106_literature review stabilised sub-bases

Literature Review

Stabilised Sub-Bases for Heavily Trafficked Roads

PROJECT REPORT

LITERATURE REVIEW

STABILISED SUB-BA

DFID Project Source RefSubsector: T

Theme: T

Project Title: D

Project Reference: R

ORIGIN PR/INT/202/00

SES FOR HEAVILY TRAFFICKED ROADS

erencesransport

2

esign of stabilised sub-bases for heavily trafficked roads

6027, R8010

Copyright Transport Research Laboratory, UK and the Bureau of Research and Standards,Department of Public Works and Highways, Philippines.

This document is an output from a co-operative research programme between the Departmentfor International Development (DFID), of the UK and the Department of Public Works andHighways (DPWH), Philippines. The project was funded from both the DFID Knowledge andResearch Programme which is carried out for the benefit of developing countries, and from theresources of the Bureau of Research and Standards of DPWH. The views expressed are notnecessarily those of DFID or DPWH.

The Transport Research Laboratory and TRL are trading names of TRL Limited, a member of the TransportResearch Foundation Group of Companies.

TRL Limited. Registered in England, Number 3142272. Registered Office: Old Wokingham Road, Crowthorne,Berkshire, RG45 6AU, United Kingdom.

The information contained herein is the property of the Transport Research Laboratory and theDepartment of Public Works and Highways, and does not necessarily reflect the views or policies of DFIDor DPWH. Whilst every effort has been made to ensure that the matter presented in this report is relevant,accurate and up-to-date at the time of publication, neither the Transport Research Laboratory nor theDepartment of Works and Highways accept liability for any error or omission.

Literature Review: Stabilised Sub-Bases for Heavily Trafficked Roads

CONTENTS

1 INTRODUCTION ............................................................................... 1

2 STABILISATION IN ROAD PAVEMENTS............................................... 2

2.1 The role of the sub-base ................................................................... 3

2.1.1 The role of a stabilised sub-base in a flexible pavement....................... 3

2.1.2 The role of a stabilised sub-base in a concrete pavement...................... 4

3 TYPES OF STABILISATION................................................................. 5

3.1 Mechanical Stabilisation................................................................... 5

3.2 Cement Stabilisation ....................................................................... 5

3.2.1 Soil Cement............................................................................ 6

3.2.2 Cement Bound granular Material (CBM) ........................................ 6

3.2.3 Lean concrete ......................................................................... 6

3.3 Lime Stabilisation .......................................................................... 7

3.4 Bitumen or Tar stabilisation .............................................................. 8

3.5 Other types of stabilisation................................................................ 8

3.5.1 Blastfurnace slag...................................................................... 8

3.5.2 Pozzolanas ............................................................................. 9

3.5.3 Non-pozzolanic chemical soil stabilisers ......................................... 9

4 ELASTIC MODULUS.......................................................................... 9

5 TESTING AND MIX DESIGN ..............................................................10

5.1 Suitability of materials for stabilisation ................................................10

5.2 Mix design ..................................................................................12

5.2.1 Post Construction - Strength.......................................................13

5.2.2 Durability .............................................................................14

5.2.3 Construction equipment ............................................................14

5.2.4 Pre-construction trials ..............................................................15

6 PROBLEMS ASSOCIATED WITH STABILISATION .................................15

6.1 Construction ................................................................................15

6.1.1 Quantity of stabiliser ................................................................15

6.1.2 Mixing.................................................................................16

6.1.3 Compaction and limited time ......................................................16

6.1.4 Rapid setting..........................................................................16

6.1.5 Curing time ...........................................................................16

6.1.6 Variability.............................................................................16


6.1.7 Testing.................................................................................16

6.2 Durability ...................................................................................16

6.2.1 Carbonation...........................................................................16

6.2.2 Sulphate and salt damage...........................................................17

6.2.3 Cracking ..............................................................................17

6.2.4 Break-up ..............................................................................17

7 CURRENT STABILISATION PRACTICE AROUND THE WORLD...............17

7.1 UK Practice.................................................................................17

7.1.1 Concrete pavements .................................................................17

7.1.2 Bituminous pavements ..............................................................18

7.2 TRL ORN31 Practice .....................................................................18

7.3 USA Practice ...............................................................................19

7.3.1 Designs for concrete pavements ..................................................19

7.3.2 Designs for flexible pavements....................................................20

7.4 Australia.....................................................................................20

7.4.1 Austroads Pavement Design Guide...............................................20

7.5 South Africa ................................................................................21

7.6 The Philippines.............................................................................21

8 PAVEMENT DESIGN FOR HEAVILY TRAFFICKED ROADS ....................22

9 CONCLUSIONS................................................................................24

10 RECOMMENDATIONS FOR PILOT TRIALS IN THE PHILIPPINES. ...........25

11 ACKNOWLEDGEMENT.....................................................................26

12 REFERENCES / BIBLIOGRAPHY.........................................................27


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EXECUTIVE SUMMARY

Stabilisation is the process of mixing a stabiliser, for example cement, with a soil orimported aggregate to produce a material whose strength is greater than that of theoriginal unbound material. The use of stabilisation to improve the properties of amaterial is becoming more widespread due to the increased strength and load spreadingability that these materials can offer. Stabilisation technology is extremely relevant forheavily trafficked pavements where its' benefits are beginning to be appreciated.

This report describes the basic types of stabilisation, indicates when it should be used,and discusses the main advantages and disadvantages of its use. The role of the sub-base and other pavement layers are also discussed for both flexible and rigidpavements.

An extensive literature review of international publications was carried out and thisreport describes some of the latest research and design methodology associated withstabilised materials used for sub-bases on heavily trafficked roads. As well asreferences to the literature it also contains an extensive bibliography of work on thissubject.

Many of the pavement design manuals from other countries were examined. Theseinclude manuals from the UK, USA, Australia and South Africa; many of whichinclude in their specifications the design of asphalt pavements with stabilised sub-bases.In these design manuals, stabilised sub-bases are used with either stabilised or granularroadbases. This report discusses advantages and disadvantages of these designs. Thevarious pavement design manuals also showed that stabilised sub-bases are often usedunder concrete pavements, which is presently not the case in the Philippines where agranular sub-base is still specified. The benefits of this form of construction are alsodiscussed.

The report notes that few of these design manuals produce savings in pavementthickness from the use of stabilised sub-bases even though they are frequentlyrecognised to have higher strengths than unbound granular materials. They are merelysubstitutes. Their use also permits the use of lower-grade, marginal materials aftersuitable stabilisation, which may reduce haulage of high quality unbound materials anddepletion of resources. The report concludes that there is a role for stabilised sub-basesin the Philippines, especially for heavily trafficked pavements where they couldimprove performance and hence reduce maintenance costs.

Finally, the report outlines technical recommendations for pilot trials of stabilised sub-bases in the Philippines. These trials would be constructed under the auspices of theBureau of Research and Standards of the DPWH and monitored under a DPWH/DFIDjointly funded research project being undertaken by staff from BRS and TRL.


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STABILISED SUB-BASES FOR HEAVILY TRAFFICKEDROADS

1 INTRODUCTION

The main objective of stabilisation is to improve the performance of a material byincreasing its strength, stiffness and durability. The performance should be at leastequal to, if not better than that of a good quality natural material.

This report describes the basic types of stabilisation, the main advantages anddisadvantages of the technique and the latest research and design methodology for suchmaterials.

The term ‘heavily trafficked roads’ varies between design standards and countries. Inthis report, as an approximate guide, the term is applied to roads with a design life ofmore than 10 million equivalent standard axles (ESA).

The term ‘stabilisation’ is the process whereby the natural strength and durability of asoil or granular material is increased by the addition of a stabilising agent. . Inaddition, it may provide a greater resistance to the ingress of water. There are manytypes of stabiliser that can be used, each with their own advantages and disadvantages.The type and quantity of stabiliser added depends mainly on the strength andperformance that needs to be achieved.

The addition of even small amounts of stabiliser, for example up to 2 per cent cement,can modify the properties of a material. Larger amounts of stabiliser will cause a largechange in the properties of that material, for example 5 to 10 per cent of cement addedto a clean gravel will cause it to behave more like a concrete.

The strength of a stabilised material will often continue to increase for a period ofseveral years from the time it is constructed, as shown in Figure 1 (Croney, 1998).

Figure 1 Rate of increase of strength with age for cemented material (AfterCroney, 1998)


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The strength of a stabilised material will depend on many factors. These include:

• the chemical composition of the material to be stabilised;

• the stabiliser content;

• the degree of compaction achieved;

• the moisture content;

• the success of mixing the material with the stabiliser;

• subsequent external environmental effects.

When small quantities of stabiliser are added, the material is often described as‘modified’ rather than ‘bound’. There are no fixed criteria for these definitions, but alimit of 80kPa (indirect tension) or 800kPa (Unconfined Compressive Strength after 7days moist curing) for a reasonably graded material is suggested by NAASRA (1986).

2 STABILISATION IN ROAD PAVEMENTS

There are many different reasons for using stabilisation, ranging from lack of goodquality materials to a desire to reduce aggregate usage for environmental reasons.Ultimately the main reason for using stabilisation will usually be cost savings. Theengineer is trying to build a problem-free pavement that will last for its intended designlife for the most economic price. The cost savings associated with stabilisation can takemany forms including reduced construction costs, reduced maintenance coststhroughout the life of the pavement or an extension of the normal pavement life.

The location of suitable materials for road construction will become increasinglydifficult as conventional high-quality materials are depleted in many areas. The costs ofhauling materials from further away may also increase, thus compounding the problem.One solution is to stabilise locally available materials that presently may not conform toexisting specifications.

From the point of view of bearing capacity, the best materials are those which derivetheir shear strength partly from friction and partly from cohesion. For stabilisation tobe successful, the material should attain the desired strength (i.e. be capable ofsustaining the applied loads without deformation) and should retain its strength andstability indefinitely.

Not all materials can be successfully stabilised, for example if cement is used as thestabiliser then a sandy soil is much more likely to yield satisfactory results than a softclay (Watson, 1994). The material to be stabilised must be tested to ensure that it iscompatible with the intended stabiliser – the subject of testing will be discussed later inthis report. It is also recommended from experience that layers which are less than150mm thick should not be stabilised (Lay, 1986/88).

Netterberg (1987) reports that unless proven by experience or durability testing, amaterial should not be improved too much. For example a material for use as a baselayer should only be stabilised if it could be used unstabilised for a sub-base layer.Another recommendation from the same report is to “discount any increase in strengthof more than 100 per cent.”


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Capping and sub-base layers can usually be stabilised without significant problems.One of the main problems with stabilised layers is that they crack to a greater or lesserdegree. This cracking is caused by changes in moisture content and temperature andcannot be avoided. The amount of cracking will depend on many factors, but generallya stronger material will produce wider cracks at a greater crack spacing than a weakermaterial.

A cement stabilised granular base directly under an asphalt surfacing will frequentlyresult in reflection cracking as shrinkage cracks in the base propagate through theasphalt surfacing. If cracks are left unsealed, then water penetration can lead to furtherdeterioration, particularly if the underlying sub-base is not stabilised.

Stabilisation of the sub-base under a granular base, however, can have many benefitswithout causing reflection cracking in the surface of an asphalt pavement. It is reportedthat a thickness of 125-150mm of granular cover over a stabilised sub-base is generallysufficient to substantially delay or stop reflection cracking (NAASRA, 1987).

2.1 The role of the sub-base

The sub-base is an important layer in both flexible and rigid pavements. It mainly actsas a structural layer helping to spread the wheel loads so that the subgrade is not over-stressed. It also plays a useful role as a separation layer between the base and thesubgrade and provides a good working platform on which the other paving materialscan be transported, laid and compacted. It can also act as a drainage layer. Theselection of material and the design of the sub-base will depend upon the particulardesign function of the layer and also the expected in-situ moisture conditions (TRL,1993).

Stabilised sub-bases can be used for both flexible and rigid road pavements, althoughthe reasons for doing this can vary. In order to identify the benefits of stabilising sub-bases, it is necessary to examine the role of the sub-base for each pavement type.

2.1.1 The role of a stabilised sub-base in a flexible pavement

A stabilised, and therefore stiffer, sub-base provides greater load spreading ability andhence reduces stresses imposed on the subgrade. When stabilised the sub-base providesmuch of the structural rigidity in the pavement, and also assists during the compactionof the upper granular layers and hence increases their ability to withstand deformation.

If the sub-base is stabilised, reflection cracking in an asphalt surface layer can beminimised by having an unbound granular roadbase. This unbound roadbase providesnot only a large proportion of the structural load spreading but also assists in delayingor preventing reflection cracking from the shrinkage and movement of the stabilisedlayer. The granular roadbase is subjected to relatively high traffic stresses and crushedaggregate is often used to withstand attrition and to assist in achieving a high value ofelastic modulus, limiting the horizontal tensile strains at the bottom of the bituminoussurfacing.

The use of a stabilised sub-base with a granular base is often referred to as an ‘upside-down pavement’ (Lay 1986). It is reported (LCPC, 1997) that a typical mode ofdeterioration for this type of pavement, based on experience from France, is slightrutting attributed to the unbound granular layer and eventually fine transverse crackingwhich occurs after much trafficking.


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2.1.2 The role of a stabilised sub-base in a concrete pavement

For a concrete pavement, the term ‘sub-base’ refers to the layer immediately below theconcrete slab. In a concrete road, the high elastic modulus of the concrete layer causesmost of the traffic-induced stresses to be taken in the concrete layer in the form ofbending stresses.

According to O’Flaherty (1994), there is a common misunderstanding about the mainfunction of the sub-base beneath a concrete slab. He states that the main function of thesub-base is to ensure uniform support to the concrete, counteracting the effect ofunsatisfactory subgrade support, rather than increasing the structural stability (i.e.strength) of the pavement.

If the subgrade could be relied upon to provide uniform support throughout the life ofthe pavement then a sub-base may not be required and the slab could be cast directly onthe prepared in-situ soil, providing it is good quality and naturally uniform. Thisuniform support appears to be crucial, especially where the subgrade is either weak orexpansive because the non-uniform support will eventually lead to the fatigue failure ofthe pavement.

It has been found that substitution of the top layer of a weak subgrade by a strongerunbound granular layer has little influence on the stresses at the bottom of the slab(TRRL, 1978). For example, a gravel sub-base 150mm thick on a weak subgrade willonly reduce the tensile stress by about 10 per cent in a thin slab and less in a thickerslab.

For a concrete pavement with a granular sub-base, the two major modes of damageare:

1. tensile stresses at the base of the concrete layer due to inadequate strength and/orthickness of the concrete and

2. lack of bearing capacity – mainly at joints or cracks where pumping and erosion ofthe support can aggravate the problem.

Use of a stabilised sub-base, provided it has adequate strength and durability, can helpto alleviate this second mode of damage. The problem of ‘pumping’ mainly occurs onroads built on subgrades with a high fines content. With a granular sub-base, fines inthe subgrade or sub-base can go into suspension if water is present and this finematerial can be pumped out of a joint or crack under the passage of heavy wheel loads.This eventually leads to a void under the slab, resulting in slab cracking, rocking orfaulting. Use of a stabilised sub-base can frequently prevent pumping by a) stopping orreducing water penetration to underlying layers and b) ensuring that there are no freefines available immediately beneath the concrete slab.

Stabilised sub-bases provide a uniform, stable and permanent support for concrete slabsthroughout their design life. They can also aid construction of the concrete slabs byproviding a low permeability surface, which minimises water loss from the freshconcrete and also provide a hard layer beneath the slabs to aid compaction.

The stress generated in a concrete slab partly depends on the stiffness ratio between theslab and the underlying support. In many countries, including the UK, the nationaldesign standards specify that all rigid pavements must be constructed with a cemented


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sub-base of adequate stiffness. “This type of sub-base erodes less than an unboundmaterial and is less water-susceptible should join sealants fail” (UK DOT, 1995).

3 TYPES OF STABILISATION

There are a number of different types of stabilisation, each having its own benefits andpotential problems. The types described below are those most frequently used,however, it must be noted that not all of them are appropriate for all situations.

3.1 Mechanical Stabilisation

The most basic form of mechanical stabilisation is compaction, which increases theperformance of a natural material. The benefits of compaction, however, are wellunderstood and so they will not be discussed further in this report.

Mechanical stabilisation of a material is usually achieved by adding a different materialin order to improve the grading or decrease the plasticity of the original material. Thephysical properties of the original material will be changed, but no chemical reaction isinvolved. For example, a material rich in fines could be added to a material deficient infines in order to produce a material nearer to an ideal particle size distribution curve.This will allow the level of density achieved by compaction to be increased and henceimprove the stability of the material under traffic. The proportion of material added isusually from 10 to 50 per cent.

Providing suitable materials are found in the vicinity, mechanical stabilisation is usuallythe most cost-effective process for improving poorly-graded materials. This process isusually used to increase the strength of a poorly-graded granular material up to that ofa well-graded granular material. The stiffness and strength will generally be lower thanthat achieved by chemical stabilisation and would often be insufficient for heavilytrafficked pavements. It may also be necessary to add a stabilising agent to improve thefinal properties of the mixed material.

3.2 Cement Stabilisation

Any cement can be used for stabilisation, but Ordinary Portland cement is the mostwidely used throughout the world.

The addition of cement to a material, in the presence of moisture, produces hydratedcalcium aluminate and silicate gels, which crystallise and bond the material particlestogether. Most of the strength of a cement-stabilised material comes from the physicalstrength of the matrix of hydrated cement. A chemical reaction also takes placebetween the material and lime, which is released as the cement hydrates, leading to afurther increase in strength.

Granular materials can be improved by the addition of a small proportion of Portlandcement, generally less that 10 per cent. The addition of more than 15 per cent cementusually results in conventional concrete. In general, the strength of the material willsteadily increase with a rise in the cement content. This strength increase isapproximately 500 to 1000 kPa (UCS strength) for each 1 per cent of cement added(Lay 1986/88). The elastic modulus of an unbound natural gravel or crushed rock willbe in the range 200-400 MPa. When stabilised, this will increase to a range ofapproximately 2,000 to 20,000 MPa.


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Cement stabilised materials can be mixed in-situ or mixed at a plant and transported tosite. To achieve stronger cement bound materials, i.e. greater than about 10 MPa cubestrength at 7 days, the materials should generally be plant mixed (DETR, 1998).

One of the main problems with stabilising a material is mixing in the cement. Theparticle size of ordinary Portland cement is quite well defined with a range of 0.5-100microns and a mean of 20 microns (Ingles & Metcalf, 1972). The larger particles ofcement never completely hydrate, and it has been found that the same amount of amore finely ground cement will produce higher strengths. Finely ground cements are,however, expensive to produce and it has been suggested (Ingles & Metcalf, 1972) thatthe larger particles of cement could be replaced with smaller particles of an inert filler.The greater bulk would aid the distribution process so that the same amount of activecement would be available throughout the material. Thus producing an equallyeffective binder, which could be cheaper than ordinary cement.

The use of cement as a stabiliser is more widespread than lime. This is due to manyreasons, but the main factors are likely to be the cost and the higher strengths that areattainable using cement. Other factors include availability, past experience and themore hazardous nature of lime. The price of cement is often similar to that ofquicklime or hydrated lime, however cement can be used on a wider range of materialsand the strengthening effect of cement is much more than that of an equal amount oflime. Hence either higher strengths are possible using an equal amount of cementinstead of lime or the same specified strength can be achieved using a lower quantity ofcement than lime. The effects of lime and cement on the 7-day strength of various soiltypes was presented graphically by Sherwood (1993) and Dumbleton (1962), as shownin Figure 2.

There are three main types of cement-stabilised materials:

3.2.1 Soil Cement

Soil cement usually contains less than 5 per cent cement. (Lay, 1986). It can be eithermixed in-situ (usually up to 300mm layer at a time) or mixed in plant. The techniqueinvolves breaking up the soil, adding and mixing in the cement, then adding water andcompacting in the usual way. Croney (1998) recommends that a minimum strengthshould be 2.5 MPa (7 day cube crushing strength) or, if this material is used to replacesub-base then the strength requirement should be increased to 4 MPa.

3.2.2 Cement Bound granular Material (CBM)

This can be regarded as a stronger form of soil-cement but uses a granular aggregate(crushed rock or natural gravel) rather than a soil. The process works best if the naturalgranular material has a limited fines content. This is almost always mixed in plant andthe strength requirement is 5-7 MPa (7 day cube crushing strength), (Croney, 1998).

3.2.3 Lean concrete

This material has a higher cement content than CBM and hence looks and behavesmore like a concrete than a CBM. It is usually made from batched coarse and finecrushed aggregate, but natural washed aggregate (e.g. river gravels) can also be used.The UK specification for this material gives a normal strength of 6-10 MPa or a higherstrength of 10-15 MPa (7 day cube crushing strength).


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Figure 2 Effect of lime & cement on 7-day strength of various soil types. AfterSherwood (1993) and Dumbleton (1962).

3.3 Lime Stabilisation

The stabilisation of pavement materials is not new; with examples of lime stabilisationbeing recorded in the construction of early Roman roads. However, the invention ofPortland cement in the 19th Century resulted in cement replacing lime as the main typeof stabiliser. The use of lime is still widespread particularly in certain parts of Africaand North America.

Lime stabilisation will only be effective with materials which contain enough clay for apositive reaction to take place. Attempts to use lime as a general binder in the sameway as cement will not be successful (Watson, 1994).

Lime is produced from chalk or limestone by heating and combining with water. Theterm ‘lime’ is broad and covers the following three main types:

a) quicklime i.e. calcium oxide (CaO),

b) slaked or hydrated lime, i.e. calcium hydroxide (Ca(OH)2) and

c) carbonate of lime, i.e. calcium carbonate (CaCO3).

Only quicklime and hydrated lime are used as stabilisers in road construction. They areusually added in solid form but can also be mixed with water and applied as a slurry. Itmust be noted that there is a violent reaction between quicklime and water andconsequently operatives exposed to quicklime can experience severe external andinternal burns, as well as blinding. Careful handling and protective clothing are,therefore, essential.


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Hydrated lime is used extensively for the stabilisation of soil, especially soil with ahigh clay content where its main advantage is in raising the plastic limit of the clayeysoil. Very rapid stabilisation of water-logged sites has been achieved with the use ofquicklime.

There is little experience with lime stabilisation for road pavements in the UK wherethe process is intended primarily for treating wet, heavy clays. Small quantities(typically 1-3 %) are used to reduce the plasticity of the clay. It is reported that suchsmall quantities usually result in a small increase in CBR strength although nosignificant increase in compressive or tensile strength should be expected (Paige-Green,1998). Paige-Green reports that typically, a minimum of 3 to 5 per cent stabiliser isnecessary to gain a significant increase in the compressive and tensile strength.

Although the use of lime stabilisation is widespread, the reported performance of thetechnique is often variable. In fact, many parts of Australia stopped using limestabilisation in the 1970’s due to some major problems. More recently the techniquehas regained favour and is being used in on-going road trials; e.g. Killarney RoadTrials and Freestone Creek to Eight Mile Intersection (Evans 1998). However, Evansconcluded that “…it may be prudent to continue to assume that lime stabilisedsubgrades do not contribute greatly to pavement strengths.”

The strengthening effect of cement is significantly greater than the equivalent quantityof lime unless the host material contains a significant quantity of clay, and so,generally, to achieve the higher strengths necessary for heavily trafficked roads,cement appears to be a more practical stabiliser.

3.4 Bitumen or Tar stabilisation

Bitumen and tar are too viscous to use at ambient temperatures and must be made intoeither a cut-back bitumen (a solution of bitumen in kerosene or diesel), or a bitumenemulsion (bitumen particles suspended in water). When the solvent evaporates or theemulsion ‘breaks’, the bitumen is deposited on the material. The bitumen merely actsas a glue to stick the material particles together and prevent the ingress of water. Inmany cases, the bituminous material acts as an impervious layer in the pavement,preventing the rise of capillary moisture.

In a country where bitumen is relatively expensive compared to cement and where mostexpertise is in cement construction, it appears more reasonable to use a cementstabiliser rather than a bitumen/tar based product.

3.5 Other types of stabilisation

Materials in this group do not, on their own, produce a significant cementing actionand may need to be used in conjunction with cement or lime (O’Flaherty, 1985).

3.5.1 Blastfurnace slag

This is a by-product of the iron industry. It cannot be used on its own as a stabiliser butwhen it is ground into finer particles the product, known as ground granulatedblastfurnace slag (ggbfs), can be used as a cement replacement, with up to 85 per centof the cement replaced with the slag.


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3.5.2 Pozzolanas

Pozzolanas possess little or no cementitious properties in themselves but will in certaincircumstances chemically react with lime to form compounds possessing cementitiousproperties. Natural pozzolanas are mainly of volcanic origin; artificial pozzolanas areproducts obtained from heating natural products. Examples of artificial pozzolanas arepulverised fuel ash (pfa) which is obtained from the burning of coal in power stationsand rice husk ash (Sherwood, 19930, (Montgomery, 1991).

3.5.3 Non-pozzolanic chemical soil stabilisers

These chemical stabilisers mostly take the form of strongly acidic, ionic, sulphonated,oil-based products. A cementitious reaction does not usually occur, but due to manyfactors including ionic exchange, the absorbed water can be reduced leading to bettercompaction and increased strength. The material must have an appropriate clay contentfor the stabiliser to have a beneficial effect. When correctly utilised, these products canbe very cost effective (Paige-Green, 1998).

Products containing chemicals such as sodium chloride and ligno-sulphonates purely‘stick’ the material or soil particles together, while other products such as thosecontaining enzymes act biologically to achieve the same effect.

Although non-pozzolanic stabilisers are usually cheaper, they are usually not aseffective as traditional stabilisers such as cement or lime, which produce significantlygreater strengths.

4 ELASTIC MODULUS

In a pavement engineering context, one of the most fundamental engineering propertiesof any material is the elastic modulus. The term ‘elastic modulus’ is defined as the ratioof stress to strain and is a measure of the material’s stiffness properties. In addition tothe modulus of a material, it is also important to know its strength because a materialmay be very stiff, but not very strong and could crack or break under heavy traffic.

The modulus of elasticity of a cemented material can be measured by several differentmethods including: dynamically (Ed) using electrodynamic excitation of long beams of150mm section or statically (Es) by loading 150mm diameter cylinders fitted withextensiometers. Croney (1998) reports that comparative studies have consistentlyshown the dynamic modulus to be higher than the static value. An approximateconversion is given below (for values of Ed >5):

Ed = 10 + 0.8Es (in GPa)……….. (Croney, 1998)

There is also much discussion about whether to use dynamic or static modulus valuesin pavement calculations and often the average of both values is used.

A relationship between dynamic modulus and compressive strength at 28 days is shownbelow in Figure 3.


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Figure 3 Relationship between dynamic modulus and compressive strength (at 28days) for some cement treated materials (Croney, 1998)

Materials cemented with pozzolanic stabilisers such as lime and cement, perform in amore elastic, semi-brittle manner under traffic than unbound materials. Ideally,knowledge of a material’s stiffness modulus and shear strength are required todetermine an appropriate thickness for the overlying pavement layers. The number offactors involved in knowing these variables are high, for example the shear strengthwill depend on factors including the effective stress which is dependant on the stresshistory, etc. To simplify matters, index tests are often used. Historically, the CBR hasbeen used but it is now often thought to be useful only for modified materials where thestrength of the materials measured in the CBR test would not exceed 100 per cent. TheUnconfined Compressive Strength test is considered a more useful guide to the elasticmodulus and many correlations exist, for example TRH13 (CSRA, 1986) andAustroads (1992). In the move towards mechanistic design there is a driving force touse more direct measurements. Such testing however may be beyond the resources ofmany laboratories.

5 TESTING AND MIX DESIGN

5.1 Suitability of materials for stabilisation

Before stabilising a material, especially a soil, it must be tested to ensure thecompatibility and the effectiveness of the intended stabiliser. These initial tests willvary between countries, but often take the form of determining the particle sizedistribution, liquid and plastic limits, soil acidity and sulphate content. One suchchemical test is for the Initial Consumption of Lime (ICL). The test is used when limeor cement is added to a clayey soil. For strength gains to occur, the chemical reactions


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require a high pH (>12.4) to be maintained, which is the ICL value. This will varyconsiderably for different soils. After sufficient lime has been added to satisfy the ICLof the soil, additional lime will be required for the formation of cementitiouscompounds. Hence, further testing is still required to establish the optimum stabilisercontent for the required strength. The test for soil acidity and sulphate content iscarried out to indicate any potential problems with the hydration of the cement orpossible chemical attack of the hydrated cement. Typical specifications are given inTable 1 and Table 2.

Table 1 Typical specifications for cement stabilisation of a granular material toform capping in UK (Watson, 1994).

Test specifications

Maximum liquid limit (LL) 45

Maximum Plasticity Index (PI) 20

Maximum organic matter content 2 %

Maximum total sulphate content 1 %

Saturation moisture content (chalk) 20 %

Grading

sieve size

% passing

(by mass)

125 mm

90 mm

10 mm

600 um

63 um

100

85-100

25-100

10-100

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Table 2 Guide to the type of stabilisation likely to be effective (From TRL ORN31, 1993 adapted from NAASRA, 1986)

Soil properties

More than 25% passing the 0.075mm sieve

Less than 25% passing the 0.075mm sieve

Type of

StabilisationPI <10 10< PI <20 PI >20 PI <6, PP <60 PI <10 PI >10

Cement Yes Yes * Yes Yes Yes

Lime * Yes Yes No * Yes

Lime-pozzolan Yes * No Yes Yes *

Notes *indicates that the stabiliser will have marginal effectivenessPI = Plasticity Index, PP = Plasticity Product = PI x per cent passing the 0.075 mm sieve

It should be noted that in the USA, grading limits are not strictly defined for sub-basematerials. Instead, an initial cement value e.g. 7-10 per cent is assigned according tothe material category (according to ASTM M145-82). If these tests indicate thatstabilisation of the material is likely to be successful, then further testing is required todetermine the required moisture and cement contents.


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5.2 Mix design

Before stabilisation is used in road construction, a laboratory testing programme mustbe carried out on the material in order to determine a) the amount of water and b) theamount of stabiliser to be added to achieve the specified strength. Care must be takento avoid excess quantities of stabiliser because this can cause wide shrinkage cracksduring curing which can lead to extensive reflection cracking through overlyingasphalt.

One test method suggested, (Croney 1998) is to first calculate the amount of water tobe added, by determining the optimum moisture content (OMC) that will give themaximum density, and then adding approximately one per cent to this value. Thisaddition is necessary because the OMC of the cement and material will differ from thatof the material alone because the fine grained cement will demand proportionatelymore water than the unbound material.

The amount of stabiliser needed to achieve the specified strength can then bedetermined using cubes made up with various cement contents which are cured for afixed perod; usually 7 or 14 days before testing, usually by crushing. For example, asuggested Unconfined Compressive Strength requirement for a stabilised sub-base is 4MPa at 7 days (Croney, 1998). This value is further qualified as the average strengthof five cubes with a minimum value of 2.5MPa for any individual cube (MCHW 1000,1998).

In general, the strength of the material will steadily increase with a rise in the cementcontent. This strength increase is approximately 500-1000 kPa (UCS strength) for each1 per cent of cement added (Lay 1986/88). Some additional stabiliser may be necessaryto take account of the variability in mixing that will occur on site. For example, anextra 1 per cent of cement is proposed in TRL ORN31 (1993).

It should be noted that in the UCS test the results can be affected by both the size andshape of the sample tested, e.g. a cube or cylinder specimen. The results are oftenconverted to those for a 150mm cube by multiplying the result with a correction factor.Some correction factors are given in Table 3.

Table 3 Conversion Factors for UCS Test (after Sherwood, 1993).

Specimen shape and size Correction factor

(to 150mm cube)

Cube - 150mm

Cube - 100mm

Cylinder - 200 mm x 100 mm diameter


Cylinder - 115.5 mm x 105 mm diameter


1.00

0.96

1.25

1.25

1.04

0.96

The effect of cement content on strength will vary depending on the type of material tobe stabilised. This can be seen in Figure 4 (NAASRA 1986).


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Figure 4 Effect of cement content on strength of various soils stabilised withOrdinary Portland cement and cured for 7 days at 25oC . (NAASRA, 1986 and

Metcalf, 1977)

5.2.1 Post Construction - Strength

To ensure adequate strength during construction, the quality of a cement stabilisedmaterial is usually determined by strength tests on the material after it has been allowedenough time to sufficiently harden (usually 7 days). The strength can be tested in manyways, but some of the most popular tests are the Unconfined Compressive Strength(UCS) test, sometimes known as cube crushing, and the California Bearing Ratio(CBR) test. As mentioned above, many practitioners now prefer to use the UCS test.

For strength and performance testing, NAASRA (1986) reports that: ‘It should benoted that the CBR test is not relevant to cement-bound materials and it cannot be usedfor design purposes. The unconfined compressive strength (UCS) test has beenextensively used to determine the relative response of materials to cement stabilisation.However, the UCS has little direct application to pavement design and it is better to usesome form of tensile strength testing as this will have a bearing on pavement design.Cemented materials are relatively brittle, and fail in tension under relatively low strain.The critical strain usually decreases with increasing modulus. Hence modulus is morerelevant to performance than UCS’.

South Africa has recently introduced tests to determine the tensile strength of stabilisedmaterials, particularly for stabilised sub-bases beneath concrete pavements (Paige-Green, 1998). In the test, a load is applied to the curved surface of a cylindricalspecimen until failure occurs. A flexural test (3 point beam test) can also be carriedout. Minimum limits for the Indirect Tensile Strength (ITS) of cemented materials havebeen set in the latest of the South African series of Technical Recommendations forHighways (COLTO, 1996).


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5.2.2 Durability

As well as ensuring that an adequate strength and stiffness has been achieved by thestabilisation process, it is also necessary to ensure that this strength is maintained overthe design life of the pavement. It should be noted that the UCS and CBR tests do notactually measure the durability of the stabilised material. This can be determined bydurability testing which could take the form of either a soaked CBR test or a wet/drybrushing test (South Africa). In more temperate climates a freezing/thawing test mayalso be appropriate. A recent revision to the South African wet/dry brush test has beenrecommended by Paige-Green (1998), who proposed that the mechanical wet/dry brushtest should be used as it removes some of the operator variability that was apparentlypresent with the previous test. After this testing has been carried out, if any doubtremains about the durability of the material then a further carbonated UCS test could becarried out (de Wet & Taute, 1985).

5.2.3 Construction equipment

Stabilisation may take the form of mix-in-plant or mix-in-situ. Mix-in-plant is mostappropriate where imported granular materials are being used and mix-in-situ is moreappropriate for the stabilisation of native soils.

In-plant mixing may take place on or off site, but an important requirement forstabilised materials such as cement-bound material, CBM, (ie where the water contentis much lower than for concrete) is that the plant must have a positive mixing action tothoroughly mix the constituents – “a simple tumbling action is not sufficient” (Watson,1994).

In-situ mixing plant consists of a rotovator which uses rotating tines to break and mixthe soil. Machines in highway construction are generally much more powerful thanagricultural machinery and hence are capable of stabilising clay and granular materialsup to 350mm thick. Some are also capable of breaking bound material. Agriculturalrotovators may be used for thinner layers up to 150mm in conjunction with suitable soiltypes (Watson, 1994).

In the United States, the process of in-situ stabilisation of soils is used far more than inEurope. A wide range of multiple and single pass plant have been developed which hasled to a cost saving which often cannot be realised in smaller countries.

Lay (1986/88) reports on equipment called ‘stabilisers’ that are capable of cutting intoin-situ material up to depths of 500mm, extracting the material which is then mixedwith stabiliser from a hopper and then replaced. The amount of additive placed is afunction of the mechanical operation and the speed of travel. Lay quotes (Grahame andGoldsborough, 1980) as containing further information.

Stabilisation of deep lifts, up to 400mm thick, are now possible due to the recentdevelopment of;


15

• Large mixing and pulverising machinery, such as the CMI RS500 (Australia)

• Large capacity purpose-built binder spreaders with automated spread control,

• High performance compaction equipment; and

• Slow setting binders.

Useful information about equipment can be found in NAASRA (1986, 1998) Chapter9: Construction.

If a thick layer e.g. 300mm is to be stabilised, then problems in achieving adequatecompaction could require that the material is placed in two lifts. An Australian designmanual (Queensland, 1990) recommends use of a cement slurry to bond the two layerstogether. This publication also reports that the second layer must never be stabilisedusing ‘in-situ’ stabilisation methods even if the first layer was stabilised ‘in-situ’, sincethis method will usually cause damage to the first layer. The manual also recommendsthat the first layer of a two part layer process is never less than 150mm thick, so that itcan support the plant that will lay the second layer.

5.2.4 Pre-construction trials

A field trial should be carried out ahead of the main work in order to determine theactual strength and density that can be achieved using the same plant that will beinvolved in the main contract. Paige-Green (1998) recommends the use of proof rollingon trial sections that incorporate density or strength testing after each roller pass. Thesetrials can identify the optimum number of roller passes that are necessary and alsoprovides an indication of the target density or strength that is required for qualitycontrol testing after rolling.

6 PROBLEMS ASSOCIATED WITH STABILISATION

Previous sections of this review have identified the advantages of using stabilisedpavement layers. However, the use of stabilisers can result in an increase in the cost ofconstruction and will only be cost effective if the increased cost can be traded offagainst the improved performance of the road.

Also before selecting stabilisation techniques, the engineer must be aware of thepotential problems of stabilisation as well as its advantages. This section discussessome of the more common problems in relation to cement and lime, the most usedstabilisers. Most of the problems can be avoided or reduced with careful materialselection and testing.

The problems listed below are in approximate order of occurrence, rather thanseriousness.

6.1 Construction

6.1.1 Quantity of stabiliser

It is important that the correct amount of stabiliser is added to the material. If too muchof the stabiliser is added, it can cause excessive shrinkage cracks. Too little stabiliserwill produce a material with insufficient strength or durability.


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6.1.2 Mixing

The stabiliser and material must be thoroughly and evenly mixed throughout the fulldepth of the layer. For in-situ stabilisation, this is best achieved with a pulvimixer,rotavator or a disc harrow, however an experienced grader operator can also obtaingood results. A common problem is that an incorrect depth of material is mixed, thusaltering the rate of application of stabiliser. Paige-Green (1998) recommends thatspecialist equipment is used for mixing rather than agricultural equipment.

6.1.3 Compaction and limited time

It is essential that the correct degree of compaction is achieved if the material is toreach the required strength. Compaction must be completed within the limited timeperiods set in the specifications, which is often only a few hours for cement.

6.1.4 Rapid setting

A number of problems have been reported where a lime stabiliser has reacted veryquickly with certain materials (typically calcretes and tillites containing amorphoussilica, aluminium and/or high clay contents), causing a rapid set to occur and thuspreventing satisfactory compaction.

6.1.5 Curing time

It is essential to cure the material under correct conditions so that an adequate initialstrength is achieved before trafficking. For curing to occur a moist environment mustbe provided by light water spraying, the application of curing membranes or theplacement of the next layer of material. If the periodic water-spraying method is used,then care must be taken to ensure that the surface does not dry out between sprayingsas carbonation can occur (Netterberg and Paige-Green, 1984), (Netterberg 1987). Thecuring period, usually 7 days before use by construction traffic, can cause delays whichshould be planned for.

6.1.6 Variability

Small changes in the chemical composition of the material to be stabilised, or exposureto harmful compounds after hardening can have large influences on the strength ofcement or lime stabilised materials. These compounds include organic matter,sulphates, sulphides and carbon dioxide. Sulphate attack can cause volume changes(swell) of the material. Work in the USA (Mitchell, 1986) and the UK (Dept. ofTransport, 1976) have placed limits on the total water-soluble sulphate content of thematerial to be stabilised at 0.5 per cent and 1.0 per cent, respectively.

6.1.7 Testing

The amount of quality control testing that is required for stabilised materials is muchgreater than for granular materials and this will add extra time, effort and cost to theconstruction process.

6.2 Durability

6.2.1 Carbonation

Carbon dioxide in the atmosphere can attack the stabilised layer resulting in largestrength reductions over time. The influence of carbonation can be minimised byensuring that the stabiliser content of the material exceeds the initial consumption of


17

lime (ICL) value by at least 1 per cent and that curing is carried out carefully and fully(Paige-Green et al, 1990).

6.2.2 Sulphate and salt damage

If the material to be stabilised shows high contents of soluble salt or sulphates orsulphides, then poor cementation can occur. The South African technical specificationsrecommend that stabilised materials should be at least 500mm away from materialswith a pH of less than 6 (CSRA, 1997) and the water-soluble sulphate (SO3) contentshould be less than 2.0 grams/Litre (CSRA, 1986).

6.2.3 Cracking

Cracking in stabilised layers due to changes in moisture content (drying shrinkage) andthermal stresses cannot be avoided. Cracking can also occur due to excessive trafficloading. The cracks are mostly transverse, and the number will increase with ageresulting in a typical ‘block’ cracking pattern (Chandler, 1985).

If a stabilised granular material is used directly beneath an asphalt surfacing, shrinkagecracks in the stabilised layer can rapidly reflect through the asphalt surfacing.

6.2.4 Break-up

If the stabilised layer is directly under a thin asphalt layer or bituminous seal, crushingof the stabilised layer can occur due to the low abrasion resistance of the material.Research has shown that the contact stress pattern of a tyre are concentrated at the edgeof the tyre (De Beer, 1997).

7 CURRENT STABILISATION PRACTICE AROUND THE WORLD

The following information is a summary of the most relevant points. If required,further information should be obtained from the guides.

7.1 UK Practice

The authority that issues pavement specifications in the UK is the Department of theEnvironment and Transport (DETR). There are two main series of publications:

1. Manual of Contract Documents for Highway Works (MCHW) – particularly usefulis Volume 1: Specification for Highway Works (e.g. Series 1000 (1998): RoadPavements – Concrete and Cement Bound Materials) and

2. Design Manual for Roads and Bridges - particularly useful is Volume 7: PavementDesign and Maintenance (eg HD 25/94: Foundations, and HD26/94: PavementDesign).

7.1.1 Concrete pavements

In the UK, a cement bound sub-base is required under a concrete pavement to minimisethe risk of erosion and weakening of the sub-base caused by water that has penetratedthrough joints or cracks. Cement bound sub-bases also aid compaction of the overlyingconcrete layer. For jointed concrete, an impermeable separation membrane (plasticsheet) is also required over the sub-base to prevent loss of moisture from the concreteto underlying layers and to act as a slip layer. For reinforced concrete, a waterproofmembrane consisting of a sprayed bituminous material is required.


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Sub-base material specifications:

For pavements with a design life up to 12 million standard axles (msa), a cement boundmaterial (CBM2) or wet lean concrete (C10) is specified whereas for designs greaterthan 12 msa, a cement bound material (CBM3) or wet lean concrete (C15) is specified.The range of material categories and strength requirements are given in Table 4.

Cement stabilisation of the subgrade can be used instead of importing a granularcapping material, as long as this stabilised layer has a minimum equivalent CBR of 15per cent. It is also specified that compaction must take place within 2 hours of theaddition of cement.

Table 4 Strengths of UK cemented materials and moduli used for calculations(DETR, 1998 & Croney, 1998)

Material category (in UK)

Minimum 7 day Cube Compressive strength

(MPa) (= N/mm2)*(Ref 1)

Modulus of elasticity for use in structural analysis

(GPa) **(Ref 2)

Average of 5 Individual Dynamic

(Ed)

Static

(Es)

Mean

CBM 1 Soil-cement (granular)

(silty PI ‹ 10)

(clay PI › 10)

4.5 2.5 18

7

1

10

4

0

14

5

0.5

CBM 2 Cement-bound material 7 4.5 23 13 18

CBM 3 Normal lean concrete 10 6.5 27 19 23

CBM 4 Stronger lean concrete 15 10 30 23 27

C7.5 Wet lean concrete 5.5

C10 Wet lean concrete 8

C15 Wet lean concrete 13

* Ref. 1: DETR, 1998: MCWH Series 1000, **Ref. 2: Croney and Croney, 1998.

7.1.2 Bituminous pavements

For flexible construction, weak cemented sub-bases may be used: CBM1, CBM2, orC7.5, see Table 4, but, as reported by (Chaddock, 1997), current specifications requirethese materials to be constructed with the same thickness as unbound granular sub-basematerials.

7.2 TRL ORN31 Practice

This design guide is for bituminous-surfaced roads in tropical and sub-tropicalcountries. The design of concrete pavements is not included. The design catalogues forvarious pavement types allow for stabilisation of the roadbase, sub-base and cappinglayers using cement or lime.

The materials recommended in the guide are roadbase (CB1 and CB2) and sub-base(CS), with unconfined compressive strength (UCS) values as shown in Table 5.


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Table 5 Properties of cement (or lime) stabilised materials

Material code Description Unconfined CompressiveStrength - UCS (MPa)

CB1 Stabilised roadbase 3 – 6

CB2 Stabilised roadbase 1.5 – 3

CS Stabilised sub-base 0.75 – 1.5

Specifications for these materials (CB1, CB2, CS) also include grading envelopes,maximum values for Liquid Limit (LL), Plasticity Index (PI), and Linear Shrinkage(LS) as well as recommended values for the coefficient of uniformity (i.e. the ratio of:sieve size that 60 per cent material passes to sieve size that 10 per cent of materialpasses).

For cement-stabilised materials, the amount of cement to add is determined bylaboratory trials according to BS 1924, using initial values of 2, 4, 6 and 8 per centcement. Cubes or cylinders are then made and cured for set times before a strength testis carried out. The UCS test is usually used to determine the optimum cement content.The procedure for lime stabilised materials is similar, but a longer curing time isallowed. For stabilised sub-base material, the CBR test can be used as an alternative tothe UCS requirement. A minimum value of CBR 70 per cent after seven days moistcuring is recommended.

In the design charts given in ORN31 the traffic loading is given in several categoriesup to 30 million standard axles. It is important to note that where a stabilised roadbaseis shown, the surfacing is a thin surface dressing and not asphaltic concrete (Chart 8).This is mainly to reduce the effects of reflection cracking. In Charts 1 to 6, a stabilisedsub-base is allowed but there is always an overlying granular roadbase, again to reducethe possibility of reflection cracking.

7.3 USA Practice

The main design manuals used in the USA are the AASHTO Design of PavementStructures (AASHTO, 1993) and part II rigid pavement Design (1998).

Initial cement contents are recommended for the various soil types (classified underAASHTO designation M145-82) as follows:

A1-A3 soils (granular materials): 3.5 – 7.0 % (by weight)

A4-A7 soils (silt clay materials): 7.0 – 10.0 % (by weight)

These are expected to give 7-day strengths of at least 2 MPa. The cement contentsgiven above only form a start point from which laboratory testing is required to achievethe required strength.

7.3.1 Designs for concrete pavements

Extensive research on base support for concrete roads has been carried out in the USA(Darter et al, 1995). This showed that the support provided to the concrete slab by theunderlying layer (called the base or sub-base) was found to have a very significanteffect on the performance of the pavement. Amongst the findings it was reported that:


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i “On a soft subgrade (27 kPa/mm) changing from an aggregate base to a treatedbase produces a large increase in the load carrying capacity (in this case 13 to26 million ESALs)”.

i For an untreated granular base, increasing its thickness does not affect trafficlife. This is supported by earlier findings from the AASHO road test (1962)which concluded that “the effect on performance of varying the thickness of thesub-base between 3 and 9 inches was not significant”. For a treated base,however, with a modulus of approximately 6900 MPa, the thickness has a verysignificant effect.

7.3.2 Designs for flexible pavements

The AASHTO (1993) pavement design manual has adopted the use of elastic modulusas the standard materials quantification measure. However instead of using a whollymechanistic approach, the elastic modulus of each layer is correlated with a strengthcoefficient to develop designs using the Structural Number approach. For the sub-base,the manual also offers correlations between elastic modulus and CBR, R-value andTexas triaxial test results. To utilise benefits in terms of utilising a higher structuralnumber coefficient for a stabilised sub-base compared with a granular sub-base theirelastic modulus would be required. It may still not be possible to interpolate astructural number coefficient because of the range of elastic moduli given in themanual.

7.4 Australia

“State Road Authorities have been stabilising heavily trafficked roads to about 400mmin depth for many years and Local Government Authorities are typically stabilising atdepths in the order of 150-200mm” (Pike 1998). The design method for a stabilisedpavement typically greater than 200mm is documented in the comprehensive AustroadsPavement Design Guide (1992).

7.4.1 Austroads Pavement Design Guide.

The Australian guide to pavement design (Austroads, 1992) uses the mechanisticapproach to road design, which it emphasises has been developed for Australianconditions. Pavement materials are characterised by the modulus of elasticity eitherdirectly or through correlation with other tests. Eight test methods are given forcharacterising stabilised pavement materials. These are ranked in order of preferencefrom flexural testing to presumptive values, being the most and least preferred ,respectively.

Stabilised sub-bases, below either a stabilised or crushed stone base material, areutilised extensively in the manual as optional pavement materials. There is a substantialsaving in sub-base thickness when cemented instead of granular materials are used.Should the cemented sub-base layer fail through fatigue, the manual permits acontinuance of the service life of the sub-base as a granular layer when estimating thetotal traffic loading that the pavement will survive. Although a number of exampledesigns are given in the manual, it is necessary to compute the suitability of alternativedesigns and select on their relative merit. To do this, a computer program is requiredto calculate the various stresses and strains in the trial pavement.


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7.5 South Africa

The stabilisation of different pavement layers is widely used in South Africa. Thestandards include the Technical Recommendations for Highways series especiallyTRH13: Cementitious Stabilisers in Road Construction (1986) and TRH 14 Guidelinesfor Road Construction Materials (1985). As shown in Table 6, there are four classes ofstabilised material C1-C4, where C1 is the strongest. The specification limits becomeless strict as the material is used further below the road surface. C1 materials areseldom used because of their tendency to form wide shrinkage cracks (Paige-Green,1998). Material Class C2 (usually cemented crushed stone) is used for a high qualitysub-base. The lower strength materials C3 and C4 (cemented natural gravels) are usedfor lower layers or for bases on low volume roads.

Table 6 Strength requirements for stabilised materials (TRH 14, 1985)Laboratory soaked UCS (MPa) after 7 days

100% mod AASHTO 97 % Mod AASHTOStabilisedMaterial

Classification Minimum Maximum Minimum Maximum

Minimum ITS* (kPa)

C1 6 12 4 8 -C2 3 6 2 4 400C3 1.5 3 1 2 250C4 0.75 1.5 0.5 1 200

Note *ITS = Indirect Tensile Strength (COLTO, 1996)

7.6 The Philippines

The Philippines has a materials and construction manual: Standard Specifications forPublic Works and Highways, Volume 2 (DPWH, 1995). Most of the materials tests arebased on the American AASHTO methods. It should be noted that the manual does notcontain pavement design information. Included in the manual are several specificationsfor the use of stabilisers in the roadbase. These are:

1. Lime stabilised - Road Mix Base course (Item 203)

2. Cement stabilised - Road Mix Base course (Item 204)

3. Cement stabilised - Plant Mix Base course (Item 206)

Included in the specifications is a strength requirement. The appropriate strength test isdependent upon the type of material, which is either:

a) For gravelly soils: CBR test. Material passing the 19mm sieve shall have aminimum soaked CBR of 100 per cent (AASHTO T193), obtained at maximum drydensity (AASHTO T180).

b) For fine textured soils: UCS test. Seven day compressive strength = Minimum of2.1 MPa (ASTM 1633).

In the 1995 specifications the use of stabilised materials for sub-bases is not specifiedfor either flexible or concrete pavements. However, the new Interim Pavement DesignGuide (DPWH, 1998) allows stabilised materials to be used for the base or sub-base inasphalt pavements. In the pavement design catalogue, assumptions are made for thelayer coefficients of the materials, their elastic modulus and equivalent CBR values.For stabilised sub-bases, an elastic modulus of 700,000 psi is assumed, although this


22

seems high. Existing specifications for these materials are used, as given in DPWH,1995. The new Interim Pavement Design Guide does not include the use of stabilisedsub-bases under concrete pavements.

8 PAVEMENT DESIGN FOR HEAVILY TRAFFICKED ROADS

The definition of ‘heavily trafficked roads’ varies between different design standards.For example in South Africa ‘heavily trafficked roads’ are those which carry in excessof 12 million standard axles (Freeme et al, 1987). In this report, as an approximateguide, it has been assumed that ‘heavily trafficked roads’ are those with a design life ofmore than 10 million equivalent standard axles.

For any pavement, it may be desirable to stabilise the base or sub-base in order toprotect the subgrade such that it can withstand the vertical loads imposed by traffic.This is particularly true for heavily trafficked pavements, where high traffic loads orvolumes inevitably mean that stronger and thicker pavement layers are required.

Examination of the major pavement design guides from around the world has shownthat the use of stabilisation is widespread. All of the design guides studied allowedstabilisation of at least one pavement layer and most of the guides reported that the useof stabilisation became more beneficial for higher traffic levels.

Most pavement design manuals for heavily trafficked roads are based on a mechanisticapproach which models the pavement as a multi-layered elastic structure. Thestresses/strains at various points in the structure that result from the applied loads arecompared to establish stress/strain criteria. It is then necessary to calibrate these modelswith observed performance data, i.e. empirical correlations, hence the procedure iscommonly referred to as mechanistic-empirical design.

The use of stabilised sub-bases in several design manuals is compared in Table 7. It canbe seen that pavements with granular sub-bases and stabilised sub-bases can bespecified in almost all of the design manuals listed for traffic levels up to 100 millionESA.


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Table 7 Comparison of Pavements with Stabilised Sub-bases.Country: USA UK

(a)UK(b)

Australia SouthAfrica

Philippines

Design Guide Source: AASHTO TRLORN31

DETRHD26/94

Austroads CSRATRH4,TRH13

DPWHInterimdesignguide

Year 1993 / 98 1993 1994-98 1992 1985 / 86 1998ASPHALTDoes the specificationinclude: Granular base withstabilised sub-base?

Y Y N Y Y Y

Maximum traffic for abovepavement design (millionESA)

50 30 n/a 100 50 30

CONCRETEDesign guide includesconcrete?

Y N Y Y Y Y

Sub-base type allowed:i) Granular materialii) Stabilised material

YY

-NY

NY

NY

YN

Maximum traffic for abovepavement design(million ESA)

>500 - 400 300 50 30

As previously discussed, the stabilisation of the sub-base layer beneath a concretepavement can minimise problems caused by poor materials, difficult constructionconditions and, in some cases, low standards of construction quality control whereinadequate slab support can lead to premature cracking. The Philippines design manualdoes not specify the use of stabilised sub-bases beneath concrete pavements (Table 7).Although it may not be possible to justify them at low levels of traffic, further studycould determine whether stabilised sub-bases would be economically beneficial athigher levels of trafficking.

Apart from the pavement design manuals and specifications described earlier, there arerelatively few published reports concerning the use of stabilised materials for heavilytrafficked pavements. One of the few reports on this subject (Freeme et al, 1987) givesdetails of accelerated loading trials in South Africa using the Heavy Vehicle Simulator(HVS) on pavements with stabilised bases and sub-bases. One of the major results ofthis study was the confirmation of the in-situ moduli (i.e. layer stiffnesses) forcemented materials of different strengths and in different states of deterioration. It wasfound that weakly cemented materials, having UCS strengths of less than 3 MPa, canbreak down quite rapidly into small blocks under trafficking. The report includes tablesof the moduli of strongly cemented and weakly cemented materials in their new (i.e.uncracked) state and then at varying stages of their life. These values may be useful forgeneral mechanistic design of road pavements with stabilised layers. It was alsoreported that many of the weakly cemented materials cracked and some of themappeared to break down into a near-granular state. The report estimates that theuncracked state for weakly cemented materials lasts for only approximately 10 per centof the life of the pavement.

A new form of erosion was also reported whereby the top of the stabilised base waseroded by mechanical interaction with the asphalt surfacing. This loose material was


24

being broken down into fines and pumped out from cracks in the asphalt. It must benoted that most of the pavements in this study had a cemented base and cemented sub-base. It is likely that a stabilised sub-base with a strong unbound granular base wouldnot suffer from this type of deterioration and that the break-down of a stabilisedmaterial could be avoided by using a higher cement content.

It was also reported that the thickness of the cemented layers must be sufficient to copewith overloaded axles as well as cumulative repetitions of legal axle loads.

In the Philippines the amount of traffic will continue to increase, as will the demandsfor high strength pavements that are able to carry even greater traffic. It can be arguedthat no particular form of pavement construction is necessarily the best. The choice inany situation will depend on factors such as the funding that is available for the project,the local cost of the different forms of construction, the likely future maintenancelevels, the volume and composition of traffic, subgrade conditions, climate, and thedesign life of the road pavement.

Before a new road is built, a detailed cost benefit analysis should be carried out todetermine the most appropriate form of construction. The use of a stabilised materialcan help with whole-life cost reduction, but care should be taken to ensure that thematerial, its construction and the environment are suitable. For the sub-base layer, thedecision whether to use unbound granular materials or cement-bound materials willdepend principally on the availability of good quality aggregates. If they are readilyavailable, their use will usually be cheaper than the alternative of stabilising a lowerquality material.

9 CONCLUSIONS

Stabilised sub-bases are now used by many road authorities for the design of heavilytrafficked roads. The primary benefits include the material’s increased load spreadingability, which is highly relevant to the Philippines with its increasing traffic levels, andthe material’s increased ability to resist water penetration and hence to be more durablein areas with less effective drainage. The use of stabilised sub-bases in the Philippinesis now included in the recent publication of the Interim Pavement Design Guide(DPWH, 1998) which allows the use of stabilised sub-bases under asphalt surfacedroads for design traffic levels up to 30 million ESA.

The stabilisation of pavement materials is a fairly straightforward operation and withgood construction techniques the properties of poor materials can often be significantlyimproved. It is essential that the amount of stabiliser to be used with a material is firstestablished in the laboratory and that there is an appropriate level of constructionsupervision and quality control to ensure that similar strengths are achieved in the road.

Cement stabilised materials, in particular, offer the possibility of both increasingpavement performance whilst utilising materials that may not generally meet acceptedsub-base specifications. However, increasing the cement content to achieve a higherstrength or to improve the material will also increase the possibility of reflectioncracking and hence the pavement designer must seek a balance between these twoconflicting factors.

The performance of both rigid and flexible road pavements in the Philippines wouldalmost certainly be improved by the use of stabilised sub-bases. What is not presently


25

established, however, is whether this improvement in performance would be costeffective after taking into account all factors, including some often missed costs such asthe additional quality control measures that are required.

It is therefore recommended that laboratory and pilot scale trials should be carried outto:

1. establish how indigenous marginal materials can be effectively stabilised to producegood quality sub-base materials;

2. establish the improved road performance that can be achieved by using stabilisedmaterials rather that granular sub-base materials; and

3. quantify the cost effectiveness of this type of road construction in the Philippines.

10 RECOMMENDATIONS FOR PILOT TRIALS IN THE PHILIPPINES.

In the review few guidelines or specifications take advantage of the potential increasedstrength and stiffness of stabilised sub-bases. This section summarises an approach forcarrying out pilot scale pavement design trials, using stabilised sub-bases, in thePhilippines.

1. Locate trial sites and identify materials for the various pavement layers and, inparticular, those for possible use as the sub-base. The roadbase layer should be ahigh quality crushed stone and the surfacing should be asphaltic concrete.

2. Carry out laboratory tests of sub-base materials to be stabilised to establish whetherthey are appropriate for stabilisation and establish the relationship between strengthand cement content.

3. Define the strength requirement and select the cement content accordingly. Note:This strength can be determined using a computer program such as ELMOD,GENSTRESS, BISAR or ELSYM to get stresses and strains below the acceptedasphalt and subgrade criteria.

4. Design the pavement trials. The design of the trials will depend on whatopportunities exist to incorporate pilot studies into ongoing construction. There areprimarily three options.

a) Maintaining pavement thickness. A trial to quantify the benefit of stabilised sub-bases over that of granular materials can be designed by having similar pavementthickness to the control section and varying only the strength of the stabilisedmaterials. In this case the construction costs may be higher but the benefits wouldaccrue from an expected improvement in pavement performance.

b) Varying pavement thickness. A trial to establish suitable pavement thicknesses canbe designed by comparing the performance of thinner pavements, incorporatingstabilised sub-bases, to that of normal practice. The design of these experimentalpavements will be based on analytical methods. In this case, construction costs maybe comparable and performance is likely to be enhanced.

c) Improving marginal materials. A trial to quantify the benefit of stabilised sub-basescan be designed by comparing the performance of marginal materials, modifiedwith cement/lime, to that of a more costly imported sub-base material that meets


26

normally accepted material specifications. In this case construction costs should bereduced and performance may well be enhanced.

5. Carry out FWD testing after construction to determine in-situ moduli. Other tests,including DCP tests, coring and testing of cored samples may also be required. Alltests should be repeated periodically to establish the change in strength with time.

6. Compare results and performance with control section. From these results it shouldbe possible to determine the theoretical future load carrying capacity of thepavement by comparing the stresses and strains or Structural Number of theexperimental pavement to existing criteria (LR 1132 and AASHTO). Theseestimates would then be compared to actual performance measured during themonitoring period. It should be noted that this analysis can only be done on a sitespecific basis where traffic volumes and load are carefully monitored.

11 ACKNOWLEDGEMENT

The work described in this report forms part of the Knowledge and Research (KAR)programme of TRL (Director Mr S W Colwill), and part of the Research andDevelopment Division programme of Bureau of Research and Standards (Director RaulC. Asis) of DPWH, Philippines. Any views expressed are not necessarily those ofDFID or DPWH.


27

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