condition to be carried out whilst ravelling at variable speeds, of up to 100 km/h, to atch prevailing traffic, and hence cause minimum

isruption to other road users.

cs vehicle

.6 TRACS surveys are controlled by an end result pecification for the survey equipment and a detailed uality Audit procedure for the surveys includes regular

ndependent checks to maintain quality assurance.

.7 The TRACS survey vehicle measures:

Texture Profile in the nearside wheel-track at approximately 1mm longitudinal intervals;

Transverse Profile across a 3.2m width at approximately 0.15m longitudinal intervals;

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• Longitudinal Profile in the nearside wheel-track at approximately 0.1m longitudinal intervals;

• cracking over a width of 3.2m (continuous monitoring);

• vehicle geographical position (Northing, Easting and altitude) as well as Road Geometry (gradient, crossfall and curvature) at discrete 5m intervals;

• a forward-facing video record of the road being surveyed;

• a downward-facing video record of the road being surveyed.

2.8 All data collected by the TRACS survey vehicle is referenced to the network sections to a longitudinal accuracy of ± 1m. The start and end points of sections are defined by Location Reference Points (LRPs) in Highways Agency Pavement Management System (HAPMS) referencing as described in the Network Management Manual (NMM).

2.9 The TRACS survey data is delivered in raw form as TRACS Raw Condition Data (RCD). The Highways Agency’s Machine Survey Pre-processor (MSP) software is used to process the RCD to generate the TRACS Base Condition Data (BCD), which contains:

• Rut Depths in the nearside and offside wheel-tracks calculated from the measured Transverse Profile, over 10m lengths;

• 3m, 10m and 30m Enhanced Longitudinal Profile Variance calculated from the measured Longitudinal Profile, averaged over 10m lengths for the nearside wheel-track only;

• Intensity of Cracking calculated from the crack map, over 10m lengths;

• Intensity of Wheel-track Cracking calculated from the crack map, over 10m lengths;

• Sensor Measured Texture Depth (SMTD), calculated from the measured Texture Profile, averaged over 10m lengths;

• crack map (refer to Annex 2B, clause 2B.17 for an explanation);

• an estimate of the level of Fretting present on the pavement, calculated from the measured Texture Profile, over 10m lengths;

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• an estimate of the Surface Type, over 10m lengths;

• the road Geometry (gradient, crossfall and curvature), over 10m lengths;

• an estimate of the retroreflectivity of the road markings, over 100m lengths.

2.10 The TRACS BCD is loaded into HAPMS. HAPMS provides maintenance engineers with access to the condition data collected from their network, and enables them to identify potential maintenance schemes and to monitor network performance. The TRACS BCD can be queried in HAPMS and reported using the database facilities, and can be displayed against a map background. HAPMS also provides reports and information in support of the development of the Road Renewals maintenance programme. The on-line facilities in HAPMS provide a fuller guide to the presentation of project information.

2.11 TRACS survey data is reported within HAPMS in relation to the Sections defined for network referencing, but there may be gaps in the data. These gaps arise for a variety of reasons (e.g. where the survey vehicle drove out of lane due to obstructions or road works, or where the survey data has been identified as Unreliable).

2.12 From an examination of the TRACS condition data in HAPMS, lengths of road with deteriorating surface condition can be identified. Examples of the use of TRACS condition data include:

• Rut Depths can be used to evaluate safety and structural aspects of the pavement surface condition;

• the Longitudinal Profile Variance can be used to assess Ride Quality;

• Texture Depth data can be used to indicate a potential loss of skid resistance, in connection with SCRIM data, or provide warning of some modes of surface failure;

• the Cracking and Fretting data, together with the Surface Type data can be used to evaluate the condition of the surface.

assessment of road condition using tracs data

2.13 An overview of the condition, or the trend in condition, of road pavements is required to give an indication of the scale of possible maintenance

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requirements and to identify changes in the general level of service provided over a period of time. At a more detailed level, lengths of road requiring further investigation need to be identified and prioritised.

2.14 The results of an analysis of the TRACS survey data may be used as a coarse sift to identify lengths of road in need of further investigation or to supplement other road condition data to provide a robust road maintenance proposal. To undertake the assessment, the TRACS condition data (with the exception of TRACS Surface Type) must be obtained from HAPMS using the

category Definition

1 Sound – no visible deterioration.

2 Some deterioration – lower level of concer(project level) investigations are not needeparameters are at this category at isolated

3 Moderate deterioration – warning level of needs to be investigated. Priorities for morextent and values of the condition paramet

4 Severe deterioration – intervention level ofrequently on the motorway and all purposprevented this state from being reached. Ainvestigations should be carried out on theaction taken if, and as, appropriate.

table 2.1: condition categories for text

2.16 For any 100m length in Condition Category 4, a more detailed investigation should be carried out at the earliest opportunity and the need for maintenance assessed. Similarly, where two or more 100m lengths in Condition Category 3 fall within any 1km, the cause of the deterioration should be investigated to determine if maintenance or other actions are necessary. Priority for treatment will depend on the type, extent, distribution and severity of deterioration and the strategic objectives for road maintenance.

2.17 It is not appropriate to apply the classification system defined in Table 2.1 for the assessment of Cracking and Fretting. Annex 2A of this Part gives Guidance Levels which may be used in the interpretation of the values of these parameters reported in HAPMS.

2.18 When carrying out the assessment it is recommended that reference be made to the detailed descriptions and further guidance concerning the

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most recent TRACS Combined Length Weighted (LW) Averages data source. The TRACS Surface Type may be obtained using the HAPMS TRACS – Base Surface Type data source.

2.15 For Texture Depth, Rut Depth and Ride Quality, the TRACS survey data can be assessed by means of four Condition Categories, as shown in Table 2.1. These Condition Categories are defined by threshold values applicable to each parameter measured by the TRACS survey vehicle, which are set out in Annex 2A of this Part.

n. The deterioration is not serious and more detailed d unless extending over long lengths, or several positions.

concern. The deterioration is becoming serious and e detailed (scheme level) investigations depend on the ers.

f concern. This condition should not occur very e trunk road network as earlier maintenance must have t this level of deterioration more detailed (scheme level) deteriorated lengths at the earliest opportunity and

ure depth, rut depth and ride Quality

TRACS measurement of the Texture Profile, Transverse Profile, Longitudinal Profile, Cracking, Fretting and Noise given in Annex 2B of this Part. In particular, when determining the cause of significant levels of Cracking, it is recommended that the crack map be examined to determine the distribution and type of cracking present.

2.19 The Highways Agency must be contacted if more information is required about the interpretation of data collected under the TRACS contract.

assessment criteria for tracs data

general

2.20 This Section describes the interpretation of each of the condition parameters measured by the TRACS survey vehicle. The threshold levels or assessment criteria for evaluating the extent of pavement

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deterioration are given in Annex 2A of this Part. The levels and criteria are based on experience gained from the HA’s ongoing research and development programme, and are currently considered to be the most appropriate criteria for condition assessment. However, they may change in the future as a result of further research.

2.21 All the threshold values and guidance levels are based on characteristic values associated with 100m lengths and are for the assessment of current TRACS data collected from in-service roads, as opposed to newly constructed roads.

tracs texture depth

2.22 Texture Depth values in the TRACS Combined Length Weighted (LW) Averages data source in HAPMS are calculated using the Sensor Measured Texture Depth (SMTD) method. The threshold values for TRACS (LW) Average Texture Depths are given in Table 2A.1 of Annex 2A of this Part.

2.23 Changes in the Texture Depth of the road surface can indicate a potential loss of skid resistance or some other mode of surface failure, e.g. Fretting (resulting in a high Texture Depth) (see also 2.37 to 2.39) or Fatting-up (resulting in a low Texture Depth). Advice on the interpretation of Texture Depth data in connection with skid resistance is given HD 28 (DMRB 7.3.1). HD 28 requires the Investigatory Level for skid resistance to be increased for surfaces (except High Friction Surfacing materials - HFS) with Texture Depth below 0.8mm SMTD (Category 3). Therefore, any location where the Texture Depth (except for HFS) triggers Category 3 or above must be reviewed in the context of HD 28.

2.24 Locations where the Texture Depth (except for HFS) triggers Category 4 will require urgent intervention if the nature of the surface condition means that further loss of texture can reasonably be expected, e.g. Fatting-up of a surface. Conversely, no action may be required if the Texture Depth is stable and a risk assessment undertaken in the context of HD28 does not indicate an elevated level of risk. Therefore, a more detailed investigation will be needed to determine the appropriate response.

2.25 Different thresholds are applied to High Friction Surfaces because of the different ways in which texture is provided by these materials. For these materials, the SMTD value is not an appropriate means of defining Condition Category 2 or higher. In this case, maintenance decisions must also take account of SCRIM results and the results of visual surveys.

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racs rut depth

26 HAPMS stores Rut Depth information as e average Rut Depth for each wheel-track over a m length. This base data is then used to calculate presentative values of rutting for the required porting lengths. HAPMS uses the stored rut formation to calculate the following LW Average lues:

Left Rut (using the left wheel-track values only);

Right Rut (using the right wheel-track values only);

Average Rut (using both wheel-track values);

Maximum Rut (using the maximum wheel-track values from each 10m).

27 Table 2A.2 of Annex 2A of this Part shows reshold values for the TRACS LW Average Maximum ut measurements. Note that concrete surfacings ould give negligible Rut Depths. It is recommended at, where any length has been identified for further vestigation as a result of high levels of rutting, mparison be made between the Left Rut and Right

ut LW Average values contained within HAPMS as a eck on the self-consistency of the rut measurements – e Annex 2B of this Part.

racs ride Quality

28 The measure used for the assessment of Ride uality, or Profile Unevenness, is the Enhanced ongitudinal Profile Variance of individual deviations the profile relative to a datum obtained by removing r filtering) longer wavelengths from the measured ngitudinal profile. This measure replaces the simpler ongitudinal Profile Variance and removes the effects of vement geometry (gradient, crossfall and curvature).

fuller description of the differences between the two easures is given in Annex 2B of this Part.

29 The assessment of Ride Quality is carried out ing three Enhanced Longitudinal Profile Variance lues, that indicate the level of profile unevenness ithin wavelength ranges less than or equal to 3m, m and 30m. It has been found that measurements

ithin these wavelength ranges may be broadly related levels of ride comfort. The Enhanced Longitudinal rofile Variance data is therefore reported within the RACS Length Weighted Averages data source in APMS as LPV 3m, LPV 10m and LPV 30m.

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2.30 The threshold values for the assessment of the TRACS LW Average Enhanced LPV measurements within the three wavelength ranges are given in Table 2A.2 of Annex 2A of this Part, and must be applied to all TRACS Enhanced LPV data reported in HAPMS. Note that different threshold values are specified within Table 2A.2 for different road classifications (which can be abstracted from HAPMS), e.g. a Motorway requires a better standard of Ride Quality than an urban single carriageway, where traffic is generally travelling at a lower speed.

tracs cracking

2.31 Guidance levels for the TRACS LW Average Whole Carriageway Cracking intensities are given in Table 2A.3 of Annex 2A of this Part. No guidance levels are currently specified for the intensity of Wheel-track Cracking

2.32 As described in Annex 2B of this Part, the TRACS survey vehicle relies on crack identification software to automatically identify cracks in the road surface. The measurement and the interpretation of the types of Cracking made by the crack identification software may differ from that made by an inspector carrying out a visual survey over the same site. As a result of this, the intensities of Cracking measured by the TRACS survey vehicle are generally lower than those recorded by a visual inspection. This behaviour is reflected by the low magnitudes for the guidance levels given in Table 2A.3 of Annex 2A of this Part.

2.33 Monitoring of the behaviour of the TRACS Cracking intensities recorded on the network has shown that they can be affected by variations in the survey conditions, which thereby influence the relative intensities of cracking reported. As a result, the intensities recorded in surveys carried out in consecutive survey years can vary and, when applying analyses based on threshold levels, the categories within which the Cracking measurements fall may change from survey year to survey year. Therefore:

• as the variability in the TRACS cracking intensities can introduce a degree of uncertainty in the cracking measurements, the thresholds defined in Table 2A.3 are provided for guidance only, to aid in identifying lengths in need of further investigation;

• it is not recommended that the intensities of cracking be used in the trending of pavement condition;

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• it is essential that, where any length has been identified for further investigation, examination be undertaken of the crack map data contained in HAPMS for the length under investigation before further action is taken;

• further advice regarding the assessment of the cracking data is provided in Annex 2B of this Part.

2.34 No thresholds are specified in Table 2A.3 of Annex 2A for the intensities of Whole Carriageway Cracking measured on concrete surfaces. Although the TRACS survey vehicle provides a measure of the extent of Cracking present on concrete surfaces it has been found that the system may falsely record the presence of joints or grooves in concrete roads as cracks. These false cracks are added to the Cracking area and lead to a higher level of Cracking being reported. Therefore, it is recommended that Cracking intensities derived from the TRACS crack data are not used to directly assess the condition of concrete surfaces within Condition Categories. However, for concrete surfaces the crack map may be utilised to aid in the assessment of the condition of the pavement.

tracs fretting

2.35 The Texture Profile data can be used for the estimation of the intensity of Fretting present on HRA surfaces. The guidance levels for the TRACS (LW) Average Fretting data are presented in Table 2A.4 of Annex 2A of this Part. However, the application of such data is relatively new and thresholds are provided for guidance only.

2.36 As the Fretting measure applies only to Hot Rolled Asphalt surfaces, the Surface Type must be known.

2.37 The Fretting measure can be used to assess the presence of minor deterioration and also aid in the assessment of Cracking. Guidance on the use of the Fretting measure is given in Annex 2B.

tracs predicted surface type

2.38 An estimate of the Surface Type can also be made from the TRACS Texture Profile data, within four categories:

• HRA;

• Thin Surfacing Systems;

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• Brushed Concrete;

• Grooved Concrete.

2.39 The TRACS predicted Surface Type information may be obtained from the HAPMS TRACS – Base Surface Type Data data source in HAPMS.

2.40 The TRACS predicted Surface Type may be subject to error. The TRACS predicted Surface Type algorithms will always generate a predicted Surface Type, even when the true Surface Type does not fall within the four categories listed in paragraph 2.39. For example, the TRACS predicted Surface Type often reports High Friction Surfaces (which are not contained within the current list of identifiable surfaces) as brushed concrete.

2.41 The TRACS predicted Surface Type is probably of most use for defining where surface changes occur. Users may consider using HAPMS to plot the surface layer of the construction data alongside the TRACS predicted Surface Type to assist in the assessment of the accuracy of their construction records within HAPMS, which will remain the primary source of surface type information.

tracs road geometry

2.42 The TRACS survey vehicle records the instantaneous gradient, crossfall and radius of curvature of the pavement at intervals of 5m. The data values contained in the TRACS – Base Geometric Data are reported in units of percent (for gradient and crossfall) and metres (for radius of curvature). There are no thresholds specified for the assessment of geometric data.

TRACS Retroreflectivity

2.43 Retroreflectivity data is used to support the description of the condition of road markings. Its use is described in detail in DMRB Volume 8, Section 2, Part 2 (TD 26: Maintenance of road markings and road studs).

TRACS Video Records (Forward and Downward)

2.44 The TRACS survey vehicle is equipped with digital forward- and downward facing video cameras that collects a video record of the road being surveyed. The digital video image data is not accessible from within HAPMS but is transferred to hard disks (separate HA Areas being stored on separate disks), and the disks

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distributed to each MA/MAC. The TRACS Forward Facing Video system, provided with the video images, is used to view the digitised video record.

2.45 The downward facing video records can be manually assessed to identify cracking and other surface defects which can be used as a preliminary stage to carrying out a full visual condition survey.

surface condition assessment of the national NEtwork of Roads (SCANNER)

2.46 This survey system has been developed by the UK Roads Board to provide a consistent method of measuring the surface condition of Local Authority road carriageways, using automated road condition survey machines, throughout the United Kingdom. Full details of the system are given in the User Guide and Specifications published by the UK Roads Board (2007). The following is only a brief overview of the system.

2.47 For information on any variations from the above Specification, applicable to Scotland or Wales, the relevant Overseeing Organisation should be consulted.

2.48 SCANNER consists of a Specification for the survey machines, a Specification for carrying out the surveys and a method of reporting road surface condition (the SCANNER Road Condition Indicator).

2.49 Before a survey vehicle can be used to carry out a SCANNER survey, it has to pass a very stringent set of accreditation tests each year. It must be operated with a defined Quality Assurance procedure and with an independent Auditor.

2.50 The survey data produced by the survey machines is loaded into a UKPMS-compliant pavement management system where it is processed to produce the SCANNER Road Condition Indicator (RCI) (England and Wales only) and for other highway maintenance and management purposes.

2.51 SCANNER was introduced to provide consistent, reliable survey data on the condition of road carriageways to support four separate requirements to:

• indicate the overall condition of a length of road carriageway, or of an area of a road network, to establish long term trends in road maintenance condition;

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• indicate the overall condition of a defined road network, as an outcome measure of local authority management and maintenance of their carriageway asset;

• produce indicative treatments and budget estimation, to plan carriageway maintenance at a network level;

• determine the optimum treatment timing, to prioritise treatment and minimise the whole life cost of maintenance at a scheme or project level.

2.52 SCANNER surveys are machine-based surveys that make a number of different measurements and process the measurements to provide a number of parameters that describe the condition of the road surface the:

• longitudinal profile along the road;

• transverse profile across the road;

• condition of the edge of the road;

• texture depth;

• presence and extent of surface cracking.

2.53 The SCANNER survey equipment makes many thousands of measurements within each 10m subsection along the carriageway. These are analysed and combined into a set of parameters which are reported as SCANNER parameter values for every 10m subsection along the road network. Even after the reduction of data to 10m SCANNER survey parameters, there is still too much data to be analysed by hand and the SCANNER data must be analysed through a pavement management system.

scanner road condition indicator

2.54 The SCANNER Road Condition Indicator (RCI) has been developed to characterise the overall condition of the road carriageway. The RCI is calculated from some of the parameters measured by SCANNER, in three stages:

• Each ‘measured parameter average value’ over a 10m subsection length is scored on a scale of 0 to 100 using an upper and lower threshold.

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• The scores of each separate parameter are combined using weightings to obtain a total number of points for each 10m subsection of the road. Each 10m subsection is then assigned to a condition “category” on the basis of the total points.

• The number of 10m subsections in each “category” are then totalled to give an overall figure for the section, the route or the network.

In the second stage, each 10m length is allocated to one of three condition categories – Red (poor overall condition – plan maintenance soon), Amber (some deterioration – needs investigation soon) and Green (in a good state of repair – no need to plan maintenance) – based on the total number of points. The overall condition assessment of a network is based on the proportions of the three condition categories of the total length of the network.

2.55 The thresholds, weightings and factors to be used in the RCI calculations are published on the UKPMS website and may vary over time.

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urveYs

chapter 3 visual condition surveys

3. visual condition s

Ha visual condition surveys

3.1 The Highways Agency Network Management Manual (2007) requires that visual surveys of carriageways and adjacent areas are carried out as follows:

Carriageways: Scheme level only

Off-Carriageway: Network and Scheme leve(Footways, cycleways, kerbs and paved verges)

This does not include surveys of unpaved verges and earthworks, which are covered in HD 41 (DMRB 7.4.

3.2 On the HA network, network level carriagewaysurveys are carried out with traffic-speed equipment only (TRACS (Chapter 2 of this Part) and SCRIM (HD 28, DMRB 7.3.1)). From these surveys and otherinformation such as routine inspections, candidate maintenance schemes are identified. More detailed investigations are carried out on these, including scheme level visual surveys. The HA has developed a computer based survey system known as Highways Agency Visual Surveys (HVS) for this purpose. The system is applicable to flexible carriageway pavementall types of Off-Carriageway paved surfaces and to rigcarriageway pavements with asphalt surfacing where defects are similar to those associated with all-asphaltconstruction.

3.3 The system requires the use of hand held Data Capture Devices (DCD) together with software provided by the HA to facilitate the input of defect dawhilst on the road, and the subsequent downloading of the data into the Highways Agency Pavement Management System (HAPMS).

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pavement visual surveys

3.4 For asphalt surfaced carriageways on the HA network, the HVS described in the HAPMS Visual Survey Manual must be used for all scheme level surveys. In cases of asphalt surfaced rigid pavement displaying defects associated with all-rigid pavements, the survey system for Rigid pavements should be used, or a combination of the Rigid and Flexible pavement survey systems, as appropriate.

3.5 HVS must only be carried out by surveyors who have been trained in the relevant survey techniques, and who are able to record defects accurately and consistently, in accordance with the definitions and procedures described in the survey manual. Surveyors are not expected to make decisions about the cause of defects, required treatments or to make other engineering judgements. Objectivity and consistency are paramount considerations in carrying out HVS.

3.6 Visual surveys of all types of highway pavement involves examination of the road surface by surveyors on foot and must always be carried out in accordance with Health and Safety legislation and with appropriate traffic management in accordance with the Traffic Signs Manual Chapter 8.

3.7 For rigid pavements the visual survey procedure detailed later in this chapter is recommended.

3.8 The defects of asphalt surfaced carriageways that are noted during HVS surveys are listed in Table 3.1. Full details of these and the manner in which they are recorded are given in the HAPMS Visual Survey Manual.

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defect Definition

1 Major Area Cracking Single or multipleas Transverse Crac

2 Minor Area Cracking Single or multipleclassified as Trans

3 Major Crazing Interlocking patter

4 Minor Crazing Interlocking patter

5 Major Fatting Bitumen in the sur

6 Minor Fatting Bitumen in the suraggregate.

7 Mud Pumping The visible presenpavement. The occof at least 1 square

8 Patching Failure Patches or reinstatover any part of thcracking.

9 Major Surface Defectiveness

Loss of material frsurface is not discechippings.

10 Minor Surface Defectiveness

Loss of material frsurface is still disc

11 Major Transverse Crack Single or multipleangles to the centr

12 Minor Transverse Crack Single or multipleangles to the centr

Table 3.1: HVS C

Off-Carriageway Visual Surveys

3.9 Visual Surveys of Off-Carriageway items (footways, cycleways and paved verges) are used at network level to:

• assist in the identification of areas requiring planned maintenance, and to provide information to enable potential maintenance schemes on Off-Carriageway features to be assessed and prioritised;

• allow the overall conditions of the Trunk Road Off- Carriageway network to be monitored for reporting purposes.

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non-interlocking cracks (>1mm wide) and not classified king.

non-interlocking cracks (<=1mm wide) and not verse Cracking.

n of cracks (>1mm wide).

n of cracks (<=1mm wide).

face course is flush or covering the coarse aggregate.

face course is close to but below the top of the coarse

ce of fines emanating from a crack/hole/joint in the urrence is accompanied by a cracked or depressed area metre in asphalt pavements.

ements that have subsided or rutted more than 10mm e patch/reinstatement or that exhibit more than 20%

om the wearing surface to a degree that the original rnible. Includes Chipping Loss from Surface applied

om the wearing surface to a degree that the original ernible.

cracks (> 0.1m spaced) cracks, >1mm wide, at right e line.

cracks (> 0.1m spaced) cracks, ≤ 1mm wide, at right e line.

arriageway Defects

3.10 All Off-Carriageway network or scheme level visual surveys on the HA network must be carried out using the HVS described in the HAPMS Visual Survey Manual.

3.11 The Off-Carriageway elements in HVS may be constructed from asphalt, concrete or block paving. Non-paved surfaces are not included. The defects to be recorded depend to some degree on the type of surfacing and include:

• major and minor cracking;

• major and minor local settlement or subsidence;

• major and minor fretting;

• longitudinal trip;

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• spot defects;

• damaged, cracked, depressed or missing blocks.

Full details of these defects and the manner in which they are recorded are given in the HAPMS Visual Survey Manual.

output for Hvs

3.12 After loading survey data into HAPMS, it is possible to produce text reports of the defects or coloured strip plans showing the occurrence of all defects. Adjacent lanes can be shown together, provided there is survey data for each. The scale and aspect ratio of the strip plans can be varied to suit the scheme length and complexity of defects. The carriageway visual survey data is not processed to produce any general condition parameters but the data is used by the HAPMS software when carrying out SWEEP Whole Life Costing analyses in support of bids for HA maintenance schemes.

surveys of rigid pavements

3.13 For the visual assessment of rigid pavements at scheme level, the Overseeing Departments have not yet developed a computerised system such as HVS. Instead, a graphical procedure is used to obtain as accurate a record as possible of all observed relevant features, i.e. carriageway condition, edge features, earthworks and drainage problems.

3.14 For rigid pavements on the HA network, faults and defects may be recorded on the field recording form shown in Figure 3.1 and using the symbols given in Table 3.2. Reference may be made to the Concrete Pavement Maintenance Manual (Highways Agency and Britpave, 2001) for detailed descriptions and photographs of each type of fault. The reference chainages used must be based on network sections as described in the Network Maintenance Manual and not on Marker posts or ad hoc systems. Figure 3.2 is an example of a completed survey form.

3.15 The occurrence of alkali-silica reaction must be recorded. This may be inferred from the presence of areas of map cracking containing a white or creamy powdery material which streaks the surface after heavy rainfall.

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3ictbvtpcatruO

3pbmccibpmo

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.16 A data capture device may be used if it is found to e cost effective and is capable of producing a graphic resentation of the defects similar to Figure 3.1.

.17 The current method of carrying out visual surveys s laborious, requires traffic management which may ause traffic disruption and exposes highway personnel o some risk of injury. Carrying out visual surveys y manual or automatic processing of traffic speed ideo images of the pavement would remove or reduce hese disadvantages. Research has shown that manual rocessing of video images is a practical method for arrying out visual surveys of rigid pavements, as an lternative to on-site visual surveys. Advice on which raffic-speed systems are approved for use on the trunk oad network will shortly be published. Any requests to se such a system should be referred to the Overseeing rganisation.

3.18 Referencing must be supplemented by numbering the bays on jointed concrete pavements; not only is this a positive referencing system which can be applied on site, but it can be used for contract preparation.

3.19 The size and extent of faults and defects may be estimated but must be plotted accurately at these estimated dimensions. Concrete bay lengths must be checked at the start of the survey and at every tenth bay. Running chainages must be maintained to an accuracy of ± 1 m.

.20 Surveys of concrete roads should whenever ossible be carried out in the cooler months of the year etween mid October and mid April when cracks are ore noticeable and when the efficiency of joint seals

an be better assessed. To assess the significance of racks, an accurate record of atmospheric temperature s required and weather conditions should always e noted. (Cracks are most readily visible when the avement surface is drying out after wet conditions; this ust be borne in mind when comparing surveys carried

ut in different weather conditions).

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figure 3.1: rigid pavem

ent survey form

Figure 3.1 Rigid Pavem

ent Survey Form

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table 3.2: standard symbols for recording condition of rigid pavements

Table 3.2 Standard Symbols for Recording Condition of Rigid Pavements

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Table 3.2 (continued): Standard Symbols for Recording Condition of Rigid Pavements

TABLE 3.2 (continued) Standard Symbols for Recording Condition of Rigid Pavements

Asphalt - B Cementitious - C Epoxy - E Add (OK) if sound or (F) if failed

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figure 3.2: exam

ple of com

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Figure 3.2 Exam

ple of Com

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continuously reinforced concrete pavements (CRCP)

3.21 Experience with visual assessment of continuously reinforced concrete pavements to date is limited. The Overseeing Organisations can therefore only give general guidance on visual survey methods.

3.22 CRCP construction has no transverse movement joints to accommodate longitudinal movement, and as a consequence transverse shrinkage cracks spaced between 1m and 4m develop shortly after construction over the total area of the slab. As time passes additional transverse cracks slowly develop between the wider spaced cracks. All these cracks are held closed by a continuous layer of heavy reinforcement, thus maintaining aggregate interlock and ensuring transfer of load across the cracks. Such cracking is considered normal for this type of construction and its presence does not indicate significant failure or weakness.

3.23 These “normal” cracks are defined as follows:

• the cracks are exclusively transverse with no spalling or bifurcations;

• less than 1mm width;

• spaced at least 1m apart.

3.24 Significant defects, indicating a weakened structure or need for maintenance, include:

• transverse cracks at spacings of less than 1m;

• transverse cracks with widths greater than 1mm;

• longitudinal cracks;

• areas of polygonal cracking;

• loose or missing blocks of concrete;

• crack bifurcations;

• failing repairs;

• spalling.

3/8

3.25 Similar visual survey methods to those used for jointed pavements should be used for surveying CRCP except that:

• the “normal” cracking is only recorded for the first 10m in every 100m length;

• the significant defects defined in 3.24 are recorded for the entire length;

• at CRCP terminations (involving ground beams), all “normal” cracking, significant defects and slab features must be recorded for the adjacent 100m of the pavement.

3.26 Guidance on the interpretation of visual survey data is given in HD 30 (DMRB 7.3.3).

presentation

3.27 Field survey sheets, or a fair copy, must be retained. The general condition of the road must be summarised on a plan to a suggested scale of 1:500. A typical form of presentation is shown in Figure 3.3. A record at this scale must also include notes about particular areas where the condition or rate of deterioration needs to be monitored during subsequent inspections.

3.28 The distribution and incidence of faults and defects is likely to be an important pointer to future performance. It may be useful to show the percentage distribution of defects in the following three categories:

a) defects likely to lead to safety hazards or serious surface deterioration within the next year;

b) defects which immediately affect user comfort or convenience, e.g. faulting or settlement;

c) defects likely to affect structural integrity within the next 3 year period.

3.29 This distribution should be compared with those given in earlier surveys. Summaries must also make clear whether existing repairs fall into the category of temporary or permanent. The material used for the repair must be recorded.

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figure 3.3: typical presentation format for v

isual surveys of concrete

Figure 3.3 Typical Presentation Form

at for Visual Surveys of C

oncrete

Asphalt

Surface


ing

chapter 4 Deflectograph Testing

4. deflectograpH test

general

4.1 This chapter describes the Deflectograph and the processing of its output. The Deflectograph is used to assess the structural condition of flexible pavements. It works on the principle that as a loaded wheel passes over the pavement, the pavement deflects and the size of the deflection is related to the strength of the pavement layers and subgrade.

4.2 The survey speed is slow (2.5 km/h) and consequently Deflectograph surveys cause considerable disruption, particularly on heavily trafficked roads. On the HA network this type of survey is no longer carried out at network level and is only used in support of individual maintenance schemes. However, other Overseeing Organisations may continue to use the Deflectograph at a network level. Ongoing research of a Traffic Speed Deflectometer (TSD) offers the potential to obtain Deflectograph type data from a survey vehicle traveling at speeds up to 80 km/h.

4.3 The assessment procedure used depends on the type of pavement and its mode of deterioration. Some thick, well constructed flexible pavements with asphalt base have been found not to deteriorate in the conventional way and with timely attention to surface defects can have a long but indeterminate life.

TwctD

d

4dldiIsp

4Dmfstttpf

may 2008

Figure 4.1

hese potentially long-life pavements are identified ith deflection and thickness criteria. The structural

ondition of other flexible pavements is assessed in erms of residual life using a long-established Deflection esign Method based on deflection and traffic loading.

eflectograpH

.4 The Deflectograph (Figure 4.1) is an automated eflection measuring system. It is a fully self-contained orry-mounted system, whereby measurements of eflection are taken at approximately 4m intervals n both wheel-tracks while the machine is in motion. t is regarded by the Overseeing Organisation as the tandard deflection measuring device for use on flexible avements.

.5 The transient deflection is measured as the eflectograph travels slowly along the line of twin easurement beams which are attached to a reference

rame. The measurement is not an absolute value of urface deflection since the reference frame sits within he wheelbase of the lorry and is itself influenced by he load. It represents a repeatable measure but since he analysis method is empirical, it is important that the rocedures for the use of the Deflectograph are closely ollowed.

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: Deflectograph



Figure 4.2: Deflectograph

4.6 It is essential that Deflectograph surveys are carried out as part of an overall assessment of highway condition. Further details of the analysis and interpretation of Deflectograph results in conjunction with other types of pavement condition data are included in HD 30 (DMRB 7.3.3).

machine calibration and approval

4.7 For use on the HA network, all Deflectographs must be tested and approved in an annual Deflectograph correlation trial to check their adequacy for trunk road testing. A copy of the certificate to confirm that the equipment is approved for use must be available to those commissioning Deflectograph surveys.

4.8 Static and dynamic calibration of the machine must be carried out as described in Annex 4A of this part. The records of these calibrations must be available for inspection at the annual correlation trial.

4.9 For use on other road networks, the relevant Overseeing Organisations must be consulted on the equipment approval requirements.

su

4.emun

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Measuring Beams

rveY categorY

10 As the Deflection Design Method is based on pirical data its use requires surveys to be carried out der prescribed conditions defined by survey category.

4.11 The survey category defined by time of year, and temperature limits must be as specified in Table 4.1 and Figure 4.3 respectively. The use of the categories must be as specified in Table 4.2. The reasoning behind these categories is explained in paragraphs 4.12 to 4.15. In certain circumstances it may be necessary to vary these standards, see paragraph 4.24.

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Volume 7 Section 3 Chapter 4 Part 2 HD 29/08 Deflectograph Testing

MONTHS OF YEAR (Shaded area refers) range of pavement

temperature

survey category

J f m a m J* J a s* o n d

Band 1 1A

Band 2 1B

Bands 1 & 2 2

Bands 1 & 2 3

* Period ends 15th of month, starts 16th of month.

table 4.1: time of Year and survey categories

figure 4.3: temperature limits for survey categories

MONTHS OF YEAR (Shaded area refers) Range of Pavement

Temperature

Survey Category

J F M A M J* J A S* O N D

Band 1 1A

Band 2 1B

Bands 1 & 2 2

Bands 1 & 2 3

* Period ends 15th of month, starts 16th of month.

Table 4.1 Time of Year and Survey Categories

Figure 4.3 Temperature Limits for Survey Categories

may 2008 4/3

January 2008 4/3


minimum category required

e a change in use is

1A with not greater than

10% in 1B

(at least one further survey 2


purpose of survey

To finalise the details of a Maintenance Scheme

To identify cause of surface damage already evident or wherproposed which will result in increased traffic loading

To provide advanced information for maintenance planning expected before strengthening measures defined)

Relative assessment within a site

Table 4.2: Survey Specificat

4.12 CATEGORY 1A defines the preferred conditions for deflection surveys. The highest confidence may be placed on the results of surveys in this category. The identification of potentially long-life pavements must be based on a Category 1A survey.

4.13 CATEGORY 1B extends the upper and lower limits of the temperature range allowed by including Band 2. This category is intended to allow for the situation that may arise when a survey planned for Category 1A does not comply with the specification because of unexpected changes in temperature taking place during the course of a survey.

4.14 CATEGORY 2. The wider temperature range obtained by adding Band 1 to Band 2 also applies to this category. The first part of September is included in Category 2 because after a hot dry summer, drying out of the subgrade may lead to measured deflections which do not truly reflect the weakest condition of the pavement.

4.15 CATEGORY 3. Surveys must not normally be carried out during the summer months specified for this category because of the difficulty of obtaining reliable, reproducible results.

equivalent thickness of sound Bituminous material (ESBM)

4.16 Deflection of asphalt pavements varies with temperature. The susceptibility to change in temperature is dependent on the thickness, age and condition of the asphalt layers. The parameter ESBM attempts to embody these factors and is only calculated for the purposes of correction of deflection to a standard temperature. It does not in any way reflect the structural contribution of the asphalt (bituminous) layers.

4.toaucobesoorwpothstr

ea

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3

ion for Trunk Roads

17 ESBM is required for correction of deflection the standard temperature of 20oC and is calculated tomatically during processing following input of nstruction information. The asphalt type needs to defined as dense or non-dense and its condition as und or unsound. Materials such as hot rolled asphalt dense bituminous macadam are defined as dense hilst materials such as open textured macadam or rous asphalt are non-dense. Unsound materials are ose showing cracking, disintegration or evidence of ipping.

4.18 Asphalt pavement layers comprising multiple surface dressings (where the total thickness is less than 25mm), any asphalt layers beneath hydraulically-bound layers and any asphalt layers with their upper surface at greater than a 200mm depth and separated from the higher asphalt layers by granular layer, must be ignored.

rly life surveys

4.19 Surveys carried out within two years of a new road being opened or a road having had a major strengthening treatment must not be used to determine residual life or strengthening requirements. This is because the early life deflections can be more variable and are not a reliable indicator of future structural strength of a pavement until the pavement and subgrade have stabilised, which usually takes at least two years.

may 2008



permitted temperature range

4.20 The road temperature specified in Table 4.2 is that measured at a depth of 40mm below the road surface at a position on or very close to the line of the nearside wheel-track.

4.21 For practical purposes the temperature of the pavement structure, in which considerable temperature gradients can occur, is represented by the measurement at a single depth of 40mm. Equations have been established between deflection and this characteristic temperature for a wide range of ESBM. To ensure that the correction of deflections to the standard temperature of 20oC remains within the validity of these equations, the temperature range within which a survey may take place becomes more restrictive as the ESBM of the pavement increases. The limiting rate of increase of temperature of no more than 2.5oC per hour, as specified in paragraph 4.33, is applied for the same reason.

4.22 Surveys may take place, within the permitted temperature range, at any time of day or night. ESBM values, for the sections of road to be surveyed, must be given to the Deflectograph operator prior to any survey work being undertaken in order that the acceptable temperature range for the survey may be calculated. See paragraph 4.25.

Flexible Pavements with Hydraulically Bound Base

4.23 The deflection behaviour with temperature of pavements with strong hydraulically-bound layers covered by asphalt can be significantly different from that of all asphalt construction. As the pavement temperature of a composite pavement increases the pavement deflection can decrease rather than increase as would be expected from the behaviour of flexible pavements with asphalt base. This effect is due to the hydraulically bound layer expanding with increasing temperature causing the cracks to close and the slabs to start locking together so stiffening the structure. Although the stiffness of any asphalt layers will be reducing, at the same time, the overall effect can be an increase of the total pavement stiffness and hence a reduction in measured deflection to relatively low values.

t

4.apAwfofocadeprwnoJu

s

f

4.capaneisex

4.recotomvainm

p

4.plwreinth

4.wthcoinwdi

may 2008

ime of Year

24 It is accepted that weather conditions may vary preciably in different regions and from year to year. lso, unusual conditions such as prolonged hot, dry eather may occur during periods of the year specified r Category 1. Where it is thought that there is a case r reclassifying a survey, either up or down by one tegory, the Overseeing Organisation must be given tails, including a description of the weather conditions evailing in the period prior to the survey. Due to the eather conditions generally associated with the more rtherly latitudes of Scotland the last two weeks of ne are included in survey Category 1 for this Region.

urveYs

requency and timing

25 A knowledge of the trend of average deflection n be very useful in the assessment of condition rticularly when the deflections are predicting low or gative residual lives. If this residual life assessment

valid, deflections measured one year later would be pected to show a deterioration.

26 Deflection measurements are inherently variable, flecting the variability of pavement materials, nstruction tolerances, the degree of compliance with lerances and, more problematically, changes in the oisture content of the subgrade. Whereas all factors ry with location, subgrade moisture content varies relation to seasonal changes of water table, drainage alfunction and ingress of water through the pavement.

lanning considerations

27 Knowledge of the ESBM is required at the anning stage in order to define the temperature range ithin which a survey may take place. The category quirements of Tables 4.1 and 4.2 and Figure 4.3 will fluence the timing of surveys on particular sites within e overall survey plan.

28 On single carriageway roads where remedial ork is envisaged from visual assessment, or where e traffic split is unequal in terms of numbers of mmercial vehicles or known loading pattern, a survey each direction is normally undertaken. However here OGV traffic is split approximately 50:50 in each rection, a deflection survey in one direction is usually

sufficient for maintenance planning purposes.

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4.29 On multi-lane roads surveys are required in lane 1 in both directions as a minimum. Surveys of the other lanes may be necessary where:

• visual defects are markedly different;

• there is a significantly different construction;

• the traffic loadings in these lanes are greater than in lane 1.

4.30 The Deflectograph operates at a nominal speed of 2.5km/h. Seasonal and temperature constraints allow a period of about 100 days in a calendar year for surveys in Categories 1 and 2, and in this period a typical Deflectograph output on continuous lengths of road, using an experienced operating team, is unlikely to exceed 1000 lane km.

4.31 The requirements of traffic management, including lane closures, may restrict the working day for survey purposes. Any such limitations on access must be determined at the planning stage.

surveY procedure

test procedure

4.32 The following operating procedure must be followed:

a) the Deflectograph must be positioned so that the nearside beam-tip follows the centre of the nearside wheel-track of the lane to be surveyed;

b) the machine must operate at a constant speed not exceeding 2.5 km/h;

c) the operator must reference the deflection record to the network sections. Additionally, the location of easily identifiable features must be marked at intervals of at least 0.5km so that deflection values may be related to their positions on the road;

d) the operator must monitor the recorded output at regular intervals and note any inconsistencies. If these occur, running checks are to be carried out. If faulty records persist, the survey must be terminated.

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road temperature measurement

4.33 The road temperature, as defined in paragraph 4.24, and its location and time of measurement must be recorded and entered on the survey record at the start and finish of the survey and at least every 30 minutes during the survey. Temperatures must also be recorded when passing into or out of continuously shaded areas on the carriageway and areas having significantly differing surface characteristics. Surveys must not continue if the temperature at any one point is changing at a rate exceeding 2.5oC per hour, measured over a period of at least 15 minutes.

4.34 It is most important that accurate road temperatures are recorded. Small errors in measured temperature can lead to large errors in corrected deflection especially if the structure includes considerable thicknesses of new asphalt. Before taking a temperature measurement any heat generated in making the necessary hole in the road must be allowed to dissipate. It may be advantageous to pre-drill these holes before the survey starts. Accuracy of measurement will be improved if the hole is filled with glycerol or other suitable liquid to aid heat transfer. Care must be taken to ensure that the temperature value indicated on the gauge has stabilized before a representative value is recorded.

use on Jointed concrete pavements

4.35 A specially-adapted version of the Deflectograph has been developed to assess the condition of joints in concrete pavements. This involves measuring the deflection either side of a joint as the wheel passes. The difference in deflection measurements may then be related to the load transfer properties of the joint and any joints which demonstrate poor behaviour may be readily picked out. Slab temperature will have a substantial effect on load transfer and testing should be carried out at pavement temperatures less than 10oC, when joints may be expected to have opened up. The ratio of deflection each side of the joint appears to be the most useful parameter to indicate the load transfer properties. It is not possible to define a standard value for this above which the joint may be considered satisfactory. This would have to be determined for each site taking into account visual defects such as cracking and pumping of fines from below. This also applies to the slower Falling Weight Deflectometer method of joint assessment, described in Chapter 5 of this Part.

may 2008



pavement condition inspections

4.36 Results from a recent and representative visual condition survey are required for all sites where a deflection survey is undertaken. It may be advantageous to undertake this visual survey at the time of the deflection survey. In Scotland and Northern Ireland however, the need for such visual surveys must, in all cases, be ascertained by enquiry to the Overseeing Organisation. The type, thickness and condition of the component pavement layers must also be determined by ground-penetrating radar or cores as appropriate. The amount of detailed information to be collected must be determined by the category of survey, the variability of construction in the pavement and its condition. Advice on more detailed investigations is contained in HD 30 (DMRB 7.3.3).

data processing

4.37 For the HA network all Deflectograph survey data must be processed centrally within the HAPMS system. The survey organisation first pre-processes the survey data using the HA’s MSP stand-alone software prior to loading into HAPMS, part of the HAPMS. Further details are given in the Network Maintenance Manual and in HAPMS documents.

4.38 The other Overseeing Organisations require Deflectograph survey data to be processed using the PANDEF computer program, or other programs approved by the Overseeing Organisation. Users should be aware that PANDEF is no longer supported by either the HA or the Department for Transport and that different versions of PANDEF may give different results for the same data. Details of PANDEF processing are given in Annex 4B.

pr

may 2008

esentation of results

4.39 For the HA network, summary processed deflection data from the HAPMS processing must be provided in support of bids for maintenance. The details of the parameters to be supplied are given in the annual HA guidance document – Value Management of the Regional Roads Programme. These are required to enable comparisons to be made between the maintenance requirements of different sites and for an order of priority to be established. In either case, a summary of all input parameters affecting the final design solution is to be provided for use as an audit trail.

4.40 For other networks, summary data from PANDEF in support of bids for maintenance must be as specified by the Overseeing Organisation. These are required to enable comparisons to be made between the maintenance requirements of different sites and for an order of priority to be established. They may be in hard copy form or computer files for transfer to a designated Maintenance Management System. In either case a summary of all input parameters affecting the final design solution is to be provided for use as an audit trail.

4.41 When making an assessment of the structural condition of a pavement, deflection measurement is to be considered as only one element of the total information to be assembled, and used in accordance with the advice given in HD 30 (DMRB 7.3.3), in deciding the most appropriate maintenance treatment.

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lectometer

chapter 5 Falling Weight Deflectometer

eight Deflectometer

5. falling weigHt def

general

5.1 This Chapter gives guidance on the use of the Falling Weight Deflectometer (FWD) for assessing the structural condition of road pavements. It describes the principles of the methods of analysis available and also sets out the requirements for calibration and operation of the FWD. Advice on the interpretation of results is given in HD 30 (DMRB 7.3.3).

5.2 A common approach to the assessment of the structural condition of a road pavement is to measure

Figure 5.1: Falling W

may 2008

its deflection under a known load. Application of this load is normally by one of two methods: by the action of a rolling wheel as in the Deflectograph, or by dropping a mass using a device such as the FWD. The deflection measured relates to the combined stiffness of the component layers in the pavement and its ability to distribute traffic loading. The FWD is normally a trailer mounted device, towed behind a vehicle (Figure 5.1). Van-mounted devices are also in use.

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5.3 The current policy for strength testing of flexible pavements described in this Part and in HD 30 (DMRB 7.3.3), requires that deflection is measured with a Deflectograph. The associated Deflection Design Method enables the residual life of the pavement to be predicted and strengthening overlays to be designed to extend that life. For rigid pavements, the assessment of structural maintenance requirements currently depends solely on Visual Condition Surveys (VCS). The FWD can provide additional detailed information on the structural condition of flexible and rigid pavements.

5.4 The impact method of load application used by the FWD is fundamentally different from the rolling wheel system employed by the Deflectograph. As yet, no satisfactory relationship has been found to convert FWD deflections to equivalent Deflectograph deflections so they cannot be input to the Overseeing Organisation’s design method. Whereas the Deflectograph system normally only uses the maximum deflection recorded at each measurement point, FWD measurements allow the deflected shape of the pavement surface to be derived. Estimates of layer stiffness can be made from knowledge of this deflected shape and the layer thicknesses.

5.5 There are many different methods of analyzing FWD measurements. Although these can produce relatively consistent results for layer stiffness, there is currently no standard approach for estimating residual life or overlay thickness using FWD results. The analytical process is subjective and calculating standard wheel load strains in the pavement and using these in conjunction with strain/fatigue life relationships is unreliable. The high sensitivity of the fatigue relationships used in this process can produce very different residual lives for the same data, when analysed by different engineers.

5.6 For the HA network the FWD must only be used for the following purposes:

a) to assess the stiffness of pavement layers of all pavement types;

b) to determine the load transfer efficiency across joints and cracks in rigid pavements.

These two types of surveys require the FWD to be configured differently and the results to be analysed in a completely different way. FWD data must not be used in isolation to other pavement condition indicators; it is important to characterise the

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material properties and understand the pavement deterioration mechanisms (see HD 30 DMRB 7.3.3).

surveYs

5.7 Surveys may be commissioned for the specific purposes described in paragraph 5.6. They must be carried out on whole lengths, or sample lengths, of the road in need of structural maintenance, as identified by approved assessment methods, and on sample sections in sound condition to enable comparisons to be made. Advice on aspects to be considered when drawing up a survey specification are given below.

measuring equipment

5.8 In principle the FWD generates a load pulse by dropping a mass onto a spring system. The mass and drop height can be adjusted to achieve the desired impact loading. Peak vertical deflections are measured at the centre of the loading plate and at several radial positions by a series of geophones. Figure 5.2 shows a typical deflection bowl (with the FWD configured for evaluating layer stiffness). These deflections and the peak impact load are stored electronically.

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Volume 7 Section 3 Chapter 5 Falling Weight Deflectometer

Figure 5.2: FWD D

machine calibration and approval

5.9 For use on the HA network, all FWDs must be tested and approved in an annual FWD correlation trial to check their adequacy for trunk road testing. A copy of the certificate to confirm that the equipment is approved for use must be available to those commissioning FWD surveys.

5.10 Consistency checks of the dynamic response of the machine as a whole must take place at intervals of between four and six weeks during periods of operation and after any major service involving replacement parts. Details of the requirements are given in Annex 5A of this Part. The records of these checks must be available for inspection at the annual correlation trial.

5.11 For use on other road networks, the relevant Overseeing Organisations must be consulted on the equipment approval requirements.

Part 2 HD 29/08

Figure 5.2 FWD

Machine Calibration and Approval

5.9 For use on the HA network, all FWDs must be testcheck their adequacy for trunk road testing. A copy of thuse must be available to those commissioning FWD surv

5.10 Consistency checks of the dynamic response of thebetween four and six weeks during periods of operation aDetails of the requirements are given in Annex 5A of thisinspection at the annual correlation trial.

5.11 For use on other road networks, the relevant Oversapproval requirements.

LAYER STIFFNESS EVALUATION

5.12 It is recommended that the results of FWD layer stthe detailed structural investigations, see HD 30 (DMRB

a) To assess the relative contribution to pavement stre

b) To provide estimates of layer stiffness of sufficientor require further consideration;

c) To assess the severity of cracks in hydraulically bo

Load

d2 d3 d1

Geophone

d

Effect of bound layers

may 2008

January 2008

d) To assess the Equivalent Surface Foundation Modescribed in Annex 5B of this Part;

eflection Bowl

laYer stiffness evaluation

5.12 It is recommended that the results of FWD layer stiffness surveys are used for the following purposes within the detailed structural investigations, see HD 30 (DMRB 7.3.3):

a) to assess the relative contribution to pavement strength of bound and unbound materials;

b) to provide estimates of layer stiffness of sufficient accuracy to indicate any weak areas that need replacing or require further consideration;

c) to assess the severity of cracks in hydraulically bound mixtures. This is described in HD 30 (DMRB 7.3.3);

d) to assess the Equivalent Surface Foundation Modulus, prior to the design of a concrete overlay. This is described in Annex 5B of this Part;

e) to determine the effective stiffness modulus of cracked and seated and saw-cut, cracked and seated concrete pavements as required in the Specification for Highway Works.

Deflection Bowl

ed and approved in an annual FWD correlation trial to e certificate to confirm that the equipment is approved for eys.

machine as a whole must take place at intervals of nd after any major service involving replacement parts. Part. The records of these checks must be available for

eeing Organisations must be consulted on the equipment

iffness surveys are used for the following purposes within 7.3.3):

ngth of bound and unbound materials;

accuracy to indicate any weak areas that need replacing

und mixtures. This is described in HD 30 (DMRB 7.3.3);

dulus, prior to the design of a concrete overlay. This is

s

4 d5

d6 d7

Effect of whole pavement

Effect of subgrade

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test procedure

5.13 On flexible and composite pavements the load level is set at a nominal 50kN + 10%. On concrete pavements, where deflections may be very low, i.e. d1 is less than 100 microns, this may be increased to a nominal 75kN + 10%. The load pulse must be applied through a 300mm diameter plate and have a rise time from start of pulse to peak of between 5 and 15 milliseconds. Most FWDs in the UK have a 60 Hz smoothing filter option. The use of this filter has been shown to improve the agreement between machines and, where available,

type of pavement Dista

inner

d1 d2

Flexible and flexible composite 0 300

Rigid and rigid composite 0 300

Table 5.1: Recommended D

5.15 Normally the loading plate is be located in the nearside wheel-track of the left-hand lane to assess the line of greatest deterioration. Additional measurements with the plate set between the wheel-tracks or in the middle of the right-hand lane of dual carriageways, if of the same construction, can provide a valuable indication of the condition of the largely untrafficked area of the pavement that has been subjected to the same environmental conditions as the trafficked wheel-tracks.

5.16 Typically, longitudinal spacing of measurements is between 5 and 20m for sample lengths of flexible pavements with asphalt base. On flexible pavements with hydraulically bound base some close spaced testing at 0.2 to 1.0m intervals may be carried out to assess crack frequency and severity in the underlying cement bound material. Details of this method are given in TRRL Report RR189 (1990) and in HD 30 (DMRB 7.3.3). Measurements on rigid pavements to assess slab and foundation properties must be taken in mid-slab locations and away from cracks.

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smoothing must be activated. Deflections must be measured to a resolution of at least 1 micron over the range 0-2mm by a minimum of 7 sensors situated at radii up to a distance of 2.25m from the centre of the loading plate.

5.14 There must be no standing water on the road surface and care must be taken to ensure that the whole area of the plate is in contact with the surface. Recommended sensor positions are set out in Table 5.1. At least 3 drops, plus a small initial drop for settling the load plate, must be made at each test point and checks made for consistency before analysis.

nce (mm) from Centre of Loading Plate

>>>> outer

geophone number

d3 d4 d5 d6 d7

600 900 1200 1500 2100

600 900 1350 1800 2250

eflection Sensor Positions

pavement temperature

5.17 The temperature of the bound layer of the pavement is normally taken at a depth of 100mm using an electronic thermometer to an accuracy better than 0.5oC and a resolution of 0.1oC. Holes for temperature measurements must be on the line of the test points and pre-drilled some time before measurement so that the heat energy created by drilling has had time to dissipate. Glycerol, or similar substance, in the bottom of the hole will ensure good thermal contact between the thermometer and the bound material.

5.18 Measurements must be taken at the start and end of each test section and at least every 30 minutes during the survey. Temperatures must also be recorded when passing into or out of continuously shaded areas on the carriageway and areas having significantly different surface characteristics.

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Deflection Profiles

700 800 900 1000 1100 1200 1300

inage (m)

Deflection Profiles

n the type and condition of the material present, such plots

5.19 The preferred temperature range for stiffness evaluation testing on flexible pavements with asphalt base is between 5oC and 30oC. At very low temperatures (<5oC), ice may be present in the unbound materials which can significantly affect the results. At high temperatures (>30oC) the response of asphalt becomes increasingly viscous and it is more difficult to distinguish between sound and unsound materials. In addition, since the stiffness of asphalt layers needs to be adjusted to the standard reference temperature of 20oC, additional uncertainty is introduced when testing takes place at temperatures significantly above or below 20oC. On flexible pavements with asphalt bases, greatest confidence can be placed on surveys carried out within the temperature range 10 to 25oC.

5.20 Additional care needs to be taken when assessing flexible pavements with a hydraulically bound base. The effective stiffness of hydraulically bound bases can increase with temperature due to cracks locking together and stiffening the structure. This effect is dependent upon the extent of cracking present and is difficult to predict. However it is not very significant on severely cracked hydraulically bound bases. Where the primary aim of the survey is to assess the condition

Figure 5.3: FWD

Chapter 5 Falling Weight Deflectometer

0

50

100

150

200

250

300

0 100 200 300 400 500 600

Cha

Def

lect

ion

(mic

rons

)

Figure 5.3 FWD

5.22 Although actual values of deflection will depend o

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will show relative differences in the condition of the layeidentified and give an indication of where structural wea

Cracks in Hydraulically Bound Mixtures

5.23 Results of tests on bases formed from hydraulicallenable the spacing and severity of primary transverse craThe crack spacing is a function of the age and strength ostronger material. Guidance on interpretation of such me

Calculating Layer Stiffnesses

5.24 The deflected shape of the surface, generated by acondition of the construction layers. Computer programsmodel the pavement structure. Essentially this analysis iswhich predicts the surface deflection under a given appliis used to match the computed deflections to the measureuntil a reasonable match is obtained.

5.25 There are a number of different programs availablstiffness estimates of pavement layers. The results of theused and the way in which the pavement is modelled (incproperties of those layers). For this reason, the followingperforming back-analysis for the HA’s network. The useconsistent results independent of who is performing the aanalysis procedures may be required but their use must bstandard procedure.

Back-analysis program

of the concrete base, it is recommended that testing be carried out below 15oC (but above 5oC). When testing rigid pavements, periods when there are significant temperature gradients must be avoided, as hogging or warping of the slabs can seriously affect the results.

analYsis

Initial Deflection Assessment

5.21 The first stage in the analysis of the data is to prepare simple longitudinal plots of selected deflections. The FWD deflection data “normalised” to a standard load, of 50 or 75kN as appropriate, may be tabulated and plotted to show the variation of pavement response along the road. Certain parts of the deflection bowl are influenced by the different pavement layers. With reference to Figure 5.2, the chosen criteria are usually d1, d6 and (d1-d4). The central deflection d1 gives an indication of overall pavement performance whilst the deflection difference (d1-d4) relates to the condition of the bound pavement layers. Deflection d6 is an indication of subgrade condition. A typical plot of these three deflection indicators is shown in Figure 5.3.Volume 7 Section 3

Part 2 HD 29/08

d1d1-d4d6

5/5

January 2008

rs, enable lengths of road with similar behaviour to be kness may be present.

y bound mixtures showing peaks in the central deflection cks to be determined even if not visible on the surface. f the cement bound material, a wider spacing indicating a asurements is given in HD 30 (DMRB 7.3.3).

n FWD impact load, depends upon the type, thickness and using linear elastic multi-layered analysis can be used to based on a mathematical model of the pavement structure ed load. An iterative procedure known as “back-analysis” d values. The layer stiffnesses are adjusted in this process

e for performing back-analysis of FWD data to produce analysis are strongly influenced by the type of program luding the number of layers used and the assumed back-analysis procedure must be followed whenever of this procedure is expected to produce reasonably nalysis. There will be occasions where alternative back-e clearly justified and they must be used in addition to the



5.22 Although actual values of deflection will depend on the type and condition of the material present, such plots will show relative differences in the condition of the layers, enable lengths of road with similar behaviour to be identified and give an indication of where structural weakness may be present.

cracks in Hydraulically Bound mixtures

5.23 Results of tests on bases formed from hydraulically bound mixtures showing peaks in the central deflection enable the spacing and severity of primary transverse cracks to be determined even if not visible on the surface. The crack spacing is a function of the age and strength of the cement bound material, a wider spacing indicating a stronger material. Guidance on interpretation of such measurements is given in HD 30 (DMRB 7.3.3).

calculating layer stiffnesses

5.24 The deflected shape of the surface, generated by an FWD impact load, depends upon the type, thickness and condition of the construction layers. Computer programs using linear elastic multi-layered analysis can be used to model the pavement structure. Essentially this analysis is based on a mathematical model of the pavement structure which predicts the surface deflection under a given applied load. An iterative procedure known as “back-analysis” is used to match the computed deflections to the measured values. The layer stiffnesses are adjusted in this process until a reasonable match is obtained.

5.25 There are a number of different programs available for performing back-analysis of FWD data to produce stiffness estimates of pavement layers. The results of the analysis are strongly influenced by the type of program used and the way in which the pavement is modelled (including the number of layers used and the assumed properties of those layers). For this reason, the following back-analysis procedure must be followed whenever performing back-analysis for the HA’s network. The use of this procedure is expected to produce reasonably consistent results independent of who is performing the analysis. There will be occasions where alternative back-analysis procedures may be required but their use must be clearly justified and they must be used in addition to the standard procedure.

B

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1 The general theory of stresses and displacements in layered s1945.

ack-Analysis Program

5.26 For use on the HA’s network, the back-analysis program must comply with the following criteria:

i. it must model the pavement structure as a number of horizontally infinite linear elastic layers;

ii. it must use elastic multi-layer analysis based on Burmister’s equations1 with all layers modelled linearly including an infinite depth subgrade and no slip between layers;

iii. it must be able to model at least three independent layers;

iv. it must be able to handle at least seven deflection sensors;

v. it must be able to report the computed surface deflection values.

avement model

5.27 For the HA’s network, the following rules must be applied when determining how to model the pavement:

i. the minimum thickness of any single layer must be 75mm;

ii. the maximum number of independent layers (including the subgrade) must be three;

iii. asphalt layers must be combined and modelled as a single layer;

iv. concrete layers must be combined and modelled as a single layer;

v. where an asphalt layer overlies a concrete layer, these must be modelled as separate layers provided that neither is less than one-third the thickness of the other (subject to constraints i and ii);

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oil systems. Burmister. Journal of applied physics, Volume 16,



vi. Poisson’s ratios used must be those shown in Table 5.2.

material poisson’s ratio

Asphalt Hydraulically bound mixture Pavement quality concrete

Crushed stone Soils (fine-grained)

0.35 0.35 0.20 0.45 0.45

Table 5.2: Poisson’s Ratios for Use in Back-Analysis

layer thicknesses

5.28 Stiffness results from back-analysis are extremely sensitive to the layer thicknesses assumed for the analysis. Underestimating the thickness of bound layers will generally result in an over-estimate of the stiffness of that layer and overestimating bound layer thickness will similarly result in an under-estimate of the stiffness of that layer. For example, a 15 per cent underestimate of the thickness of a bound layer can result in a fifty per cent overestimate of the stiffness of that layer. It is therefore essential that accurate and reliable thickness information be obtained prior to analysis.

5.29 While coring can provide thickness and material-type information of sufficient accuracy, many pavements exhibit significant variations in thickness and material type along their length (and sometimes across the pavement too) and coring of every test point is usually impractical and uneconomic.

5.30 In situations where the construction is expected to be consistent, e.g. where a road section has been constructed all at one time, and coring has shown thicknesses within, say, ±5% of the mean thickness, the core thicknesses alone should be a sufficient basis for back-analysis. In this situation, if the thickness variation is random, then analysis of the section using the overall mean core thickness would be the best option. If there is a progressive increase or decrease of thickness along the section, or there are groups of similar values, it would be preferable to analyse in sub-sections and use the mean thicknesses of these for back-analysis.

5.31 In the more usual situation, where coring shows a range of thicknesses greater than ±5% of the mean thickness and there is no other information to indicate where changes in pavement thickness occur, a Ground

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enetrating Radar survey (Chapter 6 of this Part) is ggested. In some instances further coring might also considered.

32 Where the cores and GPR data show significant ickness variation, ideally, each point must be analysed ing the corresponding thickness at the test location. owever, this will seldom be practicable and therefore will usually be necessary to split test sites into ctions of homogeneous construction for the purposes back-analysis. The measured bound-layer thickness ithin a homogeneous section should lie within ±5 r cent of the bound-layer thickness used for that ction. The determination of the homogeneous section undaries may be determined graphically by Cusum alysis or by computation.

33 For sections where there is a continuous change thickness, either a separate thickness must be ed for the analysis of each deflection bowl, or a ngle thickness is used for the section. In this case a nsitivity analysis must be helpful in indicating the pendence of the derived layer stiffnesses on assumed yer thicknesses. In some circumstances e.g. very high very low values of stiffness, the conclusion on the yer quality will be the same regardless of the value of ickness used in the back-analysis.

djustment for temperature

34 The stiffnesses derived from back-analysis present estimates of the in situ values at the time testing. The stiffness of asphalt is very dependent temperature. Therefore, in order to compare the

iffnesses obtained with those expected from standard aterials, it is necessary to first adjust them to the andard reference temperature of 20oC.

35 The temperature dependency of stiffness can ry quite considerably and is a function of a number different material properties. Consequently, no single lationship between stiffness and temperature exists at can be applied universally to all asphalt. However, sts on a wide range of materials have indicated that e relationship given below can be used to provide satisfactory adjustment to asphalt layer stiffnesses ovided that testing is carried out in the preferred range 15 to 25oC. The relationship may also be used where easurements are taken at other temperatures although e absolute values of the adjusted asphalt concrete iffnesses will need to be treated with caution.

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5.36 The temperature dependency of the stiffness of severely cracked asphalt tends to be far less than that of intact materials. Therefore, where a layer is known to be severely cracked throughout its depth, temperature adjustment should not normally be applied.

5.37 For use on the HA’s network, the stiffness of asphalt layers, unless severely cracked, must be adjusted to the standard reference temperature of 20oC using the following relationship:

E20 = ET .10(0.0003 x (20-T)² - 0.022 x (20-T))

Where: E20 = Stiffness at 20oC; ET = Stiffness at temperature T ;

T = Temperature of the asphalt at the time of testing (measured at 100mm depth).

goodness of fit

5.38 There are two goodness of fit parameters that are commonly used for indicating how well the program has matched the data. These are the Absolute Mean Deviation (AMD) and the Root Mean Squared Deviation (RMS), defined as:

Absolute Mean Deviation (AMD) = |∑(dci - dmi)/n|

Root Mean Squared Deviation (RMS) = √(∑(dci - dmi)

2/n)

Where: dmi are the measured deflections in microns at positions i =1 to n;

dci are the calculated deflections in microns at positions i =1 to n;

n is the total number of sensor positions used in the analysis (normally seven).

The AMD indicates whether or not there is an overall bias to the calculated deflection bowl relative to the measured bowl. The RMS indicates how well, on average, the calculated bowl matches the measured bowl. Although a good fit does not in itself indicate that a correct solution has been obtained, a poor fit does indicate that the solution found is suspect.

5.39 Different back-analysis programs vary in their ability to match calculated to measured deflections. Poor fits can also be obtained where cracks or other

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discontinuities are present in the pavement, where incorrect assumptions about layer thicknesses or material types are made, or where layer de-bonding is present. In addition, increasing the number of layers improves the level of fit. Table 5.3 contains guide values for AMD and RMS for pavements modelled with two or three layers. Bowls for which the AMD or RMS exceed these values must be treated with caution. Isolated results which exceed these limits must be discounted when assessing the overall condition of a section.

number of layers maximum values (microns)

amd rms

2 4 11

3 2 5

table 5.3: guide values for goodness of fit

ALTERNATIVE BACK-ANALYSIS APPROACHES

5.40 There will be occasions where the standard back-analysis procedure may not produce the most representative estimates of stiffness and an alternative analysis technique may be more appropriate. Table 5.4 lists some example scenarios where alternative techniques may be appropriate. Where an alternative back-analysis method is used, it needs to be performed in addition to the standard procedure and both analyses need to be reported.

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problem

A non-linear (i.e. stress-dependent) subgrade or the presence of bedrock is indicated by surface modulus plots. (See Annex 5B for information on surface modulus analysis.)

A layered sumay be app

On a flexible-composite (or rigid-composite) pavement modelled using a three-layer model, the stiffness of the asphalt layer is unrealistically high (or low) or the stiffness of the concrete is unrealistically low (or high).

Use a two-lforward-anaof the materlayer is resp

or

Fix the stiffneed to be don the mainand tempera7.3.3 for fur

Presence of poor quality bound materials, i.e. a sub-layer of bound material is known to be in a severely deteriorated condition and to have very low stiffness (e.g. cracked/stripped lower asphalt concrete layers in an evolved pavement).

It may be aptwo (intact materials w

Table 5.4: Example Situations where Alternmay be app

load transfer efficiencY

5.41 The main use of the FWD in relation to Pavement Quality (PQ) concrete pavements is the evaluation of Load Transfer Efficiency (LTE) and underlying slab support at joints and cracks (discontinuities). This is assessed by loading the slab on one side and measuring the deflections on each side of the joint or crack. More details on the assessment of PQ concrete pavements is given in HD 30 (DMRB 7.3.3).

test procedure

5.42 The load level must be set at a nominal 50kN + 10%. On pavements with very low deflections, ie d1 less than 100 microns, this may be increased to a nominal 75kN + 10%. The load pulse must be applied through a 300mm diameter plate and have a rise time from start of pulse to peak of between 5 and 15 milliseconds. Deflections must be measured to a resolution of at least 1 micron over the range 0-2mm by a minimum of three sensors.

5cwobTodsbHftt

may 2008

possible alternative analysis methods

bgrade model or a model using a stiff-layer at depth ropriate.

ayer model combining all the bound materials and use lysis to determine revised guide limits for the quality ials (supplementary testing will need to identify which onsible for any low stiffnesses).

ness of the asphalt layer in the analysis. This would etermined from a number of ITSM tests undertaken asphalt layers and adjusted to the FWD loading time ture at the time of the survey. (See HD 30, DMRB ther details.)

propriate to sub-divide the asphalt layers into and poor) or in extreme cases to combine the poor ith the foundation layers.

ative (Additional) Back-Analysis Models ropriate

5.43 There must be no standing water on the road surface and care must be taken to ensure that the whole area of the plate is in contact with the surface. At least 3 drops, excluding a small initial drop for setting the load plate, must be made at each test point and checks made for consistency before analysis.

.44 The relative degree of load transfer at joints and racks may be assessed by loading the slab on one side hilst deflections are measured on each side of the joint r crack. Ideally, the LTE of each joint or crack should e assessed by loading each side of the discontinuity. his is because the load transfer efficiency can depend n the support under the edge of the discontinuity. The ownstream side (“leave” side) of the discontinuity ometimes is the weaker side. Ideally the joints should e tested in both directions if resources and time permit. owever, this will require the FWD testing vehicle to

ace traffic in adjacent lanes and this recommendation is herefore subject to health and safety considerations and raffic management constraints.

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Volume 7 Section 3 Chapter 5 Part 2 HD 29/08 Falling Weight Deflectometer

Ja

5.44 The relative degree of load transfer at joints and cracks may be assessed by loading the slab on one side whilst deflections are measured on each side of the joint or crack. Ideally, the LTE of each joint or crack must be assessed by loading each side of the discontinuity. This is because the load transfer efficiency can depend on the support under the edge of the discontinuity. The downstream side (“leave” side) of the discontinuity sometimes is the weaker side. Ideally the joints must be tested in both directions if resources and time permit. However, this will req nt lanes and this recommendation is therefore subject to he t constraints.

5.4 own schematically in Figure 5.4. Ideally the load plate sh e deflection sensors d2 and d3 placed either side, 200 and 30 be taken to ensure that the sensors are positioned to avoid sp llow for this. The test would normally be carried out in the ou

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d3 Geophones

Traffic

5.45 The preferred arrangement of the equipment is shown schematically in Figure 5.4. Ideally the load plate should be placed 250mm from the discontinuity with the deflection sensors d2 and d3 placed either side, 200 and 300mm from the load centre respectively. Care needs to be taken to ensure that the sensors are positioned to avoid spalled material; spacings may need to be adjusted to allow for this. The test would normally be carried out in the outer (near side) wheel-track.

Figure 5.4: Load Transfer Efficien

The load transfer efficiency is defined as:

LTE = deflection of unloaded slab = d3 x 100% deflection of loaded slab d2

d2 and d3 are as shown in Figure 5.4.

pavement temperature

5.46 The temperature condition of the pavement will have a major effect on the measured LTE. Generally, higher values are obtained at high temperatures as the slabs expand and lock together. In order for joints or cracks to be compared they must be tested at a similar, low temperature, without temperature gradients, when the widths of the discontinuities are greater and the relative movement is larger. It is difficult to give a standard test temperature or range below which the locking effect is minimised as this will depend on the roughness and nominal width of the joints or cracks and the coefficient of thermal expansion of the slabs. An

uire the FWD testing vehicle to face traffic in adjacealth and safety considerations and traffic managemen

5 The preferred arrangement of the equipment is should be placed 250mm from the discontinuity with th0mm from the load centre respectively. Care needs toalled material; spacings may need to be adjusted to ater (near side) wheel-track.

Figure 5.4 Load Transfer Efficie

e load transfer efficiency is defined as:

LTE = deflection of unloaded slab = deflection of loaded slab

and d3 are as shown in Figure 5.4.

vement Temperature

6 The temperature condition of the pavement will lues are obtained at high temperatures as the slabs exmpared they must be tested at a similar, low temperacontinuities are greater and the relative movement isge below which the locking effect is minimised as thnts or cracks and the coefficient of thermal expansiothe past but this may not apply to all sites. Even at loe to temperature gradients which will produce slab c

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d1

Leave slab

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cy: FWD Set-up (“Leave” side test)

upper limit of 15oC has been suggested in the past but this may not apply to all sites. Even at low temperatures there may be some degree of slab locking due to temperature gradients which will produce slab curling.

Reporting of FWD Survey and Back-analysis Results

5.47 For surveys carried out on the HA’s network, the following information must be reported:

survey details

i. The date and time of the survey and the identities of the operator and company.

ii. Location of the survey including road number, lane number, transverse position, geographical location, and precise details of the longitudinal referencing in relation to the network section reference.

ncy: FWD Set-up (“Leave” side test)

d3 x 100% d2

have a major effect on the measured LTE. Generally,pand and lock together. In order for joints or cracks tture, without temperature gradients, when the widths larger. It is difficult to give a standard test temperatis will depend on the roughness and nominal width o

n of the slabs. An upper limit of 15oC has been suggw temperatures there may be some degree of slab locurling.

or crack Approach slab

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iii. Make, model and serial number of the FWD used.

iv. The mass of the falling weight and the diameter of the loading plate.

v. Type, number and positions of the geophones.

vi. Locations, depths (should be 100mm) and times of pavement temperatures recorded during the survey.

vii. Test type, leave or approach for Load Transfer Efficiency (LTE).

viii. “Supplementary” and “F20” files to be provided for loading into the Highways Agency Pavement Management System (HAPMS), for all trunk road surveys.

survey results

i. Tabulated deflections (normalised to 50kN or 70kN as appropriate).

ii. Graphs of the deflection parameters d1, (d1-d4) and d6 (normalised to 50 or 75kN as appropriate).

iii. Temperature applicable to each test point.

analysis details

i. Details of the FWD back-analysis program used (name, version, mode of operation).

ii. Details of the model used (thicknesses for each layer, number of layers, type of material assumed, details of the source of the construction information used e.g. cores, GPR, as-built drawings etc.).

analysis results

i. Tabulated back-analysed stiffnesses (including “as measured” and “adjusted to 20oC” for asphalt layers).

ii. Graphs of the layer stiffness results (asphalt layers adjusted to 20oC).

i

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5.49devlayeare

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ii. Tabulated layer thicknesses used for the analysis (i.e. for each test point).

v. AMD and RMS values for each test point.

.48 The template given in Annex 5B must be sed to record the details. Where an alternative nalysis is used, the report must include a clear xplanation of the reason for using the additional nalysis. Both sets of analyses must be presented n the format specified and it must be made clear hich set of results relates to which analysis ethod.

htweight Devices

Lightweight dynamic plate or falling weight ices are also useful for testing foundation (unbound) rs. Their use is more limited than the FWD and they not to be used for overall pavement assessment.

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ADAR

chapter 6 ground-Penetrating Radar

6. gROUND-PENETRATINg R

introduction

general

6.1 Ground-penetrating radar (GPR) is a non-destructive tool that can be used to obtain information about the construction of a pavement and its internal features. This information can be used to enhance pavement condition information obtained from visual condition, deflection surveys, coring and trial pits.

6.2 Typically, GPR can provide information about changes in pavement construction, layer thicknesses and defects/features within the pavement. The quality of the information obtained from ground radar is largely a function of three factors:

• the electrical properties (dielectric constant and the conductivity) of the materials forming the pavement;

• the type of GPR equipment employed;

• the processing software and analysis methodology including calibration procedures employed.

scope

6.3 This Part gives guidance on the appropriate use of GPR on paved roads only and does not cover the monitoring of services such as subsurface drains, buried pipes and any other non-pavement related features. The advice sets out the requirements for successful operation of a GPR survey, for quality control of the survey, and for the presentation of results.

6.4 Guidance on the use of GPR in connection with the assessment of highway structures is given in BA 86 (DMRB 3.1.7).

legislation

6.5 Section 1(1) of Wireless Telegraphy Act 1949, makes it an offence for any person to operate any equipment for wireless telegraphy if not used in accordance with the license granted

US

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6.8 idenrobthe survclasaccin Tconreli

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by the Office of Communications (OFCOM). All GPR operators operating in the UK must hold an OFCOM license and operate as required in respect of the EuroGPR Code of Good Practice. Also, the use of GPR on roads near radio astronomy sites requires specific permission from OFCOM.

E OF gROUND-PENETRATINg RADAR

eral Principles of ground-Penetrating Radar

The general principles of the use of GPR and r antenna types are explained in Annex 6A of this t.

ation referencing

6.7 Accurate location referencing is fundamental to the collection of good quality ground-penetrating radar data particularly as thicknesses will have to be calibrated or checked against pavement cores. All GPR surveys carried out on the HA network must be referenced against network sections to an accuracy of better than ± 5m.

If the surveys are carried out at traffic speed then particular care will need to taken to achieve the above requirement, the use of an automatic location referencing system may well be needed such as a sophisticated GPS and inertial guidance system as used with TRACS.

ommended uses

GPR systems available at present are able to tify pavement features to differing degrees of

ustness for assessment purposes depending on feature and the survey speed. The ability of GPR eys to identify pavement features are therefore sified into four classes A to D, depending on how urately and reliably they can be identified; as shown able 6.1. Surveys with the classification A are sidered routine and have sufficient accuracy and ability to be used regularly for pavement assessment.

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Surveys with the classification B are considered to be suitable for Confirmation that this pavement condition exists with the assistance of evidence from other techniques. Surveys with the classification C should only be used with caution as a guide and together with other data to indicate the possible construction or condition of the pavement. Surveys with the classification D are unproven and have yet to be demonstrated as suitable for use on the network. Table 6.1 also gives the constraints and other requirements when utilising GPR to ascertain a specific pavement feature. Ongoing developments in GPR systems make it likely that more features will be detected accurately and reliably in future. This advice on the limitations of the technique will be kept under review and any significant developments will be incorporated when they are ready for implementation.

6.9 The sampling rate of a GPR system influences the size of feature that can be detected and the sampling rate depends largely on the survey speed of the equipment amongst other parameters. This is mainly why surveys that detect discrete features in the pavement, such as voids or cracks, have been given different classifications in Table 6.1 depending on the survey speed. The surveys at slower speed, ie less than 30 km/h, will normally require appropriate traffic management. This subject is covered in more detail in Annex 6A in this Part.

6.10 It is recommended that surveys for the HA network be commissioned only if they meet the A and B level classifications as shown in Table 6.1 Further details of how GPR can be used to assess these pavement features are explained from paragraph 6.13 onwards.

6.11 It is also recommended that the GPR surveys should be undertaken prior to any coring to ensure that the core extraction occurs in homogeneous road sections.

limitations of use

6.12 GPR cannot penetrate metal, closely spaced reinforcement or highly conducting materials.

6.13 GPR surveys must not be carried out when it is raining or when standing water is present on the surface of the pavement. This is because a thick film of surface water may affect the radar signal making interpretation of the data more difficult. Calibration of the radar may also be less certain as explained in Annex 6B of this Part.

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6.14 GPR surveys must not be carried out on salted (de-iced) roads in case there is significant penetration of salt water into the subsurface materials beneath the road. Salt increases the conductivity of the pavement materials and the transmission of ground radar signals is heavily dependent on the pavement’s conductivity. High conductivity will lead to an attenuation of the radar signal and therefore reduces considerably the depth of penetration of the radar.

Construction Changes (Class A)

6.15 The following are typical examples of changes of pavement constructions which GPR can detect:

• changes from hydraulically bound to asphalt base and vice versa;

• haunches;

• hidden trenches covered by bituminous surfacing.

However if the construction change is outside the line of the survey, such as haunch construction, or of a short length then the survey may not be able to detect the change.

6.16 Direct evidence of construction changes must be confirmed by coring. Ideally, this must be carried out after the GPR survey has been carried out at locations of homogeneous construction (determined from the GPR) and where the GPR interpretation is unclear.

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volume 7 section 3

part 2 Hd

29/08pavement features Classification (see below) constraints and requirements

Slow speed <30km/h

Traffic speed >80km/h

Construction changes A A If the construction changes are outside the line of the survey or of a short length they may not be

ution is required for interpreting disintegrated lean te is 20mm (2.5GHz antenna). tection. For measurement at traffic speed, the e and the length of the voids. Depth of the feature curacy of measurements.r reliable detection. For measurement at traffic mpling rate and the length of the voids. Depth of ffect the accuracy of measurements.oint to assess dowel and tie bars. With a single d two further scans each side of the joint on a line e of survey will also allow the determination of re present. ture content. To eliminate uncertainty of data ry’ season when the sub base is likely to be in an ey carried out when the sub-base is deemed to be between the sub-base and the subgrade must be

erator and a slow survey speed are required. k depth measurements. No equipment available for

ining the signal attenuation, amplitude of onse from within the material, and automatically he material. However, the only certain way to

t slow speed with high frequency antenna, higher he debonded area.el but if the steel has corroded and damaged the e.d by reflections from the overlying reinforcement.

identify. Water filled voids less than 25mm deep

stems. Not proven to date.vement condition based on other data. ture research.

by gPR

chapter 6

ground-Penetrating R

adar

detected.Bound and unbound layer thicknesses and profiles

A A Low speed surveys needed for reinforced layers. Caconcrete layers. The best depth resolution for concre

Deep air-filled voids directly beneath unreinforced concrete slabs

B C Void depths need to be at least 80mm for reliable desuccess in detection will depend on the sampling ratand chosen antenna frequency will also affect the ac

Water-filled voids directly beneath unreinforced concrete slabs

B C Water-filled void depths need to be at least 25mm fospeed, the success in detection will depend on the sathe feature and chosen antenna frequency will also a

Depth and gross misalignment of joint dowel bars; detail of steel reinforcement in concrete slabs

B D Slow-speed scans are required along the line of the jantenna, one scan is required just beside the joint, anjust above the ends of the dowel or tie bars. This typthe depth and spacing of reinforcing steel mesh whe

Variation of sub-base moisture content (duplicate surveys required)

B C Signal velocity changes with material and with moisinterpretation, one survey must be carried out in a ‘dequilibrium moisture condition and an identical survwet (i.e. in the ‘wet’ season). Note that the interface visible in the signals for the technique to work.

Depths of surface cracks in fully flexible pavements

C D Specialised GPR equipment, a specifically trained opSample cores are required for calibration of the cractraffic speed survey.

Broad types of pavement materials C C Some idea of material type can be obtained by examreflections at material boundaries, continuity of respdetermined or self-calibrated signal velocity within tidentify materials is to use core data.

Debonding of pavement layers D D This feature might be visible in the bound material achance to be detected with the presence of water in t

Condition of steel in concrete D D Unlikely to indicate directly the condition of any stesurrounding concrete the radar may detect the damag

Voids and wet patches beneath reinforced concrete slabs

D D Reflections from voids or wet patches may be maske

Shallow voids directly beneath unreinforced concrete slabs

D D Air filled voids less than 80 mm deep are difficult toare also difficult to identify.

Debonding of joint sealant D D Might be detected at slow speed with special GPR syA - Sufficient accuracy and reliability to be used for pavement assessment. B - Use to confirm assessment of paC - Use with caution and as a guide, along with other data, to indicate possible D - Unproven and candidates for fuconstruction/condition of pavement.

Table 6.1: Accuracy and Reliability of Identification of Pavement Features

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Layer Thickness (Class A)

6.17 Correct interpretation of deflection measurements is heavily dependent on having accurate and reliable construction information. When processing Deflectograph measurements, information on layer thicknesses and material types are needed for determining generic base type (see Annex 4B, Paragraph 4B.10) and for classifying flexible pavements as to whether or not they are likely to be long-life pavements (see Annex 4B, Paragraph 4B.4). When analysing Falling Weight Deflectometer measurements accurate layer thicknesses are required to achieve the correct interpretation (see Chapter 5, Paragraphs 5.28 to 5.33) of layer stiffnesses.

6.18 GPR will, in general, detect all adjacent layers constructed of the same basic material as a single layer. For example, asphalt surfacing and base layers will, in general, appear to GPR as a single layer. Therefore, caution should be exercised when a GPR Contractor reports the thicknesses of individual layers within the bound layers. For reinforced layers it will be necessary to survey at a slow speed in order to reliably detect the bottom of the layer through the gaps in the reinforcement.

6.19 Layer thickness determination of lean concrete can be difficult if the concrete has disintegrated. In this situation it will have similar dielectric properties to granular material and the bottom of the disintegrated lean concrete could be misinterpreted as the bottom of the sub-base.

6.20 For use with the PANDEF program layer thicknesses are required to an accuracy of around ±10 per cent or better, but for the accurate interpretation of Falling Weight Deflectometer results accuracies of ±6 per cent or better are required. GPR trials have shown that at slow speed (<25km/h), GPR could determine the combined bound layer thickness with an accuracy of approximately 5 per cent of the real thickness. However, at traffic speed (70km/h) the error could increase to approximately 9 per cent. For underlying layers of hydraulically bound and unbound material, the expected accuracies are approximately ±15 per cent and ±30 per cent, respectively, of the real thickness.

Deep Voids Beneath Concrete Slabs (Class B or C)

6.21 GPR can give good estimates of the depth of deep air filled voids and shallower water filled voids under unreinforced concrete slabs and indicate their position and relative plan size. However, surveys to measure

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ids should only be undertaken if a visual condition rvey has shown that there are problems (e.g. rocking bs) and it is thought that they are related to poor pport under the concrete slab. If a survey is required, s advised that GPR with distance driven sampling d multiple antennas be used as this will give the eatest coverage and accuracy.

2 The reported depths of voids should be treated th caution if the report states that the depth of an air led void is less than 80 mm and the depth of the wet tch less than 25 mm. A difficulty arises if the slab is nforced because the reinforcement gives strong radar ections that can distort or mask the signals reflected m voids.

6.23 A GPR result indicating the presence of voids must not be used, on its own, to justify treatment. Other evidence must be obtained that voids exist and are causing problems, such as deterioration of joints or movement of slabs, before maintenance treatment is considered.

pth and gross Misalignment of Joint Dowel Bars d Details of Reinforcement (Class B)

4 A GPR survey of dowel bars in concrete vements should only be undertaken if there is idence that joints are not working properly and it is ught that the problem is due to the dowel bars.

5 GPR will give an indication of the depth of the wel bars and whether they are grossly misaligned. is possible to wrongly infer, from radar data, the tent of misalignment of the bars, so any report of salignment should be treated with care. If a survey required, it is advised that multiple antennas be used give greatest accuracy. Surveys aimed at providing tails of the reinforcement in the pavement, such as pth and spacing, should be carried out at low speed.

riation of Sub-base Moisture Content (Class B C)

6 Variations in sub-base moisture appear as riations in sub-base signal velocity and this velocity n be measured providing the interface between the b-base and subgrade is visible in the radar signals. y variation in sub-base material will produce a ilar variation in velocity so in order to eliminate this

ect a second survey on the same section should be dertaken and the results of two surveys compared.

may 2008



One survey should preferably be carried out in the ‘wet’ season, when the sub-base moisture content is likely to be at its highest. The other survey should preferably be carried out in the ‘dry’ season, when the sub-base moisture content is likely to be at its lowest. Areas where there are significant differences between the signal velocities measured during the two surveys are likely to indicate that water may be present. Trial pits or dry coring should be undertaken to confirm the findings of the GPR results.

Crack Depth in Asphalt Pavements (Class C)

6.27 Cracks in fully flexible pavements usually initiate at the surface and propagate downwards. Knowledge of crack depths can aid the maintaining agents to better target any maintenance. A Crack Detection Head (CDH) has been developed for use by GPR contractors to measure surface crack depth in flexible pavements. This equipment is used at slow speed and requires cores for calibration and control purposes.

6.28 When crack depth surveys are carried out, the equipment must be used in accordance with the manufacturer’s instructions, and operated by a technician who has attended the equipment manufacturer’s training course.

Material Types (Class C)

6.29 Experienced GPR contractors can obtain sufficient information to suggest broad material types from the survey results by considering such information as signal attenuation, the amplitude of the reflection at the material interfaces and signal velocity within the layers. However such interpretations are not 100% reliable and should be confirmed from core samples.

surveY plan

6.30 The GPR contractor must prepare a Survey Plan and comply with it at all times during the contract. This is to ensure that the information produced by a GPR survey is sufficiently accurate and reliable for use by the highway/pavement engineer.

6.31 The Survey Plan must include the following information in the submission:

cfe

6.tyatcath

6.deA

may 2008

• equipment specification;

• serial number of GPR equipment;

• calibration of the radar system;

• quality control procedures for both survey and analysis;

• work programme;

• survey procedure;

• risk assessment of the site work;

• form of presentation of the GPR results (Paragraphs 6.33 to 6.36).

alibration of gpr for determination of depth of atures

32 Cores will be required to identify or confirm each pe of construction and determine layer thicknesses specific locations within the survey site in order to librate or confirm the calibration of the GPR system at will be used in the survey.

6.33 Cores must be located to an accuracy of ± 1m in the longitudinal direction with respect to the network sections and to ± 0.1m in the transverse direction from the lane edge. Cores must be taken following the initial analysis of the data when changes in construction and layer thicknesses have been identified and located. Coring must be carried out in areas where the material appears to be homogeneous in both quality and thickness.

34 The methods of calibrating a GPR system for termination of layer thicknesses are explained in nnex 6B of this Part.

6/5



REPORTINg RESULTS OF A gROUND-penetrating radar surveY

6.35 The results from a GPR survey must be presented by the GPR contractor in a format which can be readily understood by the highway/pavement engineer and referenced to the network sections to allow easy comparison with other pavement condition data from the same site. In addition, the data must be provided in electronic form such that it can be easily used with the commonly available types of spreadsheet programs.

6.36 The GPR survey report must include:

• a text section summarising the results of the survey, assumptions used to interpret the radar data, measurement accuracy achieved, problems encountered, etc;

• a graphical display of the survey results;

• tabulation of the survey results;

• core logs where appropriate.

6.37 The media (electronic and/or hardcopy) for the display of radar results should be as specified or agreed with the survey customer and may require viewing software to be supplied by the GPS contractor. The format of the tabular data should be suitable for input to a spreadsheet program.

6.38 Graphs and tables should be produced to a standard format, displaying as much information as possible. The following information should appear on the graphs and tables:

• road number;

• network section identifier;

• chainage in metres along road from the start of the network section;

• date of survey;

• direction of survey;

• road type;

• construction;

•

•

•

•

•

•

•

•

•

6.39 this Pa

6/6

lane/s surveyed and track – nearside or offside wheel-track and/or between wheel-tracks;

survey length;

sampling of radar system – average sampling interval and sampling method (time driven or distance driven);

surface moisture condition;

location of any construction changes and broad identification of materials;

pavement depth in millimetres and pavement width in metres;

location and size of subsurface defects;

location of roadside features where this aids location referencing of the survey data;

location of cores, core details, and corresponding radar depths.

An example of reporting is given in Annex 6C of rt.

may 2008


ion sites

chapter 7 testing at investigation sites

7. testing at investigat

location referencing

7.1 Accurate location referencing of all pavement condition data is essential to allow reliable comparison between each type of data. The locations of all cores, test pits, in situ tests and material samples must be referenced against network sections to an accuracy of ± 1m longitudinally and ± 0.1m transversely from the nearside lane edge.

coring

7.2 When assessing pavements, coring of the bound layers is normally required for one or more of the following purposes:

a) determination of layer and total pavement thicknesses (usually in conjunction with Ground-Penetrating Radar);

b) determination of the material type and condition of the layers;

c) determination of the depth of cracking;

d) determination of the condition of rigid pavement joints;

e) provision of samples for compositional or physical tests;

f) provision of access for carrying out Dynamic Cone Penetration tests in granular foundation layers.

7.3 Coring on highway pavements must always be carried out in accordance with Health and Safety legislation and with appropriate traffic management in accordance with the Traffic Signs Manual Chapter 8.

7.4 150mm diameter cores are preferable to 100mm diameter as they will normally provide sufficient material for any laboratory testing and, if sited on cracks, are more likely to be successfully extracted.

Hsoc

7popcocdreJuaddoehthm

7idathotothtrn

may 2008

owever, if only layer thicknesses are required from und pavement, 100mm diameter or smaller cores

ould be satisfactory.

.5 Cores will sometimes be required on undamaged avement (for general construction details) and also n defects such as cracks, ruts and joints (in rigid avements) through which fines are pumping. Coring on racks is usually slower and more difficult than coring n sound material as the core pieces may jam in the ore barrel. If some of the layers are both cracked and e-bonded from each other; it can be very difficult to move the core from the barrel without further damage. dgment will be needed to select core locations which

re likely to reflect the deepest cracking but are not so isintegrated as to make coring impractical. If there is oubt over the direction of crack propagation (upwards r downwards) cores can be sited near and beyond the nd of a crack to establish whether the crack depth as reduced or not. They must penetrate completely rough all bound layers, asphalt or hydraulically bound ixtures.

.6 The asphalt layers responsible for rutting may be entified by extracting a set of three cores sited across

rut at spacings of approximately 0.5m, and comparing e layer thicknesses. The distances of the top surfaces

f the cores to a straight edge must also be recorded assist in matching the layer thinning deduced from e cores to the actual rut depth. Alternatively, a large ansverse slab may be sawn from the pavement if the ecessary equipment is available.

7.7 For each core, a full record of the core details must be made in the form of a Core Log, an example of which is shown in Figure 7.1. The log must include a good quality colour photograph with a scale strip and the core reference clearly visible. Natural lighting usually produces the best detail in the photographs. Flash photography, particularly when the core surface is wet, should be avoided as it produces strong highlights which obscure the details.

7/1



Volume 7 Section 3 Part 2 HD 29/08

Aggregate

t'l Max Size mm

Type

Comments

A 14 GNT Full depth crack in layer

M 28 GNT Full depth crack in layer. De-bonded from layer 3

M 28 GNT Sound. De-bonded from layer 4

A 28 GNT Sound

Asphalt resting on crushed limestone sub-base

figure 7.1: sample c

7.8 The following reference information must be stated on the log sheet for each core:

a) core reference;

b) section reference and chainage;

c) traffic direction;

d) lane and offset;

e) coring date;

f) pavement condition at core location including presence of cracks and their orientation.

7.9 The following details must be stated on the log sheet for each core:

a) thickness of each bound layer;

b) any missing layers;

c

Chapter 7 Testing at Investigation Sites

Figure 7.1 Sample C

7.8 The following reference information must be stated on the

a) Core reference;

b) Section reference and chainage;

c) Traffic direction;

d) Lane and offset;

e) Coring date;

f) Pavement condition at core location including presence of

7.9 The following details must be stated on the log sheet for ea

a) Thickness of each bound layer;

b) Any missing layers;

c) For each layer as appropriate:

• Type of material present;

• Possible presence of tar bound layers (if smell or sta

• Condition of the material, e.g. sound, cracked, friabl

Layers No.

Top mm

Btm mm

Thkn mm

Ma

1 0 40 40 HR

2 40 120 80 DB

3 120 225 105 DB

4 225 353 128 HR

Pavement Condition: Crack Key: HRA=Hot Rolled Aspha

CORE LOG

Project: Area 15

Section : 2400A1/357 Section Chain

Mp: 10/1 Lane: 1 Direction:

A1 / L1 / W

B / 10

/ 1

Transv

7/27/2

• Stripping of binder from the aggregate (if presen

• Condition of the bonding between layers;

• Presence of detritus where there is a lack of bon

ore log

) for each layer as appropriate:

• type of material present;

• possible presence of tar bound layers (if smell or staining is present).

• condition of the material, e.g. sound, cracked, friable etc;

• stripping of binder from the aggregate (if present);

• condition of the bonding between layers;

• presence of detritus where there is a lack of bond between layers;

• voiding and segregation (if present);

• crack depth and severity, soft or otherwise deleterious aggregate, bleeding and any other peculiarities.

ore Log

log sheet for each core:

cracks and their orientation .

ch core :

ining is present).

e etc;

t);

ed lt; DBM=Dense Bituminous Macadam; GNT=Granite

Location/Road: A1

age: 137m

WB Offset: NSWP

Date: 7th August 2006 Ref: A1/L1/WB/10/1

erse crack

may 2008January 2008

d between layers;



d) the depth of cracking, providing the cores are suitably located in relation to the pattern of cracking; this may require additional cores in some cases;

e) the nature of the material at the bottom of the core hole, e.g. crushed stone, gravel or further bound material.

7.10 Where pavement material has disintegrated during coring and there is only partial recovery of material, the layer thicknesses must be determined from the core hole if this is possible.

test pits

7.11 Excavating test pits is a much slower and more expensive method of obtaining pavement information compared to coring and must therefore be used only when necessary data cannot be obtained by other means. The test pit reinstatement is a significant task and may not be very durable unless hot-mix surfacing material is used. Test pitting is normally required for one or more of the following purposes:

a) obtaining bulk samples of the bound or unbound layers for laboratory testing;

b) detailed examination of the unbound layers or subgrade including density measurement;

c) investigating the causes of rutting;

d) examination of joints in concrete pavements;

e) investigating drainage problems within or beneath the pavement.

Information on bound layer thicknesses and quality is normally best obtained from cores. In test pits, bound layer details can only be reliably observed on the sawn edges of the excavation, which may only extend to 50 to 100mm.

7.12 Excavation of test pits on highway pavements must always be carried out in accordance with the Health and Safety Regulations applicable to the area and with appropriate traffic management in accordance with the Traffic Signs Manual Chapter 8 (2006). The necessary risk assessments must be carried out before work

7.thcotraprexrecusacahaplofexdi

7.coif sua 30

7.opcacuww

7.excadaapus

may 2008

commences. In any locations where there may be buried services the public utility organisations must be contacted for details of the locations of their plant. Cable location devices must be available and be used by a competent operator. In situations of doubt, excavation must proceed with caution, by hand methods.

13 The lateral location of the pit will depend on e nature of the distress being investigated but it is mmon for the pit to include the nearside wheel-ck of lane 1 and part of the hard shoulder, where esent. The plan dimensions of the pit are to some tent controlled by the excavation method and the quired final depth. Usually the test pit boundary is t to a depth of at least 50mm with a rotary diamond w, to ensure a neat reinstatement. Excavation is then rried out using a combination of pneumatic tools, nd labour and sometimes a mini-excavator. A typical an size would be 0.6m wide x 1.0m long for a pit 0.6m depth. If a greater pit depth is required or the cavator bucket width is greater than 0.5m, larger plan mensions will be needed.

14 If large samples of asphalt are required for mpositional analysis or tests on recovered binder, or hidden cracks in lower layers are being sought, the rfacing and base must be removed layer by layer and large piece of material from each layer, approximately 0mm square, retained for analysis as required.

15 In flexible pavements, where a test pit is being ened to investigate which layers have deformed and used rutting, a rotary diamond saw of at least 150mm tting depth, must be used to obtain a clean cut face ithin the asphalt layers. A steel straight edge across the idth of the pit can be used as a datum line.

16 The surface of each layer should be closely amined before excavation is continued and particular re taken in removing the lower layer of base, to avoid maging the surface of the sub-base. The general pearance of each layer must be noted. An air line is eful to clear away detritus and observe any cracking.

7.17 The pavement layer details revealed in the pit sides must be recorded on a test pit log similar to the example shown in Figure 7.2. If the construction variation across the pit is complex, a diagram must also be provided.

7/3


t log date: 26 July 2006

Rut Depth (mm): Near Side = 3mm Off Side = 0mmtest mple no.

required tests comments

678 1. Bitumen content 2. Bitumen properties 3. Grading

Surface condition sound

679 1. Bitumen content 2. Bitumen properties 3. Grading

680 1. Bitumen content 2. Grading

-

-

681

1. Grading Slight water seepage @ 0.80m at centre of N/S lane. Terram between sub-base and subgrade

5682 5683

1. Atterberg limits 2. Field moisture content

5684 5685

1. Atterberg limits 2. Field moisture content

Trial hole terminated @ 1.30 - 1.40m with water seepage @ 0.80m as above


test pitest pit: 17

location: M1 CH. 113 + 45 Northbound Slow Lane material depth from

surface (mm)

thickness of layer (mm)

sa

n/s o/s n/s o/s14mm slag HRA

Surface course

35 45 35 45 5

20mm limestone HRA Overlaid Surface course

99 95 64 50 5

28mm limestone HRA Binder course

159 151 60 56 5

40mm limestone DBM Base

219 191 60 40

40mm limestone DBM Base

329 306 110 115

Granular sub-base material Type 1 crushed limestone

850 870 521

564 5

Firm mottled grey & olive brown silty CLAY with traces of fine to medium sand and medium gravel

1100

1100

250

230

NSOS

Firm dark grey silty CLAY with traces of sand with some medium gravel size ironstone cobbles

1300

1400

200

300

NSOS

figure 7.2:samp

7.18 Photographs of the pit faces are desirable but the confined space within the pit and indistinct material boundaries and characteristics sometimes make it difficult to produce useful images.

7.19 The collection and testing of samples from the granular layers will depend on the purpose for the test pit and the materials discovered during excavation. However, generally it would be prudent to take samples of all distinct foundation layers for possible testing. Samples of sub-base material should be retained for grading, classification and the determination of moisture content, and the final layer again carefully removed to reveal the sub-grade or capping.

7/4

le test pit log

7.20 Density testing of foundation layers is only recommended where the stiffness or strength is unexpectedly low and low compaction or high voids are suspected, as it is a laborious and slow process. The sand replacement method is preferred to the nuclear test, as the latter will require calibration against the former unless the results are only to be used on a comparative basis. In order that the density value can be interpreted, it will also be necessary to carry out a Proctor test of the material in the laboratory to determine the Maximum Dry Density (MDD) and hence determine the relative compaction.

7.21 Capping, if present, may also be investigated in the same manner as for sub-base if this is possible

may 2008



in a safe manner. The record of the distribution of any moisture is particularly important. If the foundation layers in roads containing statutory undertakers equipment are holding water, samples may be taken for analysis, e.g. the presence of chlorine suggests that a water main may be leaking. Note must be made of any contamination between the subgrade and the sub-base or capping layers.

7.22 Samples of subgrade should be taken at 50mm depth and at 300mm depth for classification, including the determination of plastic limit, liquid limit and moisture content.

test pits in rigid pavements

7.23 The purposes for excavating test pits in rigid pavements are to investigate the following:

a) stepping/differential movement at joints;

b) very poor ‘load transfer’ at joints;

c) the possibility of suspect material beneath the concrete;

d) evidence of pumping at joints.

7.24 The reinstatement of concrete pavements after pitting is more difficult and disruptive than for asphalt materials. The new concrete will have to be left un-trafficked until a strength of 25N/mm2 is reached, thus increasing the delay and traffic disruption of the operation. Reinstatement with asphalt material must only be considered a temporary solution. For this reason, it is recommended that test pits should be made in concrete pavements only when it is considered absolutely necessary and no more than two joints are examined on any section. Consideration must be given to using large diameter (≥ 300mm) cores as an alternative to pitting.

may 2008

7.25 The recommended procedure is to saw cut the outline of the excavation, ensuring that it is taken far enough back from the joint to fully include the dowel bars (if any). Usually, only one side of the joint need be broken out, over a half lane width. Care must be taken when breaking out around dowel bars. The test pits, including any exposed dowels, should be photographed.

7.26 When replacing the concrete, it is essential that the joint is reformed, with intact dowel bars included, and the joint resealed (see HD 32 (DMRB 7.4.2)).

field tests on foundation materials

7.27 The Dynamic Cone Penetrometer (DCP) is recommended as the most suitable method for use in the bottom of core holes and test pits to indicate the strength and thickness of the foundation layers. The equipment is simple, fast and low cost. The essential features are shown in Figure 7.3.

7.28 The DCP uses an 8 Kg hammer dropping through a height of 575mm and a 60° cone having a maximum diameter of 20mm shown in Figure 1. The strength of the material is assessed on the rate of penetration per drop or “blow”. In practice the depth of penetration is recorded at increments of about 10mm, together with the number of blows to achieve this. The number of blows between readings will vary depending on the strength of the layer being penetrated.

7/5


one penetrometer


C Volume 7 Section 3 T Part 2 HD 29/08

7

60° INC

Ø 20mm

7

8

9

5

4

9

1

2

3

6

one Penetrometer

7anblt

7spbb

7faidrd

figure 7.3: dynamic c

7.29 The DCP must always be used in accordance with the Health and Safety Regulations applicable to the area and with appropriate traffic management in accordance with the Traffic Signs Manual Chapter 8 (2006). The necessary risk assessments must be carried out before work commences. In any locations where there may be buried services the public utility organisations must be contacted for details of the locations of their plant. Cable location devices must be available and be used by a competent operator. In situations of doubt, the location of the test must be moved or the test abandoned.

7.30 The DCP will penetrate most types of granular or lightly stabilised materials fairly easily. However, in strongly stabilised layers, very dense, high quality crushed stone and granular materials with large particles progress will be much slower or negligible. If there is no measurable penetration after 20 consecutive blows it can be assumed that the DCP will not penetrate the material. The cone must be replaced when its diameter is reduced by 10 per cent.

7winToytholaafonthR

hapter 7 esting at Investigation Sites

·

Key:-1 Handle2 Hammer (8kg)3 Hammer shaft4 Coupling5 Handguard6 Clamp ring7 Standard shaft8 1 metre rule9 60° cone

Figure 7.3 Dynamic C

.29 The DCP must always be used in accordance with the nd with appropriate traffic management in accordance with ecessary risk assessments must be carried out before work curied services the public utility organisations must be contaocation devices must be available and be used by a competeest must be moved or the test abandoned.

.30 The DCP will penetrate most types of granular or lighttrongly stabilised layers, very dense, high quality crushed strogress will be much slower or negligible. If there is no meae assumed that the DCP will not penetrate the material. They 10 per cent.

.31 Figure 7.4 give an example of a field sheet which mayor each test together with typical data. The data is normallyxis) against depth of penetration (-ve y axis) as shown in Findicates a change of strength and/or material type. The thicketermined by inspection and the average penetration rate, inate can be converted to a nominal California Bearing Ratio (eveloped by the Transport Research Laboratory:

7/6

/6

CBR = 10 (2.48 – 1.057 × Log10

P) ; wher (NB. The accuracy of this relationship

.31 Figure 7.4 gives an example of a field sheet hich may be used to record the general reference formation for each test together with typical data. he data is normally plotted as the cumulative number f blows (+ve x axis) against depth of penetration (-ve axis) as shown in Figure 7.5. A change in slope of e plotted data indicates a change of strength and/

r material type. The thicknesses of different strength yers are usually determined by inspection and the

verage penetration rate, in mm per blow, calculated r each. The penetration rate can be converted to a

ominal California Bearing Ratio (CBR) value using e following relationship developed by the Transport esearch Laboratory:

CBR = 10 (2.48 – 1.057 × Log10

P) ; where P = the penetration rate in mm per blow.

(NB. The accuracy of this relationship reduces for CBR values below 10 per cent.)

Health and Safety Regulations applicable to the area the Traffic Signs Manual Chapter 8 (2006). The ommences. In any locations where there may be cted for details of the locations of their plant. Cable nt operator. In situations of doubt, the location of the

ly stabilised materials fairly easily. However, in one and granular materials with large particles surable penetration after 20 consecutive blows it can cone must be replaced when its diameter is reduced

be used to record the general reference information plotted as the cumulative number of blows (+ve x

gure 7.5. A change in slope of the plotted data nesses of different strength layers are usually mm per blow, calculated for each. The penetration CBR) value using the following relationship

may 2008

January 2008

e P = the penetration rate in mm per blow. reduces for CBR values below 10 per cent.)



dcp field sHeetProject: M1 J10/11 Job No: 45678 Operator: KG

Date: 4/6/06 Time: 22.30

Depth from road surface to start of test: 375mm

Notes: Drizzle

Test ID: C Lane: 1 Section: 50/8 Chainage: 31m Direction: NB Offset: 0.7m

No of blows

∑ blows

Penetration (mm)

No of blows

∑ blows

Penetration (mm)

No of blows

∑ blows

Penetration (mm)

0 0 393 2 21 593 2 69 742

1 1 431 2 23 605 2 71 751

1 2 451 2 25 611 2 73 765

1 3 466 2 27 614 1 74 778

1 4 475 4 31 621 1 75 805

1 5 486 4 35 627 1 76 840

1 6 496 4 39 635 1 77 850

1 7 504 4 43 642 1 78 863

1 8 511 4 47 650 1 79 870

1 9 516 4 51 665 1 80 878

2 11 525 4 55 676 1 81 888

2 13 534 4 59 694 1 82 906

2 15 545 4 63 710 1 83 920

2 17 561 2 65 721 1 84 935

2 19 581 2 67 732

figure 7.4: dynamic cone penetrometer field sheet example

may 2008 7/7



netrometer plot example

Chapter 7 Volume 7 Section 3 Testing at Investigation Sites Part 2 HD 29/08

80 100

Layer 1DCP = 6.9mm/blowCBR = 39%



170mm

60mm

110mm
figure 7.5: dynamic cone pe
laboratory tests

7.32 Depending upon the nature of the materials found and the type of distress evidenced by the pavement, laboratory testing can provide valuable data for the evaluation process. It must always be remembered that it is often only possible to test material of good integrity and that this may introduce a bias into the results. The following paragraphs give some of the tests available but are not intended to represent a complete list.

7.33 Testing merely to determine whether or not a material complies with past or present standards is not usually very useful as the precise standard applicable to the material is not usually known and even if non-compliant the material may be performing satisfactorily.

asphalt

7.34 The general condition of the asphalt layers and the depth of any deterioration should be evident from a visual inspection of the cores. The elastic stiffness of the asphalt can be determined using the indirect tension to cylindrical specimens test specified in BS EN 12697-26 (formerly known as the Indirect Tensile Stiffness Modulus test (ITSM)). The stiffness modulus can be used to quantify the load spreading ability of the material as well as to help to confirm or

elatod

7aoble

7dcsmowic

7aD(t

7/8

-1000

-900

-800

-700

-600

-500

-400

-3000 20 40 60

No. of blows

Dep

th (m

m)

Figure 7.5 Dynamic Cone

Laboratory Tests

7.32 Depending upon the nature of the materials foundlaboratory testing can provide valuable data for the evaloften only possible to test material of good integrity andfollowing paragraphs give some of the tests available bu

7.33 Testing merely to determine whether or not a mavery useful as the precise standard applicable to the matmaterial may be performing satisfactorily.

Asphalt The general condition of the asphalt layers and the depthinspection of the cores. The elastic stiffness of the asphacylindrical specimens test specified in BS EN 12697-26test (ITSM)). The stiffness modulus can be used to quanhelp to confirm or explain measured deflection values oresistance of asphalt may be assessed using the Wheel-Tconsideration of the asphalt layer thicknesses across a w

7/8

xplain measured deflection values or FWD derived ayer moduli. Although the deformation resistance of sphalt may be assessed using the Wheel-Tracking test his would be judged more reliably by consideration f the asphalt layer thicknesses across a wheel-track as escribed in paragraph 7.6.

.35 Compositional Analysis provides information bout the particle size distribution and binder content f asphalt. The information obtained can, for example, e used to assess whether the material is too rich or too ean in binder which may help to explain higher than xpected material deformation.

.36 The recovered binder may also be tested to etermine the Penetration and Softening Point. This an be helpful to identify where binder is particularly oft (prone to deformation) or particularly hard which ay be associated with cracking or excessive loss

f chippings from HRA surfaces. Binder properties ill be required if recycling is being considered. If it

s suspected that tar may be present, further tests to onfirm this may be required.

.37 The state of compaction of asphalt may also be ssessed through density measurements. For historical BM materials, the Percentage Refusal Density test

PRD) compares the achieved level of compaction with he maximum achievable level. If the Relative Density

January 2008

Penetrometer Plot Example

and the type of distress evidenced by the pavement, uation process. It must always be remembered that it is that this may introduce a bias into the results. The t are not intended to represent a complete list.

terial complies with past or present standards is not usually erial is not usually known and even if non-compliant the

of any deterioration should be evident from a visual lt can be determined using the indirect tension to (formerly known as the Indirect Tensile Stiffness Modulus tify the load spreading ability of the material as well as to r FWD derived layer moduli. Although the deformation racking test this would be judged more reliably by heel-track as described in 7.5.

may 2008



of the mixture is measured together with the Bulk Density then volumetric proportions may be calculated including air void content.

Hydraulically Bound mixtures

7.38 The Cube Strength usually has to be deduced from cylinder tests. It also enables an estimate of flexural strength to be made, since direct testing in flexure is unlikely to be possible. The Density can be used to assess the degree of compaction achieved, particularly if an estimate of the Relative Density of the aggregate is available. As for asphalt, the Stiffness Modulus can be measured on either beam or cylindrical specimens, although it remains a specialist test.

unbound materials

7.39 Laboratory testing of the foundation layers, subbase or subgrade, should not normally be necessary as the DCP measurements should indicate the general condition and strength. However, tests may occasionally be necessary to explain the reasons for high or low strength or stiffness or to compare the material properties with specification standards. The most useful of these are:

• Grading;

• Liquid Limit, Plasticity Index and Linear Shrinkage;

• Moisture Content.

7.40 Where pavement failure is believed to be caused primarily by a weak foundation, a laboratory California Bearing Ratio (CBR) test of the material may be carried out, preferably by removing an undisturbed CBR mould-sized sample. (A disturbed CBR sample will also require an in situ density test to be carried out so that that the CBR material can be re-compacted to the in situ density.) An in situ CBR test is generally not very practical as it requires a large-plan test pit and takes considerable time, both of which add considerably to the cost.

may 2008 7/9


ograpHY

chapter 8 references and Bibliography

8. references and BiBli

references

1. design manual for roads and Bridges (DMRB)

BA 86 (DMRB 3.1.7) Advice Notes on the Non-Destructive Testing of Highway Structures

HD 24 (DMRB 7.2.1) Traffic Assessment

HD 28 (DMRB 7.3.1) Skidding Resistance

HD 30 (DMRB 7.3.3) Maintenance Assessment Procedure

HD 31 (DMRB 7.4.1) Maintenance of Bituminous Roads

Traffic Appraisal Manual (DMRB 12.1)

2. transport research laboratory documents (TRL)

LR834; Kennedy, C.K., Fevre, P. and Clarke, C.S., Pavement deflection: equipment for measurement in the United Kingdom, TRRL

3. British standards

BS EN 12697-26:2004 Bituminous mixtures. Test methods for hot mix asphalt. Stiffness (2004)

4. statutory publications

Construction (Design and Management) Regulations (Northern Ireland), Statutory Rules of Northern Ireland, 2007 No. 291

Construction (Design and Management) Regulations, Statutory Instrument 2007 No. 320

Health and Safety at Work Act (1974)

Health and Safety at Work (Northern Ireland) Order 1979

Management of Health and Safety at Work Regulations (1999)

5.

Bi

may 2008

Management of Health and Safety at Work Regulations Statutory Rules of Northern Ireland, 2000 No 87

other publications

Network Management Manual, Version 1.5a, March 2007, Highways Agency

National Assembly for Wales Trunk Road Manual, National Assembly for Wales Transport Directorate

Concrete Pavement Maintenance Manual, 2001, Highways Agency and Britpave (published by the Concrete Society)

HAPMS Visual Survey Manual, Version 2.03, 2005, Highways Agency Pavement Management System

Scanner Surveys for Local Roads – User Guide and Specifications, 2007, UK Roads Board

bliography

LR833; Kennedy, C.K. and Lister, N.W., Prediction of Pavement Performance and the Design of Overlays, TRRL, 1978

LR1132; Powell, W.D., Potter, J.F., Mayhew, H.C. and Nunn, M.E., ‘The Structural Design of Bituminous Roads’, TRRL, 1984

Report 250; Nunn, M.E., Brown, A., Weston, D. and Nicholls, J.C., ‘Design of long-life flexible pavements for heavy traffic’, TRL, 1997

Daniels, D., Ground Penetrating Radar 2nd Edition (Radar, Sonar, Navigations & Avionics), 2004, The Institution of Electrical Engineers, London. (ISBN 0 86341 360 9)

Hopman, V. and Beuving, E., Repeatability, reproducibility and accuracy of GPR measurements. Bearing Capacity of Roads, Railways and Airfields, Correia & Branco (eds), 2002 (ISBN 905809 396 4)

8/1

may 2008


9. enQuiries


Chief Highway Engineer The Highways Agency 123 Buckingham Palace Road London G CLARKE SW1W 9HA Chief Highway Engineer

Director, Major Transport Infrastructure Projects Transport Scotland 8th Floor, Buchanan House 58 Port Dundas Road A C McLAUGHLIN Glasgow Director, Major Transport Infrastructure G4 0HF Projects

Chief Highway Engineer Transport Wales Welsh Assembly Government Cathays Parks M J A PARKER Cardiff Chief Highway Engineer CF10 3NQ Transport Wales

Director of Engineering The Department for Regional Development Roads Service Clarence Court 10-18 Adelaide Street R J M CAIRNS Belfast BT2 8GB Director of Engineering

9/1

chapter 9 enquiries

may 2008


annex 1 not currentlY used

a1/1

annex 1

may 2008


annex 2a tracs assessment criteria and guidance levels

category →

1

threshold value

↓

2

threshold value

↓

3

threshold value

↓

4

Maximum Rut (mm) 6 11 20

Texture Depth (mm)

Anti-skid surfacing HFS 0.6 N/A N/A

All other surfaces 1.1 0.8 0.4

Table 2A.1: Rutting and Texture Depth Criteria for TRACS Measurements (100m lengths)

category →

1

threshold value

↓

2

threshold value

↓

3

threshold value

↓

4

1. MOTORWAYS AND RURAL DUAL CARRIAgEWAYS(†)Enhanced Variance (mm2) 3m 0.7 2.2 4.4 10m 1.6 6.5 14.7 30m 22 66 1102. URBAN DUAL CARRIAgEWAYS (†)Enhanced Variance (mm2) 3m 0.8 2.2 5.5 10m 2.8 8.6 22.8 30m 30 75 1213. RURAL SINgLE CARRIAgEWAY ROADS (†)Enhanced Variance (mm2) 3m 0.8 2.2 5.5 10m 2.8 8.6 22.8 30m 30 75 1214. URBAN SINgLE CARRIAgEWAY ROADS (†)Enhanced Variance (mm2) 3m 1.4 3.8 9.3 10m 6.1 18.3 36.6 30m 48 97 193

(†)Based on the road classifications in HAPMS

table 2a.2: ride Quality criteria for roads of all types of construction for tracs measurements (Enhanced Longitudinal Profile Variance, 100m Lengths)

a2a/1


may 2008


category →

Low

threshold Value (%)

↓

moderate

threshold Value (%)

↓

High

Asphalt Surface – Hot Rolled Asphalt Thresholds for cracking have yet to be decided for the new TRACS contractor. These will be made available once further research has

been completed.Asphalt Surface – Other

Concrete Surface

Table 2A.3: guidance Levels for TRACS Whole Carriageway Cracking Intensities (100m Lengths) (Refer also to Annex 2B when Assessing the Cracking Data)

category →

Low

threshold Value (%)

↓

moderate

threshold Value (%)

↓

High

threshold Value (%)

↓

severeAsphalt Surface – Hot Rolled Asphalt

0.20 1.15 1.80

Bituminous Surface – Other N/A N/A N/A

Table 2A.4: guidance Levels for Fretting (100m Lengths)

a2a/2



ription of tracs

annex 2B detailed description of tracs condition data

annex 2B detailed desccondition data

texture depth

2B.1 The surface Texture Depth measured by the TRACS survey vehicle is the coarser element of macrotexture and the finer element of megatexture formed by aggregate particles in the asphalt surfacing or by the brushing or grooving of concrete surfacing. Texture Depth contributes to skidding resistance, primarily at medium and high speeds, in two ways. Firstly, it provides drainage paths to allow water to be removed rapidly from the tyre/road interface. Secondly, the projections, which contribute to hysteresis losses in the tyre, are an important factor in the braking process.

2B.2 The Machine Survey Pre-processor (MSP) software calculates Sensor Measured Texture Depth (SMTD) from the raw Texture Profile measurements collected by the TRACS survey vehicle for the near side wheel-track and averages them over 10m lengths for storage as Base Data within HAPMS.

rut depth

2B.3 To measure Rut Depth, the TRACS survey vehicle records the Transverse Profile of the road surface over a width of 3.2m using 20 laser sensors. The Transverse Profile measurements are processed in MSP, which uses an algorithm to simulate placing a notional 2m straight edge on the recorded Transverse Profile, and measuring the largest deviation from the straight edge to the Transverse Profile. This calculation is carried out for each wheel-track. MSP calculates the average of the individual Rut Depths for each wheel-track over 10m lengths for storage within HAPMS.

2B.4 The Transverse Profile measurement method of Rut Depth determination used by the TRACS vehicle has been shown to be highly comparable with the measurements made using a conventional straight edge and wedge.

2B.5 TRACS Rut Depth measurements can sometimes be affected by variations in driving line. When the survey vehicle drives to the nearside of the traffic lane the lane edge marking may be included in the Transverse Profile measurement, giving rise to nearside Rut Depths that are higher than those actually

may 2008

present on the pavement. Similarly, driving to the offside can result in lower nearside Rut Depths.

2B.6 It is recommended that, where any length has been identified for further investigation as a result of deep Rut Depths, that comparison be made between the Left Rut and Right Rut length weighted average values contained within HAPMS. This is to check that there are no excessive differences which could be caused by the inclusion of the edge line in the Left Rut measurements.

ride Quality

2B.7 The parameter currently used for the assessment of Ride Quality, or Profile Unevenness, is the Enhanced Longitudinal Profile Variance of individual deviations of the profile relative to a datum obtained by removing (or filtering) longer wavelengths from the measured longitudinal profile. This parameter was introduced in June 2004 and uses sophisticated filtering which removes the effects of the pavement geometry (gradient, crossfall and curvature), which had previously caused high levels of variance at, for example, the approaches to roundabouts. The previous simpler method of filtering produced the parameter Longitudinal Profile Variance.

2B.8 The Enhanced Longitudinal Profile Variance value reflects the unevenness associated with profile features that are equal to or less in wavelength than the length of the filter used to calculate the Enhanced Longitudinal Profile Variance. For example, the variance of deviations from a 3m filter reflects the unevenness of profile features with wavelengths equal to or less than approximately 3m.

2B.9 The measurement of Profile Unevenness can be used to investigate the Ride Quality of the pavement. The short, medium and long wavelength features are found to relate with the perceived effect on vehicle ride are represented by variance from 3m, 10m and 30m filters respectively. The 3m, 10m and 30m Enhanced Longitudinal Profile Variance values are calculated from the TRACS longitudinal profile measurements by MSP and averaged over 10m lengths for storage within HAPMS.

a2B/1



2B.10 Extremes of 10m Enhanced Longitudinal Profile Variance may arise from poor reinstatements along the wheel-track, the presence of high and/or variable levels of rutting, and bay length irregularities (in concrete roads). High levels of 30m Enhanced Longitudinal Profile Variance may be associated with subsidence. Changes of crossfall along the length of a road may also result in slightly higher 30m ELPV being reported.

2B.11 High levels of Profile Unevenness do not only affect Ride Quality. High levels of Profile Unevenness, particularly in the 3m and 10m wavelength ranges, have been shown to contribute to increased dynamic loading of the pavement, hence accelerating the deterioration of the road pavement. Extremes of Profile Unevenness can also lead to increased stopping distances, and can have an adverse effect on vehicle manoeuvrability and safety.

2B.12 The values of Enhanced Longitudinal Profile Variance calculated from TRACS Longitudinal Profile measurements must not be compared directly with either values of Moving Average Longitudinal Profile Variance (produced by TRACS surveys prior to 1st June 2004) or, the values of Moving Average Longitudinal Profile Variance provided by earlier HRM surveys. This is because there are significant differences in the scaling of the data and the Enhanced Longitudinal Profile Variance measure generally reports lower values than the other two systems. However, the Moving Average Longitudinal Profile Variance and Enhanced Longitudinal Profile Variance methods are well correlated. On lengths where there is little variation in the geometry they will, in general, agree with regard to the locations at which higher values are reported. On lengths containing variations in the geometry the Enhanced Longitudinal Profile Variance is likely to report lower values.

a2B/2

2B.13 Further research carried out on TRACS measurements has shown that the TRACS survey vehicle may provide Longitudinal Profile data having lower levels of accuracy when surveying at slow speed, or under conditions of significant acceleration or (more commonly) deceleration. To reduce the occurrence of low accuracy data, limits have been specified for the TRACS surveys for speed and acceleration/deceleration beyond which the data is considered invalid. Where these limits have been exceeded the data is marked as Unreliable in the TRACS Base LPV Data. The presence of a significant proportion of Unreliable values within any 100m length will result in missing values when the data is expressed as 100m Length Weighted Averages in HAPMS.

2B.14 Ride Quality measurements provided by TRACS surveys prior to 1st June 2004 were reported as Moving Average Longitudinal Profile Variance. As the TRACS Length Weighted Averages data source in HAPMS reports the most recent TRACS data, the data source may contain data collected prior to 1st June 2004.

2B.15 To distinguish between Moving Average LPV and Enhanced LPV, the TRACS Length Weighted Averages data source in HAPMS provides an additional label for the LPV data called “Enhanced”. Where the label is not present this indicates that the value reported in the HAPMS TRACS Length Weighted Averages data source is the old Moving Average Longitudinal Profile Variance.

2B.16 Where it is required that an assessment be made of TRACS measurements of Moving Average Longitudinal Profile Variance (for TRACS surveys carried out before 1 June 2004) it will be necessary to apply the thresholds given in Table 2B.1.

may 2008


2

threshold value

↓

3

threshold value

↓

4

YS(†)

4 816 36165 275

4 1021 56187 300

4 1021 56187 300

7 1745 90


category →

1

threshold value

↓1. MOTORWAYS AND RURAL DUAL CARRIAgEWAVariance (mm2) 3m 1.3 10m 4 30m 552. URBAN DUAL CARRIAgEWAYS (†)Variance (mm2) 3m 1.5 10m 7 30m 753. RURAL SINgLE CARRIAgEWAY ROADS (†)Variance (mm2) 3m 1.5 10m 7 30m 754. URBAN SINgLE CARRIAgEWAY ROADS (†)Variance (mm2) 3m 2.5 10m 15 30m 120

(†)Based on the road classifications in HAPMS

table 2B.1: ride Quality criteria for roads of all tyObtained Before 1 June 2004 (Moving Average L

The TRACS Crack Identification System

2B.17 The measurement of Cracking by the TRACS survey vehicle is made using four downward facing video cameras that continuously collect images of the road surface over a transverse width of 3.2m. The video images from each camera are passed to a data processing system onboard the TRACS survey vehicle that automatically interprets the images to detect Cracking.

2B.18 The TRACS image collection system has a transverse and longitudinal resolution of approximately 2.5mm (the “pixel size”). The system is therefore unlikely to detect cracks in the road surface having widths less than 2.5mm.

2B.19 Cracking is reported as a “crack map” which describes the transverse and longitudinal position of each crack (reported as a straight line), the length of

edc

2thstCuocv

2toCwtr00

may 2008

240 480

pes of construction for tracs measurements ongitudinal Profile Variance, 100m Lengths)

ach crack and the angle of each crack relative to the irection of travel of the survey vehicle. The crack map an be viewed using facilities in HAPMS.

B.20 Crack maps are processed by MSP to obtain e intensity of Cracking over each 10m length, for orage within HAPMS. To obtain the intensity of racking, MSP simulates the placing of a grid, made p of 0.2m squares, over the crack map. The percentage f grid squares containing at least part of one or more racks is then evaluated over 10m lengths to give a alue of Cracking as a %.

B.21 Crack maps are also processed by MSP obtain a measure of the intensity of Wheel-track racking over 10m lengths in both the left and right heel-tracks. To calculate the intensity of left Wheel-ack Cracking, MSP simulates the placing of a grid .8m wide over the left wheel-track, made up of .2m by 0.8m grid rectangles. The percentage of grid

a2B/3



rectangles containing at least part of one or more cracks along the 10m length is evaluated to give a value of Wheel-track Cracking intensity (in %). The procedure is repeated for the right wheel-track.

2B.22 Due to way that the road surface images are processed to identify cracking and the need to suppress spurious reports of cracking, the total intensity of Cracking recorded by the TRACS survey vehicle will be lower than that recorded by an inspector in a manual crack survey.

2B.23 Furthermore, where a manual inspection will estimate the extent of the cracking by placing a theoretical frame around the cracking and recording the area within the frame, the MSP software will only record the total area of grid squares that contain cracks in the crack map. Since the grid squares are relatively small, the TRACS measurement will not tend to “fill-in” between cracks, as is the case when an inspector estimates an area of cracking. It has been found that

category →

Low

tV

Asphalt Surface – Hot Rolled Asphalt

Asphalt Surface – Other

Concrete Surface

Table 2B.2: guidance Levels for TRACS Whole Cartracs measurements obta

initial assessment of tracs crack data

2B.25 The TRACS measurement of cracking is subject to a degree of variability that can result in changes in the intensities of cracking reported over individual 100m lengths from survey year to survey year, even when the actual surface condition (cracking) has not changed significantly. Reasons for these changes in intensity include:

• Variations in driving line can result in the identification of road edge features that contribute to the cracking intensity recorded in one survey year, but not the next.

• Changes in surface appearance (e.g. darkness, detritus etc) may affect the sensitivity of the crack identification system to different types of cracks.

a2B/4

this contributes to an additional reduction in the area of cracking recorded by the TRACS survey vehicle, in comparison with the results of a manual survey, regardless of the surface type.

2B.24 The TRACS contractor changed in 2006 and this resulted in a change in performance of the crack detection system, thus new thresholds were required. Where it is required that an assessment be made of TRACS measurements of cracking, for TRACS surveys carried out before 1st April 2006, it will be necessary to apply the thresholds in Table 2B.2. The sensitivity of the previous contractor’s TRACS crack detection system was lower on finer textured surfaces, such as thin surfacing systems and surface dressing. It was found that the intensity of Cracking on these Surface Types was reported at a level that is proportionately lower than on Hot Rolled Asphalt (HRA) surfaces, and in particular fretted HRA surfaces. Therefore separate guidance levels for the assessment of Cracking data are specified for HRA and other asphalt surfaces in Table 2B.2.

hreshold alue (%)

↓

moderate

threshold Value (%)

↓

High

0.45 1.5

0.15 0.5

N/A N/A

riageway Cracking Intensities (100m Lengths) for ined Before 1 april 2006

2B.26 Although variability has been identified in the TRACS crack measurements, research has shown that the system reports, with a reasonable level of reliability, locations on the network that are cracked.

2B.27 However, because of this variability, it is essential that a careful approach be taken when assessing the TRACS Cracking data. Specifically, it must be noted that the use of the Cracking Intensities to trend the changes in cracking from year to year is not considered a reliable method to assess the crack data.

2B.28 Therefore, it is advised that detailed investigations are targeted at locations where consistently high Cracking intensities are observed in TRACS data from successive survey years. The average Cracking intensities over two or more years should be compared in terms of the guidance levels provided in Table 2A.3 of Annex 2A. Note that this method is only

may 2008



applicable for lengths where no maintenance has been carried out during the averaging period.

2B.29 This approach must enable more reliable identification of lengths with high levels of cracking present. Further investigation of these lengths must begin with the examination of the crack map data contained in HAPMS. This must be carried out using the most recent survey year and, where available, at least two preceding survey years.

fretting

2B.30 On pavements having coarse textures with large chip size, such as Hot Rolled Asphalt (HRA), the presence of severe Fretting (loss of stone from the surface) may be reported as Cracking by the TRACS survey vehicle. This can lead to lengths reported as containing high levels of Cracking that, in a visual survey, may have been reported as containing high levels of minor deterioration. TRACS surveys only measure fretting in the near side wheel-track.

2B.31 An estimate of the extent of Fretting can be derived from the Texture Profile by identifying characteristic shapes resulting from missing surface aggregate. Algorithms have been developed on this basis and incorporated into the MSP software to provide an indication of the presence of Fretting.

2B.32 The Fretting algorithms are currently optimised for the identification of Fretting of HRA. On other Surface Types the algorithms will generally report levels close to zero. Therefore, the Fretting data reported in HAPMS must only be used in the assessment of HRA surfaces.

2B.33 The Fretting algorithm reports the intensity of Fretting as a percentage value. The guidance levels for the assessment of Fretting have been obtained by undertaking manual surveys of fretted HRA pavements and comparing the reported levels of Fretting with those reported by the Fretting algorithm. It can be seen that the threshold levels have reasonably low absolute values. This is a result of the Fretting calculation process and its relationship to the identification of missing aggregate particles. The values must not be compared directly with measures obtained from manual visual condition surveys.

2utCha

2p

may 2008

B.34 The Fretting data is, in particular, intended for se when assessing the condition of HRA pavements hat have been reported to contain high levels of racking. Where both the Fretting and Cracking report igh levels it is probable that the crack measurements re actually reporting the presence of Fretting.

B.35 The crack map may be also used to show the resence of Fretting in the crack data.

a2B/5

may 2008


annex 3 currentlY not used

a3/1

annex 3


annex 4a Deflectograph Calibration

annex 4a deflectograpH caliBration

4A.1 This annex aims to explain briefly the principles of the Deflectograph for those unfamiliar with the apparatus. It includes details of the calibration procedure.

apparatus

4A.2 The details and dimensions for the chassis of the vehicle are given in Figure 4A.1 and for the beam assembly in Figure 4A.2. The equipment must be made to a design approved by the Overseeing Department, full details are given in LR 834 (1978).

Figure 4A.1: Chassis Details for Deflectograph Vehicles

Volume 7 Section 3 Annex 4APart 2 HD 29/08 Deflectograph Calibration

ANNEX 4A – DEFLECTOGRAPH CALIBRATION 4A.1 This annex aims to explain briefly the principles of the Deflectograph for those unfamiliar with the apparatus. It includes details of the calibration procedure.

Apparatus

4A.2 The details and dimensions for the chassis of the vehicle are given in Figure 4A.1 and for the beam assembly in Figure 4A.2. The equipment must be made to a design approved by the Overseeing Department, full details are given in LR 834 (1978).

Figure 4A.1 Chassis Details for Deflectograph Vehicles

Dimension A Dimension B Dimension C Dimension D Dimension E Dimension E Front axle load Rear axle load Twin rear wheel load Tyres Tyre pressures

210 – 260 mm 170 – 220 mm 120 – 190 mm 4445 – 4510 mm 1830 – 1875 mm 1960 – 2015 mm 4500 kg ± 5% 6350 kg ± 10% 3175 kg ± 10% 12.00x20 or 13Rx22.5 6.9 bar (100psi)

may 2008 a4a/1

January 2008 A4A/1


c

a

annex 4a Deflectograph CalibrationAnnex 4A Volume 7 Section 3

Deflectograph Calibration Part 2 HD 29/08

each day's work, or every twelve hours, and must comply rd of all calibrations must be kept for reference, see Table

Tyre “footprints”

Figure 4A.2: Deflecto

alibration

4A.3 Particular attention must be paid to calibration as deflection measurements are small, typically in the range 0 to 0.5mm.

4A.4 Static and dynamic calibrations are required. Deflectographs must be tested and approved annually in a group calibration trial by the Overseeing Organisation.

FIGURE 4A.2 Defle

Calibration

4A.3 Particular attention must be paid to calibratiorange 0 to 0.5mm.

4A.4 Static and dynamic calibrations are required.group calibration trial by the Overseeing Organisat

4A.5 Static calibration must be carried out before with the specification in Table 4A.1. A written reco

A*

4a/2A4A/2

4A.2.

graph – Beam Assembly

4A.5 Static calibration must be carried out before each day’s work, or every twelve hours, and must comply with the specification in Table 4A.1. A written record of all calibrations must be kept for reference, see Table 4A.2.

ctograph - Beam Assembly

n as deflection measurements are small, typically in the

Deflectographs must be tested and approved annually in a ion.




Beam tip input level (mm x 10–2)

Digital Output (mm x 10–2)

tolerance* max range**

10 +1 - 2

2 2

20 ±1 2

30 ±1 2

40 ±1 2

50 ±1 2

60 ±2 3

80 ±2 3

100 +4 - 2

3

Three separate movements at beam tip are required for each input value making up a calibration set.

* for each reading ** for each set of readings

Table 4A.1: Calibration of Deflectograph

4A.6 The static calibration must be carried out in the following manner:

a) the calibration rig used for transmitting movements to the beam tip must be the type approved by the Overseeing Organisation;

b) the dial gauge must be mounted on a separate baseplate and must be positioned vertically with the measuring point placed on the beam over the point of contact of the beam tip with the road.

c) three separate movements at the beam tip of offside and nearside beams must be made for each input value between zero and 1mm.

d) the dial gauge and the recording system must be returned to zero between each input movement.

may 2008

4A.7 Static calibration is influenced by the state of maintenance of the machine and calibration equipment and the accuracy with which the operator achieves input movements at the beam tip. The conditions under which calibration is carried out can also affect the result achieved. In particular, the roadside is sometimes not a suitable place to achieve the calibration specified. It may be preferable to do the required daily calibration at the overnight base. The recording equipment may need a warming-up period prior to calibration (see makers instructions).

Non-Compliance

4A.8 The results of the calibration set must be compared with the appropriate limits specified in Table 4A.1. If the results are within the limits, the survey may proceed. If the results at only one input level are out of specification, calibration at that input level must be repeated but 5 input movements are then required. The tolerance and range of 4 out of 5 of these are to be in the specification before the survey may proceed. Otherwise the calibration rig must be removed and repositioned under the beam tip and a second calibration set obtained – see the flow chart given in Figure 4A.3. If the second calibration set is out of specification reference must be made to the Deflectograph manufacturers’ Operators Instruction Manual to determine the likely cause and recommended course of action. A minimum of two calibration sets which comply with the specification must then be achieved before the machine may be considered fit for surveys. If this is not achieved it should be reported to the Overseeing Organisation who will decide on whether to proceed with the survey or require a new Deflectograph to be used.

consistency

4A.9 The written records of daily static calibration must be examined by the operators for consistency at least every two weeks during periods of operation. If any serious trends or substantial variations are found in the calibration, reference must be made to the Deflectograph manufacturer’s Operators Instruction Manual.

a4a/3


Date:

Number:

(of calibration)

n Mean Calibration

◊

N/S O/S

ecord


DEFLECTOGRAPH STATIC CALIBRATION

Purpose: SURVEY/MACHINE CHECK*

Location: BASE/ROADSIDE*

Beam Tip Input

(mm x 10-2)

Digital Output

Compliance with Specificaito

Nearside Offside N/S O/S

10

20

30

40

50

60

80

100

* Delete as necessary ◊ Values for calibration correction program should be to one decimal place

Table 4A.2: Deflectograph Static Calibration R

calibration correction by computer program

4A.10 The calibrations to be entered must be the mean of the results of all calibration sets taken on the day of the survey that comply with the Specification.

servicing of calibration equipment

4A.11 The calibration rig must be kept in good condition. The dial gauge must be serviced and checked against a standard gauge at least annually.

may 2008a4a/4



Annex 4A Volume 7 Section 3 Deflectograph Calibration Part 2 HD 29/08

rn e. er

safety

ach 5-20,

Reposition and reset rig.

Recalibrate

Meets required specification?

Refer to manufacturer’s handbook.

Rectify as necessary. Recalibrate

Recalibrate



NO

NO*

NO

YES

NO*

YES

Figure 4A.3: Flowchart for Checking the

dynamic calibration

4A.12 The dynamic response of the machine must be checked on a calibration site by comparing the pattern and general level of deflection obtained from a minimum of 3 runs with that previously established for the site. This check must be carried out at intervals of not more than 6 weeks during periods of operation and whenever any major service involving replacement parts has taken place.

c

4Abesa

FIGURE 4A.3 Flowchart for

Dynamic Calibration

4A.12 The dynamic response of the machine and general level of deflection obtained from a mThis check must be carried out at intervals of noany major service involving replacement parts h

Calibration Site

4A.13 The choice of dynamic calibration site shoof operation.

4A.14 Three sections on stiff, medium and weaksection should have a reasonably uniform deflec25-35 and 40-50 x 10 mm respectively. Ideally,possible separate sites should be located.

Carry out daily calibration

Proceed with survey


* If the Deflectograph cannot meet the specification after a second recalibration, this should be reported to the Overseeing Organisation. They will decide on whether to proceed with the survey or require a new Deflectograph to be used.

may 2008 A4A/6

4A.15 On each section there should be s

Calibration of the Deflectograph

alibration site

.13 The choice of dynamic calibration site should guided by factors such as convenience of access and fety of operation.

4A.14 Three sections on stiff, medium and weak pavements must be chosen in order to check the machine. Each section should have a reasonably uniform deflection with average deflections preferably within the range of 15-20, 25-35 and 40-50 x 10 mm respectively. Ideally, the sections should be in one length of road, but if this is not possible separate sites should be located.

Checking the Calibration of the Deflectograph

must be checked on a calibration site by comparing the patteinimum of 3 runs with that previously established for the sit

t more than 6 weeks during periods of operation and whenevas taken place.

uld be guided by factors such as convenience of access and

pavements must be chosen in order to check the machine. Etion with average deflections preferably within the range of 1 the sections should be in one length of road, but if this is not

ufficient reference points to ensure accurate location of deflections.

YES

a4a/5January 2008



4A.15 On each section there should be sufficient reference points to ensure accurate location of deflections.

4A.16 The sections should not be too heavily trafficked, thus giving a long useful life as calibration lengths and obviating the need for frequent updating of the deflections to allow for traffic carried. A recently by-passed section of road would appear appropriate.

4A.17 The drainage should be satisfactory so as to keep the seasonal variation of results to a minimum.

4A.18 Each section should have a uniform road surface and not be subject to sun/shadow effect making it difficult to determine pavement temperatures accurately.

4A.19 If the pavement contains hydraulically bound material it should be uncracked otherwise the deflection profile is liable to be excessively variable.

4A.20 The surface course of any type of pavement should be in sound, uncracked condition, preferably with wheel-track rutting less than 5 mm deep. This requirement may be difficult to attain where a pavement in the highest deflection range is required and a lesser standard may have to be accepted. However, rut depths of 10 mm should be considered the absolute maximum.

4A.21 The gradient of the road should not exceed 4 per cent. A steeper gradient could have an adverse effect when checking the machine by measuring the same wheel-tracks in different directions.

4A.22 The deflection profile of the test sections at the calibration site should be established for a range of temperatures using a Deflectograph of known satisfactory performance. This will involve making at least five runs with the machine on each of several occasions during the year when suitable temperatures occur. For each wheel-track the overall mean deflection of each section and the variability should be calculated for each temperature at which measurements are taken.

procedure

4A.23 At least one calibration set must precede and follow the dynamic calibration, which comprises a minimum of 3 runs over the test sections of the calibration site.

a

f

4Aarininita iswofde31toprTenan

4Avebewdeth

a4a/6

ssessment of results

4A.24 Results of the calibration runs must be compared with those already established for the site at the temperature of the test. It should be remembered that measurements with a Deflectograph may be taken at slightly different locations on each run, therefore the mean deflection values for different runs are not from identical samples. However, if there are any major discrepancies between the results these must be investigated by checking that there have not been any changes in conditions at the calibration site or in factors affecting the dynamic response of the machine. If an imbalance between nearside and offside results is suspected, further runs in which the nearside beam follows the track previously measured by the offside beam must be made. (This may be achieved by testing the site in the reverse direction).

actors affecting dynamic response of machine

.25 The permitted weights given in Figure 4A.1 e for the vehicle plus crew with the beam assembly the carrying position. The load on the front wheels fluences the magnitude of the measured deflection. As is not possible to use a simple scaling factor to apply correction, it is most important that the front axle load as close as possible to the value specified. If the rear heel loading is different from the recommended value 3175 kg, but within the tolerance allowed, measured flections must be multiplied by a correction factor of 75/(actual wheel loading). This correction is made all measured deflections by the PANDEF computer ogram when actual rear wheel weights are entered. he intention is that typical average values should be tered, as weights will vary depending on fuel carried d the number of crew on board.

.26 The position of the T frame relative to the hicle axles is important because the front skids can within the deflection bowl generated by the front heels of the machine. The shape of this bowl varies pending on the type of pavement material used and e magnitude of the deflection.

may 2008



4A.27 Although recent developments have made it possible for the Deflectograph to operate at higher speeds, measured deflection decreases with increasing vehicle speed. The limiting speed of 2.5 km/h must therefore not be exceeded during testing.

4A.28 Components which operate during the recording cycle, i.e. pivot bearings, transducers and amplification circuits should be particularly well maintained. The winch cable must be slack when the beam frame is stationary.

Ha annual group calibration trial

4A.29 The specifications for static and dynamic calibration are intended to ensure that any one machine gives a consistent output. Variations in performance which occur must be evident from the calibration records. The object of the HA Annual Group Calibration Trial is to establish, at least annually, a bench mark of the performance of every machine operating on Trunk Roads and to ensure that there is a common base from which to assess the results from different machines. Tests include machine inspection, static calibration and measurements on a dynamic calibration site. Owners/operators are advised of the results and a certificate of acceptability given.

coded events

4A.30 A typical format of coded events used on the Deflectographs operated for the Overseeing Organisations is as follows:

1. = left junction 2. = right junction 3. = Traffic lights 4. = Start/End slip roads 5. = Surface change 6. = Small patch 7. = Crack 8. = Telephone 9. = post box 10. = Field gate 11. = Bridge over 12. = Bridge under 13. = Marker post 14. = Start of patch 15. = End of patch

16.17.18.19.20.

mo

4Abe termoccfaupos

may 2008

= Road sign = Advance Direction Sign = Gantry = Start of area cracking = End of area cracking

nitoring of the equipment

.31 The correct functioning of the equipment can monitored by reference to the recorder output in

s of the display of incremental change in deflection urring in each recording cycle. Before attributing lts to the machine a check must be made on all sible external causes.

a4a/7


ing using pandef

annex 4B data processing using pandef

annex 4B data process

4B.1 PANDEF is no longer used on the HA network to process Deflectograph data but is still used on the networks of other Overseeing Organisations.

4B.2 PANDEF is a modular program with four main elements:

a) a database of road network definition, construction and traffic data and verified Deflectograph surveys referenced to the network;

b) deflection processing: validation of measured deflection, correction to standard deflection, matching of surveys to the network, categorisation of pavement type and derivation of residual life and overlay thickness, where appropriate, by reference to the database information;

c) deflection analysis: selection of maintenance sites based on homogeneity of residual life values and ranking of sites;

d) pavement overlay design: interactive design facility.

4B.3 PANDEF identifies two broad categories of pavement, those with the potential for a long life and those with a finite, or determinate life, which deteriorate in the conventional way. This categorisation must be considered as a first sift in the assessment process. Research is ongoing to refine the criteria and extend them to a wider range of construction types.

Long-Life Pavements

4B.4 This category includes strong flexible pavements with asphalt base and some flexible pavements with hydraulically bound base with substantial asphalt thickness. They are identified as being potential long-life pavements from their deflection level and total thickness of bituminous material (asphalt) (TTBM). More details on this category of pavement are given in HD 30 (DMRB 7.3.3). Assessment of these pavements and their maintenance treatment is not considered further in this Chapter – see HD 30 (DMRB 7.3.3).

may 2008

determinate life pavements

4B.5 Flexible pavements which do not fall into the long-life category and pavements with granular roadbases are categorised as having determinate life. For these pavements, PANDEF uses the Overseeing Organisation’s Deflection Design Method to predict residual life to the onset of investigatory conditions and provide a facility for designing strengthening overlays if all the condition parameters show this to be necessary. Some flexible pavements with asphalt base can potentially be upgraded to long-life pavement status by overlay. PANDEF will identify these pavements and calculate the overlay thickness required. More details are given in HD 30 (DMRB 7.3.3).

4B.6 The Deflection Design Method relates standard deflection to traffic loading for three different types of construction. The date of the construction of the road or its most recent strengthening and the type of material in each layer of the roadbase and surfacing together with its age, condition and thickness must be known. Details of traffic are also required.

4B.7 Outputs from PANDEF in terms of residual life and overlay thickness for a specified future design life, are automatically updated from the time of the selected survey to the enquiry report date entered. Past and future traffic loadings are calculated from this date.

data required

4B.8 The construction and traffic data for a site entered into the PANDEF database must be kept up to date so that the data is applicable to the survey being processed. Reliable construction information is particularly important because the categorisation of pavements as long-life or determinate-life is dependent on the thickness of bituminous material (asphalt) present in the pavement.

4B.9 The standard deflection value used by PANDEF is that measured at a pavement temperature of 20oC at 40mm below the surface. Traffic is expressed in terms of equivalent standard axles and must be derived using the method in HD 24 (DMRB 7.2.1).

a4B/1


nts depending on actual thickness of bituminous bound

tion of Base Type


Classification of Roadbase Type

4B.10 The Deflection Design Method includes deflection/performance relationships for the following generalised pavement construction types:

a) unbound ie granular base with no cementing action (GNCA);

b) flexible with hydraulically bound base (CEMT);

c) flexible with asphalt base (BITS).

PcttbTb

generalised pavement

construction type

Thickness of Material in the Pa

Cement-bounda Bituminous

Flexible composite

> 100 < 27

> 100c 275 – 2

> 100d 275 – 2

> 100 > 30

Fully flexible

< 100 > 15

< 100 < 15

< 100 < 15

Granular Roadbase

< 100 < 15

a - This does not include naturally cementitious mb - Deduct thickness of stripped material and surfa Sum total of remaining layers. c - If cement-bound layer is not extensively cracked - If cement-bound layer is extensively cracked. e - May be identified as potential long-life paveme material (asphalt) and deflection levels.

Table 4B.1: Classifica

a4B/2

avements do not always fit neatly into one of these onstruction types. PANDEF draws on details of the ype, thickness and condition of the pavement layers in he construction database to determine the appropriate ase type classification using the criteria given in able 4B.1. Some pavements with base type BITS may e identified by PANDEF as being potentially long-life.

vement Construction (mm) pandef Base type Classification boundb granular without

cementing action

5 ANY CEMT

99 ANY CEMT

99 ANY BITS

0 ANY BITSe

0 ANY BITSe

0 < 150 BITS

0 UNKNOWN BITS

0 > 150 GNCA

aterials, e.g. some crushed limestones. ce dressing layers less than or equal to 25mm thick.

d.

may 2008



The following functions in PANDEF must be used when processing data for the Overseeing Organisation:

a) validation of surveys and referencing to the highway network;

b) correction of Deflectograph measurements to standard conditions using temperature values entered on the survey record and machine calibration data, construction type and ESBM appropriate to the survey from the database;

c) calculation of traffic loading from the time of construction or the last major maintenance to the end of the chosen future design life, from traffic flows in AADF, preferably in disaggregated form;

d) classification into potentially long-life or determinate life pavement;

e) for determinate-life pavements, estimation of residual life and calculation of overlay thickness for a twenty year future design period or to upgrade the pavement to long-life status, as appropriate, for each deflection record;

f) division of the deflection record into maintenance sites of a minimum length to be specified by the Overseeing Department, based on the homogeneity of the residual life values using the automatic splitting algorithm.

Calculation of Traffic Loading

4B.11 The Deflection Design Method requires, at a minimum, a value of cumulative traffic loading from the year of construction or last major strengthening to the date of the deflection survey. If future design lives are to be estimated in years, then the annual traffic loading, in standard axles, is also required for each year from the date of the survey to at least twenty years into the future.

Traffic Module

4B.12 The traffic module in PANDEF which is used to calculate the annual standard axle flow for each year in each lane is based on two basic look-up tables. The first table provides default wear factors for each year from 1955 to 2040 for the following seven vehicle classes: buses and coaches, 2 axle rigid, 3 axle rigid, 3 axle articulated, 4 axle rigid, 4 axle articulated and 5 axle articulated. The second includes default relative flow rates for each of the years given in the first table

may 2008

and the corresponding vehicle classes.

4B.13 The minimum input required is a daily total commercial vehicle flow in one direction for a base year, normally at, or close to, a survey year between 1980 and 2010. The program uses default proportions by which flows are disaggregated into the seven vehicle classes. If a growth rate is not entered, a value from the default flow rates is assumed. When flows are entered for more than one year, even growth rates are applied between the available data. Consideration must be given to the effects of significant planned network changes in determining traffic growth which can either be positive or negative.

4B.14 Facilities are also provided for the user to enter fully disaggregated flow data and overall growth rates for total traffic flow.

available sources of information

4B.15 For the period up to 1978, the General Traffic Census (GTC) provides information on 16 hr flows of commercial vehicles for 6,300 (non-random) sites on motorway, trunk and principal roads. This information is in the form of flows on an average August day and in some cases an average April/May day.

4B.16 In England and Wales, factors to convert these flows to 24 hr AADF are available from the Department for Transport, Directorate of Statistics, Transport Statistics Roads 2 Division, Zone 2/14.

4B.17 In order to obtain the required one direction flow of commercial vehicles, it will be necessary to apply a directional split. Where possible this must be calculated from the census data. A 50/50 split must only be adopted in the absence of more specific information. The growth rate applying to commercial traffic will not necessarily be the same as that applying to all traffic.

4B.18 In Scotland and Northern Ireland, all enquiries relating to traffic data must be referred to the Overseeing Organisation.

interpretation and application of results

4B.19 The precision of estimates of residual life and thickness of strengthening overlays given in PANDEF are limited by the accuracy of the input data. Machine calibration and operation are closely controlled by the Overseeing Organisation, but errors in construction and traffic data entered will have a significant effect on program outputs.

a4B/3

may 2008


annex 5a fwd reQuirements for consistencY cHeck

Long-Term Repeatability Verification

5A.1 As well as the requirement to take part in the annual FWD correlation trial, referred to in Section 5.9, it is recommended that reassurance of the continuing consistent operation of the equipment is obtained by carrying out regular deflection and temperature measurements on a well-characterised test pavement at intervals of between four and six weeks whilst the survey equipment is in routine operation. Well-characterised means that similar tests have been carried out on the site over at least a continuous twelve month period. Ideally the test site should encompass a wide range of deflections on the three main pavement types but in reality this may not always be possible.

5A.2 Comparison of the consistency of the measured deflections, corrected for load, from each of the geophones, taking account of the measured pavement temperature and previous deflection temperature history of the site, will give an early indication of any significant faults developing that need further investigation.

a5a/1

annex 5a fwd requirements for consistency check


deflectograpH is

eter on Pavements Part 1 – Harmonisation of FWD valuation. FEHRL Report No. 1996/1.TRL, Crowthorne,

annex 5B Falling Weight Deflectograph Surface Modulus Analaysis

annex 5B falling weigHtsurface modulus analYs

5B.1. Deflection measurements can be used to produce surface modulus plots. The surface modulus at a point, distance r from the centre of the loaded area, is roughly equal to the “weighted mean elastic stiffness” below a depth R on the load centre line. Note that the depth R is based on the “equivalent pavement thickness”, where the thickness of the pavement layers is converted to an equivalent thickness of a material with an elastic stiffness equal to the subgrade stiffness. At a point sufficiently far from the loaded area, the deflection is not influenced by the upper pavement layers. Therefore the surface modulus calculated at the outer points on the deflection bowl is approximately equal to the subgrade modulus. Such plots give an indication of the stiffness of the pavement at different equivalent depths and can be used as guidance for the selection of further investigation and analysis methods. Further details of this method are given elsewhere (FEHRL, 1996)2.

The surface modulus at the top of the pavement (equivalent depth = 0mm) is calculated as:

Eo = 2(1-ν2) σo a/δo

The surface modulus at the equivalent depth R (valid for r>2a) can be calculated from:

Eo (r) = (1-ν2) σo a2/(r.δr)

Where: Eo = the surface modulus at the centre of the loading plate (MPa) Eo (r) = the surface modulus at a distance r (MPa) ν = Poisson’s ratio σo = contact pressure under the loading plate (kPa) a = radius of the loading plate (mm) r = distance from sensor to loading centre (mm) δr = deflection at a distance r (microns)

5B.2to ascomin F

i.

ii.

iii.

iv.

v.

2 FEHRL. Harmonisation of the Use of the Falling Weight DeflectomMeasurements and Data Processing for Flexible Road Pavement E1996.

may 2008

For surface modulus analysis, it is normal sume a value of 0.35 for Poisson’s ratio. Five mon examples of surface modulus plots are shown igure 1.1:

A continuously decreasing value of surface modulus with increasing distance. This indicates that the outermost deflection measurement points were not far enough away from the load.

A decreasing value which becomes constant. This indicates a normal pavement structure overlying a linear elastic subgrade.

A decreasing value which starts to gradually increase for the outer deflection measurement points. This indicates a normal pavement structure on a non-linear elastic subgrade, or a layered subgrade which increases in stiffness with depth.

A decreasing value with a sudden large increase for the outermost measurement points. This indicates that a very stiff subgrade layer underlies the pavement (e.g. bedrock).

A minimum value close to the surface. This indicates a weak interlayer somewhere in the upper bound layers.

a5B/1


a5B

annex 5B Falling Weight Deflectograph Surface Modulus AnalaysisAnnex 5B Volume 7 Section 3 FWD Surface Modulus Analysis Part 2 HD 29/08

A5B

figure 5B.1: typical surface modulus plots

Figure 5B.1. Typical Surface Modulus Plots.

may 2008/2/2 January 2008


annex 5B Falling Weight Deflectograph Surface Modulus Analaysis

TEMPLATE FOR REPORTINg FWD SURVEYS AND BACK-ANALYSIS

Survey - general

Start date: . . . . . . . . . . . . . . . . . . . Finish time: . . . . . . . . . . . . .

Survey Company: . . . . . . . . . . . . . . . . . . . . . . . . . . Finish date: . . . . . . . . . . . . . . . . . . .

Operator: . . . . . . . . . . . . . . . . . . . . Start time: . . . . . . . . . . . . . .location

Road No: . . . . . . . . . . . Lane No: . . . . . Start Section: . . . . . . . . . . Ch: . . . . . . .

Location: . . . . . . . . . . . . . . . . . . . . . . . . End Section: . . . . . . . . . . Ch: . . . . . . .

Pavement type: FF / FC / Rigid Total length: . . . . . . . . . mfwd geophones

Make: . . . . . . . . . . . . Model: . . . . . . . . . . . . Type: . . . . . . . . . . . . . . . . Serial number: . . . . . . . . . . . . .

Load : . . . . . kN Plate size: . . . . . .mm Number: . . . . . . . .

Test type: Standard / LTE Positions: . . . . . . . . . . . . . . . . . . . . . . . . . .temperatures

Locations: . . . . . . . . . . . . . . . . . . . . . . . . . . Maximum temperature: . . . . . .oC

Depths (if not 100mm): . . . . . . . . . . . . . . . . .mm Minimum temperature: . . . . . .oC

Times of measurement: . . . . . . . . . . . . . . . . . .Hapms files

“Supplementary” and “F20” files attached: Yes / No survey results

1. Normalised tabulated deflection data attached: Yes / No 2. Normalised d1, [d1-d4] and d6 plots attached: Yes / No 3. Temperature data attached: Yes / Noanalysis details

Name of Program: . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . Version: . . . . . . . . Mode: . . . . . . . . . . . . . . . . . . . .

No. of layers: . . . . . . Asphalt thicknesses - Max: . . . . . . . . . mm Min: . . . . . . . . . . mm Hydraulically bound mixture thicknesses - Max: . . . . . . . . . mm Min: . . . . . . . . . . mm

Sources of thickness information: Cores (No. . . . .) / GPR / As built Drgs / Other (Please specify)

Type of analysis: Standard / Non-standard (give reasons and details)analysis results

1. Tabulated back-analysed stiffnesses (at field temperature) attached: Yes / No 2. Tabulated back-analysed stiffnesses (adjusted to 20oC for asphalt layers) attached: Yes / No 3. Tabulated layer thicknesses used for each test point for analysis attached: Yes / No 4. AMD and RMS values for each test point attached: Yes / No 5. Layer stiffness (adjusted to 20oC for asphalt layers) plots attached: Yes / NoIf any data not attached please provide explanation:

may 2008 a5B/3


ION TO gROUND-

nal at Pavement Interfaces and Signal Waveform

annex 6a Introduction to ground-Penetrating Radar

Volume 7 Section 3 Annex 6A Part 2 HD 29/08 Introduction to Ground-Penetrating Radar

ON TO GROUND-PENETRATING

nal at pavement interfaces and signal waveform

Reflected signal

X

Layer 1

Layer 2

Surface

The reflected signals are collected by the receiver and create the waveform.

ANNEx 6A INTRODUCTpenetrating radar

general principles

6A.1 Ground-penetrating radar (GPR) operatesby transmitting a pulse of electromagnetic radiationfrom an antenna into a pavement. The electromagnradiation penetrates down into the pavement as an energy wave, with an envelope in the shape of a co

6A.2 As the wave travels through the various pavement layers, its velocity is changed and its streis attenuated. Part of the signal will be reflected bacat buried discontinuities or interfaces between diffematerials such as different pavement layers. These reflected signals and the two-way travel time contaithe information about the interior of the pavement. The strength of the reflected wave depends mainly the difference in the dielectric constant of the adjacmaterials in the pavement, the greater the differencestronger the reflection. In addition, polarisation of twave and its angle of incidence will affect the strenof the reflected signal.

6A.3 The pulse of electromagnetic radiation cabe visualised as a single cycle of a sinusoidal wavetravelling into the pavement in a straight line and bpartly reflected at each layer interface. The path of

Figure 6A.1: Reflection of Radar Sig

ANNEX 6A – INTRODUCTIRADAR General Principles

6A.1 Ground-penetrating radar (GPR) operates byantenna into a pavement. The electromagnetic radiaan envelope in the shape of a cone.

6A.2 As the wave travels through the various paveattenuated. Part of the signal will be reflected back materials such as different pavement layers. These information about the interior of the pavement. Thein the dielectric constant of the adjacent materials inreflection. In addition, polarisation of the wave andsignal.

6A.3 The pulse of electromagnetic radiation can bthe pavement in a straight line and being partly reflreflection at layer interfaces is shown in the left hanaddition to some of the wave being reflected, part oattenuation depends on frequency and on the type abe too weak for any reflections to be picked up by tpenetrate into the pavement is limited, although at tusually adequate to determine the thickness of mos

6A.4 The radar receives, via its antenna, the reflecand arrival time relative to when the signal pulse wadisplayed as a radar waveform as shown in the righsignal amplitude against time. The amplitude wavepavement layers and represent the reflections of thedepth, is shown by the decrease in the amplitude of

Figure 6A.1 Reflection of radar sig

As the signal travels down the pavement, part of the signal is reflected at interfaces of different materials.

Transmitted signal

T X R

Radar antenna


etic

ne.

ngth k rent

n

on ent the

he gth

n eing the

wave and its reflection at layer interfaces is shown in the left hand part of Figure 5.1. As the signal penetrates the pavement, in addition to some of the wave being reflected, part of the energy is absorbed and the wave is attenuated. The attenuation depends on frequency and on the type and condition of the pavement material. Eventually the wave will be too weak for any reflections to be picked up by the radar receiver. For these reasons the depth to which GPR can penetrate into the pavement is limited, although at the frequencies used for road surveying the penetration is usually adequate to determine the thickness of most pavements.

6A.4 The radar receives, via its antenna, the reflected wave, recording its amplitude (strength), phase, frequency and arrival time relative to when the signal pulse was transmitted from the antenna. The reflected signal may be displayed as a radar waveform as shown in the right hand part of Figure 5.1. The waveform shows the reflected signal amplitude against time. The amplitude wavelets on the waveform are caused by the interface between the pavement layers and represent the reflections of the sinusoidal radar signal. The attenuation of the signal, with depth, is shown by the decrease in the amplitude of the wavelets as time increases.

transmitting a pulse of electromagnetic radiation from an tion penetrates down into the pavement as an energy wave, with

ment layers, its velocity is changed and its strength is at buried discontinuities or interfaces between different reflected signals and the two-way travel time contain the strength of the reflected wave depends mainly on the difference the pavement, the greater the difference the stronger the

its angle of incidence will affect the strength of the reflected

e visualised as a single cycle of a sinusoidal wave travelling into ected at each layer interface. The path of the wave and its d part of Figure 5.1. As the signal penetrates the pavement, in f the energy is absorbed and the wave is attenuated. The nd condition of the pavement material. Eventually the wave will he radar receiver. For these reasons the depth to which GPR can he frequencies used for road surveying the penetration is t pavements.

ted wave, recording its amplitude (strength), phase, frequency s transmitted from the antenna. The reflected signal may be t hand part of Figure 5.1. The waveform shows the reflected lets on the waveform are caused by the interface between the sinusoidal radar signal. The attenuation of the signal, with the wavelets as time increases.

a6a/1A6A/1




transmitting (firing) pulses at fixed time or distance waveforms, a waveform graph representing the pavement ce of reflected waveforms recorded as a GPR system is

6A.5 By moving the radar along or across the pavement, transmitting (firing) pulses at fixed time or distance intervals, and recording and storing digitally the reflected waveforms, a waveform graph representingthe pavement structure is built up. Figure 5.2 is an example of a sequence of reflected waveforms recorded as a GPR system is moved along a pavement.

figure 6a.2: waveform graph

6A.6 Electronic distance measuring devices should be used to make GPR systems fire at fixed distance intervals rather than time intervals. This is important in situations where the speed of the survey is likely to be affected by traffic. It also should improve the accuracy of measurement of the alignment of dowel bars in concrete roads.

6A.7 The waveform is made up of a number of measured amplitude points equally spaced down the time axis; the more points that make up the waveform, the better the data quality. Conversely, however, as the number of measured points per waveform is increased, the sampling rate the radar system can achieve in the direction of travel decreases.

6A.8 Interpretation of the pavement structure and features from a waveform graph requires:

Annex 6A Introduction to Ground-Penetrating Radar

6A.5 By moving the radar along or across the pavement,intervals, and recording and storing digitally the reflectedstructure is built up. Figure 5.2 is an example of a sequenmoved along a pavement.

Figure 6A.2 Waveform graph

6A.6 Electronic distance measuring devices should be usrather than time intervals. This is important in situations wtraffic. It also should improve the accuracy of measurem

6A.7 The waveform is made up of a number of measuredmore points that make up the waveform, the better the datpoints per waveform is increased the sampling rate the rad

6A.8 Interpretation of the pavement structure and feature

• the wavelets to be correctly related to changes in m

• the signal travel time interval between the wavelets

Note that signal velocity will depend on the material the w

a6a/2

A6A/2

may 2008

of longitudinal section of road

• the wavelets to be correctly related to changes in material or other features; and

• the signal travel time interval between the wavelets to be converted to thickness using signal velocity.

Note that signal velocity will depend on the material the wavelets are passing through.

6A.9 The general procedure used by GPR Contractors to identify layer interfaces and the equation to convert the time data to depth data is given in Annex 6B of this Part. The conversion process, commonly known as calibration, requires a value for the velocity of the signal in each detected layer.

6A.10 There are four methods of obtaining the signal velocities in the layers and these are described in more detail in Annex 6B of this Part:

of longitudinal section of road

ed to make GPR systems fire at fixed distance intervals here the speed of the survey is likely to be affected by

ent of the alignment of dowel bars in concrete roads.

amplitude points equally spaced down the time axis; the a quality. Conversely however as the number of measured ar system can achieve in the direction of travel decreases.

s from a waveform graph requires:

aterial or other features; and

to be converted to thickness using signal velocity.

avelets are passing through.

January 2008



• Use of published velocity data – This is the least accurate method because, for a given material, the range of possible velocities is relatively wide, as indicated in Table 6B.1 of this Part, and it is difficult to estimate where within the range the material will be. In addition any moisture present in the material will greatly affect the velocity.

• The ‘core’ method – This method can give an accurate estimate of velocities and material types providing that the whole of the layers are recovered. The limitations are that the estimate relates to the core’s position and applies over the area of the core which is different to the area of the antenna footprint. The method cannot take into account material variations between core points.

• The ‘common depth point’ method – This method takes into account material variations along the pavement but relies on averaging over many measurements to reduce errors and therefore cannot deal with very localised variability in materials. Also, when surveying at high speed very shallow layer boundaries can present difficulties when wider separations of the antennas are used.

• The ‘reflection coefficient method’ – This method also takes into account material variations along the pavement, but determines the velocity at the upper surface of a material layer and this velocity may not always represent the velocity in the whole of the layer. As layers lower and lower in the structure are measured the accuracy decreases because the method assumes no energy loss by scatter and absorption in the pavement material.

6A.11 The interpretation of the raw data should either be carried out by the GPR Contractor undertaking the GPR survey or by another Contractor specialising in the interpretation of GPR surveys of pavements.

6A.12 Highway Engineers should not undertake, without training, interpretation of raw radar data, such as waveform graphs.

6A.13 A measure of the accuracy of the results of a survey can be obtained by comparing layer thicknesses obtained from cores, not used to calibrate the radar system, with the radar reported thicknesses at the same points. It should be noted however that the core data will not always correlate perfectly with the radar results. This can be due to errors in location, material loss from the core, radar measurements giving the average depth

may 2008

over the antenna footprint rather than the area of the core, and internal bound layer boundaries in the core being incorrectly identified as the base of the bound layer.

types of radar and their operation

6A.14 GPR measuring systems are generally defined by the:

• method of data storage (analogue or digital);

• single or multi-channel system;

• measured points per waveform;

• sampling rate;

• method of sampling (per fixed interval of time or per fixed interval of distance);

• antenna operating frequency;

• antenna signal coupling;

• antenna type.

6A.15 Analogue radar systems record data on rolls of paper while the digital systems allow storage of data direct to DAT tape or hard disk as well as to roll printers. Analogue recording has some disadvantages in that the data cannot be filtered to increase quality, settings such as signal gains cannot be varied at the analysis stage to take account of variations in the road, and the process of digitising the data from the paper printouts can introduce additional errors.

6A.16 The surface area, or footprint, that a radar signal examines depends mainly on the antenna design. The area will typically be about 300 mm diameter for a dipole antenna and about 100 mm diameter for a horn antenna. Multi-channel systems allow a wide range of data collection options ranging from, one measuring line being scanned with antennas operating at different frequencies in one run, to a number of parallel measuring lines being scanned with antennas operating at the same frequency in one run. The first option is useful for network level surveys where data is only gathered from one line (generally the nearside wheel-track) and the second option is useful where large areas need to be surveyed in detail.

6A.17 Each waveform is made up of a number of measured amplitude points equally spaced down the time axis; the more points that make up the waveform,

a6a/3



olution

(mm)

depth of penetration

(mm)

(mm)

resolution

(mm)

depth of penetration

(mm)37 447 134 34 40234 408 122 31 36631 378 113 28 33929 353 106 27 31828 333 100 25 30026 316 95 24 28425 302 90 23 27124 289 87 22 260

gHz 4gHzepth of olution

(mm)

min. depth of

penetration (mm)

wavelength

(mm)

depth of resolution

(mm)

min. depth of

penetration (mm)

17 201 34 8 10115 184 31 8 9214 170 28 7 8513 159 27 7 8013 150 25 6 7512 141 24 6 7111 136 23 6 6811 130 22 5 65

resolution for various types of radar

the better the data quality. Conversely however, as the number of measured points per waveform is increased the sampling rate that the radar system can achieve in the direction of travel decreases.

6A.18 The sampling rate in the direction of travel depends on the speed of travel, the firing rate of the radar pulses, the number of points per waveform, and the number of channels available. The sampling rate controls the size of pavement features that the radar can detect. For example with a single channel radar that has a typical firing rate of 90 pulses per second, 512 points on the waveform, and a typical speed of travel of 80 km/h, a measurement is made every 250 mm of travel and at this sampling rate features, such as voids, smaller than 250 mm in length may be missed. It is therefore necessary to select an appropriate sampling rate for the size of features that need to be detected.

6A.19 Some radar systems take samples of the road at fixed time intervals. This will cause some location

erThatelra

6Aofin

6Aresigththbopainpem

450mHz 90dielectric constant

wavelength

(mm)

depth of resolution

(mm)

min. depth of

penetration (mm)

wavelength

(mm)

dres

5 298 75 894 1496 272 68 816 1367 252 63 756 1268 235 59 705 1189 222 56 666 11110 211 53 632 10511 201 50 603 10112 192 48 577 96

1.5gHz 2dielectric constant

wavelength

(mm)

depth of resolution

(mm)

min. depth of

penetration (mm)

wavelength

(mm)

dres

5 89 22 267 676 82 20 245 617 76 19 227 578 71 18 212 539 67 17 200 5010 63 16 189 4711 60 15 181 4512 58 14 173 43

table 6a.1: typical values of penetration and

a6a/4

rors when the speed of travel of the system changes, e problem can be reduced by recording survey events

frequent intervals and avoided altogether by using ectronic distance measuring systems that control the dar system to take samples at fixed distance intervals.

.20 The operating frequency will affect the depth penetration and the resolution of the radar as shown Table 6A.1.

.21 The positioning of the radar antenna in lation to the pavement surface determines how the nal is coupled. If the antenna is in contact with

e surface, the radar signal is ground coupled, if not en it is air coupled. The method of coupling affects th sensitivity and depth of penetration into the vement. Ground coupling introduces more signal to the pavement, enhancing sensitivity and increasing netration depth, but it makes traffic speed operation ore difficult.

0mHz 1gHzepth of min. wavelength depth of min.

may 2008



6A.22 There are two types of antenna design, dipoles and horns. Dipoles operate most effectively when they are ground coupled, however, they can be air coupled but the gap between the antenna and pavement surface has to be small, ideally one-tenth of the transmitted wavelength to give satisfactory results. This means that the antenna mounting has to be carefully designed to enable surveys to be done at traffic speed. Horn antennas are air coupled and operate with a large air gap and so are more easily adapted to surveys at traffic speed.

6A.23 Radar can be used to detect other features such as buried services and drains but this is outside the scope of this Annex.

6A.24 Depending on the coupling and design of the radar system, a survey can either be carried out at low speed, typically between 0.5 and 20 km/h, or at traffic speed, typically between 50 and 80 km/h. Figure 6A.3 illustrates radar systems operating over this range of survey speeds. The sampling rate, in the direction of travel, achieved with high speed surveys will be lower so only larger features will be detected.

may 2008 a6a/5



Radar with ground Coupled Dipole Sntenna – Crack Depth Investigation at 0.5 km/h surveying at 0.5 km/h

Radar with Horn Antenna – Surveying at 80 km/h Radar with a Pair of Air-Coupled Dipole Antennas – surveying at 80 km/h

Figure 6A.3 ground-Penetrating Radar Systems

may 2008a6a/6


f gpr for r tHickness

annex 6B calibration of gpr for determination of layer thickness

annex 6B caliBration odetermination of laYe

general

6B.1 This annex gives details of the methods used by the GPR Contractors to determine the thickness of the pavement layers from the radar data. The information is presented to illustrate the strengths and weakness of the different approaches.

6B.2 The process of determining the layer thicknesses from a GPR waveform is carried out in two stages:

• identifying layer interface wavelets on the waveform;

• using the times between the wavelets to calculate the layer thicknesses.

Stage 1 – Identifying Layer Interface Wavelets

6B.3 The layer interfaces appear on the waveform as wavelets and the following processes are carried out to identify and characterise all the layer interface wavelets:

• relate wavelets to material boundaries (it may be necessary to filter the data to clearly resolve the wavelets);

• track along pavement, wavelets relating to material boundary;

• record time to wavelet at each sample point along the pavement;

• repeat the above processes for all material boundaries forming the pavement.

While it is theoretically possible to carry out the above processes manually it is very impractical to do this due to the excessive time required and associated high cost incurred. The processes are much more accurately and rapidly carried out with a computer system that can automatically track clear boundaries but can be manually overridden, e.g. with a mouse pointer, where the signals are difficult to track automatically.

may 2008

Stage 2 – Calculating Layer Thickness

6B.4 The thickness of a layer is related to the signal travel time in the layer by the equation:

=

2T T D ...........................................................(1)

where:

D = layer thickness (mm); V = velocity of radar signal in layer (mm/ns); T = two way travel time of the signal in layer (ns).

Two way travel time of the signal in the layer is the time interval between the wavelets occurring at the layer’s lower and upper interfaces at the same location along the pavement.

signal velocity in layer

6B.5 The velocity of the radar signal within a layer is related to the layer material’s dielectric constant ε by the equation:

ε=

299 V .................................................................(2)

where:

V = velocity of radar signal in layer (mm/ns).

There are four basic methods of calculating velocities:

(a) use published data of mean velocities of pavement materials;

(b) use cores to establish layer thicknesses and then calculate velocities by rearranging equation (1) above;

(c) calculate velocity from recorded radar signal using ‘common depth point’ method;

(d) calculate velocity from recorded radar signal using ‘reflection coefficient’ method.

a6B/1


y (mm/ns) dielectric constant

299 1

– 160 4 - 10

– 130 5 - 9

– 120 6 - 9

– 120 6 - 18

– 110 7 - 18

33 81


6B.6 Method (a) uses published data and typical velocities for different pavement materials which are given in Table 6B.1. These have been calculated from the material’s dielectric constant using equation (2). Anymoisture in the material will alter the dielectric constant and hence affect the signal velocity greatly.

pavement material Velocit

Air

Asphalt 90

Concrete 100

Hydraulically bound mixture 100

Granular 70

Capping 70

Water

table 6B.1: range of velocities and dielect

6B.7 Method (b), often called the ‘core method’, enables velocities to be accurately calculated if done with care. Aspects requiring particular attention are:

• ensuring that the core position is accurately located within the GPR data;

• correctly estimating the layer thicknesses from a core;

• ensuring that the complete core is extracted from the pavement.

6B.8 It is known that layer thicknesses can change significantly over longitudinal distances of as little as a metre. If a core is not located on the radar data to an accuracy better than a metre it is possible that the wronglayer thickness will be used to calculate the signal velocities.

6B.9 Often the interface between pavement layers is quite rough. This means that it can be quite difficult to measure, on a core, the layer thickness. Typically, the accuracy with which the actual thicknesses may be measured from a core is ± 5 mm if the core is fully extracted and not damaged.

6B.10 Care must be taken when coring to extract the full depth of the bound layer because if the core is incomplete there is a risk of underestimating the thickness of the bound layer by incorrectly identifying an internal layer interface as the bottom of the layer.

a6B/2

ric constants for various pavement materials

6B.11 Method (c) is based on calculating the velocity using a common depth point and requires a multi-dipole antenna system. With this system, the various return signals for the different antennas, which have different travel times through a layer, are analysed to obtain values for the velocities in each layer. The method is illustrated in Figure 6B.1 for the surface layer of the pavement. It can be seen that, for a fixed antenna spacing, as the thickness of the layer increases and also as the layer being measured moves lower in the pavement structure, T1 approaches T2 thus decreasing the accuracy of the method. This can be compensated for by increasing the antenna separation but accepting that shallow layer boundaries will then be difficult to resolve. Care must also be taken when moving down the pavement to ensure that each dipole pair samples the same spot.

6B.12 Method (d) is based on calculating the velocity using the reflection coefficient and requires a horn antenna. Before making measurements a metal plate is placed on the pavement surface to determine the amplitude of the signal returned from a perfect reflector. This amplitude is compared with the amplitude of the signal returned from the pavement surface and the other layer interfaces in the construction to obtain the velocity at the top of each layer. The method, illustrated in Figure 6B.2 for the surface layer, assumes that the receiving antenna collects all the transmitted energy. This assumption becomes less valid with increasing depth due to the combination of the horn’s

may 2008


er of pavement



of pavement

narrower receiving aperture and the increasing scatter and absorption of the radar signal by the pavement materials. For this method it is particularly important that conditions where there is standing water on the pavement surface be avoided as the water layer may affect the calibration.

21

22

21

22

TTSS V

−−

=

where:

V = Signal velocity in surface layer of pavement (mm/ns)

T1 = Signal travel time along path XOY (nsec)

T2 = Signal travel time along path WOZ (nsec)

S1 = Antenna spacing XY (mm)

S2 = Antenna spacing WZ (mm)

figure 6B.1: common depth point method for signal velocity in surface lay

Annex 6B Calibration of GPR for Determination of Layer Thickness

21

22

21

22

TTSS V

where:


T1 = Signal travel time along path XOY (nsec)

T2 = Signal travel time along path WOZ (nsec)

S1 = Antenna spacing XY (mm)

S2 = Antenna spacing WZ (mm)

Figure 6B.1 Common depth point method for signal velocity in surface layer

may 2008 a6B/3

A6B/4 January 2008


annex 6B calibration of gpr for determination of layer thicknessVolume 7 Section 3 Annex 6B Part 2 HD 29/08 Calibration of GPR for Determination of Layer Thickness

−+

=

21

21

AAAA

299 V

where:


A1 = Signal amplitude reflected from metal plate

A2 = Signal amplitude reflected from pavement surface

299 = Signal velocity in air (mm/ns)

Figure 6B.2: Reflection Coefficient Method for Signal Velocity in Surface Layer of Pavement

21

21

AAAA

299 V

where:


A1 = Signal amplitude reflected from metal plate

A2 = Signal amplitude reflected from pavement surface

299 = Signal velocity in air (mm/ns)

Figure 6B.2 Reflection coefficient method for signal velocity in surface layer of pavement

may 2008a6B/4January 2008 A6B/1

may 2008


annex 6c reporting tHe results of a gROUND-PENETRATINg RADAR SURVEY

6C.1 Examples of how the data may be presented in graphical form and tabular form are shown in Figure 6C.1 and Tables 6C.1 and 6C.2.

a6c/1

annex 6c Reporting the Results of a ground-Penetrating Radar Survey

may 2008

volume 7 section 3

part 2 Hd

29/08

Figure 6C.1: An Example of a graph Showing the Longitudinal Depth Profile of the Bound and granular Layers of a Pavement

a6c

/2

annex 6c

R

eporting the Results of a g

round-Penetrating Radar Survey

Volume 7 Section 3 Annex 6C Part 2 HD 29/08 Reporting the Results of a GPR Survey

January 2008 A6C/1

ANNEX 6C. REPORTING THE RESULTS OF A GROUND-PENETRATING RADAR SURVEY 6C.1 Examples of how the data may be presented in graphical form and tabular form are shown in Figure 6C.1 and Tables 6C.1 and 6C.2.

Figure 6C.1 An example of a graph showing the longitudinal depth profile of the bound and granular layers of a pavement

Construction changes Calibration cores Section identifier/Nodes

Road number: A123Date of survey: 26/10/2006 Road type: dual carriage way lane 1 SB Construction: Flexible/composite Survey length :2000m Surface condition: dry

may 2008


Network section identifier: 600A123 12 lane/s surveyed and track: Nearside lane, nearside wheel-track

road number: A123

date of survey: 19-6-2006 length surveyed: 900 m

direction of survey: South

road type: Dual 2 lane APTR surface moisture condition: Dry

construction: Flexible with asphalt base

chainage from start of network

section (m)

Bound layer depth (mm)

granular layer depth (mm)

comments (defect features, construction changes and core information)

100 245 225

200 245 220

300 250 215 Calibration core taken from nearside wheel-track at chainage 320m

400 255 215

500 255 210

600 250 210

Table 6C.1: An Example of How Layer Thickness Data from a gPR Survey Should be Presented

Network section identifier: 1900M21 10 lane/s surveyed and track: Nearside lane, nearside wheel-track

road number: M21

date of survey: 19-6-2006 length surveyed: 1200 m

direction of survey: South

road type: Dual 2 lane motorway surface moisture condition: Damp no standing water

construction: Concrete

chainage from start of network

section (m)

Bound layer depth (mm)

granular layer depth

(mm)

Comments (defect features, construction changes and core information)

120 260 180 Indications that some dowel bars are misaligned

130 260 175

140 260 180

150 270 180 Possible void starting about 2 m from nearside edge of lane, approximate area 2 x 1.5 m. Indications that some dowel bars within area of void are too close to the surface

160 270 180

170 270 180

180 280 175

190 280 175

Table 6C.2: An Example of How the gPR Survey Data for a Concrete Pavement Should be Presented

a6c/3

annex 6c Reporting the Results of a ground-Penetrating Radar Survey


part 3

Hd 30/08

maintenance assessment procedure

summarY

This standard includes the general procedure for assessing the need for maintenance of road pavements and a general guide to the design of maintenance treatments. It has been revised to reflect changes in HD 29/08 and value management procedures.

instructions for use

1. Remove existing HD 30/99, which is now replaced by HD 30/08 and archive as appropriate.



Note: A quarterly index with a full set of Volume Contents Pages is available separately from The Stationery Office Ltd.


may 2008

design manual for roads and Bridges Hd 30/08 volume 7, section 3, part 3

tHe HigHwaYs agencY

scottisH government

welsH assemBlY government llYwodraetH cYnulliad cYmru

tHe department for regional development nortHern ireland

Maintenance Assessment Procedure

Summary: This standard includes the general procedure for assessing the need for maintenance of road pavements and a general guide to the design of maintenancetreatments.IthasbeenrevisedtoreflectchangesinHD29/08and value management procedures.

may 2008



amend no

page no signature & date of incorporation of

amendments

amend no page no signature & date of incorporation of

amendments



part 3

Hd 30/08

maintenance assessment procedure

contents

Chapter

1. Introduction

2. Pavement Deterioration Mechanisms

3. Network Survey Data

4. Collection and Review of Existing Data

5. Scheme Level Surveys and Investigations

6. Interpretation

7. Treatment Design


9. Enquiries


may 2008


1/1


may 2008

1. introduction

mandatory sections

1.1 Sections of this document which form part of the Standards of the Overseeing Organisations are highlighted by being contained in boxes. These are the sections with which the design organisations must comply, or must have agreed a suitable departure from Standard with the relevant Overseeing Organisation. The remainder of the document contains advice and enlargement which is commended to design organisations for their consideration.

general

1.2 Road pavements do not last indefinitely. At some stage in their lives signs of wear such as polishing, rutting, fretting or ravelling (surface disintegration), and cracking may show on the surface. Maintenance is required when these signs of wear are judged to affect the standards of service provided to the road user and the integrity of the pavement structure. To accomplish this task in the most cost-effective manner, it is necessary to use a logical assessment procedure to enable the correct maintenance treatment to be carried out at the most advantageous time. Regardless of whether strengthening or just resurfacing is being contemplated, carefully considered and designed pavement assessment is essential. This is to ensure that the strengthening is warranted and is of the right degree, and to avoid re-surfacing being laid on a structurally inadequate pavement. For the trunk road network in England, maintenance proposals are reviewed through the Value Management process which requires documentary evidence for each scheme of appropriate investigation, interpretation and treatment selection.

1.3 In-service roads, even those built up over very many years, usually conform to one of the flexible or rigid construction types described in HD 26 (DMRB 7.2.3). A uniform approach to the collection of condition information, its presentation and assessment is described in this Part which can be applied to all these pavement types, although detailed procedures will vary depending on the specific type of construction present. Deterioration is caused by a combination of factors and this Part is not intended to be a substitute for the expertise and judgements of the designer. Every case

mufee

sco

1.4assThetec7.3resPartreamain HIntetec

1.5roaunube in tthethepra

1.6thisappWeconresmu

im

st be treated on its merits and the designer must not l constrained to use only the methods described.

pe

This Part outlines the general procedure for essing the need for maintenance of road pavements. various types of road survey and investigation

hniques involved are described in HD 29 (DMRB .2). The details of routine testing for skidding istance are contained in HD 28 (DMRB 7.3.1). This t also provides a guide to the design of maintenance tments but does not cover routine or minor

intenance, some aspects of which are mentioned D 31 (DMRB 7.4.1) and HD 32 (DMRB 7.4.2). rpretation of the various surveys and investigation

hniques is contained in this Part.

This Part does not deal with the assessment of d pavements which have failed or deteriorated in an sual manner or where contractual warranties may

in force, e.g. substantial defects appearing very early he expected life of the pavement. In these situations investigation and assessment methods would reflect specific circumstances but could be based on the ctices of HD 29 (DMRB 7.3.2) and this Part.

The various references made within the text of Part to the Network Maintenance Manual (NMM) ly only to England. For guidance in Wales, the lsh Trunk Road Maintenance Manual must be sulted. Guidance on requirements applying to the

t of the UK, where these are not already in the text, st be obtained from the Overseeing Organisation.

plementation

1.7 This Part must be used forthwith on all schemes for the improvement and maintenance of trunk roads including motorways, currently being prepared, provided that, in the opinion of the Overseeing Organisation, this would not result in significant additional expense or delay. Design organisations must confirm its application to particular schemes with the Overseeing Organisation.



use in northern ireland

1.8 For use in Northern Ireland, this Standard will apply to those roads designated by the Overseeing Organisation.

mutual recognition

1.9 The construction and maintenance of highway pavements will normally be carried out under contracts incorporating the Overseeing Organisations’ Specification for Highway Works (SHW) which is contained in the Manual of Contract Documents for Highway Works Volume 1 (MCHW 1). In such cases products conforming to equivalent standards and specification of other Member States (MS) of the European Economic Area (EEA) or a State which is party to a relevant agreement with the European Union and tests undertaken in other MS of the EEA or a State which is party to a relevant agreement with the European Union will be acceptable in accordance with the terms of Clauses 104 and 105 (MCHW 1.100). Any contract not containing these Clauses must contain suitable clauses of mutual recognition having the same effect, regarding which advice must be sought.

Health and safety

1.10 All survey and data collection on or in the vicinity of highway pavements must be carried out in accordance with:

• Health and Safety at Work Act (1974);


• Construction (Design and Management) Regulations (2007) (CDM Regulations);



m

1.pa

1/2

1.11 In Northern Ireland, the relevant Health and Safety documents are:

• Construction (Design and Management) Regulations (Northern Ireland) 2007;

• Health and Safety at Work (Northern Ireland) Order 1978;




1.12 For the Highways Agency network, further information on Health and Safety is given in Part 1 of the Network Management Manual.

aintenance assessment process

13 The principal stages in assessing the need for vement maintenance are shown in Figure 1.1.

may 2008



1. Network Survey Data and identification of possible maintenance schemes TRACS or SCANNER, SCRIM, safety, routine or other visual inspections and Deflectograph (for some networks).

Carry out HD 28 investigation if there are SCRIM values < Investigatory Level.

2. collection and review of existing data e.g. Traffic, records of age of construction, maintenance, defects and repairs, and previous survey data.

3. plan scheme surveys and investigation

Chapter 5

4. carry out scheme surveys • Pavement: Visual Condition Survey, cores etc. • Other (as required): Earthworks, drainage etc.

Chapter 5

5. interpretation of data

Chapter 5

6. treatment design

Chapter 5

figure 1.1 – investigation and assessment process

glossary

1.14 A glossary and list of principal abbreviations is given in HD 23 (DMRB 7.1.1).

1.15 In this Part, the term ‘asphalt’ replaces ‘bituminous material’ as the generic term for pavement material consisting of mineral aggregate combined with a bitumen binder and which is normally laid by a paver. ‘Asphalt’ includes all bitumen bound base, binder course and surface course mixtures, except surface dressing. The exception to this is indicated below, where ‘bituminous material’ continues in use:

•

1.Mpabocobocoin

may 2008

Deflectograph processing uses the technical terms ‘Equivalent Thickness of Sound Bituminous Material (ESBM)’, ‘Total Thickness of Bituminous Material (TTBM)’ and Base Type Classification ‘BITS’. Use of the term ‘asphalt’ would require changes to these acronyms and to text in the associated software.

16 In this Part, the term ‘Hydraulically Bound ixture’ or ‘HBM’ is used as the generic term for vement material consisting of mineral aggregate und with cement, slag, lime or fly ash; or a mbination thereof. The terms ‘lean concrete’, ‘cement und material’ or ‘CBM’ are no longer used except in nnection with Base Type Classification ‘CEMT’ used Deflectograph processing.

1/3


tion mecHanisms

chapter 2 pavement deterioration mechanisms

2. pavement deteriora

introduction

2.1 Although there are many common factors, there are also some differences in the surface and structural deterioration mechanisms of pavements depending on whether they are, generally, of flexible or rigid construction.

2.2 The surfaces of all pavements eventually suffer from loss of skidding resistance. Loss of texture and rutting can also occur, particularly for surfacing materials which have relatively high binder contents. For surface courses which lose aggregate as a result of environmental deterioration (see Paragraph 2.3) the texture depth may not decrease, indeed it may even increase. The deterioration mechanisms of specific types of pavement, commonly found on the road network are discussed below.

fleXiBle pavements witH aspHalt Base

2.3 Deterioration in flexible pavements with asphalt base is generally associated with traffic loading and/or with environmental factors. Deterioration due to traffic loading is normally associated with the following mechanisms:

• Repeated cycles of tensile strains generated within the bound layers under vehicle loading cause fatigue cracks to initiate in the asphalt. Classical pavement analysis indicated that these cracks would generally initiate at the underside of the asphalt base and then propagate upwards through the material. However, the view of how this mechanism operates in practice on the thick pavements that comprise the greater part of the trunk road and motorway network has been revised following extensive observation and investigation, and is further discussed in Paragraphs 2.4 to 2.6.

• Rutting due to the permanent, cumulative deformation of one or more of the various layers within the pavement structure including the foundation. Where the rutting emanates from the subgrade or pavement foundation and the entire pavement structure is deformed, this is referred to as structural deformation. Rutting that is confined to the asphalt surfacing is termed non-structural.

may 2008

The main environmental causes of pavement deterioration are:

• The asphalt binder may harden over time with consequent effect on the fatigue resisting properties of the mixture. One of the principal mechanisms of binder hardening is held to be oxidation of the bitumen, and this predominantly occurs at the surface of the pavement exposed to air and solar radiation. Strains at the pavement surface caused by both thermal cycling and vehicle loading can eventually lead to cracks appearing at the surface. Over time these may propagate downwards and could ultimately reach the base of the bound layers. The hardening of the bitumen may also affect the cohesion of the mixture and may lead to the loss of aggregate in the surfacing (fretting or surface disintegration).

• As bitumen is a visco-elastic material the performance of asphalt mixtures is influenced by the service temperature. The risk of the accumulation of permanent deformation in the surfacing (non-structural rutting) will, therefore, be increased during periods of hot weather and further exacerbated by slow moving and/or stationary traffic. This risk can be mitigated by the selection of appropriate, well-designed and placed materials (see HD 37).

Less common environmental causes of pavement deterioration include the variation in foundation strength caused by seasonal changes in moisture levels and the action of a freeze-thaw cycle, particularly on cracked pavements of thin construction.

long-life flexible pavements with asphalt Base

2.4 Thick, well constructed flexible pavements with asphalt base on strong foundations do not suffer bottom-up, fatigue cracking of the base or structural deformation (Nunn et al., 1997). Environmental factors can cause cracking to develop at the surface, which will gradually increase in depth. Deformation in these pavements also tends to be limited to the surfacing layers (i.e. is non-structural). Very long pavement lives can be achieved by the removal of any cracked or severely rutted material, before the defect has progressed too deeply, and its replacement with new material.

2/1



2.5 Criteria based on measured deflection and total thickness of bituminous material may be used to identify flexible pavements with asphalt base with the potential for long life. This identification is carried out as part of Deflectograph data processing, either within HAPMS or with the PANDEF software, described in

Figure 2.1 – Deflectograph-Ba

2.6 The Total Thickness of Bituminous Material (TTBM) shown in Figure 2.1 is the combined thickness of all the contiguous intact asphalt layers present in the pavement, subject to the following criteria:

a) Asphalt surfacing layers (i.e. those within the top 100mm of the existing pavement) are included in TTBM regardless of their condition.

b) Asphalt layers which are known to be severely deteriorated and whose upper surface is at a depth greater than 100mm are not included in TTBM.

c) Any intact asphalt (or deteriorated surfacing material) that is separated from other intact asphalt materials by either a severely deteriorated asphalt layer or any granular layer (either of which must be greater than 25mm thick and have their upper surface at a depth greater than 100mm) is not to be included in TTBM.

2/2

HD 29 (DMRB 7.3.2). The criteria, see Figure 2.1, are conservative in that pavements which plot just above the boundary curve, in the Determinate-life Pavements zone, may also be either Upgradeable to LLP or Potentially Long Life.

sed Pavement Life Categories

2.7 Although the pavement life categorisation in Figure 2.1 can be applied to individual deflections, the classification of a length of pavement as long-life should normally be based on the 85th percentile of the maximum deflection of both wheel-tracks within each 100m length.

determinate-life flexible pavements with asphalt Base

2.8 Flexible pavements with asphalt base that do not meet the ‘long-life’ criteria in Figure 2.1 are subject to both traffic induced and environmental deterioration. These pavements are referred to as determinate life as their life to investigatory condition may be estimated using the Deflectograph-based design method described in HD 29 (DMRB 7.3.2).

may 2008



fleXiBle pavements witH HYdraulicallY Bound Base

2.9 These pavements consist of a lower base of hydraulically bound mixture (HBM) designed to withstand traffic induced stresses and an asphalt upper base and surfacing which insulate the HBM and contribute to load spreading. The strength and thickness of the HBM layer has a significant influence on the progression of deterioration, which is also associated with the effects of traffic and the environment as described above.

2.10 Thermal effects in this type of pavement usually give rise to primary transverse shrinkage cracks in the HBM during construction. In time, these cracks can lead to cracking in the overlying asphalt, known as reflective cracking. Generally, reflective cracking starts in the surfacing, like environmental cracking, and does not necessarily penetrate to the full depth of the asphalt layers. Reflective and environmental cracks may, if left untreated, allow the ingress of water to materials beneath the surface which may be moisture susceptible. Transverse cracks in the base which are wide enough to significantly reduce granular interlock can give rise to poor load transfer which can cause significant pavement deterioration.

2.11 The asphalt surfacing may also develop environmentally induced defects such as surface cracking and loss of aggregate due to hardening of the bitumen.

2.12 Occasionally, surfacing failures occur as a result of incorrectly installed reinforcing grids or separation membranes appearing at the surface.

rigid pavements

2.13 These include the following types, which are detailed in HD 26 (DMRB 7.2.3):

• Unreinforced Jointed Concrete (URC);

• Jointed Reinforced Concrete (JRC);

• Continuously Reinforced Concrete Pavement (CRCP), which may have been surfaced with an asphalt Thin Surface Course System;

• Continuously Reinforced Concrete Base (CRCB), which will have an asphalt overlay of at least 100mm.

2wretoin

2ppthincod

J

2uinoinisRtrm

2inopd

2aTreb

c

2(Ccloeinb1reCtethb

may 2008

.14 The major surface-only defect in rigid pavements ithout asphalt surfacing, in addition to loss of skidding sistance and texture, is surface spalling. This is related the durability of the concrete but is not normally dicative of structural deterioration of the pavement.

.15 Structural deterioration mechanisms in rigid avements are very different to those in fully flexible avements. Horizontal tensile stresses are generated by e combined effects of wheel loading and thermally duced internal and warping stresses. These stresses

an, under certain conditions, lead to cracking. This is ften associated with poor support of the slab caused by rainage problems or water ingress at joints.

ointed rigid pavements

.16 The two types of jointed concrete pavement, nreinforced (URC) and reinforced (JRC), both corporate joints which are designed to minimise the

ccurrence of uncontrolled, random cracking. Cracking unreinforced pavements is a major problem as there no reinforcement to hold the material together. einforced pavements can tolerate small amounts of ansverse cracking provided that good load transfer is aintained.

.17 Structural defects manifest themselves mainly the form of various types of cracking. Settlement

r failure of joints to operate properly may also occur, roblems which, if not remedied, can lead to the evelopment of cracks and subsequent failure.

.18 Where expansion joints have lost their capacity to bsorb movement “blow ups” may occur in hot weather. wo consecutive slabs rise up in an inverted vee as a sult of debris filling the expansion gaps or dowels

ecoming locked.

ontinuously reinforced rigid pavements

.19 Continuously reinforced concrete pavements RCP) and pavements with continuously reinforced

oncrete bases (CRCB) effectively contain continuous ngitudinal reinforcement with no intermediate

xpansion or contraction joints. Internal thermally-duced stresses within the concrete slab are relieved

y transverse cracks which normally occur at -2m spacings and are held tightly closed by the inforcement. The central portion of a long slab of RCP does not move when subjected to changes in mperature; longitudinal movement takes place only at e ends. This end-movement can be partly restrained

y ground beams in a ground beam anchorage or accommodated by a special joint.

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2.20 One form of defect that can occur in CRCP is punchouts. These can occur when closely spaced transverse cracks are connected by parallel longitudinal cracks causing small blocks of concrete to become loose and eventually detach from the pavement under repeated traffic load applications.

2.21 Where the concrete has asphalt surfacing this may also develop the environmental defects of surface cracking and loss of aggregate due to hardening of the bitumen, see Paragraph 2.11.

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a

chapter 3 network survey data

3. network surveY dat

introduction

3.1 The network survey data generally available are from SCRIM (which measures skidding resistance) and either TRAffic-speed Condition Surveys (TRACS) or Surface Condition Assessment of the National NEtwork of Roads (SCANNER) system. Deflectograph data may also be available for the non-Highways Agency networks.

3.2 The first stage in pavement maintenance assessment is to review the network survey data to decide whether the condition of the pavement has deteriorated to a state that may require remedial action within the next few years. For the HA network, all network survey data should be stored electronically on the Highways Agency Pavement Management System (HAPMS).

3.3 Visual surveys are no longer carried out as network surveys on the HA network (except for off-carriageway paved areas). However, the observations of the regular safety inspections to identify Category 1 or 2 defects (refer to the Network Management Manual for details) and other route inspections should be used to supplement the above machine surveys to assist in the identification of sections of the network which have the worst levels of deterioration.

scrim

3.4 SCRIM measures the skidding resistance of the road surface, which is reduced by the polishing action of traffic. Details of SCRIM surveys are given in HD 28 (DMRB 7.3.1).

tracs

3.5 For the HA network, TRACS measures longitudinal profile variance, texture, transverse profile, cracking and fretting. TRACS surveys now include downward-facing video images which can be supplied by the survey contractor, on request. These images would be very useful in assessing the observations from the safety and detailed inspections, described below.

3.6 Details of TRACS surveys are given in HD 29 (DMRB 7.3.2). Rutting, fretting, cracking and texture data are of most interest in deciding whether further maintenance investigations are required. The longitudinal profile variance data is not usually

may 2008

a strong indicator of pavement deterioration as the structural defects have to be quite severe before the profile is significantly affected. However, a poor profile (particularly high 10m or 30m variance values) may indicate foundation settlement problems such as compressible peat layers or mining subsidence. TRACS data are processed, validated and standardised before being supplied to Managing Agents. Software is available to Maintenance Agents to enable this data to be viewed and reported against criteria related to four categories of condition. It should be noted that fretting and texture are measured only in the nearside single wheel-track, ruts are measured in both wheel-tracks and cracking is measured over the whole carriageway.

3.7 Other Overseeing Organisations may use SCANNER surveys which measure similar parameters to those in TRACS. A summary of the SCANNER system is given in HD 29 (DMRB 7.3.2).

safety, detailed and other route inspections

3.8 Safety, detailed and other route inspections are regular visual inspections designed to identify the presence of Category 1 defects (safety inspections) and Category 2 defects (detailed inspections). The inspections are usually carried out by two trained personnel, operating together from a slow moving vehicle. In particular circumstances (e.g. in town centres, principal shopping areas, subways, footbridges and at complex road junctions) inspections may need to be carried out on foot. The occurrence of Category 1 and 2 defects and the inspectors’ observations on other defects not immediately affecting safety, can be of assistance when assessing the results from machine surveys (SCRIM, TRACS and SCANNER). These visual defects should be recorded on the HA’s RMMS system.

other available condition data

3.9 Much useful condition data are gathered during surveys which are primarily designed to cover areas of activity in which work is generally short term or cyclic (e.g. repair of potholes or need for control of vegetation) to ensure that the highway is kept in good working order. Instructions on Routine Maintenance Inspections in England are given in the Network Management Manual (NMM), Volume 2.

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chapter 3 network survey data

summary

3.10 Results from network surveys and safety inspections are expected to identify those parts of the network which are showing signs of potential surface or structural deterioration, for which further assessment is required. Further assessment will start with the collection and review of data as described in Chapter 4 and lead to a scheme investigation, as described in Chapter 5.

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ew of eXisting data

chapter 4 collection and review of existing data

4. collection and revi

introduction

4.1 To reach this second stage of assessment, the network survey data and safety inspections will have shown that there are surface defects in the carriageway.

4.2 The object of this stage is to assemble all existing information relating to the pavement and its condition in order to identify potential sites for maintenance and to determine what additional monitoring or scheme level investigations are necessary.

4.3 Accurate pavement layer thicknesses and material types should be established, if possible. In the case of flexible pavements with asphalt base, these are essential in determining whether the pavement falls into the long-life or determinate-life category. If this information is unavailable, incomplete or unreliable the true picture will have to await the results of coring and/or ground-penetrating radar surveys carried out as part of the scheme level surveys.

4.4 The review and collation of data should provide the information necessary to make certain that any further action is well planned and appropriate so as to ensure that any remedial treatment is effective and economic.

data required

4.5 The following data should be assembled, if available:

• Data from all available network condition surveys – TRACS (or SCANNER) and SCRIM;

• Schedules of Cat 1 and Cat 2 defects and any other reports from the safety and routine inspections;

• Generalised construction type (flexible or rigid, base type, etc.);

• Pavement layer materials and thicknesses;

• Dates of construction and maintenance history;

• Local topography, geology and soil conditions;

• Location of cut or fill;

•

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may 2008

General drainage details;

Current and past traffic to enable the cumulative traffic carried to be estimated;

Photographs – showing general views and typical defective areas.

or Overseeing Organisations other than the HA, eflectograph data may also be available.

eview

.6 The first step in the assessment of pavement ondition is to set out all data that is relevant to a otential scheme on a linear basis and to a common cale to allow easy comparison of the various data ets. It is essential that all data is referenced to the etwork sections and chainages. The priority for further nvestigations can be gauged from the extent to which he same maintenance sections are identified as possibly eeding treatment by more than one of the separate ssessment surveys.

.7 Where there is low skidding resistance in relation o investigatory levels but where no other defects have een identified by routine surveys, an investigation hould be carried out in accordance with HD 28 DMRB 7.3.1) to determine whether remedial surface ork is likely to solve the problem.

.8 Possible associations between the various ndicators of condition and other data applicable to he site are discussed below for flexible pavement onstruction.

.9 The age of an asphalt surfacing and the frequency f minor repairs can give a good indication of its likely uture performance.

.10 All lengths of road showing signs of significant racking, fretting or rutting should be considered for nvestigation to determine the depth and extent of the racking and/or rutting.

.11 The pattern of cracking can give an indication f the composition of the pavement. Regularly spaced ransverse surface cracks, are typical of flexible avements with a hydraulically bound base. These racks may mirror cracks which develop in the ydraulically bound layer soon after construction and

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chapter 4 collection and review of existing data

investigation should be undertaken to establish whether they extend through all the bound layers. Note that a similar pattern of cracking may occur on overlaid jointed concrete slabs, for which the approach to treatment could be different, so positive identification of construction type is important.

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Ys and investigations

chapter 5 scheme level surveys and investigations

5. scHeme level surve

introduction

5.1 To reach this stage of assessment, the review of the network survey and safety inspection data will have shown that problems exist in the surfacing and/or structure of a significant length of pavement and that substantial remedial works are probably needed. Further detailed surveys or investigations will be necessary to determine the causes of the defects and the appropriate remedial works.

5.2 The types of survey or investigation likely to be required at scheme level are summarized in Figure 5.1 and are described in this chapter.

aims

5.3 The scheme surveys and investigation have two objectives:

• To determine or confirm the type of pavement deterioration (surface defects and/or loss of structural integrity); and

• To provide information to enable any strengthening, resurfacing and other maintenance works to be designed economically.

may 2008

surveys

5.4 The types of survey normally required for pavement assessment on the HA network are summarized in Figure 5.1 and described in the following sections of this Part. When investigating a cracked, seated and overlaid rigid pavement, the flexible pavement procedure is likely to be most appropriate. In other situations where a rigid pavement has been overlaid with asphalt, judgment will be needed as to which of the two processes, or a combination of both, will be most effective.

5.5 For non-Highway Agency areas the network scheme level surveys may include Deflectograph or visual surveys. In these cases the procedure should be modified to suit the circumstances.

5/1

may 2008



and investigation

scheme surveys – rigid pavements

● Visual Condition Survey ● Cores and Dynamic Cone Penetrometer Note. The surface characteristics of CRCB and CRCP pavements with thin surfacing are assessed as for flexible pavements.

OPTIONAL ● Ground-Penetrating Radar ● FWD or Deflectograph Load Transfer Efficiency ● Test pits or large diameter cores ● Laboratory testing

ecessary)

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figure 5.1 – scheme level surveys and

plan scheme surveys

scheme surveys – flexible pavements

● Visual Condition Survey ● Deflectograph ● Cores and Dynamic Cone Penetrometer

OPTIONAL ● Ground-Penetrating Radar ● Falling Weight Deflectometer (FWD) survey ● Test pits or large diameter cores ● Laboratory testing

non-pavement surveys (as n● Earthworks ● Drainage ● Vehicle Restraint Systems ● Lighting ● Signs ● Topographical surveys

surveYs and investigation reQuired for different tYpes of pavement

flexible pavements

5.6 On the HA network, three types of scheme survey must be carried out on flexible pavements:

• Visual Condition Survey (HAPMS or equivalent);

• Deflectograph;

• Coring and Dynamic Cone Penetrometer (DCP);

Other types of survey and testing may also be required, depending on circumstances, and are shown in Figure 5.1.

data interpretation and

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investigations on the Ha network

treatment design

.7 Pavements that have been designed for heavy raffic, such as the majority of dual carriageway and otorway pavements are usually of substantial and

niform pavement thickness. Where the construction s flexible with an asphalt base these pavements are ikely to be predominantly long-life. The common efects are surface cracking, rutting, crazing and loss of ggregate (fretting). On this type of site, investigations re usually uncomplicated and generally cores would e located to determine general pavement thicknesses if not already reliably known), the depth of cracks and he depths of rutted layers in order to define the required epth of inlay. DCP measurements would be made in at east one-third of the core holes. If the cored pavement hicknesses are fairly uniform, a GPR survey may not be equired. If the pavement appears to be long-life, neither WD surveys nor test pits may be required.



5.8 Flexible pavements with hydraulically bound base of any age are usually more complex because, in addition to the environmental deterioration mentioned above, reflection cracking can be a major defect. Reflection cracks can be of variable depth and may conceal substantial cracking or disintegration of the underlying hydraulically bound layer. Usually, considerably more cores will be needed to establish the severity of the variable pavement damage. DCP measurements should be made in most of the core holes. GPR, FWD surveys, strength testing of Hydraulically Bound Mixture (HBM) layers and test pits at locations of complex cracking may be required.

5.9 Pavements with an evolved construction of any type are often variable and complex. The original construction was probably quite thin but will have been strengthened or reconstructed several times over a long period of time. Thicknesses and materials may be very variable which can lead to a variety of defects and also variable deflections. Sections of these pavements may be long-life, upgradeable to long-life or determinate-life. In such circumstances, investigations will be complicated and require considerably more cores. DCP measurements would be required in most of the core holes, particularly where the pavement is thin. GPR would be essential to identify varying pavement thickness. FWD surveys and test pits may also be required.

rigid pavements

5.10 On the HA network, two types of scheme survey must be carried out on rigid pavements:

• Visual Condition Survey;

• Coring and Dynamic Cone Penetrometer (DCP).

Other types of survey and testing may also be required, depending on circumstances, and are shown in Figure 5.1.

5.11 The visual survey will normally provide the most useful information in determining the causes of failure and deciding on the appropriate treatment.

5.12 Cores should be located both in undamaged areas and also on a representative number of cracks. Cores will provide information on slab thickness and quality, the degree of interlock across cracks and provide access for DCP testing of the foundation layers. Test pits can be

may 2008

used to investigate various aspects of joint performance including differential movement, poor ‘load transfer’ and loss of subbase material due to ‘pumping’.

5.13 The joint condition in jointed concrete pavements can contribute significantly to pavement condition. Stepping/differential movement, evidence of pumping and Load Transfer Efficiency (LTE) as measured by the FWD (HD 29, DMRB 7.3.2) give clues to the condition of joints. If faulty joints are suspected a section of pavement at a joint should be carefully broken out and logged in order to examine dowel bar condition and note any moisture at the slab/subbase interface.

5.14 An FWD survey to assess the LTE of the joints may also be carried out where this is believed to be unsatisfactory. (A similar type of survey can also be carried out by the Double Beam Deflectograph, refer to HD 29 (DMRB 7.3.2).) However, great care is needed in interpreting the results of either survey type as these are very temperature dependant. LTE values to indicate satisfactory joints will have to be established on a site specific basis – refer to Chapter 6 of this Part.

5.15 GPR may be used to provide information on slab thicknesses and also on dowel alignment at joints which can be useful in determining the causes of suspected joint “lock-up”.

general advice on surveY and investigation tecHniQues

5.16 Details of how to carry out the various surveys and investigations are given HD 29 (DMRB 7.3.2). All measurements must be properly recorded as detailed in HD 29.

5.17 The primary components of the investigation are good observations and records of the surface defects, cores and possibly test pits, depending on the construction type and whether the investigation is confined to the surfacing or the structure of the pavement. These can be supported, if required, by in situ tests of the strength or stiffness of the foundation and optional laboratory tests on any of the materials.

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5.18 The scheme level surveys and investigation will involve stationary or slow moving vehicles and personnel on foot occupying part or all of a carriageway for several hours. These activities must all be carried out in accordance with the requirements of paras 1.10 to 1.12 of this Standard, as appropriate.

visual surveys

5.19 During the visual survey, photographs should be taken of some of the defects (including a recognisable object or feature to give scale) and the general context of the pavement. Although rut depths are measured as part of the TRACS (or SCANNER) surveys, it would be useful to measure local maximum rut depths in areas with significant rutting. (For the HA network, average rut depths over 10m lengths for each wheel-track are available from HAPMS.) Drainage features, the crossfall, gradient and depth of cutting or fill should be observed at the principal defect areas. All data shall be referenced by chainage within the network sections.

5.20 The visual survey photographs are essential to limit the subjectivity of the visual surveys and also to clarify some descriptive terms such as ‘surface defectiveness’, ‘fretting’ and ‘ravelling’. Photographs of the defects are an essential source of information neededin the later reviews of the schemes, when maintenance funding is allocated, e.g. the Value Management workshops for the Highways Agency.

5.21 For the HA network, conventional visual surveys may be supplemented by assessment of the downward-facing video images obtained as part of the TRACS survey. In situations where visual surveys can only be carried out at night, the results of manual processing of the video images may be preferable to a conventional survey carried out in poorly lit conditions.

coring

5.22 For time and cost reasons it will never be possibleto carry out all the coring necessary to fully explain all defects and determine the proper remedial treatment at all locations. A limited number of core locations will have to be selected which represent all the defective or weak areas, but biased to the worst areas where remedial works are likely to be more substantial. The strategy for deciding the locations for coring or test pitting will vary depending on the specifics of each site. Factors to be considered are:

5/4

• the extent of existing information;

• consistency of construction throughout the site;

• GPR layer thickness profile (if available);

• types and locations of defects;

• consistency of defects and deflections;

• whether or not defects and deflections at a given location are consistent with each other;

• locations of high and low Deflectograph deflection (assuming that the Deflectograph survey has already been carried out); and

• proximity of live traffic lanes and the safety of operatives and road users.

5.23 Ideally, the coring of the pavement should be carried out after both the visual and Deflectograph surveys have been completed in order that the most effective coring locations can be selected. However, if traffic management or other operational considerations require that the coring is carried out concurrently with the other two surveys, it is essential that a reconnaissance or simplified visual survey is carried out from the verge or hard shoulder to define, as a minimum, the principal areas of deterioration so that cores can then be positioned so as to provide the maximum amount of information as to why the pavement is deteriorating.

5.24 Cores should be taken at representative cracks to determine their depth and whether or not any adjacent material has disintegrated. Extreme locations such as intersecting cracks or cracks where the adjacent asphalt or concrete is disintegrating should be avoided as successful core recovery is unlikely. It is recommended that, where core location is critical, the intended core positions are paint marked on the road surface to avoid confusion.

5.25 Cores will also allow the detection of any tar binder in the existing construction which can result in significant extra cost and complication if the tar-bound layer needs replacement. The presence of tar will usually be evident by smell but some chemical tests will be required to confirm this and to justify any expensive measures to deal with this material.

5.26 Ruts or deformation should be straddled by a set of three cores to determine which of the asphalt layers have reduced thickness. Where intensive rutting

may 2008



is present, it may be necessary to open a test pit to determine which layers are deformed.

5.27 Although the defects and pavement construction may be similar over the whole scheme, cores should be taken at defects over the whole length of interest, normally at least one core per 200m per lane. This is to ensure that the causes and depth of defect are indeed similar.

5.28 Cores should normally extend the full thickness of bound layers to determine the total thickness and to ensure that the full depth of cracking is recorded. The cores should also indicate any loss of integrity of materials, such as stripping of the binder. It is important that the core logs are recorded by a competent person in accordance with the requirements given in HD 29 (DMRB 7.3.2) including an accurate location relative to the network sections.

dynamic cone penetrometer

5.29 Dynamic Cone Penetrometer (DCP) tests would normally be carried out in some of the core holes. This is the quickest and cheapest way to determine the approximate strengths of the foundation layers. Where there are significant surface defects, high deflections, or thin pavement thicknesses, it is desirable to test at every core hole to assess the contribution of the foundation to overall pavement strength. Where defects appear non-structural and Deflectograph deflections are low, testing at every third core hole would be acceptable.

ground-penetrating radar and falling weight Deflectometer Surveys

5.30 On sites where pavement thickness or type of construction vary significantly, it may also be necessary to carry out a Ground-Penetrating Radar (GPR) survey which can be carried out at traffic speed. This data is needed to interpolate thicknesses or construction type between core locations.

5.31 A Falling Weight Deflectometer (FWD) survey may also be considered useful to confirm and explain high or variable Deflectograph deflections through comparison of layer stiffnesses with benchmark values.

5.32 The decision on whether or not to commission these additional surveys should logically be taken after the visual condition and Deflectograph surveys have been reviewed. However, this staged strategy may delay the conclusion of the scheme investigation or increase traffic management costs during the survey work. For flexible pavements, if the defects are complex, and the

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vement construction is expected to be variable, it ould be reasonable to include either or both of these ditional surveys with the three essential ones stated Clause 5.4. For proper back-analysis of FWD data, curate pavement thicknesses are essential (refer to D 29, DMRB 7.3.2). Any decision to carry out an WD survey will usually require an associated GPR rvey unless the core-derived pavement thicknesses e very uniform and interpolation is simple. For gid pavements, where FWD surveys are carried out imarily to measure Load Transfer Efficiency, there usually less need to carry out a GPR survey. Also, rigid pavements there is usually less variation of nstruction thickness.

est pits

33 Test pitting is also described in HD 29 (DMRB 3.2) but is not encouraged for general use as this is a uch slower and more expensive method of obtaining vement information compared to coring and DCP sts. Test pits should only be used when necessary ta cannot be obtained by other means. A decision on hether test pits are required would usually be taken ter all the network data and the three essential types scheme data have been assessed. Consequently, any st pits are likely to be excavated as a separate, later eration. Large diameter (300 to 450mm) cores could

so be used as an alternative to pits.

aboratory testing

34 Decisions on the type and number of laboratory sts should be made after the assessment of the field ta. Testing of materials to compare results between iled and intact areas can often be useful. For this ason it is prudent to retain cores and other samples for least three months following the investigation while y requirement for laboratory testing can be assessed.

35 If there is doubt over the adequacy of phalt layers it can be useful to carry out Indirect

ensile Stiffness Modulus (ITSM) tests, (BS EN 697-26:2004), on asphalt core samples. If any form of rface material recycling, such as Repave, is proposed, en it will be necessary to carry out tests on the layers be recycled in order to determine their gradation d binder properties to assess their suitability for the ocess.

36 There is little value in testing apparently rviceable layers purely to check compliance ith current specifications, or those at the time of nstruction, if known. Materials not complying with ch specifications may nevertheless be performing

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satisfactorily. An adequate assessment of asphalt or HBM quality can usually be made from a visual examination of the cores to judge voids, gradation, binder content and toughness.

5.37 Where sets of cores across ruts have failed to identify the layers which have deformed and caused the rut, it may be useful to carry out wheel-tracking tests on samples taken from asphalt cores to judge the deformation of each of the suspect layers. However, poor wheel-tracking results should not be used to attribute poor rutting resistance to asphalt layers when no actual rutting has occurred on the pavement.

non-pavement scHeme level surveYs

5.38 In addition to surveys and investigation of the pavement, surveys of other parts of the highway should also be considered and carried out as necessary, including:

• Earthworks;

• Drainage;

• Vehicle Restraint Systems (VRS) (safety barriers);

• Lighting; and

• Signs.

Any justified maintenance works should be coordinated with the pavement works and carried out at the appropriate time which will depend on the nature of the works and the specific site circumstances.

earthworks

5.39 For the HA’s network, the procedure for geotechnical inspections is given in HD 41 (DMRB 4.1.3). If the required annual inspections have been carried out with no geotechnical features (defects) reported, then no further action is needed. If the inspections indicate some defects then the appropriate remedial works should be developed under the process defined in HD 41 (DMRB 4.1.3). However, if the geotechnical inspections have not been carried out, an annual inspection should be carried out for the proposed scheme length as soon as possible.

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drainage

5.40 A visual inspection of manholes, catchpits and gullies after rainfall or a water test will reveal whether water is standing in the system. Examination of the outfall pipes will confirm whether they are functioning correctly. If there is evidence of blockages within the system, a Closed Circuit Television (CCTV) survey, with jetting as required, should be carried out.

5.41 Where the edge drains are of the combined filter drain type, the presence of excessive growth and detritus over the filter media may suggest that they have become contaminated and rendered ineffective or partially ineffective. Should there be any doubt, a short length should be excavated down to pipe level for further examination.

5.42 Current standards require sub-surface drains to be provided where subbase and capping terminate. In embankments where sub-surface drains are not present, the subbase and capping will need to be extended to the side slopes. If this has not been done, there is a risk that the lower unbound layers of the pavement construction will have formed a sump for retention of water, thus weakening the pavement foundations. If there is evidence of water in the foundation layers, a trench cut through the verge will reveal whether the correct measures have been taken during construction.

5.43 When assessing moisture contents of the soil or unbound materials measured at the time of investigation, allowance should be made for their variation with time, for example between summer and winter or over shorter periods following rainfall, particularly for cracked pavements.

vehicle restraint systems

5.44 For the HA’s network all vehicle restraint systems within a scheme need to be inspected for structural integrity and compliance with current standards as required by TD 19 (DMRB 2.2.8). Carrying out any substantial pavement maintenance may require automatic rectification of any structural weakness or non-compliances of the adjacent VRS. The cost of rectifying deficiencies can be substantial and needs to be identified at an early stage of scheme development.

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chapter 6 interpretation

6. interpretation

introduction

6.1 The results of the scheme level surveys and investigations, together with the other assembled condition data and information for the site provide evidence for determining the following:

• The nature, extent and degree of the defects;

• The probable causes of the defects;

• Whether the defects are in the surface or are structural;

• The types of remedial treatment needed.

6.2 A major part of the interpretation process is the comparison of the different types of data and to note where they support or conflict with each other. It is usual to find that for at least part of the scheme length there are inconsistencies between the data.

presentation of data

6.3 At an early stage all the relevant data needs to be assembled and laid out in strip form so that comparisons of any of the data can easily be made for any location. This summary should indicate:

• Summary of the visual condition survey;

• Deflectograph results for flexible pavements – both residual life and temperature corrected deflections;

• Core information – layer type, thicknesses, condition and bond between layers;

• Summary values of the most important in situ and laboratory tests;

• FWD profiles for flexible pavements (if available); and

• Load transfer results (FWD) for jointed rigid pavements (if available).

6.4 It is essential that all survey or investigation details are referenced by chainages based on the network sections to allow easy and accurate comparison of the different types of data.

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etermination of long-life or determinate life

.5 For flexible pavements, the first step in the nterpretation of the scheme level data is to assess hether or not the pavement is likely to be long-life or eterminate life (refer to Chapter 2 of this Part). For the A network this will be determined during the HAPMS rocessing of the Deflectograph data or, for other verseeing Organisations, by the PANDEF software.

n the event that the pavement is determinate life the rocessing software will estimate the residual life (in ears) of the pavement. If the Deflectograph survey is eing processed before cores or GPR survey data are vailable, it may be necessary to reprocess the data if he layer thicknesses turn out to be different from those ssumed initially.

.6 The identification of the life type should be ased on values derived from the 85th percentile eflectograph deflection of 100m lengths.

imitations of Deflectograph Analysis

6.7 The precision of determination of pavement life type or estimates of residual life is limited by the accuracy of the input data such as measured deflection, construction and traffic details, and experimental error within the empirical relationships used. Deflectograph analysis must never be considered in isolation, but as one of several types of data to be used in the process of assessment of the structural maintenance requirements of a site. Information from visual inspection surveys, cores and any other data relating to the scheme must also be considered.

.8 Sometimes there is no clear correlation between eflections and other indicators of pavement condition visual survey and cores). This could be due to a number f factors:

Errors in the measurement of road temperature;

Deterioration of the road surface is only superficial and does not significantly increase the deflections;

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• Barely noticeable deterioration of the road surface has resulted in some degree of detachment or de-bonding of the surface course resulting in increased deflections;

• The pavement is supported on an unusually strong subgrade; and

• Temporarily higher or lower than normal subgrade moisture contents have reduced or increased the pavement strength relative to normal.

assessment of data: fleXiBle pavements

long-life pavements and flexible determinate-life pavements with >5 Years residual life

6.9 Long-life pavements possess a structure of adequate strength as shown by a substantial thickness of asphalt and low deflections. Determinate-life

pavement type Bound

poor integrithroughou

Asphalt < 3 GPa

Hydraulically Bound Mixture (HBM) < 8 GPa

PQ Concrete < 20 GPa

(Note. These stiffnesses apply to layers

table 6.1 – condition relate

flexible determinate-life pavements with <5 Years residual life

6.12 The reasons for the surface defects and short residual life need to be determined. Are they indicative of the whole structure or the condition of only one or some of the layers? The quality of the layers should be assessed based on core logs, FWD back-calculated stiffnesses, laboratory tests of sections of core and DCP for the foundation layers. Reference values of FWD back-calculated stiffnesses are given in Tables 6.1 and 6.2.

6.13 Defects in the wheel-tracks, may indicate either structural damage to the base caused by traffic alone or environmental damage to the surfacing, exacerbated by traffic. If the defects occur over the whole lane or

6/2

pavements in this group will also have low deflections which are indicative of a generally sound pavement with good foundation support, but probably with thinner construction. The comparison of information gathered on the test areas should concentrate on depth of cracking, rutting and other material deterioration. The findings will give an indication of the rate of progress of the damage and will assist in deciding on the urgency and extent of remedial treatment.

6.10 The quality of the layers should be assessed based on core logs, FWD back-calculated stiffnesses, laboratory tests of sections of core and DCP for the foundation layers. Reference values of FWD back-calculated stiffnesses are given in Table 6.1.

6.11 If no evidence of damage is found below the surface layers, this will confirm that the pavement is structurally sound and hence a surface treatment should be considered (See Chapter 8 of this Part). If damage extends downwards into the lower layers, partial reconstruction (deep inlays) should be considered.

layer stiffness at 20oc derived from fwd

ty t

some deterioration

good integrity

3 - 7 GPa > 7 GPa

8 - 15 GPa > 15 GPa

20 - 30 GPa > 30 GPa

consisting of only one material type.)

d to Bound layer stiffness

carriageway and not just in the wheel-tracks, the cause is probably not traffic related.

6.14 If the foundation layer is adequate or strong, as indicated by results from the DCP, then the analysis should concentrate on looking for deterioration in the bound layers. If the foundation support is inadequate, the required maintenance is likely to be substantial. The lack of support may be caused by:

• poor quality materials,

• inadequate compaction or

• ingress of water. (Attention should be directed to possible drainage problems.)

may 2008



6.15 Cores will indicate whether deterioration such as cracking, de-bonding of layers or stripping of the binder is present in one or more layers, all of which will affect road performance. Some of the weak or partially disintegrated materials found in older (>35 years) pavements, e.g. tar-bound slag, may have been in this state for a considerable length of time and the current performance of the pavement may not be adversely affected as a result. If there are no associated surface defects it should not be necessary to replace or overlay such weak materials. The cause of such poor strength is usually poor aggregate grading, high voids and low binder content.

6.16 A comparison of properties of materials taken from areas of minor or major surface defects may help to explain the reasons for the difference in performance.

6.17 If deterioration is confined to the surface layers, then it can be assumed that the lower intact pavement structure can be used with confidence as a basis for a surface maintenance treatment, including an overlay if a more significant extension of life is required.

6.18 Knowledge of the cause of the defects will provide a good basis for the design of structural maintenance. The primary factors to determine treatments are the condition of the layers and the causes of defects. Decisions on the type and timing of structural maintenance for all pavements will also be affected by consideration of skid resistance and the level of minor repairs, particularly patching and crack overbanding.

6.19 For dual carriageways, differences in the condition and deflection levels between different traffic lanes should be interpreted so that a comparison can be made when ascribing contributions to overall deterioration. This information also helps to optimise maintenance treatments across the carriageway width since not all lanes will necessarily justify the same approach.

additional factors applicable to flexible pavements with Hydraulically Bound Base

6.20 The assessment of pavements with hydraulically bound base requires special consideration because it is highly dependent on the findings of the visual survey and the coring. The primary transverse shrinkage cracks, which form in a layer of medium to high strength HBM at the time of construction, often cause reflection cracks in the road surface. The timing of the appearance of such cracks in the surface is partly influenced by the age and thickness of the overlying asphalt layers and

may 2008

partly by other factors such as strength of mix, subgrade strength, weather conditions during and immediately after construction and traffic loading.

6.21 With HBM, defects first develop in the vicinity of cracks, the pavement structure on either side retaining high structural stiffness. Deflection measurements on composite pavements tend to be very low (less than 0.15 mm) unless they happen to coincide with cracks.

6.22 Caution must apply to the forecasts of long residual lives which are derived for HBM bases in combination with low deflections and moderate traffic loadings, because these forecasts depend on the pavement remaining substantially uncracked.

6.23 When HBM is used for more than one layer of the pavement structure, shorter lives may be achieved for a given deflection than would be indicated by deflection analysis. Much depends on the condition of the lower hydraulically bound layer. Deflections may be kept low by the undamaged lower layer concealing progressive deterioration of the upper layers. Cores will indicate the presence of a lower hydraulically bound layer and the condition of both layers.

6.24 Table 6.2 summarises the structural features of flexible pavements with hydraulically bound bases divided into four condition classes. The principal determining factor for each class is the type, extent and severity of cracking in the HBM base. The table also gives details of the type of treatment likely to be appropriate. The FWD stiffnesses apply to the combined asphalt and hydraulically bound base layers. In addition to the features shown in Table 6.2, these pavements may also exhibit surface defects such as rutting, fretting and surface cracking. Such defects should be assessed in the same way as for flexible pavements with asphalt bases.

6.25 The determination of condition classes must be based on all the criteria in Table 6.2 and not just on one, e.g. FWD layer stiffness.

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class a class B class c class d Visual Observation Surface cracking not

evident or confined to widely spaced minor transverse cracks unless associated with construction joints in the HBM.

Surface transverse cracking confined to left hand lane. No, or very minor, longitudinal cracking in the wheel-tracks.

Transverse and longitudinal cracking in the wheel-tracks are both evident with a medium or high frequency.

Transverse and longitudinal cracking in the wheel-tracks are both evident with a high frequency.

Deflection Consistent deflection measurements which are low in relation to foundation stiffness.

Measurements peak at regular intervals and the average is as expected in relation to foundation stiffness.

Measurements are variable and the average is as expected in relation to foundation stiffness

Measurements are high in relation to foundation stiffness.

Crack Severity (See below)

Transverse crack severity generally 1.

Transverse crack severity generally 2.

Transverse crack severity 2 or 3, longitudinal crack severity generally 1.

Transverse and longitudinal crack severity generally 3.

HBM strength ≥ 10MN/m2 ≥ 10MN/m2 < 10MN/m2 < 10MN/m2 Cores Consistent sound HBM

with no wide cracks. Some occasional cracking in HBM but material generally sound.

Wide longitudinal cracks but material between cracks is sound.

Wide cracks for the full depth of some cores.

FWD - mean pavement layer stiffness modulus (< 20oC)

Consistent results >10GPa with a few individual results below 7GPa

>10GPa with some individual results below 7GPa

Variable results average >10GPa with successive results below 7GPa

<7GPa

Probable CBM condition • Little deterioration beyond initial transverse cracking due to early shrinkage and thermal warping, with good load transfer across transverse cracks.

• Deterioration has gone beyond initial transverse cracking. HBM slabs are large with movement at transverse cracks.

• Longitudinal cracking is slight or absent, with good load transfer across cracks.

• HBM slabs are large with significant movement at transverse cracks.

• Longitudinal cracking is present.

• HBM slabs are small, probably < 4m maximum dimension.

• Multiple transverse and longitudinal cracks with poor load transfer.

Implications for strengthening

• Structure has very little deterioration and pavement may be indeterminate with potential traffic capacity between 20 and 80 msa.

• If less than about 10 years old and determinate, it may be worthwhile overlaying to achieve an indeterminate pavement design.

• Structure has some deterioration and so cannot be assessed for an indeterminate design.

• See Chapter 8 overlay design procedure.

• Treat severity 2 cracks by trenching and replacing with asphalt.

• See Chapter 8 for overlay design procedure.

• Treat transverse cracks of severity 2 and 3 by trenching and replacing with asphalt.

• Locally reconstruct areas of badly cracked HBM with asphalt.

• Pavement will need to be removed to top of subbase, or lower, and reconstructed. The HBM will continue to deteriorate towards the condition of an unbound granular layer.

• Consideration should be given to retaining the pavement until the amount of patching becomes unacceptable.

• Thick overlay may be an alternative.

Crack Severity Ratings: 1: Widely spaced cracks (>10m), generally ≤ 0.5mm wide, without fretting, and no evidence of vertical movement. 2: Regularly spaced cracks (5 to 10m), generally ≤1.0mm wide, with some fretting, and evidence of horizontal and vertical movement. 3: Regularly and irregularly spaced cracks, generally >1.0mm wide, with some fretting, and evidence of horizontal and vertical movement.

table 6.2 – assessment of treatment of flexible pavements with Hydraulically Bound Base

may 20086/4



assessment of data: rigid pavements

6.26 Similar guidelines to those for flexible pavements should be adopted when examining the data for rigid pavements. An indication of the effects on performance of slab thicknesses and the strength of underlying layers for rigid pavements may be determined using HD 26 (DMRB 7.2.3). When dealing with jointed pavement, special emphasis should be placed on the condition of the joints, but the same careful analysis of the condition of the foundation is necessary.

6.27 The adequacy of the Load Transfer Efficiency (LTE) of joints or cracks can be assessed by Falling Weight Deflectometer measurements as described in HD 29 (DMRB 7.3.2). Joints or cracks with perfect load transfer should give a transfer efficiency of just under 100%. Defining a general LTE percentage above which the joints or cracks may be considered satisfactory is very difficult as it will vary from site to site and depend on temperatures through the depth of the slab at the time of testing.

6.28 Benchmark values of LTE to indicate acceptable crack or joint performance must be established for each site by comparison with visual condition information and coring data. The absolute values of the deflections must also be taken into account as low LTE values based on low absolute deflections may have little significance.

6.29 The Concrete Pavement Maintenance Manual (2001) gives further information on the assessment and interpretation of rigid pavement defects.

6.30 The options for maintenance will depend on the condition of the joints, the state of the concrete and the condition of the foundation. Maintenance of concrete pavements is considered in full in HD32 (DMRB 7.4.2).

falling weigHt deflectometer (fwd)

reference stiffness values

6.31 Typical values of layer stiffness related to likely condition are given in Tables 6.1 and 6.2. Conclusions regarding layer weaknesses must be supported by more than one type of observation or measurement. Layer stiffnesses must always be checked for correlation with pavement visual condition, the layer condition evident in cores, Deflectograph results and any laboratory

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test results. Materials which fall in the ‘Some deterioration’ stiffness category are not necessarily unserviceable. Depending on the other indicators, they could remain in the pavement with or without further strengthening.

.32 The reference stiffness values in Table 6.2 apply o pavement layers consisting of only one type of aterial. Flexible pavements with hydraulically bound

ase may also be analysed with all the bound layers asphalt and HBM) combined as a single layer and ompared to the stiffness values given in Table 6.1.

.33 Some of the factors that influence layer stiffness f various materials are given in Table 6.3.

material stiffness decreases stiffness increasesAsphalt High voids

Cracking Layer debonding Stripping

Low voids Binder-hardening

Concrete Joint nearby Cracking Debonding Poor compaction

-

Granular High moisture Clay contamination

Low moisture Natural-cementing

Subgrade High moisture Low moisture

table 6.3 – factors affecting layer stiffness

.34 Although FWD back-analysis can provide an ndication of the layer stiffness, core or DCP data (in the ase of unbound material) will be needed in all cases o determine the cause of any low values. Comparisons f the layer stiffness derived from measurements made here the material is relatively untrafficked, with those

rom the line of the wheel-track can indicate whether he weakness is due to trafficking or not.

.35 For the foundation layers of existing pavements, layer stiffness of at least 0.1GPa (=100MPa), has een found to be associated with good performance of exible pavements with asphalt base. It is also thought

o be a reasonable criterion for the unbound foundation ayers of flexible pavements with hydraulically bound ase and rigid pavements. More than 100MPa would be xpected for hydraulically bound subbase below a rigid avement. Large variations in the measured foundation upport are usually associated with a change in drainage fficiency, subbase/capping layer or subgrade material r a construction change such as a cut/fill line.

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comparing fwd stiffnesses with itsm

6.36 There is a strong association between Indirect Tensile Stiffness Modulus (ITSM) values and asphalt layer stiffnesses estimated from FWD back-analysis. As the stiffness of asphalt is loading-time dependent, the shorter pulse of the FWD results in stiffnesses greater than ITSM values. As an approximate guide, ITSM values at 20oC should be multiplied by 1.5 when comparing with FWD-derived asphalt layer stiffnesses at 20oC. However, depending on the type and age of the asphalt material this factor has been found to vary between 1.0 and 2.4. Therefore, although the ITSM values can indicate the in situ layer stiffness, core data will be needed in all cases to determine the causes of any low values.

report

6.37 The findings and recommendations of the investigation into the causes of deterioration of all types of pavements must be stated in a report. This should also include summaries of the Network survey data and the results from the Scheme survey investigation in graphical or tabular format as appropriate.

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chapter 7 treatment design

7. treatment design

introduction

7.1 This Chapter gives advice on treatment options for flexible and rigid pavements, including surface and structural maintenance. The design of overlay strengthening measures is covered in detail and includes advice on the factors to be considered in order to arrive at an appropriate and economic maintenance solution. Advice on the application of treatments is given in HD 31 (DMRB 7.4.1) for flexible pavements and HD 32 (DMRB 7.4.2) for rigid pavements. The surfacing materials permitted by the Overseeing Organisations, for use on both flexible and rigid pavements, are given in HD 36 (DMRB 7.5.1).

procedure

7.2 The decision-making process for the selection of an appropriate maintenance treatment for flexible pavements with asphalt base is illustrated as a flowchart in Figure 7.1. The process for flexible pavements with hydraulically bound bases is given later in this chapter. The following paragraphs follow the flowchart and explain what is involved at each stage for a flexible pavement with asphalt base. Similar principles apply to rigid pavements.

Necessity of Treatment and Whole Life Costs

7.3 The choice of treatment must be based on safety, serviceability, financial, environmental and traffic disruption considerations as well as on a purely technical assessment. Various treatment options should be considered. For the HA network this should consider Do Nothing, Do Minimum and Do Something options over the next five year plan. Treatments for Do Something options may be only locally required and may vary from lane to lane. A Whole Life Cost analysis should be used when comparing the costs of different options. For the HA network the SWEEP system (part of HAPMS) is used for this purpose.

7.4 The other Overseeing Organisations use several other systems including WDMPMS, SAS and UKPMS. Further information on which system should be used on a particular network should be obtained from the relevant Overseeing Organisation.

may 2008

Surface Damage

7.5 Where deterioration is found only in the surface or binder course (approximately the top 100mm of the pavement) and there is an adequate total pavement thickness, no strengthening is normally required. A surface treatment or inlay would be suitable treatments depending on the extent of deterioration and how far it extends downwards into the surfacing layers. Crack sealing should be considered for small widely scattered areas of cracking. Where permitted by the Overseeing Organisation, a surface dressing will be more appropriate to treat areas of more extensive shallow cracking or to maintain a skid resistant surface.

7.6 Inlays, involving the replacement of the surface course and possibly the binder course, may be necessary to remove more deeply cracked, fretted or rutted surfacings to prevent these defects affecting the lower layers of the pavement and its structural condition or, in the case of cracks, from reflecting through into the new surfacing material.

Structural Deterioration

7.7 If the assessment process concludes that strengthening is required because of say, deteriorated or weak material deep within the pavement or a lack of pavement thickness, the following options should be considered: overlay, partial reconstruction or full reconstruction. Overlays and possibly the other options may raise the finished road surface relative to existing levels. Where this is unacceptable, the use of stiff EME2 asphalt as the new structural material can provide substantial strengthening for less thickness compared to traditional materials.

7.8 On financial grounds, it is preferable that overlays to pavements should be applied while its structure is still essentially intact. If the existing surfacing shows signs of surface deterioration, which will usually be the case, it is generally desirable to plane the surface to a depth of 15 to 20mm before overlaying, particularly where the overlay is less than 100mm. This is to remove material with hardened bitumen and provide a sound, uncracked surface to which the new asphalt can firmly bond. Where the existing surfacing is cracked or damaged to a depth greater than 20mm, the defective material must be removed and replaced with new material before the overlay is applied. Damaged or sub-standard asphalt

7/1



ns for Flexible Pavements with Asphalt Base

layers lower in the pavement could be left in place depending on the degree of damage and the depth relative to the new surface.

figure 7.1 – maintenance treatment optio


Figure 7.1 Maintenance Treatment Optio

Can a lowtreatment bedelay streng

Is overlafeasible

Are foundation layers giving

adequate support?

Is drainage effective?

N

YES

YES N

NO

NO

YES N

START Surface defects evident

Defects conto surface l

Is strengthening required?

NO

RECTIFY DRAINAGE See HA 44

(DMRB 4.1.1)

7/2

January 2008

7.10 Once excessive deterioration has taken place so aeconomically desirable to delay reconstruction work as

7.9 Where tar-bound materials are encountered it is desirable to leave these in situ, even if they are not in very good condition, to avoid the complications and costs of the proper disposal of this material.

ns for flexible pavements with asphalt Base

Chapter 7 Treatment Design

PLANE OFF OR PATCH ANY DAMAGED

MATERIAL

RECONSTRUCT

FOUNDATION AND BOUND LAYERS

PARTIAL OR FULL RECONSTRUCTION

(Ideal depth of reconstruction may have to be reduced to limit traffic

disruption – monitoring will then be required.)

PATCH LOCALLY, APPLY LOW COST

SURFACE TREATMENT - MONITOR REGULARLY

See HD 31 (DMRB 7.4.1) cost used to thening?

y ?

O

YES

YES

O

O

RESURFACE (INLAY) AREAS OF SIGNIFICANT

SURFACE DEFECTS WITH PERMITTED

MATERIAL - See HD 36 (DMRB 7.5.1)

fined ayers?

YES

PARTIAL RECONSTRUCTION

(INLAY TO DEPTH OF DEFECTS)

may 2008

7/3

s to exclude overlaying as a viable option, then it is long as possible, to gain maximum use from materials



7.10 Once excessive deterioration has taken place so as to exclude overlaying as a viable option, then it is economically desirable to delay reconstruction work as long as possible, to gain maximum use from materials which will have to be replaced. Clearly, safety, legal and environmental considerations place a limit on such delays as does the rising quantity and cost of day to day minor maintenance.

7.11 The exception to this delaying of reconstruction work is where local reconstruction would be likely to forestall the spread of deterioration significantly. This also applies to concrete pavements where longitudinal cracks, once initiated, can propagate relatively rapidly along the carriageway.

Feasibility of Overlay

7.12 There are some instances where an overlay is not economically feasible or has to be of limited thickness. This may be due to the constraints of overbridge clearances, parapet and safety fence heights, kerbs, etc. Such restrictions should be checked at an early stage of the evaluation process. The necessary headroom must be maintained under all gantries and overbridges. The possibility of overloading at underbridges and adjacent to retaining walls must also be considered. Allowance must be made for expansion joints (if concrete overlays are used), kerbing and drainage at under bridges. Heights of copings and parapet walls will also need consideration adjacent to retaining walls and at under bridges. Safety barrier heights will need to be examined and the barrier adjusted or re-erected if necessary.

7.13 If an overlay is to be carried out then the original pavement will be retained together with all its defects. These defects have to be allowed for in overlay design. If a serious weakness exists in one of the layers, then it may be economic to reconstruct down to and including that layer, rather than to apply a relatively thick overlay. If a layer is found to be in the process of rapid deterioration which cannot be halted, then reconstruction may be preferable.

7.14 Where some of the lanes of a carriageway have substantial remaining life and do not require treatment, the additional cost of a structurally unnecessary overlay over satisfactory lanes will have to be considered. It may be cheaper to partially or fully reconstruct the nearside lane.

may 2008

Partial Reconstruction (Inlays) or Full Reconstruction (All bound layers and possibly the foundation layers replaced)

7.15 Where reconstruction, partial or full, is required, then it is generally necessary to take out only those layers which are defective, i.e. if the granular layers are satisfactory, they should normally be retained. As much as possible of the sound, existing material should be retained. This will not only save materials and expenditure, but also provide a firm basis for the new layers. Retained layers should be left undisturbed. However, the dual constraints of finished pavement level and design thickness may require the removal of some sound material. In the case of flexible pavements with hydraulically bound base, consideration should be given to retaining any severely deteriorated HBM and using it as the foundation for a reconstructed pavement. In certain cases, cracking and seating of an existing hydraulically bound base may also be cost effective (see HD 32 (DMRB 7.4.2)).

7.16 In some circumstances it may not be possible to carry out the full depth of partial or full reconstruction indicated by the survey and investigation data, because of the excessive traffic disruption that this will cause. This is likely to arise where traffic levels are high, in an urban or residential environment (which may preclude or limit night working) and where there is a lack of suitable diversions. In these conditions lesser scale works may have to be carried out, despite the knowledge that this is not a lasting solution and further major treatments may be required in 5 to 10 years rather than 10 to 15 years. More regular condition monitoring will be required for road lengths treated in this way.

Drainage

7.17 Drainage failures can lead to significant weakening of the unbound layers, as well as the subgrade. This reduces the support to the bound layers, causing failure of the pavement as a whole. If drainage faults are found, it is essential that they are rectified as soon as possible and action taken to prevent recurrence. The extent and degree of strengthening should only be finalized after the effect of the drainage measures has been assessed. Reference should be made to HA 44 (DMRB 4.1.1) for further detailed advice.

surface treatment

7.18 Timely surface treatment can be effective in halting deterioration before serious damage to the remaining structure takes place. If it is timed to coincide with a need for improvement to surface texture or

7/3



skidding resistance, then it is economically even more attractive. Details of surface treatments are given in HD 31 (DMRB 7.4.1).

asphalt overlay design

7.19 For flexible pavements, strengthening by overlay and/or partial reconstruction is normally designed to extend the life for a further 20 years or with a treatment agreed with the Overseeing Organisation appropriate to the condition of the existing pavement.

7.20 For flexible pavements with asphalt base, overlays may also comprise the additional thickness of material required to convert a determinate life pavement (with low deflections) into a long-life pavement, i.e. an ‘Upgradeable to LLP’ pavement as shown in Figure 2.1 of this Part.

7.21 Several options for strengthening by overlay may be considered. For flexible pavements, a first indication of the range of thicknesses of material required along the site may be obtained from Deflectograph data, processed using either PANDEF or HAPMS. The 85th percentile deflection applicable to each 100m length should be used for this purpose, see HD 29 (DMRB 7.3.2).

7.22 Overlay design must not be based on deflection results alone. The analysis of Deflectograph data is a starting point, not an end point as Deflectograph analysis does not take into account all factors relating to pavement performance. There must be confirmatory evidence of surface defects and material condition.

7.23 Another approach to overlay design is to compare the thickness of the existing pavement structure with that required for new construction designed to carry both past and future traffic in accordance with HD 26 (DMRB 7.2.3); the difference in thickness is an estimate of the necessary overlay. Allowance should be made for any deterioration or initial deficiencies, as well as the varying materials in the existing pavement.

Asphalt Overlay to Flexible Pavements

7.24 For flexible pavements, overlay design thicknesses are based on the use of traditional Dense Bitumen Macadam (DBM125) material. This type of asphalt is now designated “AC 20 dense bin 100/150 rec” or “AC 32 dense base 100/150 rec” as defined in BS EN 13108, depending on the required thicknesses.

7/4

Guidance in the application on the use of the new asphalt mixtures is given in PD 6691:2007.

7.25 If stiffer asphalt materials specified in BS EN 13108:2006 are used, then some reduction in overlay thickness is possible, except in the case of flexible pavements with hydraulically bound base. An assessment of the potential savings in thickness can be gained from a comparison of the recommendations given in HD 26 (DMRB 7.2.3) for the design of new roads. (The traditional asphalt designations used in HD 26 will have to be matched to the new designations given in BS EN 13108.)

7.26 Where investigations indicate that there are areas of localised, more severely deteriorated material within a length otherwise identified for overlay, it is recommended that full or partial depth reconstruction is carried out in those areas prior to overlaying to ensure as uniform a standard of road as possible.

7.27 Guidance on treatments for flexible pavements with hydraulically bound bases is given in the categorisation described in Chapter 6, Table 6.2 of this Part and Figure 7.2 of TRL Report TRL 657 (2006). Pavements in Class A require no action except where it is desirable to increase the original design life of the road. Figure 7.1 of this Part may be used to design an overlay for Category 1 pavements to produce an indeterminate life. Some flexible pavements with hydraulically bound bases which have thick asphalt cover, and cracked, seated and overlaid pavements may be closer in performance to a flexible pavement with a strong asphalt base. This should be taken into account both when analysing deflection data and when considering strengthening options. For example, the Deflectograph processing algorithms in HAPMS and PANDEF assume that flexible pavements with hydraulically bound bases and more than 300mm of asphalt can be treated as flexible pavements.

7.28 Flexible pavements with hydraulically bound bases in Classes B and C of Table 6.2 of this Part, generally require an overlay, but are also likely to need local reconstruction at severe cracks in the hydraulically bound material. The treatment selection chart in TRL 657 may be used to decide the appropriate treatment. For flexible pavements with hydraulically bound bases, an asphalt overlay provides additional thermal insulation to the HBM layer, as well as preventing ingress of water to the HBM layer and the foundation.

may 2008



Overlays to Rigid Pavements

7.29 TRL Report TRL 657 (2006) also provides treatment options flow charts for jointed un-reinforced (URC) and reinforced (JRC) pavements. The following major treatments are recommended, depending on the nature of the deterioration and other site features:

• asphalt overlay, with saw-cut and seal;

• crack and seat and overlay with a minimum of 150mm asphalt;

• CRCP overlay and thin surfacing;

• full depth reconstruction.

7.30 The Overseeing Organisation currently has no standard method for assessing the thickness of an asphalt overlay required to strengthen rigid pavements other than the indicative minimum thicknesses shown in the TRL 657 treatment options flow charts.

7.31 Designs for a new CRCB pavement assume that 15 mm of concrete is equivalent to 100 mm of asphalt which makes allowance for the different thermal stresses generated in an asphalt overlaid concrete pavement. This equivalence may be used for asphalt overlay design over intact concrete, using the design charts in HD 26 (DMRB 7.2.3).

concrete overlays as a renewal treatment

7.32 Concrete overlays have not been widely used in the UK. However, a thick concrete overlay or inlay can provide improved strength, longer life and improved surface characteristics and will benefit from a good foundation provided by the existing pavement. The following paragraphs provide information on:

• the circumstances where a thick concrete overlay/inlay would be suitable;

• how to assess the strength of an existing pavement;

• what measures are necessary to prepare for a thick concrete overlay/inlay.

7.33 Concrete overlays may be designed using the rigid pavement design chart (Figure 2.2) given in HD 26 (DMRB 7.2.3). The Surface Modulus (SM) of the pavement to be overlaid is used in place of the foundation class shown on the chart. The SM is measured using the FWD as described in Annex 5B of HD 29 (DMRB 7.3.2). A representative value for

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he SM is obtained for each section of road being onsidered for treatment, with values being taken from oth across and along the carriageway. Generally, the 5th percentile modulus value (ie the value exceeded y 85% of the sample values) should be used for each reatment length.

.34 Thick concrete overlays can be considered for exible and most types of rigid construction that would therwise require reconstruction. The existing pavement nd foundation are retained to form part of the oundation of the new road structure. Designers should onsider this alternative to complete reconstruction hich uses the remaining inherent strength of the

xisting pavement. Benefits are achieved by not equiring the removal of the existing pavement or the rovision of new foundation layers.

.35 A thin bonded concrete overlay is a pre-emptive reatment as it is only appropriate for rigid pavements n relatively good condition. It will probably only e justified where a substantial increase in traffic is nticipated. Advice on the use of thin bonded concrete verlays is available in HD 32 (DMRB 7.4.2). Further dvice is available from the Overseeing Organisation. esign should be based on the thickness charts in D 26 (DMRB 7.2.3) with allowance for traffic already

arried.

imitations of Concrete Overlays

.36 The use of a thick concrete overlay for extending he pavement life is not suitable where the existing oundation is in very poor condition with evidence of weak subgrade. It is important that the foundation is n good condition and able to provide a sound basis for trengthening the pavement.

.37 If an existing rigid or flexible pavement is everely cracked, to the extent that water has penetrated he foundation, it is possible that deterioration of he subbase and subgrade has occurred. Complete econstruction including the foundation layers may be ecessary in this situation.

ocal Treatments

.38 Following the full analysis of the survey data (see hapter 7), it may be necessary to carry out remedial r improvement works to ensure that the pavement s brought to a uniform and satisfactory standard uitable for a thick concrete overlay. This may include he reconstruction of heavily cracked existing slabs. t is possible that the crack and seat method may be ppropriate, as described in HD 32 (DMRB 7.4.2).

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7.39 If the existing pavement is of rigid construction and there is robust evidence of voids beneath the slab then under slab grouting (with lifting if necessary) should be considered. For further advice refer to HD 32 (DMRB 7.4.2). Where the existing pavement is of flexible construction, any large cracks should be treated to ensure a sound structure and surface suitable for an overlay. Advice on the treatment of cracks in flexible surfacing is available in HD 31 (DMRB 7.4.1.4). It may also be necessary to plane off the upper layers if they have become seriously weakened.

7pr

E

7ttmTa

overlay material

e

flexible urcJrc

CRCP None Not recomm

CRCB None Separatmembra

table 7.1 – preparation treatm

7.42 For each situation where a thick concrete overlay is being considered, it is important that full allowance is made for all thermal movements of the pavement. Overlay slabs should be constructed to the requirements of the Specification and the Highway Construction Details (MCHW 1, 2 and 3).

7.43 If a separation membrane is required, the Overseeing Organisation’s requirements for such membranes are contained in the Specification: MCHW 1, Series 1000.

reconstruction design

7.44 Strengthening by reconstruction involves the removal of a certain depth of the structural layers of the existing pavement and replacing these with a thickness of new or recycled material. It could involve replacement of some layers only – partial reconstruction. Alternatively it could involve total reconstruction and the replacement of some of the subgrade with fill material – full reconstruction. The degree to which existing materials should be replaced generally depends upon the degree of deterioration of each of the layers.

Structural Layer Replacement

7.45 If the main structural (bound) pavement layers, whether asphalt or concrete, are in a seriously

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.40 For both flexible and rigid pavements, all otholes, large irregularities and deep spalls should be emedied before overlaying.

xisting Surfaces

.41 Depending on the existing surface material and he design of the thick overlay slab, certain preparation reatments may be necessary. Use of a regulating layer ay be required, either asphalt or concrete. able 7.1 sets out the various options that are possible nd indicates if any surface treatment is required.

xisting pavement

crcp crcB

ended None None

ion ne

None None

ents for concrete overlays

eteriorated condition, then replacement rather than verlaying may be the most economical solution. In ome instances, only partial replacement is necessary, epending on the depth to which defects are present. herever possible, the existing subbase should be

etained to provide protection to the underlying layer. or flexible pavements with hydraulically bound base,

he HBM layer should be retained if it is capable of roviding long-term support.

.46 Flexible pavements with hydraulically ound base in Class D of Table 6.2 will need to be econstructed because the HBM will have deteriorated o small slabs with poor load transfer. Deterioration is ikely to continue until the HBM becomes little more han a granular subbase. In such cases, particularly f the asphalt is thick and in reasonable condition, ome years of useful life may be obtained from the xisting pavement before reconstruction is carried out. he importance of the route and the consequences of

egular small scale maintenance interventions will be ssential inputs into the decision as to the timing of econstruction.

.47 The design of the reconstruction may be carried ut using the charts in HD 26 (DMRB 7.2.3). The tiffness of the foundation (the remainder of pavement nce the layers to be replaced have been removed) an be taken into account and classed as 1, 2, 3 or 4 as efined in HD 26 (DMRB 7.2.3).

may 2008



Whole Pavement Replacement

7.48 If the subbase is considered inadequate (e.g. low CBR, contamination, etc) then it will require replacement. The reconstruction required can be designed wholly in accordance with HD 25 (DMRB 7.2.2) and HD 26 (DMRB 7.2.3). The subgrade then requires assessment as to whether it provides a satisfactory platform for this reconstruction. If not, then capping may also be required.

7.49 Where the subbase is considered satisfactory and the stiffness modulus provided by the foundation as a whole is adequate, then there is no need to excavate, even if the subgrade itself is of low CBR. This is because the provision of capping in the construction of new roads is primarily to enable the subbase and upper layers to be adequately laid and compacted, and to ensure that no damage to the subgrade occurs as a result of construction phase trafficking.

may 2008 7/7


grapHY


8. references and BiBlio

references

1. design manual for roads and Bridges (dmrB)

HD 23 – General Information (DMRB 7.1.1)

HD 24 – Traffic Assessment (DMRB 7.2.1)

HD 25 – Foundations (DMRB 7.2.2)

HD 26 – Pavement Design (DMRB 7.2.3)

HD 28 – Skidding Resistance (DMRB 7.3.1)

HD 29 – Data for Pavement Assessment (DMRB 7.3.2)

HD 31 – Maintenance of Bituminous Roads (DMRB 7.4.1)

HD 32 – Maintenance of Concrete Roads (DMRB 7.4.2)

HD 36 – Surfacing Materials for New and Maintenance Construction (DMRB 7.5.1)

HD 41 – Maintenance of Highway Geotechnical Assets (DMRB 4.1.3)

HA 44 – Earthworks: Design and Preparation of Contract Documents (DMRB 4.1.1)

TD 19 – Requirement for Road Restraint Systems (DMRB 2.2.8)

2. manual of contract documents for Highway works (mcHw)

Volume 1: Specification for Highway Works

Volume 2: Notes for Guidance on the Specification for Highway Works

Volume 3: Highway Construction Details

3.

4.

5.

may 2008

transport research laboratory documents (trl)

TRL 250; Nunn, M.E., Brown, A., Weston, D. and Nicholls, J.C., ‘Design of long-life flexible pavements for heavy traffic’, TRL, 1997

TRL 657; Coley, C. and Carswell, I., ‘Improved design of overlay treatments to concrete pavements. Final report on the monitoring of trials and schemes’, TRL, 2006

British standards

BS EN 12697-26:2004 Bituminous mixtures. Test methods for hot mix asphalt. Stiffness

BS EN 13108 Various Parts: 2005 and 2006 Bituminous mixtures. Material specifications

PD 6691:2007 Guidance on the use of BS EN 13108 Bituminous mixtures – Material specifications

statutory publications

Health and Safety at Work Act (1974)

Health and Safety at Work (Northern Ireland) Order 1979

Management of Health and Safety at Work Regulations (1999)

Management of Health and Safety at Work Regulations Statutory Rules of Northern Ireland, 2000 No 87

Construction (Design and Management) Regulations, Statutory Instrument 2007 No. 320

Safety at Street Works and Road Works – A Code of Practice, Department for Transport, 2002

Traffic Signs Manual Chapter 8, Department for Transport, 2006

Construction (Design and Management) Regulations (Northern Ireland), Statutory Rules of Northern Ireland, 2007 No. 291

8/1



6. other publications

Network Management Manual, Highways Agency

National Assembly for Wales Trunk Road Manual, National Assembly for Wales Transport Directorate

Concrete Pavement Maintenance Manual (2001), Highways Agency and Britpave (published by the Concrete Society)

Bibliography

LR1132; Powell, W.D., Potter, J.F., Mayhew, H.C. and Nunn, M.E., ‘The Structural Design of Bituminous Roads’, TRRL, 1984

RR87; Mayhew, H.C. and Harding, H.M., ‘Thickness Design of Concrete Roads’, TRRL, 1987

may 20088/2

may 2008


9. enQuiries


Chief Highway Engineer The Highways Agency 123 Buckingham Palace Road London G CLARKE SW1W 9HA Chief Highway Engineer

Director, Major Transport Infrastructure Projects Transport Scotland 8th Floor, Buchanan House 58 Port Dundas Road A C McLAUGHLIN Glasgow Director, Major Transport Infrastructure G4 0HF Projects

Chief Highway Engineer Transport Wales Welsh Assembly Government Cathays Parks M J A PARKER Cardiff Chief Highway Engineer CF10 3NQ Transport Wales

Director of Engineering The Department for Regional Development Roads Service Clarence Court 10-18 Adelaide Street R J M CAIRNS Belfast BT2 8GB Director of Engineering

9/1

chapter 9 enquiries

HD 31/94

Maintenance of Bituminous Roads

THE HIGHWAYS AGENCY

THE SCOTTISH OFFICE DEVELOPMENT DEPARTMENT



DESIGN MANUAL FOR ROADS AND BRIDGESincorporating AmendmentNo. 2 February 1998

Summary: This amendment revises Chapter 1 "Introduction" and Chapter 2 "SurfaceTreatments". A paragraph on headrooms has been added to Chapter 1 andChapter 2 has been amended to include the latest advice on Crack Sealingand information on other surfacing treatments.

Maintenance of Concrete Roads

Summary:

THE HIGHWAYS AGENCY HD 32/94

THE SCOTTISH OFFICE DEVELOPMENT DEPARTMENT


THE DEPARTMENT OFTHE ENVIRONMENT FOR NORTHERN IRELAND

Volume 7 Section 4Part 2 HD 32/94 Registration of Amendments

ELECTRONIC COPY - NOT FOR USE OUTSIDE THE AGENCY

January 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED




Volume 7 Section 4Registration of Amendments Part 2 HD 32/94


PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED January 1994






January 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED

VOLUME 7 PAVEMENT DESIGNAND MAINTENANCE

SECTION 4 PAVEMENTMAINTENANCEMETHODS

PART 2

HD 32/94

MAINTENANCE OF CONCRETEROADS

Contents

Chapter

1. Introduction

2. Surface Treatments

3. Joint Repairs

4. Structural Repairs

5. Strengthening


7. Enquiries

Annex

1. Maintenance and Repair Procedures

Volume 7 Section 4 Chapter 1Part 2 HD 32/94 Introduction

1. INTRODUCTION

ayracts

chds andean

r states then.

ain

t

General M

1.1 Proper maintenance of rigid and rigidcomposite pavements is important if the structure is tohave a reasonable expectancy of remaining in asatisfactory condition and achieving the design life. Toassess the need for maintenance and repair work astandard procedure of regular inspection and faultrecording is necessary, as described in HD 29 (DMRB7.3.2) and HD 30 (DMRB 7.3.3). This should ensurecorrect diagnosis of the various types of defect that maybe encountered.

1.2 This Part gives recommendations formaintenance, repair and structural strengthening ofconcrete slabs in rigid and rigid composite pavementsbased on current experience using methods which aretried and proven. In most instances a more detaileddescription of faults and advice on treatments is givenin Mildenhall and Northcott (1986). Procedures formaintenance and repair are given in Annex 1 unlessreferred to in the Specification (MCHW1).

1.3 In addition, some methods of treatment arediscussed which have had little use to date, such as,Cracking and Seating of concrete pavements,Thin-bonded Concrete Overlays and ContinuouslyReinforced Concrete Overlays. Consequently the latesadvice should be sought from the OverseeingDepartment if such methods are to be considered. Maintenance of the bituminous surfacing in rigidcomposite pavements is covered in HD 31 (DMRB7.4.1).

Implementation

1.4 This Part shall be used forthwith on all schemesfor the improvement and maintenance of trunk roadsincluding motorways, currently being preparedprovided that, in the opinion of the OverseeingDepartment this would not result in significantadditional expense or delay. Design organisationsshould confirm its application to particular schemeswith the Overseeing Department.

su

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utual Recognition

1.5 The construction and maintenance of highwpavements will normally be carried out under contincorporating the Overseeing Department'sSpecification for Highway Works (MCHW1). In su

cases products conforming to equivalent standarspecifications of other member states of the EuropCommunity and tests undertaken in other membewill be acceptable in accordance with the terms of104 and 105 Series of Clauses of that Specificatio

Any contract not containing these Clauses must cont

effect regarding which advice should be sought.itable clauses of mutual recognition having the same

NOT FOR USE OUTSIDE THE AGENCY

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Chapter 1 Volume 7 Section 4Introduction Part 2 HD 32/94

STRENGTHENING

(Chapter 5)

STRUCTURAL

REPAIRS

(Chapter 4)

JOINT

REPAIRS

(Chapter 3)

SURFACE

TREATMENT

(Chapter 2)

SURFACE

JOINTS

STRUCTURAL

STRENGTHENING

START

(Chapter 1)


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Volume 7 Section 4 Chapter 2Part 2 HD 32/94 Surface Treatments

2. SURFACE TREATMENTS

al
2.1 HD 28 (DMRB 7.3.1) discusses the assessmeof skidding resistance using the SCRIM machine orPortable Pendulum Tester, although SCRIM onlymeasures skidding resistance at low speed (50km/hr)
2.2 There are several different methods of restoriskidding resistance to concrete roads, but the mostsuitable depends on the type of road, speed of traffic,the risk factor and the aggregate used in the concreteSome methods will restore macrotexture while othersonly improve microtexture, and some do both. Thefollowing gives some general comments:-

a) Surface dressing - Restores microtexture and

macrotexture - Type dependent on road

b) Transverse grooving - Suitable for high speed roads - Restores braking force and

macrotexture - Prevents aquaplaning

c) Mechanical rougheningFlailing-transverse - Improves macrotexture and removes

polish from aggregates.

Bush hammering - Improves macrotexture - Removes polish of exposed aggregate - Risk of damage to surface and

microcracking of the matrix

Grit blasting - Removes surface polish - Short term improvement of micro-

texture - Suitable for roads with lighter traffic

with less polishing effect

d) Thin bonded surface repairs - Restores microtexture and

macrotexture - Time consuming and relatively

expensive

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nt - Suitable for damaged slabs and locareas

. Methods b) and d) above are detailed in the

ng further in this Part.

.

Specification (Series 1000) and are not discussed

SURFACE DRESSING

2.3 Design of bituminous surface dressings,including those on concrete roads, is covered in Section4, Part 1, which in turn refers both to Road Note 39(1992) and the Specification (Series 900) for furtherdetails.

2.4 A very high level of low speed skiddingresistance can be achieved by the application of asurface dressing consisting of an epoxy resin basedbinder and highly abrasion resistant calcined bauxitechippings. Its comparatively high cost is likely to limitits use to small areas. However, the performance of thistype of treatment on concrete may not be as good as onbituminous surfaces because of the difficulty ofobtaining good bond between the binder and theconcrete surface over large areas. The main reason forbond failure is the different coefficients of thermalexpansion for concrete and resin.

2.5 Surface dressings may not have a very long lifewhere turning heavy commercial traffic is likely toscour the surface. An advantage of bituminous surfacedressing is the speed at which it can be applied, but it isa weather susceptible operation which is restricted to alimited season. Care must be taken in the selection ofthe right binder to suit the circumstances and controlapplied to achieve an even distribution of binder andchippings. Traffic control measures are extensive andcomplex because of the need for controlled slow speedtrafficking of newly applied dressing, followed bysweeping to dislodge and remove any loose chippings. Surface dressing is likely to need renewing at least onceduring the structural life of the slab.

MECHANICAL ROUGHENING

2.6 Improved skidding resistance can be achievedby roughening the worn surface by the use of abrasiveblasting, scabbling, grinding or milling equipment. Abrasive blasting is effective in restoring slow speedskidding resistance and equipment is available which issuitable for treating both large and small areas.

FOR USE OUTSIDE THE AGENCY

ONIC DOCUMENT ARE UNCONTROLLED 2/1

Chapter 2 Volume 7 Section 4Surface Treatments Part 2 HD 32/94

2.7 The effectiveness of the surface textureproduced by scabbling and milling will be influencedby the the properties and characteristics of the coarseaggregate in the concrete that is exposed.

2.8 Any retexturing treatment gives an increase inhigh speed skidding resistance resulting from a greaterdepth of texture. Treatments are likely to beaccompanied by some increase in the amount of tyrenoise, the nature of which will depend on the type oftreatment that is adopted.



Volume 7 Section 4 Chapter 3Part 2 HD 32/94 Joint Repairs

3. JOINT REPAIRStheins.

ey

,

.

re

3.1 Concrete slabs expand and contract as thetemperature rises and falls. They also warp, or curl,when the temperature of the slab surface is substantiadifferent from the underside. Longitudinal andtransverse joints enable the different types of movemeto occur and it is therefore essential that they are bothwell constructed and maintained in an effective workincondition.

DEFECTIVE JOINT SEALS

Consequences

3.2 Defective joint seals allow silt, grit, stones andwater to enter between the slabs and infiltrate the lowelevels of the pavement. An accumulation of detrituscan prevent the joint closing and lead to spalling ofconcrete or, if several slabs are affected, "blow-up"expansion type slab compression failures. Penetrationof water into the joint can lead to softening of the sub-base or subgrade, and corrosion of steel dowels andtie-bars, especially in the presence of de- icing salt. T

T

3ctCag

3(Owinslihwe



presence of water is also a contributory factor in disruptive alkali-silica reaction, that can occur with

lly concrete composed of certain reactive aggregate

nt

g

r

he

ypes

.3 The life expectancy of most joint seals is shortompared with that of the concrete pavement, since thend to harden and become brittle with age. onsequently, joint seals have to be replaced regularlynd a guide to the main types, relative life and usage isiven in Table 3.1.

.4 The type of seals given in the SpecificationMCHW1) are expected to have a life of up to 10 yearsn some older roads a bituminous sealant was used,ith which the present specified hot applied sealants acompactible. Therefore, it may be preferable and

peedier to rake out and top up the joint with the shortfe bituminous seal at more frequent intervals, than ave the expense of sawing out the joint and resealingith new materials. The choice would be made onconomic grounds.

Classification Chemical Physical Life Type of ComplianceType Joint

Hot Applied PVC/pitch polymer Elastomeric Medium All BS2499 (1993)

Polymer/Bitumen Elastomeric Medium All BS2499 (1993)

Cold Applied Polysulphide Elastomeric Medium All BS5212 (1975) and

Polyurethane Elastomeric Medium All BS5212 (1990)

Silicone Elastomeric Medium Warping BS5889 (1989)

BS4254 (1983)

Compression Polychloropene Elastomeric Longest All BS2752 (1990) andASTM D2628

TABLE 3.1 Main Types of Joint Sealing Material



Chapter 3 Volume 7 Section 4Joint Repairs Part 2 HD 32/94

3.5 Either hot or cold applied elastomericmaterials or compression seals are permittedfor general re-sealing, but gun grade coldapplied materials are probably the mostappropriate when small quantities of materialare involved. At some locations, e.g. bus lay-bysor parking areas, a fuel resistant sealingmaterial should be used. At joints betweenconcrete and bituminous pavements only hot applied polymer modified bituminous sealants,Type N1 to BS2499 (1993) or preformedpolymer modified bituminous strips shall beused.

3.7 Overbanding materials shall not beused to seal joints or cracks in concretepavements. The seal tends to crack along thejoint and that remaining on the surface hidesdefects in the joint arrises.

3.6 Sealants of the hot applied type can become brittleif overheated at the time of application. Inadequatemixing of cold applied sealants or the use of incorrectproportions of components will result in poorperformance.

3.8 For fast-track construction, compression typeseals can be installed as soon as the groove is sawn. hot or cold applied sealants are to be used they can beapplied after the concrete has reached sufficient strength for grit blasting without damageoccurring to the joint grooves. Refer to theSpecification (MCHW1) Series 1000 and to HD27(DMRB 7.2.4.3).

3.9 Joint seals suffer from adhesion failure betweethe seal and the groove, cohesion failure causingtransverse or longitudinal cracking within the seal, andextrusion. In each case the seal must be replaced. Possible causes of these defects are given in Table 3.2and the correct amount of sealant to apply is shown inFigure 3.1.

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3.10 Preparation and sealing of joint groovesshall be in accordance with the relevant BritishStandard for hot or cold applied sealants andwith the Specification (MCHW1) andmanufacturer's recommendations forcompression seals.

3.11 For the seal to function properly it mustadhere to the sides of the sealing groove. Thisnecessitates use of an appropriate primer,ensuring that the sides of the sealing groove arescoured by abrasive blasting, clean, dry and nottoo cold at the time of application. Somesealants are incompatible with others (e.g. hotapplied bituminous based and pitch modifiedmaterials) and this may adversely affectadhesion if some of the old sealant remains inthe groove.

3.12 Hot applied sealants shall not beapplied when the temperature in the groove isless than 7 C. Heating the concrete to raise itso

temperature is not recommended because theeffect will be temporary and it will cool veryquickly causing moisture to condense on thesurface. However, the application of hot airthrough a lance may be used to dry surfacewater from within the sealing groove.

3.13 The groove dimensions must beappropriate both for the amount of movementthat is expected to take place at the joints,which is a function of the distance betweenthem, and the type of sealing material that isused. A width to depth ratio of between 1:1 and2:1 shall be

If

n

,

reparation of the sealing groove

ealing groove dimensions

R USE OUTSIDE THE AGENCY

IC DOCUMENT ARE UNCONTROLLED January 1994


n

Type of defect Causes Remedies

Adhesion failure (lack of adhesion Inadequate preparation of the sealing Remove old seal, thoroughly cleabetween the seal and the sides of the groove out, prepare groove and re-sealsealing groove)

Faulty or inappropriate sealingmaterial

Incorrect sealing groove dimensions

Chilling effect of cold concrete

Moisture in sealing groove

Cohesion failure (cracks within the Age - do -seal either transverse or parallel tothe joint groove) Faulty or inappropriate sealing

material


Lack of bond breaking strip beneathseal

Extrusion Overfilled sealing groove - do -

Lack of compressible caulking stripin bottom of sealing groove


TABLE 3.2 Joint Seal Defects : Causes and Remedies


January 1994 PAPER COPIES OF THIS ELECTRONIC DOCUMENT ARE UNCONTROLLED 3/3


Direction of application

Too little Too much Correctcompressiblecaulking strip

sealinggrove

FIGURE 3.1 Application of Gun Grade Cold Applied Sealant

al

d

used for plastomeric materials and from 1:1 to1:1.5 for elastomeric materials, with a minimumdepth of seal of 15mm.

3.15 During hot summer weather, sealswhich have been applied at a cooler time of theyear may be extruded from the joint. If thishappens, the seals may be damaged by trafficand can be lost altogether. Joint seals shalltherefore initially be not less than 5mm belowthe surface of the slab, except in the case of corkseals.

3.14 The dimensions of grooves appropriate to hotand cold applied sealing materials are given in theSpecification (MCHW1) Series 1000. Theuncompressed width of compression seals and the initiwidth of the sealing groove are related to the distancebetween joints, and in accordance with themanufacturer's recommendations, so that when inserteinto the sealing groove they remain in compression atall times.

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HALLOW SPALLING

16 Spalling of the arrises of joints (or the edge ofabs) is likely to impair the effectiveness of the jointal. Possible causes are given in Table 3.3.


ONIC DOCUMENT ARE UNCONTROLLED January 1994


.

3.17 Spalling due to the ingress of incompressiblematerial into the joint groove usually occurs suddenlyand is often in the form of 'wedge' shaped pieces ofconcrete which typically taper towards the back andsides.

3.18 Shallow spalling should be repaired before thejoint seal has been affected to the extent that it willpermit an appreciable amount of surface water etc topenetrate into the joint groove. It is likely that thiswould happen if the spalling is more thanapproximately 20mm deep.

3.19 Correct identification of this defect is veryimportant because it can appear to be very similar to theearly stages of deep spalling, the causes and remedy ofwhich are very different.

Thin Bonded Arris Repairs

3.20 Removing the old joint seal and groove formerat the commencement of a thin bonded arris repairenables the full extent of shallow spalling to bedetermined. This can be confirmed by tapping with asteel rod, a hollow sound indicating the presence ofcracked material, whilst a ringing tone indicates intactconcrete.

3.21 Wherever possible thin bonded arris repairsshould be carried out using either cement mortar or fineconcrete depending upon the depth of the repair. Thepractical minimum depth is approximately 10mm. Cement mortar should be used for repairs up to 20mmdeep and fine concrete for thicknesses greater than this

d

Type of defect Causes Remedies

Shallow spalling of transverse and Weak concrete lacking in Minor spalling should be removelongitudinal joint arrises or the durability or compaction by widening the joint grooveedges of slabs locally by sawing up to 40mm

Infiltration of silt or other fine wide at transverse joints andmaterial into the joint groove 30mm wide at longitudinal joints

Penetration of stones into the joint necessarygroove

Tilted joint groove formers with in this way should be rectified

Mechanical damage caused by repairremoval of formwork etc

in conjunction with flat grinding as

Spalling which cannot be dealt

by means of a thin bonded arris

TABLE 3.3 Shallow Spalling : Causes and Remedies




he slabired, a

with

e.

.

of

3.23 Thin bonded repairs to surfaces of slabsand to joint arrises shall be carried out inaccordance with the Specification (MCHW1)Series 1000. Thorough preparation, attentionto detail and good workmanship are essentialfor this type of repair.

3.22 The use of epoxy concrete, or other 'concretes'having different thermal properties and strengths fromthe existing concrete, is not recommended, sincedebonding of the repair or further cracking of theexisting concrete often ensues. They may however, beused with care on small repairs.

3.24 The procedure requires a delineating groove tobe chased out rather than sawn in order to provide aroughened vertical edge around the repair, againstwhich the repair material can be properly bonded(Figure 3.2). Sawing produces a polished surface whicinhibits good bond and there is also an undesirable

deincoreThsu

D

3.slOmthsl

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tendency for sawn grooves to be extended into tbeyond the corners of the repair. However if requshallow delineating groove may be sawn to start

and subsequently chased out to the full depth.

3.25 The success of thin bonded arris repairs

h

pends largely upon developing good bond at theterface between the repair material and the existingncrete. This is best achieved by compacting thepair material against a freshly scabbled, clean surface finished repair must be flush with the existing slabrface and must not bridge the joint.

EEP SPALLING

26 Deep spalling usually extends to at least halfab depth and possible causes are given in Table 3.4ne cause, dowel bar restraint, may be due toisalignment and/or excessive bond along the length e bar which should be free to move in one of theabs.

Mortar or fineconcrete repairmaterial

Collapsible groove former(surface treated to preventadhesion) firmly fixed inlocating groove

Min. 150mm

(100 mm for resin mortar)

Delineating groove chased outaround perimeter with smallsingle headed scabbingtool or router

Joint crackLocating groove sawn20 - 30mm wide to encompassjoint crack and locate grooveformer

Break out to sound concreteusing scabbing tool ensuringthat prepared surface isreasonably level, clean andall loose material is removed

FIGURE 3.2 A Thin Bonded Arris Repair

Min

. 10m

m

OR USE OUTSIDE THE AGENCY

NIC DOCUMENT ARE UNCONTROLLED January 1994


Type of defect Cause Remedy

Deep spalling at contraction and Dowel restraint Transverse full depth repairexpansion joints

Ingress of solids into the jointcrack

Deep spalling at the corners of - do - Full depth corner repairbays

TABLE 3.4 Deep Spalling : Causes and Remedies

mer,ionaligh

e

3.27 Full depth repairs shall be carried outin accordance with the Specification (MCHW1)Series 1000. Full depth repairs form small bayswhich should be at least equivalent to the mainslab in all respects (Figures 3.3 and 3.4). Irrespective of whether the main slab isreinforced or not, it is advisable to reinforce therepair and this must be done when the ratio ofthe longest to the shortest dimension is greaterthan 2 which will often be the case. Eithersquare or long mesh reinforcement ofappropriate weight may be used. In the case ofthe latter, the main bars shall be positionedparallel to the longest dimension and if squaremesh reinforcement is used, its weight per m2

shall be approximately twice that of the longmesh reinforcement.

Full Depth Repair

3.28 Small bays of 1m length (along thecarriageway) constructed over a granular sub-base mafail around the dowel bars and settle due to heavy trafloading and lack of compaction of the sub-base. It isrecommended that when the existing sub-base isgranular the length of bay is increased to at least 2m sthat recompaction of the sub-base is easier and thetraffic load is spread over a longer bay, so eliminatingthe punch down effect on a short bay.

3rferaca

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3.29 It is essential that the corners of therepair are broken out either to a sharp rightangle, or a 45 chamfer in the case of cornero

repairs, and that the sides are vertical anddressed smooth, otherwise it will be difficult tofix the expansion material in position and any"bridging" or projections in the corners arelikely to result in spalling. Longitudinal sides ofrepairs shall not occur along the wheelpaths. Intransverse full depth repairs the expansionmaterial shall be extended around the corners toensure complete separation at these locations. Expansion material shall be provided aroundthe entire perimeter of full depth corner repairs.

yfic

o3.31 As repairs are mainly carried out in sum

removal of slabs or parts of slabs can cause additcracking due to the compressive stresses during h

.30 When undertaking all types of full depthepairs every effort should be made to prevent slurryrom sawing, repair material and other debris fromntering any joint cracks and grooves in the sides of thepair. Prior to placing the repair material, joint cracksnd grooves should be cleaned out using oil freeompressed air if necessary and taped over withdhesive masking tape.

emoval of Slabs

emperatures being concentrated on less than half thewidth of slab, once saw cuts are made. This may caulongitudinal cracking or localised compression failuresat joints.

T FOR USE OUTSIDE THE AGENCY


Chapter 3 Volume 7 Section 4

Full depthsawn face

Joint groove formingstrip (see Note 1)

Min. cover 50mm(60mm for slabs> 200mm thick)

Min length approx 1.0m

Longitudinal meshreinforcement (see Note 2)

Sealing groove sawn,prepared primed and sealed

20 or 25mm O h.y.d.stie bars (see Note 4)

Existing slab containing wide crack broken outand replaced with pavement quality concrete

Min. 200mm

Holes drilled to receive tie bars(see Note 3)

Resinmortar

Notes:-1. Polyethylene foam joint groove forming strip 5mm wide x 40mm deep stuck along top edges of remaining slab.2. Appropriate longitudinal mesh reinforcement, main bars parallel to longitudinal dimension.3. Hole drilled 150-200mm deep to receive tie bars at 600mm c/c.4. Tie bars to be 20mm dia. for slabs up to 225mm thick.

FIGURE 3.3 Longitudinal Full Depth Repair - Jointed Slabs

100min

Joint Repairs Part 2 HD 32/94

Full depthsaw cut

Min. cover 50mm(60mm for slabs> 200mm thick)

Min length approx 1.0m

Longitudinal meshreinforcement (see Note 2)

Sealing groove sawn,prepared primed and sealed

Existing slab containing wide crack or deep spalling broken outand replaced with pavement quality concrete

Resinmortar

Notes:-1. Polyethylene foam joint groove forming strip 5mm wide x 40mm deep stuck along top edges of remaining slab.2. Appropriate longitudinal mesh reinforcement, main bars parallel to longitudinal dimension.3. Hole drilled 150-200mm deep to receive 20 or 25mm O x 400mm burr free dowel bars in Grade 250 steel at 300mm c/c.4. Tie bars to be 20mm dia. for slabs up to 225mm thick.

FIGURE 3.4 Transverse Full Depth Repair

100min

Sawn sealing groove15mm wide and25mm deep

Joint groove formingstrip (see Note 1)

ExpansionJoint

ContractionJoint

50mm

Min. 5mm disc of compressible materialto be provided in end of dowel sheathwhen expansion joint filler board is used

Bond prevented along projecting end ofdowel bars aligned parallel to the surfaceand the longitudinal axis of the slab

Min. 5mm thick polyethenefoam expansion joint fillerboard stuck with contactadhesive.




t

rsrial

crack

3.32 To reduce this risk:-

a). Saw full depth cuts at cooler periods of the day(ie. at night or early morning).

b). Saw along the joint before making cuts eachside to eliminate a badly spalled joint.

c). Cool the concrete with water.

d). If a series of repairs is required makeintermediate cuts to relieve stress at intervalsrather than cutting sequentially along the road.

3.33 Saw cuts should not be extended into adjacentslabs. To ease removal of the slab in the corners andprevent under-cutting or breakout where the saw cannoreach, holes can be drilled full depth.

3.34 When removing an old joint in one lane andforming new joints at new positions staggered withthose in other lanes, the new slab should be isolatedfrom adjacent slabs longitudinally. No tie bars arenecessary and a separating compressible material

O

3.caTashA

3.rewpr

3.isharesammloinsere



should be placed along the longitudinal joint.

t

THER CRACKS AT JOINTS

35 The main types of these cracks, their likelyuses and appropriate remedies are summarised inble 3.5. The two types of stitched crack repair areown in Figure 3.5, and procedures described in

nnex 1.

36 The reason for carrying out a stitched crackpair is to convert the crack into a tied warping jointhich will allow the slab to "hinge" at that point whilseventing the crack from becoming wider.

37 The use of resin mortar to bond-in the tie ba recommended to try to ensure that the repair materdens before movement at the crack disrupts thepair. It may be necessary to use a purpose made w to cut the sealing groove along the line of aeandering crack. If the crack occurs within theiddle third of the length of the tie bars at angitudinal joint, it will not normally be necessary tostall new 'staple' tie bars and only the sawing andaling of a groove along the crack will usually bequired.

Type of Defect Cause Remedy

Transverse or diagonal cracks at Dowel restraint - gross Transverse full depth repairtransverse joints misalignment

Late sawing of joint groove

Misaligned top and bottom crackinducers

Longitudinal cracks at transverse Compression failure Transverse, longitudinal or cornerjoints full depth repairs as appropriate

Ingress of incompressible materialinto joint crack

Edge restraint

Longitudinal cracks at longitudinal Misaligned top and bottom crack Longitudinal full depth repairjoints inducers

Omission of bottom crack induceror

Stitched crack repair

TABLE 3.5 Other Cracks at Joints : Causes and Remedies




Longitudinal

Tie bars located at 600mm centres

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TYPE 1 REPAIR

TYPE 2 REPAIR

FIGURE 3.5 Stitched Crack Repairs



Volume 7 Section 4 Chapter 4Part 2 HD 32/94 Structural Repairs

4. STRUCTURAL REPAIRS

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4.1 Structural defects manifest themselves mainin the form of various types of cracks in the slab butsettlement or movement at joints may also occur whiwithout remedial action, can lead to the developmencracks and subsequent failure. Also considered asstructural defects are compression failures of the'blow-up' expansion type. Due to the unique featurescontinuously reinforced slabs and roadbases they ardealt with separately in paragraphs 4.32 to 4.36.

4.2 Structural cracks are classified according totheir severity which is defined in terms of the unspallwidth of the crack measured on the surface of the slain cold weather (see Table 4.1).

TRANSVERSE AND LONGITUDINAL CRACKS

4.3 Narrow transverse cracks are a normal featuof all reinforced slabs and roadbases. They areconsidered to be structurally insignificant, are notexpected to deteriorate any further, and consequentlare not likely to require any remedial treatment. However, longitudinal cracks are not expected and mwell deteriorate and develop further unless someremedial action is taken.


January 1994 PAPER COPIES OF THIS ELECT

ly Where longitudinal and transverse cracks cross tha risk of spalling occurring, particularly if they cross

ch, obliquely. Reinforcement at medium transverse ct of may not have yielded completely and action shoul

taken to prevent the ingress of water, brine etc whcould lead to corrosion and spalling. When sealin

of cracks, a sawn groove is preferred to one that is ce out by a router or single headed scabbing tool, be

it is much more regular and can be made narrowe

yielded. In effect the cracks are likely to be acting ed either undowelled or untied joints and consequentlb require at least a full depth and perhaps a bay

replacement repair.

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reinforcement at wide cracks will almost certainly have

4.4 No cracks of any type are expected to occurbetween the joints in unreinforced slabs and, althoughnarrow transverse cracks may not require anyimmediate treatment, it is quite likely that in this type ofslab they will become wider in a fairly short time. Consequently at the very least they need to be regularlinspected. Medium and wide cracks should be treatedin the same way as similar cracks in reinforced slabs.

4.5 The most likely cause and appropriate remediefor structurally significant cracks are given in Table 4.2.

Crack Definition Width (mm) Condition Assumed

Narrow < 0.5 Full aggregate interlock and loadtransfer

Medium 0.5 - 1.5 Partial load transfer. Permitsingress of water

Wide > 1.5 No load transfer. Permits ingressof water and fine detritus

TABLE 4.1 Crack Classification


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Chapter 4 Volume 7 Section 4Structural Repairs Part 2 HD 32/94

Type of defect Cause Remedies

Transverse cracks Excessive bay length Medium width cracks - form a

Dowel bar restraint at joints

Late sawing of joint grooves

Inadequate reinforcement lap repair

Sub-base restraint (lack ofseparation layer or excessiveirregularity of sub-base)

groove and seal

Wide cracks - transverse full depth

Longitudinal cracks Excessively wide bays Narrow cracks in reinforced slabs

Omission of bottom crack inducerat longitudinal joint Narrow cracks in unreinforced

Compression failure of all types should be remedied by

Settlement

require no immediate action

slabs and medium cracks in slabs

means of a stitched crack repair

Wide cracks in all slabs should beremedied either by a longitudinalfull depth repair or by means of abay replacement repair

TABLE 4.2 Transverse and Longitudinal Cracks : Causes and Remedies

4.6 Prior to breaking out the affected bay afull depth saw cut shall be made around theperimeter of the repair to minimise damage tothe surrounding slab. This shall include theexisting transverse joints, care being taken toensure that the saw cuts do not extend intoadjacent bays. The concrete may then be sawninto smaller pieces before being broken up andremoved from the bay. The concrete thatremains in the corners of the repair after sawcutting shall be broken out carfefully to avoidundercutting the remaining slab.

Bay Replacement



4.7 Any reinstatement of the sub-base thatis necessary shall be undertaken before the newdowel and tie bars are fixed at the transverseand longitudinal joints respectively. Particularcare is necessary to ensure that any newsub-base material is fully compacted especiallyin the corners, and a heavy plate vibrator shallbe used to compact either granular or cementbound sub-base material.




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4.8 An existing cement bound subbase maybe reinstated and regulated as necessary usingsand/cement mortar, fine concrete or fine coldasphalt.

4.11 The procedure for bay replacement isgiven in Annex 1. Note should be made ofChapter 3, paragraphs 3.31 to 3.34 regardingremoval of slabs in summer.

4.9 If an existing granular sub-base is to bereplaced with cement bound material, any potentialdeleterious effect, due to creating a discontinuity in theunderslab drainage, should be considered andappropriate action taken.

4.10 To avoid surface water ponding in the repair, ishould either be prevented from entering by the use osand bags or provision made for it to drain away.

DIAGONAL AND CORNER CRACKS

4.12 The term diagonal cracks is intended to includall multi- directional full depth cracks in the slab whichare neither generally transverse, nor longitudinal, noracross the corners of bays. Corner cracks includesingle, full depth cracks varying in length fromapproximately 0.3 m to 2 m across the corners of bayswhich, if not repaired, may lead to localiseddeterioration of the sub-base and perhaps subsequentmud pumping. The most likely causes and appropriateremedies are given in Table 4.3.

4.13 If a repair to the full width of a slab is notappropriate then a corner repair is carried out. Whenundertaking corner repairs it is desirable that as large 'chamfer' as possible is provided across the corner toreduce the risk of a crack subsequently developingacross the slab from that point. This means that it maynot be possible to extend the saw cuts that are madearound the corners of the repair through the full depthof the slab necessitating quite a lot of careful breakingout to the smooth vertical face that is required in thecorners. Particular care should be taken to avoiddamaging the remaining top edges of the slab.

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longitudinal joint and/or edge of the slab.

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.14 It is essential that the repaired slab should nohibit either contraction or expansion movement in thxisting slab. For this reason it is recommended that owel or tie bars are provided in the edge which isarallel to the longitudinal axis of the slab, and that amm thick expansion filler board is provided around therimeter of each repair.

RACKS AT MANHOLES AND GULLIES

.15 If recesses in the slab which house surfaceater gullies or manholes etc are incorrectly positione

elative to transverse or longitudinal joints, cracks arekely to develop across the slab. Similar cracks arelso likely to occur if the slab is 'propped' or resting on

he gully or manhole construction. To ensure this doeot happen, either the recess must be made sufficientrge to encompass the shaft of the structure and any

oncrete surround to it, or the top of the shaft andurround, must not be brought closer than 200 mm tohe underside of the sub-base.

.16 To reduce the risk of these cracks occurring,he recesses should be positioned in the corners of thays, either astride or alongside a transverse joint, anave 'chamfered' corners. When this is not possible,

ERTICAL SLAB MOVEMENT

.17 Vertical movement of the slab may developither in the form of dynamic movement which occursnder passing traffic or permanent movement in the

orm of settlement of the slab or 'stepping' at joints orracks.

.18 Dynamic movement may be associated withud-pumping, the usual signs of which are muddy

tains on the surface of the slab which, unless remedi likely to eventually result in multiple cracking of thelab. Mud-pumping is probably also indicative of pooravement or sub-soil drainage which should beorrected before any remedial work to the slab isndertaken. Seepage of water up through joints orlong the edges of the slab may also indicate poorrainage.

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Diagonal Cracks Settlement or heave of the sub-base Narrow cracks in unreinforcedor subgrade slabs and medium cracks in all

slabs will need to be either sealedor remedied by means of a stitchedcrack repair

Wide cracks will necessitate eithera bay replacement repair or a fulldepth repair

Corner Cracks Lack of load transfer at joints Corner or transverse full depth

Dowel bar restraint near edge ofslab

Ingress of solids into joint at edgeof slab

Acute angles in non-rectangularslabs

Loss of sub-base support

repair as appropriate

TABLE 4.3 Diagonal and Corner Cracks : Causes and Remedies

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4.19 Dynamic movement may be measured asdeflections of the slab at joints or cracks under a stator dynamic load. A dynamic load may be applied by moving lorry, Deflectograph or Falling WeightDeflectometer. In each case, high absolute deflectionrelative deflection across joints or cracks is indicativepoor support and possible voiding.

4.20 Settlement is most likely to occur as a result consolidation or compaction of the fill material inembankments, particularly in the back-fill behindstructures or when the pavement is constructed onground which has a low bearing capacity. It may alsooccur where there are shallow mine workings etc.

4.21 `Stepping' in the form of permanent relativevertical movement at joints and wide cracks is aphenomenon which can occur in slabs where there iseffective load transfer in the form of dowel or tie barsjoints, and in which the reinforcement, if any, hasyielded the cracks.

4.22 These defects, their likely causes andappropriate remedies are described in Table 4.4. However, it should be noted that the remedy for the


PAPER COPIES OF THIS ELECT4/4

immediate problem may not remove the original caic eg. ground softening due to water ingress. It is esa that the cause is understood before ordering repai

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Slab Lifting

4.23 Raising the level of slabs by slab lifting is avery controllable process in which the slab is connectedto a lifting frame which straddles the bay and is raisedto the required level in increments of a few millimetresat a time by the operation of hydraulic jacks (Proceduregiven in Annex 1). Whilst the slab is still connected tothe lifting frame, the void that has been createdunderneath should be filled by either pressure orvacuum grouting. When slab lifting is undertaken overa long length, it may be necessary to install stitched tiebars across the longitudinal joint to prevent this fromopening subsequently.


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Dynamic movement at joints and Lack of support from sub-base Pressure or vacuum groutingcracks

Lack of, or ineffective, loadtransfer dowels or tie bars at joints

Mud-pumping Poor pavement or sub-soil drainage Renew or improve existing surfacwater and/or sub-soil drainage asnecessary

Settlement Compaction or consolidation Localised settlement may bewithin the sub-soil drainage remedied by means of slab lifting

Movement in underlying ground or vacuum groutingin conjunction with either pressure

Severe settlement can be remediedeither by reconstruction of the slabor by the construction of anoverlay as described in 7.4.2.5.

Stepping at joints and cracks Lack of effective load transfer Slab lifting undertaken indowels and tie bars at joints conjunction with either pressure or

vacuum grouting and/or bumpcutting

TABLE 4.4 Vertical Slab Movement, Mud Pumping and Settlement : Causes and Remedies

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4.24 Slab lifting can also be used on continuouslyreinforced concrete pavements, but requireslongitudinal saw cutting between lanes in order toisolate each section of pavement. In contrast to othertypes of concrete pavement, both the slab and theunderlying cement bound sub-base should be cutthrough if lifting is to be successful, as a result of thebonding created between these two layers.

Pressure grouting

4.26 Pressure grouting is used either to fill smallvoids and stabilise dynamic movement of the slab or tfill the voids that are created when slabs are raised tocorrect settlement or stepping at joints and cracks(Procedure given in Annex 1). As well as cementitiouand resin grouts a dry mix mortar may also be used tofill voids, but it may be necessary to raise the slabinitially to a slightly higher level than is actuallyrequired to allow for future compaction once traffickedFluid grout is more suitable for the filling of smallervoids under the slab.

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injected and the use of low viscosity grout enao small voids to be penetrated. There is also lit

of inadvertently filling service ducts.

s 4.28 Care should be taken never to grout uexpansion joints, particular care is necessary

operating during colder weather. Preventing frmovement of slabs could lead to compression ty

. "blow-up" failures.

4.29 The unit of measurement of grout in both

Vacuum grouting

4.26 In this process, normally a low viscosity regrout is induced to flow into voids beneath the slabthe application of a vacuum. Holes approximately mm in diameter are drilled through the slab on 1 mm grid to provide the vacuum suction and groutinjection points (see Annex 1, Figure 1.1).

4.27 The advantages of this process are that anwater beneath the slab is drawn off before the grou

pressure and vacuum grouting should be the litre.

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COMPRESSION FAILURES

4.30 Compression failures can occur in concretepavements either in the form of longitudinal cracks, oras a series of short longitudinal and transverse cracksclose to a joint giving a 'crazed' appearance, or as"blow-up" expansion failures in which the slab iscrushed and may buckle as part of it lifts up from thesub-base. These defects occur as a result of excessivhigh compressive stresses which develop in the concrdue to restrained expansion of the slab during periodshot weather, the effects of which may be accentuated a high moisture content in the concrete.

4.31 Among the defects found at joints where"blow-ups" have occurred are defective joint seals,misaligned or badly corroded dowel bars, severespalling, longitudinal cracks, a lack of bond betweenupper and lower layers of concrete (in two layer slabconstruction) and poorly compacted concrete. "Blow-ups" normally extend the whole width of at leastone traffic lane and require immediate temporary repato enable traffic to continue using the carriageway. Permanent repair is effected by means of bayreplacement or transverse full depth repairs across thefull carriageway width incorporating a 20mm fillerboard to enable a larger amount of expansion to takeplace subsequently.

CONTINUOUSLY REINFORCED CONCRETE

4.32 Continuously reinforced concrete pavements,and in particular those which have a concrete runningsurface, are generally more difficult and costly to repaithan other types of rigid pavement because of the largquantity of heavy steel reinforcement in the slab and thhigh levels of stress that are generated in it. The mostappropriate time of the year to carry out work requiringpart of the slab to be demolished (eg full depth repairsis during the Spring and Autumn months, which avoidsworking during particularly hot weather whencompressive stresses are high and the slab may bucklor during cold weather when the slab is in tension.

4.33 Preventive maintenance to prolong thestructural life of the slab is highly desirable. Measuressuch as the sealing of medium to wide cracks, repair ospalled cracks, and grouting to stabilise vertical slabmovement may all be necessary.

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4.36 The appropriate remedy for both deepspalling and the "punchout" type of defect is afull depth repair (Procedure given in Annex 1). Where possible, the repaired area shall be notless than 0.5m from the nearest adjacenttransverse crack that is to remain, nor closerthan 3m to an end-of-day construction joint. Because of the relatively high level of stress thatcan develop in this type of pavement, full depthrepairs shall be carried out one lane width at atime and succeeding repairs not made until thefirst has achieved sufficient strength to betrafficked. The continuity of the reinforcementshall be maintained where necessary byadequate laps.

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.34 A defect peculiar to continuously reinforcedlabs is that known as a "punchout" in which fragmentsf broken concrete may be "punched" by the action of

raffic downwards into the underlying subbase layer. his type of defect usually occurs at locations whereevere differential settlement has taken place or failuref the subbase is evident. "Punchouts" occurarticularly where closely spaced transverse cracksave developed along with longitudinal cracks in acalised area. This subsequently results in therogressive disintegration of the slab under the action

raffic.

.35 Construction defects, such as poorly compacteoncrete or inadequate laps of the longitudinaleinforcing bars, can result in localised damage toontinuously reinforced slabs. Extensive spalling ofoncrete above the reinforcement is rare in continuouseinforced slabs unless there is insufficient cover to thteel.

EMPORARY REPAIRS

.37 On some occasions it may be necessary tondertake temporary or emergency repairs quickly,sing materials which can be trafficked in a short time.owever it is advisable to carry out permanent repairss soon as practicable, otherwise further deterioration

he slab may occur. Temporary repairs may be requireo perform satisfactorily for several years and thereforedequate preparation, such as removal of all loose and

damaged concrete, should be carried out.




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4.38 For full depth temporary repairs, either densebituminous macadam or hot rolled asphalt may be useAlternatively, suitably sized precast concrete slabs canbe used, provided they are properly levelled and seateon the existing sub-base to prevent rocking. Anypotential difference in finished road level may be madeup by a layer of CBM3 or 4 between the precast slaband existing sub-base.

4.39 Partial depth repairs using either normalbituminous, special proprietary bituminous orthermoplastic material may be undertaken as atemporary remedy to shallow or deep spalling at jointsor to surface scaling. If compacted by hand it may benecessary to lay the repair material slightly higher thanthe surface of the surrounding slab to allow for furthercompaction under traffic. It is recommended that theminimum thickness of this type of repair should be20mm and if the repair is a deep one, it is advisable toapply the repair material in layers approximately50-100 mm thick.

4.40 Materials that are applied across a joint shouldhave elastic properties that will enable them to accommodate movement at the joint. If necessary,the surface of temporary repairs may be 'dusted' withcement or sand to prevent the repair materials beingpicked up by the tyres of vehicles which traffic it soonafter the repair.

TRENCH REINSTATEMENTS

4.41 Advice on the excavation and reinstatement oopenings in a rigid pavement are given in theSpecification (MCHW1) Series 700. The use of foameconcrete as backfill material may be appropriate insome circumstances.



Volume 7 Section 4 Chapter 5Part 2 HD 32/94 Strengthening

5. STRENGTHENING

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5.1 Strengthening of an existing rigid or rigidcomposite pavement may be required to extend its lifdue to an increase in traffic because the structuralcondition of the existing pavement has deteriorated tothe point where it is no longer able to carry thepredicted future traffic. HD 29 (DMRB 7.3.2) and HD30 (DMRB 7.3.3) describe methods of assessing theresidual life of rigid pavements and the selection (andbasic thickness design) of maintenance measures. Tchapter concentrates on the practical considerations carrying out such works.

OVERLAYS

5.2 If the existing slabs are relatively intact then iis usual to recommend their retention in anystrengthening measures. However before overlayingthe pavement must be brought up to a relativelyuniform standard, by replacing failed bays, grouting ovoids, rectifying joint defects, etc.

5.3 Overlaying of concrete pavements (excludingcontinuously reinforced concrete) represents greaterdifficulties than overlaying bituminous pavements dueto the discontinuities which occur at joints or widecracks. Joints (and wide cracks) represent a source oconcentrated movement in the pavement brought aboby a combination of:-

a) Load induced movements;

b) Long term temperature induced movements(seasonal);

c) Short term temperature induced movements(diurnal);

d) Drying shrinkage movements.

5.4 An overlay placed over existing jointedconcrete slabs will be subjected to a concentration ofstrain at locations of joints (and wide cracks) in theunderlying pavement. Therefore the overlay must bedesigned to accommodate this movement.

5.5 For concrete overlays this can be carried out two basic ways:-



a) By forming joints in the overlay at the samee location as joints (and wide cracks) in the

underlying pavement;

b) By separating the overlay from the underlypavement using, say, as regulating layer o

bituminous material.

his 5.6 It should be noted that a concrete overlof is sufficient to strengthen the road may be of ina

thickness to provide cover to reinforcing steel, and m

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therefore need increasing in depth.

5.7 To help delay/resist reflection cracking ofbituminous overlays one or more of the followingmeasures are required:-

a) Using thicker bituminous layers than requiredfor the predicted traffic loading only;

b) Creating a "joint" in the bituminous surfacingby sawing and applying a suitable sealant;

c) Using modified binder to improve the elasticrecovery and fatigue cracking properties of thebituminous materials;

d) Using stress-absorbing or "reinforcing"materials to distribute strains above joints (orwide cracks) by partial debonding or othermechanisms.

5.8 In method b) reflection cracks do not alwaysfollow the line of the formed joint and several parallelcracks can be created. To help overcome this, a bandsurfacing material is sometimes removed, and replacewith a modified bituminous material.

5.9 Modified binders and some stress-absorbing o"reinforcing" materials are relatively new and there isinsufficient evidence to date to prove their long termeffectiveness and overall economy.

5.10 If the existing concrete slabs are in poorcondition, with many cracks, underslab voiding,stepping at joints, etc. then it may be advantageous tobreak up the slabs further, followed by seating theresulting concrete "blocks", to give a more uniformsupporting layer for either a concrete or bituminousoverlay and to effectively eliminate the large strain



Chapter 5 Volume 7 Section 4Strengthening Part 2 HD 32/94

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5.12 It is essential that a good bond isachieved between the overlay and the existingconcrete, since bond failure could lead to majordistress, with cracking and spalling of theconcrete across the carriageway width. Specialsurface treatment is therefore required, theextent depending on the existing surfacecondition, aggregate type, etc.

5.13 This may consist of two passes of a gritor shot blaster, the second one just prior tooverlaying. A cement grout may be used toassist in creating the bond, but, if used, thismust be applied immediately before the overlayconcrete is placed, otherwise premature settingof the grout may create a slip layer. Theexisting joints shall also be cleaned and resealedand any spalling at joints repaired prior tooverlaying.

5.14 Joints and wide cracks in theunderlying pavement should be reproduced inthe bonded concrete

concentrations at joints (and wide cracks). This isdiscussed further in 5.24 to 5.31.

Bonded Concrete

5.11 Bonded concrete overlays are only appropriatefor existing concrete pavements in good condition andwhere structural and level considerations dictate thatonly a thin overlay (eg. 50-100mm) is required. Arelatively small increase in slab thickness can greatlyincrease pavement life, but is not consideredworthwhile to apply overlay thicknesses less than50mm since the material cost is only a fraction of theoverall strengthening cost. Any assessment ofincreased life should be based on the properties (eg.strength) of the existing slab, making allowance forfatigue damage to date due to past traffic.

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overlay, ensuringthat the appropriate joint type(eg. contraction or expansion) is used. Thereare problems in achieving this in practice whichmeans that the work is labour intensive andrequires close supervision.

5.18 Prior to the construction of the overlayit is necessary to stabilise any verticalmovement that might be occurring at joints orcracks in the existing slab by grouting or fulldepth reconstruction. Any spalling at jointsshall also be rectified with bituminous orcementitious materials and the surface levelsregulated if necessary. A partially unbondedoverlay must not be attempted since this cannotbe specified nor produced in a uniform mannerin practice.

.15 Consideration should be given to using amestone aggregate in the overlay since this has a looefficient of thermal expansion than, say, flint gravelr granite aggregates and will therefore shrink lessuring curing. Use of fabric reinforcing mesh in theverlay may be required to help control shrinkageduced cracking.

.16 Since there is no reduction in the total numbef joints, this method of overlaying can lead toontinuing maintenance expenditure, and associatedraffic delays, in the future.

nbonded Concrete Overlays

.17 When structural and level consideration dictahat a thicker overlay is required, then an unbondedverlay may be used. Such concrete overlays may bither unreinforced, jointed reinforced or continuouslyeinforced slabs. Reinforced concrete overlays areuitable for strengthening of existing pavements thatave deteriorated to a considerable degree as well as

hose that are still in good structural condition.

.19 Use of an unbonded overlay does not constrahe designer to the same slab shape and size as the

underlying pavement. Therefore continuouslyreinforced concrete may be used.


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Volume 7 Section 4 Chapter 5Part 2 HD 32/94 Strengthening

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5.20 For an unbonded overlay in jointedreinforced concrete, joints need not bereproduced in the same locations as the existingslab. However, the pavement design mustensure that debonding occurs by some positivemeans (eg. by use of a bituminous regulatinglayer of nominal thickness 40-50mm).

5.21 The minimum practical thicknessrecommended to achieve cover to reinforcement, toenable proper compaction and to help preventreinforcement ripple (for continuously reinforcedconcrete overlays) is 200mm.

Bituminous Overlays

5.22 The thickness of bituminous overlays isdetermined not only by structural requirements but alsby the need to minimise or avoid reflection crackingwhich may result from movement in joints or cracks inhe underlying slab. Recommended thicknesses aregiven in HD 30 (DMRB 7.3.3).

5.23 Considerable care must be taken before theoverlay is applied. Joints will require checking andremedial works carried out if necessary. Depressionsand potholes should be filled and all cracks sealed. Iparticular, measures must be taken to limit the amounof horizontal and vertical movements at joints and widcracks.

Cracking and Seating

5.24 If the existing pavement is too distressed andvariable to repair to a relatively uniform condition, thencracking and seating in situ may be appropriate prior toverlaying. This method has also been proposed as ameans of delaying or controlling reflection cracking in bituminous overlay to a rigid pavement. The aim is toreduce the size of each concrete element so thatmovements are no longer concentrated at joints.

5.25 The process also helps to seat the concrete,eliminating any voiding which may have developedbelow the slab during its life. However the resultingload spreading ability of the concrete is considerablyreduced. Consequently there is a contradictoryrequirement between the need to increase the numbecracks to help control reflection cracking, and the neeto retain some integrity to help load spreading andoverstressing of the subgrade.



d) Climatic Factors;

e) Trafficking Factors.

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5.26 An optimum crack spacing for each site hastake into account the following:-

a) Existing Thickness and Type of ConcretePavement;

b) Existing type and quality of Subgrade;

c) Proposed Overlay Thickness and MaterialType;

5.27 The first site using cracking and seating in thUK was in 1978 and several other sites havesubsequently been treated. However, there appearsbe insufficient experience to choose an optimum craspacing, as evidenced by USA practice whererecommendations for the size of the broken concreteelements range from 450mm x 600mm to 2m x 2m. Where the pavement is lightly trafficked and only a tbituminous overlay is envisaged, cracking the slabs little more than a granular sub-base may be appropr

5.28 Cracking should be carried out usingequipment designed for the process to produce conc`blocks' of relatively uniform shape. Equipmentcurrently available includes:-

a) Guillotine type drop hammer;b) Pneumatic or hydraulic hammer;c) Whip hammer;d) Demolition ball.

5.29 For 1m to 2m size blocks a guillotine hammedemolition ball or heavy impact hammer would besuitable. With the smaller whip hammers and hydratype hammers the impact spacing is less, but the oveffect is to produce cracks at up to 1m spacing. Theare suited to more lightly trafficked roads, or in urbanareas, where heavy percussion equipment would beundesirable for environmental reasons.

5.30 Trials should be carried out at each site todetermine the optimum energy for each "drop" andspacing of "drop" to achieve the desired objective. Iessential that the cracked slabs are properly seated appropriate use of both deadweight and vibratoryrollers, to ensure that all voids are filled and that nofuture "rocking" of concrete elements will occur.



Chapter 5 Volume 7 Section 4Strengthening Part 2 HD 32/94

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5.31 Prior to overlaying a regulating layer isrequired and this can be provided by either abituminous or cement bound material. The overlayitself may consist of a flexible or rigid pavement, butcontinuously reinforced concrete in particular offersgood load spreading properties which enable it toaccommodate some localised variation in support fromthe underlying materials.

Reconstruction

5.32 Strengthening can also be achieved by thedemolition and reconstruction of the existing pavemenThis will be necessary when the existing slab hasdeteriorated structurally to the extent that it isconsidered unsuitable for use as the sub-base orroadbase beneath a new overlay or in those cases whan increase in the level of the running surface of theroad cannot be accommodated.

5.33 Slabs may be demolished in a variety of wayswith either pneumatic, mechanical or resonant breakethe latter being claimed to be particularly appropriatefor the demolition of reinforced slabs.

5.34 Consideration should be given to recycling theexcavated slabs by crushing and using as aggregate fcapping, sub-base, cement-bound materials or concre



Volume 7 Section 4 Chapter 6Part 2 HD 32/94 References and Bibliography


,

.

e.

References

1980

BS5889; "Silicone Based Building Sealants", BSI.

1981

D2628-81; "Specification for Pre-formedPolychloroprene Elastomeric Joint Seals for ConcretePavements", ASTM.

1983

BS4254; "Specification for Two-Part Polysulphide-Based Sealants", BSI.

1985

D3406-85; "Specification for Joint Sealant, Hot Pouredfor Concrete and Asphalt Pavements", ASTM.

1990

BS2752; "Specification for Chloroprene RubberCompounds", BSI.

BS5212; Part 1; "Specification for Joint Sealants", BSI

1992

Road Note 39; "Recommendations for Road SurfaceDressings", TRL and Department of Transport.

1993

BS2499; "Hot Applied Joint Sealants for ConcretePavements", BSI.

1994

HD 28 (DMRB 7.3.1) Skidding Resistance.

HD 29 (DMRB 7.3.2) Structural Assessment Methods.

HD 30 (DMRB 7.3.3) Structural Assessment Procedur

HD 31 (DMRB 7.4.1) Maintenance of BituminousRoads.

S

B

1

MD



Undated

Mildenhall H S and Northcott G D S, "A Manual for the

pecification for Highway Works (MCHW1).

ibliography

986

aintenance and Repair of Concrete Roads",epartment of Transport and Concrete Society.



Volume 7 Section 4 Chapter 7Part 2 HD 32/94 Enquiries



7. ENQUIRIES

All technical enquiries or comments on this Part should be sent in writing as appropriate to:-

Chief Highway EngineerThe Department of TransportSt Christopher HouseSouthwark Street T A ROCHESTERLONDON SE1 OTE Chief Highway Engineer

The Deputy Chief EngineerThe Scottish Office Industry DepartmentRoads DirectorateNew St Andrew's House J INNESEDINBURGH EH1 3TG Deputy Chief Engineer

The Director of HighwaysWelsh OfficeY Swyddfa GymreigGovernment BuildingsTy Glas RoadLlanishen K J THOMASCARDIFF CF4 5PL Director of Highways

Chief Engineer - Roads ServiceDepartment of the Environment for Northern IrelandRoads Service HeadquartersClarence Court10-18 Adelaide Street W J McCOUBREYBELFAST BT2 8GB Chief Engineer - Roads Service

Volume 7 Section 4Part 2 HD 32/94 Annex 1

MAINTENANCE AND REPAIR PROCEDURES

1.1 Procedure (see Chapter 3, Fig 3.5)

a) Chase out slots 25-30mm wide by470mm long at 600mm centres and atright angles to the line of the crack. The depth of the slots shall be such as toensure that, when bedded, the tie barslie between 1/3 and 1/2 the depth of theslab below the surface.

b) Drill holes 25-30mm in diameter by50mm deep at each end of the slots.

c) Clean out the slots using oil-freecompressed air.

d) When in a dry state, prime the slots,place the staple tie bars into beds ofepoxy resin mortar and cover to aminimum depth of 30mm with the samematerial.

e) Prepare the sides and complete thefilling of the slots with thoroughlycompacted resin or cementitiousmortar.

f) Cure and open to traffic.

g) Saw a groove along the line of the crackand seal.

1.2 Procedure (see Chapter 3, Fig 3.5)

a) Ascertain the depth of the slab.

b) Mark out drilling points at a distancefrom the crack equivalent to the depthof the slab, at 600mm

Stitched Crack Repair - Type 1

Stitched Crack Repair - Type 2



intervals along the crack with alternatepoints on opposite sides of the crack.

c) Drill holes (min. 16mm diameter) atapproximately 26 to the surface of theo

slab to a depth which allows 50mmcover at the bottom of the slab.

d) Place in cartridges of epoxy resin typeadhesive.

e) Insert 12mm diameter deformed tiebars through the cartridges.

f) Rotate the bars for about 1 minute toensure adhesive is well mixed.

g) Cut the bars so that the end isapproximately 50mm below the surface.

h) Alternatively, the length of the tie barsmay be pre-determined by measuringdown the hole and notching the bars ata point 50mm below the surface. Afterthe bars have been driven in, rotatedand the mortar set, the surplus can bebroken off by twisting. Any bars whichcontinue to twist after the mortarshould have set shall be deemed to beunbonded. They shall be withdrawnand the hole redrilled.

i) Plug the remainder of the hole with anepoxy resin mortar.

j) The road may be opened to traffic assoon as the mortar in the holes has set.


ONIC DOCUMENT ARE UNCONTROLLED A1/1

Volume 7 Section 4Annex 1 Part 2 HD 32/94

1.3 Procedure

a) Make a vertical saw cut around theperimeter of the repair through the fulldepth of the slab taking care to ensurethat saw cuts do not extend into theadjacent bays. Further saw cuts maybe made to enable the concrete to beremoved in convenient pieces.

b) Carefully break out and remove theexisting concrete from within the repairarea without damaging the remainingslab and with the minimum amount ofdamage to the sub-base.

c) Reinstate or regulate the sub-base asnecessary.

d) Drill for and fix new dowel and tie barsat transverse and longitudinal joints.

e) Provide new separation layer.

f) Position reinforcement if required.

g) Stick groove forming strips along thetop edges of the existing slab.

h) Place, spread, compact and finishpavement quality concrete inaccordance with the requirements ofthe Specification (MCHW1) Series1000, flush with the surface of theadjacent slab and to within a toleranceof 3mm and with a difference of notmore than 4mm between the surface ofthe repair and a 3m straight edge.

Bay Replacement

S


PAPER COPIES OF THIS ELECTRA1/2

i) Apply a wire brushed surface textureand cure immediately by theapplication of a sprayed resin based,aluminized curing compound.

j) Seal joints as specified.

1.4 Procedure

a) Make a full depth saw cut along thelongitudinal joint or joints to separatethe bays that are to be raised from theadjacent slab.

b) Drill 50mm diameter lifting holes intothe bay at positions to suit the liftingframe, and to straddle joints or cracks.

c) Drill 32mm to 36mm diameter holesthrough the slab and bound sub-base ona 1m x 1m grid for grout injection.

d) Position hydraulically operated liftingframes transversely astride the bay withlifting bolts over the lifting holes.

e) Fix threaded female sleeves into thelifting holes with resin grout or mortar. Ensure the sockets remain verticalduring setting of the mortar.

f) Screw the lifting bolts into threadedsleeves at the lifting points.

g) Establish a level reference datum acrossthe bay that is to be raised.

h) Slowly raise the bay by controlledoperation of the hydraulic jack at eachcorner of the lifting frames in sequencefor a few millimetres.

lab Lifting using Space Frames.




i) Strat grout injection, using a PortlandCement/PFA dry mix in the ratio of 1 to1, using the pressure grouting systemfor larger voids, or the vacuumgrouting system for small voids (seeprocedure for pressure or vacuumgrouting).

j) Lift and grout in stages until the correctlevel is reached or for a maximum lift of50mm. If more than a 50mm lift isneeded, raise adjacent areas along thecarriageway, before returning tocomplete the process to the requiredlevel.

k) Repeat the process for adjacent lanes.

l) Clean out the injection and lifting holesand plug them with a cementitiousmortar. Remove any excess grout ordry material.

m) For long lengths of more than 15mwhere the longitudinal joint tie-barshave been cut, use either of the crackstitching methods to tie the joint, with aminimum of 4 bars at 600mm centresplaced in the centre of eachunreinforced slab. The spacing of barsin reinforced slabs should be agreed foreach site.

1.5 Procedure (see Figure 1.1)

a) Avoiding services, drill 32mm to 36mmdiameter grout injection holes throughthe slab and any bound sub-base on a1m x 1m grid extending over the wholearea of the void under the slab.

b

c

d

e

1

a

b

Pressure Grouting

Vac



) Remove any water from the void byblowing with compressed air at eachend of the grout holes in sequence,working progressively across and alongthe bay, down crossfalls andlongitudinal gradients.

) Inject fluid grout or dry mix mortarunder pressure at each of the groutholes in sequence, workingprogressively across and along eachbay. Grouting shall continue at eachhole until refusal. Temporarily plugadjacent holes when excess grout orplumes of dry mortar emanate fromthem.

) Upon completion of the pressuregrouting process any surplus grout shallbe removed from the surface of the slaband the holes cleaned out and madegood with resin or cementitious mortar. Any resin grout which cannot beremoved from the surface of the slabmay be blinded with calcined bauxite ifthis can be done before the grout hasgelled.

) Open to traffic after the appropriateminimum curing period has elapsed.

.6 Procedure (see Figure 1.1)

) Avoiding services, drill 32mm to 36mmdiameter grout injection holes throughthe slab and any bound sub-base on a1m x 1m grid extending over the wholearea of the void under the slab.

) Temporarily plug the holes and sweepthe surface clear of all debris.

uum Grouting.



Volume 7 Section 4Annex 1

c) Remove the temporary plugs and placevacuum channels in position.

d) Place clear, flexible plastic sheetingover the area to be grouted and on topof the vacuum channels.

e) Effectively seal around the vacuum andinjection holes and around manholes togully openings to prevent ingress of airinto the voids or grout into the services.

f) Apply a vacuum to holes at edge oftreatment area and draw off any waterfrom the void beneath the slab.

g) With the vacuum applied, puncture theplastic sheeting at the

ELECTRONIC COPY -

PAPER COPIES OF THIS ELEA1/4

Part 2 HD 32/94

injection holes, pour grout into them inthe required sequence and continue theprocess until the grout ceases to bedrawn into the void. If grout begins tobe drawn up through any of thevacuum points, that hole should beplugged to avoid grout flowing on to thesurface of the slab.

h) Upon completion of the vacuum processany surplus grout shall be removedfrom the surface of the slab and theholes cleaned out and made good withresin or cementitious mortar. Any resingrout which cannot be removed fromthe surface of the slab may be blindedwith calcined bauxite if this can be donebefore the grout has gelled.

i) Open to traffic after the appropriateminimum curing period has elapsed.

Vacuum

Slab

GroutGroutGroutVacuumPolytheneSheet

FIGURE 1.1 Vacuum Grouting Diagram

NOT FOR USE OUTSIDE THE AGENCY

CTRONIC DOCUMENT ARE UNCONTROLLED January 1994


1.7 Procedure

a) Mark out a square or rectangular areaencompassing the defect. The edges ofthe repair shall be at least 1.5m fromthe nearest transverse crack and 3mfrom any transverse construction joint. The minimum length of repair shall be1.0m.

b) Saw a groove around the perimeter ofthe repair not more than 40mm deepand to a depth less than that of thereinforcement. (Particular care shall betaken to ensure that the reinforcementis not cut at the edge of the repair).

c) Carefully break out the defectiveconcrete in order to form a soundvertical edge to the repair, care beingtaken to limit damage to and bending ofthe reinforcement to a minimum. Should it become necessary to cut thereinforcement in order to remove theconcrete, the cut must be made at leastone lap away from the edge. Bars madeof high yield steel shall not be bent andsubsequently straightened.

d) Reinstate the sub-base as necessary.

e) Lap and tie or weld in any new oradditional reinforcement that may berequired to replace that which has beendamaged or removed. The length oftied laps shall be at least 35 bardiameters or 450mm, whichever is thegreater for longitudinal bars and300mm for transverse bars. Theminimum

Continuously Reinforced Slabs and Roadbases - FullDepth Repair



length of any welded lap shall be150mm.

f) Erect side forms where necessary.

g) Stick groove forming strips or corkseals along the top edges of thesurrounding slab.

h) Clean out the area with compressed airand thoroughly dampen the sub-baseand edges of the repair.

i) Place and evenly spread high earlystrength pavement quality concrete tothe appropriate surcharge, compactusing internal and surface vibrationand finish flush with the surface of theadjacent slab and to within a tolerance± 3mm and with a difference of notmore than 4mm between the surface ofthe repair and a 3m straight edge. Particular care shall be taken to ensurethorough compaction around thereinforcement and the edges of therepair.

j) Apply a wire brushed surface textureand cure immediately by theapplication of resin based, aluminizedcuring compound.

k) Seal joints as specified.



SCOTTISEI DEVELOPbfENT DEF’ART?dENT

CHIEF ROAD ENGINEER

TECHNICAL MEMORANDUM SH 4/86

SCOlTISH ROUTINE MAINTENANCE MANAGEMENT SYSTEM

Summary

This memorandum gives details of the requirements and procedures to be adopted by Agent Authorities for inventory data collection and inspections to comply with tbe Code of Practice for Routine Maintenance.

. I l I TABLE OF CONTENTS

1. SCOPE

2. IN’IRODUCTION

3. SCOTTISH ROUTINE MAJNTENANCE MANAGEMENT SYSTEM

4. PROCEDURES

5. TIMETABLE

6. FUNDING ARRANGEMENTS

7. ENQUIRIES

I l 8. APPENDICES

Appendix A

Inventory items and details to be collected

Appendix B

Inspection record form

Appendix C

Checklist of items to be inspected, activity codes and details of inspection defects

Appendix D

Section summary sheet for detailed surveys

Appendix E El

Inspection procedure flow chart

PAGE NO

1

1

Al

Dl

1. SCOPE

1.1 The purpose of this Technical Memorandum is to advise Agent Authorities of the

new requirements for routine maintenance on motorways and all purpose trunk roads, the

plans for implementation of the Code and the new management procedures to be introduced.

1.2 Reasonable costs incurred by Agent Authorities in setting up the new maintenance

procedures as described in Section 4 of this Memorandum will be reimbursed.

1.3 The cost of carrying out inspections and the procedures required by the Code will be

reimbursable under the terms of the existing Agency Agreement: details are given in

Section 6 - Funding Arrangements.

1.4 The Department have published the Code of practice for Routine Maintenance after

consultation with the County Surveyors Society (Scottish Branch). The Code takes effect

from 1 April 1986.

2. INTRODUCTION

2.1 The Code of Practice is a working management aid which is intended to ensure

consistency of standards and value for money. Together with an inventory of highway

infrastructure and furniture the Code will provide a realistic base for resource planning and

distribution of maintenance funds.

2.2 Agent Authorities are now requested to set up agreed procedures for collecting and

maintaining an inventory of highway infrastructure and furniture, recording inspections and

defects encountered and the remedial action taken. Where required by the Code, Agent

Authorities should prepare and maintain record drawings and schedules.

2.3 The implementation of the Code will be monitored to determine whether the

objectives of consistency and value for money are being achieved. The Code has been

produced in ring binder format to facilitate modifications should these prove necessary.

3. SCOTTISH ROUTlINE MAINTENANCE MANAGEMENT SYSTEM a

3.1 The new system of routine maintenance management which Agent Authorities are

now asked to adopt has three main components:

(i) a Code of Practice which sets out inspection and reporting procedures,

action to be taken and, where appropriate, standards to be met;

(ii) an inventory of highway infrastructure and furniture, collected and stored in

a consistent manner;

(iii) a data filing/handling system which allows the interrogation and cross-

referencing of the information collected as detailed in (i) and (ii) above. The system

should relate to procedures for costing and programming work and allow checks to

be made on performance.

3.2 A computerised inventory collection system has been developed using the Husky

Hunter data capture device with data collected to be sent to the Department for direct

input to CHIPS. Development of a data handling/filing system for Agent Authority use is in

hand and in the first year, before the system is available Agent Authorities should follow the

procedures set out in Section 4 of this Memorandum.

4. PROCEDURES FOR ROUIZNE MAINTENANCE MANAGEMENT

4.1 Inventory: Data Collection and Storage

4.1.1 An inventory of trunk road infrastructure and furniture is an essential part of the

Scottish Routine Maintenance Management System which is to be applied to the network.

To be of maximum benefit, inventory data MUST be collected and stored in a consistent

manner.

4.1.2 The inventory items and details to be collected and stored are set out in the list at

Appendix A. This information should be stored in a computerised filing system and, until

development of the system for Agent Authority use has been completed, all inventory

details will be stored on the Department’s CHlpS database.

4.1.3 Agent Authorities are therefore sked to collect data in accordance with the list at

Appendix A. Priority should be given to the motorway network.

2

I‘

.I

l 4.1.4 Before collecting data, Agent Authorities should discuss and agree with the

Department (HQ Section) the procedures to be adopted for collection and storage of data

and the timescale over which these activities arc to take place. Data capture devices are to

be used and the information stored in a suitable filing system which allows convenient

access and has appropriate safeguards against loss of information.

4.1.5 An Inventory Collection Manual based on field-tested methods of data collection has

been produced and will be issued to Agent Authorities before inventory surveys are

commenced.

4.2 Inspection Reports: Data Collection Storage and Retrieval

l 4.2.1 In 24 areas of maintenance activity the C.ode calls for 2 types of inspection - Safety

and Detailed -at frequencies which reflect the importance of a particular road. In addition

daily safety patrols are called for on the most important motorways and trunk roads, but no

special arrangements axe required for these safety patrols - they should be carried out

during other maintenance activities.

4.2.2 Detailed Inspections form part of the procedures necessary to establish a

programme of planned maintenance. Where defects are found they should, depending on

their category, either be corrected immediately or as soon as possible after inspection, or be

noted as items requiring attention within a programme of planned maintenance.

4.2.3 Agent Authorities are therefore required to carry out both types of inspection and

to record the type, extent and location of defects found and all subsequent action taken.

Safety inspections should commence as soon as possible after 1 April 1986; detailed

inspections should commence after inventory details have been collected. All records must

be retained for 6 years.

4.2.4 Until the computerised handlkg system has been fully developed it will be necessary

to adopt a conventional paper-based recording system. A record form devised for this

purpose is at Appendix B. Once the computerised system is operaticmal recording will be

undertaken using data capture devices such as the Husky Hunter.

4.2.5 The Code refers to checklists of items to be inspected and defects to be recorded.

Sample checklists are at Appendix C.

4.2.6 A separate f,orm should be completed for each defect found and used to record all

subsequent action taken. Nil returns should be made in respect of Detailed Inspections and a

Section Summary Sheet for that purpose is at Appeadix D. Nil returns, in the form of a

completed Snspection Record, will only be required in respect of Safety Inspections if they

are generated by reports and complaints received outside the normal inspection procedures.

Suitable records will have to be kept to demonstrate that Safety Inspections are being

carried out in accordance with the requirements of the Code. That will preclude the need

for nil returns in respect of planned Safety Inspections.

4.2.7 Following inspectioas, completed records should be suitably filed.

4.2.8 The filing system should be interrogated at regular intervals to determine where

maintenance work is outstanding and to prepare works programmes. Where inspections show

the need for immedtate action (Category 1 defect) which cannot be taken at the time of

inspectioa the defect must be brought to the attention of those responsible with a view to

their carryiug out remedial action at the earliest opportunity. With a computerised data

filix@handling system such notifications can be generated automatically. With a

conventional paper system special action such as the immediate generation of a works order

will be neceesarp. Inventory information is essential in the preparation of such orders, of

works programmes in general and of specifications for works.

4.2.9 A flow chart illustrating the procedure for recording inspections and subsequerrt

action taken is shown at Appendix E.

4.2.10 The Department is preparing an Inspection Manual which will be available in the

near future.

4.3 AV- OF INFORMATTON

4.3.1 The Department will call for reports and make performance checks based on the

data held within the systems previously des&bed.. The checks will be in the nature of spot

checks with no prior warping given either to timing or to the areas to be considered.

4.3.2 The nature of the information sought would be: checks on the extent of compliance

with the Code; identifying difficulties in attaining standards set out in the Code; summary

4

data for inclusion within a wider maintenance management system; information on

individual defects which will include the time of their occurrence and the type and timing of

the remedial action taken.

5. lIMETABLE FOR IMPLEMENTATION

5.1 Agent Authorities are to start working to the Code of Practice from 1 April 1986.

5.2 A change from present methods of routine maintenance management to a method

based on the code will not be immediately possible, but the requirements in respect of

cyclical maintenance activities and frequencies of safety ispections are to be adopted from

the outset. It will not be possible to comply with the Detailed Inspection requirements of

the Code until such times as the Inventory surveys have been completed and the Inspection

Manual has been issued.

5.3 The procedure described in Section 4 of the Memorandum will therefore be brought

into operation over a period of time. H the first year of operation Agent Authorities will be

required to set up procedures for collection of inventory information and for the recording

of information from inspections and subsequent action to remedy defects found.

5.4 The preparation of record drawings and schedules called for by the Code should also

start during the first year of operation. In certain areas it will not be possible to complete

schedules until Detailed Inspections have revealed the existence of infrastructure or

furniture not immediately visible or previously known about.

5.5 Development of a computer-based Routine Muatmance Management System to

take account of the Code of Practice is well in h& T& system will not, be fully

developed for use by all Agent Authorities in the 1986/67 fiPancial year but it is hoped to

implement a full scale trial of the system with one of the Agent Authorities during the year.

The aim is to have a fully developed system available for ue by all Agent Authorities in the

1987/88 financial year.

6. FUNDING ARRANGEMENTS FOR ROUTINE MAINTENANCE OPERATION

6.1 Bids for the forthcoming financial year have to be made not later than 15 November

in the preceding year. It will not be possible for the bids for the 1987/88 financial year to

5

be based totally on any consolidated experience of working to the Code but bids made in 0

1987 for the 1988/89 financial year should be based on data and experience obtained in

1986187.

6.2 In the first year of operation Agent Authorities will still be required to manage

within the routine maintenance allocation issued by the Department before the start of the

financial year.

6.3 In bidding for the 1987/88 financial year Agent Authorities should make every effort

to take account of the requirements of the code,

6.4 Revision will be made for bids for carrying out inspections under the Code (other

than Safety Patrols as described in Section 2.2 of the Code) and for assembling the collected

data on a suitable filing system. Such activities, will be directly reimbursable under the

terms of the existing Agency Agreement.

6.5 The cost of operating the Code-based system of Scottish Routine Maintenance

Management including data handling.sucb as interrogation of inventory and inspection record

data to generate works programm’es, will be deemed to be included in the Agency Fee. The

cost of subsequent recording of action taken and the Safety Patrols will also be deemed to

be covered by the Agency Fee.

7. ENQUIRIES

7.1 Enquiries in connection with this Memorandum should be addressed to the Chief

Road Engineer, Scottish Development Department, New St Andrew’s House, Edinburgh EHl

3S2, telephone 031 556 8400 Ext 5719. I

M MACKENZIE

Chief Road Engineer

Scottish Development Department

New St Andrew’s House

Edinburgh

EHl 3SZ 6

APPENDIX A

SCOTTISH DEVELOPMENT DEPARTMENT

CODE OF PRACTLCE FOR ROUTINE MAINTENANCE

INVENTORY ITEMS AND DETAILS TO BE COLLECTED

Balancing Pond

Bollard

a

Bridge Over

Bridge Under

Carriageway

Catchpi t

Central Island

Central Reserve

0 Channel

Communications Cabinet

Counterfort Drains

Crossover

Culvert

a C$cle Track

link, section, date, xsect, chainage, distance

link, section, date, xsect, chainage, identity no, type,

diagram no, electrical ref. no.

link, section, date, start, end, identifier, type

link, section, date, start, end, identity no, type

link, section, date, start, end, surface, width

link, section, date, xsect, chainage

link, section, date, start, end, surface, width

link, section, date, xsect, start, end, surface, width

link, section, date, xsect, start, end, type

link, section, date, xsect, chainage, identity code, type

link, section, date, xsect, start, end

link, section, date, xsect, chainage, surface, width, sweep,

text

link, section, date, chainage, length, diameter


Al

Ditch link, section, date, xsect, start, end, type

Embankment and Cuttings

Fences and Barriers

Footway

Filter Drain

Grip

Gullv

Hardshoulder

Hedge

Interceptor

Kerb

Lighting Point

Manhole

Pedestrian Crossinq

Pedestrian Guardrail

Piped Grip

Reference Marker Point

link, section, date, start, end, xsect, angle, height

link, section, date, xsect, start, end, type

link, section, date, xsect, end, start, surface, width,

sweep


link, section, date, xsect, chainage, width, length, type a

link, section, date, xsect, chainage, type, access, locked




link, section, date, xsect, start, end, material, type

link, section, date, xsect, chainage, identity code, column

type, height, supply typ, position of column, installation

type, mounting brackets, electrical ref. no.



link, section, date, Chainage, type, material

link, section, date, xsect, start, end, material

link, section, date, xsect, chainage, length

link, section, date, xsect, chainage, type, identity code

A2

Retaining Wall link, section, date, xsect, start, end, type, height, position

Road Markings (Hatched) link, .section, date, xsect, start, end, width, material,

pattern, type of edge line

Road Markings (Longitudinal) link, section, date, xsect, start, end, material,

classification, colour, type, width, length, gap

Road Markings (Transverse & Special)

link, section, date, xsect, cbainage, classification, colour,

material, height

0 Road Studs

Safety Fence

Sims

a

Telephone Box

Traffic Control Barrier

Traf fit Signals

Trees

Verge

link, section, date, xsect, start, end, type, classification,

spacing, colour

link, section, date, xsect, start, end, type, shape, cross

section, post

link, section, date, xsect, chainage, identity code, type,

illuminated, diagram no, mounting height, mounting

method, width, height, ownership, electrical ref no,

photograph no

link, section, date, xsect, chainage, identity code

link, section, date, chainage, location, type, arrangement,

control

link, section, date, xsect, chainage, identity code,

mounting method, type, number of lamp units, layout no,

electrical ref no

link, section, date, xsect, chainage, number, length

link, section, date, xsect, start, end, actual width,

maintained width, angle, number of obstacles

A3

.

1. The range of inventory items and details collected may be expanded to meet

particular local requirements.

2. Date of last inspection is to be recorded with each inventory item.

3. “Link”, “section”, “start”, *end’ all relate to the CHART system of locational

referencing.

4. “Xsect” relates to cross sectional position.

5. More information on the collection of inventory is contained in the Inventory

Collection Manual referred to in section 4.1.5 of the Memorandum.

A4

r . _ -

.’ - CODE OF PRACTlCE FOR ROUTINE MAINTENANCE - INSPECTION RECORD klPml% 8

GENERAL

!I tn b IfI Rrrn tl fin ?I! R0pi0ll Agonl coo*

Elm Aor8 MulYtmr Twr Inmltm Ssnrl Number

coo* SID NRMlPOL/PBL

El lb PoIm EXISI~ hmpoctor VYYMOD

mmr oalm

LOCATION

il fJ-/ I-, m j&TJg f*, b

Aetlvily SW IOenlllier (Link 6 Soctknl No01 A NoOa 8 cnrrnroe x-Ssctional coor

5’1 I I 1 1 I I 1 i”l I I I I I

(ml Pormon

1 I I 1 TWlW odomlry Nummr Mbaram Numbar

iJ ~2~l~~~~~~~ll~~~llrlll~ll~II11~~~~~[[~~~~

D@s~r~ptmn Of Location ( OplIonrl I

IMMEDIATE - CATEGORY 1 20 TEXT

kl &El

i/lmGh” Works OrOw No. Rmctbw0m Actual Co11

YIN

1fMPORARr - CATEGORY I

kJ 2

Illllll YYMLtDD

b ifrrrrn

0810 fun0 Estlmsled COSI

$rmwcle COUO Works Order No. Rocnat~o AClUb CDS1

YIN

PEhANENf - CATEGORY 1 1

7

II irrrrn km r”rrrrm YVYUOO

D8W f”l I , , Pr I , I’;, B rof’T;“Tc;; I

coum Works Ororr No. Rscnmoe Aclwl Carl YIN

PERYANENT - CATEGORY 2

rl lIr71 ffnn ifmm’ TIWU Dale

Es~~mamo Cow

bfmnJtTii”rrrrm Code works Or&r NQ~ Rbcherge Aclual Coat

YIN

81

APPENDIX c

SCOTITSH DEVELOPMENT DEPARTMENT

CODE OF PRACTICE FOR ROUTINE MAINTEN~CE

CHECKUX OF mMS TO BE INSPECTED, AC- CODES AND DIXAILS OF

INSPECTION DEFECTS

rl-EM

Minor carriageway repairs

Footways and cycle tracks

Covers, gratings, frames and boxes

Kerbs, edgings and preformed channels

Highway drainage: Piped drainage systems

Highway drainage: Gullies, Catchpits and

Interceptors

Highway drainage: Piped grips

Highway drainage: Grips

Highway drainage: Ditches

Highway drainage: Filter drains

Highway drainage: Culverts

Highway drainage: Balancing ponds

Highway drainage: Ancillary items

Highway drainage: Flooding

Fences and barriers

Grassed areas

Hedges and trees

Sweeping and cleansing

Roadstuds

Road markings

Road traffic signs

Traffic signals

Road Lighting

a Motorway communications installations

ACXTVITY CODE

MC

FC

CG

KC

PD

GC

PG

GP

DI

FD

cv

BP

AI

FL

FE

GA

HT

SC

RS

RM

SG

TS

LP

CI

Cl

DETAIU OF INSPECTION DEFECTS

MINOR CARRIAGEWAY REpAfRS

Localised cracking

Localised edge deterioration

Surf acing joints

Cracking around ironwork

Patch - adjacent cracking

Patch - loss of material

Patch - diff in level

Trench RI - adjacent cracking

Trench RI - loss of material

Trench RI - diff in level

Pothole

FOOTWAYS AND CYCLE TRACKS

Standing water

Slab profile - uneverr/trips/gap>20mm

Slab cracked

Slab rocking

Block profile

Black top - pothole>25mm

Black top - local cracking

Black top - extensive cracking

Black top - fretting

Failed patch - adjacent cracking

Failed patch - loss of material

Failed patch - diff in level

Overgrown by vegetation

Adopted trench RI - adjacent cracking

Adopted trench RI - loss of material

Adopted trench RI - diff in level

LOCK

LODT

SRJT ’

CKlR

PACK

PLMT

PDLV

TACK

TLMT

TDLV

POTH

STWT

SLPF

SLCK

SROK

BKPF

BPOT

BLCK

BECK

BFRT

FPCK

FLMT

FDLV

OVGV

RACK

RLMT

RDLV

axea

length

length

area

area

area

axea

area

area

area

length

area

area

area

area

area

area

area

area

area

area

area

length

area

area

area

c2

. -

COVERS, GRATINGS, FlUMES AND BOXES

Difference in level with road

Difference in component levels

Rocking under load

Cracked or broken

Missing

Parallel gratings

Smooth surf ace

Blockage

IDLV

ICLV

IRLD

IECK

MISS

PARL

SMTH

BLOK

KERBS, EDGING AND PREFORMED CHANNELS

Vertical projection>ZOmm EVPJ

Horizontal projection>50mm EHPJ

Loose/rocking ELRK

Damaged DAMG

Channel block alignment CHAL

Missing MISS

Impeded water flow (detritus) IMWF

HIGHWAY DRAINAGIS: PIPES DRAINAGE SYSTEMS

Blockage

Other malfunction

Flooding

Drainage damage to road/verge

Flood nuisance to properties

Flood nuisance to services

BLOK

OMAL

FLOD

DRRD

NPRP

NSER

HIGHWAY DRAINAGE: GULUEIS, CATCHPITS AND INTZRCEF’TORS

Damaged

Collapsed

Silting

Blockage

DAMG

COLP

SILT

BLOK

0 c3

percentage

length

length

length

length

length

length

length

length

length

length

HIGHWAY DlUlNAGEt PIPED GRIPS

Blockage

Detritus

Broken

XUGHWAY DRAINAGE: GRIPS

Weed growth

Detritus “refuse”

Blockage

Flooding

HIGHWAY DRMNAGE: DlTCEES

Weed growth

Collapsed bank

Obstruction

Deposited rubbish

Silted

Flooding

HIGHWAY DIUINAGE: FILTER D-S

Weed growth

Filter material damaged

Filter material displaced

Silted

Flooding

HIGHWAY DmAGEt CULVERTS

scour Free flow

BLOK

DETR

BROK

WEED

DETR

BLOK

FLOD

WEED

CLBK

OBST

DRUB

SILT

FLOD

WEED

FMDM

FMDS

SILT

FLOD

SCOR

FRFL

percentage

length

length

length

length

length

length

length

length

length

leagth

c4

HIGHWAY D-AGE: BALANCING PONDS

Function outfall regulating device OUTF

Blockage of inlet INLT

Blockage of outlets OUTL

Silting SILT

Erosion of banks/walls/bunds ERSN

Surcharge SURC

HIGHWAY DRAlNAGE: ANCUY TITEMS

Pump function

Sluice function

Tidal flap function

Headwall/apron condition

PUMP

SLUJ

TIDL

HAFL

HIGHWAY D-AGE: FLOODING

Flood

I FENCES AND BARRERS

Rotten - wood fence

Rotten - wood post (fence/barrier)

Corroded - metal (fence/barrier)

Corroded - metal post (fence/barrier)

Corroded - concrete fence

Corroded - concrete post

Missing

Damaged/deformed

Loose panels

Loose anchors

No tension (metal fence)

Not stockproof

FLOD

RWDF

RWDF

CMTF

CMTP

CCTF

CCTP

MISS

DAMM

LOSP

LOSA

NTEN

NSTK

Cause, length (external

source of information)

length

length

length

length

length

length

length

CS

GRASSED AREAS

Inadequate visibility MS

Risk to pedestrians RPED

Overgrowing footway/carriageway OVER

Noxious weeds l-WED

HEDGES AND TREES

Unstable/overgrown

Dead tree

Dying/diseased tree

Dying/dead bran&

Obstructed sightline

Obstructed sign/lamp post etc

Hedges not stockproof

SWEEPING AND CLEANSPJG

Excessive muck

Need for herbicide

Debris in traffic lane

Debris in hard shoulder

ROADSTUDS

Loose catseye casing

Loose catseye rubber

Loose studs

Poor reflective conspicuity/catseye

Poor reflective conspicuityjstud

Damaged catseye

Damaged stud

Missing catseye

Missing stud

UNST

DTRE

DYTR

DBRA

OBSL

OBSN

HNST

MUCK

HERB

DBTL

DBHS

LCAS

LCAR

LSTUD

REFC

REFS

DAMC

DAMS

MISC

MISS

length

length

1ePgth

area

height

height

length

length

length

length

length

length

C6

ROAD TRAFFIC SIGNS

Target distance (warning/regulatory) TRED

Legibility distance (directional etc) LEGD

Surf ace luminance SFLM

Surf ace colour SFCL

Physical condition of fittings COFT

Physical condition of frame COFR

Lamp failures LAMP

Moving parts MOVP

Electrical condition COEL

Exposed wiring

Surface corrosion

EXPW

SFCO

Accident damage ACCD

Missing MISS

Damaged DAMG

-C SIGNALS

Equipment wiring and earh condition

Equipment cabinet condition

Condition of base seals

Presence of gas

Hardware physical condition

Condition of buttons/detectors

Condition of reg signs/illumination

Alignment or obscuration

Condition of pole wiring/earth

Condition of loop/feeder

Audible circuits

Damaged

EQWE

EQCB

CBSL

PGAS

HPCD

CBDT

CRSI

ALOB

CPWE

CLOF

AUDC

DAMG

length

c7

ROAD LfGHTING

Lighting failure

PECU failure

Time switch failure

Electrical condition

Wiring deterioration

Exposed wiring

Corrosion of columns

Need for tree pruning

Missing (door/bowl/bulb)

Damaged post/column

Accident damage

Physical condition of fittings

No electricity supply

ROAD MARKINGS

LFAL

PECU

TMSW

ELCN

WDET

EXPW

CCOR

NTPR

MISS

DAMG

ACCD

con

NOSP

Wear WEAR

Spread SPRD

c 010Lu COLR

Skid resistance SKID

Retro-reflectivity REFL

MOTORWAY COhdMUNICATtONS INTALLATIONS

Not water tight

Damaged

WTGT

DAMG

Defect Codes OTHR

NONE

are to be added to each activity code.

length, % rem

length, % orig

length, %

lePgth, SFC

length

text

C8

4

rnne ne 00 APTIrE FnR Rnl ITINF MAINTFNANCF APPENDIX DT-

a

SECTION SUMMARY SHEET FOR DETAILED SURVEYS

a Region -o GE !zz Agml CO80 Dtv CO80 Cod0

l~lllllllllll

I II1Ill Sit0 Identiiier CLlnh. h Soclionl

YYMMDD D810 lIIm_I

- .- NOOl A Moo0 B

Ro4d frattle Slpns

1

i . ’

..’ . BPPl!NDIXE S a CODE OF PRACTICE FOR ROUTINE MAINTENANCE FLOW CHART FOR COMPLETION OF INSPECTION RECORDS

Momh6 for gths calms hlercrotors 6 road mark&s

RECORD PERMANENT

CATEGORY 2 ACTON

a

t

CATEGORY 1

lmme late

I

I

N ADD TlME

7’ AND , DATE

I

N ADD TIME

!? AND DATE

I 7 I tr I

Record Record

fenwwa?Y PlJlWWMll AClbl ACnDn

,

NOT35 NOr@ information on the c0IQletion of inspection records is contained ill the Inspection Ikuual referred to ia paragraph 4.2.10 of the I-iemorandum.

El Prmted in rhe UK for HMSO Demand 298675 CZ 7195

May 2001




PART 3

HD 40/01

FOOTWAY MAINTENANCE

SUMMARY

This part provides guidance on footway maintenance. Itprovides advise on inspection of footways, the likelycauses of defects and the possible treatment options thatare available.


This is a new document to be incorporated in theManual.




HD 40/01

Footway Maintenance

Summary: This part provides guidance on footway maintenance. It provides advise oninspection of footways, the likely causes of defects and the possible treatmentoptions that are available.


* A Government Department in Northern Ireland

THE HIGHWAYS AGENCY

SCOTTISH EXECUTIVE DEVELOPMENT DEPARTMENT

THE NATIONAL ASSEMBLY FOR WALESCYNULLIAD CENEDLAETHOL CYMRU

THE DEPARTMENT FOR REGIONAL DEVELOPMENT*


May 2001







PART 3

HD 40/01

FOOTWAY MAINTENANCE

Contents

Chapter

1. Introduction

2. Inspection Methods

3. Causes of Defects

4. Treatment Options

5. Treatment Techniques and Materials


7. Enquiries

Annex A Description of DefectsB Laying Natural Stone Setts and

Decorative SurfacesC Laying Concrete SlabsD Worked Examples


May 2001



1. INTRODUCTION

General

1.1 This part provides guidance on footwaymaintenance to ensure that the surface and structure ofthe footway provides safe passage by day and night forpedestrians, including those with mobility difficulties.

1.2 Footways deteriorate in a variety of ways,depending upon the cause of distress and the type ofconstruction. In some cases timely maintenance mayprevent general deterioration, due to ageing andpermeability, from progressing to a state wherereconstruction becomes necessary. Some features ofdistress are indicative of structural failure and anymaintenance treatment will need to take this intoaccount. If the deterioration presents a safety hazard tousers then maintenance becomes essential. Theappropriate response time will depend on the risk of anincident occurring. It is outside the scope of this Part tospecify intervention levels and response times.

1.3 Regular inspections are necessary to monitor thecondition of the footway so that any defects can berectified. The maintenance treatment adopted shoulddepend not only on the type of defect, its severity andextent, but also on the mechanism that has resulted inthe defect. In many instances it will be sufficient totreat the symptom(s) of footway distress by repairingthe surfacing. However, if it is suspected that thefootway construction is failing at depth, a siteinvestigation may be required to assess the causes anddepth of the failure. An informed engineeringjudgement can then be made about the strategy to beadopted.

1.4 For one particular mode of failure, that causedby frequent overrun of vehicles, treating the symptomonly will lead to recurrence of failure. Considerationshould first be given to re-designing the trafficmanagement of the area so as to prevent future vehicleoverrun. If this is not possible, the maintenance strategyshould be reconstruction using the designs given inHD 39 (DMRB 7.2.5) for light or heavy-vehicle use asappropriate.

May 2001

1.5 The other major consideration before amaintenance strategy is adopted is to decide whetherthe maintenance treatment should be carried out usingthe same material type as that of the failed footway. If achange in material type is required (eg. for aestheticreasons) then the guidelines given in HD 39 (DMRB7.2.5) should be followed. This may necessitate a fullreconstruction.

1.6 A series of worked examples are provided inAnnex D of this Part to demonstrate the use of thetechniques and flow-charts.

Implementation

1.7 This Part shal be used forthwith on allschemes for the construction, improvement andmaintenance of trunk roads includingmotorways currently being prepared, providedthat, in the opinion of the OverseeingOrganisation this would not result in significantadditional expense or delay. Designorganisations should confirm its application toparticular schemes with the OverseeingOrganisation.

Mutual Recognition

1.8 Where Parts of Volume 7 give the OverseeingOrganisation’s requirements for products, they makeprovision for the acceptance of equivalent productsfrom other member states of the European Community.Reference should be made to the statement in each Partconcerned.

1/1


Chapter 2Inspection Methods

2. INSPECTION METHODS

Introduction

2.1 Regular inspections are necessary to monitor thecondition of the footway network so that any defectscan be identified and rectified. The Local AuthoritiesAssociation Code of Good Practice (LAA, 1989)considers that two levels of inspection are sufficient forfootways (safety and detailed) and gives warning andintervention levels for recorded defects. Refer to theOverseeing Organisation for their maintenance policy,setting out the inspection frequencies, interventionlevels and response times. Taking responsibility for andadhering to a sensible maintenance policy can reduceinsurance premiums and litigation claims.

2.2 There are various Codes of Practice and systemsavailable giving details of defects to note, inspectionfrequencies and response times. Examples are:

• LAA Code of Good Practice

• MARCH (Maintenance Assessment, Rating andCosting for Highways)

• CHART (Computerised Highway Assessment ofRatings and Treatments)

• Trunk Road Maintenance Manual: Routine andWinter Maintenance Code

• UKPMS Pavement Management System andvisual inspection guide

There are also a number of proprietary systemsavailable. CHART is primarily intended for themanagement of trunk roads and is less applicable tofootways. Table 2.1 summarises the frequencies ofinspections currently recommended in the LAA Code ofGood Practice which relate to the use of the footway.For Northern Ireland, inspection frequencies aredetailed in Table 2 of Roads Service’s Policy andProcedures Guide RSPPG_019.

2.3 Safety inspections should note all defects thatare a hazard, such as trips, potholes, broken flags,rocking flags, missing pavers or flags, ruts ordepressions. Safety inspections provide evidence thatthe authority takes a responsible attitude to its dutiesunder the Highway Act 1980 (or The Roads (NI) Order1993 in Northern Ireland) and inspection records formthe core of defence against claims.

May 2001

Category Type of Footway Inspection Frequencyfor Category Shown

Detailed Safety

1 Pedestrian precincts 12 months 1 week

1 Main shopping areas 12 months 1 month

2 Busy urban areas 12 months 3 months

3 Less used urban and 3 years 6 monthsbusy rural

4 Little used rural 5 years 1 year

Table 2.1 Frequency of Inspections

2.4 Detailed inspections or condition surveysshould be carried out using a standardised system. Inaddition to the factors that are noted in safetyinspections, other defects such as fretting, irregularitiescaused by plant growth, slipperiness and water seepageor pumping should be noted. The inspections can beused as a basis for assessing the current state andcondition trends of the footway network. They help toidentify the structural maintenance required and enablethe engineer to prioritise maintenance in order to spendthe budget more effectively.

2.5 In maintenance management systems all defectsnormally have warning and intervention levels andassociated response times that vary according to thefootway usage. A complaint received from a member ofthe public should initiate a very quick response.

2/1


Chapter 3Causes of Defects

3. CAUSES OF DEFECTS

Introduction

3.1 The remedial treatment for a footway will notonly depend on the defect but also on the cause of thatdefect. Surface deterioration that has no underlyingstructural cause can be remedied by replacement oroverlaying of the surface. If not treated, surfacedeterioration may allow the ingress of water, which canthen lead to structural deterioration. If surface distressis the result of a problem at depth it may not be costeffective to merely repair the surface, as the defect islikely to recur.

3.2 A visual condition assessment should enable ashortlist of likely causes of defects to be drawn up.However, the cause may no longer be evident, eg. if itwas caused by flooding or the diversion of traffic due toa nearby accident. Investigation of the history of thefootway, with respect to its construction, maintenanceand rehabilitation, should provide further information.

3.3 To help establish the cause of the defect,inspections should note:

• Drainage

• Kerb and channel condition

• Trees - size and type

• Nature of use - pedestrian only, light-vehicleoverrun, heavy-vehicle overrun

• Undertakers’ apparatus

3.4 The cause of primary distress may be difficult toascertain, particularly in cases where a number ofdistress mechanisms are evident. It may be difficult todecide whether rutting has resulted from crackingcombined with the ingress of water and trafficking, orwhether the footway is of structurally unsound designand the deformation has led to cracking of the surface.It may be necessary to dig a trial hole to check layerthicknesses, material condition and drainage adequacy.Care should be taken to avoid all undergroundapparatus.

May 2001

CAUSES OF DETERIORATION

3.5 Some possible causes of deterioration infootways and footway materials are listed in Table 3.1.Many of these could be avoided by careful planning andsupervision of footway construction and maintenance.

Cause of FeaturesDeterioration

Poor specification Inadequate thickness, edge restraint,and design drainage or frost protection

Faulty Incorrect materials or methods leadingconstruction to inadequate layer thickness, poor

surface regularity or inadequatecompaction

Abuse Poor trench reinstatement leads tosettlement or unevenness, weakeningof the structure, loss of interlock onmodular surfaces. Vehicle overrunleads to over-stressing materials and/orthe foundation and hence structuralfailure

Weathering Ageing leads to hardening of bitumen.Freeze/thaw cycles cause surfacedeterioration. Clay shrinkage orswelling causes ground movement.

Damage by Weeds and tree roots cause surfacevegetation unevenness.

Fair wear and tear Heavy usage causes loss of surfacetexture and fretting.

User perceived Spalling, cracking, poorly matchedfailure materials, etc. lead to an unacceptable

appearance

Table 3.1 Causes of Deterioration

Poor Specification and Design

3.6 The footway may fail because it has not beendesigned to withstand the loadings to which it issubjected or because the specified layer thicknesses areinsufficient to provide protection to a frost susceptiblesubgrade. Insufficient attention may have been paid tothe risk of vehicle overrun, or the use of the footwayhas changed since it was built. Inadequate parkingfacilities may lead to an increase in overrun as carownership increases.

3/1



Faulty Construction

3.7 Deterioration may be due to substandard orincorrect materials or to non-compliance with specifiedlevels or tolerances. Better education and supervision ofthe work force would reduce the deterioration due tofaulty construction.

3.8 Inadequate compaction is a common problem infootways because of the restricted space, the smallrollers used and the short workability time of thin layersof bituminous materials. Powell and Leech (1983)showed that a three per cent reduction in the voidcontent of bituminous materials markedly increases thestructural performance of pavements.

Abuse

3.9 Non compliance of trench reinstatements withthe Code of Practice Specification (DOT et al, 1992),commonly known as the HAUC Specification, issuedunder the New Roads and Street Works Act 1991, maycause deterioration of the footway surface or of thestructure at greater depth. (In NI the NIRAUCSpecification, issued under the Street Works (NI) Order1995 applies). Overrun is also a form of abuse, andheavy vehicle overrun (such as might occur due tobuilding development), is particularly damaging, as arethe feet of skip lorries. Spillage of diesel fuel can causesoftening of bitumen and loss of aggregate from aflexible surface.

Weathering

3.10 A dense, well compacted bituminous material isless susceptible to the detrimental effects of weatheringthan a less dense material with more air voids. Surfacefretting and spalling due to freeze/thaw cycles isunlikely to be a problem for precast concrete and clayproducts manufactured in accordance with appropriatestandards. Natural stone products with high waterabsorption levels are more susceptible.

Fair Wear and Tear

3.11 Loss of surface texture due to normal foot orcycle traffic cannot be eliminated, but it can be reducedto an acceptable level by using the appropriate materialsfor the wear properties required. If the material is wellsuited for its situation the loss of texture due to wearcan be balanced by increase in texture due to surfaceweathering.

3/2

Damage by Vegetation

3.12 Damage by tree roots occurs where trees aresited too close to footways (or vice versa) and wherethe roots are spreading rather than deep. Correct siting,planting and choice of tree species would avoid thesituation. Damage by weed growth occurs mostfrequently on little used footways; pedestrian trafficgenerally destroys weeds soon after germination. Weedinfestation takes advantage of pre-existing damage suchas cracks and gaps, so treatment of surface defects canhelp prevent weed growth.

User Perceived Failure

3.13 Care taken in reinstatement and choosing widelyavailable varieties of modular surfacing, such thatsufficient stocks of replacement modules to match canbe kept, should limit user perceived failure. Onbituminous footways an application of slurry seal canrestore the surface appearance. If there is no hazard andthe structural performance of the footway is notaffected, then no action is needed.

DISTRESS MODES

3.14 The typical distress modes which result fromthese causes of deterioration are listed in Table 3.2 forthe three generic types of surface. A detaileddescription, including photographs, is provided inAnnex A of the Part.

Bituminous Modular (Pavers and Flags) Concrete

Block cracking Broken/cracked CrackingDepressions and modules Damaged jointsbumps Depressions and Depressions andLinear cracking bumps bumpsLoss of surface Missing modules Surface failureaggregate Rocking flagsPotholes Spalling of arisesStructural Rutting or surface(sub-grade failure) Widening of jointsNon-structural Rutting (Pavers)rutting (material Unzipping (Pavers)deformation)

Table 3.2 Distress Modes for Surface Types

SURFACE DETERIORATION

Bituminous Surfaces

3.15 One reason for deterioration of bituminoussurfaces is ageing due to oxidation. This process will be

May 2001



accelerated if good compaction is not achieved andthere are high air voids in the bituminous material. Asthe bitumen ages surface cracking or fretting maydevelop with eventual formation of potholes. Potholesfrequently result from a small defect being worsened bythe infiltration of water into the base material.

3.16 Stripping of binder from the aggregate will alsolead to surface deterioration. Stripping may occurbecause the bitumen has become brittle with age; theaggregate has deteriorated; insufficient bitumen wasused in the mix; or absorption of bitumen into theaggregate has taken place. The process of stripping ishastened by the presence of water in the voids.Stripping which occurs soon after construction may bedue to insufficient bitumen in the mix; poor adhesion ofbitumen to stone due to wet, dusty or water attractingaggregate; or to cold or wet weather at the time ofconstruction.

3.17 Surface deterioration due to delamination mayresult from poor cleaning or insufficient bond coat priorto placing the new surfacing. Cracking of thebituminous surfacing may be a result of ageing of thesurface or may indicate structural problems.Longitudinal cracks may result from insufficient edgerestraint allowing the footway to spread, or fromshrinkage of clay subgrades. Cracks can also arise atjoints, leading to ingress of water and deterioration.Reflective cracking can occur on footways with a leanconcrete base.

Modular Surfaces

3.18 A rocking module indicates displacement or lossof the bedding sand which may be caused by ingress ofwater as a result of joint failure. This can be due to lackof edge restraint and opening of joints as the footwayspreads, or to removal of jointing sand by suctioncleaners. Cracked modules are usually the result ofpoint loads or traffic loading of modules which haveinsufficient tensile strength to resist the applied loading.Rocking or cracked modules may result in trips.

3.19 Loss of material from the exposed surface of amodule or surface spalling, may be due to amanufacturing defect, impact loading, abrasion,weathering or chemical attack. Heavy trafficking ofmodules by pedestrians may cause loss of slipresistance. Vehicular trafficking of modules set inmortar may break the bond and loosen the module.

May 2001

Concrete Surfaces

3.20 Concrete surfaces can become roughenedbecause of loss of the cement paste due to acid attack orsulphate attack. This can also apply to the surfaces ofconcrete pavers or flags. Frost attack may cause scalingand delamination of poor quality concrete. Airentrained concrete is resistant to frost attack.

3.21 Cracks in concrete footways may be due toinsufficient thickness of slab, shrinkage, low concretestrength or inadequate compaction, loss of support andsettlement, natural weathering, temperature changes andfreeze/thaw cycles. If the cracked concrete is providingadequate load spreading to the underlying foundationthe cracking can be considered as surface deterioration,otherwise reconstruction is required.

STRUCTURAL DETERIORATION

3.22 Structural deterioration occurs when the footwayconstruction has insufficient strength to support theloading to which it is subjected. This may be because ofabuse, such as vehicle overrun on a footway notdesigned to support overrun; poorly reinstatedundertakers’ trenching works; or weakening of theconstruction due to drainage failure or material failure.

Overrun

3.23 In a footway designed for pedestrian-only usage,overrun causes over-stressing of the footway surfacingand the foundation since the construction hasinsufficient strength to support the loading. This islikely to lead to rutting and/or tilting kerbs. Onfootways, overrun may be associated with cracking inbituminous or concrete surfaces where there isinadequate thickness for the loading. The cracking thenincreases the ingress of water and leads to softening ofunbound layers which accelerates the rut formation.

Statutory Undertakers’ Works

3.24 Long narrow depressions are usually a sign offailure in an statutory undertaker’s trench and are oftenaccompanied by cracks and steps along the edges of thedepressions. Local depressions may also indicate failureof a reinstatement. Depressions may be the result ofsettlement due to poor compaction or volume change ofsubgrade materials. Surface crowning or heave may bedue to poor reinstatement of trenches or to groundmovement caused by water ingress.

3/3



Subgrade Moisture

3.25 Drainage is a critical factor and, if it fails toperform, structural damage often ensues. As a generalrule, it is preferable to remove surface water to theadjacent carriageway where it will be dealt with inconjunction with carriageway drainage.

3.26 Tree roots can cause considerable damage. Byextracting water from the subgrade in periods of dryweather, the roots can cause the subgrade soil to shrink,particularly where the soil is an expansive clay. Thiswill cause the footway surface to deform. Widening ofjoints in modular paving may be indicative of groundmovement.

Cracking

3.27 Mosaic cracking (crazing) of a bituminoussurface may be the result of excessive deflections dueto lack of support from underlying materials, or toembrittlement of the bituminous material. Crackingmay also be due to poor quality material. Reflectivecracks may be caused by shrinkage or other movementin the base (especially if cement bound), sub-base orsubgrade.

3.28 Cracking of modular paving may be due to thefoundation layers providing insufficient support. Thelarger and thinner the flags, the larger the warping andtraffic induced stresses. Note that a stabilised bed willnot provide adequate support to a warped flag.

May 20013/4


Chapter 4Treatment Options

4. TREATMENT OPTIONS

Introduction

4.1 The treatment options will depend upon theseverity and extent of the defect, the consequences ofleaving it untreated, the hazard presented and theavailable budget. Consideration should also be given tothe amenity value of the area. Tables 4.1 to 4.5summarise the range of treatment options for temporaryand long-term works. To ensure that the maintenancestrategy is cost effective, consideration should be givento the following:

• Maintenance history of the footway

• Expected life of the footway

• Past footway use and future use

• Mechanism of deterioration

• Risk factors such as overrun

• Range of material options

• Recycling possibilities

Temporary, Urgent or Short-term Works

4.2 Temporary, urgent or short-term treatmentscannot always be expected to remain in place for morethan 1 year. The need for an urgent treatment may resultfrom a report, complaint, safety inspection or anemergency such as a chemical or fuel spillage. Theobjective is to make safe and to protect the public untilthe long-term treatment has been effected. Defectswhich may be identified as requiring emergencytreatment are significant trips, depressions, bumps,cracks, potholes, missing modular units orcontamination. Urgent treatment will depend on theseverity and, to some extent, on the cause. Amaintaining authority will normally have a maintenancepolicy statement in which the response period to thosedefects which constitute a hazard and requireemergency treatment will be defined.

4.3 If it is not possible to eliminate the defect in theshort term, then pedestrians should be warned of thedefect by barriers being placed around it. This may bethe case, for example, when trips are caused by treeroots and the remedy may involve raising the surface of

May 2001

the footway, root pruning or removal of the tree;processes which need some time to carry out. If rootpruning is considered, advice should be sought from anarboriculturist.

4.4 Trips in modular footways should be dealt withby lifting and replacing the offending modules.Bituminous material can be used as a temporaryreplacement in order to remove the hazard in the shortterm but “unzipping” of modules will occur if it is leftin place for too long. Depressions can also be taken outusing a bituminous mixture, and surface unevenness onconcrete footways can be regulated in the same manner.An alternative is to use cement bound material.Replacement of surfacing modules using temporarymaterials is occasionally necessary becausereplacement modules are unavailable or the isolatedreplacement of small quantities of modular surfacing isuneconomic. Any interim surfacing should providesimilar comfort levels for pedestrian traffic as thenormal surfacing modules.

4.5 In footways surfaced with bituminous material itis better to patch the footway than merely to fill apothole, as infill material is unlikely to be durable.Guidance on patching is provided in Chapter 5 of thisPart.

Long-term Treatments

4.6 Sealing bituminous footways prevents furthersurface deterioration such as formation of potholes,fretting or stripping of aggregate from the surface, andrestores slip resistance (although a slippery surface mayjust need cleaning). Stripping needs to be treatedpromptly as the footway surface can deteriorate rapidly.Sealing the surface will also reduce the infiltration ofwater into the pavement and may thus reduce the rate ofdevelopment of rutting and depressions. Slurrysurfacing restricts the rate of infiltration of water intopavements better than a thin asphalt overlay.

4.7 Sealing or replacing the surface may be theappropriate action for mosaic cracking (crazing), ifthere is no associated deformation resulting fromstructural weakness. Depressions or ruts in bituminousfootways can be removed by resurfacing whilst, inmodular footways, the modules will need to be liftedand relaid. However, if the depressions or ruts arecaused by weakness in the formation, or by footway

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under-design, the problem is likely to recur unless in-depth repairs are performed. If rutting is due to ingressof water and weakening of the subgrade, the provisionof subsoil drainage may solve the problem. (Surfaceand structural repairs are described in Chapter 5 of thisPart).

4.8 Broken and uneven large flags adjacent to thekerb, resulting from vehicle overrun, can be replaced byin-situ concrete or bituminous material until such timeas the whole footway can be resurfaced. Concretefootways with uneven joints may have their lifeextended by grinding off the “lips” to restore safety(ie trips on very large slabs).

4.9 Deformation which continues to occur aftersurface treatment has been carried out needs to beexamined in more detail. Where deformation is causedby tree roots, a root barrier should be installed ifpossible. Where deformation is the result of overrun,reference should be made to HD 39 (DMRB 7.2.5) sothat the footway can be reconstructed with theappropriate layer thicknesses and materials for thesituation. Alternatively, some means of preventing theoverrun may be employed.

Consequences of Not Treating Defects

4.10 If defects are not treated there are consequencesboth for the user and for the footway itself. Defects inthe surface condition of a footway will cause pedestrianinconvenience. If these defects have been identified bysafety inspections as being potentially hazardous to theuser, then not treating them within a reasonable timewill make it difficult to defend any subsequent claimsmade by pedestrians. Even defects which do not presentan immediate safety hazard may do so in the future.(An example of this would be a depression withstanding water, insufficient to cause a safety hazarditself, but which may become one if freezing conditionsprevail). The surface condition is the most importantcharacteristic applying to pedestrian safety and comfort.

4.11 The judgement of the inspector, reflecting themaintenance policies of the authority, must be exercisedin deciding when to treat defects. As far as the footwayitself is concerned, the treatment required is likely to beeasier to carry out and less costly if the defect isrepaired before major deterioration has occurred.Sealing the surface of a bituminous footway with slurrysurfacing, while fretting or cracking is minor, will delayfurther deterioration. Not sealing an aged bituminoussurfacing may lead to serious fretting, potholes formingand the need for resurfacing or even reconstruction.

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4.12 Where there is minor cracking (a single crack),with no other associated defects discernible, nomaintenance action is required unless a good reasonexists to seal the crack - such as to prevent infiltrationto a water sensitive material below. However, weedsmay grow in the detritus that collects in unsealed cracksmaking them unsightly and causing them to widen.Damage may also be caused by water freezing in cracksand expanding, thus enlarging the cracks.

4.13 In modular footways, any unevenness or rockingmodules will worsen if left untreated, since debris getsinto gaps and uneven load spreading by the modulescauses greater localised stress on the underlyingfoundation which may consequently deform.

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TreatmentsDefect Problem

Temporary Long-term

Depressions Hazard for users Fill or ramp. Contact Reshape surfacing& bumps Statutory Undertaker if

cause is failed reinstatement.

Rutting Hazard for users Fill Reshape surfacing. Preventoverrun or reconstruct if necessary

Slippery Hazard for users Warn users or Clean, restore texture or renewsurface restore texture surfacing

Surface Hazardous Clean or neutralise Replace surfacing if necessarycontamination

Unsightly - Clean

Vegetation Tree roots cause Warn users or ramp Reshape surfacing. Considertrips replacing trees with a more

suitable variety

Vegetation obstructs Trim growth Consider increasing frequencyfootway of maintenance

Surface slippery Warn users and clean

Water Surface water Salt, if freezing Reshape surfacing to correctgradients and crossfalls.

Blocked drainage Clear blockage Rectify any damage caused

Burst mains Inform Statutory Undertakerand make safe

Reshape = relay modular paving, replace bituminous wearing course (and basecourse) or overlay with newwearing course, overlay with thick slurry seal or use Retread (refer to Chapter 5 of this Part).

Table 4.1 Treatments Common to all Types of Footway Surfacing

Note for Tables 4.1 - 4.5: Temporary, includes urgent or short-term works eg to remove a hazard. Further treatmentshown under “long-term” may also be required.

May 2001 4/3




Temporary Long-term

Mosaic cracking Embrittled surfacing - Seal or replace surfacing

Structural failure - Replace surfacing. Reconstruct if problemrecurs

Linear cracking Water ingress, trips - Seal, replace surfacing or reconstruct ifnecessary

Loss of surface Loose or uneven - Seal or replace surfacingaggregate surface

Potholes Trips, water ingress Fill Patch or replace surfacing

Table 4.2 Treatments: Bituminous Surfacing


Temporary Long-term

Broken or damaged Unsightly - Replace damaged modules, re-surface ormodules appearance, water reconstruct if problem is due to overrun

ingress

Loss of jointing Water ingress, - Replace jointing material, seal jointsmaterial modules can move/

rotate

Missing modules Holes, trips Fill holes Replace modules

Rocking modules Hazard to users Relay modules Reconstruct if problem is due to overrun

Spalling of arrises or Unsightly - Replace damaged modulessurfaces appearance

Trips Hazard to users Relay modules Reconstruct if problem is due to overrun

Widening of joints Water ingress, Fill wide gaps Relay surfacing and provide edgemodules can if a hazard restraint if necessarymove/rotate

Note: When relaying modular surfacing it is preferable to relay the whole area between edge restraints.

Table 4.3 Treatments: Modular Surfacing

May 20014/4




Temporary Long-term

Damaged joints Spalling, cracking, - Seal cracks. Overlay or replace jointswater ingress and/or concrete

Cracking Unsightly, water - Seal cracks. Overlay or reconstructingress

Weathering Scaling, delamination, - Overlay or reconstructaggregate exposure

Corrosion of steel Cracking, rust - Seal cracks. Reconstruct

Trips Hazard for users Ramp or feather Overlay or reconstruct

Table 4.4 Treatments: Concrete Surfacing


Temporary Long-term

Damaged or Loss of edge support - Replace kerb or edgingmissing item

Horizontal or Hazard for road users Relay item Seal cracks. Overlay or reconstructvertical steps and/or pedestrians

Poor channel Drainage not - Relay or replace kerb or edgingalignment, functioning properlydelamination,aggregateexposure

Vegetation Water run-off Remove Increase vegetation control frequencyprevented vegetation, apply

weedkiller

Trips Hazard for users Relay item Replace kerb or edging

Table 4.5 Treatments: Kerbs and Edging

May 2001 4/5



4.14 Depressions or rutting caused by overrun arelikely to worsen if untreated, particularly if thesubgrade is soft.

Defect Category and Response Time

4.15 The same defect may be assigned a differentresponse time according to its location and footway use.The response times for defects requiring urgenttreatments may be shorter in a busy urban situation thanin a rural situation, although they should be repaired asquickly as resources allow. Response times may alsovary according to the location of the defect on thefootway. For example, a trip behind a kerb is more of ahazard than a trip next to a wall. It is important that thetimes set out in the maintenance policy can be achievedin practice.

4.16 Where the defect can be left for a period beforetreatment, interim treatments are generally carried outto restore the surface of the footway. Interim treatmentscan be part of a planned maintenance programme.Where the form of deterioration is likely to recur, long-term treatments should be carried out. It may be morecost effective to reconstruct a footway, in accordancewith HD 39 (DMRB 7.2.5), than to carry out frequenturgent or interim treatments.

May 20014/6


Chapter 5Treatment Techniques and Materials

S AND MATERIALS
5. TREATMENT TECHNIQUE
5.1 If any treatment necessitates temporarily closingthe footway, appropriate traffic management proceduresmust be initiated. Similarly, occupiers of premises withfrontages on the footway, should be notified if they willbe inconvenienced. It is then important to keep to theplanned timetable.

Vegetation Control

5.2 Vegetation control ranges from cleaning mossand lichen off the footway surface to felling trees. Mossand lichen growth on the surface does not cause muchdamage but can seriously reduce safety by making thesurface slippery, especially in wet weather. If trees aregrowing within or very close to the footway the rootscan cause irregularities in the surface. The upwardexpansion of the roots of forest trees planted in urbanareas can cause excessive heave of footways (up to300mm is not uncommon). Expert advice must be takenbefore any root pruning, felling or tree replacement isundertaken. The interests of road safety must bebalanced against the environmental cost of losing thetree. Further information on the management of treesclose to the highway may be found in the KindredsAssociation report on highway liability claims (TheKindreds Association, 1994).

5.3 Weed growth is normally the result of poorworkmanship during construction or reconstruction, andpoor routine management, combined with the proximityof weeds near the footway. An ideal weedkiller woulddeal with both broad leaf perennials and annual grass-type weeds and would have the following attributes:

(a) systemic - taken in through leaves

(b) translocating - travels through plant to kill root

(c) residual/persistent - active in soil for six to ninemonths.

5.4 Weedkiller should be applied both to theformation at the construction stage and to the walkingsurface, the latter being most effective during April toSeptember. The application of weedkiller should formpart of the maintenance programme.

5.5 Increasing environmental pressures have led tonegligible use of residual herbicides, although there isno formal legislation against their use. Environmentally

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t is argued that it is desirable to use a contact herbicide,nd generically this means the use of Glyphosate basedormulations. Effective control of weeds now requiresreatment both in Spring and Autumn on the same site,hereby increasing the cost. At formation level thehoice of chemicals is broader, but care must be takeno ensure that the product used will not translocate todjoining properties, hedgerows, fields, parks, etc.

.6 Extra care needs to be taken in applyingeedkiller to non-porous paved surfaces in order to

educe possible contamination of streams and rivershich receive water from paved areas. Spray should be

upplied by accurately calibrated equipment using theowest appropriate rate and avoiding run-off. Productshould not be applied over drains or in drainagehannels, gullies or similar structures. Personnelpplying pesticides must receive appropriate training.

.7 Full details on all pesticides approved under theontrol of Pesticides Regulations is published annuallyy the Ministry of Agriculture Fisheries and Food andhe Health and Safety Executive (MAFF and HSE,996). Advice on the safe use of pesticides for non-gricultural purposes is given also in the Health andafety Commission’s Approved Code of PracticeHealth and Safety Commission, 1994) and in theesticides Users Handbook (Watterson, 1988). Inman1992) gives useful information regarding weed controln the highway.

rgent Treatment

.8 When a defect is identified as a safety hazard its necessary to carry out urgent treatment to ensure theafety of footway users. As the objective is to removeanger it is not usual to investigate the cause, but rathero undertake a robust repair, as rapidly as possible.

here the defect cannot be treated quickly pedestriansust be warned of its presence.

.9 Removal of trips: Trips in bituminous oroncrete footways can be taken out by constructing aemporary ramp of bituminous material. This techniquean also be applied to trips around ironwork. Inodular footways the module causing the trip should be

aken out and relaid. Alternatively the module can beemoved and the resulting hole filled with temporaryituminous material.

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5.10 Rocking modules or ironwork: Rockingmodules can be lifted and relaid. If the problem isrocking manhole covers the appropriate statutoryundertakers should be informed. If necessary, barrierscan be erected until the problem is remedied.

5.11 Temporary filling of depressions: Potholes anddepressions can be filled with fine bituminous material,(cold mix or asphalt). Prior to placing, all crackedsurfacing and loose material should be removed fromthe area to be repaired. A vertical face should beformed along the edge of the existing material. Any softor yielding base material should be removed andreplaced.

5.12 The base and edges of the area to be filledshould be lightly and uniformly tack-coated withbitumen emulsion. When the emulsion has broken(colour change from brown to black) the cold mix orasphalt is placed in layers by hand. (Cut-back ordelayed set material may be used for emergency worksbut durability is poor). It is important to ensure that thesurface of the filled depression after compaction is levelwith the existing footway surface, so the loose level ofthe infill material needs to be above the surface. Theloose thickness of the layer is approximately 1.25 timesthe compacted thickness.

5.13 Compaction can be carried out using a handrammer but is best achieved using a vibrating plate. Toproduce a smooth joint, the edges should be compactedfirst, with the compactor overlapping the existingsurface. A pothole temporarily filled with handcompacted material may be voided with poor durability.It will therefore deteriorate rapidly and patching may berequired later to effect a permanent repair.

5.14 Cleaning of spillage: A maintaining authorityshould have an established procedure for thiseventuality, based on the COSHH regulations. Spillageshould be removed as appropriate, which may requirethe use of an absorbent medium (such as sand) if thespillage is liquid. All debris should be removed fromthe surface and disposed of in a controlled andapproved manner. The surface should then be washedusing an appropriate detergent if necessary. In the eventof a chemical spillage the emergency services willadvise. Depending on the nature of the spillage it maybe necessary to replace the whole or part of the wearingcourse after the spillage has been dealt with.

5.15 Barriers: If a defect presents a safety hazard,and it is not possible to make a temporary repairquickly, it may be necessary to erect some form ofbarrier. Pedestrian barriers are described in Chapter 8

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Paragraph 3.4.4.6 of the Traffic Signs Manual(Department of Transport et al, 1991 or subsequentdocument). They should be of reasonably solidconstruction to guide the blind and partially sighted.They should have a robust tapping rail fixed at a heightof approximately 150mm above ground level, measuredto the underside of the rail, and a robust handrail at aheight of between 1.0m and 1.2m, measured to the topof the rail. In both cases the rail should be at least150mm deep and high.

5.16 Patching: Where appropriate, patching shouldbe carried out immediately to remedy hazardousdepressions or unevenness in the surface. Patching withbituminous material can also be carried out on modularor concrete surfacing as an emergency repair. Furtherdetails on patching may be found in the documentPreferred Method 1 Patching (Department of Transport,1988) and in the Highways Authorities StandardSpecification No. 10 (County Surveyors Society, 1995).Patching is discussed in more detail later.

SURFACE TREATMENTS

Bituminous Footways

5.17 Surface treatment has two objectives, firstly, toseal the surface and secondly, to provide a non-slipsurface. It also has the advantage of restoring aconsistent appearance to the footway. Proprietaryproducts can be painted onto the footway to increaseslip resistance in problem areas such as steep slopes.Sealing of a cracked and porous surface can beundertaken using either a surface dressing or slurrysurfacing, an extremely cost effective and rapid process.Application of both surface dressing and slurrysurfacing is confined to a “seasonal window”. However,with the development of polymer modified slurry sealsit is becoming possible to use them throughout the yearwith the exception of the worst period of winter.

5.18 Whilst patching can be used as a remedy forlocalised defects it does not prevent deterioration overthe whole surface, and leads to the problem ofdifferential deterioration rates and poor overallappearance. Nevertheless, surface repairs to abituminous footway will usually take the form ofpatching of one form or another to regulate the surfaceor remove serious defects, prior to surface treatmentwork. Patching can be categorised as surface (overlay)or inlaid. Alternative surface treatments are givenbelow.

May 2001



5.19 Surface patching or overlay: Surface patchingconsists of laying new wearing course material over theexisting surface. Since the material should not befeathered out at the edges, this method is onlyrecommended where the existing material can bechased out at the edges and the new material graded tomeet the existing surface without causing a trip. It ispossible to remove minor irregularities with a surfacepatch but judgement must be exercised to decidewhether a shaping layer is needed first. The processmay be described as follows :

(a) Chase out edges of patch.

(b) Brush off any loose material.

(c) Apply emulsion and wait for it to “break”.

(d) Paint vertical face with 50 to 100 pen bitumen toeffect seal.

(e) Lay wearing course material.

(f) Compact.

5.20 Inlaid patching: Inlaid patching is thereplacement of defective flexible materials with newmaterial, hand laid, to any depth not less than thewearing course thickness. Each layer of the defectivematerial should be replaced with the relevant materialand compacted as specified in HD 39 (DMRB 7.2.5).

5.21 Planing on a footway is becoming much morepracticable with the advent of milling attachments forsmall items of plant, especially rubber tyred skid-steertype. If this type of planer is available, the sequence ofoperations is as follows:

(a) Plane rectangular areas and step in by 50 - 75mmat the basecourse level if more than a single layeris to be used. (The important consideration hereis to ensure that the patch is of sufficient size toallow subsequent rolling. It is pointless trying tocarry out basecourse patching if the roller isgreater than the width of the wearing coursecavity).

(b) Remove any loose materials from the cavity.

(c) Apply bituminous emulsion to the base of thecavity and wait for it to “break”.

(d) Paint the vertical edges of the cavity with 50 or100 pen bitumen.

(e) Fill the cavity with bituminous material.

May 2001

(f) Compact.

(g) Repeat stages (c) - (f) if wearing course is used inaddition to basecourse.

5.22 Surface dressing: Surface dressing is a thinveneer suitable for restoring skidding resistance,arresting further fretting and restoring waterproofingproperties of the surface. The main limitation of thistreatment is that it should only be applied during thewarmer months (between April and September).Surface dressing involves the application of a binder tothe surface followed by the application of aggregate onto the wet binder. As the binder can be picked up bypedestrians’ footwear it is particularly important thatoccupiers of properties fronting the footway are notifiedif surface dressing is to be used.

5.23 Care should be used in the selection of sites forfootway surface dressing. Since the loading is almostwholly pedestrian there is no post applicationembedment of the aggregate by vehicles, resulting in arough surface. Footway surface dressing is most viablefor low pedestrian flows and rural areas.

5.24 The surface dressing does not affect any verticaldiscontinuities in the surface and will leave an unevensurface, if one existed previously. Pre-patching canremove surface irregularities. Surface dressing has theadvantage of user colour selection of the chippings,which may be of benefit in some situations. It isrecommended that chipping size be kept to less than6mm with a 3 - 6mm range being preferred.

5.25 Proprietary systems have been developed whichuse bituminous emulsions and chopped glass fibrestrand combined as part of the binder applicationprocess. This has the advantage of providing a degreeof tensile strength at surface level. The process also hasthe advantage of “bridging” cracks in a way notpossible without the reinforcing glass fibre strand. Thishas the benefit that less surface patching is needed thanwith unreinforced systems.

5.26 The process of footway surface dressing issummarised below:

(a) Remove trips and areas of badly degradedwearing course by surface patching.

(b) Apply masking tape to ironwork and streetfurniture.

(c) Apply binder at determined rate.

(d) Apply chippings at determined rate.

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(e) Remove masking tape.

(f) Remove loose chippings within 1 week.

5.27 The design method for surface dressing is givenin Road Note 39 (Transport Research Laboratory,1996); the Road Surface Dressing Association has aCode of Practice (Road Surface Dressing Association,1995) and a specification for footway dressings is givenin the Highway Authorities Standard Specification No.4 (County Surveyors Society, 1995b). Furtherinformation is provided in HD 37 (DMRB 7.5.2.8).

5.28 Slurry surfacing: Slurry surfacing is used tostabilise the surface area and to ensure that it isimpermeable, of consistent appearance and provides auniform, slip free, textured walking surface. Theprocess involves the application of a slurry comprisinga bitumen emulsion, polymer modifiers, fillers andaggregates blended to control the viscosity and settingtime of the mixture as required. The existing footwaysurface must be cleared of debris, loose material andvegetation by the use of high pressure washing toensure good adhesion of the slurry.

5.29 Over the last seven years there have beensignificant developments including the use of polymermodifier technology, which allows for on-siteformulation to control the setting time; this means theprocess is less dependent on the weather. It is alsopossible to add filler and aggregate to enhance therobustness of the mix. These facilities havesignificantly increased the capability of slurrysurfacings and the period over which they can beapplied.

5.30 Some of the more recent formulations containchopped fibre reinforcement to permit thicker build upof the layers. With the most robust mixes a single layerthickness of up to 40mm is attainable, with multiplelayers being used for deeper regulating. It is necessaryto allow the first layer to set prior to the application of asubsequent layer; a practical proposition, as settinggenerally takes no more than 30 minutes. During thesetting time it is necessary for pedestrians and vehiclesto be kept off the footway, and it is especially importantto keep occupiers of properties fronting the footwayfully informed.

5.31 The main criterion for the selection of a site forslurry sealing is that it should not be in need of anystructural strengthening. A fine, single coat of slurrysurfacing can be used if the main problem is confined tosurface fretting with no shaping required, while acoarse material with reinforcement can be used toprovide up to 40mm of shaping.

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.32 Using modern slurry surfacings it is nowossible to treat a footway which previously wouldave had to be considered for partial reconstruction.hether a fine or coarse slurry seal should be used is aatter of experienced judgement based on the amount

f regulating or damage expected after high pressureashing (if the existing bituminous surface material isrittle). Regardless of the mix the same degree ofurface smoothness can be achieved.

.33 Thin slurry surfacing is more suitable for use onightly used urban and rural footways. Use of thicklurry surfacing may necessitate the lifting of ironwork.epending on usage, the life of slurry seal is in the

ange of five to nine years, with seven years beingommonly observed.

.34 A performance based specification is given inhe Highway Authorities Standard Specification No. 3County Surveyors Society, 1995c) and furthernformation can be obtained from BS 434 Bitumenmulsions for Road Use (British Standards Institution,984) and HD 37 (DMRB 7.5.2.10).

.35 Overlay: An alternative surface treatment is toverlay the existing wearing course with dense thinearing course material, ensuring that the degree of

ompaction is in accordance with that specified inD 39 (DMRB 7.2.5). When overlaying it may beecessary to chase-out the existing wearing course athe kerb edge and profile the new material to kerb level.his is to avoid a step between overlay and kerb, whileaintaining the minimum layer thickness with respect

o the aggregate size. Ironwork will need to be raisednd care must be taken not to interfere with the efficientse of damp-proof courses and air-ducts on adjacentroperties.

.36 Overlaying with bituminous material can also bearried out on cracked or eroded concrete surfaces,roviding the load spreading properties of theeteriorated concrete remain adequate.

.37 Guidance on the supply and laying of materialsan be found in the Highways Authorities Standardpecification No. 10 (County Surveyors Society,995a). A list of cold lay surfacing materials currentlypproved by NJUG (National Joint Utilities Group) cane obtained from NJUG at 30, Millbank, London,W1P 4RD.

odular Footways

.38 Surface repairs to a modular footway are carriedut when there are local defects such as trips,

May 2001



depressions and broken or missing modules. Modularfootways may also need treatment to repair joints or torestore their slip resistance.

5.39 Lifting and relaying: Relaying flags and paversshould be carried out in the same way as laying thesemodules; according to BS 7263, BS 6717 and BS 6677for flags, concrete pavers and clay pavers respectively.Guidance on laying setts is given in Annex B of thisPart. Useful guidance on reinstating concrete pavers isalso produced by Interpave (1999).

5.40 Removing a damaged single module prior torelaying can be difficult, particularly if it is a smallelement, as breaking it out may disturb adjacentmodules. It may be necessary to consider relaying allmodules that abut the damaged module, or to relay afull line of modules back to an edge restraint.

5.41 If the surface requires regulating, modules willneed to be lifted over the whole of the affected area. Inall cases a complete renewal of the bedding materialshould always take place. It is unusual to need manyreplacement modules, and this helps to minimise thecost of maintenance. An experienced pavior is requiredto execute a good reinstatement.

5.42 There is increasing opinion which suggests thatmodules need to be held apart by some form of struttingwhen excavation takes place. This prevents a relativelyminor inward movement, which would cause problemswhen replacing those modules that had been taken out.It is also thought that the use of vibratory compactionduring the reinstatement of an excavation can bedamaging to the remaining modules by increasing theirtendency to move inwards to the free edge. This may beavoided if foamed concrete is used for reinstatement ofexcavations instead of granular backfill.

5.43 If a progressive failure is taking place, it may benecessary to accept the use of a temporary bituminousinfill material which will become part of the walkingsurface, possibly for a considerable time. Although thiswill provide a safe and comfortable surface for the user,if this treatment increases in area it will becomeprogressively more visually intrusive. An alternative,depending on the size of the failing modules, may be touse a smaller module in the cavity, of a similar colourand texture to the parent surface. This may be feasiblewhere large flags are broken and can be replaced withconcrete or brick pavers of size 200mm x 100mm.

5.44 Joint repairs: Where a modular bed has beenlaid with a cement mortar joint, and degradation hasoccurred, it will be necessary to refill the joint. If

May 2001

necessary the joint should be raked out, which is timeconsuming and costly. It may be more appropriate touse a small hand-held grinder with an attachment toremove the dust and debris, in a controlled manner, intosome form of hopper.

5.45 Once the joints have been cleaned out they canbe refilled with a 4:1 sand/cement mixture of normalbuilding consistency. Care is required to confine themixture to the joint and prevent staining of the surface.Alternatively, a semi-dry mix can be used which isbrushed into the joint; the free water chemicallyactivates the cement. The process of hardening takeslonger than with a wet mortar but is less likely to stainthe surface of the modules. Care should be taken tokeep suction sweepers off the surface until the mortarhas set and hardened.

5.46 Modular footways with sand filled joints maysuffer from loss of jointing sand as a result of suctionsweeping or water action. If replacement of the jointingsand is followed by application of a joint sealant it willhelp prevent future loss of sand. Joint sealant can beused to retain jointing sand; to inhibit weed growth injoints; to reduce porosity, thus enhancing theappearance of pavers by preventing stain penetration;and to inhibit infiltration of water and fuels throughpaver joints. Three types of sealer are generallyavailable; water based, solvent based acrylic andmoisture cure urethane. Emery and Lazar (1996) claimthat the urethane sealer is the most effective. Problemscan arise with sealant affecting the underlyingbituminous layer.

5.47 Relaying setts and decorative surfaces: Theuse of natural stone setts and other decorative surfacesis becoming more common. Any form of repair willgenerally involve a degree of relaying. On the basis ofthe increasing numbers of sites showing failure, furtherresearch is needed to ascertain the best method oflaying setts. Various alternatives are discussed in AnnexB of this Part.

5.48 Retexturing: Re-texturing of modular surfacesthat have become slippery due to heavy pedestriantraffic, can be undertaken by various methods includingscabbling, sand or shot blasting, high pressure waterjetting or application of a weak acid. Scabbling is slowand tends to crack large concrete flags. It can be usedsuccessfully on natural stone flags, especially wherethere is some form of surface growth. Sand blasting canbe successful, but causes a lot of fine particles anddebris, which have to be removed separately.

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5.49 Shot blasting with steel shot involves the use ofspecialised machinery, which sucks up the debris andshot by vacuum. The steel shot is separatedmagnetically in the return loop for continuous re-use.This process tends to form a lightly-exposed aggregatesurface on the flags, giving a pleasing appearance.However, regular use of this technique can sometimescause problems, and retexturing areas around streetfurniture or at the edges of footways is difficult. Jointsmay need resealing with sand after any high pressuremethod has been used.

5.50 Washing of the surface: In feature areas it maybe necessary to wash the walking surface. This is doneby adapted small suction sweepers using hot water withadded detergent. In addition to the cleaner appearancethere is a significant improvement in the slip resistance.

Concrete Footways

5.51 When a trip has formed this may be regulatedusing a fine bituminous mix to form a temporary“ramp”. Alternatively, if this is unacceptable, it may bepossible to reduce the face of the trip using surfacegrinding equipment. Care should be taken to ensureeffective dust suppression, using either water spray orducted suction. As the process of grinding will renderthe surface relatively pervious a waterproof sealing coatshould subsequently be used.

5.52 Where concrete has become spalled, thedamaged area should be removed by scabbling. Thesurface can be reinstated using a hand-applied epoxymortar to replace the damaged concrete surfacing.Alternatively, a fine concrete mix can be used if thearea needing filling is at least 30mm deep. In this casethe edges should be sawn and a bonding agent used toassist adhesion. The new surface should be covered andprotected from traffic for at least three days.

5.53 The need for a joint repair at expansion jointswill be seen when the joint sealer becomes ineffective,usually by losing adhesion with one side of the joint. Inthese circumstances it will be necessary to remove andreplace the sealer. In the event of spalling of a jointtaking place this can best be treated by making a localrepair. Further advice on the repair of concrete is givenin Section 4 of Volume 7 (DMRB 7.4.2).

5.54 If a section of concrete footway becomespolished due to heavy pedestrian traffic it can be re-textured by shotblasting in a similar manner to thatdescribed in the repair of footways comprising modularconstruction.

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STRUCTURAL REPAIRS

5.55 Where defects are due to failure at greater depththan the wearing course alone, deeper repairs will needto be carried out. This may involve replacing allbituminous material, concrete or modular paving and,possibly, the sub-base.

Bituminous Footways

5.56 Basecourse replacement: When replacing thewearing course alone is judged inadequate (eg. due tothe need to replace more than 40 per cent of thebasecourse at the same time) it becomes more costeffective to consider a complete replacement of all thebituminous layers. The existing wearing course andbasecourse material is removed by planing or diggingout, the granular base is rolled and restored to line andlevel, and each layer is replaced with the relevantmaterial as specified in HD 39 (DMRB 7.2.5). Theprocess brings the footway up to its original structuralcondition.

5.57 If required, the wearing course and basecoursemay be combined as one layer when replaced; this willprovide a longer working time in cold weather, butcompaction will require more effort due to the greaterthickness. The main benefit is that there is also a betterchance of achieving a higher degree of compactionwhich results in a potentially longer service life. Thecost of this is less than the conventional two-courseconstruction since there is a lower labour cost due to asingle layer being laid.

5.58 Recycling: This may be considered wheninvestigation indicates that the construction andcondition of the existing footway materials are suitable.The primary factors are that the footway exhibitscracking and general degradation and is not suitable forslurry surfacing. If the footway has had slurry surfacingapplied twice previously, a third application is notrecommended, but recycling will rejuvenate theremaining bituminous material.

5.59 Cold mix-in-situ techniques are used to re-usethe existing bituminous materials, which are thensurfaced with a new wearing course or slurry surfacing.The footway is restored to near its original condition.Where the footway is not subjected to overrun this maybe a cost effective treatment having a similar lifeexpectancy to reconstruction. The economics will varyconsiderably depending on the area to be treated. Theprocess is also considerably more environmentallyfriendly than reconstruction as it makes use of theexisting material rather than disposing of it.

May 2001



5.60 The use of this process (“Retread”) is becomingmuch more common with the introduction of smallscarifying plant. The process consists of scarifying boththe wearing course and basecourse to a depth of up to75mm, followed by grading for level and crossfall.Bitumen emulsion is then introduced into the body ofthe material with specialised pressure harrows.Depending on circumstances, it may be necessary toadd further crushed stone to “fill-out” the scarifiedmass, this being a matter of experienced judgement. Itis also necessary to remove a certain amount of materialto permit the placing of a new wearing course.

Modular Footways

5.61 An in-depth repair of a modular surfacecomprises the lifting and relaying of the existingmodules followed by replacement and re-levelling ofthe laying course. Partial replacement or re-levelling ofthe sub-base may be required to remove depressions orbumps. The major advantage of modular construction isthe probability that few new modules will be required.This process will not enhance the structural capabilityof the footway and a careful evaluation will be requiredto ensure that it is not necessary to reconstruct the baseto strengthen the footway. (Refer to Chapter 4 of thisPart).

5.62 Relaying of modular footways may be regardedas environmentally friendly as it results in substantialrecycling of in situ materials. Even if the materials arenot to be used on the site in question they may be usedelsewhere.

Concrete Footways

5.63 An in-depth repair of a concrete footway willinevitably lead to the replacement of a concrete slab orslabs. After breaking out the existing slab(s) the processof laying the new slabs is described in Annex C of thisPart.

In-situ Stabilisation

5.64 Where it is possible to treat a poor subgrade and/or unbound granular layer by stabilising it, rather thanexcavating and replacing it, this course of action shouldbe considered if it is economically justified. Cementand lime are the stabilising agents most frequently used.The process consists of scarifying to the depth of thelayer to be stabilised, reshaping as required to correctthe profile, lightly compacting, spreading and mixing in

thtr

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May 2001

e stabilising agent and water, compacting andimming.

.65 Stabilised patches normally require dampeningd a curing coat to ensure that enough water isailable for the chemical reactions necessary for

abilisation to proceed. A typical curing coat wouldnsist of a light application of either cut-back bitumen

r bitumen emulsion, with a sand or light aggregatever.

econstruction

.66 Due to the range of repair and enhancementchniques available the need for reconstruction isenerally only required when it is necessary to upgradee structural capability of the footway to deal with thefects of vehicular damage. This will inevitably requirethicker construction and design recommendations areiven in HD 39 (DMRB 7.2.5). Since reconstructionvolves a considerable investment it is appropriate torefully consider alternative constructions and theirhole life costs.

ERBS

.67 Replacement of kerbs almost always involvesisturbance of the carriageway surface. It may beecessary to remove more kerbs than those immediatelyfected to get a good alignment. Where kerbs andges have sunk as a result of an excavation, it may be

ecessary to provide a regulating patch on the channelr footway. Care should be taken to ensure a waterproofint is achieved between the kerb and the carriageway prevent ingress of water to the sub-base. The joint

etween the kerb and carriageway wearing courseould be painted with a 50 pen bitumen before layinge wearing course.

.68 The need to replace a kerb should normally bedged against the danger it causes to pedestrians. Inme maintaining authorities, however, a spalled kerbay be replaced for environmental reasons although itay not be causing a safety problem.

.69 The repair or replacement involves the removalf the existing kerb, regulating the bed with semi-dryncrete, (Grade C7.5P) and the relaying of the kerb orging, prior to reinstating the adjacent surfaces of therriageway and footway in materials identical to theisting ones, if available.

5/7



TYPICAL FLOWCHARTS

5.70 A series of flowcharts is presented below toassist the choice of appropriate maintenance. Flowchart1 is applicable to all types of footways and leads intoFlowcharts 2 and 3, which apply to bituminous andmodular surfacings respectively. There is no flowchartfor concrete surfacing as such footways are notcommon.

May 20015/8



START

Isdefect a

safety hazard?

Isroot growtha problem

?

Isfailure due to

overrun?

Isreinstatement

with in guaranteeperiod ?

Isproblem caused

by an undertaker’sreinstatem ent

?

Isfailure of

edge restrain ta problem

?

Issurface

contaminationor slip aproblem

?

Is localdrainage failure

indicated ?

Isfoundation

failureindicated

?

Urgentrepair or sign

Trim or contain roots.Resurface footway .

C lean.Retexture if necessary.

Replace surface if necessary.

Replace or constructedge restrain t

Contactutility

Repairdrainage

Investigatefurther

Reconstruct to DMRB 7.2.5.3Reconstruct

foundation locallyif necessary

Go to Flowchart 2 for bituminous surfacingGo to Flowchart 3 for modular surfacing

No

No

No

No

No

No

No

No

No

Yes

Yes Yes

YesYes

Yes

Yes

Yes

Yes

FLOWCHART 1 : GENERAL

May 2001 5/9



S T A R T

A redefec ts

con fined toiso la tedare as ?

A redefec ts

con fined tow earingcourse

?

Is s lu rrysurfacin gsuita b le ?

D oessurface ne edreg ulating ?

Is foo tw aysub je c t to H G V

overru n ? (D M R B7.2.5 .2 )

H asre trea d bee n

use d ?

Is s lu rrysurfacin gsuita b le ?

Is <40m mreg ulatingreq uired ?

Is re treadan eco nom ica l

so lu tion ?

P a tch

R epla ce w earin gcourse o r ove rla y.

R e trea d

U se th ick s lurrysurfacin g o r rep lacew earing co urse o r

overla y

U se su ita b les lu rry su rfa c in g

U se su rfacedress ing o r th ins lu rry su rfa c in g

R epla ceb itum inous la ye rs

Ye s

Ye s

Ye s

Ye s

Ye s

Ye s

Ye s

Ye s

Ye s

N o

N o

N o

N o

N o

N o

N o

N o

N o

FLOWCHART 2 : BITUMINOUS

Notes If slurry surfacing has already been used twice for maintaining the surface, it is probable that thebituminous surface needs replacing rather than another application of slurry surfacing.

Any overlay, including thick slurry surfacing, can only be used if levels of damp proof courses etc allow.

May 20015/10



S T A R T

Is surfacelarge fla gs ?

Is surfaceuneve n ?

Is overruna problem ?

A re flagscracke d ?

Is loss ofjointing sa nda problem ?

A re flagsre quired foraestheticreaso ns ?

Is surfaceuneve n ?

R eplace jointing sand.P revent furthe r loss.S eal if nece ssary.

R ep la ce w ith sm allelem ent m odules orbitum inous m a terialto D M R B 7.2.5.3

R elay on sand bed dingrep la cing dam a ged m o dules

Investigate furth er

Y es

Y es

Y es

Y es Y es

Y es

Y es

N o

N o

N oN o

N o

N oN o

FLOWCHART 3 : MODULAR SURFACING

May 2001 5/11



IOGRAPHY
References


1994

HD31 (DMRB 7.4.1) Maintenance of BituminousRoads.

HD32 (DMRB 7.4.2) Maintenance of Concrete Roads.

1999

HD37 (DMRB 7.5.2) Bituminous Surfacing Materialsand Techniques.

2001

HD39/01 (DMRB 7.2.5) Footway Design.




Highway Construction Details (MCHW3).


1983

Powell, W.D. and Leech, D., “Compaction ofBituminous Road Materials using Vibratory Rollers”,LR1102.

1995

Burtwell, M(Ed), “A Study of Footway Maintenance”.Report 134. (Research Project Funded by TRL/CSS/HA).

1997

“Design Guide for Road Surface Dressing”. Road Note39, 4th Edition (Revised 1997).

May 2001

1997

“Footways : Design and Maintenance Guide”,Application Guide 26


1984

BS434: Bituminous Road Emulsions (Anionic andCationic): Part 1: Specification for bitumen roademulsions (Confirmed 1997).

BS434: Bituminous Road Emulsions (Anionic andCationic): Part 2: Code of Practice for use of bitumenroad emulsions (Confirmed 1997).

5. Others

1988

Department of Transport, The Welsh Office and TheScottish Office, “Preferred Method 1: Patching. Reportof the Standing Committee on Highway Research”,Cornwall County Council, Truro.

Watterson, A., “Pesticides Users Health and SafetyHandbook”, Gower Technical, London.

1989

“Joint Study of Highway Maintenance: A Code ofGood Practice”. Association of County Councils,Association of District Councils, Association ofMetropolitan Authorities, The Convention of ScottishLocal Authorities, London.

1991

Department of Transport, The Scottish Office, WelshOffice & Department of the Environment for NorthernIreland, “Traffic Signs Manual Chapter 8. Traffic SafetyMeasures and Signs for Road Works and TemporarySituations”, HMSO, London.

1992

Department of Transport, The Welsh Office and TheScottish Office, “Highway Authorities UtilitiesCommittee Specification for the reinstatement ofopenings in highways (Code of Practice)”, HMSO,London.

6/1



Inman, J.D., “Highway Weed Control by Pesticide”,Municipal Engineer, Dec 1992, p229-232.

Health and Safety Commission, “The Safe Use ofPesticides for Non-agricultural Purposes: Control ofSubstances Hazardous to Health Regulations. ApprovedCode of Practice”.

1994

The Kindreds Association, “Report of the HighwayLiability Claims Task Group”, Merlin Communications(UK) Ltd.

1995

“Highway Authorities Standard Specification No. 3:Slurry Surfacing (Works)”, County Surveyors Society.

“Highway Authorities Standard Specification No. 4:Surface Dressing (Works)”, County Surveyors Society.

“Highway Authorities Standard Specification No. 10:Reinstatement of trenches, patching and footwaysurfacing (Works)”, County Surveyors Society.

“Code of Practice”, Road Surface Dressing Association.

Street Works (Northern Ireland) Order 1995. NorthernIreland Road Authority and Utilities Committee(NIRAUC). “Specification for the Reinstatement ofOpenings in Roads”. The Stationery Office, Belfast.

1996

Emery, J.A. and Lazar, M., “Block Paving : To Seal ornot To Seal”. Proceedings of the Fifth InternationalConference on Concrete Block Paving, Tel Aviv, Israel.

1999

Pritchard, C., “Precast Concrete Paving : A DesignHandbook”, Interpave, Leicester.

2000

“Good Practice Guide for the Use of Natural StoneSurfacings for Roads and Paths”, The Society of ChiefOfficers of Transport in Scotland (SCOTS).

6/2

Undated

Ministry of Agriculture Fisheries and Food and TheHealth and Safety Executive, “Pesticides ApprovedUnder The Control of Pesticides Regulations 1986.Reference Book 500”. (Revised Annually). TheStationery Office, London.

Bibliography

Atkinson, K. (Ed)”, Highway Maintenance Handbook”,Thomas Telford Ltd., London, 1990.

Mays, G.C. (Ed) “Durability of Concrete Structures -Investigation, Repair, Protection”, E & F.N. Spon ofChapman & Hall, London, 1992.

Neville, A.M., “Properties of Concrete”, PitmanPublishing, 1991.

Potter, J. (Ed), “Road Haunches : A Guide to the Re-useof Materials”, TRL Report 216, 1996.

Wingate, P.J.F. and Peters, C.H., “The CHART Systemof Assessing Structural Maintenance of Highways”,TRRL Supplementary Report 153, Crowthorne, 1975.

City Engineer’s Group, “The MARCH HighwayMaintenance System”, 1975.

“Non-structural Cracks in Concrete”. The ConcreteSociety, Technical Report Number 22, Third edition,1992.

Highways Agency, “Trunk Road Maintenance Manual.Volume 2, Routine and Winter Maintenance Code”:Stationery Office.

National Assembly for Wales, “Trunk RoadMaintenance Manual”, 1998.

Highways Agency, “User manual for the HighwaysAgency’s Routine Maintenance Management System”,Stationery Office, 1996.

May 2001


May 2001

7. ENQUIRIES


Chief Highway EngineerThe Highways AgencySt Christopher HouseSouthwark Street J KERMANLondon SE1 0TE Chief Highway Engineer

Chief Road EngineerScottish Executive Development DepartmentVictoria QuayEdinburgh J HOWISONEH6 6QQ Chief Road Engineer

Chief Highway EngineerThe National Assembly for WalesCynulliad Cenedlaethol CymruCrown BuildingsCathays Park J R REESCardiff CF10 3NQ Chief Highway Engineer

Director of EngineeringDepartment for Regional DevelopmentRoads ServiceClarence Court10-18 Adelaide Street G W ALLISTERBelfast BT2 8GB Director of Engineering

Chapter 7Enquiries

7/1


May 2001

Annex ADescription of Defects

ANNEX A DESCRIPTION OF DEFECTS

A.1 DEFECTS COMMON TO ALL TYPES OFFOOTWAY CONSTRUCTION

Depressions and bumps:

Circular or oval shaped depressions are usually a signof local failure at sub-base or subgrade level. Thecauses are many and varied and should be investigatedif the problem is more than an occasional depression.Local depressions can occur where vehicles park oroverrun the footway, especially where the foundation isweaker than surrounding areas, possibly as a result of adrainage problem. Long narrow depressions are usuallya sign of settlement in an undertaker’s trench and areoften accompanied by cracks and steps along the edgesof the depressions.

Bumps may be due to root growth or heave of thefoundation. Heave may result from the ground swellingdue to tree removal or the formation of ice lenses inwinter due to insufficient protection to a frostsusceptible soil. Due to the varying nature of the soiland moisture condition, the heave will be uneven,resulting in bumps. Surface irregularities can also resultfrom faulty construction or drainage faults.

Rutting:

Longitudinal rutting usually occurs close to the kerb,caused by vehicles parking on the footway to avoidcausing an obstruction in the road. Transverse ruttingmay occur at vehicular accesses. Although longitudinalruts are more frequent and often deeper than transverseruts, the latter are noticed more by pedestrians who maytrip over them. In flag footways rutting is generallyassociated with cracked flags. It is important to identifywhether the rutting is structural (due to failure of thefoundation as for ‘depressions’) or non-structural(confined to surface layers only, exacerbated by longloading times of parked vehicles, and both hightemperatures and high penetration binders inbituminous materials).

A/1


May 2001

Slippery surface:

The majority of materials used for footway surfacinghave a satisfactory inherent slip resistance. However,slip resistance can be affected by polishing of thesurface due to constant trafficking, use of inappropriateaggregate, or growth of moss or lichen. Slurrysurfacings, in particular, may become polished with ageand wear.

Surface contamination:

Surface contamination by foreign substances affects theaesthetic appearance of a footway by staining ordiscoloration. Fuel spillage may seriously lower theskid resistance, or soften bituminous material. Itrequires rapid removal, or the application of aneutralising agent, if damage is to be avoided.

Vegetation:

Vegetation is remarkably persistent and will grow inany crack or gap which becomes filled with debris.Vegetation will therefore tend to be less of a problem inthose footways which are cleaned and maintainedregularly. Tree roots cause heave of the footway withassociated cracks and trips. Weed growth in cracks andgaps causes them to widen leading to unevenness. Mossand lichen may make the surface slippery when wet.


A/2


May 2001

Water:

Water emerging from the footway is a nuisance topedestrians and will lead to rapid deterioration of thefootway if left untreated. It could be caused by a springor, in urban areas, it is probably due to a leaking watermain or a blocked drain. During freeze/thaw cycles thewater may cause safety hazards. Standing water mayoccur in depressions, due to deterioration or toinsufficient gradient on the surface profile, and maybecome a hazard to pedestrians in freezing conditions.

Cracks or joints in the footway surface may allow theingress of water which will subsequently weaken thestructure. Freezing water in cracks will cause furtherdamage.

A.2 DEFECTS: BITUMINOUS FOOTWAYS

Excess binder:

This may be due to high temperatures or to binder richmaterial having been used. It requires treatment becauseof the inconvenience to pedestrians.

Mosaic cracking:

This may be due to structural failure of the foundationor embrittlement of the bituminous material. (Bitumenhardens in the presence of air and sunlight, and can leadto material which becomes brittle). Mosaic crackingappears as an area of approximately hexagonal plateswith the dimension of the plates of the order of 50 to100mm; the thicker the bituminous construction thelarger the plate.


A/3


May 2001

Linear cracking:

Cracking can vary from occasional hairline cracks, onlyvisible in a drying surface, to major problems with gapsover 15mm and a difference in level across the crack ofa similar dimension. The cause of the cracking needs tobe established in order that appropriate maintenance becarried out. Linear cracking can occur due to groundheave or to differential settlement alongside areinstatement. It may be reflective cracking, occurringif a bituminous surfacing is placed on a stabilised base,where cracks can be initiated by thermally inducedstresses and by vertical movements between adjacentcementitious slabs caused by loading.

Loss of surface aggregate:

This can be due to fretting, stripping or loss of binderdue to local pollution. Superficial fretting appears as a‘dryness’ of the surface and a slight dustiness whicharises as fine particles of aggregate in the surfaceloosen from the bitumen. This usually occurs in winterwhen the bitumen is more brittle. If it remains untreatedit is likely to lead to loss of coarse aggregate on thesurface and to ingress of water. Loose surface materialcan become a safety hazard. When aggregate is lostfrom slurry surfacing it can result in a smooth and,sometimes, slippery surface.

Stripping occurs when the binder has poor affinity tothe aggregate. Water penetrates between the bitumenand the aggregate causing the aggregate to be strippedof binder. This occurs most frequently at the bottom ofthe wearing course layer when it has a high voidcontent. Once the surface is broken, failure can progressrapidly, as the loose material at the bottom of the layeris no longer contained.

Delamination may be a common problem with thinslurry surfacing. The thin layer peels off due to lack ofadhesion with the underlying material. This can occurbecause of poor workmanship or when the thinsurfacing becomes worn through.


A/4


May 2001

Potholes:

If fretting or stripping are allowed to continue unabatedthen it is likely that potholes will develop due to loss ofmaterial. This is the most obvious sign of failure of theupper layers of the footway and can be dangerous. Thedepth of a pothole usually equates to one or morecomplete bituminous layers.

A.3 DEFECTS: MODULAR FOOTWAYS

Broken or damaged modules:

Cracking can be merely unsightly or can be a hazardwhere it leads to wide gaps or trips. It can occur due toa combination of thermal warping and/or trafficloading. Thermal warping will lessen the support underthe flag and traffic loading may then produce stresseshigher than the modulus of rupture of the flag. Thelarger and thinner the flag, the larger the warping andtraffic induced stresses. Cracking can also be a result offatigue due to repeated loading. A stabilised bedprovides no support to a warped flag and is thusunsuitable.

Loss of jointing material:

Loss of jointing material can be caused by trafficking,weather, water ingress, vegetation growth or the suctionaction of mechanical sweepers. For rigid jointingmaterial the defect mechanisms leading to loss are thesame as those of concrete. Mechanical breakdown orloss of jointing material causes modules to lose anyinterlock, thus reducing the load spreading ability of themodular surfacing. Modules also become free to movelaterally and to rotate, which may result in rockingmodules or widening of joints.


A/5


May 2001

Missing modules:

Missing modules leave holes and trips, allow water toaccumulate and weaken the structure of the remainingmodules.

Rocking modules:

A paver, flag or sett which is not supported evenly willrock when loaded. The problem is exacerbated if therocking pumps water and fines out from under themodule, reducing the support further. This fault isusually only detectable if the module is walked on.

Spalling of arrises or surfaces:

Spalling of arrises can occur when modules rotate dueto loading during construction or use, or lack ofsupport, especially if the joint gap is small. Spallingoccurs as adjacent modules exert pressure upon eachother. Chips may be knocked off the top corners of flagsor pavers when laying or during the life of the footway.This can detract from the visual appearance but rarelybecomes a hazard.

Spalling of the surface, or loss of surface material, mayoccur if inappropriate natural materials are used and isunsightly rather than hazardous. It may be due to amanufacturing defect, impact loading, abrasion,weathering or possible chemical attack. York stonesuffers from delamination which may result in anuneven surface.


A/6


May 2001

Trips:

Trips are caused where modules settle unevenly orcrack. Where the difference in level is marked, tripspresent a hazard to pedestrians. This is more likely tobe a problem with large flags.

Unzipping:

Loss of interlock and movement of blocks or paversmay occur due to disturbance by excavation or to poorinitial construction.

Widening of joints:

This may be indicative of ground movement, loss ofjointing material or failure of footway edge constraint.


A/7


May 2001

A.4 DEFECTS: CONCRETE FOOTWAYS

Defects which occur in concrete flags or blocks, such asspalling and cracking, may also occur in mass concretefootways. Other defects are:

Cracking:

If large areas of concrete are laid without joints theconcrete will crack as it cures. Cracks may also occurdue to the concrete having insufficient tensile strengthto support the stresses caused by loading andtemperature changes. If the concrete has crackedsufficiently to impair its load spreading ability,depressions or ruts may result.

Damaged joints:

Damaged joints may occur as a result of movement ofthe concrete slabs due to loading or expansion. Spallingand cracking can result.


A/8


May 2001

Surface damage:

The surface appearance can change and deteriorate dueto weathering (sun, temperature changes, rain, snow,atmospheric pollution, freeze/thaw) which can result incracks, and sometimes weed growth. Disruption causedby expansion of water on freezing can cause shallowsurface scaling and delamination, leaving a roughsurface. The effect is more pronounced for concreteswith lower cement contents, because these have lessstrength and are more porous (interconnecting voids).However, air entrained concrete has excellent resistanceto frost attack, since the discrete voids created by theadmixture allow room for freezing water to expandwithout spalling the concrete. Chemical attack can leadto the surface aggregate becoming exposed or largeareas of surface concrete being destroyed. Zones moreprone to attack are near gullies where there may be aplentiful supply of contaminated water.

Poor constituents:

In normal concrete, strength development is derivedfrom chemical reactions between water and cement. Ifbagged cement is stored under damp conditions it willchemically react and become solid. These reactions arenot reversible. Hence the content of the bag should notbe used because the product will have low strength andvery poor durability. Contaminated aggregates are alsoa common cause for severe deterioration of concrete,particularly those contaminated with sulphate. Ifcontaminated aggregates are used the concrete willshow significant deterioration after winter weathering.As wood or other forms of contamination decompose,cracks will appear, caused by the ingress of waterfollowed by freeze/thaw cycles.


A/9


May 2001

Broken or missing items: This may result in a loss ofedge support to the footway. York stone kerbs maysuffer from delamination

Horizontal or vertical steps:

If the kerb projects into the carriageway it may bedangerous for road users. Vertical steps may be a hazardfor pedestrians.


A/10

A.5 DEFECTS: KERBS AND EDGING

Many of the defects which may be noted during an inspection of kerbs and edgings are common to modularsurfacing, such as trips, loose or rocking kerbs or damaged kerbs. Vehicles colliding with kerbs are a commoncause of defects. Apart from immediate repairs undertaken for safety reasons, defective kerbs should be replaced inan annual programme, generally in association with other carriageway or footway defects.


May 2001

Poor alignment:

If preformed channels are part of the kerb, pooralignment can lead to drainage problems and watercollecting in puddles. Poor kerb alignment may be asymptom of overrun, and may lead to ingress of water.

Spalling of arrises or faces:

Chips may be knocked off the top corners during thelife of the footway. This can become a hazarddepending on the extent of the damage. Spalling of thesurface, or loss of surface material, may occur ifinappropriate natural materials are used, and maybecome hazardous if vehicles mount or “knock” thekerb.


A/11


May 2001

Vegetation growth:

Growth of vegetation along the edge of a footway or indrainage channels may prevent over edge run-off ofwater causing inconvenience to footway users.


A/12


Annex BLaying Natural Stone Setts and Decorative Surfaces

AL STONE SETTS AND

The traditional Victorian method of laying settsinvolved setting them to about mid depth in a drycrushed stone of 6mm nominal size. The top half of thejoint was then filled with a poured bitumen, care beingtaken not to stain the surface. This process has thebenefit of robustness, impermeability and long lastingjoints. The joints can easily be repaired using pouredbitumen if degradation takes place.

An alternative method is to fill the top half of the jointswith a crushed granite of 0 - 6mm grading. Followingthe initial tamping, usage of the footway progressivelycauses the aggregate to “lock up” and becomeimpermeable.

Present day practice is to use a stabilised sand at 6:1 or8:1 sand/cement for the bedding, and fill the top half ofthe joint with a semi-dry mix of 3:1 or 4:1 sand/cement.Initially, this provides an impervious and frost resistantjoint. However, thermal stresses will inevitably causecracking leading to eventual degradation of the jointand reduced service life; hence this method cannot berecommended.

Widely used in Europe, but much less so in the UK, isthe practice of bedding and filling the joints with a2 - 5mm crushed stone, used dry when laid, and wateredon completion. The process of watering assists filling ofthe joint. The crushed stone “tightens up” in the jointwith time and successive load applications, and it alsofacilitates very easy relaying as the jointing materialand setts can generally be re-used. It must beemphasised that there is a considerable degree of layingskill required. Judgement must be exercised aboutwhich sett will be a better fit, against those already laid,because each one has a unique shape.

The laying course is 50mm thicker than thecharacteristic half depth of the setts. On selection of asett, the mason forms a cavity in the laying course witha snipe nosed hammer, places the sett and settles it intoplace with the hoof of the hammer. Final settlementmay be done using a hand rammer as a separateprocess. If a straight joint is being used, each line is laidto a string line. In the event of more decorative designsbeing used, for example the fan pattern, the setting outis somewhat more complex and requires skilledjudgement to ensure the pattern is achieved.

ANNEX B LAYING NATURDECORATIVE SURFACES

May 2001

A development of these two methods is to lay themodule in a semi-dry sharp sand/cement mix (4:1 or3:1) and for the top half to be filled with a liquidmortar. The appropriate mix for this is 1 part sharpsand, 4 parts building sand, 2 parts ordinary Portlandcement. The mix should be to the consistency of“custard” and it should be poured into the joint toensure complete filling. There is the need to spend timewiping the surface clean to prevent staining. Thismethod of laying is also appropriate where localdecorative feature panels have been included in thefootway surface.

Further advice will be available in the SCOTS “GoodPractice Guide for the Use of Natural Stone Surfacingsfor Roads and Paths” to be published in 2000.

B/1


May 2001

Annex CLaying Concrete Slabs

C/1

A typical concrete slab footway consists of 100mmthickness of 40 N/mm2 concrete laid in maximumlengths of 3m on a waterproof surface membrane on anappropriate base.

Prior to placing the concrete, a waterproof membraneconsisting of either waxed building paper or 500gpolythene sheet should be placed. This prevents suctionof the moisture in the wet concrete into the granularsub-base.

Concrete of medium workability, maximum 50mmslump, is laid in alternate bays with construction jointsbetween each bay and expansion joints every third bay.The concrete should be tamped before being floated andfinally finished with a lightly-drawn transverse broom.

Details of the expansion joint is similar to Drawingnumber C2 in Highway Construction Details(MCHW3) except the dowel bar is not used. Theconstruction joints are formed against a stop end boardwhich is removed prior to laying the adjacent slab.

ANNEX C LAYING CONCRETE SLABS


May 2001

Annex DWorked Examples

D/1

ANNEX D WORKED EXAMPLES

EXAMPLE 1

A narrow bituminous footway is adjacent to a relatively narrow carriageway which is on a bus route in a residentialestate. The damage has occurred over the last 5 years, since the introduction of the bus route. The footway suffersoverrun by buses as well as private cars and light delivery vehicles. The kerbs are less than 75mm in height and theback of the footway is poorly supported.

Flowchart 1:

Is urgent treatment required? Chapter 2, Clause 2.3

Although the footway is in a generally bad state of repair it does not present an immediate safety hazard.

Investigate cause of defect(s). Chapter 3, Annex A

Following Flowchart 1, root growth and surface contamination or slip are not a problem, but there is some failureof the edge restraint as the kerb is low and the back edge of the footway is not well supported. Defects are not dueto reinstatement failure, but foundation failure is indicated as there is rutting behind the kerb.

Maintenance Option:

The footway should therefore be reconstructed according to DMRB 7.2.5.3 (Heavy-vehicle design). Considerationshould be given to lifting the kerbs to help prevent future overrun. Replacement of the surfacing will also berequired.


May 2001


D/2

EXAMPLE 2

A bituminous footway is situated in a cul-de-sac in a residential area approximately 30 years old, and has receivedlittle past maintenance. It suffers overrun by private cars and delivery vehicles. The bituminous material showssigns of a combination of thermal and age induced cracking. There is a longitudinal rut, over 25mm deep, on thefootway.

Flowchart 1:

Is urgent treatment required?

Although the footway is in a bad state of repair there is no immediate safety hazard.

Investigate cause of defect(s).

Following Flowchart 1, there is no root growth, surface contamination or slip, failure of edge restraint or failure ofreinstatement. The longitudinal rut indicates foundation failure due to overrun (Chapter 3, Clause 3.23).

Further considerations:

As the footway has received little maintenance over 30 years, and the deformation appears to have beenprogressive during that time, it is worth considering removing the deformation by replacing only the bituminouslayers. However if the foundation is not sufficiently strong, the problem may recur (Chapter 4, Table 4.2).

Maintenance Options:

Use of a thicker bituminous layer would strengthen the footway construction. The footway may need to bereconstructed according to DMRB 7.2.5.3 (Light-vehicle design).


May 2001


D/3

EXAMPLE 3

A bituminous footway in a residential area presents an uneven surface due to considerable loss of wearing coursematerial. Some of the kerbs are slightly out of line due to undertakers’ activities. There is no rutting.

Flowchart 1:


There is no immediate safety hazard.


Following Flowchart 1, there is no root growth, surface contamination or slip, failure of edge restraint,reinstatement or foundation.

Flowchart 2:

Assess extent of defect(s).

Defects are not confined to isolated areas but, apart from the detail of kerb alignment, are confined to the wearingcourse.

Maintenance options:

The use of a thick slurry surfacing, to seal and regulate the surface, is indicated. This will need to be keyed in at theback of the kerb (Table 4.2 and Chapter 5, Clauses 5.28-5.34).

As the carriageway also appears in need of attention, the footway and carriageway works could be carried out at thesame time and the kerbs could be realigned. As the kerbs are only slightly out of alignment they would not berealigned unless carriageway works were being undertaken.


May 2001


D/4

EXAMPLE 4

A bituminous footway in a late 1950’s residential estate is in need of maintenance. The general view of the estateshows depression of kerbs due to a vehicular crossing and cracking in an old undertakers’ reinstatement. Thephotograph of the corner shows that the kerb in the centre has been replaced in the past and it appears that the finalpatching at the back and front was not completed. The replacement kerb is shorter than the original and the kerbadjacent to the gully is tilting. In the upper part of the picture, overlaid patching following undertakers’ activitieshas failed leaving a trip. There is no rutting.


May 2001


D/5

Flowchart 1:


If the trip height presents a safety hazard a temporary ramp of bituminous material should be placed until apermanent repair can be carried out (Chapter 5, Clause 5.9).


Following Flowchart 1, there is no root growth, surface contamination or slip, but there is failure of edge restraintand the tilting and unsuitable kerbs should be replaced (Chapter 5, Clauses 5.67 - 5.69). The failed reinstatement iswell over 2 years old and is not, therefore, in the guarantee period. There is no indication of foundation failure.

Flowchart 2:

Assess extent of defect(s).

Defects appear to be confined to the wearing course. The worst defects are in isolated areas.


Defects could be treated with more overlay patching (Chapter 5, Clauses 5.19 - 5.21, 5.35 - 5.37). However, theappearance of the footway would be enhanced if it were resurfaced rather than receiving further patching.

The use of a slurry surfacing would give the footway an homogenous appearance and could be beneficially used aspreventative maintenance on the remainder of the footways in the estate. A thick slurry surfacing will need to bekeyed in at the back of the kerb (Chapter 5, Clauses 5.28 - 5.34).


May 2001


D/6

EXAMPLE 5

The photographs show the footway on both sides of the road in an urban residential area with terraced houses. Itappears to have been originally surfaced in stone flags which have been replaced with bituminous construction.The stone kerb is believed to be original. The kerb has been displaced because of growth of tree roots which havealso caused cracking of the bituminous surface.


May 2001


D/7

Flowchart 1:


The displaced kerb is a safety hazard and should be removed until it can be replaced (Chapter 5, Clauses 5.8 -5.16).


Following Flowchart 1, root growth and subsequent failure of edge restraint and cracking are the only problems.


Trees in residential areas, while undoubtedly environmentally attractive, cause many ongoing maintenanceproblems, some of which are illustrated here. A permanent reconciliation of the maintenance problems is notpossible without removal of the trees. Experience indicates that this is unlikely to be possible. Maintenance istherefore likely to be ongoing to keep the footway and channel safe.


The tree roots should be pruned, if possible, to minimise future problems. The displaced kerb can be shortened tofit around the roots and narrower edging can be used immediately adjacent to the tree. The concrete edging forminga box around the tree can be replaced leaving more room for root growth, providing that the footway remainssufficiently wide for pedestrians. The bituminous footway needs relaying where it has become uneven (Table 4.1and Chapter 5, Clauses 5.55 - 5.60).


May 2001


D/8

EXAMPLE 6

A footway in a residential street some 40 years old has not received any maintenance since its construction. Thesurfacing is worn and uneven with some loose material, significant loss of wearing course material, cracking andweed growth. Missing kerbs have been patched with bituminous material.


May 2001


D/9

Flowchart 1:


There are no specific safety hazards but loose material and unevenness make walking on this footway difficult. Thepriority given to maintenance will depend on its location.


Following Flowchart 1, the surface should be replaced and the edge restraint reconstructed. Foundation failure isnot indicated.

Flowchart 2:

The deterioration is general, not confined to isolated areas or to the wearing course. The footway is too degradedfor slurry surfacing to be used. The footway is not subject to HGV overrun and retread has not previously beenused, so it is one possibility.


The kerbs and the bituminous layers need replacing (Table 4.5 and Chapter 5, Clauses 5.56 - 5.60). Retread couldbe an option, depending on the economics of the situation.


If the level of undertakers’ works is very high then the minimum cost treatment should be used.

If the whole area is likely to be part of an improvement scheme a change in surfacing, to modular, could beconsidered.


May 2001


D/10

EXAMPLE 7

A footway in a small market town suffers from overrun, especially from heavy delivery vehicles, which has causedthe kerbs to sink. This has allowed the formation of a trip adjacent to an undertaker’s cover.

Flowchart 1:


The trip does not appear to constitute an immediate safety hazard, but could be taken out, temporarily, by rampingwith bituminous material if necessary (Chapter 5, Clauses 5.8 - 5.16).


Following Flowchart 1, the kerb should be replaced. Foundation failure is indicated as the kerb has sunk androtated and the footway behind the kerb is depressed.

Flowchart 2:

The deterioration is confined to an isolated area and patching is therefore indicated.


The footway should be strengthened locally to prevent further deterioration due to overrun. When the kerb is beingrealigned the sub-base under the corner of the footway could be replaced with concrete to give additional strength(Chapter 5, Clauses 5.67 - 5.69). This could be brought to the surface level or an inlaid patch of bituminousmaterial could be applied over the concrete (Chapter 5, Clauses 5.20 - 5.21).


If the cover to the undertaker’s apparatus is not suitable for an overrun situation the appropriate undertaker shouldbe informed and the cover replaced.


May 2001


D/11

EXAMPLE 8

A flagged footway on a 100 year old urban street is in a bad state of repair. The footway is surfaced with largenatural stone flags. Most kerbside flags are cracked and the kerbs have sunk. A combination of overrun by deliveryvehicles and laying of undertaker’s apparatus, with inadequate backfill to the trench before the flags were relaid,has caused breaking and spalling of the flags.

Flowchart 1:


Safety hazards such as significant trips or rocking flags need to be dealt with as a matter of urgency. Modules canbe replaced, temporarily, with bituminous material (Table 4.3 and Chapter 5, Clause 5.9).


The edge restraint has failed and the kerb should be replaced (Table 4.5 and Chapter 5, Clauses 5.67 - 5.69). It isunlikely that reinstatement failure is the only cause of the cracked flags, because of the known problem of overrun.Foundation failure is indicated as the kerb has sunk and flags next to the kerb are cracked, with some rutting.


The footway should be reconstructed following the recommendations of DMRB 7.2.5.3 (Heavy-vehicle design).


Consideration needs to be given to the choice of surfacing materials. The footway could be surfaced withbituminous material or small element modules. If natural stone flags are required for environmental reasons thenoverrun should be prevented. This could be achieved by the installation of bollards (Refer also to Chapter 5, Clause5.47)

When the kerbs are relaid the roadside face will be higher than at present. The actual height selected will dependupon the threshold levels of the adjacent building, and the need to maintain falls towards the carriageway.


May 2001


D/12

EXAMPLE 9

A flagged footway is adjacent to a public house. Damage has been caused by delivery vehicles, more particularlydue to the impact loading caused by the off-loading of barrels.

Flowchart 1:


Although many flags are broken there are no significant trips, but heels may catch in the open joints.


Defects are not due to root growth, surface contamination, failure of edge restraint, reinstatement or foundation.

Flowchart 3:

Flowchart 3 indicates that, as overrun is a problem, the footway should be resurfaced with small element modulesor bituminous material (Table 4.3). However, as the surface is subject to severe impact loading the surfacing willneed to be capable of resisting this.


One option would be to resurface the footway with 80mm thick concrete blocks (Chapter 5, Clauses 5.38 - 5.43).An alternative would be to use pavement quality concrete with the addition of polypropylene fibres, or,alternatively mastic asphalt could be used (Refer DMRB 7.2.5.4).


May 2001

EXAMPLE 10

A flagged footway in a suburban street requires attention because of trips around mature trees. The footway isotherwise in a good state of repair.

Flowchart 1:


There are some significant trips which should be removed as soon as possible (Table 4.3 and Chapter 5, Clause5.9).


Defects are due to root growth making the flags uneven, creating large trips because of the size of the flags.

Flowchart 3:

Flowchart 3 suggests that the large flags should be replaced with bituminous material or small element modules.


In this situation the use of block paving could be appropriate (Chapter 5, Clauses 5.38 - 5.43). The small moduleswould minimise both the trips caused by future root growth and the area to be relaid on each occasion whenfuture maintenance is necessary. Bituminous material is also an appropriate surfacing and would minimise thetrips (Refer DMRB 7.2.5.4).


D/13


May 2001

EXAMPLE 11

A footway in a shopping centre is surfaced in concrete blocks. It has a depression along the line of an undertaker’sreinstatement that was carried out three years ago.

Flowchart 1:


No safety hazards reported so maintenance can be part of planned programme.


Defects are due to reinstatement failure, but three years is outside the HAUC guarantee period. However, theengineer should consider which undertaker is responsible and enlist their co-operation to investigate the problem(Table 4.1).

Foundation failure along the reinstatement is indicated. This may be due to faulty construction or to drainageproblems such as water tracking along utility pipes.

Flowchart 3:

As far as the surfacing is concerned, loss of jointing sand is also a problem (Table 4.3).


The cause of the problem with the reinstatement needs investigating and appropriate action should be taken.(Chapter 3, Clause 3.4). The footway surfacing can then be relaid and jointing sand replaced. (Chapter 5, Clauses5.38 - 5.46).


D/14


May 2001

EXAMPLE 12

A footway has been built out into the road as part of traffic calming measures. A gap has now opened between theconcrete blocks and the kerb and the surface of the pavers has become uneven. An adjacent reinstatement indicatesthat the problem may be due to the pavers having not been properly replaced on completion of the reinstatement.

Flowchart 1:


The trip by the kerb is a safety hazard and should be temporarily taken out by ramping with bituminous material(Table 4.1 and Chapter 5, Clause 5.9).


Defects appear to be due to a combination of overrun and reinstatement failure. If the reinstatement is within theHAUC guarantee period the appropriate undertaker could be requested to carry out repairs. The pavers should belifted and an investigation made as to whether the problem is due to lack of foundation support, or confined to thesurfacing (Chapter 3, Clause 3.4).

Flowchart 3:

To obtain an even surface the pavers will need relaying.

Maintetance Options:

As the area of pavers is small, the best solution would be obtained by lifting all pavers and relaying, using newpavers where necessary (Chapter 5, Clauses 5.39 - 5.43). If the foundation requires strengthening this can becarried out at the same time, reconstructing the foundation according to DMRB 7.2.5.3 (Heavy-vehicle design).


D/15

November 2006



SECTION 5 PAVEMENT MATERIALS

PART 1

HD 36/06

SURFACING MATERIALS FOR NEWAND MAINTENANCE CONSTRUCTION

SUMMARY

This revision primarily updates Chapter 3 of thisStandard. This Standard provides a summary ofsurfacing options available for use on both flexible andrigid pavements and advises on current requirements forsurfacing. It also details requirements for aggregatespreviously covered in HD 28 (DMRB 7.3.1) and givesadvice on surface texture.


1. Remove Contents pages from Volume 7 andinsert new Contents pages for Volume 7 datedNovember 2006.

2. Remove HD 36/99 from Volume 7, Section 5which is superseded by this Stadard and archiveas appropriate.




HD 36/06

Surfacing Materials for New andMaintenance Construction

Summary: This revision primarily updates Chapter 3 of this Standard. This Standardprovides a summary of surfacing options available for use on both flexible andrigid pavements and advises on current requirements for surfacing. It alsodetails requirements for aggregates previously covered in HD 28 (DMRB 7.3.1)and gives advice on surface texture.


THE HIGHWAYS AGENCY

TRANSPORT SCOTLAND




November 2006






SECTION 5 PAVEMENT MATERIALS

PART 1

HD 36/06

SURFACING MATERIALS FOR NEWAND MAINTENANCE CONSTRUCTION

Contents

Chapter

1. Introduction

2. Surfacing Options

3. Texture and Aggregate Properties


5. Tyre/Road Surface Noise


7. Enquiries


November 2006



1. INTRODUCTION

1.1 This Part provides a summary of surfacingoptions available for use on both flexible and rigidpavements and advises on current requirements forsurfacings. The Part also details the requirements foraggregates to ensure that satisfactory skiddingresistance is provided on roads and should be read inconjunction with HD 28 (DMRB 7.3.1). This Part alsoincludes details of surface texture and how this affectssurface noise at the tyre/road interface.

1.2 Supplementary information on bituminousmaterials is given in HD 26 (DMRB 7.2.3). Furtherinformation on the maintenance of bituminous roadscan be found in HD 31 (DMRB 7.4.1) and in HD 32(DMRB 7.4.2) for the maintenance of concrete roads.

1.3 Detailed information on bituminous materialtypes, and surfacing processes, together with advice ontheir use, is presented in HD 37 (DMRB 7.5.2). Detailsof concrete surfacing and materials are given in HD 38(DMRB 7.5.3). Reference should be made to theSpecification (MCHW1) Series 700, 900 and 1000,together with the Notes for Guidance (MCHW2). Forsome materials there are British Standards and otherpublished documentation and these are referenced inthe appropriate chapters.

Implementation

1.4 This Part shall be used forthwith on all schemesfor the construction, improvement and maintenance oftrunk roads including motorways, currently beingprepared provided that, in the opinion of the OverseeingOrganisation this would not result in significantadditional expense or delay. Design organisationsshould confirm its application to particular schemeswith the Overseeing Organisation.

November 2006

Mutual Recognition

1.5 The construction and maintenance of highwaypavements will normally be carried out under contractsincorporating the Overseeing Organisation’sSpecification for Highway Works (MCHW1). In suchcases products conforming to equivalent standards andspecifications of other States of the European EconomicArea and tests undertaken in the other States will beacceptable in accordance with the terms of the 104 and105 Series of Clauses of that Specification. Anycontract not containing these clauses must containsuitable clauses of mutual recognition having the sameeffect regarding which advice should be sought.

1/1


Chapter 2Surfacing Options

2. SURFACING OPTIONS

2.1 The choice of surfacing materials/systems plays avital role in providing roads that meet the needs of theuser, are safe and give value for money. For many yearshot rolled asphalt with chippings rolled into the surfacewas the most widely used surfacing on trunk roads,including motorways, for both new construction andmajor maintenance. However, recent years have seenthe development of new materials and techniques, manyof which are proprietary, which offer significantadvantages not just to the road user but also to theenvironment. For example, noise generation may bereduced, delays at road works curtailed, ride qualityimproved and deformation resistance enhanced, allwhile maintaining existing safety levels. Furthermore,new products such as energy efficient ‘cold-lay’materials are in their development phase. This Chaptergives guidance on the range of surfacing options thatare now available for both new construction andmaintenance.

Performance Specifications

2.2 To remove the barriers to trade and to encourageinnovation, the Construction Products Directive (CPD)of the European Union requires the introduction ofperformance related specifications wherever possible.Specification clauses of this type have been included inthe Specification for Highway Works (MCHW1&2)covering surfacings such as surface dressings (Clause922), slurry and micro-surfacings (Clauses 918), highfriction surfacing (Clause 924), (Clause 938), thinsurface course systems (Clause 942) and hot rolledasphalt (Clause 943). Performance is assessed either bytesting samples from the laid material, testing the laidmaterial in-situ or, for proprietary systems, byassessment and approval under the British Board ofAgrément Highway Authorities Product ApprovalScheme (BBA HAPAS).

2.3 Where BBA HAPAS certification isspecified but certificates are not in place, or inEngland, HA type approval has not been given, theapproval of the Overseeing Organisation must besought and a Departure agreed.

Ch

2.4in pavbeeindaccfac

•

•

•

•

•

2.5be nolocgenthethecon

2.6of thaemhoSp

2.7givinfproassnetinf2.2

November 2006

oice of Surfacings

Apart from the suitability of surfacing materialsterms of safety and robustness, the permittedement surfacing options for use on trunk roads, haven determined by the Overseeing Organisations, asicated in Tables 2.2 (E), (S), (NI) and (W), takingount of the variations across the UK of a number oftors:

the nature of the existing network;

population density;

traffic intensity;

climatic conditions;

availability of materials.

The decision on which permitted options are toincluded should be made on a site-specific basis butne should be ruled out without justification. Inations where speeds are limited and tyre/roaderated noise low, or where traffic intensity andrefore the overall noise level is not very great, then full range of suitable surfacings should besidered

Where noise levels are high due to the intensityhigh-speed traffic, surfacing materials are availablet can significantly reduce tyre/road generated noiseission compared to hot rolled asphalt. These includet, paver-laid thin surface course systems,ecification Clause 942 (MCHW 1).

Advice on the different types of surfacings isen in HD 37 and 38 (DMRB 7.5.2 & 3). Althoughormation on various surfacings and treatments isvided in HD 37 and HD 38, it should not beumed that their use is permitted on the trunk roadwork. Advice is provided for certain treatments forormation only. Reference should be made to Table to check permitted options.

2/1



2.8 In England, no surface treatment should beconsidered without taking into account theHighways Agency’s requirements for low-noisesurfacing. Retexturing of existing surfaces is notpermissible without Departure approval. Thisapproval will not be unreasonably withheld forsmall lengths of pavement with a particularskidding or other safety concern.

2.9 The surfacing options permitted shall bethose shown in Tables 2.2E, 2.2NI and 2.2W, forEngland, Northern Ireland and Wales respectively.Where an option is permitted with “DepartureRequired”, a Departure from Standard will berequired from the Overseeing Organisation.

2tkd

2/2

England

For Table 2.2S, for use in Scotland, where anoption is permitted subject to “Approval toProceed”, such approval is required from theOverseeing Organisation.

.10 In Table 2.2, high-speed roads are defined ashose with an 85th percentile traffic speed exceeding 65m/hr. The various pavement construction types areefined in HD 26 (DMRB 7.2.3).

This table is for use

in England

only

All construction types

Use without restriction Departure required

New Yes Thin Surface Course System Porous AsphaltConstructionor No High Yes Thin Surface Course System Hot Rolled AsphaltMajor (minor) speed? Porous AsphaltMaintenance? (85%ile Surface Dressing

above 65 Exposed Aggregate Concretekm/hr) (note 1)

Brushed/Burlap Drag/TinedConcrete (note 1)

No Thin Surface Course System Hot Rolled AsphaltPorous AsphaltSurface DressingSlurry/MicrosurfacingExposed Aggregate Concrete(note 1)Brushed/Burlap Drag/TinedConcrete (note 1)

Note 1: Rigid construction only

Table 2.2E (England): Permitted Pavement Surfacing Materials forNew and Maintenance Construction

November 2006




in Wales

only

Wales



New Yes Thin Surface Course System Hot Rolled AsphaltConstruction Porous Asphaltor Exposed Aggregate ConcreteMajor (note 1)Maintenance?

No High Yes Thin Surface Course System Hot Rolled Asphalt(minor) speed? Porous Asphalt

(85%ile Surface Dressingabove 65 Exposed Aggregate Concretekm/hr) (note 1)

Brushed/Burlap Drag/TinedConcrete (note 1)

No Thin Surface Course System Hot Rolled AsphaltPorous AsphaltSurface DressingSlurry/MicrosurfacingExposed Aggregate Concrete(note 1)Brushed/Burlap Drag/TinedConcrete (note 1)

Note 1: Rigid construction only

Table 2.2W (Wales): Permitted Pavement Surfacing Materials forNew and Maintenance Construction

November 2006 2/3




in Scotland

only

Scotland


Use without restriction Approval to Proceedrequired

New Yes Hot Rolled Asphalt Thin Surface Course SystemConstruction (note 1) Porous Asphaltor Generic SMAMajor Exposed Aggregate ConcreteMaintenance? (note 3)

No High Yes Hot Rolled Asphalt Thin Surface Course System(minor) speed? (note 1) Porous Asphalt

(85%ile Surface Dressingabove 65 Generic SMAkm/hr) Exposed Aggregate Concrete

(note 3)Brushed/Burlap Drag/TinedConcrete (note 3)

No Hot Rolled Asphalt Thin Surface Course System(note 2) Porous AsphaltSurface Dressing Generic SMA

Slurry/MicrosurfacingExposed Aggregate Concrete(note 3)Brushed/Burlap Drag/TinedConcrete (note 3)

Note 1: Not permitted on rigid constructionNote 2: Refer to Overseeing Organisation on rigid constructionNote 3: Rigid construction only

Table 2.2S (Scotland): Permitted Pavement Surfacing Materials forNew and Maintenance Construction

November 20062/4




in Northern Ireland

only

Northern IrelandFlexible and flexible composite


New Yes High Yes Thin Surface Course System Porous Asphalt (note 1)Construction speed? Hot Rolled Asphaltor (85%ileMajor above 65 No Thin Surface Course System Porous Asphalt (note 1)Maintenance? km/hr) Hot Rolled Asphalt Generic SMA

Coated macadam

No High Yes Thin Surface Course System Porous Asphalt (note 1)(minor) speed? Hot Rolled Asphalt

(85%ile Surface Dressingabove 65km/hr) No Thin Surface Course System Porous Asphalt (note 1)

Hot Rolled Asphalt Generic SMACoated macadamSurface DressingSlurry Surfacing

Note 1: not permitted on flexible composite construction

Rigid


New Yes High Yes Exposed Aggregate Concrete Brushed ConcreteConstruction speed? Burlap Drag Concreteor (85%ile Tined ConcreteMajor above 65Maintenance? km/hr) No Exposed Aggregate Concrete Brushed Concrete

Burlap Drag ConcreteTined Concrete

No High Yes Exposed Aggregate Concrete Brushed Concrete(minor) speed? Hot Rolled Asphalt Burlap Drag Concrete

(85%ile Surface Dressing Tined Concreteabove 65 Porous Asphaltkm/hr) Thin Surface Course System

No Exposed Aggregate Concrete Brushed ConcreteHot Rolled Asphalt Burlap Drag ConcreteSurface Dressing Tined ConcreteSlurry Surfacing Porous Asphalt

Thin Surface Course SystemGeneric SMA

Rigid composite


New Yes High Yes Hot Rolled Asphalt Porous AsphaltConstruction speed? Thin Surface Course Systemor (85%ileMajor above 65 No Hot Rolled Asphalt Porous AsphaltMaintenance? km/hr) Thin Surface Course System

Surface Dressing

No High Yes Hot Rolled Asphalt Porous Asphalt(minor) speed? Thin Surface Course System Generic SMA

(85%ileabove 65 No Hot Rolled Asphalt Porous Asphaltkm/hr) Thin Surface Course System Generic SMA

Surface DressingSlurry Surfacing

Table 2.2NI (N Ireland): Permitted Pavement Surfacing Materials for New and Maintenance Construction

November 2006 2/5


Chapter 3Texture and Aggregate Properties

ATE PROPERTIES
3. TEXTURE AND AGGREG
3.1 Friction between the tyre and road surfaceconsists of two main components, both of which arerelated to speed.

a) Sliding resistance between tyre and road surfacewith its magnitude determined by the nature ofthe materials in contact.

b) Loss of energy caused by deformation(hysteresis) of the tyre.

Therefore, during a single braking operation the frictionavailable to the vehicle is not constant.

3.2 In dry conditions all clean, surfaced roads have ahigh skidding resistance. The fine scale Microtexture(see Figure 3.1) of the surface aggregate is the maincontributor to sliding resistance and is the dominantfactor in determining wet skidding resistance at lowerspeeds. Coarse Macrotexture, which provides rapiddrainage routes between the tyre and road surface, andtyre resilience are important factors in determining wetskidding resistance at high speeds. Annex 3 of HD 28(DMRB 7.3.1) contains further discussion of skidresistance and the influence of micro- and macro-texture. Megatexture relates to the roughness of theroad and has no effect on skidding resistance but affectsnoise, (see Chapter 5 of this Part for details).

Figure 3.1: Surface Texture

3.3 The skidding resistance of wet roads is reducedby the lubricating action of the film of water on the wetroad surface. Drainage channels provided by the largescale texture (macrotexture) and/or the pattern on thetyre, assist in getting rid of the bulk of the water and areof increasing importance the higher the speed.

November 2006

Penetration of the remaining water film can be achievedonly if there are sufficient fine scale sharp edges(microtexture) on the road surface on which the tyrecan build up high contact pressures to establish areas of‘dry’ contact between the road and the tyre.

3.4 Aquaplaning is the condition where the vehicletyres are completely supported by a layer of water andthere is no contact with the road surface. High speedand a thick film of water on the road surface encouragea vehicle to aquaplane, but a relatively thin layer ofwater could cause a problem if combined with lowtexture depth and ‘smooth’ tyres. Although aquaplaningitself is not regularly identified, conditions may oftenexist where a high proportion of tyre/road contact islost.

3.5 Because of the effects of weight transfer whenbraking and/or cornering some wheels are likely to skidearlier than the skidding resistance of the road surfacealone indicates. In addition, if brakes are out ofadjustment and hence the distribution of braking efforton the wheels is uneven, the minimum skiddingresistance required to avoid skidding will be increasedstill further, as more of the retarding force will have tobe taken by the wheels of the functioning brakes.

MICRO-TEXTURE

3.6 The micro-texture characteristics of a particularstone depend on its polishing susceptibility under theaction of tyre forces.

Measurement

3.7 The accelerated polishing machine (Figure3.2) is used on aggregates to simulate the polishingaction of traffic. The Polished Stone Value (PSV)test, must be carried out and is specified inBS EN 1097-8:2000. It requires six hours ofpolishing designed to produce a state similar tothat which the aggregate would be subjected tounder actual traffic when equilibrium conditionsare reached.

3/1



Figure 3.2: Accelerated Polishing Machine

3.8 The portable skid-resistance tester (Figure3.3) must used to determine the skid resistancevalue of the aggregate after polishing. This istermed the PSV.

Figure 3.3: Portable Skid Resistance Tester

Ag

3.1parextis uandecoposressucjun

3.1600excspeinccom

3/2

3.9 Aggregate durability must be measured bythe Aggregate Abrasion Value (AAV) test asdefined in Annex A of BS EN 1097-8:2000. TheAAV is a measure of the durability or resistance toabrasion of an aggregate under the action of traffic.

gregate Selection

0 To determine the correct PSV and AAV for aticular site the designer should have regard to theent and scale of the work. When specifying a PSV itndesirable to have too frequent changes of aggregate the aim should be to specify and provide the mostnomical aggregate available over the longestsible lengths. The highest PSV aggregates should betricted to those locations where they are requiredh as on bends and gradients, and at intersections andctions.

3.11 The minimum PSVs to be applied todifferent categories of site and related to trafficflow are given in Table 3.1. The appropriate AAVsare given in Table 3.2. Tables 3.1 and 3.2 refer toboth new works and maintenance and values ofPSV and AAV must be inserted into theappropriate part of Appendix 7/1 of theSpecification (MCHW1). The minimum values ofPSV given in Table 3.1 are the values to be used ifno other information is available. On an existingsite, if the life that has been achieved by theaggregates, the skid resistance and the skiddingaccident rate have all been satisfactory, then thecontinued use of the same aggregate source, albeitwith a lower PSV than that given in Table 3.1 maybe considered. If, however, the measured skidresistance of the site when related to the lifeachieved and the skidding accident rate are belowexpectations for an aggregate from a particularsource, then a higher PSV than that given in Table3.1 may be specified.

2 Although some motorways carry in excess of0 commercial vehicles per lane per day, PSVs iness of those shown in Table 3.1 must not becified. Although minimum PSV values have beenluded for all types of site and traffic level, some

binations are unlikely to occur in practice.

November 2006



0.45 60 65 65 68+ 68+ 68+ 68+ 68+ 68+ HFS

0.5 65 65 65 68+ 68+ 68+ HFS HFS HFS HFS

0.55 68+ 68+ HFS HFS HFS HFS HFS HFS HFS HFS

0.45 50 55 60 60 65 65 68+ 68+ HFS HFS

0.5 68+ 68+ 68+ HFS HFS HFS HFS HFS HFS HFS

0.55 HFS HFS HFS HFS HFS HFS HFS HFS HFS HFS

Minimum PSV required for given IL, traffic level and type of siteSite Site

category description IL Traffic (cv/lane/day) at design life

0-250 251- 501- 751- 1001- 2001- 3001- 4001- 5001- Over500 750 1000 2000 3000 4000 5000 6000 6000

Motorways where traffic is generally 0.30 50 50 50 50 50 55 55 60 65 65A1 free-flowing on a relatively straight

line 0.35 50 50 50 50 50 60 60 60 65 65Motorways where some braking

A2 regularly occurs (eg. on 300m approach 0.35 50 50 50 55 55 60 60 65 65 65to an off-slip)Dual carriageways where traffic is 0.3 50 50 50 50 50 55 55 60 65 65

B1 generally free-flowing on a relatively 0.35 50 50 50 50 50 60 60 60 65 65straight line 0.4 50 50 50 55 60 65 65 65 65 68+Dual carriageways where some braking 0.35 50 50 50 55 55 60 60 65 65 65

B2 regularly occurs (eg. on 300m approachto an off-slip) 0.4 55 60 60 65 65 68+ 68+ 68+ 68+ 68+Single carriageways where traffic is 0.35 50 50 50 55 55 60 60 65 65 65

C generally free-flowing on a relatively 0.4 55 60 60 65 65 68+ 68+ 68+ 68+ 68+straight line 0.45 60 60 65 65 68+ 68+ 68+ 68+ 68+ 68+Gradients >5% longer than 50m as 0.45 55 60 60 65 65 68+ 68+ 68+ 68+ HFS

G1/G2 per HD 28 0.5 60 68+ 68+ HFS HFS HFS HFS HFS HFS HFS0.55 68+ HFS HFS HFS HFS HFS HFS HFS HFS HFS

K Approaches to pedestrian crossings 0.5 65 65 65 68+ 68+ 68+ HFS HFS HFS HFSand other high risk situations 0.55 68+ 68+ HFS HFS HFS HFS HFS HFS HFS HFSApproaches to major and minorjunctions on dual carriageways and

Q single carriageways where frequent orsudden braking occurs but in agenerally straight line.

R Roundabout circulation areas 0.45 50 55 60 60 65 65 68+ 68+ HFS HFS

0.5 68+ 68+ 68+ HFS HFS HFS HFS HFS HFS HFS

Bends (radius <500m) on all types ofS1/S2 road, including motorway link roads;

other hazards that require combinedbraking and cornering

Notes:1. Site categories are grouped according to their general character and traffic behaviour. The Investigatory Levels (IL) for

specific categories of site are defined in HD 28 (DMRB 7.3.1). The IL to be used here must be that which has beenallocated to the specific site on which the material is to be laid, as determined by following the procedures in HD 28.

2. Motorway or dual carriageway slip roads may fit in a number of groups depending on their layout. For example, a free-flowing section close to the main line would be in Group 1 whereas the end of an off-slip approaching a give way line orthe point at which a queue develops would be in Group 3. Some slip roads with gradients may be in Group 4. Use themost appropriate Group depending upon the Site Category from HD 28 that was used to determine the IL.

3. Where ’68+’ material is listed in this Table, none of the three most recent results from consecutive PSV tests relating tothe aggregate to be supplied must fall below 68. See paragraph 3.21.

4. Throughout this Table, HFS means specialised high friction surfacing, incorporating calcined bauxite aggregate andconforming to Clause 924 of the Specification (MCHW 1) will be required. Where HFS is required on the approachesto a hazard, the minimum treatment length must be 50m. This may be extended where queuing traffic or sightlinesindicate that 50m may not be sufficiently long.

5. For site categories G1/G2, S1/S2 and R any PSV in the range given for each traffic level may be used for any IL andshould be chosen on the basis of local experience of material performance. In the absence of this information, the valuesgiven for the appropriate IL and traffic level must be used.

6. Where designers are knowledgeable or have other experience of particular site conditions, an alternative psv value canbe specified.

Table 3.1: Minimum PSV of Chippings, or Coarse Aggregate in Unchipped Surfaces, for New Surface Courses

November 2006 3/3



1001- 1751- 2501- >32501750 2500 3250

12 10 10 10

14 14 12 12

Traffic (cv/lane/day) at <250 251-design life (see 3.15) 1000

Max AAV for chippings 14 12for hot rolled asphalt andsurface dressing, and foraggregate in slurry andmicrosurfacing systems

Max AAV for aggregate 16 16in thin surface coursesystems, exposedaggregate concretesurfacing and coatedmacadam surface course

Note 1: For roads carrying less than 1750 cv/lane/day, aggrehas shown that satisfactory performance is achieved

Note 2: The maximum AAV requirement for porous asphalt(MCHW 1).

Table 3.2: Maximum AAV of Chippings, or Cofor New Surface

3.13 The PSVs in Table 3.1 are related to the IL fordifferent traffic flows set out in Chapter 4 of HD 28(DMRB 7.3.1). A margin of safety has been added foreach of the following reasons:

a) to allow for variability of aggregates, theprecision of the PSV test and variations inestimating traffic flows;

b) to allow for turning movements and traction/braking forces at junctions, on bends and ongradients;

c) where possible, to ensure that the skiddingresistance achieved on trunk roads does not fallbelow the requirements within the lifetime of thesurfacing. This avoids frequent maintenance onhigh speed and other trunk roads with consequenttraffic delays.

3.14 Using the appropriate PSV for a particular siteand traffic loading should result in a surfacing givingsatisfactory performance before reaching theinvestigatory level of Characteristic SCRIMCoefficient. See Chapter 4 of HD 28 (DMRB 7.3.1).

3/4

gate of higher AAV may be used where experience by an aggregate from a particular source.

is specified in Clause 938 of the Specification

arse Aggregates in Unchipped Surfaces, Courses

3.15 The traffic flow used to determine theappropriate PSV and AAV for a particularsurfacing must be the maximum volume of trafficmeasured as commercial vehicles per lane per day(cv/lane/day) based on the Average Annual DailyFlow (AADF) predicted to be using the lane at theend of the anticipated life of the surfacing – seeHD 24 (DMRB 7.2.1). Estimates of traffic growthrates and life of the surfacing may be based onlocal experience.

3.16 The same levels of PSV and AAV must beused on different traffic lanes across thecarriageway and in the hardshoulder except that,where aggregates are used for demarcation, amaximum difference of 5 PSV points may beallowed.

3.17 The PSVs given in Table 3.1 apply to roadsconstructed within current design standards, andwill provide satisfactory skid resistance on sites ofaverage difficulty requiring the given investigatorylevel within the general site group for the life ofthe surfacing.

November 2006



3.18 For site categories G1/G2, S1/S2 and R a rangeof ILs and corresponding PSVs for each traffic level isgiven in Table 3.1. For sites in these groups, the PSV tobe specified should be based upon local experience ofmaterial performance. For maintenance resurfacing, thecurrent skid resistance in relation to the life achieved,the investigatory level and the skidding accident rateshould be considered. If satisfactory, the PSV and AAVof the new surfacing should be the same as theaggregates used previously. If consideredunsatisfactory, the PSV must be increased to that for ahigher IL within the range given for the appropriatetraffic level. For new construction, existing sites withsimilar traffic flows, IL and site geometry should beused to assist in determining the initial values of PSVand AAV to be specified. In the absence of any suchsuitable information, the values given for theappropriate IL and traffic level must be used.

3.19 The actual PSVs, AAVs and texture depthsbuilt into schemes of new construction and theassumptions on which the minimum values wereselected must be recorded and maintained in areadily available form, (eg. the schememaintenance manual). Standards to be adopted insubsequent renewal work may then be determinedin the light of the skidding resistance performanceset against those initial recorded values.

3.20 The requirements of Tables 3.1 and 3.2cover:

a) chippings for surface dressing;

b) the coarse aggregate in thin surface coursesystems, porous asphalt, bitumen macadamsurface courses and surface courses of rolledasphalt without coated chippings applied tothe surface;

c) coated chippings applied to the surface ofrolled asphalt, to mastic asphalt and to finegraded macadam;

d) coarse aggregate in slurry surfacing andmicrosurfacing systems; and

e) the coarse aggregate in non-surface dressedbinder courses of bitumen macadam or stonemastic asphalt and bases of bitumenmacadam or rolled asphalt used astemporary surfaces by general traffic forprolonged periods and not subject to speedrestrictions or without warning signs.

November 2006

3.21 Samples of the aggregate representative ofthose to be incorporated into the Works must betested in accordance with BS EN 1097-8 forcompliance with the specified PSV and AAVproperties. Alternatively, except where a PSV of68+ is specified, the aggregate must be deemed tocomply if the mean of the three most recent resultsfrom consecutive tests, relating to the material tobe supplied, is greater than or equal to thespecified PSV and less than the specified AAV.Where a PSV of 68+ is specified, none of the threemost recent results from consecutive tests shall beless than 68. Tests must have been carried out inthe previous six months by a laboratory accreditedby UKAS or equivalent for these tests or by alaboratory in a Member State of the EuropeanEconomic Area or a State which is party to arelevant agreement with the European Union thatcan demonstrate suitable and satisfactory evidenceof technical and professional competence andindependence for such tests. The latter requirementmust be satisfied if the laboratory is accredited in aMember State of the European Economic Area or aState which is party to a relevant agreement withthe European Union in accordance with therelevant parts of EN45000 series of standards forthe tests carried out.

3.22 It is essential that the aggregate supplied tosite must be the same in all respects to the samplesubmitted for acceptance. If it is considered thatthere is a change in the material delivered to site,further tests must be ordered.

3.23 There are few quarries that can supply aggregatewhere PSV is consistently over 68, together with amaximum AAV of 10. In order to achieve values inexcess of this, it is necessary to specify a high frictionsurface treatment as described in Clause 924 of theSpecification (MCHW1). Although highly skidresistant, material complying with Clause 924 is unableto meet the requirement of a texture depth of 1.5mm(measured by the volumetric patch test). Therefore, onhigh speed roads, this type of material must only beused where strictly necessary, eg. for braking sectionsand tight curves. When such materials are to be used onhigh speed roads, attention must be given to the need todrain water off the surface by profiling or by othermeans.

3/5



3.24 The PSV of 70+ is considered to be the highestpractical level that can be consistently achieved usingartificial aggregate such as calcined bauxite. Forheavily stressed sites the use of a small size, hardaggregate with a PSV of 70+ effectively increases theinitial skidding resistance provided and thereby extendsits ‘life’, ie. the period before the investigatory level isreached. This effective increase in skidding resistancealso increases the stress on the chippings, hence thenecessity to use a binder modified with an epoxy or asimilar resin. Advice is given in Chapter 9 of HD 37(DMRB 7.5.2) and also in Series NG 900 of the Notesfor Guidance to the Specification (MCHW2).

3.25 To decide whether a high PSV stone should beused for renewing a surface, consideration should begiven to the PSV and AAV of the existing aggregate inrelation to the life achieved, the current skid resistanceof the surface and the skidding accident rate of the site.If all are satisfactory, the use of stone from the samesource and of the same PSV may be appropriate. Whererecords of PSV and AAV are not available,identification of the source of an aggregate may enablevalues that are sufficiently accurate for assessmentpurposes to be estimated.

MACRO-TEXTURE

3.26 Adequate macro-texture is required for the rapiddrainage of surface water from the tyre and roadpavement interface thereby reducing the chance ofaquaplaning. The texture depth is a measure of themacro-texture and is an important factor influencingskidding in wet conditions on high speed (>65km/h)roads.

3.27 Surface texture takes two forms:

a) ‘positive’ texture: a cluster of angular peaks orseries of ridges above a datum level, typical ofsurface dressings, hot rolled asphalt with chips,slurry and microsurfacings and brushed concrete;

b) ‘negative’ texture: a network of depressions orseries of grooves below the general level, typicalof thin surface course systems, and porousasphalt.

3.28 Ideally, choice of an appropriate texture depthwould be made on the basis of values related toaccident occurrence that could then become part of amaintenance policy. However, further research isrequired into this area and until such results areavailable, the approach is to specify minimum levels of

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re depth for new higher-speed roads to apply attruction or major maintenance. This is given ins 900 of the Specification (MCHW1).

For speeds in excess of 65km/h, the texture depthe surface should be that required by theification (MCHW1). This will ensure that theease in skid resistance that occurs at higher speedsinimised and will facilitate the rapid drainage ofr from the road surface. At lower speeds, textureh is less important and compliance with the moreral specification requirements or with specified of spread of chippings should be sufficient. Withr speed roads, micro texture is the major factor intaining skid resistance, although texture depth is

important. In bituminous and exposed aggregaterete roads, micro texture is provided by the use of ace aggregate with a specified resistance tohing given by the PSV.

surement

For many years texture depth has been measurede volumetric patch method in which a knownme of solid glass spheres or sand is spread into alar patch. The diameter of the patch is measured

the average depth under the peaks in the surfacelated. The technique is described inN 13036-1:2002.

More recently, laser-based techniques haveme available which determine the texture depthit by a different methodology.

Measurement of Texture at High Speed

It is possible to measure texture indirectly usings and reflected light at speeds up to 100km/h. Thisod has been incorporated in a number of devicesding the SCRIMTEX, the Highways Agencyrch tool HARRIS (Highways Agency Roadarch Information System: Figure 3.5) and Trafficd Condition Surveys (TRACS).

November 2006



Figure 3.5: Highways Agency Road Research Information System (HARRIS)

November 2006 3/7


OISE

Chapter 5Tyre/Road Surface Noise

5. TYRE/ROAD SURFACE N

GENERAL

5.1 Noise from road traffic has become, over the lastfew years, a very contentious environmental issue.Where traffic speeds are lower than 50 km/hr, trafficnoise is mainly attributable to engine, transmission andexhaust noise, especially from lorries. Where speeds arehigher, the major component of traffic noise comesfrom the tyre/road interface. This noise comes from,amongst other things, vibration of the tyre wall,compression of air within the contact area of the tyre,and the snapping out of the tread blocks as they leavethe road surface. The quality of the road surface, tyredesign and vehicle speeds all have an effect on tyrenoise.

5.2 Details of the available low noise surfacings andwhere they may be used is given in Chapter 2 of thisPart.

5.3 For many years it has been the UK practice toensure that there are interconnecting drainage pathswithin the surface over which the tyre runs to helpdisperse water and improve skidding resistance,particularly at high speeds. It was also recognised thatthe coarseness of the surface contributes to traffic noise.This coarseness has traditionally been measured by thesand patch test, which gives the average depth oftexture over an area similar to the contact patch of atyre.

Definitions of Texture Depths

5.4 It is now recognised that there are a number offactors within the road surface texture that playsignificantly different roles in improving skiddingresistance and generating noise. It is helpful todistinguish different scales of texture by defining theroles of the texture ranges as follows:

i) The fine scale microtexture of the surfaceaggregate is the main contributor to skiddingresistance and is the dominant factor indetermining skidding resistance at lower speeds.

ii) Macrotexture provides rapid drainage routesbetween the tyre and the road surface andcontributes to the wet skidding resistance athigher speeds. It also allows air trapped beneaththe tyre to escape.

November 2006

iii) Megatexture at a scale comparable with the tyrecontact patch is mainly associated with tyrenoise. Surfaces with high mega-texture includeHRA with gaps between groups of chippings andthe old-style cobbled surfacings.

iv) Unevenness in the longer ranges cause large tyreand suspension movements that affect thehandling of vehicles.

5.4 Fig 5.1 shows the difference between micro-,macro-, and mega-texture lengths and depths.

5.5 The texture depth is the average deviation of aroad surface from a true planar surface within anycategory of texture. It is represented at the road surfaceas:

i) Microtexture

Describes the roughness of the surface aggregate, whichis associated with the crystalline structure of the coarseaggregate and the sand particles in the surface laitanceof a brushed concrete surface.

ii) Macrotexture

Represents the height above a road surface of theaggregate chipping (eg. for HRA, surface dressing andbrushed concrete), or the depth of texture below theroad surface (eg. for porous asphalt, thin surfacings,tined and exposed aggregate concrete surfaces,(EACS)).

iii) Megatexture

Represents the degree of smoothness of the surface.

iv) Unevenness

Describes amplitudes of longer wavelengths, whichaffect vehicle suspensions.

5/1


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Figure 5.1: Details of Textu

.6 The effect of texture on noise and skidding isiven in Table 5.1.

.7 With recently developed laser based equipmenthe depth of the texture can be measured separatelyithin each texture range. The objective of modern

urfacing techniques is to reduce the depth of texture inhe megatexture range as much as possible, whileetaining an adequate depth of macrotexture to provideigh speed skidding performance. Low speed skiddingerformance is mainly controlled by the microtexture.he interrelationship of the effects of different types of

exture on skidding resistance and noise generation arehown in Figure 5.2.

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/2

re Length and Depths

ositive and Negative Texture Depths

.8 An important difference between surfaces, whichas a strong effect on noise generation, is the degree tohich the surface aggregate particles protrude abovee plane of the tyre contact patch. Surfaces that arermed by rolling aggregate chippings into the softrface of an underlying matrix during construction are

escribed as positive texture. Those in which theggregate chippings are embedded at the surface withine matrix, leaving voids that are generally below the

lane of the contact patch, are described as having aegative texture. For the same texture depth the latterenerate much less tyre noise. Brushing concrete roadrfaces also produces a positive texture but this

process may, unless care is taken, build up unwantedmegatexture depths (see paragraph 5.20 for furtherdetails).

November 2006


Texture depth/ Skidding resistancenoise properties

Little noise contribution Low speed

Deep texture = High speedlow noise

Low texture = High speedlow noise

Low texture = High speedlow noise

Suspension noise Ride quality/handling


Range Texture length (mm)

Micro texture <0.5

Macrotexture 0.5-10

10-50

Megatexture 50-500

Uneveness >500

Table 5.1: Contribution of Textur

5.9 Positive and negative texture types are shown inFig 5.3. Hot rolled asphalt, surface dressing andbrushed concrete surfaces are generally considered tobe positively textured whereas porous asphalt, thinsurfacing and exposed aggregate concrete surfaces aregenerally considered to be negatively textured.

Aggregate Shape

5.10 The shape of the aggregate particles that areprovided at the surface to provide for skiddingresistance also can have an effect on noise. Particles ofa more cubical nature with a lower flakiness index packbetter into the surface to provide a flatter area on whichthe tyre can run. At a detailed level it can be seen thatthe tyre contact is spread more evenly over the contactarea, which in itself reduces the apparent contact patch.Conversely a rougher surface increases the contactpatch, which exacerbates tyre/road noise as the noise isrelative to the length of the escape path for the trappedair.

November 2006

e Depth to Noise and Skidding

5.11 Road surfaces with negative textures, providedthere is sufficient interconnection between the voidsbelow the running surface, reduce the amount of noisegenerated by reducing the air pressures within thecontact area. At high speed the compression and releaseof air trapped under the tyre is a significant componentof tyre noise.

5.12 These observations can be translated intopractical advice for the design and construction of roadswith lower noise surfaces. Advice for controllingtexture ranges is given in paragraphs 5.13 to 5.21.

Microtexture

5.13 The amplitudes of microtexture for bituminoussurfaces and EACS come from the roughness of thesurface of the coarse surfaces, the microtexture comesfrom the fine aggregate (sand). High amplitudes ofmicrotexture have a minimal, if any, effect on the tyre/road noise, but provide low speed skidding resistance.

5/3



Figure 5.2: Effects of Texture Depth on Skidding and Noise

Figure 5.3: Details of Surface with Positive or Negative Texture

November 20065/4



Macrotexture

5.14 Macrotexture amplitudes on surface dressed andHRA roads come from the space between individualstones. This is a factor of the size and the evenness ofthe stones on the surface of the road. With PA, thinsurfacings and EACS, the macrotexture depths aredependant on the shape of the aggregate at the surfaceand the voids between adjacent stones. The voidsbetween the stones allow the air and water beneath thetyre to dissipate rather than be trapped. The cubicnature of stone with a low flakiness index enables aflatter surface of stone to be presented at the roadsurface with the benefits outlined in paragraph 5.10.Water trapped between the tyre and the road causesaquaplaning at high speed and trapped air causes noisewhen the pressure is released. At larger lengths ofmacrotexture vibrations in a tyre wall, which are asignificant cause of tyre noise, are excited. The ideal isto produce a macrotexture with high depths in the 0.5 to10mm lengths and low depths in the 10 to 50mmlengths.

5.15 The texture of traditional concrete roads isformed by transverse brushing the surface of theconcrete while it is still plastic. The aim is to producean even texture without occasional transverse ridges.The bristles form the macrotexture during thetransverse brushing operation. Brushing when theconcrete surface is either too wet, the brush pressure isincorrect or the bristles are of an inappropriate stiffnesscan form too deep a texture depth. A mix that has lostits workability or brushes that are clogged with mortarcan produce a shallow texture.

5.16 With EACS, porous asphalt and thin surfacings,the macrotexture is a function of the packing and size ofthe surface aggregate. A low flakiness index is specifiedto obtain more cubic aggregate that packs closelytogether to produce small voids. In the UK, a 10 to6mm coarse aggregate with a 1.5mm texture depth hasbeen selected to provide adequate skidding resistance.In Austria, an 8mm maximum sized aggregate was usedto reduce the macrotexture in the texture range >10mm,and achieve good noise reducing properties. An 8 to4mm sized aggregate with a 1.0mm texture depth isrecommended for lower speed roads (90km/hr) wherethe risk of aquaplaning is less than for high speed roads.

5.17 Porous asphalt and some thin surfacings havevoids that interconnect with the surface. The voidspermit water to drain to below the running surface ofthe road thereby giving these surfaces their sprayreducing qualities. Noise entering these voids is tosome extent trapped within the voids. The untrapped

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November 2006

oise tends to be in the lower frequencies that givehese surfaces their more distinct lower tonal qualities.hese surfaces tend to reduce both tyre/surface noisend engine/transmission noise.

.18 With voided surfaces the sand patch test does notive a true indication of the surface texture, or itsotential lower noise properties, due to the sand partlyntering the voids. The texture is better assessed bysing close proximity laser based systems to determinehe profile at the tyre contact surface. It has been foundhat the noise increases as the hydraulic conductivityeduces, indicating that the less porous surfaces giveigher noise levels. The test for hydraulic conductivityives an indication of the noise reducing properties oforous surfaces.

egatexture

.19 It has been found that high megatexture depthsause a tyre wall to deflect and vibrate under load. Thiss a major cause of tyre/road noise. Megatexture onRA surfaces comes from gaps between the groups of

hippings. This can be caused by the way the chipperpreads the chippings. The chipper dispenses chippingss a series of transverse bands, with the possibility ofaps between those bands. These gaps are often in theigh macrotexture to megatexture ranges (> 10mm). IfRA surfaces are allowed to cool excessively, such that

he chippings are not properly embedded, high depthsf macrotexture and megatexture can result.

.20 Concrete surfaces laid with a slipform or fixedorm paver may have megatexture undulations causedy the paver. These arise from the natural irregularitiesf the paver method of working. There are slightertical movements in the surfacing whenever the pavertops and starts, or the machine compensates for levelhanges. The vertical movements of the transverseinishing screed combined with the forward movementf the paver can cause regular depressions in theegatexture range. These can be reduced by the

ongitudinal oscillating float (super smoother) whichives the surface a final smoothing.

onclusion

.21 When examining the causes of tyre/road noise it ismportant to be aware of the various interacting factors.he aggregate at the surface makes a significantontribution to both the skidding and noise performancef the road. The construction techniques, that are underhe control of the contractor, also provide a majorontribution to the safety and the tyre/noise generated

by the surface.

5/5


Chapter 6References

IOGRAPHY
References

Design Manual for Road and Bridges (DMRB): TheStationery Office (TSO)

(http://www.official documents.co.uk/document/deps/ha/dmrb/index.htm gives access to the HighwaysAgency’s DMRB documents)

HD 24 Traffic Assessment (DMRB 7.2.1)

HD 26 Pavement Design (DMRB 7.2.3)

HD 28 Skidding Resistance (DMRB 7.3.1)

HD 29 Structural Assessment Methods(DMRB 7.3.2).

HD 31 Maintenance of Bituminous Roads(DMRB 7.4.1)

HD 32 Maintenance of Concrete Roads(DMRB 7.4.2)

HD 37 Bituminous Surfacing Materials andTechniques (DMRB 7.5.2)

HD 38 Concrete Surfacing and Materials(DMRB 7.5.3)

Manual of Contract Documents for Highway Works(MCHW): The Stationery Office (TSO)

(http://www.archive2.official-documents.co.uk/document/deps/ha/mchw/index.htm gives access to theHighways Agency’s MCHW Volumes 1 and 2documents)

Volume 1: Specification for Highway Works (MCHW1)Volume 2: Notes for Guidance on the Specification forHighway Works (MCHW2)

2000

BS EN 1097-8: Tests for mechanical and physicalproperties of aggregates - Determination of the PolishedStone Value (PSV), BSI.

November 2006

2002

BS EN 13036-1: Road and airfield surfacecharacteristics - Test methods - Measurement ofpavement surface macrotexture using a volumetricpatch technique, BSI.

Bibliography

1970

Road Note 27; “Instructions for using the Portable SkidResistance Tester”, HMSO.

1972

Szatkowski W. S. and Hosking J. R., “The Effect ofTraffic and Aggregate on the Skidding Resistance ofBituminous Surfacings”, LR504, TRRL.

1976

Hosking J. R. and Woodford G. C., “Measurement ofSkidding Resistance: Part II, Factors Affecting theSlipperiness of a Road Surface”, LR739, TRRL.

1998

Roe, P.G. and Hartshorne, S.A., “The Polished StoneValue of Aggregates and In-service SkiddingResistance”, TRL Report 322.

Roe, P.G., Parry, A.R. and Viner, H.E., “High and LowSpeed Skidding Resistance: the Influence of TextureDepth”, TRL Report 367.

1991

Roe, P. G., Webster, D. C. and West, G., “The RelationBetween the Surface Texture of Roads and Accidents”,RR296, TRRL.

1992

Roe, P. G., “Measurement of the Macrotexture ofRoads: Part 3 Development of the Highspeed TextureMeter”, RR297, TRRL.

6/1


November 2006 7/1

7. ENQUIRIES



Chief Road EngineerTransport ScotlandVictoria QuayEdinburgh J HOWISONEH6 6QQ Chief Road Engineer



Chapter 7Enquiries

May 1999



SECTION 5 SURFACIN G ANDSURFACING M ATERIALS

PART 2

HD 37/99 AMENDMENT NO 1

BITUMINOUS SURFACING M ATERIALSAND TECHNIQUES

SUMMA RY

This amendment is a correction to Chapter 8, Table 8.1Areas of use for surface dressing binders.


1. Remove existing page 8/8, dated February 1999,from Volume 7, Section 5, Part 2, Chapter 8 andarchive as appropriate.

2. Insert page 8/8 dated May 1999.

3. Enter the details of the amendment on theRegistration of Amendments sheet, sign and dateto confirm the amendment has been incorporated.



February 1999




PART 2

HD 37/99


SUMMA RY

This Advice Note gives advice on the suitability,specification, laying and testing of bituminous surfacecourses. It supersedes HD 37/97 and also Chapters 4and 5 of HD 27/94, revised versions of which areincorporated into Chapters 2 and 5 of HD 37/99.


1. Remove HD 37/97 from Volume 7, Section 5,Part 2, which is superseded by HD 37/99, andarchive as appropriate.




HD 37/99

Design Manual for Roadsand Bridges

Volume 7 : Pavement Designand Maintenance

Bituminous SurfacingMaterials an d Techniques






Summary: This Advice Note gives advice on the suitability, specification, laying andtesting of bituminous surface courses. It supersedes HD 37/97,HD 27/94, Chapters 4 & 5, and HD 28/94, Chapter 2.


February 1999







PART 2

HD 37/99


Contents

Chapter

1. Introduction

2. Laying Bituminous Surface Courses

3. Binders and Binder Modifiers

4. Hot Rolled AsphaltAnnex A: Method for Determination of Loss of

Chippings and Proportion of BrokenChippings

5. Porous Asphalt

6. Thin Wearing Course Systems

7. Stone Mastic Asphalt

8. Surface Dressing

9. High Friction Surfacing

10. Slurry Surfacing and Micro-surfacing

11. Retexturing (Bituminous)


13. Miscellaneous Surface Courses and Treatments

14. References

15. Enquiries


February 1999


February 1999

1. INTRODUCTION

1/1


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General

1.1 This Part gives advice on the suitability,specification, laying and testing of bituminous surfacecourses. It covers all materials using bituminous binderranging from veneer coats like surface dressing andslurry surfacing to heavy duty surfacing material such ahot rolled asphalt. It also includes high frictionsurfacing, retexturing of bituminous materials andrecycling using the Remix and Repave processes.

Implementation

1.2 This Part shall be used forthwith on allschemes for the construction, improvement andmaintenance of trunk roads including motorways,currently being prepared, provided that, in theopinion of the Overseeing Organisation, this wouldnot result in significant additional expense or delay.Design organisations should confirm its applicationto particular schemes with the OverseeingOrganisation.

Types of Bituminous Surfacing Materials

1.3 There are essentially three basic types ofpremixed bituminous material. Those with:-

• High stone content, full coarse aggregateinterlock, and a gap grading (eg SMA);

• A continuous grading (eg DBM);

• A rich mortar and a gap grading where the coarseaggregate does not interlock (eg HRA);

Intermediate materials exist such as a high stone contehot rolled asphalt with crushed rock fines, which fallsbetween the last two types. Slurry surfacing, andparticularly micro-surfacing, normally have acontinuous grading. Single chipping surface dressingdoes not fall into any of these classifications, but multi-layer dressings (like racked in, double and sandwichdressings) can be considered as a type of high stonecontent material with full coarse aggregate interlock.

igh Stone Content Gap-graded Materials.4 High stone content materials with aggregate

nterlock can be divided into two main types:

Those with very little or no fine aggregate(eg Porous Asphalt - PA), and

Those in which the voids between the coarseaggregate particles are largely filled with a masticof fine sand, filler and binder, often reinforcedwith fibres or polymer (eg a stone mastic asphalt SMA).

.5 Many of the new thin surfacing materials fall intohis second type. The high strength of these materials isroduced by the stone skeleton and they are inherentlyesistant to rutting. Because the stone skeleton is somportant the aggregates used in these materials must trong and hard, and must have a consistent shape andrading, or the final mix will vary in properties.

.6 On roads where the width varies they are best laidsing a paver with a variable width screed as they cane difficult to lay by hand and the overall grading needslose control. When properly compacted, denseaterials such as SMA can be very durable, because a

oids are low and the binder film thickness is generallyigh. If the air void content is too low however,chieving and retaining surface texture can beroblematic and the SMA can be prone to deformation.

.7 For long life, porous materials depend on theontinuing integrity of the binder film. There are twoain factors operating, adhesion of the binder to theggregate and weathering of the binder. Some aggregatesave a better affinity for bitumen than others and where

here is doubt, (eg with quartzites), then adhesion agenthould be used. The weathering of the binder and theeans of overcoming the problem are dealt with in moreetail in Chapter 5 of this Part.

oated Macadams or Asphaltic Concretes.8 Materials with a continuous grading are oftenalled asphaltic concretes or macadams. The currentEN name for all these types of material is asphalticoncrete. All the dense and close graded macadams inS 4987 fall into this group. Macadams are not

generally used as surface courses on trunk roads as thsuffer from a number of disadvantages which areoutlined in Chapter 13 of this Part.

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1.9 The mixes are ‘designed’ to give a denseaggregate matrix, often based on the ‘Fuller’ curve.However this means that the voids in the mineralaggregate (VMA) are low, and in order to avoid fillingall the voids with binder (making an unstable materialprone to deformation) the binder content must be keptrelatively low. This results in a relatively thin binder filmthickness, which reduces its durability. However a thinbinder film is not generally a problem in layers protectedfrom the weather, which is why materials of this typemake excellent load spreading layers in the roadbase anbasecourse.

1.10 Materials in this group are sensitive to variationsin grading and binder content and production will bemonitored by the supplier under the QA Sector schemeto ensure consistency. They generally have lowworkability and therefore need good compaction ifdurability is to be achieved. When properly compacted,the texture depth obtainable is very low making themunsuitable as surfacing for high speed roads.

Hot Rolled Asphalt1.11 Low stone content gap graded materials arenormally referred to as hot rolled asphalt and arecovered by BS 594. They usually have a high bindercontent and low voids. They depend for their rutresistance on the sand/filler/binder matrix and, ofnecessity, the binders are hard (low penetration grade).

1.12 With the requirements for higher stability andretention of high texture depth, asphalts have tended tobecome less workable, with lower binder contents, andhence are less forgiving of poor workmanship. Thisoften shows up initially as fretting at joints, and jointsealing within the first few years of the life is notunusual. The causes of joint fretting are low bindercontent or over-raking, and poor compaction. It istherefore important that asphalts are adequatelycompacted up to the edge and that this compactionshould be monitored.

1.13 High stone content hot rolled asphalt has anominal stone content of 55%, and no chippings arerolled into the surface.

Properties of Bituminous Surface Courses

1.14 None of the surface course materials will provideall the desirable requirements for all situations. Asurface course should be the best compromise betweenthe various available properties. The main propertiesrequired are:

a) durability,b) resistance to deformation,

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load spreading ability, texture depth, skidding resistance, low noise generation the ability to keep water out of the pavement

structure, resistance to cracking.

hese properties and others are discussed below.

nvironmental durability.15 This is the ability of a material to resist changes its properties caused by environmental effects (suchs rain, sun, frost, thaw, temperature changes, oxidationnd also by contaminants deposited on the materialuch as oil, mud and animal detritus). Long exposureill affect binders and the properties of the mixture, anay affect the aggregate. Durability may be assessed easuring the changes in the engineering properties oe material with time.

.16 The durability of a mixed material depends onither its ability to keep the weather out, if it is intended be a dense material, or its ability to resist the weathr,it is permeable. A dense material should have a voidntent of less than about 6 % in-situ. The design voidntent will not be achieved without proper control ofixing, placing and compaction. The durability ofpen-graded and porous wearing courses havingterconnected voids, (which permit the ingress and flof air and water), depends on the thickness of the bindm on the aggregate; the susceptibility of the binder toxidation; and the long term adhesion of the film to theggregate. The durability of surface dressing and somf the bonded materials depends primarily on gooddhesion to the underlying road structure. However witrface dressing, the use of dusty chippings may induc

ond failure between the chippings and the binder in thort term.

esistance to deformation.17 This is important in all layers of a bituminousad, but the need is greater for a surface course

ecause:

the surface of a road gets hotter and the bitumenbecomes softer compared to the lower layers;

the stresses generated by traffic are greatest at thesurface.

usceptibility to rutting can be measured by the wheelacking test given in BS 598 Part 110. With the veryeavy traffic being carried on major roads it is alsoecessary for the basecourse, (ie the layer immediatel


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below the surface course), to be designed for resistancto deformation.

1.18 Bitumens harden over the first few weeks afterlaying (steric hardening) and therefore a surfacingmaterial is more prone to rutting during its very earlylife. Where heavy channelised traffic on new surfacing islikely to occur during the hottest period of the year itmay be necessary to use a material with an enhancedresistance to deformation.

Load spreading ability1.19 This is assessed by measuring its stiffness. Thedesign charts in HD 26 (DMRB 7.2.3) assume aminimum stiffness for the materials that will be used inthe construction of a new road. Where only a surfacecourse is being applied in a maintenance situation theability of the material to spread load is not considered tbe a major factor. In new construction, or where astructural overlay is being added, then an assessment the load spreading ability is required.

1.20 For maintenance resurfacing the structuralcontribution of the surface course may be ignored:

• for materials with a nominal layer thickness ofless than 15 mm;

• for open graded British Standard materials; and

• for proprietary materials where the supplier or theContractor has not provided the necessaryinformation.

For materials with a nominal layer thickness of 15 mmor more and where the supplier or Contractor hassupplied the necessary information, an assessment cabe made of the contribution. For thin surfacing systemsgeneral advice on the structural contribution is given inChapter 6 of this Part.

Texture depth and skidding resistance1.21 These should be specified as described in HD 36(DMRB 7.5.1) and the standards required should beapplied in all situations. For surface dressing to Clause922 and thin surfacings to Clause 942, the Specificatio(MCHW1) requires texture depth to be maintained at oabove the levels specified, at least until the end of theguarantee period, currently two years, to ensure asatisfactory overall life is achieved.

Surface noise1.22 This can be a problem in some situations.Surfacing layers with ‘negative’ texture such as thinsurfacings and stone mastic asphalt are quieter thanconventional chipped hot rolled asphalt by 2 to 3 dBA o

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more. It should be noted that tyre/surface noisegeneration is more of a problem at high speeds. At lowspeeds engine and transmission noise are dominant.Porous asphalt - when newly laid - is currently thequietest material, with a reduction in noise (compared new HRA) of about 4 to 5 dBA, the voids absorbingsound. However this advantage reduces as the pores with detritus, giving a relative reduction of about 3dBAsimilar to some thin surfacings. Further information onsurface noise is given in HD 36 (DMRB 7.5.1)

Adhesion1.23 The adhesion of a surface course to the underlypavement structure is essential, particularly so withsurface dressing and very thin materials. These are nothick enough to carry traffic induced stresses withoutexcellent adhesion. Bond is even more important wherthere is a possibility of high braking and lateral stresseStructural strength is only fully developed when all thelayers in the pavement are well bonded, effectivelyforming a single layer. (Refer to Chapter 2 of this Partfor information on Bond or Tack Coats)

Waterproofing1.24 Sealing the surface of a pavement assists inprolonging its life. This is usually achieved in one of twways. Either the bond coat is heavy enough tosignificantly reduce the permeability of the underlyinglayer, (eg. surface dressings, some thin surfacings andporous asphalt), or the material itself is dense enoughprevent or seriously impede water draining into the roastructure. Normal tack coats to BS 594 or BS 4987should not be assumed to greatly improve thewaterproofing characteristics of the road.

Workability1.25 This is necessary to achieve the intendedproperties of the surface course after laying andcompaction. For example, workability is important in ahot rolled asphalt surface course, when chippings havto be applied to the surface. Inadequate workability wilead to poor compaction and chip retention, particularlin adverse weather conditions. Workability is alsoimportant in thin surfacings to achieve the necessaryaggregate interlock, and rapid heat loss in cold weathecan result in poor compaction.

Resistance to cracking1.26 Cracking of surfacing materials is caused by acombination of factors:

• thermal movement,

• repeated traffic loading and induced strain, and

• embrittlement of binder due to ageing.


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1.27 Cracking has often been a problem associatedwith composite road construction, where thermalmovement is concentrated above widely separatednaturally forming cracks in the underlying cement boundmaterial. Techniques introduced to induce cracks atcloser spacings may alleviate this problem.

1.28 The fatigue characteristics of a mixture arelargely governed by the volume and properties of thebinder. Because binder ageing and the consequentembrittlement is a factor, fatigue cracking is likely tooccur earlier in open graded materials than inimpermeable ones since the binder can harden due toweathering throughout the depth of the more openmaterials. Cracking in less permeable surfacingmaterials tends to initiate in the first few millimetres ofthe surface where the binder is the most exposed andtherefore the most embrittled, and propagatedownwards. Cracks often start at the interface betweenthe binder and aggregate, thus good adhesion betweenbinder and aggregate is advantageous.

Moisture damage1.29 Moisture damage, causing stripping of binderfrom the aggregate, leads to fretting and ravelling. Theimmediate cause is poor adhesion of the binder to theaggregate, which is usually related to the chemicalcomposition of both aggregate and binder. Damageoccurs more often with permeable, open gradedmaterials, and the worst affected can be materials withair void contents in the range 9-14 % which can neitherdrain nor dry out very easily.

Macrotexture1.30 Texture depth can change with time, due to anumber of different mechanisms. With surface dressingthe main cause is embedment of chippings into theunderlying layer. Where compaction during laying isinadequate, materials will undergo secondarycompaction under traffic, particularly in very hotweather. This almost invariably reduces themacrotexture. Very high texture depth is not desirable,as the noise level generated by the passage of traffic overthe surface is increased.

Ride quality1.31 This is generally improved for all materials laidwith a paver incorporating a floating screed. Thisexcludes slurry surfacing, which is laid with a spreaderbox, and surface dressing or other sprayed processes.Micro-surfacing can improve the transverse shape of thepavement but has limited effect on the longitudinalprofile.


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2. LAYING BITUMINOUS SUR FACE COURSES

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Chapter 2Laying Bituminous Surface Courses

2.1 Advice on laying bituminous materials is given inPart 2 of both BS 594 and BS 4987. Additional adviceregarding particular materials is given in the Chapterson specific materials in this Part.

Weather Conditions

2.2 The weather conditions during the construction ofbituminous surface courses affect both the layingoperation and the subsequent performance. Althoughmaterials may appear satisfactory in the short term, eveto the end of their maintenance period, the lifeexpectancy of a surface course laid in inclement weathemay be reduced. The requirements of the specificationshould be strictly adhered to.

Available Research Reports and Weather Forecasting2.3 TRL Research Reports RR 4 and RR 280 providedetails of research into the effects of various factors thatinfluence the rate of cooling of an asphalt layer. In orderof priority, the principal factors are layer thickness, windspeed and ambient temperature.

2.4 The Meteorological Office can provide historicalinformation relating to weather conditions, such asmonth by month analysis of temperatures, to form astatistical forecast of conditions that affect aspects ofroad building and in particular, the weather sensitiveoperation of surfacing.

2.5 The Meteorological Office can also provide24-hour local forecasts, including wind speeds, fromregional weather centres. Information on these servicescan be obtained from the Meteorological Office,Climatological Services (Met 03), London Road,Bracknell, Berkshire RG12 2SZ (Tel: 01344 420242).Planning the laying of surface courses should always bedone in conjunction with an up to date weather forecast

Specification2.6 The Specification (MCHW 1) Series 700 and900, with the associated Notes for Guidance (MCHW 2set out the requirements for acceptable weatherconditions for laying bituminous surface courses. Forperformance specifications Series 700 does not applybut it provides a useful guide to good practice. (Forsome special materials the requirements are included inSeries 900). Where modified binders are used advicefrom the binder supplier via the Contractor should beobtained.

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.7 Various assumptions have been made in draftinhe Specification (MCHW 1). These are:

) no allowance has been made for solar gain,

) the temperature of the substrate has been assuto be at ambient temperature initially,

) the binder is an unmodified bitumen to BS 3690and

) that for materials with 50 pen binder, thetemperature of the asphalt immediately behind tpaving machine is at least 140°C and compactiis effectively completed when the laid asphalttemperature has fallen to about 100°C. A similatemperature differential is applicable to othergrades of binder.

.8 Great care should always be taken when theemperature is, or has recently been below 0°C as iceay be present on the substrate. Under no circumsta

hould material be laid on a frozen substrate asnevitably the lower part of the layer will coolrematurely and may not be compacted properly. A zonef weak material may then be present which is likely thorten the life of the surface course.

easuring Wind Speed and Temperature.9 Except where local conditions apply, such ashelter in a cutting or exposure on an embankment, ohen conditions are transient with gusting or squalls,ind speed is fairly consistent over a sufficiently largerea to enable a single wind speed to characterise ahole site. To account for gusting it is necessary toefine wind speed as the average over the precedingour. It should be measured using a recordingnemometer with an accumulative digital output.and-held devices averaging readings over a feweconds, should only be used as approximate indicator general guidance the Beaufort scale is given inable 2.1.

.10 The Specification (MCHW 1) allows for windpeed to be measured at heights of either 2 m or 10 measurement at 2 m height, using a portablenemometer, is appropriate for monitoring on site.easurement at 10 m height is more suited for use oajor contract where there is a site office compound.



Equivalent speed at 10 m above ground

Force Description Description for use on land Knots Mean Limits

0 Calm Calm, smoke rises vertically 0 <1

1 Light Air Direction of wind shown by smoke but not 2 1-3by wind vane

2 Light Breeze Wind felt on face; leaves rustle; ordinary vanes 5 4-6moved by wind

3 Gentle Breeze Leaves and small twigs in constant motion; wind 9 7-10extends light flag

4 Moderate Breeze Raises dust and loose paper; small branches 13 10-16are moved

5 Fresh Breeze Small trees in leaf begin to sway; crested wavelets 19 17-21form on inland waters

6 Strong Breeze Large branches in motion; whistling heard in 24 22-27overhead wires; umbrellas used with difficulty

7 Near Gale Whole trees in motion, inconvenience felt when 30 28-33walking against the wind

8 Gale Breaks twigs off trees; generally impedes progress 37 34-40

9 Strong Gale 44 41-47

10 Storm 52 48-55

11 Severe Storm 60 56-65

12 Hurricane above 65

Table 2.1 The Beaufor t Scale of Wind Speeds

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2.11 The siting of an anemometer should be away fromand upwind of obstructions, and at positions agreed withe Overseeing Organisation. Wind speed measured at aheight of 10 m is more representative of prevailingconditions, being less affected by low level obstructions.For large works with a permanent site office in acompound, the installation of a recording station at 10 height is the preferred option. Measurements at a heighof 2 m on site may be subject to local obstruction,turbulence and traffic induced gusts and the anemometemay require frequent repositioning as work proceeds.Nevertheless it may be the only option for small siteswithout an office compound.

2.12 Ambient temperatures should be measured usingsuitable device calibrated to ±1symbol 176°C andreadable to ±0.5°C, which ideally should be placed in asuitably screened enclosure upwind of any heat sourceIt will reduce discussion on site considerably if allpersonnel can agree that a single thermometer will beused to determine the air temperature. It should be notthat the quoted air temperature is always in the shadeunless otherwise specified.

2.13 Surface temperature should be measured with anelectronic thermometer having a surface measurementprobe of low heat capacity and calibrated and readableto ±1°C. The temperature of the laid material should bemeasured at the mid-depth of the layer using anelectronic thermometer with reasonably quick responseand as low a heat capacity as possible, compatible witadequate robustness. Where the Specification(MCHW 1) includes measured compaction limits,(eg void contents or PRD) it is the Contractor’sresponsibility to determine the temperatures necessaryachieve full compaction. In these circumstances, excepfor the maximum mixing temperature specified to avoidundue hardening of the binder, the temperatures shouldnot be specified.

Consideration of Specific Materials

Hot Rolled Asphalt2.14 The application of coated chippings to hot rolledasphalt and their retention thereafter is particularlysensitive to the weather conditions prevailing duringlaying. The time for compaction and chippingapplication is of necessity longer, at about 10 minutes,than for materials laid in a single operation withoutchippings. Careful control of laying and rollingtemperatures is vital to ensure, with reasonable certainy,that the chippings will be retained.

2.15 In addition to normal cooling in ambientconditions, the application of cold chippings (12-15% bmass of hot asphalt) to the surface causes rapid coolin

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of the top few millimetres. If the chippings are wet or iflaying takes place during rain, surface coolingwill be accelerated. If the surface temperature is too lowduring chipping application, the bitumen coating on thechippings may not soften sufficiently to bond thechippings into the asphalt and the surface of the asphaltmay be too stiff to allow adequate embedment of thechippings. The use of frozen chippings exacerbates thisproblem and almost invariably results in rapid chippingloss. To protect chippings from frost and contaminationby dust, the sheeting of stockpiles is recommended.

2.16 Both the width of the chipping machine and thenecessity to feed it from the side using a loading shovelcause logistical problems, particularly on livecarriageways. The chipper is nearly 5 m wide andoverhangs each side of the asphalt mat by some distancreducing the width of the footway (where present) onone side and the trafficable width on the other. For safetyreasons on narrow roads a formal road closure may berequired or traffic control which may cause long delays.

Porous Asphalt2.17 From the temperatures set out in Clause 938 ofthe Specification (MCHW 1) an 8 minute compactionperiod may be assumed for a 50 mm thick layer ofporous asphalt to cool from the minimum paver-outtemperature to the temperature for substantialcompletion of compaction. This period has beenestimated in accordance with TRL Research Report 4and adjusted to take into account the difference betweenthe expected rates of cooling of hot rolled and porousasphalt surfaces.

2.18 Laying in the rain should not be permitted as thiscools the material too quickly, reducing compaction.Heavy rain may also affect the bond between the binderand the aggregate.

Thin Surfacings2.19 Since these materials are thin they cool veryrapidly, and the amount of time available for compactionis limited. As they are proprietary materials, it is theContractor’s responsibility to ensure adequate bond andcompaction is achieved. Laying these materials on a wetsurface or during falling rain may significantly reducethe initial bond to the underlying surface.

2.20 Some of the thinner variants of these proprietaryproducts cannot be laid in winter, and their applicationon cold windy days, or at night outside the winter period,should be treated with caution. Local weather forecastsshould be used to plan the works during more favourableconditions.


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Slurry Surfacing and Micro-surfacing2.21 These materials are made with bituminousemulsions and therefore should not be laid in icyconditions. It is necessary for a proportion of the initialwater content to evaporate from the slurry before it canbe trafficked. Laying in wet or humid conditions,particularly when combined with low temperatures, willsignificantly delay the setting process. It should be borin mind that even in good conditions the material takesleast 30 minutes to become sufficiently stable to open totraffic. Adverse conditions can increase this period toseveral hours. Very high temperatures may causeproblems such as pre-setting and efficient breaking ofthe emulsion may be inhibited, leading to failure.

2.22 With these constraints, slurry surfacing andmicro-surfacing, like some of the hot paver-laid thinsurfacings, have a closed season in winter. Their use isalso restricted at maximum road temperatures, similarsurface dressing.

Surface Dressing2.23 Surface dressing has a very short laying seasonparticularly for heavily trafficked roads, although theadvent of superior binder grades and improved proceshave lengthened the season. Full guidance is given inRoad Note 39. The main reason for the short season ithat the chippings have to adhere to the binder attemperatures close to ambient, while the binder mustmaintain sufficient cohesive strength to resist trafficforces when the road is opened.

2.24 For successful surface dressing, it must not beraining and the road surface should be dry. In wetconditions the binder will not adhere to the surface andrapid failure may occur. The sprayed binder film will notadequately adhere to wet or cold chippings and the timtaken for the surface dressing to gain adequate stabilitto resist the traffic forces can significantly increase.Obtaining accurate weather forecasts and making goouse of them is more important for the successfuloutcome of surface dressing than for most other surfactreatments.

2.25 When emulsion binders are being used the workshould stop if the relative humidity exceeds 80%, as thbinder will not break properly and will lack adequatecohesion for the safe opening of the road to traffic.Relative humidities between 60% and 80% can lengthethe breaking time and care should be exercised whenmaking the decision to open to traffic. The use ofbreaking agents and multiple-layer dressings mayalleviate this problem.

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Site Preparation

2.26 For all surface course materials it is essential thatan adequate bond is achieved and maintained betweenthe surface course and the underlying pavementstructure. The thinner the surface course, the greater thetraffic generated stresses at the interface. To achieve anadequate bond the substrate must be clean and free of alloose material.

2.27 Where the substrate is new or nearly new, with thebinder film still intact, and no contamination, furthertreatment should not be necessary. Where it has beencontaminated by site traffic for example, it is essential toremove the contamination by sweeping, and if necessaryby the use of water jetting or other methods. Where anexisting road is being resurfaced, all packed mud, excessoil droppings, any other accretions or organic growthmust be removed.

2.28 Treatment with a suitable weedkiller may benecessary some days or weeks prior to the work beingcarried out. The advice of the weedkiller manufacturershould be followed with regard to timing. There areareas where the use of some weedkillers is constrainedand the Overseeing Organisation should be consulted tocheck on any limitations that are in place.

2.29 Existing roads may need some pre-treatment orshaping before the proposed surface course can be used.This should be carried out using an appropriatebasecourse material with the necessary properties,strength and resistance to rutting, for the expected trafficlevels. Another way of regulating is to plane off any highareas. The most suitable method will depend on theamount of regulation necessary, threshold levels and thelevel of street furniture.

2.30 In order to obtain best value for money from theresurfacing, any local weaknesses in the underlyingpavement should be repaired and cracks sealed using anappropriate treatment, see HD 31 (DMRB 7.4.1). Ingeneral the thinner the surfacing, the lower its ability toregulate the existing surface. Surface dressing followsthe existing profile and will not smooth outirregularities. Similarly slurry surfacings, which are laidwith a spreader box following the existing profile, aretoo thin to regulate the surface, although the thickermicro-surfacings can improve the transverse profile. Hotpaver-laid thin surfacings can regulate the existingsurface to a significant degree and guidance is given inthe Notes for Guidance (MCHW 2). The limits on theregulating ability of proprietary thin surfacings may begiven in the BBA HAPAS Certificates – see Chapter 6of this Part. Porous asphalt must not be laid on a surfacethat will prevent the free drainage of water (ie theexisting surface must be free of depressions).


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2.31 For the thinnest surfacings it is best practice toremove all road markings before resurfacing. This isparticularly relevant for surface dressing and very thinmaterials as the thickness of white lines is significantwhen compared to the thickness of the surfacing. It wialso behave differently from the rest of the substrate ifoverlaid and may precipitate failure. It may be possiblto mask thicker lines, for example the ribbed linebetween the hardshoulder and the slow lane.

2.32 With hot paver-laid materials all ironwork shouldbe reset to its final level after laying any base orregulating course and before laying the surface coursis impossible to patch round any ironwork that is liftedafter laying the surface course without introducingpotentially weak areas and damaging the sealing effectof the new surfacing. With slurry surfacing and surfacedressing the ironwork or reflective studs should bemasked as described in the Specification (MCHW 1).

Bond or Tack Coats

2.33 The purpose of bond or tack coats is to ensurethat all the layers of a bituminous road behavemonolithically. It is, therefore, extremely important thatthey are specified and applied in accordance with theSpecification (MCHW 1)). When a permeable course used, the bond coat also has to seal the existing surfaagainst ingress of water into the pavement structure.

2.34 Bond or tack coats are normally required but theare occasions (eg laying a surface course on an existroad that has fatted up or on new basecourse orroadbase) when there is already sufficient bitumen at theinterface between the new and the underlying layer suthat an additional coat is not necessary.

2.35 All unmodified bond coats and tack coats arecationic bitumen emulsions. They are quick breakingand most conform to BS 434; these are designatedK1-40, K1-60 or K1-70. K1-40 and K1-60 are sprayedat ambient temperature, but K1-70 must be heated tobetween 80°C and 90°C before it is sufficiently fluid forspraying.

2.36 All polymer modified emulsions are proprietaryand will require BBA HAPAS Certification. They aretypically either 60% or 70% binder content emulsions.The binder manufacturers instructions should befollowed for spraying methods but typically the 60%emulsions are sprayed at ambient temperature and th70% ones are sprayed at between 80°C and 90°C. Soproprietary surfacing materials include the bond coat part of the process and the BBA HAPAS Certificate forthe surfacing will include the bond coat requirement.

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2.37 Conventional bitumen emulsions to BS 434 arenormally suitable for use on roads where the existingsurface is a bituminous material. Many conventionalemulsions do not adhere well to concrete and thesuitability of a particular type should be checked withthe manufacturer before such use. A polymer modifiedemulsion specifically formulated for use on concretemay be a better choice.

2.38 There are a number of techniques that can be usefor applying bond or tack coats. The traditional one withK1-40 and K1-60 is to use a hand lance, with either ahand or motorised pump, using material delivered in 200litre drums. K1-70 binder, as used for porous asphalt,must, because it is sprayed hot, be applied using surfacdressing equipment, ie. a heated tank and spray bar. Arecently introduced technique is by integral spray barattached to the paver, together with a tank for holdingthe emulsion.

2.39 The rate of spread of bond coats should bemeasured. The material usage divided by the areacovered will be sufficient for all surface courses, exceptsurface dressing or thin surfacings, where more precisemethods should be used.

2.40 All methods of spraying bond coats havedifficulties with either the control of the rate of spread,the evenness of spread, the completeness of break orpick up of material on the wheels of the deliveryvehicles.

Rate of spread2.41 It is very difficult to obtain an even rate of spreadof binder with the hand lance because it is difficult tocontrol a single nozzle. There is no correlation betweenthe amount that is pumped and the speed of swing of thlance except for the skill of the operator.

2.42 The main difficulty with integral spray bars is thelow rate in terms of volume. A conventional surfacedressing bar sprays between 70 and 100 litres/min foreach metre width, whereas an integral bar on a paverlaying at 12 metre/min will be spraying about 6 litres/min per metre width when using K1-40 emulsion for atraditional tack coat. This means that the jets on thespray bar must be smaller and are therefore moredifficult to manufacture to give the correct rate ofspread. Some integral bars use intermittent or movablejets to give the correct rate of spread. It is important thathese are checked for efficient operation.

Evenness of spread:2.43 The hand lance has problems with evenness ofspread, depending solely on the skill of the lanceoperator. However skilled, there will be significantly less


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sprayed around the edges of an area than in the middlunless it is permitted to spray outside the area required

2.44 When using surface dressing equipment there arfew problems, providing it has been properly calibratedand the jets checked for blockages. With spray barsintegral with the paver the main problem is likely to beblockage of the small jets. They should be checked prito the start of each days work and whenever the paverhas stopped for long enough for a transverse joint to bcut. Where the jets are movable or intermittent it is verimportant that evenness of rate spread of spread ischecked regularly. The manufacturers procedures shoulbe followed to ensure that the spray bar continues tooperate correctly throughout the laying period.

Completeness of break:2.45 Emulsions applied in front of the paving trainneed reasonably good weather conditions if they are tobreak prior to construction traffic using the sprayedsurface. If the emulsion is not fully broken beforetrafficking there will be serious disruption to the binderfilm by the wheels of the lorries and the paver.Emulsions will not break in the rain, they breakextremely slowly if the relative humidity is above 80%,and they form ice if the surface is below freezing.Emulsions should not be used in these conditions.

2.46 There should be no problems with the emulsionbreaking when sprayed from an integral spray bar, as thehot material following immediately behind will break theemulsion quickly and completely by the evaporation ofthe water. Provided that it does not cause problems withevenness and rate of spread, there are advantages inusing binder contents higher than 40% in the emulsionas less water needs to evaporate off for the same amountof bitumen on the road. (This will also mean that lessheat is lost from the bituminous material being laid).

Picking up:2.47 When emulsions are sprayed ahead of the pavintrain there is always a risk that construction traffic willpick up even fully broken binder which remains sticky.This is a particular problem with the thicker films suchas those used with porous asphalts. To overcome this ifit occurs to any significant extent, it may be necessaryuse a polymer modified emulsion, chosen for lowadhesivity to vehicle tyres; or to use an agent to reducthe adhesivity.

2.48 The system used should ensure that site trafficdoes not pick up the bond or tack coat and transfer it tother areas such that the process is seriously affected.(eg Filling the surface voids in thin surfacings or porouasphalt).

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Joints

2.49 Whatever surface course is used it will benecessary to construct joints, both longitudinally andtransversely. Joints are a potential source of weaknessand great care should be taken in their formation. Thebest technique to be used will depend greatly on thesurface course material concerned.

2.50 The techniques used for materials to BS 594 orBS 4987 are given in Part 2 of each standard. The sammethods are also suitable for use with stone masticasphalt.

2.51 A proprietary material will have the correctjointing method described in the BBA HAPASdocumentation. Joint formation in surface dressing,slurry surfacing and micro-surfacing are described in threlevant Clauses of the Specification and Notes forGuidance (MCHW 1 and 2) and appropriate chapters ithis Part.

Aftercare

2.52 Aftercare is the practice that occurs aftercompletion of rolling and prior to the gain of fullstrength by the surfacing. It is most important forsurface dressing and is dealt with fully in Chapter 8 ofthis Part with further information given in Road Note39. Slurry and micro-surfacing should not be traffickeduntil they have gained sufficient strength to resist theforces that are likely to be exerted, including braking.This type of surface will also need sweeping for a fewhours or days after opening as traffic continues toconsolidate the material. How quickly this happensdepends on the location, the amount of traffic and theweather conditions.

2.53 Hot, paver-laid materials, in general, do not needany treatment after the completion of rolling. Howeverheavy traffic should not run on the material until it hascooled and hardened sufficiently, because all bituminousmaterials are less resistant to deformation when they anew. As a guide, newly laid surfacing should not beopened to traffic if its surface temperature exceeds 25°Cunless the maximum temperature within the mat hasfallen below 35°C. The maximum temperature withinthe mat may be assumed to be at mid-mat. Guidance othe avoidance of wheel-track rutting in hot rolledasphalt, also relevant to other surfacing materials isgiven in the Notes for Guidance Clause NG 943(MCHW 2). Further guidance may be given in BBAHAPAS documentation for proprietary surfacingmaterials and modified binders.


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Road Markings

2.54 There are no particular difficulties with theapplication of road markings on any surfacing materialswith the exception of porous asphalt which is dealt within Chapter 5 of this Part. On some surface dressingtypes with high texture, (eg 14/6 mm racked-in), the firsapplication of lane markings can be short lived unlessthe application thickness is at the upper end of thepermitted range. This is because the majority of thematerial is below the peaks of the chippings.

2.55 Contra-flow and maintenance operations oftenrequire the application of temporary reflecting roadstuds. Many types of stud leave a sticky deposit ofbituminous adhesive that clogs and blocks the surfacevoids (of porous asphalt and thin surfacings) and sometypes can also cause pluck-out of aggregate. Wheresurface dressing is subsequently applied such depositsmay bleed through the dressing and leave fatty patchesTrials may need to be performed, at the back edge of thhardshoulder, to ensure that the studs proposed for usewill come free from the surface without plucking outsurface aggregate or leaving an excessive deposit.

2.56 Problems have also been reported with pre-formemarking tapes, in particular poor long term adhesion onnegatively textured surfacings in wet weather.

Testing Bituminous Surface Courses

2.57 The Specification (MCHW 1), Series 100 andAppendix A, requires that asphalt mixes are produced inplants that are registered to the ISO 9000 ‘SectorScheme for the Production of Asphalt Mixes’. Using thenew CEN terminology this includes all bituminousmixtures. Under this scheme, producers are required tomonitor asphalt production at predetermined frequencieto ensure the consistency of the various products. Plantare categorised as Q1, Q2 …or Q6, to demonstrate theconsistency actually achieved, the lower the ‘Q’ value,the better the consistency. Testing asphalt in the ‘asdelivered’ condition should therefore no longer benecessary. Nevertheless if obvious variations in aproduct do occur, for example as a result of binderdrainage, audit testing should be undertaken to checkwhether or not the product complies with therequirements of the Specification (MCHW 1).Non-compliance should be reported to the OverseeingOrganisation and the certification body.

2.58 When required, testing of bituminous surfacecourses should be carried out in accordance with theSpecification and Notes for Guidance (MCHW 1 and 2)Series 700 and 900. The frequency of testing is specifiein MCHW 1 with guidance given in MCHW 2 including

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eries NG 100 Table NG 1/1. Advice is also included inhe appropriate Chapters of this Part for specificurfacing materials.

nterpretation of Test Results.59 A test result should never be considered in

solation unless it is so far outside the specifiedarameters that the probability of the materialonforming to specification is minimal. Many standardest procedures give indications of the precision of theest and for a single result the figure given foreproducibility should be used as a guide to its precisionhe more test results that are available the more precise

he judgement that can be made on the compliance of thaterial as a whole.

.60 Some requirements of the SpecificationMCHW 1), for example, the void content of an HRAurface course or the binder content of porous asphalt,ave different limits set for single results and for theean of a number of results. By this means the risk ofccepting large quantities of sub-standard material iseduced, whilst recognising the fact that not only doariations occur in the materials but also in samplingnd testing, the latter not reflecting the actual valuesithin the layer. The importance of accurate samplingannot be emphasised strongly enough as this is a veryommon cause of disputes on site.

.61 Where the test results demonstrate that there is aompliance problem a number of steps should be taken:

Ensure that the samples were taken and testscarried out correctly. A NAMAS accreditedsampling and testing laboratory will have recordsto demonstrate that the sampling and testing was,or was not, carried out correctly;

If the procedures were correct then decide whethethe results are so far outside specification that thematerial definitely does not comply;

If the position is unclear, where possible carry outduplicate tests on samples already taken, or takeother samples from the same location as theoriginal samples, and repeat the tests;

Compare the two sets of results: if they confirmeach other (within normal testing variation), takethe mean value in comparison with thespecification. If the results differ significantly,give more weight to results from samples cut fromthe completed mat.


February 19992/8


e

2.62 With performance based specifications ajudgement can often be made on the likely effect ofnon-compliance on the life and performance of thesurface course. Advice should be obtained from theOverseeing Organisation. Removal and replacementshould be a matter of last resort as this processintroduces additional joints and the final product is lesslikely to conform in terms of compaction and ridequality than the original, because of the limited areabeing replaced.

2.63 Once it is decided that material must be removed,the defective area must be determined. Occasionally thelimits are obvious, eg when a delivery of lowtemperature material leads to high void contentmeasurements, the start and end of the load can often bseen. When the extent of the non-conforming materialcannot be identified in such a manner, the Contractormay reduce the remedial work by carrying out furthertesting to determine the limits of the area affected.Alternatively it will be necessary to replace all materialcovered by the result which is out of specification, up tobut not including the location of compliant results oneither side.

2.64 Whatever decisions are taken with regard toremedial works the Contractor and his supplier shouldinvestigate the cause of the non-compliance, as part oftheir normal quality assurance procedures, in order toreduce the likelihood of the problem recurring.


February 1999

3. BINDERS AND BINDER MODIFIERS

Chapter 3Binders and Binder Modifiers

3/1

Int roduction

3.1 Conventional binders have been used successffor many years in road surfacings, generally providingboth satisfactory performance and durability even on most heavily trafficked roads. However, some roads incertain areas of the country, surfaced with conventionalrolled asphalt, have deformed prematurely due to anumber of causes:

• Increasing traffic density;

• Increasing use of super single tyres;

• Early trafficking during lane rental contracts;

• Channelised traffic;

• Slow moving heavy vehicles on hills, particularlyduring prolonged hot periods.

3.2 For surface treatments such as surface dressinthe use of conventional binders, even when using theracked-in process, has limited ability to providesatisfactory performance on heavily trafficked roads andhighly stressed sites. Detailed advice on the specificaof binders for surface dressing is given in Chapter 8 othis Part. Similarly, porous asphalt using conventionalbinders (without additives or modifiers such as fibres polymers) has not provided adequate durability onheavily trafficked roads. Thin surfacings, introducedfrom France in 1990, have utilised modified binders,either in the mixture or as a special bond coat, in ordeto ensure satisfactory performance.

3.3 With the introduction of end performancespecifications for road surfacings, materials will havebe engineered to meet specific requirements. Wherematerial designs indicate that the required level ofperformance cannot be achieved with conventionalbinders and available aggregates alone, the use ofmodified binders may be necessary.

3.4 This Chapter provides a brief introduction tobinders for road surfacings used in road constructionand maintenance. It describes the modification ofbituminous binders to improve performance of mixturewhen necessary, especially for the more heavilytrafficked and highly stressed sections of the roadnetwork. With increasing commercial vehicle trafficintensity, and increasing degree of site difficulty, the

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probability that modified binders will be neededincreases. See Fig 3.1.

Figure 3.1 : The Need for Modifiers in Mixtures atDifficult Sites and at High Traffic Intensit y.

3.5 Benefits which may be obtained by the use ofmodified binders include:

• Improved resistance to permanent deformation ofmixtures at high service temperatures;

• Greater load spreading (increased stiffness) for apavement layer of given thickness;

• Reduced fatigue of mixtures, giving reducedcracking under repeated load;

• Improved ductility at low service temperatures,giving reduced thermal cracking;

• Improved adhesion to aggregates, giving reducedstripping in mixtures and in surface dressings;

• Increased cohesion, giving better chippingretention in the early life of surface dressings;

• Improved workability of mixtures, reducing therisk of poor compaction;

• Reduced hardening or ageing in service, givinglonger life in surface materials;

• Reduced temperature susceptibility throughoutservice temperature range;

INCREASING COMMERCIAL VEHICLE TRAFFIC

POTENTIALFOR MODIFIERS

NON-EVENTSECTIONS OFMOTORWAY ETC.

APPROACHES TOPEDESTRIANCROSSING/TRAFFICLIGHTS ETC.

STRAIGHT/

INCREASINGDEGREE OFSITEDIFFICULTY

CONVENTIONAL BINDERSAND MIXTURES SATISFACTORY


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• Increased viscosity at low shear rates, allowingthicker binder films to be obtained in openmixtures and reduced bleeding in surface dress

Figure 3.1 summarises some of the claimed benefitsfrom using binder modifiers.

3.6 Most modifiers will only be able to achieve someof the properties, so choice of modifier is site specific.Addition of a modifier will not automatically confer

ing.

satisfactory performance to a base binder; indeedmodifiers and bitumens have to be carefully selected toensure compatibility. Further advice is given in theReferences.

3.7 Where modified binders are used, theSpecification (MCHW 1) Series 900 requires details oftheir performance characteristics in terms of both bindeand mixture properties to be included in tenders.

Improvements

Modifier Notes Permanent Fatigue Thermal Binder Ageing Moisture Recycling Cost EnvironmentalDeformation Cracking Cracking Drainage Damage Difficulty Addition Considerations

Fillers 2 Some Some Some Low Low Dust suppressionneeded

Fibres 2 Some Yes Some Low Low Fine materialhazardous

Natural 1, 2 Yes Some Low Medbitumens

Chemical 1, 2 Some Some Med Med Possibility of leachingModifiers should be considered

Thermo- 1, 2 Yes Yes Yes Yes Yes Yes High High Harmful when uncuredsettingPolymers

Thermo- 1 ,2 Yes Some Some Yes Some Med Med-High Degradation ifplastic overheatedPlastomers

Thermo- 1, 2 Yes Yes Yes Yes Some Yes Med Med-High Degradation ifplastic overheatedElastomers

Reclaimed 2 Some Yes Yes High Med Use of waste material

Rubber

Notes

1. Where permanent deformation is improved the mixture may be designed to have higher binder content, wthe benefit of increased resistance to fatigue, thermal cracking, ageing and moisture damage.

2. Within the same generic group there is a wide range of modifier composition and performance benefit. Thgeneral distinctions between Plastomers and Elastomers are becoming less well defined as innovationproceeds.

3. This table should be used as an overview and not for selection of a modifier for a specific purpose.

Table 3.1 Summary of the Potential Benefits from using Binder Modifiers in Mixtu res


February 1999

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3.8 Binders are visco-elastic materials (that is to say,they display both viscous and elastic behaviour). Aviscous material, like all liquids, continues to flow allthe time a stress is imposed on the material, whereas aelastic material deforms instantaneously under anapplied load and does not undergo further deformationthereafter. When the stress is removed, a purely elasticmaterial regains its original shape whereas a viscousmaterial does not recover but remains in the deformedstate. Conventional binders are predominantly elasticand brittle at low temperatures and viscous fluids at higroad temperatures depending on the frequency ofloading.

3.9 The visco-elastic behaviour of a binder is mostconveniently assessed by dynamic shear testing. Thisinvolves subjecting the binder to an alternating shearstress and measuring the resulting alternating shearstrain. The ratio of the stress to the strain is known asthe complex stiffness modulus (G*). The lag or phaseangle (δ) between the stress and strain is also measuresee Figure 3.2. Elastic materials exhibit a phase angle

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zero, viscous materials a value of 90° and visco-elasmaterials some intermediate value. At low temperatuunmodified bitumens tend towards purely elasticbehaviour and their phase angle approaches 0 degreAt high temperatures, unmodified bitumens behavepurely viscously and their phase angle can reach 90degrees at temperatures above 70°C. Dynamic testinmay be carried out over a wide range of frequencies temperatures. The loading time (or frequency) and thtemperature of the material are inter-related in theireffect on the behaviour of visco-elastic materials. Henthe same response can be observed when measuremare made at low temperatures for long periods as at temperatures for short periods. It is possible to reprethe results of tests taken at different temperatures andloading times using one master curve of visco-elasticbehaviour, thereby characterising the material. Anexample of a master curve is shown in Figure 3.3. Thmain mechanisms of road failure are being studied inorder to relate these rheological properties to them sthat it may be possible to predict the relativeperformance of binders from these properties.

3.2 : Essential Parts of Dynamic Shear Rheometer and Definition of Phase Angle δδδδδ

Applied

Measured

COMPLEX MODULUSPHASE ANGLE *

Stress

Strain

F

(

Time

Time

Applied

8mm or 25mm

1mm typically

F

(

*

Torque

Binder Film

Fixed Plate

Oscillating Plate



Figure Figure 3.3 : G* for an aged Bitumen showing derivation of Master Curve at 25°C using Time –Temperature Superposition

Figure 3.4 : Phase Angle versus Temperature at 0.4Hz for three binders (after RTFOT)

101

103

105

107

109

10-7 10-4 10-1 102 105

Vialit Pendulum100km/hr TrafficPen TestStanding TrafficThermal Cracking

80/C

60/C

45/C

35/C

25/C

15/C

5/C

-5/C

Frequency (Hz)

Comp

lex S

tiffne

ss M

odulu

s G* (

Pa)

0

10

20

30

40

50

60

70

80

90

-10 0 10 20 30 40 50 60 70 80 90 100

Modified bitumen Type 2Modified bitumen Type 1Unmodified bitumen

IncreasinglyElastic

9

8IncreasinglyViscous

Temperature (/C)

Pha

se a

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d (d

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➤

➤

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3.10 Master Curves, can be produced using a DynamicShear Rheometer (DSR) which is fully computercontrolled and can produce much of the dataautomatically. As the response of binders is dependentupon temperature and loading time, it is important toselect appropriate values to allow comparison ofmaterials. In the Specification (MCHW 1) Clause 928,the frequency of 0.4 Hz has been chosen as the standa(loading time of 0.4 seconds, equivalent to slow movingtraffic). A plot of complex stiffness modulus againstfrequency at a temperature of 25°C has been selected fthe master curve.

Test Methods

3.11 The basic geometry of the DSR is presented inFigure 3.2. DSRs use very little binder, typically lessthan one gram, and tests have even been carried out onsurface dressing binders scraped from a road surface. Iorder to cover the whole range of binder properties, testare carried out over a frequency range of (typically) 0.1to 10 Hz at a number of different temperatures, usuallyat least six, ranging typically between minus 5°C and80°C. Having obtained the data at individualtemperatures the principle of time-temperaturesuperposition is used to produce a single Master Curve,as illustrated in Figure 3.3.

3.12 The effect of binder modification can be seen fromplots such as Figure 3.4 which relates Phase Angle andTemperature. The Modified Bitumens, Type 1 andType 2, represent results that might be obtained forexample, from the addition of an elastomer or aplastomer, respectively. The measurement can be used tohelp understand the engineering properties that are likelto be generated in situ. In Figure 3.4 for the unmodifiedbinder, the phase angle increases with temperature in thrange -5 to +80°C, indicating increasingly viscousbehaviour. In contrast, the Type 1 elastomeric bindershows increasing elasticity in the temperature range20 to 70°C. The Type 2 plastomeric binder showsalmost constant phase angle from 10°C to 60°C, but thebehaviour changes rapidly at both ends of thistemperature range. The results show how polymers canassist in improving the deformation resistance ofbituminous mixtures by increasing the elastic,recoverable strain component under loading at hightemperatures.

3.13 Figure 3.5 plots Complex Stiffness Modulusagainst Temperature for the same three binders presentein Figure 3.4. This shows that both modifier types canincrease binder stiffness at high service temperatures(50-70°C). The lower stiffness and more viscousresponse (see Figure 3.4) obtained from the elastomer

(T0c

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ns

ype 1 binder) at low service temperatures (less than°C) will also assist in retarding thermally inducedracking.

.14 An ‘Equi-Stiffness High Temperature’: T2kPa

°Cas been defined for use with polymer modifiedituminous binders. This is the temperature at which theomplex stiffness modulus equals 2000 Pascals at aading frequency of 0.4 Hz. This parameter indicatesigh temperature stiffness performance, much in theame way that softening point is used for conventionalitumens. Figure 3.5 shows that the Type 2 binder has aigher T

2kPa °C than the unmodified bitumen.

.15 An ‘Equi-Stiffness Low Temperature’: T2MPa

°Cas been defined. This is the temperature at which theomplex stiffness modulus is 2 Mega Pascals x 106Pa) at a loading frequency of 0.4 Hz. It provides

n indication of the relative stiffness of polymerodified binders against a standard bitumen. Figure 3.5

hows the Type 1 modified binder to have a lower T2MPa

an the unmodified bitumen. T2MPa

oC is approximatelyqual to the temperature at which the penetration valuef the unmodified bitumen is equal to 19dmm. It isossible that this temperature may provide an alternativarameter to penetration at 5°C, which is believed to beseful for prediction of low temperature performance.


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Fig 3.5 : Complex Stiffness Modulus versus Temperature at 0.4Hz for three binders (after RTFOT)

101

103

105

107

109

-10 0 10 20 30 40 50 60 70 80 90 100

Modified bitumen Type 2Modified bitumen Type 1Unmodified bitumen

T2000Pa for Type 2 modified

T2MPa

for Type 1modified T2MPa for unmodified bitumen T2000Pa for unmodified bitumen

2 MPa

25/C

2000 Pa

Temperature (/C)

Com

plex

Stif

fnes

s M

odul

us G

* (P

a)

3.16 For surface dressing and micro-surfacing, theSpecification (MCHW 1) Series 900 requires thatpenetrations at 5°C and 25°C are measured. Binder-aggregate adhesivity by the Vialit Plate Shock test(BS EN 12272-3) is also used for evaluating chipping-binder combinations.

3.17 The Vialit Pendulum test as specified in Clause939 of the Specification (MCHW 1) is used to assess tcohesion of the binder, a measure of the forces holdingthe material together. This is probably more importantfor surface dressing, thin surfacings and bond coatswhere the binder is under more direct stress than at another level in the pavement structure. A balance has tobe struck between the cohesion of a binder and its abilto adhere to the aggregate.

3.18 The Notes for Guidance (MCHW 2) ClauseNG922, classify cohesion requirements for three bindegrades; Premium, Intermediate and Conventional. Thepeak value is specified, but the plot of cohesion againstemperature must be provided so that, in future,temperature ranges can be set. Provided that the levecohesion provided by a binder is sufficient, thetemperature range over which the required level ofcohesion is maintained is more important than themaximum level attained.

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Ageing

3.19 Conventional binders, as thin films on theaggregate particles, age in the presence of air leading fretting and ravelling (loss of aggregate in the surface),cracking, and finally to failure. This is much morecritical in surface courses - stiffening in a basecourseand roadbase may actually contribute to enhancedperformance. The rate of change of binder propertiesdepends on the voids in the mixture, whether they areinterconnected, the binder film thickness and thecomposition of the binder after paving. There are somechanges of binder properties because of loss of volatileand oxidation during storage, manufacture and laying,after which the binder hardens slowly - see Fig 3.6. Thehigher the penetration of the as-laid paving gradebitumen in the mixture the longer it has before onset ofbrittle failure by the ageing process; however thepropensity to rutting increases with increasing softness(higher penetration) so a balance has to be drawn. Withdenser asphalt mixtures, ageing progresses more slowy,as the binder is less vulnerable to oxidation. This is themain reason for the limits imposed for maximum airvoids in the performance related specification for hotrolled asphalt; see the Specification (MCHW 1) Clause943.


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air

ad,


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TIME

5% VOIDS

100

PEN

10% VOIDS

CRITICAL VISCOSITY

70

50

20

STORAGE IN TANK MIXING HARDENING ON THE ROAD 5 YEARS

Fig 3.6 : Changes in Penetration of Conventional 100 pen Bitumen with Time, for Mixtu res of differing VoidsContents

3.20 Modifiers such as fibres may be used to increasbinder content and hence film thickness in order toreduce the effects of ageing in the more open mixtures,such as porous asphalt, where both air and water havease of access.

3.21 Polymer modifiers may be used to increase theresistance to ageing because softer base binders mayused. A balance between brittle elastic behaviour at lotemperatures and adequate resistance to permanentdeformation at high service temperatures must beobtained.

3.22 Adhesion of the binder to aggregate may also beimproved by the use of modifiers. Enhanced adhesionreduces stripping caused by water and thereby minimithe associated loss of mixture strength.

3.23 In order to understand better the long termproperties of binders, and set limits on which to baseperformance specifications, some form of controlledageing test is required.

Rolling Thin Film Oven Test (RTFOT)3.24 This test is used to simulate the loss of volatilesand oxidation that take place during mixing, transportiand paving the asphalt. This test is specified for all

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ses

ng

binders, although validation has not been carried out fmost polymer modified binders. Tests such aspenetration, cohesion and rheology should be carriedout, both before and after the test, to show anysensitivity of the binder to the manufacturing processand to simulate the condition of the binder immediatelyafter laying.

High Pressure Ageing Test (HiPAT)3.25 This test is used to artificially age a binderwithout applying an unduly high temperature, whichcould destroy the integrity of the binder and initiatereactions that do not occur in practice. High pressure at 2.1 MPa and a temperature of 85°C is used for aperiod of 65 hours. Cohesion and rheologymeasurements are carried out after this test, andinformation about resistance to oxidation of the bindermay be obtained.

3.26 Aggregate binder interactions may inhibit theageing of binders, but higher void contents and thinnerbinder films accelerate ageing, so a mixture test ispreferred. A test for ageing mixtures has beendeveloped, which involves storing cores at 85°C for120 hours which may simulate several years on the roprior to testing in devices such as the Indirect TensileStiffness Modulus Test.


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End Product Performance Specifications

3.27 In the case of surface dressing, micro-surfacingand thin surfacing the Contractor has responsibility forperformance during the guarantee period; this may befor only two years, whereas the life of surface dressingmay be eight years and for thin surfacing ten years ormore. It is imperative that the Overseeing Organisationhas data to ensure that modifiers used successfully in past, where durability has been confirmed in practice,are identified, so that they may be used again withconfidence.

3.28 The end-performance tests currently availablecannot fully predict durability and it is inevitable thatsome material component control will be part ofspecifications for some years to come. Polymer modifiebinders will require rheological identification, cohesiontesting to determine low temperature performance andtests to determine resistance to ageing, so thatperformance limits may be determined for futurespecifications.

3.29 Workability and compactibility of mixtures arealso of paramount importance for durability. Although itis the responsibility of the Contractor to ensure thematerial has been laid consistently, the OverseeingOrganisation may evaluate this by examining cores, tocontrol void content, and by visual assessment.

3.30 Use of a polymer modified binder to reducerutting in HRA does not necessarily make the surfacingmore durable. A modifier which provides highdeformation resistance may make laying moreproblematic, resulting in an as-laid product that has poperformance. Similarly, a polymer modified binder forsurface dressing may have a very high cohesion valueand perform well in laboratory tests, but exhibit pooradhesion to chippings in practice, under realistic weathand traffic conditions. Therefore a balance has to bestruck. The approach adopted is to encourage innovatby allowing the use of proprietary products with BBAHAPAS certification. The scheme includes monitoringand surveillance which will provide assurance on theperformance of the various products and systems.

3.31 Binder performance tests will not guarantee theperformance of all binder/aggregate combinations, butcan provide a useful indication of mixtures that arelikely to be satisfactory.


February 1999 4/1

4. HOT ROLLED ASPHA LT

tri-axle trailers with ‘super single’ tyres, which

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Chapter 4Hot Rolled Asphalt

Background

4.1 The main strength of a hot rolled asphalt (HRA)mixture comes from the stiffness of the sand/filler/bindermortar. A major factor in the performance of the mortaris the binder, normally 50 pen grade bitumen toBS 3690. Filler stiffens the bitumen which binds all theaggregates together. Some filler is contained in the fineand coarse aggregates in the mixture, but most is addedWhilst cement is a permitted filler, most if not all HRAis made with limestone dust, 75% of which currentlymust pass a 75 micron sieve.

4.2 The coarse aggregate may be crushed rock, slagor gravel. In chipped HRA wearing courses, which arelaid to nominal thicknesses of 45 mm or 50 mm, thenominal single size coarse aggregate is 14 mm. Thecoarse aggregate particles increase the volume of theasphalt and reduce its cost but, since they do not form askeletal structure, they do not add very greatly to itsdeformation resistance.

4.3 The addition of coated chippings to the HRAprior to full compaction enables the macrotexturerequirement for skidding resistance to be obtained. Goodworkmanship is required on site to ensure the correctembedment of chippings.

4.4 HRA wearing course has generally performedwell, with good durability and levels of safety. Areasonable tolerance of weather conditions at the time oflaying permits the placing of the material in winter andat night. However HRA has sometimes suffered frompoor resistance to deformation in wheeltracks (rutting),which was particularly evident on roads with highconcentrations of commercial vehicles after the hotsummers of the mid 1970s and mid 1990s. The design othe mortar is critical for adequate rut resistance.Experience has shown that the sand fraction isimportant, with rounded particles performing poorlycompared to crushed rock fines.

4.5 The rate of rutting of a particular wearing coursedepends on the temperature of the surface as well as thetraffic loading/speed and material properties. Thestability test (commonly known as the Marshall test)was introduced in 1976 to assess all sands, and criteriawere introduced for various traffic categories on thebasis of commercial vehicle traffic. There has been asignificant increase in tyre pressures in the last 20 yearswhich concentrates the load on the road, and recentlythere has been a considerable increase in the use of

comteco(Bfo

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ncentrate the rutting forces into a narrower track. Aore stringent requirement than the Marshall stabilityst has become necessary for the design of wearingurses. For this reason the wheeltracking testS 598: Part 110) has been introduced as a requiremer surface courses generally, and for HRA in particular.

operties

6 In addition to the properties of the sand, binderoperties are crucial to the production of a high ruttingsistance. The HRA must have sufficient workabilityr placing, laying, compaction and pre-coated chippingtention. It must also have durability; adequatesistance to rutting, stiffness, resistance to water ingresd resistance to binder degradation. An earlier attemp

produce improved rut resistance by using a specialade of binder (40 pen HD) was unsuccessful, becaufailed to provide the other properties required of arface course, even though its rut resistance wasproved.

7 The Specification (MCHW 1) Clause 943quirement for minimum wheeltracking rates, whichlate to the degree of site difficulty and traffic intensity,ill probably result in the increased use of polymerodified binders. Such binders have been used tohieve low wheeltracking rates and mixtures have beeund to be reasonably easy to lay and to compact. Thnefits in terms of reduced temperature susceptibilityd elasticity have yet to be fully established, especiall

the long term.

8 Performance improvements can be made by usinaterials such as Trinidad Lake Asphalt, which isvoured in some areas for special sites such asidgedecks. Principally however, thermoplasticlymers have been used and these include Ethylenenyl Acetate (EVA) and Polyethylene (PE) (oftenrmed plastomers); Styrene-Butadiene-Styrene block-polymer (SBS) and Styrene-butadiene rubber (SBRometimes referred to as elastomers). SBR is usedainly in the form of latex added at the mixing plant anre is required to ensure consistency is achieved. The

fferences between plastomers and elastomers arecoming less distinct as manufacturing processesvelop and base bitumens are engineered to suit thelymer.

9 For designed mixes, the acceptance of the finaloduct merely on an analysis of grading and binderntent is a procedure open to considerable error.

Clause 943 has been written in terms of performance


February 19994/2

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criteria that can be measured from samples taken fromthe road. This end performance specification shouldalways be used on the heavier trafficked roads. It leavesthe supplier free to use his expertise to produce amixture that will perform as required in the most costeffective manner. For less heavily trafficked roads anasphalt designed using stability criteria, Clause 911Specification (MCHW 1), may continue to be used. Inorder to ensure long term durability, resistance to ruttingmust not be the only consideration; ageing is alsoimportant and is related to the voids in the mixture,binder characteristics and binder content.

4.10 It is unlikely, but not impossible, that thecombination of properties needed at the highest levels ofperformance will be achievable using natural sand andan unmodified binder. However a great deal can be doneby assessing the effects of different sands on theperformance of the mixture and by blending sands. Asan example, the addition of a proportion of crushed rockcan both improve the resistance to rutting and may evenin some cases, improve the workability. The level ofknowledge is currently insufficient to predict the resultof blending fine aggregates, so appropriate performancetesting is required.

4.11 With the advent of more stringent requirementsfor rutting resistance the need to retain chippings shouldnot be forgotten. The less workable the mix and the moreadverse the weather conditions during laying, the morelikely the possibility of significant chipping loss. Rapidcooling of the top surface of the asphalt, forming asurface skin, can prevent adequate chipping penetrationThis is particularly relevant when laying polymermodified HRA in cold or windy conditions.

4.12 Annex A describes a suitable procedure for thedetermination of the proportion of missing and brokenchippings. This may be used initially and at the end ofthe maintenance period, to assess the degree of chippinloss and/or the number of broken chippings. Brokenchippings are more likely to be lost in the early stagesand are often a result of the asphalt cooling too muchprior to rolling in the chippings. There is currently nospecified limit for chipping loss but as a guide, a loss ofless than 5% is considered reasonable.

High Stone Content Mixtures

4.13 A variant of HRA which has been used on trunkroads and motorways (prior to surface dressing) has anominal stone content of 55%, with no added coatedchippings. Although with this level of coarse aggregatethere is some aggregate interlock, the material stilldepends primarily on the sand/filler/binder mortar for itsstiffness. Nevertheless, a 55/14 mixture will typically

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ave a stability about twice that of a 30/14 mixtureade with the same constituents. Most experience haeen gained using natural sand in the mixture (55/14Fnd there have been no reported problems of ruttinghen appropriate mixes have been used.

esign

.14 The method of design for the HRA wearingurse will depend on whether the material has beenecified to Clause 910, 911 or 943 in the SpecificationCHW 1):

Clause 910 is a recipe specification and calls upthe appropriate parts of BS 594.

Clause 911 is for a design mix using the stabilitymethod in BS 598: Part 107 to meet criteria setout in BS 594.

Clause 943 material is required to meet theperformance criteria set out in the contract, butstill requires aggregate grading to BS 594.

.15 For Clause 911 materials the stabilities given inS 594 are only appropriate for 30% stone contentaterials. Where high stone content asphalt (55% sto to be used, then the mixture should be made with thme constituent materials that would meet the stabilitquirements of BS 594 for 30% stone content asphalith appropriate adjustments to grading and binderntent.

.16 The definition for HRA Wearing Course in Clause43 is a 35% stone content material complying withlause 901 and BS 594: Part 1 except that the binderay be modified. A minimum binder content by volume also specified, in order to give some confidence thate material will be reasonably durable. The layerickness is required to be either 45 mm or 50 mm. Thicker layer may be necessary for structural or profileprovement, or during the winter period. (50 mm iscommended because of the problems caused by toopid cooling of thinner layers).

.17 For Clause 943 mixes, a ‘Job Mixture Approval’ial carried out on or off site, is necessary to checkerformance and to obtain approval of the mix. If thenstituents are then fixed as a Job Standard Mixturee approved mix may be used for a period of eighteenonths. The mixture tests are carried out on cores takom the trial site. Wheeltracking, air voids, density andmposition are recorded. The Overseeing Organisation approve as a Job Standard Mixture, the results fr

previous contract or trial carried out up to eighteenonths previously.


February 1999

4.24 The temperature at which G* is 2000Pa at 0.4is considered to be a more useful high temperatureparameter for modified binders. For conventional

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4.18 Advice on the performance criteria required fordifferent situations is given in NG943 of the Notes forGuidance (MCHW 2) for rut resistance, and HD 28(DMRB 7.3.1) for macrotexture. The material needs tomeet the criteria over the whole range of grading, bindercontents and compaction levels that would be expectedin normal production.

4.19 Basecourses should also be assessed for rutresistance. HRA basecourses should not be used underHRA wearing courses above rutting resistance Class 0,see Table NG 9/28 (MCHW 2). Class 1 wearing coursesrequire at least DBM100 basecourses and Class 2wearing courses DBM50 or HDM basecourses. Allmacadam basecourses should be designed and laid inaccordance with Clause 929 of the Specification(MCHW 1).

Clause 943 Data Requirements

4.20 Binder data is required to assist in developingfuture End Product Performance Specifications. Anoverview of some of the tests is given in Chapter 3 ofthis Part. The following are requested under Clause 943of the Specification (MCHW 1).

Penetration Test on the binder at 5oC4.21 Penetration at 5°C, used before and after theRolling Thin Film Oven Test, may provide some lowtemperature performance information. No criteria havebeen suggested as to levels or classes.

Rheological Data4.22 Rheology is principally used for productidentification. The complex stiffness modulus and phaseangle characterise the binder and give an indication ofperformance at both high and low temperatures. Therheological data can also provide a calculatedpenetration, not only at 25°C (standard frequency0.4Hz), but also at 5°C.

4.23 The Softening Point is commonly used to evaluatehigh road temperature performance, and as a guide tothe temperature of the material at which it is safe to opento traffic. However for polymer modified binders therelationship between Softening Point and performance ispoor. The high Softening Point values given for someelastomers (such as Styrene-Butadiene-Styrene modifiedbitumen) using the conventional test equipment may bedue to the significant elastic component in the complexstiffness modulus (G*) even at temperatures above60°C.

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2000Pa, may also provide a limit for

icking; ie. 20°C to 30°C below this temperature mahe maximum temperature at which it is safe to opesection to traffic without premature rutting. Former modified binders the advice of the binderplier should always be sought on this matter.

age StabilityStorage Stability is a test to determine which

ers can remain homogenous in storage with norm circulation. Suppliers of binders that do not havequate storage stability will have to demonstrate thasing a method of stirring or circulation, their binde be made effectively homogenous under practicalditions.

to-MicrographsPhoto-Micrographs of polymer modified binder

required as it is possible to identify the range ofeptable dispersions. If for any reason a markednge in performance is detected then these may heldiagnostics process. At the present time, methodsloyed are not reproducible and should not be usedredicting performance or for product identificationoses.

esionCohesion is measured by the Vialit Pendulum and

yield valuable information about the lowperature properties of polymer modified binders.ology does not examine the energy required toagate a crack nor the forces of attraction at the ve frequency of loading in this destructive test. Databe collected in order to see if performance levels be determined for wearing courses.

ss Brittle PointThe Fraass test, as determined by IP80, has no

n found to be reproducible for polymer modifieders. However new test equipment may improve theision, and this is the only established low

perature test for binders. Comparison of resultsre and after ageing may be the only indicator that

be used.

Non-mandatory Data that is being collecteddes the following:

AgeingIf Rheology, Fraass and Cohesion are examinedafter the High Pressure Ageing Test (HiPAT),some information about resistance to chainscission (degradation of the polymer) andoxidation of the binder may be generated.


February 19994/4


• Yield StrainThis test method is designed to determine whenmixes have sufficient cohesion to resist fracture.A very stiff mixture, made to produce excellentwheeltracking, could crack or fret at lowtemperatures or under high frequency trafficloading.

4.30 An asphalt testing load frame such as aNottingham Asphalt Tester may be used to determine arange of performance and durability measurements onmixes:

• Indirect Tensile Stiffness Modulus (ITSM)BS DD 213This test measures the indirect tensile stiffnessmodulus of a mix and may be used with the BBAHAPAS test procedure for water conditioning,when published, to determine mixture sensitivityto moisture. It is not known how modifiedmaterials behave under traffic in the presence ofwater. (A water immersion wheeltracking machinemay be another investigatory tool).

• Repeated Load Axial Test(RLAT)BS 598 Part 111This may be used to predict resistance topermanent deformation in conjunction with anappropriate modelling procedure.

• Indirect Tensile Fatigue Test (ITFT) BS DD ABFThis test may be used to examine the fatiguecracking associated with ageing. The proposedBBA HAPAS test procedure for ageingrecommends subjecting samples to a temperatureof 85 ±2°C in a forced draught oven for a periodof 120±0.25 hours to age the specimens, which isintended to simulate several years in the road.


February 1999 4A/1

Chapter 4Annex A - Method for determination of loss ofchippings and proportion of broken chippings

Annex A - Method for determination of loss of chippingsand proportion of broken chippings

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A.1 The method is based on the procedure used insurface dressing for the determination of chipping loss,but the method of counting is slightly different, as thechippings are not laid with 100% shoulder to shouldercoverage.

A.2 Equipment

a) A piece of plywood, hardboard or other suitablematerial approximately 400 mm square with a300 ± 5 mm square hole cut into the centre of it.

b) A tape, notebook and pencil or pen.

c) Useful but not essential would be an assortment ocoloured board chalks for marking countedchippings.

A.3 Method

a) Place the board onto the wearing course in thenearside wheeltrack with the hole sides paralleland at right angles to the direction of travel.

b) Count all the chippings that are present, entire orbroken, that are visible through the hole, ignoringall those that are partly obscured by the board.This = N

1.

c) Count all indentions in the asphalt wherechippings have been detached, again ignoring anpartly obscured by the board: = N

2.

d) Count the chippings that are broken that havealready been counted in b): = N

3. It may be easier

to mark the broken chippings with coloured chalkand then count the marked chippings.

e) Repeat in two more locations at approximately2 m intervals along the road.

A.4 Calculations

a) Percentage of lost chippings:= 100 x N

2 ÷ (N

1 + N

2)

b) Percentage of broken chippings:= 100 x N

3 ÷ N

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he results should be reported to the nearest wholeumber for both the individual readings and the mean of

he three locations.


February 1999

Chapter 5Porous Asphalt

5/1

5. POROUS ASPHALT

during mixing, transportation and laying.

.

Background

5.1 Porous asphalt, PA, consists primarily ofgap-graded aggregates held together by binder to form matrix with interconnecting voids through which watercan pass. It acts as a reservoir and, provided that thecrossfall or longitudinal fall is sufficient, acts as a lateraldrain throughout the time it is wet. It is important thatporous asphalt surfacing is laid over an impermeablelayer that protects the lower layers of the pavement fromingress of water.

5.2 Unfortunately the interconnected voids allowexcellent access to air; so ageing and embrittlement ispotentially exacerbated. Ideally a softer binder, togetherwith thick binder films are desirable, so bindermodification is necessary.

5.3 Trials of PA surfaces have considered thedurability and drainage characteristics of mixtures madewith different binders. Modifiers and additives testedhave included synthetic rubber compounds such asStyrene-Butadiene-Styrene block co-polymer (SBS),natural rubber, Ethylene Vinyl-Acetate co-polymer(EVA), epoxy resins, natural and mineral fibres. Detailsof the trials are reported in TRL Laboratory ReportLR 563, “Pervious bitumen macadam surfacings laid toreduce splash and spray at Stonebridge, Warwickshire”;Research Report 57, “Pervious macadam : trials ontrunk road A38 Burton Bypass, 1984”; Research Repor323 “Trials of pervious macadam and rolled asphalt onthe A38 at Burton” and TRL Report 264 “Review ofUK Porous Asphalt Trials”. Further findings arereported by Szatkowski and Brown, “Design andperformance of pervious surface courses for roads inBritain 1967 - 1976.”

5.4 The following findings of the trials are relevant:

• Durability of PA is improved by using softerbinders (100 and 200 pen grade) and as high abinder content as possible. The quantity of binderincorporated into PA mixtures must be optimised,using the Binder Drainage Test.

• The target binder content determined in the BinderDrainage Test represents the maximum quantityof binder that can be safely incorporated into PAwithout introducing excessive binder drainage

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Excessive binder content and/or excessive mixingtemperature causes binder drainage and mixturesegregation during transportation from the mixingplant, leading to inconsistency of the finishedsurface, with areas either rich or lean in bindercontent.

Lean areas have insufficient binder content andmay lack fine aggregate, becoming more prone topremature failure due to ravelling and fretting.

Rich areas have flushed, low skid-resistance,patches of binder on the road surface that areimpermeable.

Segregated PA is difficult to discharge fromdelivery lorries.

Temperature controls and maximum target bindercontents are to be incorporated into thespecification to reduce the above problems.

Insufficient binder content leads to a less durablematerial, as the binder film is thinner andconsequently more prone to premature hardening,leading to a shorter life expectancy for the PA.

.5 The incorporation of some modifiers during therials, such as SBS or natural rubber, or additives suchs fibres, were found to be effective in reducing binderrainage and allowed a higher binder content to be usedVA was less effective in increasing the binder carryingapacity of PA and its use did not lead to improvedurability, compared with other materials. Howeverroprietary EVA modified binders for PA are nowvailable that can achieve the required target binderontent.

.6 PA with 100 pen bitumen has an expected life of to 10 years, at traffic levels up to 6,000 commercialehicles per lane per day, compared with 10 years orore for similar surfaces constructed with an HRAearing course.

.7 The tack or bond coat used beneath PA ispecified to further waterproof the underlying pavementayers and to maintain good adhesion. If it is necessaryo improve the seal or the bond, polymer modifiedmulsions should be considered.


February 19995/2


,

Benefits

5.8 Rain water on road surfaces can be hazardoumotorists; surface skidding resistance is reduced andspray generated by moving vehicles, particularly thostravelling at high speeds, decreases driver visibility. PAreduces these problems due to its open texture whichacts as a drainage layer, removing surface water duringrainfall and reducing traffic generated spray. TheSpecification (MCHW 1) has a Relative HydraulicConductivity requirement to ensure sufficientinterconnected voids are present. Unfortunately evidefrom the Netherlands indicates that it is no safer thanconventional surfacing. The reason for this may well that drivers who are normally inhibited from driving ahigher speeds in wet weather, tend to drive faster whenspray is suppressed and in-car noise reduced.

5.9 The level of noise emitted at the tyre/roadinterface, on a PA surface, is lower than for most othesurface courses offering comparable skid resistance. Ohigh-speed roads surfaced with PA, the averagereduction in dry road surface noise levels, comparedwith conventional surfaces is approximately 4dB(A) f‘light’ and 3dB(A) for ‘heavy’ vehicles. The reduction more pronounced in wet weather. Research suggests thit reduces traffic noise by acoustic absorption and inaddition, during wet weather, the rapid drainageproperties of the material reduce the incidence of noicaused by the generation of spray.

5.10 PA reduces the glare reflected from wet surfacedue to low incident level sunshine during the day, orvehicle headlights during the night. Carriagewaymarkings are more visible in wet conditions.

5.11 PA gives most benefit on high-speed roads,particularly those with concentrations of commercialvehicles. Spray generation and dispersion is related tyre width and tread, vehicle profile, vehicle speed anrainfall intensity, as well as road surface characteristi

5.12 The most efficient method of draining water fromPA is an open, free face at the edge of the carriagewy.However a 50 mm vertical step may have undesirablsafety implications for some road users. The guidancon edge of pavement details for PA and other aspects ofdrainage given in HA 79/97 (DMRB 3.2.4) should befollowed.

5.13 PA performs best on roads such as motorwaysand rural dual carriageways. These tend to have:

• High-speed traffic;

• Good vertical and horizontal alignment;

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• An effective drainage system, and

• Few, if any, junctions.

Limitations

5.14 The durability of PA is dependent on the qualityof the laid material, the soundness of the base on whichit is laid, the site characteristics, design layout, drainageand traffic flow. When water is retained within PA,because of poor drainage, its life will usually bereduced. Frequent braking and turning movements byheavy traffic may also cause surface fretting and earlyfailure.

5.15 Some reduction in void content and a closing upof surface voids occurs during service, due to theaccumulation of detritus and surface compaction bytraffic. This causes a reduction in relative hydraulicconductivity and increased spray levels. Traffic noiselevels also increase. However even when the voids areclosed up, it still provides a good reduction in noiselevels and spray generation similar to thin surfacings,when compared to HRA surfaces, due to cross-surfacedrainage within the surface texture and soundabsorbency.

5.16 There is currently no reliable and effective methodof removing detritus from the voids. The wheel pathareas seem to remain relatively clear, possibly due to thesuction effect of passing tyres. The problem of detritusclogging the voids of PA is pronounced when it is usedon the hardshoulder. This is considered to be due todetritus migrating within the PA towards the lower edgeof the pavement.

5.17 PA should not be used in the following situations:

a) On areas where the pavement strength is sub-standard,

b) On areas where there is already ponding in rutsand depressions,

c) On areas where there is considerable accelerationbraking, turning and parking,

d) On tight radius curves, and loops with radii lessthan 75 m, or when gradients exceed 10%,without advice from the Overseeing Organisation,

e) On areas where excessive deposits of detritus oroil and fuel may be experienced; such as parkingareas, exits from farms and quarries and otherindustrial sites,


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f) On areas where the use of tracked vehicles,construction plant, farm equipment or similarindustrial vehicles is expected,

g) On areas where the crossfall is insufficient toremove water to the road edge, such that floodingmay occur in the porous asphalt,

h) At locations where free drainage cannot beaccommodated along the low edge of thesurfacing; for example abutting other types ofconstruction such as a concrete carriageway,

i) Generally on lengths of carriageway of less than100 m, because of spray carry-over from adjacensurfacings, unless special conditions prevail,

j) On steel decked bridges except where expresslypermitted by the Overseeing Organisation,

k) In urban environments - in speed restricted areaswhere tyre/road surface noise generation is low -without approval from the OverseeingOrganisation,

l) Where frequent excavations by statutoryundertakers may occur,

m) Where traffic levels exceed 6,000 commercialvehicles per lane per day, at opening, exceptwhere expressly permitted by the OverseeingOrganisation,

n) On carriageways having a 30 mph speed limitbecause there is no beneficial reduction in sprayor noise levels achieved at low speeds.

5.18 There is a carry-over of rainwater for about 100m by vehicles moving from impervious surfaces ontoPA. To ensure satisfactory spray suppression for aparticular length of road, the length of road surfacedwith PA should extend for at least 100 m upstream of thsection where treatment is required.

5.19 On multi-lane carriageways, where animpermeable surface is laid downstream of a PA surface,the lane ends of the PA should be staggered across thecarriageway, in the direction of the drainage path. Thiswill avoid excess water welling up over the transversejoint. For detailed guidance refer to HA 79/97(DMRB 3.2.4).

5.20 On slip roads, PA should be continued until aconvenient stop-point is identified, such as a straightalignment of constant gradient but not less than 50 mbefore a give-way or stop line. It should always be

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stopped before areas of high stress, such as intersectiwith roundabouts and other junctions, when high skid-resistant materials may be required. PA can be used onmerging and diverging lanes, where it should extend thefull width of the carriageway at the taper and shouldpreferably also be used on the nose area and the sliproad beyond.

Bridge decks5.21 Where PA is considered for surfacing concretedeck bridges, the sub-surface water drainage systemshould be designed to permit an adequate water flowthrough and from the PA, taking account of theexpansion joints. For detailed guidance refer toHA 79/97 (DMRB 3.2.4).

Specification

5.22 Clause 938 of the Specification (MCHW 1) offersa balance between the requirements of durability,relative hydraulic conductivity, noise reduction andstability to give acceptable overall performance. PAshould only be laid on an impermeable basecourse ordirectly on an existing uncracked wearing course.

Binder5.23 The specified target binder content of 4.5%(Table 5.1), is a balance between improved durability(thicker binder films) and relative hydraulic conductivity(lower void content). Previous editions of theSpecification (MCHW 1) permitted unmodified bindersat a lower binder content, but it has been demonstratethat the benefits of enhanced durability with modifiedbinders outweigh the extra cost. Therefore only modifiebinders or binders with fibre additives are permitted.

5.24 Modifiers or fibre additives that promote a thickebinder film on the aggregate improve durability.However, experience from previous trials indicates thatnot all modifiers offer the benefits sought, so evidence olikely performance must be provided, as described later.(Paragraphs 5.35-5.38).

5.25 For pre-blended modified bitumens, it is essentiathat the blend is made with a base bitumen having apenetration within the range of 100 to 200 and that theblend is stable. A Storage Stability Test for pre-blendedmodified bitumens is given in the Specification(MCHW 1).

5.26 All proprietary modified binders or bindermodifiers, including fibre additives, will require a BBAHAPAS Certificate or in their absence a Departure fromStandard from the Overseeing Organisation.


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TargetTraffic flow Bindercv/lane/day Binder content (%)

by mass oftotal mix

100 or 200 pen bitumen andup to 1500 modifier or fibre additive, or

pre-blended modifiedbitumen*

4.5100 pen bitumen and

Over 1500 modifier or fibre additive,or pre-blended modifiedbitumen*

*base bitumen to have a penetration grade in the range100-200 pen

Table 5.1 Binder Content and Penetrationfor Porous Asphalt

5.27 Where PA with modified binder is shown to havea degree of flexibility (elastic recovery) it may be used inareas where reflective cracking may be a problem. Allcracks and surface defects in an existing surface shouldbe repaired and sealed before overlaying with PA.

Aggregate5.28 The Specification (MCHW 1) requires the coarseaggregate to be crushed rock or steel slag, or a mixtureof both, and the fine aggregate may be either crushedrock or steel slag fines, natural sand or a mixture ofthese materials.

5.29 Comparative trials, still under study, of 20 mm,14 mm and 10 mm maximum sized PA materials showthe 20 mm material does not clog as fast as the finergradings, thereby reducing potential maintenancerequirements. It has been found to have superiorperformance in terms of retained spray suppression,relative hydraulic conductivity and acoustic durability(retained noise reducing properties). Therefore only20 mm maximum sized material, with similar, but notidentical, grading to the BS 4987 material is allowed.

5.30 A minimum of 2.0% hydrated lime is specified toassist in the prevention of binder stripping by water andto stiffen the binder. All filler may be hydrated lime.Increased hydrated lime content may enable a higherbinder content to be achieved. Some aggregates are moprone to stripping by water than others and additionalcare should be taken with such aggregates.

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.31 The Binder Drainage Test should be performedsing the combined aggregates and hydrated lime filler ensure the target binder content specified can be

chieved.

.32 Although natural sand fine aggregates areermitted, the combined aggregate may not haveufficient surface area to carry the specified amount ofinder when the mixture is tested using the Binderrainage Test. In this case, the use of alternativeggregates should be investigated.

.33 The aggregate grading should target the mid-pointf the specified grading limits. Material manufacturedwards the coarse side of the envelope will have aigher voids content but may lack cohesion and stabilitynd be more subject to binder drainage. Materialanufactured towards the fine side of the envelope willave a reduced voids content and consequently a lowelative hydraulic conductivity.

.34 The aggregate shape and grading (within theverall envelope permitted) should be such that thelative hydraulic requirement is met consistently.

ontract Requirements

formation to be Supplied.35 Whichever binders are proposed, the Overseeingrganisation requires all available information on theirpecification and previous use, including trials, to beubmitted. Rheology Master Curves produced asescribed in Chapter 3 of this Part should be providedr all proposed binders. Use of the proposed modifier oodified binder may be agreed without trials or furthervestigation if the information supplied, and previousse or trials, are considered satisfactory.

.36 The Contractor should provide two copies of alle information, including test certificates, required fore evaluation of the proposed modifier or modifiedinder. Where the original documentation is in anguage other than English, it should be accompaniedy an English translation. The information andertificates should be supplied at least four weeks prior an agreement for use is required.

.37 Where use or trials of a modifier or fibre additive,r modified binder have taken place in another state ofe European Economic Area, evidence of satisfactorysults will be acceptable, provided the results obtained

re at least equivalent to those obtained from the UKials described in TRL Research Reports 57 and 323.

.38 When information or suitable evidence ofsatisfactory use is not available for a modifier or fibre


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additive, or modified binder, as described above, theOverseeing Organisation may require trials to beundertaken before its use on trunk roads, includingmotorways, is permitted.

Manufacturing Tolerances5.39 The durability and drainage performance of PAdepends upon compliance with the Specification(MCHW 1) and careful control of manufacture andlaying. In particular, it is important to ensure thespecified maximum mixing temperature and bindercontent does not exceed those stated in the Specificatio(MCHW 1) to prevent the occurrence of binderdrainage.

5.40 Different criteria for binder content complianceare given for samples taken at the plant and samplestaken on site. This is to allow for the possibility of slightbinder drainage taking place in transit and some slightsegregation of aggregate sizes due to handling.

Mixing5.41 The maximum mixing temperatures stated in theSpecification (MCHW 1) relate to a binder viscosity,including any modifier, of about 0.5 Pascal seconds(Pa.s). Excessive mixing temperatures cause binderdrainage. In the case of natural rubber modifier, theymay also degrade the rubber. To limit hardening of thebinder in the mixed PA, and reduce binder drainagewithin loads during transit, the elapsed time betweenmixing and the completion of laying and compactionshould not exceed 3 hours, including any time spent inhot storage bins at the mixing plant.

5.42 Because of the low mixing temperatures specifiedcompared to HRA or DBM materials, longer plantdrying times are required for the aggregates, thusreducing plant production rates. Experience in othercountries has indicated that batch-type plants arepreferred over drum or continuous mix plants, as theaggregates have an extra period of time in the plant binwhere final drying of the aggregate occurs, prior tomixing.

5.43 Experience has indicated it is preferable for themixing of component materials to be carried out on acontinual basis, to avoid temperature fluctuations andcontamination caused by plant changes to produce othematerials, such as HRA and DBM.

Construction

Underlying Surface Profile5.44 To ensure transverse flow of water, and to protectthe lower layers of the pavement structure from surfacewater penetration, PA should only be laid on an

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mpervious substrate and on carriageways with adequrossfall or longitudinal fall. It should not be used toestore poor profile. It is essential to regulate underlyinurfaces to remove depressions which may trap and hater, particularly if the surface onto which PA is to be

aid has been cold-milled. Application of a tack or bonoat is specified, prior to laying PA, to further protecthe underlying pavement layers.

.45 The crossfall of each area of surface should be inimum of 2.5% to ensure rainwater will reach the

arriageway edges quickly and easily. Maximum effects obtained when the edge of the PA is free draining, withnobstructed discharge of water along its whole length,llowing water to enter the drainage system, soreventing localised flooding. Minor flooding may bexperienced at locations with camber changes and it me necessary to consider rolling crowns. Advice shoule sought from the Overseeing Organisation.

aying.46 The minimum paver discharge temperatures state

n the Specification (MCHW 1) relate to a binderiscosity, including any modifier, of about 5 Pa.s. Aominal laying thickness of 50 mm is specified not onl

o maximise compaction time but also to ensuredequate storage capacity and spray reducing life of tA in service.

.47 Laying should, where possible, commence on thow side of the carriageway and proceed towards theigher side. This is to prevent roller water, or surfaceater, draining into areas onto which PA is about to beid.

ompaction and Joint Formation.48 Joint formation in PA is critical to its success and

he procedure laid down in the Specification (MCHW 1hould be followed meticulously. Transverse jointshould be formed against a 200 mm wide and 45 mmhick hard timber stop-end nailed to the road surface idvance of paving operations. An uncut joint is binderich so it is not necessary to apply bitumen to the jointrior to laying material abutting it.

.49 Joint cutting should be avoided. However, wheret is unavoidable it should be done with a power saw,aking appropriate measures such as suction extractioo prevent contamination of the surfacing with detritusfter cutting, a light coating of bitumen emulsion, suchs K1-70 or a polymer modified emulsion, should beparingly applied to the cold joint to promote adhesionhe binder is not there to seal the joint, as for denseaterials.


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5.50 Only steel-wheel tandem drum rollers arepermitted for compaction. Vibrating rollers should notbe used because of the possibility of aggregate crushiRubber-tyred rollers are not permitted as they knead aclose the surface, thus reducing the drainageperformance of the PA. Three wheel rollers should notbe used as they have been found to leave roller marksthat can be difficult to remove. It is recommended that aleast 5 passes of each roller should be applied within tspecified temperature range. The removal of all visibleroller marks is an indication of practical completion ofcompaction.

5.51 In order to avoid foot-marks, no one should bepermitted to walk on uncompacted PA, as the differentialcompaction caused is difficult to remove.

5.52 Where two or more lanes are to be surfaced,laying in echelon is preferred, with a maximum staggeof 20 m, so that the longitudinal joints can be effectivelyrolled together whilst hot. Provided the paving machineare close together, the materials will roll togetherwithout adverse crushing or loss of relative hydraulicconductivity. Longitudinal joints should not be paintedwith bitumen, or cut by chisels or saws, except asprovided for in Paragraph 5.49 above.

5.53 To maintain drainage flow between areas of PA,construction joints should not be cut or chiselled. Theaction of cutting tends to close-up the voids and maycause local ponding. This effect is most marked in thelongitudinal direction, causing areas of water to appeaon the surface as it banks-up behind a cut transversejoint, so creating a wet area or localised flooding on thsurface of the carriageway.

Existing Concrete Roads5.54 When laying on an existing concrete road, hotspray-applied K1-70 emulsion should be applied to theconcrete, at a rate of 0.4 to 0.45 l/m2, immediately priorto laying the PA, in accordance with the Specification(MCHW 1) Series 700, 900 and 1000.

Scheduling of Work and Use of Porous AsphaltSurfaces5.55 Laying of PA requires careful scheduling andcontrol such that construction plant does not have to uthe surface unnecessarily after completion and beforeopening to traffic. Particular care needs to be taken toavoid oil and fuel spillages. Landscaping operationsshould be scheduled such that soil is not placed on thesurface, in order to prevent detritus and mud damage.PA should be allowed to cool to ambient air temperatubefore opening to traffic.

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Assessment, Maintenance and Repairs

5.56 Visual condition survey methods can, withmodifications, be used for assessment of PA surfaces. Itis also anticipated that High-speed Road Monitorsurveys and skidding resistance assessment techniqueswill be used. It is expected that some modification to themethods of survey may be required, (eg cracks are mordifficult to see) or interpretation of results may needamendment (eg investigatory SCRIM values). TheOverseeing Organisation will keep this aspect underreview and publish advice as required.

5.57 The skeletal aggregate structure of the PA reducessecondary compaction due to traffic and PA has a highresistance to wheeltrack rutting. Therefore, if ruttingoccurs it is likely to result from deformation inunderlying layers.

5.58 The repair of small potholes, or the reinstatementof utility trenches and the like, should be carried outpromptly with PA or open graded macadam complyingwith BS 4987: Part 1. Dense Bitumen Macadam mayalso be used if necessary; however it should be replacewith a permeable material when circumstances permit.

5.59 For the repair of larger potholes, the damagedmaterial should be excavated to form an irregularlyshaped section. A coating of bitumen emulsion should bapplied to the base of the patch to provide bond, and thpatched area should be filled with either PA or opengraded macadam. This should assist in minimising locaflooding caused by any restriction to the flow of waterthrough the area after repair. HRA may be used if thearea to be patched is not too large, say no more than0.5 m x 0.5 m.

5.60 Deterioration of PA may accelerate towards theend of its life. If patching requirements exceed 10% ofthe surface area, the PA may be deemed to have failed. Ifthe surfacing was originally provided as part of a majorimprovement, then commitments made in anyEnvironmental Statement regarding noise attenuationneed to be maintained when it is replaced. Consequently,in order to restore the desired road surface properties, anew PA surface may be required. This will necessitateremoval of the existing surface by cold-milling, followedby a regulating course and then replacement of the PA.When using the 10% criterion, judgement should beexercised. The failed areas should be random rather thalocalised; a localised failure can be dealt with by anappropriate treatment rather than resurfacing the wholesection.

5.61 Failure to achieve a plane surface under PA hasbeen recorded as precipitating failure of the surfacing. A


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particular problem occurs when fuel and oil spillagefrom vehicles passes through the PA and is deposited inthe depression and then not flushed out by rain. Thehydrocarbons attack the substrate and underside of thePA, stripping the bitumen and softening the pavement.

5.62 The problem manifests itself during subsequentrain, when pot holes form in the wheelpath and down thcentre of the running lane. Potential trouble spots can bidentified before potholes form, by observation of damplocations remaining on the road surface, after theremaining surface has dried.

5.63 After accidents or similar, spillage of fuel and oilsshould be promptly treated using water-based detergentsfollowed by copious water flushing. Particulate andgranular adsorbents such as grit or cement should notused as these would clog the voids in the PA.

5.64 When carrying out any form of work, on oradjacent to a PA surface, materials should not bestockpiled or deposited on the surface without firsttaking precautions, to prevent contamination.

5.65 It is essential that regular cleaning of the drainagchannel is carried out to prevent accumulation ofdetritus. Where kerbs, vehicle cross-overs and junctionare features, planned maintenance procedures arenecessary. Experience has shown that drainage channerequire cleaning at least annually. This can beaccomplished by sweeping, pressure sluicing, andsuction extraction.

Road Markings and Detector Loops

5.66 Road markings, such as sprayed thermoplasticresins and paints, or machine extruded markings, can used on PA. In general, manual screeding ofthermoplastic markings should not be permitted since thot material has more time to permeate the surfacing.However, for directional signs, arrows and similar,screed markings can be used.

5.67 Detector loops are currently proposed to beinstalled 80 mm below surfacings on motorways,therefore problems are not anticipated. Refer to HD20(DMRB 9.3.1) Loop Detectors for Motorways. Advicecan be obtained from the Overseeing Organisation.Where possible, the installation of the detector loopsbefore laying the PA will reduce the likelihood of thesaw cuts precipitating failure.

5.68 Where the slot for the detector loop is cut throughthe PA the backfilling should be completed using well-rammed and compacted 6 mm size medium gradedwearing course complying with BS 4987, in order tomaintain the drainage path through the material.

Winter Maintenance

5.69 UK experience of winter maintenance on PA hasbeen gained with only a small number of trial sitesexcluding bridges; therefore only limited advice can begiven at this time. While grit-free salt is desirable, thecurrent salt and grit spreading policy adopted for densimpervious road surfaces can also be adopted for PAsurfaces, with some modifications.

5.70 Frost and ice form earlier compared with denseimpervious surfaces, due to PA’s porosity, lower heatconductivity and reduced thermal capacity. Snow and icecan also linger longer on PA. Research also indicates thetemperature within a PA layer falls more rapidly thanthat within other surfacing materials.

5.71 Precautionary salting is recommended in advancof snowfall. The formation of brine in the PA voidslowers the temperature at which freezing will occur, sodelaying the formation of ice. There is then arequirement for more frequent applications of salt,compared with a dense impervious surfacing, to clearsettled snow. The overall rate of salting necessary maysignificantly exceed that which would have been applieto dense impervious surfaces. Prompt ploughing isrecommended, but care is required to avoid damagingthe surface. Ploughs must be fitted with rubber skirts othe blades.

5.72 Where PA is interspersed with sections ofimpervious surface, it is recommended that theindividual lengths of both types of material aremaximised to facilitate the operation of separate wintermaintenance procedures for each type of surface.Signing of the start and finish of PA sections may assistmaintenance staff to identify the location of PA duringsalting operations and snow-ploughing.

5.73 Experience in other countries, with similarclimates to the UK, indicates slightly increasedquantities of salt are needed on dense impervioussurfaces immediately following a PA section, due toreduced salt transfer along the road from the PA sectionto the dense section.

5.74 Notwithstanding that ice appears to form faster oPA surfaces, experience to date, in the UK and otherEuropean countries, appears to indicate safety in winteconditions is not adversely affected, providedpreventative measures are taken, as described above.

5.75 Winter maintenance techniques adopted for PAwill be kept under review. Further advice will be issuedas it becomes available. General advice on wintermaintenance techniques can be obtained from theOverseeing Organisation.

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6. THIN WEARING COURSE SYSTEMS

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Chapter 6Thin Wearing Course Systems

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Background

6.1 Thin wearing course systems, or thin surfacingsas they are more commonly described, are proprietarysystems in which a hot bituminous bound mixture ismachine-laid with a controlled screed paver onto a bonor tack coat to form, after compaction and cooling, atextured wearing course generally less than 40mm inthickness. Mixtures consist of aggregate, filler andbituminous binder which may be modified by theaddition of polymers, rubber, resins, fibres or fillers suchas hydrated lime or cement. The bond or tack coat maybe polymer-modified and sprayed hot or cold dependingon the proprietary system used. Other types of thinsurfacings using different techniques are described in thechapters on Surface Dressing (Chapter 8), and Slurryand Micro-surfacing (Chapter10).

6.2 Proprietary thin wearing course systems aresuitable both for new construction and for maintenanceThe British Board of Agrément (BBA) has classifiedthin surfacings into three types depending on theirthickness as follows:

Type A <18mm

Type B 18-25mm

Type C >25 to <40mm

6.3 When used on trunk roads includingmotorways, proprietary thin wearing coursesystems shall have a British Board of AgrémentHAPAS Roads and Bridges Certificate appropriatefor the site classification and the level of traffic incommercial vehicles/lane/day to be carried. In theevent that no such Certificates have been issued,thin wearing course systems shall have HA TypeApproval in England and for those proposed forschemes in Wales, Scotland and Northern Ireland, aDeparture from Standard shall be obtained from theOverseeing Organisation.

6.4 Type A and B thin wearing course surfacings, upto about 25 mm in thickness, were developed in Francein the 1980s where in excess of 100 million sq m havesince been laid. Type C thin wearing course systems,25-40 mm in thickness have been developed in the UKand are proprietary versions of stone mastic asphalt(SMA) - see Chapter 7 of this Part. Stone mastic asphwas developed in Germany over 25 years ago, originalas a very robust surfacing to combat wear from studde

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tyres in winter - the use of which has since been banneDespite this however, SMA remains the most commonlyused surfacing in Germany today.

6.5 All thin wearing course systems have beendeveloped to meet the UK’s safety requirements,necessitating the use of high PSV aggregates and theprovision of initial and retained surface texture. The firsproprietary thin wearing course systems were approvedfor use on trunk roads including motorways in 1994 andthere are now a growing number of HA Type Approvedproducts for use in England, with others undergoingassessment.

Specification

6.6 Specification requirements for Thin WearingCourse systems are set out in Clause 942 of theSpecification (MCHW 1) with accompanying Notes forGuidance in NG 942 (MCHW 2).

6.7 A Thin Wearing Course system with acurrent BBA HAPAS Roads and BridgesCertificate or Type Approval shall only be laid by aContractor approved by the System Proprietor.Installation and quality control procedures shallcomply with the requirements of Clause 942 of theSpecification (MCHW 1) and when issued, with therequirements of the BBA HAPAS Certificate andmethod statement agreed by the BBA.

Installation

6.8 Thin wearing course systems are proprietaryproducts and as such their design, manufacture,transportation, laying and compaction are theresponsibility of the Contractor.

Bond or tack coats6.9 The thinner the surfacing, the more important therole of the bond or tack coat in the performance of thesystem. The type and rate of spread of the bond or tackcoat for each type of substrate on which each system cbe laid should be specified in the BBA HAPASCertificate or method statement for the system. Emulsiobond or tack coats sprayed as a separate operation ahof the paver should be fully broken prior to surfacing.

Audit Checks

6.10 It should not be necessary to carry out routineaudit checks on proprietary products with a two yearguarantee. Nevertheless if obvious variations in a


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product are occurring, then audit tests should beundertaken to determine aggregate properties andgrading, binder content and binder characteristics. Theseshould be carried out to check that the product complieswith the requirements of the Specification (MCHW 1)and, when issued, with the requirements of the BBAHAPAS Certificate and the system proprietor’s methodstatement. Non-compliance should be reported to theOverseeing Organisation and the BBA, and may, ifserious and ongoing, result in the suspension of TypeApproval or the BBA certificate for the system.

Overlaying Concrete

6.11 Thin wearing course systems are generallysuitable for application to both old and new continuouslyreinforced concrete (CRCP) surfaces. When laying onconcrete however, conventional emulsions to BS 434may not provide a sufficient combination of adhesion,cohesion and durability, so it is likely that a polymermodified emulsion will be required. When surfacing overjointed concrete, joint sealants in the concrete substrateshould be replaced by Type N2 hard sealants toBS 2499, brought up almost flush to the surface, andexpanded polythene backing strips should not be used.These tend to be compressed by the roller and thenrecover, cracking the surface course. The thin surfacingoverlay should be laid continuously and the roadmonitored for the appearance of cracks in the surface. Ifcracks or depressions appear at a later date, the materiacan be sawn to encompass any cracks or depressions,and sealed with material complying with BS 2499 orBS 5212. It is important to ensure that, as far as ispractical, the sawn edges of the joint should coincideexactly with the underlying edges of the concrete joint tominimise spalling or ravelling of the surfacing.

High Friction Surfacing

6.12 Where high friction surfacings (HFS) are to beapplied over thin wearing course systems at approachesto roundabouts and other highly stressed sites, the deep‘negative’ texture can reduce the coverage of resinbinder to such an extent that the adherence of calcinedbauxite chippings may be reduced, resulting inpremature chipping loss. To alleviate this problem thetexture of the area of thin surfacing that will be coveredby HFS should be reduced to between 1 to 2 mm asmeasured by the sand patch test. This may be achievedby any suitable means at the discretion of the systemproprietor, for example by additional compaction withvibrating rollers whilst still hot or by the substitution ofa smaller aggregate size in these areas. Alternatively asuitably sized grit may be applied and rolled in. If thethin wearing course system is to be trafficked prior tothe application of HFS, then 3 mm grit should be appliedand rolled in to provide enhanced short-term skid

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sistance. Temporary warning signs may be appropriatesuch circumstances.

nefits

pid Construction3 Thin wearing course systems offer a very fastans of resurfacing roads and can be laid more

eedily than hot rolled asphalt and possibly surfaceessing. Faster application can result in lower costs forffic management and reduced costs of delay to thevelling public. For the thinner systems outputs up to,000 m2 per day, and sometimes even more, have beenhieved under suitable conditions.

duced working area4 Less working width is necessary to lay andmpact thin wearing course systems compared to hotlled asphalt which has to have chippings applied. Theipping machine is almost 5 m wide and can only beded from the side, sometimes necessitating a complet

ad closure for single carriageways.

wer Cost5 Being thinner and faster to lay, thin wearingurse systems can be significantly lower in cost thant rolled asphalt, but more expensive than surfaceessing. Where applicable the cost of planing andmoval is also reduced. Traffic management and delaysts are also reduced.

ise Reduction6 As a result of their flat, machine laid surfaces andiform negative surface texture, thin wearing coursestems can be significantly quieter than conventionalrfacings such as hot rolled asphalt and brushedncrete. However, at the present stage of theirvelopment, they are not as quiet as newly laid porousphalt. Some measurements have shown that slightlys noise reduction is achieved when systems using theger aggregate grading are laid very thinly.

ray Reduction7 Thin wearing course systems with adequateture depth exhibit spray suppression capability at lowels of rainfall due to their more open ‘negative’rface texture. This does not however approach that ofw porous asphalt. Like porous asphalt, spray reducingoperties diminish with time, although to a much lessertent, provided texture is maintained.

rface Regulation8 Thin wearing course systems have a skeletalucture and a high degree of compaction is achieved b paver. Most systems permit minor regulation of

isting surfaces and the recommendations of the systemoprietor should be obtained in this respect. General


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guidance is given in Notes for Guidance (MCHW 2)Clause NG 942.

Rut Resistance6.19 Due to their skeletal structure formed by thecoarse aggregate particles, thin wearing course systemgenerally have a high resistance to wheel track ruttingalthough they are vulnerable to deformation originatingin the lower pavement layers. Where a regulating layeis necessary below a thin surfacing, an SMA or amacadam basecourse, the latter designed and laid inaccordance with the Specification (MCHW 1)Clause 929, may be appropriate to maintain a highresistance to deformation.

Limitations

Life Expectancy6.20 Hot applied, machine laid thin surfacings, adaptefrom continental practice, have been in use in the UK fless than 10 years. Elsewhere in Europe the productsfrom which they have been developed have demonstrasatisfactory working lives of between 10 and 20 yearsand more. In the UK, safety considerations, in particulhigh speed skid resistance - surface texturerequirements - and low speed skid resistance (MSSC)are likely to limit the lives of thicker SMA derivedsystems to about 15 years, whilst very thin systems arunlikely to provide satisfactory working lives much inexcess of 10 years. Parallels can be drawn between thlikely deterioration of higher void content thin surfacingand porous asphalt.

6.21 This presupposes that the condition of thesubstrates on which they are laid is satisfactory. Areduced working life maybe anticipated for anybituminous surfacing applied over an existing surface base that is not in a reasonably sound condition. Thisparticularly so for thinner surfacing systems.

6.22 Satisfactory working lives between 7 to 15 yearsmay be expected for thin wearing course systems -depending on their thickness, void content, the level oftrafficking and the condition of the underlying pavemen

6.23 The present HA type approval scheme for thinwearing course systems, soon to be superseded by BBHAPAS Certification, requires a trafficking trial of atleast 1000 cv/lane/day for a minimum period of 2 yearafter which the product shall be defect free - subject tosatisfactory substrate - and shall retain a minimumtexture in the wheel tracks of 1 mm. The assumption ismade that a product on the ‘primary’ trial, and othersites taken into consideration during this period,withstanding this level of trafficking for two winters andtwo summers will be robust, and that the level of riskafter two years is acceptable. The product is then

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approved for use on trunk roads including motorways inEngland without limit to the level of trafficking, but atwo year defect free guarantee is demanded for each sincluding texture retention. To date, when correctlymixed and laid, even the thinnest surfacings have proveto be satisfactory on very heavily trafficked sites.

Strength Contribution6.24 Type A thin wearing course systems ( less than18 mm in thickness) are likely to have a lower stiffnessthan hot rolled asphalt due to the effect of their negativesurface texture. A structural contribution of 50% of thesame thickness of HRA is suggested. However whenused in new construction or for major maintenance, theincreased thickness of the stiffer roadbase required toachieve the overall design thickness given in HD 26(DMRB 7.2.3) may be assumed to more thancompensate in terms of load spreading ability for thereduced stiffness of the thin surfacing. The effect ofsurface texture reduces for thicker systems, many ofwhich are proprietary versions of stone mastic asphalt,and a similar stiffness to HRA on a pro rata basis forthickness should be assumed.

Hand Application and reinstatement6.25 It is preferable that thin wearing course systemsshould not be laid by hand except where a paver cannooperate, eg nosings of roundabouts, and then only infavourable weather conditions. Due to their low fineaggregate content, thin wearing course systems appeabinder rich and ‘sticky’ and being thin, they lose heatrapidly making them difficult to hand lay and compactsatisfactorily. Their use on urban sites for example,should be carefully planned. Ironwork should be lifted inadvance, and edge details and minor bell mouth openinwhere it is not possible to lay by machine should besurfaced by hand, but only in optimum weatherconditions. Alternatively providing prior agreement hasbeen obtained from the Overseeing Organisation, suchareas may be resurfaced with thicker hot rolled asphalt(HRA) and 14 mm nominal size coated chippings ordense bitumen macadam (DBM) laid by hand. Similarlythe borders around any ironwork that cannot be raisedadvance of the paving machine should be made good ithis manner or with a proprietary cold lay material,mechanically compacted.

6.26 Minor repairs to thin wearing course systemswhich will not significantly affect noise generation -where this is an issue - or outside the wheel tracks, mabe repaired, with the prior agreement of the OverseeingOrganisation, using HRA or DBM as described above.However major trench reinstatements for example,should be reinstated with machine laid thin wearingcourse. In some circumstances it may prove expedientand more economic to resurface a complete lane width


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Adverse weather working6.27 A high degree of the compaction necessary forthin wearing course systems is achieved by the paver,compaction being completed by the roller tight up behindthe paving machine. In this respect, and because theapplication of chippings is unnecessary, the installationof thin wearing course systems in less than perfectweather conditions might appear straightforward.However the very thin Type A systems lose heatextremely rapidly and should not be laid in the wintermonths unless the pavement is adequately pre-heated.Conversely some thicker Type C systems can be laid andcompacted successfully in temperatures as low as 2°Cand rising, provided the air is still and the substrate isdry with a temperature above freezing. If however, thewind speed at a height of 2 m is 5 km/hr, then an airtemperature of 10°C or more may be necessary toachieve full compaction. Thin wearing course systemsare proprietary products and their design, manufacture,transportation, laying and compaction are theresponsibility of the Contractor. Guidance should beobtained at tender stage with regard to any constraintson laying and compaction in adverse weather conditions


February 1999

7. STONE MASTIC ASPHA LT

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Chapter 7Stone Mastic Asphalt

Background

7.1 Stone Mastic Asphalt (SMA) is a hot mix surfacecourse relatively new to the United Kingdom but whichwas developed in Germany nearly 30 years ago to resistudded tyres. Despite a subsequent ban on the use ofthese in 1975, stone mastic asphalt has continued to bused because of its superior performance whencompared to the asphaltic concrete surfacing. It wasstandardised in the German Technical Specifications in1985 and is the most common surfacing in use inGermany today. Variants of the material have also beenadopted in many other countries including Sweden,Denmark, the Netherlands, Belgium, France andSwitzerland in Europe, and further afield in Japan,Australia and the USA.

7.2 In Germany the standard aggregate sizes used a0/11 mm, 0/8 mm and 0/5 mm, there are however nomacrotexture requirements. A TRL report by Nunn(PR65 1994 ) showed that by increasing the nominalaggregate size to 14 mm it is possible to achieve themacrotexture necessary to provide the high speed skidresistance required on trunk roads including motorwaysin the UK.

7.3 Stone mastic asphalt essentially consists ofdiscrete and almost single sized aggregate particlesforming a skeletal structure bonded together by masticAt the bottom, and in the bulk of the layer, the voids inthe coarse aggregate are almost entirely filled by themastic, while at the surface the voids are only partiallyfilled resulting in an open surface texture. Providedtexture is maintained, this provides good skiddingresistance at all speeds and facilitates the drainage ofsurface water. A careful choice of aggregate size, shapeand grading is necessary to produce a surface that wilmeet UK texture requirements for high speed roads. Thmastic consists of a blend of crushed rock, sand, filler(usually ground limestone) and an additive or modifier tprevent binder drainage while the material is hot. Themost usual additive is cellulose fibres, but mineral fibreand polymers have also been used, both separately anin combination.

Properties

7.4 SMA has proved to be durable and resistant toage hardening as a consequence of its low void contenand thick binder film. As a result it is resistant topremature cracking, ravelling and moisture damage.Other advantages claimed for the material are its abilityto shape an uneven or rutted surface, because the

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majority of the compaction is carried out by the paverand there is little further compression under rolling. It inecessary to limit the void content to ensure adequatedurability. However, if the void content is too low,deformation can occur resulting in a loss of surfacetexture.

7.5 Noise measurements in other countries haveshown that SMA surfacing is significantly quieter thanasphaltic concrete. Similarly work at the TRL hasshown that 14 mm SMA with a texture depth of1.67 mm is quieter than chipped Hot Rolled Asphalt aboth 70 and 90 km/hr; and 10 mm SMA with a texturedepth of 1.27 mm is quieter still.

7.6 Germany has used 0/16 SMA as a base-coursewhere rutting has been a problem below the surfacecourse. SMA has also been used in the UK as a rutresistant basecourse for high performance HRA and isuitable as a basecourse for proprietary thin wearingcourse systems. 10 mm SMA tested at TRL had a whtracking rate of 0.8 mm/hr and 14 mm SMA 1.1 mm/hr,both measured at 45°C. These results and othersmeasured under contract conditions have confirmed thin general well designed SMA mixes have a highresistance to deformation.

Design

7.7 It is the supplier’s responsibility to design the mixfor the constituents that it is intended to use and for thsite where the material is to be laid. The design of thematerial will depend on the aggregate grading andparticle shape and the thickness at which the materialto be laid. A change in any of these parameters maymean a change in the design of the mix. The grading abinder content must be tightly controlled once the jobmix has been agreed.

Laying

7.8 Laying the material has not proved to be aproblem, but hand laying must be avoided as the veryhigh stone content and fibre reinforced or modifiedbinder make the material extremely difficult to handwork. The final appearance of hand laid or even handworked material is very different from that of themachine laid material and may, because of higher voidcontent, be significantly less durable.


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Chapter 7Stone Mastic Asphalt

Specification

7.9 Site trials undertaken in the UK haveconfirmed German experience that SMA mixes arevulnerable to small variations in aggregate gradingand binder content, which can result in a reductionin surface texture. SMA to the same specificationbut using aggregates from different sources, laid ona variety of trial sites on roads in England have,whilst performing admirably in other respects,produced inconsistent results in terms of the textureretention essential for high speed skid resistance. Inconsequence stone mastic asphalt to a genericspecification shall not be used as a wearing courseon trunk roads including motorways in England,but it may be used as a basecourse. For schemes inWales, Scotland and Northern Ireland where SMAis proposed, a Departure from Standard shall beobtained from the Overseeing Organisation.

7.10 To take advantage of the superior properties ofSMA as a wearing course, proprietary versions designedfor particular aggregate sources have been developed asthin wearing course systems to ensure surface texture ismaintained. These require BBA HAPAS Certification, asdescribed in Chapter 6 of this Part.


Chapter 8Surface dressing

8. SURFACE DRESSING

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8.1 Surface Dressing is a principal method of routinemaintenance of road surfaces. The concept isstraightforward: in its simplest form, a thin layer ofbituminous binder is applied to the road surface andstone chippings, nominally single sized, are spread anrolled into it.

Purpose of Surface Dressing

8.2 Surface Dressing performs two functions whichrelate directly to essential requirements of the EuropeaConstruction Products Directive:

Safety - Skid Resistance

8.3 Surface Dressing increases the macro-texture amicro-texture of the road surface, with minimum usageof scarce high-quality aggregate. These propertiesdirectly influence the skid resistance of the road surfaca significant aspect of its contribution to safety.

Durability - Preventative Maintenance

8.4 Surface Dressing seals the underlying surfaceagainst the ingress of water and air, which causedeterioration of the structural courses of the road. Thisis preventative maintenance, which directly influencesthe durability and therefore the life of the road.

Tender Programming

8.5 Road hardness tests should be carried out in thesummer or early autumn of the previous year while roasurfaces are above 20°C. Experienced contractors witpremium quality plant are in limited supply and arecommitted to contracts on a first come first served basso late tendering will increase the risk of a contractbeing carried out by less experienced operators usingpoor quality equipment. In order to obtain best value fomoney and the most cost effective product, Tendersshould be issued - based on a provisional programme necessary - before the end of December preceding thesummer in which the work will be carried out. Tendererdesigning surface dressing systems will need time to vieach site to finalise their designs. After return of tenderfurther time is necessary for tender evaluation andassessment including simple whole life cost comparisowhere appropriate. Heavily trafficked roads are bestsurfaced dressed between mid-May and the end of Jul

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If work is to be carried out at night then surface dressinshould be completed earlier - by mid-July. High PSVchippings for heavily trafficked roads can be inrelatively short supply during early summer whensurface dressing is at its peak and to avoid delays andobtain best value for money contracts should be let asearly as possible.

The Process

8.6 Although the concept is straightforward, untiladhesion is ensured and an interlocked mosaic ofchippings formed, the dressing is vulnerable to traffic,especially when site conditions are adverse, too wet, hor cold. This might lead to the conclusion that, with theprevailing weather in the United Kingdom, there is littlechance of success, but in fact the majority of mediumand lightly trafficked roads are maintained by thisprocess.

Where it may be used

8.7 Surface Dressing is one of the most common ofall treatments and is a principal method of routinemaintenance of road surfaces. It is one of the mostcommon of all treatments, and one that has becomeincreasingly important since the introduction ofstandards for skidding resistance on trunk roads. It ispermitted on all types of roads from unclassified tomotorways, but demands particular care duringconstruction on high-speed roads. It is suitable for bothconcrete and bituminous roads, although hard surfaceor soft and variable substrates present special problemIf the existing road surface has a poor profile or isdeformed in the wheel tracks (a rut depth greater than10mm) then pre-treatment by planing (milling) orsurfacing may be required. Thin asphalt surfacingoverlays in these circumstances may have greatereconomy as they have some ability to improve profile.

8.8 There are circumstances where it is not possibleto provide controlled low speed trafficking which isnormally used to settle a dressing down prior tosweeping and opening to unrestricted traffic. The mostdifficult of these are motorway sites where the trafficflows and speeds are such that convoying could bedangerous. Lane switching may be permitted to enablesweeping after a period of unrestricted trafficking. Insuch circumstances it is vital to produce a very stabledressing that can be fully swept prior to trafficking withminimal risk of loose chippings subsequently.

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The Equipment8.9 For trunk roads and motorways the sprayer andchipping spreader should be capable of accuratelyplacing a complete lane width, generally up to 3.8m, insingle pass with no joints or overlaps. The abutting lanare joined longitudinally by binder overlap of up to100mm at the full spray rate at the lane markings. Thimeans that wet edges are formed and these must beprotected from traffic during construction and excellenwork has been achieved on even the busiest motorwawhere special systems and techniques are employed.

Benefits

Skid resistance8.10 By selection of a suitable surface dressingtechnique, chipping type and chipping size virtually anreasonable values for macro-texture (texture depth),micro-texture and abrasion resistance may be obtainefor the road surface. The road surface characteristicsmay thus be designed for site conditions. Chippings wa high polished stone value (PSV) are specified for areof traffic stress such as braking areas, hills or bends alow aggregate abrasion value (AAV) chippings selectefor heavily trafficked sites to reduce wear rate.

Preservation8.11 Surface dressing is a preventative maintenancetechnique as well as a repair method. Regular dressinwill keep the upper structural layers of the road sealedagainst the ingress of air and water, thus reducing therate of deterioration of these layers. Surface Dressingalso reduces the amount of water reaching the sub-grathus minimising deterioration, thereby maintaining thestructural integrity of the road pavement.

Conservation8.12 An additional benefit of surface dressing isconservation of existing materials: the dressing has athickness generally less than 10-12mm, so planingbefore treatment is not necessary when the profile isacceptable and ironwork does not normally have to beraised. Chippings with appropriate PSV and AAV arerequired to provide continuing adequate safety standaon busy roads; with surface dressing all of the highquality chippings are at the surface in contact with thetraffic.

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Environmental8.13 Emulsions are environmentally friendly: theemissions are mainly water vapour. Cutback solvent iprimarily kerosene (similar to domestic paraffin), one the least hazardous low cost organic solvents. Wastematerials (bitumen and chippings) are not currentlyclassified as hazardous.

Appearance8.14 Although chippings for surface dressing on trunroads will be chosen for their skid resistance and weaproperties depending on traffic category and trafficstress; coloured chippings may be used for delineatinhard shoulders and central reserves, or used in trafficcalming measures.

Speed of Works8.15 The speed of surface dressing is a major assetreducing road closure during maintenance. For singlecarriageway work with coned off sections and trafficcontrol, outputs of 10,000m² per day are possible. Wicontra-flow and safety lanes output is really only limiteby the supply of chippings to site and the frequency otesting as the binder sprayer is able to apply 10,000mabout one hour. The ability to surface dress at nightminimises traffic disruption.

Drawbacks

Structural strength8.16 Surface dressing does not strengthen the roadstructure.

Profile8.17 Defects such as rutting and shoving must beeliminated before the surface dressing is applied, thedressing making no improvement to the road profile.Soft patching materials, binder rich crack repair bandsealing (including some hot screeded proprietarysystems) and existing fatting in wheel tracks are likelyallow rapid embedment on heavily trafficked roadsleading to early loss of texture.

Riding quality8.18 There is no improvement in riding quality.

Noise8.19 The high texture depth (macro-texture) achievewith surface dressing may cause an increase in noiselevel, especially in the very early life before anychipping embedment and mosaic formation have takeplace. The use of multiple dressings or smaller chippisizes reduces noise generation and are often specifieurban areas for that reason.

February 1999



Sensitivity to Weather Conditions8.20 Given a proper design and good constructioncarried out at the correct time of year, surface dressinghas a high probability of success unless the weatherconditions unexpectedly deteriorate. Wet weather shortlafter construction may cause the chippings to becomedetached from the binder (binder stripping) and trafficmay dislodge them destroying the dressing in the wheeltracks. If traffic can be diverted until the road driesagain and then re-introduced carefully, the dressing maybe saved. In very hot weather during and immediatelyafter construction the chippings may turn and be pickedup by vehicle tyres. A maximum road temperature of40°C may be specified, even for modified binders, toreduce this risk. Lightly coated chippings are especiallyvulnerable. Fine material, particularly absorbent lightcoloured 1-4mm chippings, (‘dust’), applied to thedressing may prevent this mode of failure, a racked-in odouble dressing is less vulnerable due to the smallerchippings at the surface. Conventional emulsion bindersare generally less of a problem in hot weather thancutbacks except when there is high humidity. Howevercutbacks have an advantage over emulsions at the loweoperating temperatures when it is humid.

Surface Dressing Techniques

8.21 There are a number of different systems ofsurface dressing available. All of those current at thetime of publishing this part are described below butothers may be developed and their use should notnecessarily be precluded because they are not describehere.

Standard (single binder single chipping application)8.22 The basic surface dressing technique using asingle application of binder followed by a singleapplication of chippings, usually 6 or 10 mm, issatisfactory for most lightly trafficked roads. (SeeFigure 8.1). When used with an unmodified binder it issuitable for lightly trafficked roads without significantareas of stress. It is also used with modified binder onsomewhat more difficult sites such as cul de sacs whereall the traffic, although small in quantity, is turning orbraking. Modified binder dressings are more resistant todamage caused by power steering being used whenvehicles are stationary.

Racked-in (single binder spray double chippingapplication)8.23 Binder is applied at a higher rate than for singledressing, and the primary size chippings (typically 14mm on fast heavily trafficked roads) applied at a lower

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ate (90% of that for a single dressing), followedmmediately by small chippings (6 mm) to fill the gapsnd achieve mechanical interlock (see Figure 8.2). Thedvantages of the racked-in method are high initial

exture depth, early stability of the dressing, and a majoreduction in the initial loss of large chippings.

nverted double dressing (Pad coat).24 Where the existing road surface is very hard ororous with high or variable macro-texture, a firstressing using small chippings (6 mm) can be made torovide a uniform softer surface to which the mainressing is applied (see Figure 8.3). It is common to

eave the pad coat exposed to traffic for up to a yearefore applying the main dressing. The advantage ofsing a pad coat is that the main dressing chippingsmbed quickly, increasing resistance to chipping loss.

ouble Dressing (Double binder spray, doublehipping application).25 Binder is applied at a little less than the normalate (typically 1.3 l/m² of emulsion), and normal sizehippings (usually 14 mm) are applied at slightly lesshan the normal rate for a single dressing. A secondpplication of binder (typically 1.2 l/m²) and smallhippings (6 mm) follows (see Figure 8.4). Thedvantages of the double spray double chipping methodre moderately high texture depth, reduction in loss of

arge chippings, and the possibility of using largerhippings than would usually be selected for the road.nitial stability may be low, but builds up rapidly, andtressed areas may be treated using modified binders. Its usual to allow extended time for rolling and curing ofn emulsion binder before opening the dressing to

raffic. Solvent entrapment when using cutback bindersay present problems unless sufficient time is allowedetween layers to enable evaporation to take place.

.26 The use of double dressings - even where trafficlows are low - on exposed sites such as hills orountains enhances durability, and is standard practice

n some European countries. Double dressings areeneficial on high speed, heavily trafficked dualarriageways and motorways where slow speeds torientate the chippings and form a mosaic are difficult ifot impossible to ensure. Interlocked double dressingystems have a high initial stability, particularly whensed in conjunction with premium grade binders.

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Schematic Representation of the Types of Surface Dressing Prior to Embedment

Figure 8.1 Single surface dressing

Figure 8.2 Racked-in surface dressing

Figure 8.3 Inverted double dressing

Figure 8.4 Double dressing

Figure 8.5 Pre-chipping Dressing (Sandwich dressing)

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Pre-chipping Dressing (Sandwich dressing)8.27 The primary chippings are applied first (typically14 mm) followed by a single surface dressing with lowrate of binder spread (typically 1.2 l/m² of a modifiedemulsion and 6 mm chippings) (see Figure 8.5). It isimportant not to allow traffic to disturb the primarychipping layer, which has to be laid to close tolerancesThis technique creates voids enabling the system totolerate a binder-rich (or variable) surface, and allows stable dressing to be constructed using larger thannormal chippings. A variant of this with a rate of spreadof binder of typically 2.2 l/m² has been used successfulon a heavily trafficked single carriageway, normalhardness site in very hot weather.

Differential Rates of Spread and Chipping Sizes8.28 With some modern sprayers it is possible to varythe rate of spread across the road, (ie transversely)which enables lower rates of spread of binder to be usein the most heavily trafficked parts of the road, namelythe wheel tracks, where less binder will lead to a longerlife before the onset of fatting. Using larger chippings inthe wheel tracks has the same benefit and may evenimprove transverse profile.

Cured Resin Binder Dressing8.29 High performance thermosetting binder often usewith calcined bauxite chippings. These dressings aresignificantly different from normal surface dressing.Reference should be made to Specification (MCHW1)Series 900, ‘Resin Based Skid Resistant SurfaceTreatment’.

Design Principles

8.30 Road Note 39 (1996) provides a complete guideto the design and practice of surface dressing. Theinformation is based on systematic experiments andtrials by TRL over many years, in close co-operationwith both industry and the Highway Authorities. TheRoad Surface Dressing Association (RSDA) and RoadEmulsion Association have published many documentswhich complement this Chapter.

8.31 The decisions to be made when specifying surfacdressing for a particular length of road are outlined inFigure 8.6, which is targeted primarily at surfacedressings for high-speed roads carrying heavy traffic,circumstances under which a simple single dressingusing unmodified bitumen will most probably beinadequate.

8.32 In situations where a simple surface dressingusing unmodified bitumen will suffice, a traditionalrecipe specification based on the guidance in Road No39 (1996) is all that is needed.

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8.33 For high speed roads carrying heavy traffic,modified binders and multiple layer surface dressingswill almost certainly be required. For these the empiricaapproach to design is becoming less acceptable, and wend performance in mind, rational designs tailored to a‘system’ are becoming favoured. This type of design isoften best left to the contractor who has expertise with aparticular system and will carry the risk of prematurefailure. It is important for the Overseeing Organisationto ensure that the correct performance levels of thesurface dressing are specified and obtained, in particulaany parameters specified must be measured todemonstrate compliance. The Overseeing Organisationwill need to be satisfied that there is minimum risk offailure during the designed life, which may be muchlonger than the guarantee period.

Principal Chipping Size Selection

8.34 The size of chipping selected for use at aparticular location is based on the degree of embedmenof the chippings expected having regard to the hardnessof the existing road surface and traffic intensity. Themeasurement of road hardness is a critical design factoand for a particular traffic category determines the sizeof chipping to be used (see Road Note 39 (1996)). If thehardness varies considerably along the site then it maybe difficult for the contractor to achieve consistent highlevels of end performance. In such situations, agreemenbetween the Overseeing Organisation and contractorwith regard to the practicality of achieving the specifiedlevels will be necessary.

8.35 Chippings are nominally single sized. A highflakiness index value for the chippings is undesirable;this may be measured by a simple mechanical test. Theshape of the chippings influences the mechanicalinterlock between the chippings, the amount of binderneeded to secure them and more importantly, thedurability of the dressing. The contractor has to selectchippings and a system suitable for the site, to enable thspecified micro-texture and macro-texture to bemaintained. It is macro-texture that largely determineshigh speed skid resistance and reduction in spray; at lowspeeds skid resistance depends more on the micro-texture of the chippings - a function of PSV.

8.36 Dust is generated in the transportation andhandling of surface dressing chippings. Pressurewashing and heater drying, even on site, have been useto reduce the problem, to improve adhesion and reduce

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Figure 8.6 Flow Charts for Specification and Design of Surface Dressing

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the risk of failure. These techniques, as well as the useof chemically or bitumen coated chippings should beconsidered for works on trunk roads, although the use lightly coated chippings with emulsion binders areunlikely to be used, as this may slow down the rate ofbreak of an emulsion and actually inhibit chippingadhesion.

8.37 Since the main purpose in using polymer modifiebinders is to provide a durable product and minimise thrisk of loose chippings, it is pointless to use thoseexpensive binders with chippings having low PolishedStone Value, high Flakiness Index, high AggregateAbrasion Value, or high dust content. Some variation othese properties is to be expected even in chippings froa specific quarry, since there may be variation in theproperties of the rock at different faces within the samequarry.

8.38 Table 2.1 in HD28 (DMRB 7.3.1.2) shows theminimum PSV requirements for use on roads of differintraffic intensity and stress, and Table 2.2 shows the AArequirements for differing traffic levels. In order toconserve aggregates the highest PSV chippings shouldnot be specified for all roads, but to consider each siteindividually. Unfortunately wear resistance generallyimproves with chippings of lower PSV, thereforedurability and safety are a balance and savings may bemade by optimum selection. For trunk roads wherecontra-flow occurs during maintenance, each lane maybe subject to the same traffic conditions therefore thePSV and AAV are generally chosen to be the same.However much lower traffic forces result in lessembedment in the right hand lane of dual carriagewayswith three or more lanes (high speed, no commercialvehicles - except during contra-flow) and a smallerchipping may be used. The PSV requirement of over 7on some difficult sites necessitates the use of calcinedbauxite. Resin based skid resistant treatments aregenerally employed (see Specification MCHW1),although 6mm calcined bauxite with polymer modifiedbinder as a single dressing has been used successfullyOther artificial aggregates such as calcined flint, blastfurnace or steel slag may be used to both economic anconstructional benefit provided skidding characteristicsare maintained.

8.39 When Racked-In techniques are used in surfacedressing a smaller size chipping with a lower PSV thanthe larger primary chipping may be permitted, since thesmall chippings will not predominantly come intocontact with vehicle tyres. When however, the secondchipping is larger than half the principal size (for

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example 10mm followed by 6mm) then the PSV shouldbe similar. For all other multiple chipping systems allsizes should have the design PSV. On trunk roads nochippings should have a PSV of less than 45.

8.40 Adhesion between binder and chippings dependon the chemical nature of the chippings, the binderproperties (use of wetting or adhesion agents), and moof all on the amount of dust or clay/silt surroundingsome types of chippings. The contractor should test thchippings and binder using a suitable adhesion test suas the Vialit Plate Shock Test.

Specification of Surface Dressing

Recipe or performance-related specification?8.41 If consideration of Table 8.1 indicates that theproposed dressing falls comfortably in the easy type osite, a recipe specification may be used. For a difficulttype of site with higher risk rating, a modified binderwill be required (see paragraph 8.52) together with aracked-in dressing or other multiple treatment, and useof a performance-related specification is advised. Eitheroute could be followed for average types of site. Withrecipe specification responsibility for the design restswith the Overseeing Organisation; with a performancespecification this responsibility is transferred to thecontractor. It is anticipated that as experience is gainewith performance related specifications the use of recispecifications will diminish.

8.42 It is in the areas of ‘high average’ and ‘lowdifficult’ sites that the design expertise of a contractorcan make the most significant cost savings. On verydifficult sites only the highest quality best performingmaterials coupled with the highest standards ofworkmanship will be successful. The contractor’sDesign Proposal and method for execution of the workneeds careful, informed assessment particularly withregard to safety aspects.

Recipe specification8.43 Figure 8.6 gives a flow diagram for planning andspecifying dressings. A break-out point for use of aperformance-related specification is also given. RoadNote 39 (1996) provides a sound basis for generatingrecipe specification for surface dressing andSpecification clause 919 (MCHW1) sets out therequirements; no further advice will be given here.

Performance-related specification8.44 In the past specifications have been generallyrecipe specifications, with the Overseeing Organisation

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Traffic at Design Life(commerical vehicles / lane / day)

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00

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Site DefinitionSiteCategory

Dual carriageway (all purpose) non-eventsections

Motorway (main line)

Dual carriageway (all purpose) minorjunctions

Single carriageway non-event sections

Single carriageway minor junctions

Approaches to and across major junctions(all limbs)

Gradient 5-10%, longer than 50m (dualdownhill, single both ways)

Bend, radius 100-250m (not subject to aspeed limit 40mph or less)

Roundabout

Gradient >10%, longer than 50m (dualdownhill, single both ways)

Bend, radius <100m (not subject to aspeed limit 40mph or less)

Approaches to roundabout, traffic signals,pedestrian crossing, level crossing, etc

A

C

B

E

F

G1

H1

J/K

L

G2

I

II

III

IV

V

H2

D

Key:

Easy sites, conventional binders, minimum cohesion 0.5 J/cm2

over a minimum temperature range of 15oC

Fast and/or moderately difficult sites, intermediate grade binder, minimum cohesion 1.0 J/cm2

Difficult sites, premium binder, minimum cohesion 1.2 J/cm2

High friction surfacing systems

Not suitable for surface dressing or unlikely traffic levels---

Table 8.1 Areas of use for surface dressing binders

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specifying materials, quantities and procedures. Thisrequires a degree of expertise in and experience of thematerials and processes. With the introduction ofmodified binders, proprietary materials and noveltechniques, new expertise is required. The dependenceon the supplier to maintain properties of his product toensure consistent road performance and the Contractorto determine the process technique for the siteencourages the transfer of responsibility to theContractor, in the short term at least.

8.45 Performance-based specifications are the logicalconclusion of that process: all the design work is carriedout by the contractor, with the Overseeing Organisationspecifying only the level of performance required, andimposing no checks other than regular assessment ofperformance. However, this procedure presents afinancial problem: either the contractor must guaranteethe work for its design life, and accept that he will not bepaid in full until the end of the design life, or theOverseeing Organisation must pay in full before therequested life has been achieved.

8.46 In the former case specification writing would besimple: the Overseeing Organisation would state theirrequirements and the contractor would fulfil them in anyway he wished. This would be an unacceptable burdento contractors because of the financial aspect of paymedelayed for several years. In the latter case the idealsolution would be to perform predictive tests on thematerials before and immediately after application to theroad, which would demonstrate beyond reasonable douthat the work would last for the specified length of time.Unfortunately there is insufficient knowledge at thepresent time for this to be possible. The OverseeingOrganisation would therefore not be willing to pay infull for the job until near the end of the specified life,which again would be unacceptable to the contractor. Aa compromise, the specification to which this advicerelates asks the contractor to do the design work forspecified performance and carry out performanceprediction tests, and in addition imposes testingrequirements to ensure that the proposed design has becarried out with a reasonable degree of precision. Therisk element is divided between contractor andOverseeing Organisation, by having a Guarantee Periodof two years (which is much less than the Design Life ofthe dressing). During the two years, the contractor musmake good any defects, after which period theOverseeing Organisation accepts the risk and any cost future remedial work.

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.47 A further assurance of continuing performance tooth contractor and Overseeing Organisation is the usef an approved binder - one which has been through anvaluation procedure. A formal materials approvalcheme is being developed jointly by the Highwaysgency, County Surveyors' Society and the Britishoard of Agrément (BBA). Until the Highwayuthorities Product Approval Scheme BBA/HAPAS is

n place the Overseeing Organisations will continue toperate the practice of informal approval, listingurrently accepted materials and processes on the basif established track record with guidance as to categoryf performance, where possible, and new materialsnder examination for approval. The combination ofecipe tests (based on extensive past experience),redictive tests (developed comparatively recently and aet not fully proven) and use of an approved binderhould satisfy the Overseeing Organisation that there isigh probability of the work having the life estimated,hich will be very much longer than the guaranteeeriod.

ffect of existing surface

.48 The assessment of need for the maintenance of aection of road is outside the terms of reference of thishapter. Reference should be made to HD29 (DMRB.3.2) Structural Assessment Methods, HD 28 (DMRB.3.1) Skidding Resistance and the CHART visualssessment procedure. Once it has been decided that aection of road needs some form of treatment and thattrengthening is not required, then the suitability ofurface dressing for the treatment should be assessed.or surface dressing to be suitable there should be a hirobability that the treatment will produce the level oferformance required over a reasonable lifetime. The

actors affecting the decision are: traffic levels andpeed, difficulty of the site and the existing road surfacedvice on the suitability of surface dressing with respec

o traffic levels, speed and site difficulty is given in Roadote 39 (1996). The main variables of the existingurface that will affect the final dressing are porosity,oughness, amount of fatting, hardness andeterogeneity. Table 8.2 (based on French experience)hows the normally achievable performance, using bestractice, of a correctly designed and appropriate system

or different traffic levels and surface types. Twoifferent performance criteria are given depending onpeed (the difference is in the macrotexture required). Aould be expected, the heavier the traffic the moreritical is the state of the current surface. At the highest

traffic end only the normal, non-porous, homogeneous,fairly smooth road can be surface dressed with a good

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Table 8.2 Surface Dressing - Minimum Requirements andTypical achievable specifications on various existing surfaces for different sites

Minimum Requirements for Surface DressingTraffic (commercial vehicles/lane/day)

0-250 251-1000 1001-2500 2501-3250 > 3250

Category

No speed limit H 1.05 2 H 1.05 3 H 1.05 3 H 1.05 4 H 1.05 4

30 or 40 mph L 0.7 2 L 0.8 3 L 0.8 3 L 0.8 4 L 1.05 4

Existingsurface Typical Achievable Performance Specifications for Surface Dressing

characteristics

Porous X 0.8 2 X 0.8 3 X 0.8 3 X 0.8 3 X 0.8 3

Rough HL 1.05 3 HL 1.05 3 HL 1.05 3 X 1.05 3 X 1.05 3

Normal,homogeneous HL1.05 4 HL 1.05 4 HL 1.05 4 HL 1.05 4 HL 1.05 4

Soft,homogeneous HL1.05 4 HL 1.05 4 L 0.8 4 L 0.8 4 X 0.8 4

Fatting up L 0.8 2 X 0.8 2 X 0.7 1 X 0.7 1 X 0.7 1

Bleeding X fail X fail X fail X fail X fail

Heterogeneous,tracked A* HL 1.05 3 HL 1.05 3 HL 1.05 3 X 1.05 3 X 1.05 3

Heterogeneous,tracked B* L 0.8 4 L 0.8 4 L 0.8 4 L 0.8 4 X 0.8 4

Heterogeneous,patched L 0.8 2 X 0.8 2 X 0.7 1 X 0.7 1 X 0.7 1

Notes:* A is for a design to optimise macrotexture.* B is for a design to minimise chip loss.• The macro-texture is given in mm (HRM) and is that which is achievable for a given site.• The Visual Assessment (VA) Class is the attainable visual assessment class Measurements

defects at the end of the 2 year guarantee period.• H indicates that the requirements can be met on high speed roads.• L indicates the requirements can be met on low speed roads.• X and shading indicate surface dressing will not normally meet the requirements.• This table shows that it is not possible to specify 1.05 mm texture depth with the highest

performance in terms of minimal defects on roads other than for normal homogeneous sites.the higher traffic levels the achievement of the minimum requirement becomes increasinglydifficult to attain.

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probability of success over an economic life. On verylightly trafficked roads a successful outcome is possiblein virtually all cases subject only to proper design andexecution.

8.49 There are a number of factors that can widen thetype of surface that can be treated using surfacedressing. The three main ones are to accept a lowerstandard, to accept a shorter life or to pretreat thesurface in some way to improve its characteristics.Surface dressing systems are continually improving anit is likely that developments in the future will enable awider range of surfaces to be successfully treated.Surfaces which Table 8.2 shows as not making the graby only one level may well be successfully treated insome instances by an improved process, whether in thebinder or in the system. As contractors gain knowledgeof, and confidence in, performance specifications thetype of site they will be willing to guarantee may alsowiden in scope.

8.50 Table 8.3 describes the various types of site thatare stated in Table 8.2 and also describes possiblepretreatments to overcome the constraints of the existinsurface.

Specification parameters

Parameter description and limits8.51 System of surface dressing permitted - in order tallow the contractor the maximum choice the OverseeinOrganisation should allow any system unless there areparticular reasons why a specific type is needed, forexample, to reduce noise generation or for durabilityreasons.

8.52 Binder cohesivity is a measure of the ability of thbinder to cope with traffic stresses. In general termsmost manufacturers produce three levels of cohesivity their range of binders, conventional, intermediate gradeand premium grade. These are characterised bycohesivity levels of 0.5, 1.0 and 1.2 J/cm2 respectively.(Note: when comparing two binders with the samecohesivity, the one maintaining the level over the widestemperature range is likely to perform better than theother). The more stressed the site the higher thecohesivity required but to some extent it is possible tocompensate for low cohesivity values by using a morestress resistant surface dressing system. Not all standbinders necessarily meet the lowest level so that testcertificates should always be required. It is always opefor a contractor to use a higher grade than that specifieSuggested levels are given in Table 8.1 which uses thesite categories from HD 28 (DMRB 7.3.1).

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In a true performance specification no value would bespecified leaving it to the contractor but it isrecommended that in the early years of this type ofspecification until wide experience is gained, a value isalways specified.

8.53 The minimum PSV of chippings is required toensure adequate resistance to skidding and the values Table 2.1 of HD 28 (DMRB 7.3.1.2) should be used.

8.54 The maximum AAV of chippings is required toensure adequate resistance to abrasion by traffic and thvalues in Table 2.2 of HD 28 (DMRB 7.3.1.2) should beused.

8.55 The class of spraybar is a guide to the evennesstransverse distribution which the spraybar is capable ofproducing and class 4 is the most onerous. This class inot attainable except by very well maintained spraybarsand should therefore only be specified where it can bemost beneficial, ie on roads where the current surface ivery even and the traffic levels are high, wherevariations in the rate of spread would show up veryquickly. It is recommended that class 4 spraybars arereserved for motorways, class 3 for other dualcarriageways and class 2 for other roads. Where a singcarriageway is very heavily trafficked and the currentsurface is consistent along the whole length and acrossthe whole width, then class 3 should be specified. Theevenness of rate of spread should be checked at the stof, or recently before, the contract and then weekly. Inaddition to the class of spraybar, the class for toleranceon the rate of spread of binder should be specified andfor this purpose the specification of class 3 is suitablefor all sites as it is both achievable and adequate. Thefrequency of test is set out in Specification (MCHW1)Appendix 1/5.

8.56 The class of chipping spreader is a guide to theevenness of transverse distribution which it is capable oproducing, class 2 being the most onerous. This classshould be specified for the primary chipping in a multilayer surface dressing otherwise class 1 is adequate. Tevenness of rate of spread should be checked at the stof the contract and then daily. In addition to the class ofchipping spreader, the class for tolerance on the rate ofspread of chippings should be specified and for thispurpose the specification of class 2 is suitable for allsites as it is both adequate and achievable. Thefrequency of test is set out in Specification (MCHW1)Appendix 1/5.

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Table 8.3 Description of surfaces prior to surface dressing

NOTE: SMTD = Sensor Measured Texture Depth

Surface Type Description Possible pretreatments

Porous Surfaces like porous asphalt and open Surface dressing with 6mm chippingtextured macadam some weeks before the main dressing

(or even the previous season)

Rough A surface with a texture depth above Application of slurry surfacing and1mm sensor measured, usually with some mechanical surface treatments some fretting normalise or reduce texture

Normal, Minimal variation in appearace over None neededhomogeneous whole section, SMTD <1mm,

hardness at least “normal”

Soft, As above but surface hardness is less None neededhomogeneous

Fatting up Has a surface layer of free bitumen, If it is only in the wheel trackes and husually limited to the wheel tracks taken at least 5 years to develop then

is worth removing the excess binderwith high pressure water jetting

Bleeding Has a surface layer of free bitumen, No pretreatment will avoid subsequenusually extending beyond wheel tracks failure, in very bad cases even onand often happens soon after a surface virtually untrafficked roadsdressing has been applied

Heterogeneous, This is the normal state of surface See Fatting up abovetracked which has been previously surface

dressed, because of the difference intexture and traffic across the lane it ispossible to optimise design for texturein the wheel track or chip retention inthe untrafficked areas

Heterogeneous, This occurs mainly in urban areas The problem is best reduced to apatched where most roads are subject to opening minimum by insisting on proper

by statutory undertakings companies reinstatement with materials whichwith subsequent reinstatement using match the hardness and porositymaterials which may have significantly properties of the existing road. Lowdifferent properties of porosity and speed roads which are badly affectedhardness from the surrounding road can sometimes benefit from asurface pretreatment with a slurry surfacing.

The slurry surface will have a lowmacrotexture and will have to be left tomature for a sufficient period beforesurface dressing, otherwise it will bevery soft

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8.57 Macrotexture: for general guidance on the needfor macrotexture and its measurement see HD 28(DMRB 7.3.1). The generally accepted specification forhigh speed roads of 1.5 mm minimum using the sandpatch test is based on hot rolled asphalt and pre-coatedchippings. That material maintains its texture over along period with changes varying from slow increase toslow decrease depending on the relative rates of wear othe chippings, embedment of the chippings and loss ofmatrix from the asphalt between the chippings. Becausthe changes are slow in HRA it is feasible to specify thetexture depth prior to opening to traffic. With a surfacedressing, however, the chippings re-orient themselves inthe relatively fluid binder-chipping matrix and chippingembedment occurs at a rate depending on the traffic anthe hardness of the substrate, with the consequentreduction in texture depth unless the substrate is veryhard (concrete) when there is no embedment.

8.58 The decay of texture depth with time is not linearand is rapid in the first year or two; it depends on manyparameters so it is difficult to extrapolate early lifetexture depth measurements. Figure 8.7 shows texturedecay of single dressings with conventional binders onthe M40 High Wycombe by-pass, reported by Jacobs,F.A., 1983. To obtain adequate texture at 2 years theinitial texture has to be very much higher, for someprocesses over 3 mm may be necessary which generatconsiderable noise, particularly under high speed trafficIn order to keep noise down to an acceptable level innoise sensitive areas it may be necessary to specify amaximum texture at the end of the initial bedding inperiod, say at 4 weeks.

Figure 8.7 Effect of type of chipping onthe reduction in texture depth with time.

(conventional binder on a rolled asphalt substrate)

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.59 The end product performance specification tohich this Chapter refers requires texture measuremen

nitially, after mosaic formation, at between three andive weeks (only where noise is a problem), after elevenonths and before thirteen months; and then at the endf the guarantee period (two years, except new untriedroprietary materials or systems where the guaranteeeriod should be one third of the offered design life). Theduction in texture over the period between 12 and 24onths provides some indication whether or not the

exture depth will remain above the required minimumalue for the design life of the dressing.

.60 All texture depths for surface dressing should bepecified in terms of HRM measurement (other methoday be used provided they are converted to HRMquivalent values) and the level for high speed roads aears would normally be specified at 1.03 mmapproximately the equivalent of 1.5 mm in the patchest). For roads carrying traffic at speeds below 90m/hr lesser textures should be specified. See Notes fouidance to Specification, Clause NG922 (MCHW2)

or appropriate values.

.61 The decrease in texture between 12 months andonths is a guide to the life of the dressing, the lower

he value the longer the life of the dressing, unless failuechanisms intervene. A maximum figure of 40% is anppropriate specification value with a minimum of 0%s any increase in texture indicates that the surface is

osing chippings.

.62 The level of fretting, P1, as measured using theest method described below, is a guide to likelihood ofailure by continued loss of chippings. Appropriatealues of the maximum permitted percentage are givenn Notes for Guidance to Specification, Clause NG922MCHW2).

.63 The level of defects, P2, as measured using the tes

ethod described below, is a guide to the likelihood ofnsatisfactory performance of the dressing. Appropriatalues of the maximum permitted percentage are givenn Notes for Guidance to Specification, Clause NG922MCHW2).

.64 The maximum level of localised loss of chippingsn any given area, P

3, should not be greater than the

pecified percentage which is given in Notes foruidance to Specification, Clause NG922 (MCHW2).

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Tests specific to surface dressing

8.65 Tests used to check the aggregates and the bindeused for surface dressing are covered in the sections onthe constituent materials following this section. The testsused to ensure that the surface dressing system issuitable and the process is carried out correctly are dealwith in this section.

8.66 The test for accuracy of rate of spread of binderin BS prEN12272-1 (1996) measures the transversedistribution in working conditions and thereforeovercomes the problems of assessing transients that areaveraged out in the depot tray test. The test can becarried out quickly and easily on site using eithercontiguous absorbent tiles or trays that are sufficientlyrobust to take the sprayer running over them withoutlosing their ability to absorb binder. The tiles may be offoam, carpet, or any other material which can absorb allthe binder sprayed on them without loss. If the tiles arelightly stuck to a strong backing strip they can be pickedup as a unit and removed to the side of road for theindividual tiles to be weighed and any hold up of thesurface dressing train is minimised. A method usedsuccessfully during specification trials in 1996 usedsections of carpet tile (250x100 mm) stuck to a length ofaluminium sheet which was covered with ‘cling film’thus enabling the whole testing to be carried out withoutthe use of solvents. The result is expressed as a meanrate of spread of binder and a coefficient of variationwhich is the standard deviation of the mass of binder oneach tile (which should have the same area) divided bythe mean mass.

8.67 Although the test described in paragraph 8.66gives the average rate of spread of binder a simpler testis recommended to assess this: in BS prEN12272-1(1996) is a harmonised version of the test that has beenused for many years in UK - the tray test, but now itmay be carried out using absorbent tiles. These can be oany suitable material and there must be at least 5 tiles.There are a number of constraints on dimensions but 5tiles consisting of a ½ or a ¼ of a standard 0.5x0.5 mcarpet tile is suitable. The individual masses aredetermined and the average reported. The variance,which, in this case, is the highest rate minus the lowestrate, ie the range of values, measured on individual tilesdivided by the average, is also calculated and if it isabove 0.2 the test is repeated and if the repeat test givesa variable result the cause must be investigated andpossibly an accuracy test carried out.

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68 Also in BS prEN12272-1 (1996) are tests forenness of rate of spread and accuracy of rate of sp chippings. These measures are particularly importa the context of multiple chip dressing where the rate read of the primary chipping is critical to the succes the dressing and the locking up of those chippings be secondary chipping.

69 It is essential for proper control of the rate ofread of binder that the temperature and pressure ofnder at the time of spraying are within the correctnge. This means that the thermometers and pressuruges on the tanker must be working and must be

ving the correct reading. In order to achieve thisliability, all the instruments should be calibrated befoe start of season using a system traceable to nationandards. It has been a requirement for many years fese gauges to be duplicated as the operatingvironment is very harsh. There is no reason to chan

is principle and the contractor's QA scheme shouldntain a requirement for the regular reading of theplicate gauges and if the readings differ by more tha

stated amount they should, if necessary, be repairedd recalibrated, or replaced by previously calibrateduges.

70 The standard method of measuring texture deptr surface dressing is different from the method used ated materials. The reasons for this is that it iseasured after the contract has finished and the Higheed Road Monitor (HRM) or its equivalents can cart the work without needing traffic control and the

ngths to be covered are often much greater than for ated material contract. Other test methods are notecluded but the results must be converted to HRMlues before comparison with the specified levels.ere is no precision data on any method of measurin

xture depth on surface dressing and care should beercised when interpreting results, particularly those

ose to specification limits.

71 In terms of defects like fretting (generalisedipping loss) surface dressing usually either failstensively or it does not fail at all. Border line cases are both for fretting and for localised chipping loss,wever assessment methods are given in draft prEN227055. They are somewhat tedious but it is unlikel

at they will be needed very often.

nder Data

8.72 In order to gather data which enable more preciadvice to be given on binder requirements thespecification for surface dressing, both to Specification

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clauses 919 and 922 (MCHW1) requires the provisiof a wide range of data on the binder to be used andits compatibility with the proposed aggregate. It isexpected, over a period of time, that the amount of drequired will reduce as it becomes clearer that somethe data is more useful than others in defining theperformance of a binder. Most of the data is requireonce for any source and grade of binder although it recommended that tests on the binder should be repat least annually. If a manufacturer uses a number osources of bitumen or modifier he should either giverange of values that covers the variation across allsources or give the data for each individual source. would also apply if the supplier manufactures the binat a number of different plants. It will always be opethe purchaser of the binder or the OverseeingOrganisation to carry out any or all of the tests to chwhether a particular consignment conforms with thedeclared limits. The date of test for any parametershould be given when providing a data sheet.

8.73 The header data, ie manufacturer, binder nameaggregate source etc, are required simply to ensurethe data is attached to the correct binder or binder/aggregate combination. If the name of the binder ischanged, say for marketing reasons, its previous onshould also be given until all the data given is datedleast a year after the change of name. This is to enanames to be changed without unnecessary testing wenabling data to be traceable.

8.74 The binder recovery method used should be thBritish method using the RTFOT apparatus at 85°Can inert nitrogen atmosphere. When the CEN (Austrmethod has been defined so that it can meet therequirements of EN45000 then that procedure may be used for emulsions provided that all tests carriedsubsequently state the recovery method. If a suppliechanges his recovery method the whole suite of testshould be carried out using both methods at the timechangeover so that continuity of data can be maintaA standardised recovery procedure is required so thpurchaser and the Overseeing Organisation can comthe results for a given consignment of binder with thmanufacturer's claims as the purchasers of binder ofinal dressing are not in a position to check the basebinder. It is not intended that the recovered bindernecessarily reflects any particular stage in the life ofbinder after spraying although it may be found to doand is likely to reflect its state fairly early in its lifewhen it is still vulnerable to traffic damage.

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8.75 The penetration test at 25°C with 100g load for5 seconds is the test traditionally used in the UnitedKingdom to determine the grade of harder binders.Carrying out the test on the base binder determines thegrade of binder making up the bulk of the final blend.Comparison with the penetration measured on therecovered binder indicates the changes that haveoccurred during the manufacture and recovery process

8.76 The penetration test at 5°C with 200g load for60 seconds is a test, in the absence of any suitablealternative, which can give some indication of the lowtemperature characteristics using a well known andcommon piece of test equipment. It may be possible touse the result together with the standard pen test to givan indication of the sensitivity of the binder to changesin temperature (analogous to PI).

8.77 The penetration test at 5°C with 100g load for 5seconds for use on cut back binders as delivered is foruse as quick quality control test to check consistency ofa series of consignments. As an alternative a hightemperature viscosity (between 100 and 160°C) can beused for the same purpose. As it is for comparisonpurposes the same test and test parameters shouldalways be used by one supplier.

8.78 The Vialit pendulum cohesion test gives ameasure of the ability of a binder to resist trafficstresses. For simplicity only the peak value is used todetermine the grade of a surface dressing binder. Thetemperature range over which a given cohesion ismaintained is at least as important as the peak levelreached; this is why plots of the results are required. Ithas been suggested that the area under the graph abosome arbitrary value, say 0.5J/cm2, would be analternative criterion by which to compare binders.Specifiers should be aware that very high levels ofcohesion (over 2 J/cm2) are sometimes associated withpoor adhesion.

8.79 The Vialit plate shock adhesion test assesses anumber of factors depending on how the test isperformed, all the procedures are given in BS prEN12272-3 (1997).

The factors are:a) Active adhesivity which measures the bond

between the binder and damp aggregates in theirnatural state

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b) Mechanical adhesion is the adhesivity bonding tdry chippings to the binder with their natural dusor fines making an inhibiting screen

c) Wetting temperature is the lowest temperature obinder on the plate just before applying chippingfor which the number of stained chippings is atleast 90%

d) Fragility temperature is the lowest testtemperature at which 90% of the chippingsremain bonded to the plate

8.80 Active adhesivity assesses the compatibilitybetween binder and damp aggregates and may be usdetermine the effectiveness of adhesion agents or theeffect of a change of aggregate. It is carried out at 5°Cin order to discriminate between binder/aggregatecombinations; much work was carried out in France todetermine the optimum test temperature. It is notdesigned to simulate conditions on the road. This simptest should be carried out on a regular basis (saymonthly) during the season to check the maintenancecompatibility between the binder and the aggregate asdelivered. If there is a sudden change in the result furtinvestigation should take place of both the binder andaggregate and their combination to determine the cauand possible effects on the completed dressing.

8.81 Mechanical adhesion assesses the effect of duston chippings and can be used to determine the level odust and other fine material which has a deleteriouseffect on the adhesion of the aggregate to the binder.Different types of fine aggregate have different effectsClay is particularly effective at preventing bond at verylow concentrations - well below the fines contentpermitted in most Standards for chippings.

8.82 Wetting temperature is applicable to cut-backbinders only and is a measure of the lowest roadtemperature at which work should take place. It can aindicate the sensitivity of the binder aggregate to roadtemperature at the time the dressing is carried out.

8.83 Fragility temperature provides an indication ofwhether or not problems are likely to occur in the earlylife of the dressing, before embedment has taken placand whether very low temperatures will affect the bondbetween the aggregate and the binder. Therefore it givan indication of the suitability or otherwise of the systefor late season work when no embedment is likely befothe following spring.

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8.84 The product identification test data from adynamic shear rheometer is required because itdiscriminates between binders made with different basebitumens and different polymers without in any wayindicating the ‘recipe’ manufacturing process. The datarequired is a master curve of G* against frequency (Hz)at 25°C and δ against temperature at a frequency of 0.4Hz over the range -10 to +60°C. The frequency has beestandardised in order that comparison can be madebetween binders if required and to make the likely databank of information as comparable as possible. All theavailable data should be provided in tabular format.Some polymer modified binders do not permittemperature or frequency shifting to provide a singlemaster curve and in these cases the separate curvesshould be provided together with the reasons why theprovision of a master curve was not possible. The datafrom this test is only required on recovered binder.

8.85 Spray temperature range is the range oftemperatures determined by the binder manufacturerover which the binder may be satisfactorily sprayed. Therange may be different for slot jet and swirl jet spraybars, if so this should be made clear and the contractorshould insert the temperature range appropriate to thespray bar proposed for the contract.

8.86 Spray pressure range is the range of pressuredetermined by the binder manufacturer over which thebinder may be satisfactorily sprayed. The range may bedifferent for slot jet and swirl jet spray bars, if so thisshould be made clear and the contractor should insert thtemperature range appropriate to the spray bar proposefor the contract.

8.87 Weather limits should be indicated if they aredifferent from those which would be applicable toconventional as set out in Road Note 39 (1996). Themaximum and minimum road temperature and themaximum humidity should be given in all cases. Anylimits not indicted will be assumed to be as given inRoad Note 39 (1996).

8.88 The minimum orifice viscosity (STV orRedwood II) is that which the particular emulsion binderrequires to prevent it flowing down any slope in anormal road (say up to 10% gradient) before thechippings are spread. Different binder formulations mayhave different requirements as their flow behaviour on aroad may be different from that through an orifice.

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8.89 Other properties may be given if the tendererconsiders that may be useful to the OverseeingOrganisation in its consideration of the technical meritsof the tender.

Description, evaluation and avoidance of failures

8.90 Surface dressing has failed when it is either:

a) no longer able to meet the needs of thetraffic using the surface, or

b) no longer protecting the structure of thecarriageway from the ingress of water

8.91 Failure occurs in one of three different and rareloverlapping time periods: during construction or shortlyafterwards caused by extremes of weather and/or pootraffic management; during the first couple of years; ordue to old age, which may be any length of time from 5years after execution. Records exist of surface dressinperforming satisfactorily in excess of 20 years. Earlyfailures are almost always the result of inadequacies inone or more of the 4 stages in the production of asurface dressing on the road. The stages are:

a) Specificationb) Designc) Materialsd) Execution including aftercare

8.92 In a performance specification the last 3 items athe responsibility of the contractor and there are goodsources already available from which to obtain adviceand guidance on best practice. In addition to thisdocument these include Road Note 39 (1996) for desigBritish Standards, Road Emulsion Association Ltd(REAL) and manufacturers’ technical information,RSDA advice notes for materials and RN39 and RSDAcode of practice for execution. The specification is theresponsibility of the purchaser and advice onspecification and the suitability of the existing surface taccept a surface dressing is given earlier in this chapte

8.93 All surface dressings fail eventually. This is dueto a combination of factors including principally:embedment of chippings, fretting or wear of chippingsand binder hardening. This long term failure is rarelycatastrophic and appropriate maintenance surfacetreatment can be planned in advance. Surface dressindo not fail on a fixed time basis and each site should binspected regularly and treated at the appropriate time

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Failure definitions

8.94 The rest of this section deals with short termfailures: their definition, evaluation, avoidance andremedies. These failures may be anything from marginto catastrophic.

Whip-off - The normal removal by traffic of excesschippings shortly after the production of a surfacedressing.

Blacking-up - The appearance of binder at the surfacevery early in the life of the dressing, without significantloss of macrotexture.

Bleeding - The exudation of bituminous binder from aroad surface, often accompanied by “bubbling” of thesurface and often spreading to untrafficked areas.

Fatting up - The result of almost total embedment,usually in the wheel tracks only.

Fretting - Random loss of chippings from a completedsurface dressing.

Scabbing - The detachment of both binder and chippingafter application from the existing road surface.

Streaking - Loss of chippings from a completed surfacedressing such that one or more lines appear parallel tothe direction of application.

Tearing - The removal of chippings by traffic at pointsof high traffic stress.

Tracking - Fatting up or bleeding in wheel tracks causedby channelised traffic.

A summary of causes of failure, their avoidance and,where it is possible, their remedies is given in Table 8.4

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Avoidance and Remedy

If caused by hot weather then dusting may stop the problem developing.Additional sweeping may be required

Can be a sign of poor design or execution. Binder rate slightly to somewhathigh. Immediate remedy is to dust at the first sign of it occurring (do notdelay). Avoidance: ensure design is correct, check rate and evenness ofspread of binder and stop work when road surface is too hot, particularlywith cutback binders. Providing it is not too serious, the excess binder willweather off during the first winter. It most frequently occurs with cut-backbinders, subsequent loss of volatiles will reduce the likelihood of recurrence.High pressure water jetting in the spring can be used to remove excessbitumen if it does not weather off over the winter.

The only avoidance measure is not to surface dress at all. Only remedy is toplane off all binder rich material and resurface or recycle by scarifying withcement.

Usually caused by poor design. The chippings are too small for thecombination of road hardness and traffic on the site (check hardness). Canonly be avoided by proper design. No easy remedy. Sometimes a redressingusing a sandwich construction may work. Water jetting to remove binder willextend life somewhat but problem will recur usually in the next spell of hotweather. The only long term solutions are either removal of the fatteddressing using a planer equipped with chisel tips and redressing using acorrectly designed system or by overlaying with an asphalt surfacing. (Amacadam surfacing will frequently allow the bleeding to recur).

Wrong combination of chippings and binder is a design fault. Check roadhardness. Avoid late season work particularly with 14 mm chippings. Bestremedy is to redress the site using the next smaller size of chipping which willconvert it into a form of double dressing. Check that the binder is adequatelystrong for the site stresses. Check compatibility of chippings and binder usingVialit plate shock test.

The only way to avoid this is to properly clean the site and if particularlybadly contaminated with hard mud or other materials, water jetting may berequired. Proper preparation is cheaper than remedial action. The onlyremedy is spot dress areas when the problem occurs.

Avoided by proper care. The jet test must be carried out before work eachmorning, whenever there is change of binder and whenever there has been aprolonged stoppage, particularly if the jets have not been "blown" when thejets may have become slightly blocked by either cold or broken binder. Theon-site transverse distribution test giving coefficient of variation should beused regularly.

Surface dressing should not be specified on very small roundabouts used byarticulated vehicles. If a roundabout is dressed, then the design should bevery carefully carried out, the works executed at an appropriate time of day,either at dawn or in the evening when heavy traffic is at a minimum, trafficcontrol should be particularly well executed until the system has gainedadequate strength. When it occurs where braking is expected, then the samefactors apply. It can also occur at random but this is infrequent and is not acontrollable failure.

When it occurs early then the fault normally lies with the design. Wherechannelised traffic occurs, the design should allow for this, depending ondegree of channelling, and designing the dressing for that higher level.Similar allowances should be made for very slow moving traffic as theloading time is longer. Advice on this matter is given in RN39.

Failure mode

Whip-off

Blacking up

Bleeding

Fatting up (inearly life, saybefore 2 years)

Fretting

Scabbing

Streaking

Tearing

Tracking

Table 8.4 Causes, avoidance and remedies of failure for surface dressing

Cause

This normally occurs and is not a failure but can be apre-cursor to fretting and if it continues for more than afew days should be monitored

Occurs where there is sufficient binder in surface dressingsystem to enable the traffic to draw it up to the surface ofthe chippings usually when the surface is very hot, occursbefore the binder has fully cured

Caused by binder from the underlying road migrating upthrough the surface dressing to be seen first as beads.High road temperatures, low binder viscosity, excessbinder and water pressure stripping binder fromunderlying aggregate are usual reasons

Binder appearing at the surface caused by the penetrationof chippings into the underlying surface owing to traffic.Care should be taken to distinguish this from bleeding asthe cause is different.

The random loss of chippings can have a number ofcauses. The most usual are too little binder for the size ofchipping, too little embedment before the onset of winter,too weak a binder for the quantity and speed of traffic.Poor adhesion of chippings to binder can also contribute.

The usual cause of this is inadequate site preparation andis due to the presence of mud and other contaminants onthe road surface.

This is usually due to a malfunctioning binder sprayercausing variations in rate of spread across the width ofthe road. Temperature of binder too low, pressure too lowand spraybar height variations are typical causes.

This may be caused by traffic turning sharply (the usualmechanism), particularly at roundabouts. It also occursless frequently when heavy vehicles brake hard withlocked wheels. Both causes are most likely to occur in theearly life of the dressing before the binder has gainedadequate cohesion.

This may occur any time during the life of the dressingand is the usual mode of long term failure. It should notoccur early in the life of the dressing.

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February 1999

Chapter 9High Friction Surfacing

9. HIGH FRICTION SUR FACING

necessary within two to three years.

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Background

9.1 Experience over the last 30 years in the UK hasshown these surfacings to be highly effective in reducingtraffic accidents on sites with high traffic density andskidding risk. Typical sites are the approaches to signalcontrolled junctions, to roundabouts and pedestriancrossings subject to a heavy flow of vehicles. For thelength of high friction surfacing necessary, referenceshould be made to Table 3.1 HD 36 (DMRB 7.5.1) andaccompanying notes.

Systems in use9.2 High friction surface treatments are now availablebased on a variety of binders, both thermosetting andthermoplastic. Depending on the type of binder, highPSV aggregates - most commonly calcined bauxite - areither broadcast over a pre-applied binder film orpre-blended with binder and the mixture applied. Theresin binders used at present for broadcast systems areepoxy, polyurethane and acrylic all of which arethermosetting. The binders used for screeded systems arosin esters which are thermoplastic.

System classification9.3 On heavily trafficked sites, the durability ofdifferent systems can vary greatly. To avoiddiscriminating against those products that are suitableonly for moderately or lightly trafficked sites, and also toencourage innovation, the BBA HAPAS certificationscheme to assess high friction surfacings has beenintroduced. High friction surfacing systems are classifiedduring the assessment into three types, as shown inNotes for Guidance, (MCHW 2) Table NG 9/15.

Life expectancy9.4 Each type classification has an expected servicelife of between 5 to 10 years at the maximum trafficlevels shown in Notes for Guidance, (MCHW 2) TableNG 9/15. Types 1, 2 & 3 are suitable for very lightlytrafficked sites, Types 1 & 2 for moderately traffickedsites and Type 1 for heavily trafficked sites. A Type 1system used on a moderately or lightly trafficked site canoffer a much extended life, twenty years is not unknownConversely a Type 3 system used on a heavily traffickedsite will have a much reduced working life. Siteconstraints and the time of year can favour the use ofless robust systems, generally thermoplastic hot appliedmaterials, for convenience. Until thermoplastic Type 1or 2 systems are available, this should not be permittedunless safety or other reasons mean there is noalternative. In such circumstances replacement may be

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9.5 High friction surfacing systems shall bespecified in accordance with the Specification,(MCHW 1) Clause 924 and shall have a currentBBA HAPAS Roads and Bridges Certificate. Theminimum polished stone value of the aggregate,determined in accordance with BS 812: Part 114, tobe used in high friction surfacing systems, shall bespecified in accordance with HD 36 (DMRB 7.5.1)

nstallation

9.6 A high friction surfacing system with acurrent British Board of Agrément (BBA HAPAS)Certificate shall only be installed by a Contractorapproved by the BBA and the Certificate Holder asan Approved Installer for that system. Theinstallation and quality control procedures shall bein accordance with the BBA HAPAS Certificate foreach system and the current Method Statementagreed by the BBA.

.7 Systems should only be installed on surfaceshich are dry, hard and sound, and free from dust, oil,xcess bitumen or other contaminants that may caus

ack of adhesion. Surfaces not suitable for treatmentnclude slurry surfacing, micro-surfacing, fatted and

ultilayer surface dressings and surface dressings ovoft or unsound bases.

.8 To reduce the risk of premature failure, highriction surfacing systems are best applied to wearingourses that have been trafficked for some weeks prioro installation of the surfacing. Nevertheless applicatioo newly laid untrafficked wearing courses of differentypes have been made without any apparent problemor reasons that are not entirely understood, on occaracking which extends into the wearing course can bnduced by the application of high friction surfacing.he risk of this occurring is much greater when theearing course is newly applied and untrafficked,lthough opinions differ on this point. Provided the high

riction surfacing is well bonded to the substrate andith the agreement of the Overseeing Organisation, suchracking if it occurs, may be sealed using a suitablepoxy or similar resin and the high friction surfacingade good. Any cracks in excess of 0.5mm are the

iability of the Contractor under the terms of theuarantee required in the Specification, (MCHW 1)ub-Clause 924.7.


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Chapter 9High Friction Surfacing

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Overlaying concrete9.9 The bond to concrete substrates and therefore thlong term performance can be inferior to that achievedon bituminous surfacings and the suitability of eachsystem should be checked by reference to the BBAHAPAS Certificate.

Overlaying Thin Wearing Course Surfacings9.10 Where thermosetting high friction surfacings -such as epoxy resin - are to be applied over thin wearincourse systems at approaches to roundabouts and othhighly stressed sites, the deep ‘negative’ texture in thesurface can reduce the coverage of resin binder to sucan extent that the adherence of calcined bauxitechippings is reduced, resulting in premature chippingloss. To alleviate this problem, the texture of the area othin surfacing to be covered by high friction surfacingshould be reduced during or after laying to between 1 t2 mm as measured by the sand patch test. This may bachieved by any suitable means at the discretion of thethin wearing course system proprietor, for example byadditional compaction with vibrating rollers whilst thethin surfacing is still hot or by the substitution of asmaller aggregate size in these areas. Alternatively asuitably sized grit may be applied and rolled in. If thethin wearing course system is to be trafficked prior tothe application of HFS, then 3 mm grit should be applieand rolled in to provide enhanced short-term skidresistance. Temporary warning signs may be appropriatin such circumstances.

Guarantee

9.11 A two year guarantee is required from theContractor from the date of opening the surfacing totraffic. This is linked to minimum performancerequirements set out in the BBA HAPAS ‘GuidelinesDocument for the Assessment and Certification of HighFriction Surfaces for Highways.’ The guaranteeexcludes defects arising from damage caused bysettlement, subsidence or failure of the carriageway onwhich the surfacing has been applied.

Limitations

9.12 High friction surfacings are expensive,particularly if productivity is affected by the geometry ofa site and the number of areas to be treated. The use cheaper alternatives should be considered, if trafficlevels allow, such as improved road signs and markingsimproved street lighting, or surface dressing with a highPSV natural aggregate bonded with a binder capable owithstanding the braking forces generated.


February 1999

10. SLURRY SURFACING AND MICRO-SUR FACING

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Chapter 10Slurry Surfacing and Micro-Surfacing

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Background

10.1 Slurry surfacing and micro-surfacings aremixtures of aggregates and plain or polymer modifiedbitumen emulsions, which may contain fibre additives.Slurry surfacings range in thickness from about 2 mm 8 mm and micro-surfacings from about 10 to 20 mm.Slurry surfacings are suitable for footways; areas thatare trafficked only occasionally and at low speeds; andfor traffic delineation. Micro-surfacings are targetted atall roads, including high speed roads carrying significatraffic volumes, and in consequence require appropriatlevels of skid resistance and texture retention. Bothmaterials permit only limited surface regulation whenlaid in one pass. If greater surface regulation isnecessary, an initial pass may be made to fill in surfaceirregularities, such as minor rutting, followed by asecond pass to provide the complete overlay.

Specification

10.2 The Specification (MCHW 1) Series 900, withaccompanying Notes for Guidance (MCHW 2) inNG900, sets out the requirements for these materials,dividing them into two types:

• Slurry surfacing, Clause 918, covering the thinnematerials, with aggregate up to a nominal 4 mmmaximum size. It is an end performancespecification in which the slurry surfacing may bein accordance with either BS 434, or if aproprietary system, with the British Board ofAgrément Highway Authorities Product ApprovalScheme, Roads and Bridges (BBA HAPAS)certificate for the system.

• Micro-surfacing, Clause 927, which is an endperformance specification covering materials withnominal aggregate sizes of 6 mm or more, allproducts requiring BBA HAPAS certification.

10.3 Proprietary slurry surfacing systems are less tha18 mm in thickness and are classified as Type A thinwearing course systems as defined by BBA HAPAS, seeChapter 6 of the Part. Micro-surfacing usually fallswithin Type A but may also be laid thick enough to fallwithin Type B (ie 18 to 25mm).

10.4 Slurry surfacings may be used on footways andon very lightly trafficked carriageways carrying lowspeed traffic (ie in areas with a 30 mph or lower speedlimit). Some micro-surfacings have sufficient initialsurface texture to enable their use on high speed road

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and are presently undergoing trials to determine whattexture level will be retained. The performance in trialsof proprietary slurry surfacing and micro-surfacingsystems will determine their suitability for the siteclassifications and traffic levels to be set out in the BBAHAPAS ‘ Guidelines Document for the Assessment andCertification of Thin Surfacing Systems for Highways.’

10.5 When used on trunk roads includingmotorways, proprietary slurry surfacing and micro-surfacing systems shall have BBA HAPAS Roadsand Bridges Certificates appropriate for the siteclassification and the level of traffic in commercialvehicles/lane/day to be carried. Prior to the issue ofCertificates, a Departure from Standard shall beobtained from the Overseeing Organisation beforeslurry surfacings (except those complying withBS 434), or micro-surfacings, are used on schemesin the UK.

Benefits

Skid Resistance10.6 Reasonable values for micro-texture (ie low speedskid resistance) and abrasion resistance may be obtainby selection of a suitable aggregate type with a minimumpolished stone value (PSV) and maximum aggregateabrasion value (AAV) in accordance with Chapter 3 ofHD 36 (DMRB 7.5.1). Aggregates with a high polishedstone value (PSV) are required for stressed sites such abraking areas, hills or bends. Aggregates with a lowaggregate abrasion value (AAV) are necessary forheavily trafficked sites to reduce the rate of wear. Highspeed skid resistance is determined by micro-texture anretained surface texture.

Appearance10.7 Slurry and micro-surfacings may be used on roadsurfaces that have undergone a number ofreinstatements, or significant patching, in order toprovide a more uniform overall appearance. Slurry andmicro-surfacings can be used on surfaces that arefretting, and on those showing early signs of ravelling, tohalt further deterioration. Coloured aggregates ormixtures may be used for delineating hard shoulders ancentral reserves, or for traffic calming measures wheretraffic levels are appropriate.

Conservation10.8 Planing before treatment is not necessary whenthe profile is acceptable, and ironwork may not need tobe raised for slurry surfacings and Type A


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micro-surfacings. Material usage is low, components aremixed cold using damp aggregates and no energy isexpended on drying and heating the constituents.

Environment10.9 Bitumen emulsions are environmentally friendly,the emissions being mainly water vapour. Cutbacksolvent is usually kerosene (similar to domesticparaffin), one of the least hazardous low cost organicsolvents.

Profile10.10 Micro-surfacing improves the profile of theunderlying surface, particularly in the transversedirection as these products are spread using a fixedscreed mounted on either skis or a very shortwheel- base frame.

Ride quality10.11 Micro-surfacing may improve ride quality,particularly if the problem is caused by undulations ofvery short wavelength. Undulations with a wavelengthgreater than about 1 m may be slightly improved.

Noise10.12 Tyre noise is relatively low because slurrysurfacings have low or fairly low macro-texture.Micro-surfacings with a higher texture may be lessquiet. How micro-surfacings compare with low noise,hot paver-laid thin wearing course systems has yet to bedetermined.

Preservation10.13 Slurry and micro-surfacings can be used onsurfaces that are fretting, and on those showing earlysigns of ravelling, to halt further deterioration.

Limitations

Macrotexture10.14 There are currently no slurry surfacings that canmaintain adequate surface texture for high speed roadsfor more than a few weeks; therefore these materialsshould not be used on high speed roads.

Profile10.15 Slurry surfacing does not improve the profile,either transverse or longitudinal, of the existing surface,so defects of this nature should be reduced to acceptablevels before their use.

Speed of works10.16 Laying slurry and micro-surfacings, particularlythe thicker varieties, is relatively slow and the materialmust be left to break and stabilise prior to opening totraffic. In good weather conditions, warm with lowhumidity, this will take about half an hour, but in less

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atisfactory conditions it may take an hour or more.here more than one layer is used however, traffic may

se each layer as it becomes sufficiently stable.

ermeability0.17 Although slurry and micro-surfacings arresturface deterioration, most products are permeable to areater or lesser extent. They should not be assumed toe entirely waterproof.

tructural Strength0.18 Slurry surfacings do not increase the load bearingapacity of the pavement structure. Thicker micro-urfacings may add to the structural strength of theavement, but any claims made by a system proprietorhould be confirmed by the BBA HAPAS certificate.

ensitivity to Weather Conditions0.19 Slurry and micro-surfacings with emulsioninders are sensitive to high humidity and wet weatheruring construction. If heavy rain occurs before themulsion has broken the surface may be washed away,r if there is a frost within the first 24 hours, then theurfacing may be disrupted.

pplication Techniques

10.20 Slurry surfacing and micro-surfacing withBBA HAPAS Roads and Bridges Certificates shallonly be laid by Contractors approved by the systemProprietors. Installation and quality controlprocedures shall comply with the requirements ofClause 918 and Clause 927, respectively, of theSpecification (MCHW 1) and with the requirementsof the BBA HAPAS Certificate and methodstatement agreed by the BBA.

urface Preparation

0.21 The surface onto which the slurry or micro-urfacing is to be applied should be cleannd free from all contamination. All major depressionsnd potholes should be repaired. All oil deposits, dust,ose material, mud or other deposits should be removedy pressure washing or vigorous sweeping, asppropriate. Any weeds, moss, lichen or algae should bemoved by the application of a residual weed killer

pproved by the Overseeing Organisation andubsequent pressure washing or other mechanical mean water jetting has been used to prepare the surface, allee water should be removed before work begins.urfaces which have ‘fatted up’ are generally notuitable unless the excess bitumen can be removed bytexturing - see Chapter 11 of this Part. If the excess

itumen is not removed, it is likely to bleed through thelurry surfacing.


February 1999

skidding resistance is required.

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10.22 All ironwork, kerbs, edges, road studs, streetfurniture and when required, road markings should bemasked with self adhesive masking material. Turves, oil,sand and similar materials must not be used. Junctionwith surfaces not to be treated should be masked toproduce well defined, clean joints.

Weather conditions10.23 Application should not be carried out when theground temperature falls below 4°C or when standingwater is present. When spreading in hot, dry conditionit may be appropriate to slightly dampen the surface bmeans of a mist spray, to avoid the emulsion breakingtoo quickly. The surfacing should also be protected fromthe effects of rain or frost before the slurry or micro-surfacing has developed sufficient resistance.

Equipment10.24 The equipment needed for installation isdependent on the process used and can vary frombrooms and squeegees used for veneer coats of slurrysurfacing on footways, to integrated mixing and placinequipment used for micro-surfacings.

1.5 mm and 3 mm thick Slurry Surfacing to BS 43410.25 These materials are most suitable for footwayswith an acceptable profile, where the surface isbeginning to fret or is in the very early stages ofravelling. They may be mixed on site using a variety oplant, or for small areas, delivered to site as premixedmaterial in suitable containers, which may need remixior stirring before use. The material may be applied usa small spreader box (the preferred method) or by hanusing squeegees over the existing footway. The slurryshould be worked into any cracks and up to the edge okerbs and street furniture in order to seal the surface.The final finish should be made with a broom drawnacross the surface in a consistent direction, in order togive the finished product a satisfactory appearance.

Slurry Surfacing, 0/2 and 0/4 Aggregates10.26 These products are normally laid about 3-6 mmthick in a single layer and have little regulating ability.They may be used on footways in exactly the samemanner as BS 434 materials, and also on cycleways avery lightly trafficked roads (eg on housing estates),where an existing bituminous surfacing is beginning tofret. They are normally mixed in a continuous mixer onsite and immediately spread using a spreader box towby the vehicle carrying the aggregates, binder and mixr.The speed of laying can be reasonably fast but, as wiall slurries, the traffic must be kept off until it has gainedadequate strength. It is not normally necessary to adjuironwork. They generally have a very low surfacetexture and should not be used where high speed

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Micro-surfacing10.27 These products are normally laid in two layerswith a total thickness up to 20 mm. They contain coarseaggregate up to about 10 mm within a continuouslygraded material. They may be laid in two, oroccasionally more, layers using continuous flow mixersand spreader boxes, and therefore can regulatetransverse irregularities to a considerable extent andarrest fretting and ravelling.

10.28 It is probable that some ironwork will needresetting because of the thickness of micro-surfacingsystems. Care should be exercised in finishing roundironwork, to avoid an unsightly appearance. It is normalpractice for ironwork to be raised after the application ofthe first layer of micro-surfacing because of the fixedscreed in the spreader box, but before placing the seconlayer, so that an even finish is achieved. Although morenormally used on road surfaces, these products may alsbe used on irregular footways to improve their shape.Hand laying should not be used except in limited areaswhere it is not possible to use a spreader box.

Slurry surfacings to BS 434

Binders10.29 Materials produced and applied in accordancewith BS 434 should only use unmodified emulsionbinders as specified in Part 1 of that Standard. Thefollowing are suitable:

• Class A4 Rapid Setting or Class K3, capable ofproducing slurries which are sufficiently stableduring mixing and laying such that prematurebreaking of the emulsion binder is avoided, butdeveloping early resistance to damage by trafficor by rain.

• Class A4 Slow Setting. This can be used wherethe rate of set is less important, when the slurrycan be expected to dry out naturally before beingsubjected to traffic or to rain.

Aggregates10.30 The aggregates should be crushed igneous rock,gritstone or slag, blended where necessary with cleansharp naturally occurring sand which is free from silt,clay or other fine material. The aggregate grading curveshould be smooth and not gap graded. Slurry surfacingcan be very sensitive to the source, grading and type offine aggregate.

Additives10.31 Additives normally used to control consistency,mix segregation, and setting rate are Portland Cement tBS 12, hydrated lime to BS 890 and/or chemicalretardants.


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10.32 With natural bitumen as the binder, redpigmentation is the most common colour used. Theamount of filler that can be replaced by pigment has tobe restricted otherwise the slurry will be adverselyaffected, thus colours are not bright.

Mixing10.33 The precise proportions of each constituent shoulbe determined after trials at the plant to be used in theworks.

10.34 The binder content by mass should be appropriatto the bulk density of the aggregate. When blast furnaceslag is used, the bulk density of the aggregate isgenerally less than that of natural aggregates or steelslag, and a higher target binder content by mass will berequired. (The proportion by volume will be similar inthe two cases).

10.35 The component materials should be measured inta mechanical mixer and mixed until the aggregate isuniformly coated with bitumen emulsion and the slurryis of a consistency that can be laid satisfactorily.

10.36 If a bond coat is required it should be applied inaccordance with BS 434 Part 2 before the slurrysurfacing is spread. The rate of spread of the bond coawill depend on the surface to be treated. For bituminousmaterials the rate should be within the range 0.15-0.3 l/m2 and for concrete surfaces 0.4-0.6 l/m2. The use of apolymer modified bond coat may be advantageous onconcrete surfaces.

10.37 The slurry should be evenly spread such that thelayer thickness conforms to the design requirements ofthe work. Care should be taken to ensure that all surfaccracks, voids and depressions are completely filled withslurry. The slurry as discharged from the mixer shouldbe used without any further addition and should be laidcontinuously by a mobile mixing machine feedingdirectly into the spreader box. In some areas, such asconfined areas on footpaths, central reserves and thelike, it is recognised that the material may have to bespread by hand. Even so, hand laying should be avoidewherever possible.

Visual Appearance10.38 The finished surface should have a uniformtexture and colour throughout the work. The finishedproduct should be free from blow holes and surfaceirregularities due to scraping, scabbing, dragging,droppings, excessive overlap or badly alignedlongitudinal or transverse joints. Variations in the colourof slurry surfacing can sometimes occur initially butthese tend to stabilise with time, often within 24 hours,dependant on the weather conditions and trafficking.

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emain for some time. This is not necessarily anndication of uneven mixing or segregation and it shouldormally disappear after 2 or 3 weeks.

nd Performance Specification

0.39 Clauses 918 and 927 in the SpecificationMCHW 1) and associated Notes for Guidance (MCHW) are end performance specifications, with a two yearuarantee requirement, normally of two years duration.lmost all slurry and micro-surfacing systems areroprietary products and as such, require certification bBA HAPAS. Thus the capability of each proprietaryystem to provide the desired performance will bessessed in advance. The only exception to this is slururfacing which complies with BS 434.

0.40 The BBA HAPAS Certificate and methodtatement for a product will set out the assessederformance levels, the site preparation, the mixing and

aying procedure, bond coat requirements if any, andftercare. The performance requirements for slurryurfacing to Clause 918 during the guarantee periodelate to wear and to loss of the surfacing, and limits aret on the exposure of the substrate permitted. Clause27 for micro-surfacing includes similar limits and inddition, sets limits for the minimum retained textureepth in the nearside wheel track and maximum textureecay over the second year of the guarantee period, aleasured by the sand patch test. It is therefore importa

hat the actual performance of slurry and micro-urfacing systems are monitored and compared againshe specified requirements.

0.41 Clauses 918 and 927 of the SpecificationMCHW 1) call for rheological product identificationnd Vialit Cohesion of the recovered binder to berovided by the Contractor. It is unlikely thaterformance can be determined from binder data alones systems are highly dependent on the aggregaterading, its physico-chemical nature and the emulsionet. However these are all proprietary products and theata is required for product identification and thereby tonsure consistency.

udit Checks0.42 It should not be necessary to carry out routineudit checks on proprietary products with a two yearuarantee. Nevertheless if obvious variations in aroduct are occurring, then audit tests should bendertaken to determine aggregate properties andrading, binder content and binder characteristics. Thehould be carried out to check that the product complieith the requirements of the Specification and, when


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issued, with the requirements of the BBA HAPASCertificate and the system proprietor’s methodstatement. Non-compliance should be reported to theOverseeing Organisation and the BBA, and may, ifserious and ongoing, result in the suspension of the Bcertificate for the system.

Description, Evaluation and Avoidance of Failures

10.43 Slurry and micro-surfacings can fail in a numbeof ways. Table 10.1 summarises possible causes of thfailure, suggests how they may be avoided and in somcases rectified.



Defect Cause Avoidance and remedy

Lack of adhesion Inadequate preparation and cleaning The only way to avoid this is to ensure that the existingto underlying of existing surface. Incompatibility surface is properly cleaned. If badly contaminated with hardsurface between slurry and underlying road mud or other materials, water jetting will be required. There

is no remedy other than remove and repeat the work.

Lack of bond Lower layer contaminated or Check the design of mixture and ensure site is kept clean.between layers insufficient binder in the mix There is no remedy except to remove and resurface.

Failure to set Work done in adverse weather Work should not be carried out in adverse conditions, ie rain,conditions or incompatible cold or high humidity. Check the design of the mixture.constituents Resurface the site in more appropriate conditions or with a

more robust product.

Rapid wear Material inappropriate to site. Opened Check design of mixture. The material must gain sufficientto traffic too soon. Work done in stability before opening to traffic. Do not work in adverseadverse weather conditions conditions. The only remedy is re-surface the site with a more

robust product or in more appropriate conditions.

Tearing Material insufficiently strong for the Poor design of surfacing; reappraise site and traffic conditions.location or has poor cohesion or poor Only remedy is to remove and repeat the work using morebond robust materials.

Too rapid set Poor design of the mix or the work is Redesign the mix.being carried out in hot weather Keep the existing surface damp or stop work until weather is

cooler.

Pushing Insufficient cohesion or opened too Close the road to traffic until cohesion has improved; use aearly, or a defective mixture mixture with a higher cohesion. If the mixture is at fault then

the only remedy is to remove and replace with a more robustproduct.

Deformation The design of the mixture is incorrect Use a more deformation resistant mix. Only remedy is tofor the amount of traffic on the site remove and replace.

Bleeding Too much binder in the underlying Do not use slurry or micro-surfacing over a very fatty roadsurface; too much binder in the mix surface. Check design of mixture. The only long term remedy,

where the new road surface has fatted up due to excessbitumen in the underlying surface, is to remove the slurry,re-texture to remove the excess bitumen and re-apply.Alternatively, resurface using another type of surfacing. If themixture is at fault, remove and replace with a redesignedproduct.

Fatting up - The sand-filler-binder matrix is too No remedy. Remove and replace with a superior product.premature texture weak to prevent embedment of theloss coarse aggregate

Depression Usually a reflection of a low area inthe underlying layer

Ridge Check that the screed has no notches. Check that the slurry iscontained and does not flow round the end of the screed.

Longitudinal Usually caused by material adhering Ensure the screed is clean and free from adherent material,tracks to the screed check slurry is not breaking too fast (most likely in hot, dry

conditions)

Colour differences Many factors can cause colour Variations in colour may occur as a result of inadequatedifferences mixing or workmanship, or a change in material sources or

their proportions. Variations due to differing substrate porosityfor example, can often be temporary, the colour becomingmore uniform within a few days.

Table 10.1 Defects in Slurry Surfacing

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February 1999

Chapter 11Retexturing (Bituminous)

11. RETEXTURING (Bituminous)

11/1

11.1 The wet skidding resistance of an asphaltic roagenerated primarily by the microtexture of the aggregin the surface. This microtexture is gradually polishedby traffic until an equilibrium level is reached. HD 36(DMRB 7.5.1.3) provides advice on the choice ofaggregate and HD 28 (DMRB 7.3.1) on the standardfor the skidding resistance of in-service roads.

11.2 The ability of a surface to maintain adequateskidding resistance at high speeds is governed by themacrotexture of the surfacing. As a surfacing ages, thmacrotexture may fall as chippings are embedded intthe asphalt matrix or substrate or excess binder comethe surface. In some circumstances, for example overolling, the chippings in a new surfacing may beembedded too far and the requirements for texture deof a new road may not be met.

11.3 Retexturing is the mechanical reworking of asound road surface to restore either skidding resistantexture depth or both.

Retexturing techniques

11.4 The suitability and effectiveness of a retexturingtreatment depends on the condition of the road prior ttreatment. Some treatments can increase both skiddinresistance and texture depth; others may increaseskidding resistance but reduce texture depth. There aalso treatments which increase texture depth with littleeffect on skidding resistance.

11.5 Advantages include:

a) conservation of natural resources by reworkingexisting surface;

b) retexturing may be more economical than sometraditional resurfacing methods, especially whersmall areas are to be treated;

c) most processes can be carried out at any time year in all but the most severe weather conditio

d) traffic disruption is reduced compared withconventional treatments because of short lead-itimes and the speed of the processes;

e) can be used as a "stop-gap" measure to treatsmall, high-risk sites.

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11.6 Disadvantages include:

a) retexturing should not be used on unsound roadswhere there is cracking or surface irregularities,or on roads with sealing or overbanding.

b) some processes cannot treat roads with severetransverse deformation, such as heavily ruttedsurfaces.

c) road surfacing features such as ironwork, whitelining and traffic detection loops may have to beavoided or protected.

11.7 The durability of the results of a treatment willdepend on the type and geometry of road, the quantityand behaviour of the traffic, the aggregate and the rateof spread of chippings, where used, in the wearingcourse. However, just as a new surfacing will polishunder the action of traffic, the aggregate on a retexturedsurface will eventually polish back to an equilibriumskidding resistance level, close to that of the originalsurfacing. On a high stress site, where there is muchbraking and turning, the improvement may last a matterof months, but, in a low stress site, the same treatmentmay continue to show an improvement over theuntreated surface for three years or more.

11.8 The following paragraphs give some comments onavailable methods and suggestions on their applicationfor restoring skidding resistance and/or surface texturedepth. Table 11.1 gives a summary of these methods andsuggestions.

Impact methods

11.9 Processes in this category involve striking theroad surface with either hard-tipped tools or hardparticles. These treatments are effective where the lossof skidding resistance is due to polishing and affectmainly the aggregate particles and the weatheredasphaltic matrix.

a) Bush hammering: The road surface is struck by anumber of impact heads with chisel-endedhammers with hardened tips. This processenhances skidding resistance, but can sometimesreduce texture depth, depending on the conditionof the existing road surface and the severity of thetreatment.


February 199911/2


b) Shot blasting: The impact is by steel shotprojected at high speed from a rotating wheel. Asthe surface is scoured, both shot and arisings arerecovered and separated, with the steel shot storefor reuse. This process improves both skiddingresistance and surface texture depth of chippedrolled asphalt surfacings by removing theweathered asphaltic matrix and leaving thechippings (with renewed faces) exposed. There isa risk of chippings that are not properlyembedded, such as in surface dressing, becominloosened by this process, as the supporting matriis removed.

Cutting and scabbling/flailing

11.10 This category includes cutting, sawing, grooving,grinding and scabbling/flail grooving. In the latter case,the cutting action is combined with impact on the cuttingheads.

a) Grooving/grinding: Using diamond-tipped bladesassembled in configurations to suit the patterns ocutting required. This process can be used toprovide either discrete grooving patterns or forbump cutting. The treatment affects macrotextureand can reduce texture if the blades are in a closspaced configuration. Microtexture is oftenunaffected because the plateaux between grooveare the original surface.

b) Longitudinal scabbling: Hardened tips set into theedges of steel washers are loosely mountedside-by-side and drawn across the road surfacewhilst being hydraulically loaded. This processenhances skidding resistance, by removingmaterial from the tops of particles to expose newaggregate faces, but it reduces surface texturedepth by the same process.

c) Orthogonal grooving: This consists of deeperlongitudinal and transverse grooving combinedwith scabbling. The skidding resistance isimproved but, whilst the surface texture depth isimproved initially, the grooves may close up underheavy trafficking. Further, the regular groovesgenerated within the road surface can give rise toincreased road noise. Although this treatment canprovide significantly enhanced skiddingresistance, it is not a preferred option as it can beless comfortable for all road users and may reducthe ride stability of motorcyclists.

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1.11 This involves the surface being subjected to thection of a fluid at high temperature or pressure. These

reatments are not mechanical reworking of the roadurface to expose new aggregate surfaces, and as aesult do not restore skidding resistance lost through theolishing action of traffic.

ther considerations

1.12 Although retexturing is a useful option to considerhen addressing problems of skidding resistance orurface texture depth loss due to the action of traffic,here will always be other factors to be taken intoccount.

) Some treatments will be more appropriate forsome surfacings than others. For example, anaggressive cutting or flailing technique would beinappropriate for a surface dressing or othersurfacing type where small aggregate particles arerelatively loosely bound to the substrate orsurrounding matrix. There would be a risk of thesurfacing becoming separated from the substrateby direct action of the treatment of water ingressand frost action.

) The effect of an individual process of bothskidding resistance and texture depth must beconsidered in the light of what is required in aparticular situation. For example, where surfacetexture is already at an acceptable level or whereincreasing it may be undesirable, a treatment thatdoes not increase surface texture would beappropriate.

) Retexturing is most effective on road surfacingsthat are generally sound. If some sealing action isrequired in addition to improved skiddingresistance or texture, retexturing would beinappropriate. Similarly, a surface which isfretting or losing chippings may be damagedfurther by mechanical action.

1.13 As with all processes, the advice in this Chapterannot cover all contingencies, it is appropriate to getdvice from the contractors when sourcing these

reatments as to details of a particular process and itsuitability for a particular site.



ily

Suitability of t reatment processesSurfacing Original condition: effect

type required from treatment Bush Shot Grooving/ Longitudina l Orthogonal Carbonising Water-hammering blasting grinding scabbling grooving jetting

Polished aggregate: good1 texture ✔ ✔ ✔ ✔ ✔ O Orecovery of skiddingresistance

poor1 texture ✔ ✔ ✔ x x O O

Chipped Embeddedrolled chippings: good2 SR O ✔ O x ✔ ✔ ✔asphalt recovery of texture

depthpoor2 SR O ✔ O x O ✔ ✔

Excessive noise/ good SR ✔ x O ✔ x x xexcessive texture

Polished aggregate: good texture ✔ x x O x x xrecovery ofskidding resistance

Surface poor texture ✔ O x x x x xdressing

Fatted-up: good SR x x x x x ✔ ✔recovery oftexture depth

poor SR x x x x x ✔ ✔

Polished aggregate: good texture ✔ ✔ ✔ ✔ x x xrecovery of

Thin skidding resistancesurfacings poor texture ✔ ✔ ✔ x x x x

Removal ofbinder film good SR O O x x x O O

Polished aggregate: good texture ✔ ✔ O ✔ x x xrecovery of skidding

Close resistancetextured poor texture ✔ ✔ O x x x x

macadamsRemoval of good SR O O x ✔ x ✔ ✔binder film

Key: SR Skidding Resistance O Treatment may be appropriate in some circumstances✔ Appropriate treatment but effects will be limited and depend on surfacing conditionx Not recommended

Notes: 1 SMTD = Sensor Measured Texture Depth. 2 When referring to skidding resistance, "good" and "poor"When referring to texture in this context, denote above or below investigatory level respectively."good" and "poor" are approximately thefollowing: SMTD > 1.2 mm, good;SMTD < 0.6 mm, poor.

Table 11.1 Appropriate circumstances and treatments for retexturing bitumen-bound surfacings

General Except for thin wearing course systems, proprietary surface dressing and, where permitted, stone mastic asphalt surfacings,note: which have minimum in-service texture depth requirements at the end of their warranty periods, it has not been found

necessary to introduce standards for in-service texture depth, adequate texture being maintained by erosion of the bindermatrix. Nevertheless reduced texture may contribute to a reduction in low speed skidding resistance, although not necessarto below the investigatory level, and which may be enhanced by an appropriate treatment.

February 1999 11/3


February 1999

13. MISCELLANEOUS SURFACING M ATERIALS

e

Chapter 13Miscellaneous Surfacing Materials

13/1

13.1 There are a wide variety of surfacing materialsthat are not covered in the other chapters of this Part.These are products that are either undergoingdevelopment or that are little used on trunk roads.

Dense bitumen macadam

13.2 Dense bitumen macadam wearing course toBS 4987 is available with a 6 mm nominal aggregatesize. It has a very low texture depth and therefore it isonly suitable as a surfacing for roads with a low speedrestriction. With good compaction it is reasonablydurable, however its resistance to deformation does notmake it suitable for very heavily trafficked roadscarrying a large proportion of commercial vehicles.

Open graded bitumen macadam

13.3 Open graded bitumen macadam wearing course toBS 4987 is available with both 10 and 14 mm nominalaggregate sizes. They are not however, designated‘preferred mixtures’ in BS 4987. These products havelow strength and their durability is suspect. They are notsuitable as surfacings for high speed or heavilytrafficked roads.

Close graded bitumen macadam

13.4 Close graded bitumen macadam to BS 4987 isavailable in both 10 and 14 mm nominal sizes. They aredesignated ‘preferred mixtures’ in BS 4987, howevertheir void content can be relatively high and theirdurability is therefore suspect. Furthermore theseproducts are sensitive to small variations in bindercontent and aggregate grading with the result that theirvoid content can fluctuate from one load to the nextunless tight control is exercised at the mixing plant. Inconsequence, their resistance to deformation must alsobe suspect and they are not suitable for use on heavilytrafficked roads. In addition, as the texture depthobtained is low they are not suitable as surfacings forhigh speed roads.

Dense tar surfacing

13.5 Few suppliers carry tar routinely and the materialis generally only available to special order. The onlypermitted use of dense tar surfacing is in vehiclestanding or parking areas to take advantage of itsresistance to fuel spillage, where traffic speeds are low,and significant texture depth is unnecessary. Dense tarsurfacing (DTS) should comply with the requirements ofBS 5273 with a coarse aggregate content of 50%. Thenarrow temperature range specified limits the time

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vailable for compaction and a minimum depth of0 mm is required even in good conditions. If trafficking very severe, the use of crushed rock fines should beecified to obtain sufficient resistance to deformation.

ecause tar based materials stiffen rapidly with age anyeformation that occurs is more likely to happen in thest or second summer rather than subsequently. Denser surfacing has proved to be very durable although itsxture can reduce in time. Because it is little used, theaterial is unlikely to be cost effective except in rarestances.

-situ Macadams

3.6 In-situ macadams are proprietary products whichre either applied as multilayer surface dressings to form thin surfacing or as graded aggregates onto which ainder is sprayed, followed by mixing in-situ, gradingnd compaction. As these are proprietary systems theyill require BBA HAPAS certification.

ontrolled Texture Asphalt

3.7 Controlled texture asphalt is a surfacingmewhat similar to a Dense Bitumen Macadam. It is

esigned to be placed with an initial texture depth thatill be maintained for the life of the surfacing. As yet itoes not provide an adequate initial texture depth toermit its use on high speed roads and it is alsoxpensive. Controlled texture asphalt contains a mixturef two aggregates with significantly different resistance abrasion; typically with aggregate abrasion values ine ratio of 2:1. The grading of the material is based one packing properties of the selected aggregates to giv mixture with a controlled void content and surfacexture. Once the target grading has been selected ande binder content determined, production tolerancesave to be more precise than those specified for Britishtandard materials. Controlled texture asphalts areroprietary and will therefore require BBA HAPASrtification.

old laid materials

3.8 Cold laid materials are being developed forermanent reinstatements as a result of thepecification for the Reinstatement of Openings inighways’ issued by the Highways Authorities andtilities Committee (HAUC). A number of productsave HAUC approval for use in the reinstatement ofenches on roads carrying traffic up to 30 msa.


February 1999

of Hot Rolled Asphalt, Research Report 280.

d



14/1

References


HD 20 Loop Detectors for Motorways(DMRB 9.3.1)

HD 26 Pavement Design (DMRB 7.2.3)HD 28 Skidding Resistance (DMRB 7.3.1)HD 29 Structural Assessment Methods

(DMRB 7.3.2)HD 31 Maintenance of Bituminous Roads

(DMRB 7.4.1)HD 32 Maintenance of Concrete Roads

(DMRB 7.4.2)HD 36 Surfacing Materials for New Construction

and Maintenance (DMRB 7.5.1)HD 38 Concrete Surfacing and Materials

(DMRB 7.5.3)


Volume 1 : Specification for Highway WorksVolume 2 : Notes for Guidance on the Specification for

Highway Works


1973

Brown, J. R., Pervious Bitumen Macadam Surfacings toReduce Splash and Spray at Stonebridge, Warwickshire,Laboratory Report 563.

1985

Daines, M.E., Cooling of Bituminous Layers and TimeAvailable for their Compaction, Research Report 4.

1986

Daines, M. E., Pervious Macadam: Trials on TrunkRoad A38 Burton Bypass, 1984, Research Report 57

1989

Edwards, A. C. and Mayhew, H.C., Recycled AsphaltWearing Courses, Research Report 225

1991

Nicholls, J. C., Adverse Weather Conditions and Laying

1

DRR

1

N(P

1

RR

1

GRu

NS

RRP

1

RRD

HAb

4

1

Ba

1

BCe

992

aines, M. E., Trials of Pervious Macadam and Hotolled Asphalt on the A38 at Burton, Researcheport 323.

994

unn, M. E., Evaluation of Stone Mastic AsphaltSMA); a High Stability Wearing Course Material,roject Report 65.

996

oad Note 39, 4th Edition (Revised), Design Guide foroad Surface Dressing.

997

ershkoff, D. R., Carswell, J., and Nicholls, J.C.,heological properties of polymer-modified binders forse in rolled asphalt wearing course, Report 157.

icholls, J. C., Laboratory Tests on High Frictionurfaces for Highways, TRL Report 176.

oe, P. G. and Hartshorne, S. A., The Mechanicaletexturing of Roads: Study of Processes and Early-lifeerformance, TRL Report 298.

998

oe, P. G. and Hartshorne, S. A., Mechanicaletexturing of Roads. An Experiment to Assessurability. TRL Report 299.

eslop, M. W. and Gershkoff, D.R., The Properties andpplication of Modified Binders, TRL Report XXX (toe published)

. British Standards Institution

975

S 5273; Dense tar surfacing for roads and other pavereas. (Confirmed 1990)

984

S434; Bituminous Road Emulsions (Anionic andationic): Part 1: Specification for bitumen roadmulsions. (Confirmed 1997)


February 1999

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t

:

:

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14/2

BS434; Bituminous Road Emulsions (Anionic andCationic): Part 2: Code of practice for use of bitumenroad emulsions. (Confirmed 1997)

1989

BS 3690; Part 1: Specification for Bitumens for Roadsand other Paved Areas. (Confirmed 1997)

BS812: Part 114: Method for determination of thePolished Stone Value (PSV)

1990

BS 5212; Cold applied joint sealants for concretepavements: Part 1: Specification for joint sealants.

BS 5212; Cold applied joint sealants for concretepavements: Part 2: Code of practice for the applicationand use of joint sealants.

BS 5212; Cold applied joint sealants for concretepavements: Part 3: Methods of test.

BS 598; Sampling and examination of bituminousmixtures for roads and other paved areas: Part 105:Methods of test for the determination of texture depth.

1992

BS 2499; Hot applied joint sealant for concretepavements: Part 2: Code of practice for the applicationand use of joint sealants.

BS 594; Hot rolled asphalt for roads and other pavedareas: Part 1: Specification for constituent materials aasphalt mixtures.

BS 594; Hot rolled asphalt for roads and other pavedareas: Part 2: Specification for transport, laying andcompaction of rolled asphalt.

1993

BS 2499; Hot applied joint sealants for concretepavements: Part 1: Specification for joint sealants.

BS 2499: Hot applied joint sealants for concretepavements: Part 3: Methods of test.

BS 4987; Coated macadam for roads and other pavedareas Part 1: Specification for constituent materials anfor mixtures.

BS 4987; Coated macadam for roads and other pavedareas Part 2: Specification for transport, laying andcompaction.

nd

BS DD 213; Method for the determination of the indirectensile stiffness modulus of bituminous mixtures.

BS2000: Methods of test for petroleum and its productsPart 49: Determination of needle penetration ofbituminous material.

BS2000: Methods of test for petroleum and its productsPart 58: Determination of softening point of bitumen.Ring and Ball Method.

1995

BS DD ABF: Method for the determination of thefatigue characteristics of bituminous mixtures usingindirect tensile fatigue.

1996

BS 12; Specification for Portland cement.

BS 598: Sampling and examination of bituminousmixtures for roads and other paved areas: Part 102:Analytical Test Methods.

BS 598: Sampling and examination of bituminousmixtures for roads and other paved areas: Part 110:Method of test for the determination of wheel trackingrate.

BS DD 226: Method for determining resistance topermanent deformation of bituminous mixtures subjectto unconfined dynamic loading.

BS prEN 12272-1, Surface Dressing, Test Method,Accuracy of rate of spread of binder and chippings.

1997

BS prEN 12272-3, Surface Dressing, Test Method,Determination of binder-aggregate adhesivity by theVialit plate shock method.

5. Others

1977

Szatkowski, W. S. and Brown, J.R., Design andPerformance of Pervious Wearing Courses for Roads inBritain, 1967 to 1976, Highways and Road ConstructioInternational.


February 1999

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f


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1996

Experimental Procedures for the Design and Testing ofBituminous Mixtures for pavement engineering:Recommendations from the joint University/Industry/Highway Authority Bitutest project, University ofNottingham, Dept of Civil Engineering.Bibliography

6. Transport Research Laboratory

1968

Road Note 36 (2nd edition); Specification forManufacture and Use of Rubberised BituminousMaterials and Binders.

1993

Carswell, J., The Testing and Performance of SurfaceDressing Binders for Heavily Trafficked Roads. ProjectReport 12

Carswell, J and Gershkoff, D.R., The Performance ofModified Dense Bitumen Macadam Roadbases.Research Report 358

Gershkoff, D.R., Rheological Analysis of PolymerModified Bitumens for use in Rolled Asphalt WearingCourse. Project Report PR/H/24/93

Nicholls, J.C. and Daines, M.E., Acceptable WeatherConditions for laying Bituminous Materials. ProjectReport 13.

1994

Nunn, M.E. and Smith, T. Evaluation of a PerformanceSpecification in Road Construction. Project Report 55

Chaddock, B. and Pledge, K., Accelerated and FieldCuring of Bituminous Roadbase. Project Report 87.

1995

Daines, M.E., Tests for Voids and Compaction in RolledAsphalt Surfacing. Project Report 78

Nicholls, J.C., Potter, J.F., Carswell, J. and Langdale,P., Road Trials of Thin Wearing Course Materials.Project Report 79

1996

Nunn, M.E., The Characterisation of BituminousMacadams by Indirect Tensile Stiffness Modulus. TRLReport 160.

.

1997

Nicholls, J.C., Review of UK Porous Asphalt Trials.TRL Report 264.


1972

BS 3136: Specification for cold emulsion sprayingmachines for roads: Part 2: Metric units. (Confirmed1994).

1973

BS 1446: Specification for mastic asphalt (natural rockasphalt fine aggregate) for roads and footways.(Confirmed 1990)

1987

BS 598: Sampling and examination of bituminousmixtures for roads and other paved areas: Part 100:Methods for sampling for analysis. (Confirmed 1996)

BS 3195: Methods of sampling petroleum products: Pa3: Methods for sampling bituminous binders (identical toEN58).

BS 598: Sampling and examination of bituminousmixtures for roads and other paved areas: Part 101:Methods of preparatory treatment of samples foranalysis.

1989

BS 598: Sampling and examination of bituminousmixtures for roads and other paved areas: Part 104:Methods of test for the determination of density andcompaction. (Confirmed 1996)

BS 812 : Testing aggregates: Part 114: Methods fordetermination of the polished stone value.

1990

BS 598: Sampling and examination of bituminousmixtures for roads and other paved areas: Part 107:Method of test for the determination of the compositionof design wearing course rolled asphalt. (Confirmed1996)

BS 598: Sampling and examination of bituminousmixtures for roads and other paved areas: Part 108:Methods for determination of the condition of binder oncoated chippings and for the measurement of the rate ospread of coated chippings.


February 1999

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ofm

nce

t


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BS 598: Sampling and examination of bituminousmixtures for roads and other paved areas: Part 109:Methods for the assessment of the compactionperformance of a roller and recommended procedures forthe measurement of the temperature of bituminousmixtures.

BS 812: Testing aggregates: Part 113: Methods for thedetermination of aggregate abrasion value (AAV).(Confirmed 1995)

1995

BS 598: Sampling and examination of bituminousmixtures for roads and other paved areas: Part 111:Method for determination of resistance to permanentdeformation of bituminous mixtures subject tounconfined uniaxial loading.

1996

BS 598: Sampling and examination of bituminousmixtures for roads and other paved areas: Part 102:Analytical test methods.

BS DD 227: Methods of measuring irregularities onsurfaces of roads, footways and other paved areas usingstraightedges and wedges.

BS DD 228: Methods for determination of maximumdensity of bituminous mixtures.

BS DD 229: Method for determination of the hydraulicconductivity of permeable surfacings.

BS DD 232: Method for determination of the maximumbinder content of bituminous mixtures without excessivebinder drainage.

BS EN 500-1 to 500-6: Mobile road constructionmachinery.

1997

BS prEN 12594: Petroleum products- Bitumen andbituminous binders- Preparation of test samples.

BS prEN 12850: Petroleum products- Bitumen andbituminous binders- Determination of pH of Bitumenemulsions

8.

StPrIPPe

19

Vavisrop 2

19

HetheasTe

19

SzhigheIns

19

WBi

19

Stbin

StprRh

19

Huco

MemaanReBo

Others

andard Method for analysis and testing of Petroleumoducts, Appendix E: The significance and useage o precision data. Published annually by the Institute otroleum.

54

n der Poel, C.J., A general system describing thecoelastic properties of bitumen and its relation to

utine test data. Journal of Applied Chemistry, vol 4,21.

69

ukelom, W., A bitumen Test Data Chart for showing effect of temperature on the mechanical behaviour

phaltic bitumens. Journal of the Institute of Petroleuchnologists, vol 55, p 407.

79

atkowski, W.S., Rolled asphalt wearing courses withh resistance to deformation Proceedings of confereld in London 16 October 1979 (published 1980).titution of Civil Engineers.

90

hiteoak, D., The Shell Bitumen Handbook. Shelltumen UK.

93

andard specification for performance graded asphalder. AASHTO Provisional Standard MP1.

andard test method for determining the rheologicaloperties of asphalt binder using a Dynamic Sheareometer. ASSHTO Provisional Standard TP5.

94

nter, R.N. (Editor), Bituminous mixtures in roadnstruction. Thomas Telford, London.

rcer, J., Nicholls, J.C. and Potter, J.F. Thin surfacingterial trials in the UK; Paper given at Performanced Life cycle Analysis of Bituminous Thin Surfacehabilitation Techniques. Transportation Researchard, Washington D.C.


February 1999

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1995

Proceedings of the First European Workshop on theRheology of Bituminous Binders. Eurobitume.

Road Surface Dressing Association: Code of Practicefor Surface Dressing. RSDA.

The Shell Bitumen Industrial Handbook. Shell BitumenUK.

Brown, S.F., Gibb, J.M., Read, J.M., Scholz, T.V. andCooper, K.E., Design and testing of bituminousmixtures; report of the LINK/Bitutest research project.University of Nottingham, Dept of Civil Engineering.

1996

Proceedings of the Eurasphalt and Eurobitume congres1996, Foundation Eurasphalt.

Dobson, G., Rheology and Roads. Eurobitume.

1997

Cabrera, J.G. (Editor), Proceedings of the SecondEuropean Symposium on Performance and Durability ofBituminous Materials, Leeds, April 1997. Aedificatio,Zurich.

Nicholls, J.C. (Editor), Asphalt surfacings; A guide toasphalt materials and treatments used for the surfacecourse of road pavements.


February 1999 15/1

15. ENQUIRIES


Quality Services DirectorThe Highways AgencySt Christopher HouseSouthwark Street J KERMANLondon SE1 0TE Quality Services Director


The Director of HighwaysWelsh OfficeY Swyddfa GymreigCrown BuildingsCathays Park K THOMASCardiff CF1 3NQ Director of Highways

The Technical DirectorDepartment of the Environment forNorthern IrelandRoads ServiceClarence Court10-18 Adelaide Street V CRAWFORDBelfast BT2 8GB Technical Director

All technical enquiries or comments on this document should be sent in writing as appropriate to the above.

Chapter 15Enquiries

February 1999




PART 3

HD 38/97

CONCRETE SURFACING ANDMATERIALS

Amendment No. 1

SUMMA RY

This amendment includes completely revised Chapters 1and 6 and new Chapters 2 and 4 dated February 1999.


1. Insert the replacement pages listed on theAmendment sheet (Amendment No. 1), removingthe corresponding existing pages which aresuperseded by this amendment and archive asappropriate.

2. Insert new Chapters 2 and 4.

3. Enter details of Amendment No. 1 on theRegistration of Amendments sheet, sign and dateto confirm that the amendment has beenincorporated.




February 1999

Amendments

AMENDMENT No. 1 (February 1999)

Replacement Pages

Page No. Date

Front sheet

Contents page

1/1 - Introduction (replaces 1/1) February 1999

2/1 - 2/2 inc (new chapter) February 1999

4/1 - 4/4 inc (new chapter) February 1999

6/1 (replaces 6/1) February 1999

Implementation

The replacement page should be used forthwith on all schemes for the construction, improvement and maintenance oftrunk roads including motorways.

August 1997



SECTION 5 SURFACING ANDSURFACING M ATERIALS

PART 3

HD 38/97


SUMMA RY

This Standard gives advice on the materials andtechniques available for road surfacing to trunk roadsusing concrete.


This is a new document to be inserted into the manual.



Note: A quarterly index with a full set of VolumeContents Pages is available separately from theStationery Office Ltd.

HD 38/97

Concrete Surfacingand Materials

Summary: This Standard gives advice on the materials and techniques available forroad surfacing to trunks roads using concrete.






IncorporatingAmendmentsNo. 1 datedFebruary 1999



PART 3

HD 38/97

CONRETE SURFACING ANDMATERIALS

Amendment No. 1

Contents

Chapter

1. Introduction

2. Transverse Textured Concrete Surface

3. Exposed Aggregate Concrete Surface

4. Retexturing (Concrete)


6. References

7. Enquiries


February 1999

August 1997



SECTION 5 SURFACING ANDSURFACING M ATERIALS

PART 3

HD 38/97


SUMMA RY

This Standard gives advice on the materials andtechniques available for road surfacing to trunk roadsusing concrete.


This is a new document to be inserted into the manual.



Note: A quarterly index with a full set of VolumeContents Pages is available separately from theStationery Office Ltd.


February 1999

1. INTRODUCTION

1/1

General

1.1 This Part of the Design Manual for Roads andBridges gives advice on the materials and techniquesavailable for road surfacing to trunk roads usingconcrete. Similar advice relating to bituminous surfacingis found in HD 37 (DMRB 7.5.2).

1.2 This Part contains advice on providing atransverse textured concrete surface finish and anExposed Aggregate Concrete Surface (EACS) which isalso known as whisper concrete. Advice is also providedfor retexturing a concrete pavement if there has been aloss of skidding resistance.

Implementation

1.3 This Part shall be used forthwith on all schemesfor the construction, improvement and maintenance oftrunk roads including motorways, currently beingprepared, provided that, in the opinion of theOverseeing Organisation, this would not result insignificant additional expense or delay. Designorganisations should confirm its application toparticular schemes with the Overseeing Organisation.



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2. Transverse Textured Concrete Surface

Chapter 2Transverse Textured Concrete Surface

General

2.1 This Chapter explains the use of transversetextured concrete surface. It gives the background anddetails for the finish that will meet the requirements ofthe Specification for Highway Works (MCHW 1) Series1000. This is achieved by transversely brushing or tininthe slab after compaction of the concrete is completedand before commencement of the curing process. Abrushed or tined surface finish may be applied toUnReinforced Concrete (URC), Jointed ReinforcedConcrete (JRC) or Continuously Reinforced ConcretePavements (CRCP).

2.2 Brushed concrete surfaces have been successfuconstructed and have given many years of trouble freeservice, provided proper maintenance has been carriedout. In recent years it has become evident that somebrushed concrete surfaces can have high tyre/road nolevels. The advice given in this Chapter is aimed atensuring that new surfaces are not excessively noisywhilst maintaining adequate skidding resistance. Furthadvice on the noise generated on concrete surfaces isgiven in HD 36 (DMRB 7.5.1).

Scope

2.3 The requirements for the materials and theconstruction of roads with a brushed concrete surfaceare given in the Specification (MCHW 1) Series 1000.The relevant Clause should be used in conjunction withthe associated Appendix 7/1 and the related Notes forGuidance to the Specification for Highway Works(MCHW 2).

Use

2.4 For details of locations where brushed concrete isuitable, refer to HD 36 (DMRB 7.5.1).

Materials

Cements2.5 Cements used for the construction of concretepavements will be composed of Portland Cement (PC)often in combination with ground granulatedblastfurnace slag (ggbs) or pulverised fuel ash (pfa).

February 1999

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2.6 The specified minimum cement contents havebeen shown to provide the concrete with satisfactorylong term durability. The limits on ggbs and pfa havebeen set to give the concrete a resistance to spallingwhen subjected to de-icing salts.

Admixtures2.7 To assist in maintaining the workability of highstrength pavement quality concrete mixes, while keepina low water/cement ratio, plasticisers and super-plasticisers are often used. Air entrainment is required ithe top 50mm of a concrete pavement to provideresistance to surface scaling from de-icing salts.

Aggregates2.8 The coarse aggregate will be natural material,crushed concrete or air-cooled blast furnace slag. Thesoundness value will exceed 75 and the maximum sizewill be 40mm, unless the spacing of the longitudinalreinforcement is less than 90mm, when it will not excee20mm.

2.9 The use of white flints is not advised. They cancause pop-outs or can lead to D-shaped cracking. Theyconsist of nodules of cortex or harder flints covered incortex. Cortex is weathered flint and is porous as are, tsome extent, all flint aggregates. The overall porosity oa flint aggregate will depend on the proportion of whiteflints. Smaller particles tend to have higher adsorptionthan larger aggregate sizes.

2.10 The acid soluble content of the fine aggregate(sand) is limited to ensure the surface has satisfactorylow speed skidding resistance.

Construction

2.11 For a number of years new concrete pavementshave been built in both single and 2 layer constructionboth of which are permitted by the Specification(MCHW 1) Series 1000. The Contractor may choose thmethod of construction. More recently 2 layerconstruction has been used with either 2 separate paveor a double-decker paving machine. The advantage of2 layer construction is that only the top layer needs to bair entrained and the lower layer can contain the higherlevels of ggbs or pfa and sands with a higher acidsoluble content.

2/1


e

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Chapter 2Transverse Textured Concrete Surface

2.12 Both fixed form and slip formed pavers have beensuccessfully used. Recent developments appear to havemade slip forming more attractive to contractors due totheir increased capacity and flexibility on site. Modernslip form pavers also have the capability of being able tobe adapted for 2 layer construction.

Joints2.13 Joints in concrete pavements have been found tobe a frequent source of maintenance. In recent yearssteps have been taken to both improve the durability,hence the serviceability, and to reduce the number ofjoints. Now expansion joints are only required at run onslabs at the end of CRCP pavements and for themaintenance of an existing road with expansion joints.Elsewhere to improve durability transverse joints are tobe sawn contraction joints. Longitudinal joints may bewet formed or sawn. The timing of sawing the hardenedconcrete is critical. If sawn too early the aggregate willbe plucked out, if too late the concrete will have alreadycracked. With flint gravel aggregates in lower strengthconcrete pavements cracking may occur before sawingcan begin, whereas in pavements of high early strengthflint gravel concrete joints are more likely to besuccessfully sawn.

2.14 The spacing of joints in slabs may be increased by20% if a coarse aggregate with a low coefficient oflinear expansion, such as limestone, is used in theconcrete slab. If 2 layer construction is used theincreased joint spacing may be used if this aggregate isused throughout the lower layer.

Surface finish

Regularity2.15 The regularity of the surface provides a majorcontribution to the rolling resistance of that surface andthe noise generated by traffic. The micro-texture(dependant on the constituents of the concrete)contributes to the slow speed skidding resistance andsufficient macro-texture (as measured by the sand patchtest) reduces aquaplaning at high speed. Both areessential to produce a safe road. Mega-texture is foundto generate tyre/road noise. Both macro and mega-texture are controlled by the oscillating longitudinalfloat, which is also known as a ‘super smoother’.(For details of micro-, macro- and mega-texture seeHD 36 (DMRB 7.5.1.5)

2.16 The oscillating longitudinal float is an integralpart of the paving equipment, playing a vital role inensuring that a smooth, flat slab is produced. Where the

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scillating longitudinal float is part of a separate piecef equipment, as may be the case if rail mounted pavingquipment is chosen, it is essential that it keeps close toe rest of the paving train so that it is working on theesh concrete surface.

exture.17 Macro-texture is applied to the surface of the slab provide a path for water and air to be dissipated fromnder the tyres. Texture may be applied by brushing theurface or by dragging burlap and then tining the surfacf the wet concrete. Whereas both treatments createuitable macro-textures they can also produce surfacesith high mega-textures, which are unacceptably noisy.

.18 Brushed textures are applied transversely acrosse slab while the concrete is still plastic. It is importantat the texture is applied evenly with no ridges beingrmed, particularly where the brush overlaps withrevious passes. The pressure of the brush on the roadeeds to be uniform and consistent and the bristles keptlean to enable a satisfactory finish to be produced.

.19 The burlap and tined surfaces are widely used inmerica and now can be used in the UK. The burlapet hessian) is dragged over the surface after it has

een regulated to precondition the surface and give it aicro-texture. The tined texture is produced using

imilar equipment to that for applying brushed textureut with the brush head replaced by a head carrying theteel tines which is drawn transversely across theurface. The tines are 3mm wide and spaced apart at oird of those given for texturing hardened surfaces ine Specification (MCHW 1) Clause 1029. A trial length

hould confirm the actual spacing required for thearticular materials and plant being used.

uring.20 Immediately after completion of the texturing ofe slab it should be cured as required by thepecification (MCHW 1) Clause 1027 to reduce initialurface thermal cracking. During the initial period theurface should be protected against precipitation,oisture loss, contamination and dispersal of curinggent to avoid damage to the surface of the slab.

February 1999


Chapter 3Exposed Aggregate Concrete Surface

3. EXPOSED AGGREGATE CONCRETE SURFACE

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General

3.1 This chapter outlines the use of ExposedAggregate Concrete Surface (EACS) formerly known ‘whisper concrete’. It draws attention to the design,specification, construction and life of the surface andcontains guidance on its design, use and maintenance

3.2 EACS can provide a long life road surface withlower traffic noise than traditionally surfaced roads,offering comparable skid resistance. The exposedaggregate top surface forms a matrix of interconnectepaths below the plane running surface through whichwater can pass, to maintain a high speed skiddingresistance, and assist in reducing tyre/road surface noThere may be a slight reduction in spray generated byhigh speed vehicles.

3.3 The level of noise emitted at a tyre/road interfacon EACS is generally higher than for porous asphalt blower than for hot rolled asphalt, brushed concrete ansurface dressed running surfaces offering comparableskid resistance. EACS is perceived to have better tonqualities than conventional brushed concrete surfaces

3.4 The appearance of EACS is similar to that of horolled asphalt with a surface picking up its colour fromthe exposed aggregate.

Scope

3.5 Clauses specifying requirements for materialproperties, testing, laying and exposing the aggregateare given in the specification (MCHW 1) Series 1000.The relevant clauses in the Specification should be usin conjunction with the associated Appendix 7/1 and threlated Notes for Guidance to the Specification forHighway Works (MCHW 2).

Development

3.6 EACS was first trialled in Denmark anddeveloped in Belgium with the aim of producing a safelong life concrete running surface as an alternative to conventional brushed finishes used previously. Thetechnique has been successfully used since the 1970and now EACS is the normal form of surface treatmento CRCP on major Belgian roads. In the late 1980s thenvironmental problems of tyre/road noise came to thfore in Austria. Anxious to find a low noise concrete

August 1997

road surface that would stand up to studded tyre use inwinter, the Austrians refined the Belgian process,discovering that by reducing the size of the chippings inthe coarse aggregate at the road surface the tyre/roadnoise levels could be substantially reduced.Consideration was given in Austria to full depthconstruction but this would have used large quantities ofpremium aggregate. Two layer construction was chosento produce an economic solution. Now this is the normaform of EACS construction in Austria providing aneconomic, long lasting surface giving measurable noisereduction when compared with conventional concretepavements. Other countries which have used the EACSinclude France, the Netherlands, Australia and the USAAlso in Sweden it has provided a durable surface thatstands up to the use of studded tyres in the wintermonths. EACS roads in Austria were constructed usinga smaller sized aggregate, with a lower skiddingrequirement than specified in the UK. To provide anadequate low and high speed skidding resistance adeeper texture depth and the appropriate aggregate sizare specified. Other European countries, apart fromBelgium, have chosen to use jointed concrete slabswhereas in the UK, CRCP construction has been used.However, the use of jointed concrete pavements (URCand JRC) are permitted options for use on trunk roads,including motorways.

Use

3.7 EACS can be used in any location, includingheavily trafficked roads. In noise sensitive areas EACSmay be the only permitted concrete surfacing option.

Durability

3.8 Concrete roads are structurally designed for atraffic life of forty years although the surface willnormally require restoration of skid resistance at somepoint in its life. The special aggregate used in the topmix concrete is durable, with the minimum PSV andmaximum AAV specified in Appendix 7/1 of theSpecification (MCHW 1). Evidence from othercountries indicates that no particular durability problemwill be encountered. In the UK, the trials of EACS haveendured a number of winters without any sign ofdistress.

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Noise

3.9 EACS is designed to provide an adequate leveskid resistance, both at high and low speeds, as welllow level of surface noise. Work by Descornet andFuchs (1992) has identified the respective roles ofmicro, macro (see HD 23/94 (DMRB 7.1.1) GlossaryTerms) and megatexture in relation to tyre road noiseOn low speed roads, microtexture, which depends onroughness or harshness of the aggregate, is of primeimportance. On higher speed roads, macrotexturecreated by procedures such as transverse brushing oexposing the aggregate plays an important role inreducing noise. Megatexture, the variations of amplituof certain wavelengths in the 50 to 500mm range, is aundesirable irregularity. It can be a major cause of tyreroad noise because it creates radial vibration of the tMegatexture can originate during the course ofconstruction in the form of corrugation or other surfacirregularities. The use of the transverse finishing screin advance of a longitudinal oscillating float as soon athe surface is laid, plays a vital role in eliminating thecharacteristics by constructing the aggregate surfacelevel plane and hence resulting in noise reduction.

3.10 This type of surface typically exhibits noisereductions of around 2 dB(A) compared withconventional bituminous surfaces, and is the quietestconcrete surface currently available. EACS can be usin noise sensitive areas.

Structural Capacity

3.11 Depending upon the requirements of the highwascheme EACS can be applied to Jointed Unreinforce(URC), Jointed Reinforced (JRC), and ContinuouslyReinforced (CRCP) pavements.

Relative Costs

3.12 At present EACS is in the region of 10% moreexpensive than a similar pavement with conventionalbrushed concrete finishes. In certain areas of the couEACS can be cheaper than conventional asphalt.Experience shows that concrete surfaces compare wwith HRA surfaces when whole life cycle costs aretaken into account.

Materials and Mixing

3.13 In two layer construction the Specification(MCHW 1) gives a wide choice of aggregates for theconcrete in the lower layer. The Specification (MCHW1) for EACS states that:

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a) the coarse aggregate requirements are given in Specification and Appendix 7/1 (MCHW 1). Thecoarse aggregate should comprise at least 60%the fully compacted concrete (total mass of theconstituents excluding water). Attention is drawnto the special grading and flakiness requirementof the Specification (MCHW 1);

b) sand should comply with Classification F of Table5 in Clause 5.2 of BS 882 : (1983), except for thgrading requirements contained in theSpecification (MCHW 1). The reason forrestricting the maximum sand fraction is to makecertain that tyres maintain contact with the coarsaggregate, to ensure that the required level ofskidding resistance is provided;

c) the type of cement used in the concrete should blimited to Class 42.5N/42.5R Portland cement adefined in the Specification (MCHW 1).

Batching and Transportation

3.14 It is important for the contractor to take steps toprevent the contamination of the concrete for EACSwith aggregates or constituents for other concretes. Cshould be taken in the stockpiling of materials. It ispreferable that plant used for the batching and mixing EACS should be separate from that used for otherconcrete.

3.15 Dedicated, clearly identified delivery vehiclesshould be used for the transport of the separate mixesThese should be cleaned thoroughly between deliverieof separate mixes. Open tipper trucks should be sheet

Construction

3.16 Two layer monolithic construction has beenshown to be cost effective and has been successfullyused by contractors. Rail mounted equipment has beeused only in the UK. Elsewhere, slipform pavingmachines have been preferred. The requirements for atypes of concrete for road pavements are given in theSpecification (MCHW 1). The batching output capacityand transport delivery capacity should be sufficient tofeed the paving equipment continuously, enabling aconstant forward movement to be maintained during thperiod of paving.

August 1997



3.17 The oscillating longitudinal float is an integralpart of the paving equipment, playing a vital role inensuring that a smooth, flat slab is produced prior to tapplication of retarder. Where the oscillatinglongitudinal float is part of a separate piece ofequipment, as may be the case if rail mounted pavingequipment is chosen, it is essential that it keeps closethe rest of the paving train so that it is working on thefresh concrete surface.

3.18 The exposure of the surface is a two stage procincluding retarding of the surface mortar, and brushingto expose the aggregate.

a) A suitable retarder should be used which willretard sufficiently the action of the cement at thesurface of the top layer so that the target texturedepth can be achieved by the brushing operatioAdvice of retarder manufacturers should besought. The retarder should contain a non staindye, for visual identification of adequatecoverage. The application system should atomisthe retarder with a fine spray enabling total andeven coverage of the slab to be obtained withouoverdosing with excess retarder flowing over thesurface of the slab. From the time that theconcrete surface is finished and the retarder isapplied the surface of the concrete needsprotection from both drying out and rainfall. Theprotection should be maintained until immediatebefore exposing the aggregate. This may be of physical nature with protective sheeting beingtightly stretched across the slab and firmlysecured at the edges. (Such a process couldrequire a licence to avoid infringement of a pateagainst which the contractor would indemnify thclient as laid down in the Conditions of ContractAn alternative is a system of protection where achemical substance, compatible with the retarder,is sprayed on to the newly retarded surface whiafter some minutes, forms a protective skin whicprotects the retarder and slab against theelements. For this system temporary tentage isrequired to be available to protect the surfaceagainst rain before the longer term protectionbecomes operative.

b) The second stage in the creation of the exposedaggregate surface is the brushing. The time tostart brushing requires careful judgement.Brushing too early will result in chipping loss,and brushing too late will mean the surfacetexture will be very difficult to achieve. The ideal

August 1997

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surface will characteristically not only meet thetarget texture depth requirements, but also exhibita ‘shoulder to shoulder’ pattern of chippings.Experience indicates that the more cubic theshape of the chipping, the more likely that closepacking will occur.

3.19 Care should be taken to avoid fretting at dayjoints. This can be aggravated by retarder spray runningdown the vertical end of the slab. Hand finishing of theconcrete can cause problems if excess grout is pulled tothe surface, making exposure of the aggregate at thispoint more difficult.

3.20 Experience from the UK trials has shown thatlongitudinal joints can only be formed satisfactorily bysawing.

Surface Texture Depths

3.21 The required surface texture depths are stated inAppendix 7/1 in the Specification (MCHW 1). See alsoTable NG 10/3 in the Notes for Guidance (MCHW 2),which gives guidance on the requirements of average,maximum and minimum texture depths for high speedand low speed roads.

Brushing

3.22 The aggregate should be exposed by brushing in alongitudinal direction to give the texture depth as statedin Appendix 7/1 in the Specification (MCHW 1).Brushing should continue until that texture depth isachieved. Provision should be made for effective dustand laitance collection and disposal.

Curing

3.23 After brushing is completed the surface should bedampened prior to the application of the final curingagent.

Defect Repairs

3.24 If the specified minimum texture depth is notachieved by brushing, retexturing in accordance withClause 1029 of the Specification (MCHW 1) should becarried out. Failure to achieve the specified minimumtexture depths will result in unsatisfactory skidresistance for high speed vehicles and increased trafficnoise of lower speed vehicles. The full extent of anyareas still nonconforming following mechanicalretexturing should be removed and replaced.

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3.25 Where the specified average texture depth or themaximum texture depth is exceeded remedial measuresshould be agreed with the Overseeing Organisation.

3.26 Surface defects may be repaired provided:

a) The affected area is cut out to the top of the lowerlayer concrete;

b) A bonding agent is used; and

c) The same mix design used for the replacementconcrete.

The aggregate should be exposed in such manner as toachieve a texture similar to that in an adjacent compliantslab.

Renewal of Skid Resistance

3.27 After a period of years it may be necessary torestore the skid resistance to the EACS. Research iscontinuing into satisfactory in-situ ways of carrying thisout. Alternative methods of restoring the skid resistanceinclude overlay or inlay technique using EACS or one ofthe thin bituminous surface treatments now available.Surface dressing, though possible, would almostcertainly result in a sharp increase in tyre/road noise.

Winter Maintenance

3.28 There are no special winter maintenancerequirements.

Road Markings

3.29 There are no problems associated with applyingmarkings to an exposed aggregate surface.

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4. Retexturing (Concrete)

Chapter 4Retexturing (Concrete)

4.1 The wet skidding resistance of a road is dependenon the tyre interacting with the microtexture of the roadsurfacing. On a concrete road, microtexture comesprimarily from the sand and fine aggregate in theconcrete surface layer. Microtexture is graduallypolished by traffic until an equilibrium level of skiddingresistance is reached. On a concrete road, the laitancecontaining the fine aggregate may be worn away toexpose the coarse aggregate which may be unable tomaintain adequate microtexture. HD 28 (DMRB 7.3.1)provides advice on standards for the skidding resistanceof in-service roads.

4.2 The ability of a surface to maintain adequateskidding resistance at high speeds is governed by themacrotexture of the surfacing. As a surfacing ages, thelevel of macrotexture may fall as, for example, theridges created by brushing, are worn away. There mayalso be inadequate or excessive brushing of the newconcrete surface such that the minimum or maximumrequirements for texture depth of a new road may not bemet.

4.3 Retexturing is the mechanical reworking of asound road surface to restore either skidding resistancetexture depth or both. However, on a concrete road thecoarse aggregate may play a significant part indetermining the suitability of retexturing treatments.

Retexturing techniques

4.4 The suitability and effectiveness of a retexturingtreatment depends on the condition of the road prior totreatment. Some treatments can increase both skiddingresistance and texture depth; others may increaseskidding resistance but reduce texture depth. There arealso treatments which increase texture depth with littleeffect on skidding resistance.

4.5 Advantages include:

a) conservation of natural resources by reworking anexisting surface;

b) retexturing may be more economical than sometraditional resurfacing methods especially wheresmall areas are to be treated;

c) most processes can be carried out at any time ofyear in all but the most severe weather conditions;

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) traffic disruption is reduced compared withconventional treatments because of short lead-intimes and the speed of the processes;

) can be used as a “stop-gap” measure to treatsmall, high-risk sites.

.6 Disadvantages include:

) retexturing should not be used on unsound roadswhere there is cracking or surface irregularities,or on roads with sealing or overbanding;

) caution is needed with some treatments wherethere are joints within the concrete surface. Insome circumstances reinstatement of the jointsmay also be required;

) road surfacing features such as ironwork, whitelining and traffic detection loops may have to beavoided or protected;

) retexturing is of limited value where the coarseaggregate has been exposed and that aggregateeasily polished;

) some techniques may not be appropriate if thecoarse aggregate is a very hard material such asflint, which may not respond well to impact orcould cause damage or excessive wear to cuttingblades.

.7 The durability of the results of a treatment willepend on the type and geometry of road, the quantitynd behaviour of the traffic. However, just as a newurfacing will polish under the action of traffic, atextured concrete surface will eventually polish back an equilibrium skidding resistance level, close to thatf the original surfacing. On a high stress site, whereere is much braking and turning, the improvement mast a matter of months but, on a low stress site, the

ame treatment may continue to show an improvementver the untreated surface for three years or more.

.8 The following paragraphs give some comments ovailable methods and suggestions on their applicationr restoring skidding resistance and/or surface textureepth. Table 4.1 gives a summary of these methods anduggestions.

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Impact methods

4.9 Processes in this category involve striking theroad surface with either hard-tipped tools or hardparticles. These treatments are effective where the lossof skidding resistance is due to polishing.

a) Bush hammering: The road surface is struck by anumber of impact heads with chisel-endedhammers with hardened tips. This processenhances skidding resistance, regenerating themicrotexture of the concrete surface by erodingthe cementitious matrix. However, it cansometimes reduce texture depth, depending on thecondition of the existing road surface and theseverity of the treatment. In this process the forcewith which the road is struck can be controlled tosome extent.

b) shot blasting: The impact is by steel shotprojected at high speed from a rotating wheel. Asthe surface is scoured, both shot and arisings arerecovered and separated, with the steel shot storedfor reuse. This process improves skiddingresistance by eroding the cementitious matrix toregenerate the microtexture of the concretesurface.

Cutting and scabbling/flailing

4.10 This category includes cutting, sawing, grooving,grinding and scabbling/flail grooving. In the latter case,the cutting action is combined with impact on the cuttingheads.

a) grooving/grinding: Using diamond-tipped bladesassembled in configurations to suit the patterns ofcutting required. This process can be used toprovide either discrete grooving patterns or forbump cutting. Transverse sawing of randomly-spaced grooves in hardened concrete, introducedin the Specification for Highway Works(Department of Transport, 1986), has beensuccessfully used as a means of restoringmacrotexture to worn or rain-damaged areas for anumber of years. Grooving treatments can reducemacrotexture if the blades are in a close-spacedconfiguration. It should be borne in mind thatmicrotexture is often unaffected by groovingbecause the plateaux between the grooves are theoriginal surface.

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longitudinal scabbling: Hardened tips set into theedges of steel washers are loosely mounted side-by-side and drawn across the road surface whilstbeing hydraulically loaded. This process enhancesskidding resistance, by removing material fromthe tops of particles or ridges to expose new faces,but it can reduce surface texture depth by thesame process. On a concrete road, this processmay expose the coarse aggregate and this may nobe appropriate where the aggregate has anaturally-polished surface (such as flint) or a lowresistance to polishing, such as many limestones.

orthogonal grooving: This process is notrecommended for concrete surfaces.

uid action

11 This involves the surface being subjected to thetion of a fluid at high temperature or pressure. Theseatments do not involve mechanical reworking of thead surface to expose new aggregate surfaces and, as sult, do not restore skidding resistance lost through thelishing action of traffic. However, they may have aouring or cleaning action.

hot compressed air: This process scours thesurface of the cementitious matrix removingmaterial through dehydration of the cement pasteand thermal shock.

high pressure water jetting: By using highpressure water to remove the residual tyre rubberdetritus the original surface can be exposed.

ther considerations

12 Although retexturing is a useful option to considerhen addressing problems of skidding resistance orrface texture depth loss due to the action of traffic,ere will always be other factors to be taken intocount.

Some treatments will be more appropriate forsome surfaces than others. For example, anaggressive cutting or flailing technique would beinappropriate for a jointed surface where there isrisk of the joints and the egdes of slab beingdamaged, with the accomanying risk of wateringress and frost action.

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b) The effect of an individual process on bothskidding resistance and texture depth must beconsidered in the light of what is required in aparticular situation. For example, where surfacetexture is already at an acceptable level or whereincreasing it may be undesirable, a treatment thatdoes not increase surface texture would beappropriate.

c) Retexturing is most effective on road surfaceswhich are generally sound. A surface which isworn or losing surface material may be damagedfurther by mechanical action.

4.13 The use of exposed aggregate concrete as asurface type is in its early stages in the UK, with noexposed aggregate surfacing having yet reached a levelwhere remedial treatment is required. From a skiddingresistance point of view, exposed aggregate concretebehaves in a similar way to asphaltic surfacings, in thatit is the aggregate particles exposed at the surface whichprovide the microtexture. If retexturing techniques are tobe considered for such surfacings, caution must be usedto avoid loss of macrotexture due to excessive erosion ofthe exposed aggregate. There may be also a risk ofaggregate loss if the supporting cementitious matrix isdamaged.

4.14 As with all processes, the advice given herecannot cover all contingencies. It may be appropriate totake advice from suppliers regarding details of aparticular process and its suitability for a particular site.

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Suitability of treatment processesSurfacing Original condition: effect

type required from treatment Bush Shot Grooving/ Longitudinal Orthogonal Hot Water-hammering blasting grinding scabbling grooving compressed air jetting

Polished surface: good1 texture O O x O x O xrecovery of skiddingresistance

poor1 texture O O x O x O x

CRCP Brush-marks worn good2 SR O O ✔ O x x xor rain-damaged:recovery of texturedepth

poor2 SR O O O O x x x

Excessive noise/ good SR ✔✔✔✔✔ x x O x x xexcessive texture

Removal of oil/ good SR x O x x x O ✔✔✔✔✔rubber film

Polished surface: good texture O O x O x x xrecovery ofskidding resistance

Joint poor texture O O x O x x xconcretepavement Brush-marks worn good SR O O O O x x x

or rain-damaged:recovery of texturedepth

poor SR O O O O x x x

Excessive noise/ good SR O x x O x x xexcessive texture

Removal of oil/ good SR x x x x x O ✔✔✔✔✔rubber film

Polished aggregate: good texture ✔✔✔✔✔ O x x x x xrecovery ofskidding resistance

Exposed poor texture O O x x x x xaggregateconcrete Removal of oil/

rubber film good SR x x x x x x ✔✔✔✔✔

Key: SR Skidding Resistance✔ Appropriate treatmentO Treatment may be appropriate in some circumstances but effects will be limited and depend on surfacing conditionx Not recommended

Notes: 1 SMTD = Sensor Measured Texture Depth.When referring to texture in this context, "good" and "poor" are approximately the following:SMTD > 1.2 mm, good; SMTD < 0.6 mm, poor.

2 When referring to skidding resistance, "good" and "poor" denote above or below investigatory level respectively.

Table 4.1 Appropriate circumstances and treatments for retexturing concrete surfacings

February 19994/4


Chapter 6References


References:

Design Manual for Road and Bridges (DMRB):Stationery Office Ltd

HD 28 Skidding Resistance (DMRB 7.3.1)

HD29 Structural Assessment Methods (DMRB 7.3.2).HD32 Maintenance of Concrete Roads (DMRB 7.4.2).

HD36 Surfacing Materials for New Construction andMaintenance (DMRB 7.5.1)

HD 37 Bituminous Surfacing Materials and Techniques(DMRB 7.5.2)

Manual of Contract Documents for Highway Works(MCHW) : Stationery Office Ltd.

Volume 1: Specification for Highway Works(MCHW1)

Volume 2: Notes for Guidance on the Specification forHighway Works (MCHW2)

Transpor t Research Laboratory (TRL)

1997

Hewitt, A.P., Abbott, P.G. and Nelson, P.M, AlternativeTextures for Concrete Roads: Results of M18 and A50Trials, TRL Report 291.

Roe, P.G. and Hartshorne, S.A., The MechanicalRetexturing of Roads: a Study of Processes and Early-life Performance, TRL Report 298.

1998

Roe, P.G. and Hartshorne, S.A., Mechanical Retexturingof Roads: an Experiment to Assess Durability. TRLReport 299.

BSIns

BSso

Ot

DePrCo

February 1999

I Standards Publications: British Standardstitution, London

882: 1983. Specification for aggregates from naturalurces for concrete.

hers

scornet, G. and Fuchs, F., Concrete Paving Texture,oceedings of the PIARC workshop “Noise Reducingncrete Surfaces”, Vienna, pp 54 - 60, 1992

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August 1997 7/1

7. ENQUIRIES


Quality Services DirectorThe Highways AgencySt Christopher HouseSouthwark Street J KermanLondon SE1 0TE Quality Services Director


The Director of HighwaysWelsh OfficeY Swyddfa GymreigCrown BuildingsCathays Park K J THOMASCardiff CF1 3NQ Director of Highways

The Technical DirectorDepartment of the Environment forNorthern IrelandRoads ServiceClarence Court10-18 Adelaide Street V CRAWFORDBelfast BT2 8GB Technical Director

All technical enquiries or comments on this document should be sent in wirting as appropriate to the above.

Chapter 7Enquiries