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Dr Stuart Matthews, Chair fib Commission 3

Chief Engineer Construction, BRE

Building Research Establishment, Garston, Watford, UK

Safety and working life of existing structures

Overview• The challenges for existing structures and infrastructure

• Introduction to structural assessment –Why and when to do it

• Deterioration and damage

• Introduce some of the available guidance –What and where and how to do it

• Sustainability evaluation of existing concrete structures -Structural assessment and interventions

• fib MC2010: Conservation of structures

• Future look - some forthcoming international developments on the assessment of existing structures: fib MC2020

The challenges• Everyday life in Europe and the rest of the world depends on

structural materials, with concrete being the one most widely used:– Buildings, Bridges & Other Structures

• Sustainability and carbon considerations increasingly encourage life extension and adaptation of existing assets for new uses

• Through-life care is critical to sustain effective & beneficial use• Sometimes concrete can suffer from deterioration

• A whole-life perspective required, not just focus on first-cost• Proper service life planning and through-life management is critical • Interventions for preventative, remedial and upgrade works• Structural & safety assessment important management activities• Good design & construction practices are essential for new assets

to maximise their life and contribution to a sustainable society

Courtesy: Petr Hajek, Czech Technical University, Prague

Adaptation of existing assets

Courtesy: Petr Hajek, Czech Technical University, Prague

Adaptation of existing assets

Infrastructure – Some headline issues

• Over 50% (and increasing) of world’s population now live in cities• Effective infrastructure critical to economic competitiveness

• Mature economies have established but aged infrastructure and face huge costs for deferred maintenance / upgrade works

• Strong drivers for life extension and adaptation of existing assets, as well as merging existing assets with new increases in capacity

• Changing drivers: lower carbon usage, climate, rising sea level etc

• Costs of infrastructure ‘hidden’ – users fail to understand true cost• General shift to user charges seems inevitable around world

• Construction and repair costs increased by over 50% since 2000• Changing and demanding circumstances

[Ernst & Young Report Infrastructure 2007: Global Perspective]

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Structural & safety assessment -Why and when it is done

Common (but diverse) reasons include:• Purchase, insurance, or legal purposes

• Defects in design and / or construction

• Deterioration with time or from being in service

• Accidental damage due to a fire or other cause

• Assuring safety and / or serviceability for future use

• Structural alteration / change of use

• Change of environmental conditions affecting durability

Similarly diversity exists in the wide range of forms of construction, age and the technical details employed in existing concrete structures

Extension of life :Benefits of effective assessment

These may include:

• Better targeted remedial / preventative works

• Confidence in durability and expected remaining life• Seeks to avoid expenditure on actions giving marginal

contribution to increased safety and durability

• Supports investment planning and decisions required for improving habitability, energy efficiency etc

• Facilitates pro-active management of remaining life

• Contributes to enhancing value for money

• Contributes to meeting sustainability goals

The Pont du Gard Roman aqueduct, south France

Krk Bridge, Croatia

Structures built using ultra high performance fibre reinforced concrete (UHPFRC)

Toll plaza Millau Viaduct, France

Materials / forms of construction

Inst. Structural Engineers: Appraisal of existing structures: 3rd edition

1700 1800 1900 1950 2000

Increasing diversity of concrete materials & systems

Increasing rate of change

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0

50

100

150

200

250

300

350

0.1 0.2 0.3 0.4 0.5 0.6 0.7

HPC: c.1990s

RPC, UHPC: c.2000s

NSC: 1970-1980s

Com

pres

sive

str

engt

h [M

Pa]

Water-binder ratio [-]

MC 1990

MC 2010NSC: c.1950s

Development of compressive strength of concrete with time since about 1950.

[Expressed in terms of f cm, the mean value of concrete cylinder compressive strength]: Courtesy: Frank Dehn

Some structures deteriorate or are damaged in-service and need to be assessed for current safety and future performance / durability

Threats to current and future satisfactory performance

Corrosion of reinforcements - extchloridesCorrosion of reinforcements - intchloridesCorrosion of reinforcements -carbonationFreezing/thawing

Chemical attack

ASR

Plastic shrinkage

Plastic settlement

Drying shrinkage

thermal cracking

Creep/thermal movements

Abrasion/erosion

Cavitation damage

Deterioration processes affecting concrete: Jones et al [1997]

Damage due to corrosion of reinforcements

Carbonation induced reinforcement corrosion

KRK BRIDGE:Cracking and corrosion experienced

Access for intervention difficult & expensive.Management strategy needs to account for this.Ideally avoid need for frequent works on bridge.

Combined attack:Freeze-thaw damage & then reinforcement corrosion

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Accidental damage: Fire

Accidental damage:

Occurrence is not a service life / durability issue

But repair afterwards may have durability implications !!

Through-life structure management criticalCollapse of a bridge near Madrid in 2015

Accumulated layers of pavement - 4.8kN/m 2

Courtesy: Javier LEONE.T.S. Ingenieros de Caminos, Canales y PuertosUniversidad Politécnica de Madrid

Need to understand micro-climate conditions -Example of a bridge subject to de-icing salts

Hazard scenarios for prestressing steel in a typical box girder bridge

fib Bulletin 33

Guidance on the assessment of existing structures

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CONTECVET : A validated users manual for assessing the residual service life of concrete structures

JCSS: Probabilistic assessment of existing structures

Swiss Codes SIA 269-1 to 8:Assessment of existing structures

Guidance on the assessment of existing structures

And many more guides ……………

Institution of Structural Engineers: Appraisal of existing structures:1st edition 1980; 3 rd edition 2010

ISO 13822: Bases for design of structures-Assessment of existing structures

Assessment of large panel system (LPS) dwelling blocks for accidental loading: Handbook

BRE Digest 366: Structural appraisal of existing buildings, including for a material change of use

fib Bulletins - 1

Bulletin 34: fib Model Code for Service Life Design

Bulletin 22: Monitoring and safety evaluation of existing concrete structures

Bulletin 10: Bond of reinforcement in concrete

Bulletin 26: Influence of material and processing on stress corrosion cracking of prestressing steel - case studies

Bulletin 17: Management and strengthening of concrete structures

Guidance relevant to the assessment of existing concrete structures

Bulletin 24: Seismic assessment and retrofit of reinforced concrete buildings

fib Bulletins – 2

Guidance relevant to the assessment of existing concrete structures

Bulletin 62: Through-life care and management of concrete structures - Assessment, protection, repair and strengthening (part of fib Text Book)

Bulletin 44: Concrete structure management: Guide to ownership and good practice

Bulletin 59: Condition control and assessment of reinforced concrete structures exposed to corrosive environments (carbonation / chlorides) [Probabilistic approach]

Bulletin xx: Safety and performance concepts:[fib Commission 2]

fib Model Code 2010: [Chapter 9: Conservation]

Steps in the structural appraisal process

1. Establish the brief for the appraisal

2. Make initial inspection and preliminary structural appraisal of the building using information which is immediately available

3. Decide on urgent action or intervention needed on safety grounds

4. Gather & review existing documents and information (desk study).

5. Carry out a detailed survey and investigation of the structure.

6. Undertake structural appraisal of the structure

7. Consider any structural work needed

8. Prepare a report on the outcomes of the appraisal

Potentially – undertake any required interventions

Main steps in the structural appraisal processStep 1

Step 8 Prepare report on the outcomes of the appraisal

Appraisal Leve l 1: Pre liminary structural appraisal Appraisal Leve l 2: Initial detailed structural appraisal Appraisal Leve l 3: Further de tailed structural appraisal Appraisal Leve l 4: In-depth structural appraisal, including recalculation if appropriate

Step 7 Define any structural or remedial work necessary,together with any future maintenance activities required

Step 4 Gather and review available documentation / other information

Step 5 Carry out detailed investigation of building - possibly in several stages

Repeat for additional

information required

Step 6 Undertake structural appraisal for both normal and accidental loads and actions - possibly in several stages

Appraisal Leve l 0: Appraisal undertaken without calculations - ‘by inspection’

Agree brief for appraisal with clientMake initial review of any information available

Step 2 Undertake reconnaissance inspection of building Make preliminary structural appraisal using available information

Implement urgent actionStep 3 Define any urgent action required to make building 'safe '

fib Model Code 2010

Chapter 9: Conservation

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fib Model Code 2010 - Overview

PART CHAPTER

Part 1

1. Scope

2. Terminology

3. Basic principles

Part II: Design Input Data

4. Design principles

5. Materials

6. Interface characteristics

Part III: Design 7. Design

Part IV: Construction 8. Construction

Part V: Conservation9. Conservation

10. Dismantlement

fib Model Code 2010: Chapter 9: Conservation of Concrete Structures

9.1 General

9.2 Conservation strategies and tactics

9.3 Conservation management

9.4 Condition survey

9.5 Condition assessment

9.6 Condition evaluation and decision-making

9.7 Interventions

9.8 Recording

Conservation process -simplified overview

Record ing

Determination of tactic and regime for condition control (at time of des ign of new structure)

Record ing

Prov is ional spec ifica tion of conservation s tra tegy (for ex is ting structure or revised performance requi rements)

Rev iew conservation strategy and conservation tactic

Fina lise inspection regime for condition moni toring

Star t / Continue through l ife condition monitor ing

Condition survey

Condi tion assessment

Execution of chosen in tervention

Condition eva luation and dec is ion-making

inc l. spec ifica tion of intervention

Condi tion survey (a fter construction)

Condition assessment(after construction)

Provis iona l determination of tac tic and regime for condition control (for ex is ting s tructure or rev ised requirements)

Condition survey (for re -des ign)

Condition assessment(for re -des ign)

i = 1 → n

Speci fication of conservation s tra tegy (a t t ime o f des ign o f new st ructu re )

START : EXISTING

START : NEW

Recording

Determination of tactic and regime for condition control (at time of design of new structure)

Recording

Provisional specification of conservation strategy (for existing structure or revised performance requirements)

Review conservation strategy and conservation tactic

Finalise inspection regime for condition monitoring

Start / Continue through life condition monitoring

Condition survey (after construction)

Condition assessment(after construction)

Provisional determination of tactic and regime for condition control (for existing structure or revised requirements)

Condition survey (for re-design)

Condition assessment(for re-design)

Specification of conservation strategy (at time of design of new structure)

START : EXISTING

START : NEW

Recording

Review conservation strategy and conservation tactic

Finalise inspection regime for condition monitoring

Start / Continue through life condition monitoring

Condition survey

Condition assessment

Execution of chosen intervention

Condition evaluation and decision-making

incl. specification of intervention

i = 1 → n

Conservation strategies• Strategy A: Structures which are to be managed by

planned condition control activities.– Structures where deterioration would be technically unacceptable or

must not be seen.– Monumental, important or sensitive buildings & structures.

• Strategy B: Structures or parts thereof which are managed by reactive activities.– Structures where remedial measures can be taken after deterioration

becomes visible.– Buildings and other common structures.

• Strategy C: Structures or parts thereof for which condition control is not practical.– Structures where it would be difficult economically and / or

technically for preventative or remedial measures to be taken, such as foundations.

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Condition control levels / inspection regimes

The 4 levels of condition control / inspection regime are:

CCL3 is a proactive approach utilising a regular inspection, testing and monitoring regime.

CCL2 is a reactive approach which utilises a planned inspection regime but this is only visual in nature.

CCL1 is a reactive approach utilising an ad-hoc inspection and testing / investigation regime.

CCL0 involves no physical inspection, testing or monitoring.

It may be possible to gather indirect indications of performance through ancillary behaviours. For example, the performance of foundations might be inferred from structural movements.

Condition assessment

Involves consideration of:

• The deterioration mechanisms, present deterioration level and deterioration rate of materials and / or structural performance shall be determined using:

• Appropriate models to make predictions on the basis of the information obtained from:– inspection– testing and monitoring activities– the design and construction records– information upon previous interventions, and – the environmental conditions

Condition evaluation and decision-making

• Evaluation of the extent of material / structural performance deterioration shall consider:– Records in the Design File and the As-Built Documentation – Results of inspection and monitoring outlined above.– Assessment of the nature, severity, significance and rate of

deterioration.

• Depending on how the evaluation compares with the :– minimum required performance level– with prognoses for the future management of the structure

• it may be necessary to make preventative (proactive ) or remedial (reactive) interventions

Interventions

• Planned interventions: i.e. maintenance, preventive and remedial works which are planned at the time of designing new structures or repairs to existing structures .

• Unplanned interventions: i.e. remedial or strengthening interventions not planned at the time of design, but become necessary due to damage or revised requirements

Sustainability evaluation of existing concrete structures:

Structural assessment for through-life management

Sustainability evaluation of existing concrete structures

We need an approach aimed at solving the dilemma

that currently exists between:

• infrastructure, as a very long-term asset, and

• the much shorter-term approach often adopted for

design, management and maintenance planning

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Environment

Economic SocialSD

Sustainable decision-making

There should be no hierarchy between factors

Client needsEnvironmental factors

Economic factors

Functional factors

Socio-culturalfactors

Components of sustainable construction / Lifetime engineering

Economic considerations

Target reliability levels for existing structures are modified relative to the values

assumed for new structures, for

Social considerations

Sustainability considerations

Excessive conservatism / unnecessarily high reliability level

has negative economic, social and environmental impacts

Seek a satisfactory balance

Revised reliability levels for existing structures: Vrouwenvelder & Scholten (2010)

Table 9-11-4 from fib Bulletin 62

Consequence class

Minimum reference period for an existing

building (years)

β - New β - Repair β - Existing

WN [2] WD [3] WN [2] WD [3] WN [2] WD [3]

CC0 [1] 1 3.3 2.3 2.8 1.8 1.8 0.8

CC1 – Low [5] 15 3.3 2.3 2.8 1.8 1.8 [4] 1.1 [4]

CC2 – Medium [5] 15 3.8 2.8 3.3 2.5 [4] 2.5 [4] 2.5 [4]

CC3 – High [5] 15 4.3 3.3 3.8 3.3 [4] 2.5 [4] 3.3 [4]

[1] Class 0 as Class 1, but applies only to situations where no danger to human life is involved.[2] WN: Forces due to wind are not dominant.[3] WD: Forces due to wind are dominant.[4] In this case the stated reliability index level (β -value) represents the minimum acceptable level for human safety for the

circumstances and consequence class being considered.[5] Classified on the basis of the provisions in Annex C of BS EN 1990.

Structural performance to be analysed by means of

Linear elastic

analysis

Linear elastic analysis with

limited redistribution

Plastic analysis

Non-linear analysis

Selection of analysis type considering:• Structure type

• Failure consequence class• Validity of models used for new structures

• Availability of new design models

Safety verification formats

Partial factors format

Full probabilistic

Linear elastic, linear elastic with limited

redistribution, plastic

Global resistance

factors

Non-linear analysis

Perhaps considering:• Structural / material deterioration: Now and in future

• Effect of structure management actions• Effect of interventions

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Sustainability evaluation of existing concrete structures:

Interventions to extend working life

Concrete patch repairs

Concrete sprayedwith silane

Application of silane or concrete repairs?

Concrete repair scenarioConcrete repairs undertaken twice in 120 yearsAfter 50 years, then on 70 yearsTargeted repair: 30m2 in minor scheme

75m2 in major schemeWhat's modelled?

Break out of defective concreteDisposal of waste materialNew material demandsPlacement of new material – sprayed concrete

Foreground system for Foreground system for bridge service lifebridge service life

Water

Background Background system for bridge system for bridge service lifeservice life

EmissionsWastes

EmissionsWastes

End of design life

Service

Construction

120 year life cycle of Cascade

Bridge

GGBS

Sand

10mm aggregate

Cement transport

Sand transport

Agg. 10mm transport

GGBS transport

Cement

Mechanical breakout of defective concrete

DieselTransport waste spoil

and disposal in landfill

Concrete mixing

Concrete transport

Bridge maintained

through 120-year service

life

Placement of spray concrete

Diesel

Structure prepped for

spray concrete

Gunite pump

Concrete mix pre-batched in factory

Scenario for one concrete repair scheme

50 years 70

years

Bridge maintenance strategy through 120 years

Two concrete repair schemes. Each consists of the mechanical breakout of deteriorated concrete and placement of sprayed concrete. This includes 30m2 of works at 50 years and 75m2 of works at 70 years.

Silane treatment scenarioApplication at 15 year frequency for 120 years8 coatings applied over life cycle Targeted treatment – 70m2 per applicationA defined silane concentration

What’s modelled?Silane demandThe application processChemical degradation

Foreground system for Foreground system for bridge service lifebridge service life

Background Background system for bridge system for bridge service lifeservice life

EmissionsWastes

EmissionsWastes

End of design life

Service

Construction

120 year life cycle of Cascade

Bridge

Electricity

Electricity

Bridge maintained

through 120-year service

life

Scenario for one silane application scheme

15 years 30

years 45 years 60

years 75 years 90

years

Bridge maintenance strategy through 120 years

105 years 120

years Eight separate silane treatment schemes each

providing coating to 67m 2 of the

structure

Silane

Application of silane with

spray applicator

Atmospheric emissions occurring during

application

Transport of spray

applicator equipment to

bridge site

Degradation of silane through

service period

Site mixing of silane prior to

application

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020406080

100120140160

180

Co ncreteRep .@

50Years

Co ncreteR

ep .@70Y

ears

Comb ined C

o ncreteRep .

S ilane app l. o

ver 1

20 years

R Fossil fuels

EQ Acid/Eutroph

EQ Ecotox.

HH Ozone layer

HH Clim. change

HH Resp. inorg.

HH Resp. org.

HH Carcinogen.

In chosen scenario the environmental impact of use of silane to protect structure from deterioration is greater than a remedial maintenance approach utilising concrete repair works to achieve a structure design life of 120 years. However, different outcomes for 10 or 20 year repeat application of silane .

NB. This case study is only illustrative - the findings are specific to the scenario modelled. Highlights need for good realistic models of processes & outcomes.

The environmental impact of two possible options for bridge maintenance

Sustainability evaluation of existing concrete structures

• Need assessment methodology comprising a set of relevant sustainable indicators (following CEN350, ISO...)

• Need relevant data – based on actual state of the structure & estimated remaining working life

• Need strategy for through-life management of structure

• Evaluate options for remaining working life of structure

• Prediction of future lifetime of concrete structure for potential intervention / through-life care options

• Evaluate effectivity of intervention & care options versus alternative of demolition & construction of a new structure

fib Commission 7: Sustainability looking at these matters

Looking to the future:Ongoing and proposed

developments in the assessment of existing structures

Sponsored by:

CEN Eurocode project:‘New European technical rules for the assessment and retrofitting of existing structures’

Sponsored by:

Eurocodes and new technical rules for the assessment & retrofitting of existing structures

New

tech

nica

l rul

es:

Ann

exes

to e

xist

ing

code

doc

umen

ts

Summary: Current CEN situation and on-going developments

• There is a considerable body of information available to help those making structural assessments

• However, many important issues are not yet adequately addressed

• New CEN activities will increase the body of guidance and put it into formats compatible with the structural Eurocodes

• Anticipate annexes to Eurocodes providing standards for assessment of existing structures

• This will only provide high level guidance based upon mature consensus knowledge (rather than state-of-the-art knowledge).

• Length of concrete part expected to be limited (10 - 20 pages?)

• Also there are relevant ISO standards such as ISO 16311: Maintenance and Repair of Concrete Structures

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fib MC2020Advancing the fib Model Code for

Concrete Structures: A single merged structural code for both new

and existing concrete structures

fib MC2020: The journey so far - 1

• On 16 May 2015 fib Technical Council endorsed concept of preparing fib Model Code 2020 with the vision that it would be a single merged code for both new and existing concrete structures

• A workshop was held in The Hague, the Netherlands on 30 June 2015 to develop concepts

• This workshop assisted in defining the requirements and the way of delivery of this ambitious project

fib MC2020: The journey so far - 2

• A Core Group was established in autumn 2015 to develop a Roadmap for advancing the fib Model Code for Concrete Structures towards MC2020

• Core Group members are:

• Joost Walraven • Frank Dehn • Aurelio Muttoni • Agnieszka Bigaj-van Vliet• Giuseppe Mancini

• Stuart Matthews• Gerrie Dieteren• Tamon Ueda• Hugo Corres Peiretti• Harald Müller (guest)

fib Model Code 2020 –What might it look like?

Part I: General Development /Change

1. Scope Shared

2. Terminology Shared - Extended

3. Basic principles Shared - Extended

Part II: Design & Assessment Input Data

4. Principles and processes• Design principles• Principles of assessment / service life aspects• Principles of through-life management & interventions

Greatly extended• MC2010• New• Extended

5. Materials• Contemporary materials• Other / previous materials / forms of construction• Forms of damage and deterioration

Greatly extended• MC2010• New• New

6. Interface characteristics• Contemporary materials• Other / previous materials / forms of construction• Forms of damage and deterioration

Greatly extended• MC2010• New• New

fib Model Code 2020 –What might it look like?

Part III: Design & Assessment Development /Change

7. Design• Design verification• Assessment verification / re-design procedure

Verification addresses:• Structural safety, serviceability, durability, robustness,

sustainability

Greatly extended• MC2010• New

Part IV: Construction

8. Construction• New works• Works on existing structures

Extended• MC2010• New

Part V: Conservation

9. Conservation• Conservation by through-life management• Conservation by interventions – Physical works

Extended• Changed / Extended• Changed / Extended

10. Dismantlement • MC2010 - Extended

Aspirations for fib Model Code 2020 - 1

• Single merged structural code dealing with both new and existing concrete structures

• Operational model code that is oriented towards practical needs

• Includes worldwide knowledge with respect to materials and structural behaviour

• Recognizes the needs of engineering communities in different regions of the world

• Integrated life cycle perspective

• Holistic treatment of structural safety, serviceability, durability and sustainability

• Fundamental principles and a safety philosophy based on reliability concepts

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Aspirations for fib Model Code 2020 - 2

• Use of the performance based concept to remove specific constraints for novel types of concrete & reinforcing materials

• Fully integrated provisions based on generalized models and implementation of the level of approximation approach applicable for both the design of new structures and all the activities associated with the assessment etc

• Allows full advantage to be taken of information that can be acquired by testing and monitoring of existing structures

• Addresses robustness and redundancy for new and existing structures

Aspirations for fib Model Code 2020 - 3

• Considers material degradation and / or insufficient or deficient detailing of the provided material and behaviour models,

• Considers needs for model improvement and treatment of uncertainties in models and model parameters for existing structures and (phased) construction of interventions

• Gives attention to through-life management aspects

• Attention to new types of concrete / repair materials etc

• Addresses end-of-service-life issues such as demolition and disposal, recycling etc

Some technical aspirations - 1Levels of approximation / sophistication in analytics – say 4

• IV System assessment of critical existing structures / design of special cases• III In depth elemental evaluation existing structures / design of special cases• II Typical elemental design / assessment• I Preliminary design, non governing limit state (design and assessment)

Verification levels in reliability evaluation: Levels I to IV• I Partial factor methods II Updating of info / reduction in uncertainty• III Full probabilistic approach IV Full probabilistic with cost optimisation

Very general safety format

simplest safety format

Some technical aspirations - 2

• Better models for deterioration processes – especially propagation stage

• Generalised models, with coherent applications for new and existing structures

• Structural models for deterioration / damage effects• Lower safety levels for existing structures for economic factors

• Same safety levels for existing structures for human safety• Improved models for service life prediction / estimation

• Models for ‘repaired’ structures / intervention behaviour• ……………….

• The list expands, but need to establish limits

fib MC2020

The starting point isfib Model Code 2010

but could MC2020 be delivered in a different / updateable format?

Application examples: Concrete structure conservation

Introduction:Through-life management of an asset

Structural Concrete journal: 2013 - Issue 4: Conservation of concrete structures according to MC2010, Matthews et al

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Application example 1Concrete structure conservation

Application example 1

• Illustrate the overall nature of the interactions between decisions made during the service life design and construction process and the activities undertaken later in the life of the asset as part of its through-life management (asset life Stages 1 to 4)

Four types of asset for illustrative purposes:• A new building with a specified service life period of 50 years• An existing building, with a requirement for a residual service life

period of 25 years• A new bridge with a specified service life period of 100 years • A new special structure with an exceptionally long service life

period of 200 years

Application example 2Concrete structure conservation

Simplified illustration of the process for making an assessment and intervention upon a deteriorated existing concrete bridge to extend its useful service life

BEFORE INTERVENTION

Remedial / upgrade works to bridge

10.9m 10.9m27.5m 27.5m

Longitudinal elevation of bridge

Deck reinforced over intermediate supports by fibre reinforced strengthening

Carriageway widened to 9m by addition of cantilever footpaths on side of deck

Cracks in deck resin injected, membrane applied to deck slab, paint coating to all other concrete

AFTER INTERVENTION

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Dr Stuart Matthews, Chair fib Commission 3

Chief Engineer Construction, BRE

Building Research Establishment, Garston, Watford, UK

Stuart.matthews@bre.co.uk

Safety and working life of existing structures