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Fundamentals of Durable Reinforced Concrete

There have been signicant advances made in concrete technology in recentyears. However, the results of this recent research are still either widely scattered in the journal literature, or mentioned only briey in the standardtextbooks.

The aim of this series is to examine, to some depth, each topic of interest toprovide a uniform, high quality coverage which will, over a period of years,build up to form a library of reference books covering all the major topics inmodern concrete technology. Primarily the books are of single or dual author-ship although, for certain books, edited volumes consisting of a collection ofchapters has been considered more appropriate.

The material is presented at a level suitable for senior and postgraduateengineers working in concrete technology.

Also available in the series

1 Fibre Reinforced Cementitious Composites A. Bentur and S. Mindess

2 Concrete in the Marine Environment P. K. Mehta

3 Concrete in Hot Environments I. Soroka

4 Durability of Concrete in Cold Climates M. Pigeon and M. Pleau

5 High Performance Concrete P-C. Aitcin

6 Steel Corrosion in Concrete A. Bentur, S. Diamond and N. Berke

7 Optimization Methods for Material Design of Cement-based CompositesEdited by A. M. Brandt

8 Special Inorganic Cements I. Odler

9 Concrete Mixture Proportioning F. de Larrard

10 Sulfate Attack on Concrete J. Skalny, J. Marchand and I. Odler

11 Determination of Pore Structure Parameters K. Aligizaki

12 Fundamentals of Durable Reinforced Concrete M. G. Richardson

Modern Concrete TechnologySeries editorsArnon Bentur Sydney MindessNational Building Research Institute Ofce of the PresidentTechnion Israel Institute of Technology University of British ColombiaTechnion City 6328 Memorial RoadHaifa 32 000 Vancouver, B.C.Israel Canada V6T 1Z2

Mark G. Richardson

London and New York

Fundamentals of DurableReinforced Concrete

First published 2002 by Spon Press11 New Fetter Lane, London EC4P 4EE

Simultaneously published in the USA and Canada29 West 35th Street, New York, NY 10001

Spon Press is an imprint of the Taylor & Francis Group

2002 Mark G. Richardson

All rights reserved. No part of this book may be reprinted orreproduced or utilised in any form or by any electronic, mechanical, orother means, now known or hereafter invented, includingphotocopying and recording, or in any information storage or retrievalsystem, without permission in writing from the publishers.

The publisher makes no representation, express or implied, withregard to the accuracy of the information contained in this book andcannot accept any legal responsibility or liability for any errors oromissions that may be made.

British Library Cataloguing in Publication DataA catalogue record for this book is available from the British Library

Library of Congress Cataloging in Publication DataA catalog record for this book has been requested

ISBN 0-419-23780-1

This edition published in the Taylor & Francis e-Library, 2004.

ISBN 0-203-22319-5 Master e-book ISBN

ISBN 0-203-27744-9 (Adobe eReader Format)(Print Edition)

Dedicated to the memory of Tom McCormack (19492001)

The publisher makes no representation, expressed or implied, with regard tothe accuracy of the information contained in this book and cannot accept anylegal responsibility or liability for any errors or omissions that may be made.The reader should verify the applicability of the information to particular situations and is urged to consult with appropriate professionals prior to taking any action or making any interpretation that is within the realm of aprofessional practice.

Disclaimer

Preface xiAcknowledgements xii

1 Framework for durability by specication 1Context 1Key issues 2Historical review 3Specifying durable concrete: the options 11Durability and the next generation of standards 18Summary 18

2 Probabilistic approach to durability design 20Design life 20Structural design analogy 22Approach to design for durability 26Future research needs 33Application in practice 35Summary 37

3 Permeability and transport processes 38Pore structure and the hydration process 40Transport processes and rates 43Measurement of permeation properties 46Factors inuencing the permeability of site concrete 49

4 Corrosion of reinforcement in concrete 51Nature of corrosion damage 51Electrochemical process 54Polarisation curves, the Evans Diagram 60Passivity 61

Contents

Corrosion mechanism in carbonated concrete 62Corrosion mechanism in chloride-rich concrete 63Inuences on corrosion activity 65Inuence of cracking 67Modelling the rate of corrosion 69Monitoring corrosion activity 71Summary 76

5 Carbonation 77Carbonation and corrosion 77Chemistry of carbonation 78Detection of the carbonation front 79Primary factors inuencing carbonation rate 81Mathematical modelling of the rate of carbonation 85Application of models to service life prediction 94Carbonation: exposure categories in EN 2061 97Specication by performance 99Summary 100

6 Chloride ingress 101Chloride ingress and corrosion 101Detection and expression of chloride levels 105Critical chloride level for corrosion 107Primary factors inuencing chloride ingress 111Mathematical modelling of chloride ingress 114Application of models to service life prediction 121Chlorides: limitations and exposure categories in EN 2061 124Specication by performance 129Summary 132

7 Alkalisilica reaction 133Background 133Manifestation of the problem 135Mechanism of expansion and reaction 139Primary factors inuencing the reaction 141Other factors inuencing ASR occurrence 145Modelling and service life prediction 148Specications to minimise the risk of ASR 149Summary 159

viii Contents

8 Freeze/thaw effects 160Background 160Primary factors of inuence 162Air entrainment 165Developments in specication and design practice 166Freeze/thaw attack: exposure categories in EN 2061 170Developments in testing 173Specication by performance 176Summary 177

9 Chemical attack: sulfates 179Introduction 179Physico-chemical aspects 180Factors inuencing sulfate attack 184Approaches in specication and design practice 188Sulfate attack: exposure categories in EN 2061 190Developments in testing and specication by performance 192Summary 193

10 Chemical attack: acid and seawater attack 194Introduction 194Physico-chemical aspects 196Factors inuencing attack 199Mathematical modelling of acid attack 201Approaches in specication and design practice 203Acid and seawater attack: exposure categories in EN 2061 204Specication by performance 206Summary 206

11 Cracking in reinforced concrete structures 208Introduction 208Mechanism of cracking 209Chronological aspects of cracking 212Cracking and the design phase 213Cracking during the construction phase 220Cracking during the service phase 225Cracking and corrosion of reinforcement 229Summary 230

Contents ix

12 Abrasion, erosion and cavitation 232Surface deterioration 232Abrasion 233Erosion 235Cavitation 235

13 Weathering and eforescence 237Weathering 237Eforescence 238

References 241Index 255

x Contents

This book has been written at a time of change in European concrete practice,particularly in respect of specifying for durability. The introduction to European practice of non-harmonised standard EN 2061 Concrete Part 1:Specication, performance, production and conformity represents a signicantstep in raising awareness of the need to consider each potential deteriorationmechanism when specifying concrete from a durability perspective. Despitethis signicant step, those who contributed to the drafting of the standardwould not see it as reaching the journeys end in respect of the methodologyfor specifying durable concrete. It will in time be seen as a rst generationEuropean standard.

The new standard will do much to enhance the durability of future European concrete infrastructure although it is but a rst step in a new direction. The exciting demands of agship infrastructure projects, which areliterally spanning the divide between regions and countries, require furtherdevelopment of our detailed understanding of deterioration mechanisms inconcrete and reinforced concrete. Equally, the requirement that our futurestructures should represent signicant examples of sustainable development,demands that we get the balance right between optimum use of materials andthe costly risk of failure during a dened service life. Thus mathematical models of degradation are required which can be used in the probabilisticanalysis of durability and life cycle costing. The next generation of Europeanconcrete standards will hopefully embrace to a greater extent the art and science of a well-reasoned engineering solution for the design of durable concrete structures.

Current and future directions in the specication of durable European concrete form the context of the book. The emphasis is therefore on design andspecication issues to the general exclusion of site practice. It is not intended toconvey an impression that the production of a durable concrete artefact canbe achieved without due regard for the skills of those who toil in all weathersand in difcult working environments to construct the structures we can beproud of. The subject matter of the book is but one link in the quality chain.

This book is therefore intended as a basic text for three groups of readerswho will play a major role in designing the quality structures of tomorrow.

Preface

First, it provides an overview in respect of deterioration mechanisms for speciers who nd that, in applying the principles of new European StandardEN 2061, they want to know a little bit more about each phenomenon thatcan lead to durability failure. Second, it presents a state-of-the-art review forpostgraduate researchers about to embark on the task of advancing ourunderstanding and modelling of a particular degradation mechanism. Third,it is a source of reference for undergraduates of engineering, architecture andbuilding technology, and students of advanced concrete technology who nd a need to read up on a particular deterioration process as part of theircoursework.

Acknowledgements

I wish to express my thanks to Tony Moore and Richard Whitby, Spon Press,for their encouragement during the various stages of preparing the book. Iacknowledge with thanks permission from Seoirse Mac Craith to use his photographs in the production of Figures 1.1, 4.2, 7.3, 10.1 and 13.1. I amgrateful to Mary Dunphy and Myles Christian for their expert advice on matters pertaining to the illustrations. The advice of Dr Peter OConnor onchemical aspects of the subject matter is greatly appreciated. I express myheartfelt thanks to Rosemary Flynn, Anne Duffy and Joseph Duffy for theirinvaluable assistance during the preparation of the manuscript. Finally, Ithank colleagues in various sectors of the concrete technology world for theirfriendship and supportive interest in the publication.

xii Preface

Context

Economic forces make concrete the most suitable material for the majority of the worlds infrastructure. Concrete forms an indispensable element ofmotorways, airelds, harbours, canals, wastewater treatment facilities, andwater supply schemes. High-rise buildings, once the preserve of steel, are nowbeing executed in both concrete and steel/concrete composite construction.Concrete railway sleepers are displacing timber in many parts of the world.Thus concrete continues to play its crucial, if understated, role in the planetsdevelopment. Practices in the concrete industry therefore have signicantglobal effects.

The most important construction issue emerging to the fore at the turn ofthe millennium is sustainability. Can our new structures be provided in a waythat does not have a negative impact on the balance sheet of our planets niteresources? How do we determine an adequate specication for concrete,project by project, in a manner that will achieve satisfactory performance overthe required life of the structure without squandering the earths resources?Over-specication is both wasteful of resources and unjust to the client.Under-specication, on the other hand, leads to premature and costly repairwork, often with considerable disruption to third parties, such as road users,and ultimately a higher cost in nancial and environmental terms. Towardsthe end of the twentieth century over a quarter of a million concrete bridgesin the United States of America were classied as decient and the numberwas increasing by 3500 per annum. The scale of the problem in Europe was somewhat less but was not insignicant. Figure 1.1 illustrates a typicalexample of corrosion damage to the soft of a bridge. Clearly the state ofthe developed worlds highway infrastructure at the end of the twentieth century did not represent sustainable development. It well illustrates the challenge for concrete practitioners in the twenty-rst century: a fundamentalaspect of sustainability is the specication and achievement of durable concrete.

1 Framework for durability by specication

Key issues

Durable concrete is quality concrete. A quality product is one that meets predetermined expectations. These expectations may be set out in a speci-cation and can vary from one project to another depending on serviceabilityrequirements. Durability is not an end in itself. In the context of EuropeanCommission directives, durability is not specically listed as one of the essential requirements of a nished structure. However the achievement ofessential requirements, such as mechanical resistance and stability, over therequired life of the structure involves consideration of the implications ofdurability failure. It is also worth stating at the outset that durable concreteneed not be maintenance-free concrete. These views may present a differentstarting point for the achievement of durable concrete than those that mayonce have existed, when the properties of concrete were equated with those ofbedrock.

Specication of durable concrete involves a mutual understanding by both the specier and the end-user of what is meant by durability. A review oftraditional codes of practice shows that we are not yet at the stage where thespecier and end-user engage in a structured framework that includes full consideration of the key issues. These key issues are not as yet denitive butshould include the following:

2 Fundamentals of durable reinforced concrete

Figure 1.1 Evidence of corrosion and spalling in a concrete bridge soft

the design life of the structure or its individual elements; the serviceability requirement; a quantiable description of the criteria that dene serviceability failure; the acceptable level of risk; the permissible extent, if any, of maintenance.

Full application of these issues to future practice will require three developments. First, development and acceptance of mathematical models ofdeterioration that can be easily applied in practice. Second, development andacceptance of universally applicable tests for properties of concrete fromwhich the likely future satisfactory performance of the concrete may be veried. Third, the adoption of a probabilistic approach to durability designwith agreed values of the acceptable probability of failure or reliability index.The concepts involved are neither new nor complex. However they have yet togain a signicant foothold in practice due to the semi-traditional craft natureof guidance on concrete durability in codes of practice and product standards.

A short review of the key issues may be helpful in introducing the concepts.Fundamentally, the design life is a notional value determined by the designeras a function of the required service life, dened by the user, and an appro-priate factor of safety. The serviceability requirement may be descriptive, forexample the onset of a stage in the corrosionspalling cycle. Serviceabilityfailure could be quantied in a number of ways, for example a maximum limiton the percentage of surface area in a deteriorated condition. Probabilityanalysis is already implicit in the routine design for structural resistance ofload-bearing elements and could equally be integrated in design for durability.The permissible extent of maintenance would depend on access, aesthetic, oreconomic considerations. For example, foundations may be inaccessible overthe full service life; patch repairs might be unacceptable in visual concrete;functional obsolescence might render it uneconomic to invest in maintainingstructures beyond a certain age. These key concepts underpin developments inthe production of satisfactory specications for durable concrete that can provide a better level of reliability than that experienced heretofore.

Historical review

Strides have traditionally been made in the exploitation of concrete tech-nology in respect of the structural performance of the material. Even todayhigh strength concrete, part of the new generation of high performance concretes, is gaining market share. Strengths in excess of 100 MPa are notuncommon. Strength, however, should not be the main issue of interest to theconcrete practitioner. Structural failures are rare but durability failure is alltoo common. Many studies, such as that by Mac Craith (1985), have high-lighted the potential deciencies in concrete over its service life, particularly inrespect of corrosion of embedded reinforcement. Nevertheless, numerous

Framework for durability by specication 3

examples exist of durable concrete structures in even the most hostile exposure conditions. Deciencies exposed by serviceability failure must therefore point to shortcomings in execution of the concrete or inadequatespecication.

The historical decline in durability levels lay, partly, in the development of


Figure 1.2 Chronological relationship between mix parameters and durability

higher cement strengths. The sequence is illustrated in Figure 1.2. Theachievement of durable structural concrete has traditionally been linked tocement content. Prior to 1960 the cement content required to achieve speciedstrength was generally considered adequate for durability also. In selectedinstances of severe exposure the engineer might exercise judgement in specifying a higher cement content than that required for strength alone.Developments in the 1960s included increases in the strength of cement,rened tolerances in mix design and batching procedures, and increased efciency through greater employment of statistical methods in quality control. The net result was that concrete strength grades and workabilityrequirements were maintained with lower cement contents. Water/cementratios were appreciably higher than a decade before and this led to higherpermeability concretes. Concern for the serious implications regarding durability was voiced by some and the problem was addressed. An example isthat of the inclusion in the United Kingdom Code of Practice CP 110:1972 ofspecic durability recommendations for a range of exposure classes. The recommendations took the form of limits on minimum cement content andmaximum water/cement ratio.

National durability grades

The heightened awareness among designers of the need to specify for durability as well as strength led to anomalies. Specications sometimesincluded redundant parameters with the result that the producer was forcedto endure inefcient use of materials. Additionally the range included mixesthat satised the designers intention regarding strength but not the durabilityrequirement. This problem was overcome by the introduction of the nationaldurability grade concept, as proposed by Deacon and Dewar (1982). Thebasis of the concept is illustrated in Figure 1.3.


Figure 1.3 Durability grade concept

The national durability grade introduced a link between specication fordurability and a measure of its potential attainment. This was achievedthrough developing the accepted link between durability and impermeability.Achievement of impermeability was introduced in Code of Practice CP110 inthe United Kingdom in the 1970s by control of maximum water/cement ratioand minimum cement content. However this in itself was an incomplete loopbecause the most commonly specied and tested parameter was twenty-eightday cube strength. Deacon and Dewar looked at the problem from anotherangle. Compressive strength is related to water/cement ratio and cement content: if one specied a particular concrete grade could one then be assuredof meeting particular targets in respect of maximum water/cement ratio andminimum cement content? If this could be established for a large sample size,for example on a national basis, one could specify the two key durabilityparameters through concrete grade alone and test accordingly. Examples ofthe relationships determined by a survey of national practice are illustrated inFigure 1.4. This shows the results of a survey conducted by the Irish ConcreteSociety in 1998, analysed by West and Keating (1999). It may be seen that forany given grade of concrete within the national population one can establishan absolute maximum water/cement ratio and a minimum cement content ordeduce statistical values, such as those applicable to a 95 per cent condenceinterval.

The earlier free-fall in durability level was arrested by the introduction ofthe durability grade concept. Nevertheless durability grade was seen as beinga contributor to durable concrete but not the nal solution. Two signicantshortcomings existed. The rst involved the variation of national materialproperties with time and the difculty of tracking these changes in codes of practice. The second involved the continued use of exposure conditionscategorised on the general basis of environment rather than on specic deterioration mechanisms.

National durability grades can only be based on a survey of practice at aparticular time. The resulting relationships, for example Figure 1.4, are critically dependent on the characteristics of local materials and these characteristics may change with time. A key factor, for example, would be theaverage cement strength. Figure 1.5 illustrates differences between gradesestablished in different countries and at different times in the same country. Acomparison is presented of United Kingdom and Irish practice in the 1990sand a comparison between Irish practice in the mid-1980s and late 1990s. Itwill be noted that higher cement strengths in the UK led to the requirementfor higher concrete grades for a given cement content than those in the Republic of Ireland. Equally the gradual increase in strength over time ofIrish cement is reected in the higher concrete grades for a given cement content recorded in the later survey.

The second shortcoming was that national durability grades were intro-duced into an existing system of exposure classes based, in the main, on aqualitative description of exposure condition. These qualitative descriptions,


categorised for example as mild, moderate, severe etc., made reference toenvironments rather than specic deterioration mechanisms. This may havehad the effect in some instances of over-simplifying the process for some speciers who then overlooked relevant deterioration mechanisms. Thisshortcoming is being addressed in the next phase of development promptedby the introduction through the Comit Europen de Normalisation (CEN)


Figure 1.4 Example of relationship between mix parameters and characteristiccube strengths based on a survey of national practice

of European Standard EN 2061 Concrete Part 1: Specication, performance,production and conformity, and its associated national documents, for example BS 8500 in the UK and DIN 10452 in Germany.

European standard EN 206

Development of a European standard for concrete was a protracted process,indeed as it was for cement, extending over twenty years. The process beganin the early 1980s with two proposed standards that appeared as prEN 206and prEN 199 (ready-mixed concrete). These proposed standards failed to getthe required support for publication as EN documents. The drafts were latermerged into a single document and achieved European prestandard status asENV 206 in 1989. Further development over the following ten years broughtthe document to EN status with the positive vote being recorded in the springof 2000.

Development of standard EN 206, and the relevant parts of design code Eurocode 2 such as cover to reinforcement, provided an opportunity to take a more rational approach to specication and design for durability.Durability-threatening mechanisms are considered in turn: risk of reinforce-ment corrosion; and the effects of carbonation, chloride ingress, freeze/thaw,and chemical attack. This has been framed in an exposure classication system which has eighteen subclasses designated by alphanumeric codes (Figure 1.6). Durability is specied either through the traditional practice oflimiting values of concrete composition or by performance-related methods.


Figure 1.5 Variation of cement content and concrete grade relationship with timeand between regions

The former is likely to be more widely used. The standard requires that theintended working life of the structure shall be taken into account. Allowanceis included for anticipated maintenance.

In relation to the approach of limiting concrete composition, the commonparameters are permitted types and classes of constituent materials;minimum cement content; maximum water/cement ratio; and (optionally)minimum strength class. In some cases additional requirements may need tobe imposed, for example air entrainment or use of sulfate-resisting cement.Despite extensive deliberations it did not prove possible to frame a single setof values in EN 2061 for use across Europe. Standard EN 2061 could not cover all aspects of European concrete practice in a unied manner. Thestandard therefore requires or permits national standards bodies to publishprovisions valid in the place of use and the relevant limiting values are presented in national complementary documents. Such an approach is notuncommon in European standards practice. Guidance on local conditionsmay be found in national annexes to European standards and normative references may be found in complementary standards. In the case of EN 2061,for example, the British Standards Institution has published relevant valuesand framework in complementary standard BS 8500.

The performance-related method is quite different. It allows the durabilityrequirements to be determined in a quantitative way. Consideration is given tomatters such as the intended working life and the criteria that would dene


Figure 1.6 Exposure classication system in European standard EN 2061

durability failure. The required parameters of the concrete may then bedetermined in one of three ways. The rst utilises a comparison between thedurability requirements of the project and those assumed in the traditionalapproach of limiting values of concrete composition. Parameters may then be determined through renement of the values published in national complementary documents to EN 2061. The second method involves use ofperformance criteria based on approved tests and involving conditions whichare representative of those to be encountered. The third approach involves theuse of predictive models.

International research, particularly over the past two decades, has signi-cantly brought forward an understanding of the phenomena that inuencedeterioration. Massive investment in a number of high prole Europeantransport infrastructure projects has encouraged the application of thisresearch to practice but it has been on a limited basis. The approach adopteddiffers from traditional practice by its consideration of the required servicelife, the relevant deterioration mechanisms and the use of predictive durabilitymodels. The core aspects of the rst generation of mathematical models ofdeterioration are well developed but these prototypes require further develop-ment before being rened for use as routine design tools. Basically it is desiredthat one could design for durability in a probabilistic sense in a similar way to the current methods of design for structural resistance. Of equal importance, however, is that it is compatible with the drive to increase the useof performance-related specications in European practice.

It did not prove possible for the CEN Technical Committee responsible forEN 2061 to issue the rst version of the standard to be used in practice withdurability parameters determined on a more scientic basis. It must beacknowledged, however, that the introduction of exposure classes based ondeterioration mechanisms, rather than environments, is a major step forward.However the output is still based on national experience of local materials in local environments over the last couple of decades. It would have beenpreferable if the change in concrete practice could have advanced moredirectly to the extensive use of performance tests or design methods. Progressin this direction is being encouraged and is being pursued, for example, bythose engaged in DuraNet, a network for supporting the development andapplication of performance-based durability design and assessment ofconcrete structures. It is intended that later versions of EN 206 will embracedevelopments in these elds.

The development of durability-related performance tests and criteria isprogressing steadily. The primary topics being studied are carbonation,freeze/thaw performance, and sulfate resistance. Other characteristics mayprove worthy of investigation. For example, criteria for the acceptance ofconcrete cover based on a statistical evaluation of the achieved cover ratherthan a single minimum value. Tests for chloride ingress and abrasion resistancemay soon prove applicable to more widespread practice. There have beeninevitable setbacks for example a test for water penetration was evaluated


as a measure of concrete quality but it is not being pursued further. Never-theless, the core aspects of many performance tests for durability are wellestablished although a lot of work remains to be done in achieving an acceptable level of precision.

Specifying durable concrete: the options

Three main approaches may be distinguished for the specication of durableconcrete:

all-encompassing prescriptive approach; deterioration-specic prescriptive approach; durability design method and performance testing.

The method used through recent decades is represented by the rstapproach. The second approach is a key feature of the introduction to practice of European Standard EN 2061. It will be the most commonly used methodology for some time to come despite its recognised shortcomings.The third approach, introduced through an informative annex in the rst version EN 2061, provides an alternative method which is potentially morereliable but is likely to be used in a minority of cases until research progressesfurther in the areas of deterioration modelling and performance-based specications.

All-encompassing prescriptive approach

The all-encompassing prescriptive approach involves consideration ofdeemed-to-satisfy limits. It is exemplied by the durability clauses in codes ofpractice such as British Standard BS 8110 and National Standards Authorityof Ireland IS 326. The tables relating concrete quality and cover to reinforce-ment, in the form of that presented in Table 1.1, is based on the concept ofthe national durability grade. Control of the minimum cement content andmaximum water/cement ratio may be achieved through specication of anappropriate minimum grade of concrete. The most noteworthy issue is thatdeterioration mechanisms are not explicitly considered. The specier usesinstead an all-encompassing environmental classication system based on aqualitative description of the conditions of exposure (for example, moderateor severe). Consideration of specic threats, such as alkalisilica reaction, iscovered by reference to clauses or other documents containing prescriptiveadvice.

This all-encompassing methodology relies to a considerable extent on theexercise of engineering judgement. Guidance is given on interpreting thedescriptions but it cannot be exhaustive. Examples arise where the specierwill feel that their nal choice remains somewhat subjective. Doubts about theadequacy of this prescriptive approach for specication in chloride-laden


environments have been expressed by, for example, Browne (1986) and Bamforth (1994).

The dilemma faced by the specier may be considered by an analogy withstructural design. A designer could not size a reinforced concrete beam withan acceptable level of reliability if the information on span and loading wasnot quantied but was merely classied as moderate and severe respectively(Figure 1.7). It is readily apparent that such a system would lead to an unacceptably low factor of safety in certain circumstances and the wastefuluse of excessive material in others.

The all-encompassing prescriptive approach is not objective. It cannot beused in economic optimisation because it does not take account of therequired service life nor the required reliability. The method is cumbersome in adapting to the benets of emerging technologies in the form of new materials and construction techniques. A code based solely on the all-encompassing approach cannot, for example, easily harness the potential benets of new technologies. The specier, for example, would nd it difcultto do a costbenet analysis on the use of controlled permeability formworkor corrosion inhibitors because their use would not specically change theprescriptive requirements of the code.

The shortcomings of the all-encompassing prescriptive approach may beseen by an examination of four key aspects:

validity of a prescriptive approach; exposure class selection; the relationship between strength and mix parameters; site practice.

A fundamental issue is the validity of a prescriptive approach. The specieris relying solely on the perceived relationship between a descriptive exposure


Table 1.1 Format of traditional prescriptive approach to durability

Exposure classication Nominal cover

Mild NC2 NC1 NC1 NC1 NC1Moderate NC4 NC3 NC2 NC1Severe NC5 NC3 NC2Very severe NC6 NC5 NC3Most severe NC6Abrasive *

Concrete properties

Max. free water/cement ratio W5 W4 W3 W2 W1Min. cement content C1 C2 C3 C4 C5Lowest grade of concrete G1 G2 G3 G4 G5

*Additional requirements may be noted

class and prescribed mix parameters (minimum cement content and maximum water/cement ratio) to ensure adequate protection. Reliance onsimplied empirical relationships or previous satisfactory experience is total.

The second issue is the identication by the specier of the appropriateexposure classication based on descriptive clauses. The description of whatis covered by the exposure classes in national codes has usually been quitecomprehensive and so accurate judgement is likely. Nevertheless it is not themost efcient method of ensuring that all relevant deterioration mechanismshave been identied and considered. The likelihood of over- or under-specication is signicant.

The third issue is the assumed relationship in a national code between concrete grade and current industry practice regarding minimum cement content and maximum water/cement ratio. This relates to the fact that theconcrete durability grades quoted in codes are based on industry norms in acountry as surveyed at a particular time. Changes to practice due to changesin material characteristics, such as cement strength, may alter the relation-ships. It is impractical to expect codes of practice to rapidly adapt to subtlemarket changes. Thus the weakest link in the chain inuences the durability ofthe concrete but the chains themselves are not amenable to regular scrutiny.

The fourth point relates to the implicit assumption that achievement of keyparameters such as specied cube or cylinder strength will be accompanied by


Figure 1.7 The dilemma posed by a qualitative description of the design constraints

proper site practice regarding compaction and curing so as to achieve the full durability potential of a given mix. Poorly cured concrete may have anunacceptably high permeability in the cover zone despite being made with an appropriately specied concrete of high quality. The durability propertiesof the cover zone in the member as-built, other than the depth of cover, doesnot specically form part of the acceptance criteria. Prescribing the right mix does not necessarily guarantee the achievement of the required level ofimpermeability.

Recognition that the all-encompassing prescriptive approach has failed incertain cases in the past has resulted in more demanding values being placedon water/cement ratio and other criteria. There is a limit however on how farone can push the numbers without causing other problems with concrete suchas ease of placing and compaction.

Deterioration-specic prescriptive approach

The potential shortcomings of the all-encompassing approach will beaddressed, in part, by the deterioration-specic approach of European Standard EN 2061. Specication remains prescriptive in that minimumbinder content, maximum water/binder ratio, and (optionally) minimum concrete grade are output from consideration of exposure subclasses.However the specier is forced to consider the most onerous condition from acombination of deterioration mechanisms and environmental conditions inthe determination of limiting values of concrete composition. Some nationalcomplementary standards will also allow a trade-off between concrete quality and cover.

The essential advance over the all-encompassing approach is that ofmaking the exposure/environmental classications more comprehensive. Thedownside is that it makes the process more complicated through the introduction of the greatly increased number of exposure classications.Practitioners probably feel that there are too many classes while someresearchers feel that there are still too few! The numerous subclasses arerequired to take account of environmental characteristics that may inuencethe rate of deterioration, for example wetting and drying cycles in the case ofcarbonation.

The details of the eighteen subclasses are presented in Table 1.2.The basic format of the limiting values table is presented in Table 1.3. This

illustrates an overview of the structure. The limiting values of concrete composition and properties are set nationally and published in accompanyingdocuments to the European standard. Minimum strength class is presentedusing a dual designation system based on cube and cylinder strengths. Thestandard was formulated on the basis of cylinder strengths and the cubestrengths are merely an approximation to the cylinder strength hence thesomewhat awkward dual reference system, exemplied by a Grade C30/37concrete.


The approach addresses one of the four shortcomings raised in relation tothe all-encompassing prescriptive approach that of exposure class selection.By presenting the specier with an expanded suite of exposure classes, whichmore closely reect specic deterioration mechanisms, it is more likely thatspecication for durability will be more reliable. Nevertheless the other threeconcerns remain: validity of the prescriptive approach, changing relationshipsover time between concrete grades and mix composition and achievement ofpotential durability through proper site practice.

Durability design and performance testing

The third approach to the specication of durable concrete is radically different to the foregoing. The durability design method involves consider-ation of each relevant deterioration mechanism and the expected service lifeof the structure in a quantitative way. Appropriate material parameters maythen be determined based on an acceptable probability of failure.

The all-encompassing prescriptive approach was introduced to practice ata time when the deterioration mechanisms were less well understood.Research, particularly over the last two decades, has identied the dominant


Table 1.2 Summary of exposure classes and environments in EN 2061

Degradation Sub- Environmentphenomenon class

No risk of corrosion X0 Unreinforced concrete: all exposures exceptor attack freeze-thaw, abrasion, chemical attack

Reinforced concrete: Very dryCorrosion induced XC1 Dry or permanently wetby carbonation XC2 Wet, rarely dry

XC3 Moderate humidityXC4 Cyclical wet and dry

Corrosion induced XD1 Moderate humidityby chlorides other XD2 Wet, rarely drythan from seawater XD3 Cyclical wet and dryCorrosion induced XS1 Exposure to airborne saltby chlorides from XS2 Permanently submergedseawater XS3 Tidal, splash and spray zonesFreeze/thaw attack XF1 Moderate water saturation, no de-icing agent

XF2 Moderate water saturation, de-icing agentXF3 High water saturation, no de-icing agentXF4 High water saturation, de-icing agent or sea water

Chemical attack XA1 Slightly* aggressive environmentXA2 Moderately* aggressive environmentXA3 Highly* aggressive environment

*Quantied in respect of the chemical characteristics of groundwater (SO42, pH, CO2, NH4,Mg2) or soil (SO42, acidity)

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mechanisms and the key parameters controlling the rates of deterioration.Further work on the proposal and renement of mathematical models ofdeterioration is continuing. The models for carbonation, chloride ingress and corrosion propagation are the most advanced to date but still requirerenement. Excellent progress has been made in the last few years on adopting the probabilistic approach, commonly used in structural design, fordetermination of durability parameters.

The durability design approach allows consideration of the type ofstructure, material properties, microclimates, required life, quality of sitepractice and the probability of failure. Thus it addresses the shortcomings ofthe prescriptive approach.

The specier may envisage a prescriptive approach as being part ofEuropean standards for some time to come but the employment of durabilitydesign and the development of performance-based specications is to beencouraged, especially on critical projects. Standards for certain phenomenacontinue to develop along the prescriptive route, for example alkalisilicareaction. Nevertheless signicant advances are being made on topics relatedto corrosion of reinforcement.

Three avenues are available for determining the appropriate parameters:

satisfactory experience; performance test methods; predictive models.

The avenue of specication based on satisfactory experience allows accountto be taken of long-term satisfactory experience with concrete specied forsimilar works in a particular environment. The proposed works would involveapplication of similar materials and practices in an environment where the satisfactory history was established. Essentially it is a renement of theprescriptive approach but the link between specication and performance isclearly established.

The second alternative route involves performance test methods. A specied performance would be dened for a relevant deterioration mech-anism. The concrete producer would then prove the adequacy of a proposedmix by reference to a relevant approval test. The test would demonstrate thepotential of the proposed mix to meet the dened performance level. The testmight be conducted specically for the contract. Alternatively the concretemix might be accepted where adequate performance has been established by previous tests on concretes of similar materials and weaker mixes (forexample, higher water/cement ratios). Control testing could then be used to monitor an agreed key element of the mix. This would highlight any departures from the agreed mix.

The third possibility involves the use of predictive models. Considerableprogress has been made on the mathematical modelling of deteriorationmechanisms, especially those related to corrosion initiation. The route


provides the key to better design of new works and management of the durability of existing stock. The predictive models will relate the rate ofdeterioration, for a given mechanism, to key measurable parameters of theconcrete or mix constituents. Acceptable models will be calibrated againstdata from the in-service behaviour of structures.

The development of the models involves consideration of the often complex chemical and physical interactions present in a deterioration mechanism. Translation of the models into workable design aids may involveincorporation of simplifying assumptions. This would not necessarily detractfrom the integrity and sophistication of the approach since these steps wouldcontribute in a quantiable way towards the factor of safety which is an integral element of prudent design and management of risk.

The practitioner will nd more advanced treatment of these topics andsoftware in publications under development, such as that by The ConcreteSociety and Taywood Engineering Ltd.

Durability and the next generation of standards

The launch into practice of the rst generation of European concrete standards, especially EN 2061 and EN 1992, in the early years of the secondmillennium will not fully reect the state of knowledge regarding durabilitydesign which existed at the end of the rst millennium. The nature ofstandards and codes preparation is such that they must inevitably lag behindtechnical developments. Considerable progress on durability design has beenmade, notably through the DuraCrete project of the European CommissionsFourth Framework Programme and in practice in the Netherlands.

It is likely that the next generation of standards will still embrace a form ofdeemed-to-satisfy approach for routine building projects. However the inclusion of more sophisticated quantitative approaches in a normative wayfor critical projects will probably be evident. The research which will underpinthe changes to the second generation of codes and standards will be focusedon validating deterioration models from laboratory and eld trials and on thebroader issue of applying risk analysis to durability specication.

The risk analysis approach will involve a signicant change to specicationfor durability by forcing speciers and, signicantly, clients to consider service life in a probabilistic way. The client will get what they pay for. Indeedthe extension of the risk analysis approach to embrace life cycle costing willbe an obvious step.

Summary

At the turn of the millennium the risk of concrete failing to perform satis-factorily over its service life should be low, given the current understanding ofdeterioration, yet it remains unacceptably high. The cause of the problem liesin the gulf that exists between simplied descriptions of exposure conditions


and the complex deterioration mechanisms that act alone or in concert. Theneed to bridge this gulf is a pressing one. Durability of concrete needs to be ensured in future works from both an economic and an environmentalviewpoint. The economic argument is a straightforward one prematureexpenditure on repair or demolition and the cost of disruption to productionreduces a clients competitiveness and reects badly on the concrete industry.Equally important in modern society are the green issues. Unforeseen maintenance involves wasteful use of the earths scarce resources of bothmaterials and energy. Demolition due to unserviceability involves productionof construction waste for which the planet has a dwindling capacity to absorb.Future generations will not look kindly on todays speciers if load-bearingframes of reinforced-concrete buildings are not durable enough to allowbuilding rejuvenation through faade replacement and interior re-t.

Substantial research on concrete durability is in progress throughout theworld. In Europe it had been hoped that the fruits of this research would havebeen fully incorporated into the rst generation of European concrete standards EN 2061 and EN 1992. Regrettably the required package ofresearch knowledge and in-service experience is not sufciently complete toallow this. Nevertheless progress is apparent in that speciers will be requiredto consider the various deterioration mechanisms in a structured frameworkrather than relying on a qualitative exposure classication system.

The prospects for ensuring the durability of concrete structures over theperiod of their intended service lives is now promising. The introduction ofnew materials and construction techniques offer higher quality concretes withadequate impermeability. Equally exciting, however, is an enhanced under-standing of the parameters that inuence the rate of reinforced concrete deterioration. This knowledge is introducing a new way of approaching durability through design methods based on scientic and engineering principles. Prescriptive approaches still have a role to play but it behoves thespecier to make the best use of the considerable advances made in recentdecades in understanding the properties of durable reinforced concrete.

The following chapters present the specier with background to the factorswhich inuence the selection of limiting values in the EN 2061 frameworkand provide an overview of research developments to those about to engagein the challenging quest of developing predictive models, performance tests,and probability-based durability design methods.


An integral concept of specication for durability in structures is that thematerial properties should meet the performance requirements over a denedlife. This concept has been in the background in standards and codes ofpractice for over 50 years. For example in the United Kingdom, Code ofPractice CP3, Chapter IX on durability (Council for Codes of Practice for Buildings 1950) included denitions of both the designed life and thesatisfactory life. More recently British Standard BS 7543 (1992) has beenpublished to cover the durability of buildings and their elements. Signicantly,in the quest to provide a rational basis for achieving design life, a jointRILEM/CIB group developed a systematic methodology for service life prediction (Masters and Brandt 1989). This has been adopted for standard-isation by the International Standards Organisation (1998) as ISO 15686 Service Life Planning of Buildings. The RILEM/CIB methodology built onearlier work from Sweden (Sentler 1983) and in the Netherlands (Siemes et al.1985). This was further studied and a clearer focus on the way forward wasprovided by the publication of RILEM Report 14 (Sarja and Vesikari 1996)and CEB Bulletin 238 (Schiessl et al. 1997). Meanwhile a signicant impetusto the development of the tools necessary to apply the service life principleswas provided through a European project DuraCrete (Probability Perform-ance Based Design of Concrete Structures). Although the stage has yet to be reached whereby codes and standards will fully embrace the concepts,degradation models of a deterministic nature have been used on major infrastructure projects in Denmark and a stochastic model was used to document the required service life in a contract in the Netherlands.

Design life

An overview of the concepts of design life and service life is illustrated in Figure 2.1 in a development of work by Tuuti (1982) and Sarja and Vesikari(1996). This shows that the structure reaches its design life when the maximumtolerable level of damage is reached. An acceptable value of the service life may then be evaluated, for example by statistical considerations. Thisintroduces the concept but in practice the degradation mechanism can itself

2 Probabilistic approach todurability design

be modelled stochastically and the risk of reaching the limit of damage is considered probabilistically. This is explained in later sections.

The notion of design life has been described (Rostam 1984) as the combination of possible technical life with economic considerations. Possibledenitions of design life have been advanced. For example: the minimumperiod for which the structure can be expected to perform its designated function without signicant loss of utility, and not requiring too much maintenance (Somerville 1986); the period of use intended by the designer(Concrete Society 1996); and the period of time after installation duringwhich a building or its parts exceed the performance requirements (ISO1998). A denition of the working life is presented in EN 2061 (ComitEuropen de Normalisation 2000a) in terms similar to that of Somervillesdesign life.

The design life of a building would be determined based on the service lifeexpectation of the client. It is unlikely that clients will be very specic but itshould be possible for them to indicate the category of structure in descriptiveterms such as temporary, normal, or major infrastructural. The designercan then relate this to the classication of structures in standards and codesthat contain information on service life expectation. Other than the case oftemporary structures, the choice for non-renewable elements will typically bebetween a fty-year life or a 100-year life. Even this simple choice allows the specier to get the client thinking about the balance between rst cost andlife-cycle cost. The days are gone when clients could expect their reinforcedconcrete assets to be forever maintenance-free.

Probabilistic approach to durability design 21

Figure 2.1 Concept of service life prediction based on durability considerations

Design life may thus be seen in the context of supporting decisions relatingto specication of material properties and could therefore be extended to life-cycle cost analysis. However it must be seen as a notional concept thestructure will not necessarily reach the failure criterion at the appointed year. Those assessing the durability of existing concrete structures often nd variable corrosion activity, which demonstrates the variable nature of the phenomena involved. It does not mean however that a rational approach tospecication for durability is impossible. Specication for durability isamenable to mathematical analysis and design but the methods should takeaccount of variability. This implies the use of a statistical approach to theproblem.

Structural design analogy

The use of statistical methods is highly appropriate in durability designbecause the fundamental requirement is to minimise the risk of failure. Thisintroduces the concept of risk analysis. Durability design can be achieved inthe context of a dened probability of failure. Equally one may refer to adened level of reliability. The approach is directly comparable with that usedin design for structural resistance.

In the past speciers and clients or owners may have had a vague idea ofwhat their expectations were in respect of service life but it was not specicallyconsidered in practice, except in rare cases such as temporary structures.Harnessing the design life concept allows the selection of appropriate materials on a rational basis. This involves the need to model the degradationin performance with time-dependent functions in the context of an overallapproach to durability design. A number of approaches are possible and several have been advanced (Siemes et al. 1985, Sarja and Vesikari 1996,Siemes and Rostam 1996).

Consideration of the various approaches to durability design involves anappreciation of the meaning of safety factors, mean values, design values,distributions, performance functions, and capacity functions. At rst the concepts may seem new to the specier who has traditionally worked onlywith deem-to-satisfy rules for durability but a comparison with traditionalstructural design methods demonstrates that the principles are the same. Thefundamental principles that underlie the application of probability analysis to durability design may best be introduced, therefore, by rst reviewing afamiliar example from structural design codes. The example chosen fordemonstrating the principles is that of providing adequate resistance to bending in the design of a reinforced concrete beam.

Consider the case of a singly-reinforced concrete beam, simply supportedat each end, and carrying a uniformly distributed load (Figure 2.2). The beammay fail in a number of ways but for the purpose of this example only thephenomenon of bending will be considered.

The design of the beam is based on the assumption that it will fail if


the maximum demand bending moment, occurring at mid-span, exceeds the ultimate moment of resistance of the section. Thus we may impose a constraint:

R S 0

where R resistance (ultimate moment of resistance of section) and S load (maximum bending moment).

To convert this relationship into a form that yields design values for the section we formulate relationships describing the resistance of the sectionbased on concrete failing rst, the resistance of the section based on the reinforcement yielding rst, and the maximum demand bending moment.Thus:

R f ( fcomp, b, d)

and

R f ( fsteel, As, z)

while

S f (W, L)

where fcomp is the compressive strength of the concrete, b is the width, d is theeffective depth, fsteel is the tensile strength of the steel, As is the area of steelreinforcement, z is the lever arm, W is the total load, and L is the span. It maybe further noted that z is a function of d and that d is a function of the total


Figure 2.2 Conditions assumed in structural design analogy

depth h. The design problem becomes one of selecting values and either validating that the constraint of the formula is met or solving the formula foran unknown value.

The issue of uncertainty now enters. The intended values of width and totaldepth are generally assumed to remain constant during the service life and arereadily achievable, within specied tolerances, during construction. What ofthe assumed concrete strength and load, however? The actual concretestrength that will be achieved in the structure cannot be known with certaintyat the design stage. It will depend on the materials selected by the producer,the degree of compaction achieved by the operatives, and the curing conditions, which will be inuenced in part by the weather at the time ofconstruction. The load is variable also. The imposed load component in particular will uctuate on a daily basis to a degree that is primarily dependent on the function of the building but what of the unexpected loadsthat may arise over the lifetime of a building? Design based on the worst case weakest possible concrete and potentially heaviest load together with anallowance for design and construction blunders would unnecessarily resultin ungainly structural elements with a high moment of resistance (Rmax).These would be uneconomic, would severely limit the technical advancementof span, and would fail to meet the requirements of sustainable development.On the other hand, design based on the most optimistic case concreteachieving its full potential strength, no unexpected load combinations, and no allowance for blunders would result in elements with low moments ofresistance (Rmin) which would have a high probability of failure. This is


Figure 2.3 Theoretical prole of actual bending moment in service compared to maximum and minimum moments of resistance.

illustrated in Figure 2.3, which charts a theoretical prole of the bendingmoment values (Sactual) resulting from load combinations that vary with time,as may be expected in reality.

It is a question of balancing economy and safety in an acceptable way. Thisis achieved by the use of characteristic values of strength and load based onprobabilistic considerations and by the application of safety factors. Thecharacteristic values of strength and load are determined by consideration ofthe mean values encountered in practice, their variability and the applicationof statistical parameters. The characteristic strength is determined by reducing the mean strength by an amount based on a chosen multiple of thestandard deviation, while the characteristic load is based on values above ananticipated mean. These values are modied further to produce design valuesthrough the application of partial safety factors (Figure 2.4).

Thus the design problem may be solved in a deterministic way despite thefact that the values of the moment of resistance (R) and design bendingmoment (S) are calculated in an approach that incorporates allowances forthe variability encountered in practice. The underlying probabilistic nature ofthe problem is further reinforced by the fact that the magnitude of both thefactors of safety and the allowance for variability in the design codes has beenselected to yield an acceptable probability of failure. The concept is illustratedin Figure 2.5.

Thus while it appears that the problem being solved is of the form:


Figure 2.4 Concept of design strength and design load based on statistical considerations

R S 0

it is set in the wider context of ensuring that the probability of failing to satisfy this condition is less than a maximum allowable failure probability (Pf max). This may be stated as follows:

P {R S < 0} Pf max

The application of these principles to durability design was considered byRILEM Commission TC 130CSL (Sarja and Vesikari 1996). Further development in application of the principles to practice will hopefully inuencethe durability clauses in the second generation of European standards forconcrete.

Approach to design for durability

The mathematical solutions to the problem of design for durability are notquite as straightforward as those presented in the structural design analogy.Allowance for the multitude of variables and their distributions in durabilitydesign problems can sometimes lead to complex mathematical solutions.


Figure 2.5 Incorporation of probabilistic concepts into an essentially determin-istic model

Thus a number of approaches have been proposed, each of which may bemore readily applicable in any given case depending on the deteriorationmechanisms involved. The application in each case, however, involves aspectsthat are familiar from the principles of the approach to structural design.Although the terminology in the literature varies for similar approaches,broadly the approaches may be described as the Lifetime Safety FactorMethod, the Intended Service Period Design, and the Lifetime Design.The rst of these is essentially deterministic but the others are stochasticdesign methods.

Lifetime Safety Factor method

The concept of the Lifetime Safety Factor method rst involves considerationof the following:

a function (R or R(t)) which describes the resistance of the structure; itmay or may not be time-dependent; it may be based on mean values ofinput parameters;

a function (S or S(t)), which describes the load (for example, the chloride level) on the structure; it may or may not be time-dependent; itmay be based on mean values of input parameters;

the safety margin (R(t) S(t)); the mean service life (tmean).

The resistance of the structure may reduce with time until failure is reachedwhen it equals a pre-determined value of the load. In such cases R(t) isdescribed as a performance model. Equally, the resistance of the structuremay be constant but the load may increase with time until failure is reachedwhen it equals a pre-determined value of the resistance. In such cases S(t) istermed a degradation model. Note that the requirement may be phrased in a limiting manner: for example, the ability of a concrete element to keep agiven parameter below a certain level. Degradation models are common indurability design problems. For example, in the case of the carbonation phenomenon the depth of carbonation increases with time and may eventually reach the reinforcement; in the case of chloride ingress, the level ofchlorides at the reinforcement builds up over time and may reach a criticalcorrosion threshold level.

Consider the case of a degradation problem. The conditions which prevailduring the mean service life (Figure 2.6) are such that:

R S(t) 0 t tmean

and

R S(tmean) 0


It may be seen from Figure 2.6 that, due to the distribution of values ofS(t), the possibility of the condition {R S(t) 0} being met occurs inadvance of tmean and that the probability of this occurring increases with time.Thus the maximum allowable failure probability must be considered (Pf max)leading to a design constraint that:

P {R S(t) 0} T Pf max T tmean

and this leads to a target service life (tg) which is the time at which Pf maxoccurs. The challenge for the specier is to meet the target service life (tg) thatrepresents the clients expectation of service life. To achieve this duration ofsatisfactory performance it is necessary to specify based on the anticipatedmean service life (tmean), which becomes the design service life (td). This introduces the concept of the Lifetime Safety Factor, which is described bythe relationship:

td t tg

where td design service lifet Lifetime Safety Factortg target service life

Introduction of probability to durability design in a similar way to that of


Figure 2.6 Prole of a degradation model for durability design

structural design for the traditional form of loading requires a more simplied route than that of calculating probabilities. It may be shown (Sarjaand Vesikari 1996) that, once values for Lifetime Safety Factors have beencalibrated, design would involve the following steps:

agree the target service life of the structure; determine the design service life based on the Lifetime Safety Factor and

the target service life; apply the relevant degradation model or performance model using the

design service life and select appropriate material properties, sectionssizes, and/or protective measures;

check that the reduction in the safety margin, for example R S(t), fromtime t 0 to time t tg is less than an allowable value;

if the reduction in safety margin is too great, redesign using higher performance materials, or larger sections, or introduce additional protective measures;

if the redesign fails to produce a satisfactory value of the minimum safetymargin consider agreement with the client of a shorter target service life.

The Lifetime Safety Factor method provides a good introduction to the concept of extending structural design principles to durability design. Thenext development is to extend consideration of the probability of failure as asignicant criterion. Two other methods, which are essentially both sides ofthe same coin, have been described (Siemes and Rostam 1996) as theIntended Service Period Design, and the Lifetime Design. They approachthe same problem from different perspectives but yield the same outcome. Thechoice of method depends on the information available at the design stage.

Intended Service Period Design method

The principles underlying the Intended Service Period Design and the Lifetime Design methods are similar to the Lifetime Safety Factor method.However the methods extend the use of probabilistic analytical tools.

In relation to the Intended Service Period Design, consider the following:

the function (R or R(t)) which describes the resistance of the structure; the function (S or S(t)), which describes the load on the structure; the condition which represents a technical failure (R(t) S(t) 0); the maximum allowable failure probability (Pf max); the target service period (tg) during which it is expected that the prob-

ability of technical failure will not exceed the maximum allowable level.

The probability of the difference between resistance and capacity becomingnegative at least once during the target service period may be calculated at anytime t:


P {R(t) S(t) 0}T where T tg

This value may then be compared with the maximum allowable value of thefailure probability to calculate the probability of the former being less thanthe latter. This may be stated as follows:

Pf,T P {R(t) S(t) 0}T Pf max where T tg

Thus it is possible to chart a series of values of Pf,T for different values of tand check whether or not the required level of reliability is achievable duringthe target service life. The effect of changing parameters in the resistance orload functions may also be examined by plotting the probability distributioncurves. Typical curves are illustrated in Figure 2.7. The shape of the curveswill be familiar from the traditional quality control procedure of assessing bystatistical means the acceptability of a batch of goods while only testing asample.

Thus design would involve the following steps:

agree the target service life of the structure by consultation with the client; apply the relevant degradation and/or performance model using the mean

values of the input parameters together with their distributions; check the probability of the resistance falling below the required level

during the service life, or equally check the probability of the load exceeding the available level of resistance;


Figure 2.7 Probability distribution curves showing the effect of different load parameters

chart the failure probability through consideration of the acceptablemaximum level of failure;

if the failure probability is unsatisfactory, redesign the mean values of theinput parameters by specifying higher performance materials or largersections and/or reduce the distributions by specifying stricter quality control measures if this is feasible;

if the redesign fails to produce a satisfactory level of failure consider ifthe client could accept a shorter target service life or a higher risk offailure.

Lifetime Design method

The principles of the Intended Service Period Design may also be used in theLifetime Design but the problem is approached in a different way. For example in the former one might consider the probability of the level ofchloride at the reinforcement reaching 0.4 per cent by weight of binder duringthe target service life. In the latter, one considers the probability of the servicelife being less than the target service life for the condition whereby the chloride level is 0.4 per cent by weight of binder at the reinforcement. The rstmethod is used where the distributions of the performance and load areknown. The Lifetime Design method is used where the distribution of theservice life is known or may be assumed to follow a certain prole.

The Lifetime Design method uses the functions R(t) and S(t) to formulatea relationship in terms of the life of the structure (L):

L f (R(t), S(t))

Knowing (or assuming) the appropriate model of the service life distri-bution and the maximum permissible value of the probability of failure it ispossible to evaluate the target service life. The probability of the calculated lifeof the structure being less than the target service life may be determined:

P {L tg 0}

The concept is illustrated in Figure 2.8 for a deterioration process whosetime-dependency promotes a higher risk of failure in earlier years.

A further calculation may be made of the probability of failure. This is theprobability of the difference between the expected life and the target servicelife being less than an acceptable value:

Pf P {L tg 0} Pacceptable

As before, the effect of changing parameters in the resistance or load functions may be examined by plotting the probability distribution curves asillustrated in Figure 2.9.


Thus design would involve the following steps:

agree the target service life of the structure by consultation with the client; apply the relevant degradation and/or performance model, using the

mean values of the input parameters together with their distributions, todetermine the calculated life of the structure;

check the probability of the calculated life falling below the target servicelife;

chart the failure probability through consideration of the acceptablemaximum level of failure;

if the failure probability is unsatisfactory, redesign the mean values of theinput parameters by specifying higher performance materials or largersections and/or reduce the distributions by specifying stricter quality control measures if this is feasible;

if the redesign fails to produce a satisfactory level of failure consider ifthe client could accept a shorter target service life or a higher risk offailure.

Use of the Lifetime Design method involves knowing the service life distribution. The distribution will depend on the deterioration process underconsideration. The distributions will not necessarily be normal. Some phenomena lead to a situation whereby the values are not evenly distributedabout the mean with damage occurring more frequently in earlier years thanlater. This would be typical in the case of deterioration models involving the


Figure 2.8 Lifetime design: service life distribution

square root of time. These models include the signicant cases of corrosioninitiation by carbonation and chloride ingress. The service life distribution for such phenomena may be modelled by, for example, the log-normal distribution.

Future research needs

The translation of theory to practice represents a great challenge forresearchers and speciers in the eld of durability design. The reaction ofmany speciers to the introduction of the extended prescriptive approach of European standard EN 206 is likely to be that it seems overly complex.Imagine then the challenge of introducing the probabilistic approach to durability design into practice. Nevertheless speciers recognise that animprovement in the method of specication for durability is required in the case of structures which are required to have a long service life. The traditional prescriptive approach has a role to play but it cannot full the needin all cases.

The inclusion of probabilistic methods of durability design in codes andstandards involves a number of stages of development. Signicant progresshas been made in the 1990s but much work remains to be done. The pathahead may be charted as follows:

development of accepted models of deterioration and the conversion ofthese models into readily-applicable design tools;

calibration of the models with experience of real structures;


Figure 2.9 Probability distribution curves showing the effect of different resistance parameters

selection of favoured methods of probabilistic durability design; denition of applicable limit states; determination of acceptable levels of failure probability.

Many models already exist for various deterioration mechanisms and areview of the most widely referenced ones are presented in later chapters. It isclear that in some cases there is great diversity in the models available. This iscaused by the complex interaction of physical and chemical phenomenainvolved in each deterioration mechanism. Much of the research done in the production of the models is very valuable but a need exists to focus attention on a smaller number of models for development into internationally-acceptable design tools.

Models used as design tools need to be relatively straightforward so thatthey can be used on an everyday basis. Complex formulations based onparameters that cannot be readily specied or tested are unlikely to ndfavour. The renement, or to put it more correctly, the coarsening, of themodels will involve replacement of terms which are difcult to test routinely.Practitioners will want to be able to specify on the basis of formulae thatinclude properties that can be checked for compliance. Replacement of termsthrough conservative assumptions may involve loss of exactitude but this willbe compensated for by an increase in reliability in service. Coarsening of themodels need not imply that oversimplication will result. The projectsemploying probabilistic durability design methods will most likely be highprole, multi-million Euro projects where adequate design time will be affordable. Furthermore the use of software will ease the demands ofstatistical analysis. Complex models are unlikely to be required on routine lowto medium budget projects because the prescriptive approach of EN 206would sufce.

Input parameters for the models will need to be established by calibrationwith practice. The type of distribution appropriate to each input parameterwill need to be studied and agreed. The typical magnitude of standard deviations will also need to be assessed. A wealth of data exists in both published research studies and in unpublished commercial test reports ondeteriorated structures carried out as part of investigations. The challenge isto harness this data in a focused manner.

Serviceability Limit States need to be debated and agreed. A wide range of possibilities exists even in the single case of crack development in coverconcrete from corroding reinforcement. For example in some structures theserviceability limit state would be reached at the appearance of the rst crackwhereas in others a certain level of spalling would be tolerable. Circumstanceswill vary the risk of concrete spalling is less acceptable in the case of a bridgeover a motorway than for a bridge over a minor river.

Research is also required on dening the acceptable level of probability failure or, if preferred, the reliability index. The codes tend to specify the reliability index rather than the probability of failure. The relationship


between the acceptable level of failure probability and the reliability index isillustrated in Figure 2.10. The terms are related by the following formula:

Pf y ()

where Pf probability of failurey normal distribution function reliability index

For example, if is two the area outside that multiple of standard deviations from the mean, on one tail of the curve, is about 3 per cent, givinga failure probability of the order of 102.

A starting point may be made by studying the values in existing codes ofpractice and draft Eurocodes. To indicate the order of magnitude of prob-ability of failure appropriate to durability studies Bamforth (1999) referred totypical code values. To take the extreme case of collapse, the reliability indexfor the ultimate limit state was 3.8 for a fty-year life indicating a probabilityof failure of the order 104. The reliability indices for serviceability limitstates drop down to about 1.5, indicating a probability of failure of the order102. This order of magnitude may be appropriate for durability design butthe various durability limit states of serviceability need to be explored ingreater detail, particularly in the context of sustainable development.

Application in practice

Clearly much work remains to be done before probabilistic durability designmethods are fully enshrined in codes of practice. However it is heartening


Figure 2.10 Relationship between acceptable level of failure probability and reliability index

to review what is thought to be the rst case of the use of the DuraCretemethodology in practice. The example, reported by Breitenbucher et al.(1999), pertains to the design of the concrete lining for the Western ScheldtTunnel in the Netherlands. The methodology was used to full a requirementthat the contractor document the achievement of an anticipated life in excessof 100 years.

The tunnel was to be constructed in chloride-contaminated soil. The tunnelsegments were to be jointed together with interlocking nibs and recesses. Itwas recognised that in time the joints may leak, thus exposing the joint surfaces to chloride ingress. The cover to reinforcement at the joints was identied as being critical a compromise was required between minimisingthe cover for structural reasons and providing adequate cover for durabilityreasons over a service life in excess of 100 years.

The serviceability state chosen for analysis was that of the onset ofcorrosion. The acceptable probability of failure was derived by a study of thereliability indices in a Dutch code of practice and in a draft Eurocode. In the absence of better data a requirement was set that the minimum reliabilityindex would be in the range of 1.5 to 1.8, equating to an acceptable failureprobability of about 102.

Chloride ingress was modelled by a formula based on a solution to Fickssecond law of diffusion. Further detail on the background to this formula ispresented in Chapter 6. The form of the equations used was as follows:

t0 nx(t) 2C(Crit) kt DRCM,0 ke kc t ( t )kt DRCM,0 D0

CCritC(Crit) erf1 1 ( CSN)where x concrete cover

C(Crit) critical chloride contentCSN surface chloride levelDRCM,0 chloride migration coefcient measured at time t0D0 effective chloride diffusion coefcient at time t0kt, ke, kc constants to take account of method of test,

environment and curing on the value of D0n age exponenterf 1 inverse of error functiont0 reference periodt exposure period

The condition tested was for a cover equal to or in excess of 35 mm.Quality control procedures for achieving the specied cover were anticipatedto be good and so a mean value of 37 mm was adopted with a standard


deviation of 2 mm, exponentially distributed. The chloride migration coefcient was determined by a rapid chloride migration method to be 4.75 1012 m2/s (mean value) with a standard deviation of 0.71, normallydistributed. The critical chloride content was adapted from a literature review,taking account of the anticipated humidity, and was taken to be 0.70 per centby weight of binder with a standard deviation of 0.10 per cent, normally distributed. The surface chloride level was taken as 4.00 per cent by weight ofbinder with a standard deviation of 0.50 per cent, normally distributed. Meanvalues and standard deviations were assigned for the coefcients and ageexponent. The value of the reference period, 0.0767 years (28 days) was, ofcourse, deterministic.

An analysis of the use of a 37 mm mean cover yielded a reliability index of1.5 at 100 years and was therefore acceptable.

Summary

The rst generation of European standards enter practice at a time when keyconcepts of probabilistic durability design are well understood. However thebackup data required to allow inclusion of the design methodologies in codesand standards is not yet available. The need exists for renement of the deterioration mechanism models and correlation of laboratory and eld data.The precise direction that the second generation of standards will takedepends on the timing of their publication and on research developments inthe interim. These developments will be in the context of a frameworkembracing the key concepts of service life design and probabilistic analysis.Meanwhile the successful use of service life prediction models, both essentially deterministic and stochastic, in major civil engineering infra-structural projects has been demonstrated.


The durability of concrete is essentially inuenced by processes that involvethe passage, into or through the material, of ions or molecules in the form ofliquids and gases. The service life will be dependent on the rate at which thesespecies may move through the concrete. The passage of these potentiallyaggressive agencies is primarily inuenced by the permeability of the concrete.

Permeability may be dened as the ease with which an ion, molecule or uidmay move through the concrete. This denition is somewhat imperfectbecause the processes involved in uid and ion migration include the distinctmechanisms of capillary attraction, ow under a pressure gradient and owunder a concentration gradient. These mechanisms are characterised by the material properties of sorptivity, permeability and diffusivity respectively.The term permeability has often been popularly used, however, in an all-embracing manner to refer to properties that inuence ingress.

The permeability of concrete to a given agent, for example carbon dioxide,is a function of the pore structure, the degree of interconnection of the porestructure and the moisture content of the permeable pore structure. Thediameter of most ions and gas molecules are smaller than the pores in concrete so even the highest quality concrete will be permeable to some extent. The permeability of a concrete will be predominantly inuenced by the permeability of the cement paste, especially the quality of paste in thecover concrete and at the interface with aggregate particles. The capillary porestructure is particularly signicant. Permeability is a function of the degree of interconnection between the po

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