Tailor Made Concrete Structures – Walraven & Stoelhorst (eds)© 2008 Taylor & Francis Group, London, ISBN 978-0-415-47535-8

New aspects in durability bridge design

M. Empelmann, V. Henke, G. Heumann & M. WichersInstitute for Building Materials, Concrete Construction and Fire Protection (iBMB), TechnicalUniversity of Braunschweig, Braunschweig, Germany

ABSTRACT: The present paper deals with design aspects of the life-cycle optimization for reinforced andprestressed concrete bridges. At the beginning the special requirements regarding the life-cycle of bridges,in comparison to normal concrete structures, will be shown. The minimum life-cycle of the overall bridgeconstruction will then be compared with the life-cycle of single bridge elements. By linking the expenditure formaintenance and repair with the life-cycle of single bridge elements, weak points can be identified. Based onthese results effective approaches to increase the life-cycle of the total structure will then be presented. By meansof a typical example, possibilities able to quantify the effectiveness of optimization measures will be shown.

1 INTRODUCTION

Bridges are important, linking structures in the trafficinfrastructure of each country. Within the Federal roadsystem of Germany approximately 36,000 bridgesexisted until the end of 2001, with an assumed value ofalmost 40 billion euros. Most of these bridges are con-structed with reinforced or prestressed concrete. It canbe assumed that this tendency will still be valid in thefuture, which means that bridges will predominantlybe erected using concrete as the main constructionmaterial.

With regard to the aimed life-expectancy of100 years for bridge constructions, maintenance andrepair strategies have to be developed for the exist-ing bridges (e.g. figure 1), but as well for new bridgeconstructions. The durability has to be used as the

Figure 1. Example of a damaged concrete bridge.

fundamental basis, in order to ensure a sufficient ser-viceability during the life-cycle of the structure. Thisaim can be achieved by the erection of “durable bridgeconstructions”.These have a far higher life expectancyand require a lower maintenance expenditure, i.e. lowercosts for supervision, maintenance and repair.

2 BASIC REQUIREMENTS FOR BRIDGESTRUCTURES AND LIFE-CYCLEENSURING STRATEGIES

Besides the usual requirements concerning theultimate load bearing and serviceability, “DINFachbericht 102” specifies further planning objec-tives for bridge structures. These structures have tobe planned in such a way that the serviceability,with regard to adequate maintenance costs, will bemaintained with an acceptable probability during itslifetime. Reference values for the lifetimes of differ-ent structures are given in DIN EN 1990 (see table 1).

Table 1. Lifetime reference values acc. to DIN EN 1990.

Building Lifetimecategory reference value Example

1 10 years Temporary structures2 10–25 years Exchangeable components,

e.g. bearings or assembly parts3 15–30 years Agricultural and similar

structures4 50 years Normal buildings5 100 years Bridges and similar engineering

structures

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For bridges and similar structures a lifetime of 100years has to be aimed at, which is approximately thedouble lifetime of normal buildings.

The overall lifetime of complex bridge structuresis a function of the lifetimes of single componentsand their interaction. Possible starting points for theoptimization of bridge structures are mainly thosecomponents, which have the maximum influence onthe lifetime and therefore can be seen as so called “hot-spots” within the structure. Numerous approachesfor the identification of these weak spots, with regardto the life-cycle, are possible. In the scope of this paperthe identification of weak points will be based on therelative maintenance and repair necessity of singlecomponents.

3 LIFE-CYCLE AND IMPORTANCE OFBRIDGE COMPONENTS

3.1 Life-cycle of single bridge components

The life-cycle of each bridge component can bedefined as the time span from its construction phaseuntil a complete renewal becomes necessary. A multi-stage quantification of the actual state developmentis not possible, as only very few statistically relevant

Table 2. Time interval reference values for major repairmeasures.

Building material First repair measures after

Concrete 40 yearsSteel (general) 35 yearsSteel (corrosion protection) 20 years

Figure 2. Damage causes according to ENV 1504-9.

data, with the appropriate degradation models, areavailable in Germany.

From the evaluation of different sources (e.g.König, Haardt, Vollrath, Wicke), the reference val-ues of mean lifetimes for bridge structures and singlebridge components could be drawn. These referencevalues are given in table 2 for concrete and steelstructures.

Apart from the steadily increasing traffic actions,the life-cycle of concrete building members is fur-thermore influenced by chemical, physical and partlyby mechanical actions. The main damaging causesaccording to ENV 1504-9 for concrete building mem-ber are compiled in figure 2.

The values for bridge components are partly givenin table 3. Is has to be mentioned here that the life-cycleof pavements and expansion joints depends mainly onthe density of heavy lorry traffic.

3.2 Relative maintenance and repair expenditure

The expenditures for a renewal or complete repair arefar higher than those for maintenance and inspection

Table 3. Reference value for the mean life-cycle of bridgecomponents.

Bridge component Mean life-cycle

Sealing 20 yearsBearings 30 yearsConcrete side walk 25 yearsCrash barrier 20 yearsRailing 30 yearsDrainage 30 yearsNoise protection walls 20 years

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measures, and can only be assessed for the differentbridge elements. The linking of the mean lifetime ofbridge components with the expenditure for a renewalor complete repair shows, which components requirethe highest maintenance and repair expenditure andtherefore have the main influence on the durability ofthe complete structure.

It can be seen in figure 3 that the part “concretesurfaces” dominates with 35%. The parts “pavements”and “sealings” account together for 23%, “bearings”and “expansion joints” together cause 18% of therepair expenditure. This means, that by optimizationof the concrete parts a significant reduction of theexpenditure for renewal or repair can be achieved.Thisapproach will be shown in the following sections.

4 OPTIMIZATION APPROACHES FOR BRIDGECONSTRUCTIONS AND ITS CONCRETECOMPONENTS

4.1 Fundamentals

Within the scope of the own reflections, regardingan improved life-cycle for concrete and prestressedconcrete bridges, four main categories were chosen:“construction”, “structural system”, “concrete tech-nology” and “structural detailing”. As the improve-ment of the actual construction was already dealt within numerous papers, and is a fundamental erectionproblem, this aspect will not be regarded in this papermore closely. In the following the main aspects will

Figure 3. Relative expenditure for renewal or repair ofdifferent bridge components.

be the “structural system”, “concrete technology” and“structural detailing”.

4.2 Structural system

An old design rule specifies that the number of bear-ings and joints in bridges should be kept low inorder to reduce their high construction and mainte-nance costs (design aim: integral bridges). Contraryto this design aim it is often tried to reduce restrainedforces in bridges by the arrangement of bearings andjoints, in order to neglect this forces in the structuraldesign.

Besides, compact cross sections show a muchmore favourable behaviour with regard to damagingenvironmental influences. This means that compactcross sections, such as slabs, wide T-beams or sec-tions with a continues bottom surface are superiorto filigree load carrying systems with regard to theirdurability.

4.3 Concrete technology

The use of high performance concrete (HPC) is veryoften only taken in consideration for bridge con-struction, due to its higher compression strength.Considerations taking durability aspects for bridgesuper-structures into account have so far very seldomcarried out, though HPC has many advantages in thisrespect.

Contrary to normal strength concretes, HPC pro-vides a very high density, especially if silicate flouris used as an additive. Figure 4 shows the develop-ment of capillary porosity in relation to the com-pressive strength. The density has a very positive

Figure 4. Capillary porosity in relation to the compressivestrength, see Fehlhaber (1994).

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influence on the durability, especially on the resis-tance against chemical and corrosive influences on thereinforcement.

Additionally an improved resistance againstmechanical abrasion and frost and / or frost chloride(due to de-icing salts) effects can be obtained. In somecountries HPC has been used for years and favourableexperiences have been obtained e.g. in Scandinavia,America and Australia.

4.4 Structural detailing

The minimum and additional safety concrete coveraccording to DIN 1045-1 is based on the exposureclasses.The concrete cover has to fulfil different tasks:bond securing, protection of the reinforcement againstdamaging chemical influences and protection of thereinforcement against fire. An upper limit of the con-crete cover is not given in DIN 1045-1 nor in thespecial report for bridge design “DIN Fachbericht102”. With an increased concrete cover the protec-tion of the reinforcement against carbonization andchemical influences will be improved.

Damages, which occur in spite of the fulfilment ofconventional serviceability limit state design require-ments, normally are due to a lack of robustness.The robustness is normally secured by constructivemeasures, such as the arrangement of a nominal rein-forcement. Sufficient robustness means that damageswill be noticeable in time (i.e. by large deformations)but their influence will not lead to the complete fail-ure of the structure (i.e. by the formation of plastichinges).

5 EFFECTIVENESS QUANTIFICATION OFSINGLE OPTIMIZATION APPROACHES BYPROBABILISTIC LIFE-CYCLE PROGNOSIS

5.1 Basics of life-cycle prognosis or design

The design of durable bridge structures is, with regardto the economical aspects of the owner or operat-ing company, an optimization task. This means thatit should be tried, by a well considered constructionand material choice, to minimize the erection costs,later maintenance, inspection and repair expenditureand possible closure costs for the whole structure.

The aim is to obtain the overall state of the completestructure by accumulation of single building elementstates. So far, this has been, due to the lack of appro-priate numerical methods and basic data, only beenpossible with a great deal of effort and for simple appli-cations. A method, which has been developed in themeantime, is the probabilistic life-cycle prognosis andits application potential will be shown in the followingsimple example.

5.2 Differentiation between descriptive designconcept and life-cycle design

For time being, the durability of concrete constructionsis assured according to DIN EN 206 and / or DIN1045 by a descriptive design concept. The design isbased on the choice of decisive exposure classes. Tothese exposure classes minimum requirements withregard to the concrete used and the minimum con-crete cover are assigned. Contrary to the load carryingdesign of construction elements, a safety concept isnot recognizable for this procedure, which is sup-posed to secure the durability of a structural elementwith regard to the serviceability limit state design. Forthe structural engineer involved in such a design pro-cedure the meaning and/or relevancy of these limitstate elements (concrete cover, cement content) remainunclear.

These basic deficiencies can be resolved by a proba-bilistic life-cycle design.The basics of such a life-cycledesign and the design fundamentals for the main con-crete damaging factors were mainly formulated inGehlen (2000).

With regard to the durability of bridge structures,the effectiveness of single materials against damagingenvironmental effects, in connection with structuraldetails, can be assessed by a life-cycle design. Theresults of such a design are verifiable and offer the pos-sibility to compare different materials and constructiveinfluences (e.g. concrete cover) with regard to theirinfluence on the durability in a transparent way.

5.3 Example for the use of a life-cycle prognosisand design

Based on a typical prestressed concrete superstructureand by the use of the design model described in Gehlen(2000), the anticipated life-cycle of this building mem-ber and the effect of the main basic variables on thelife-cycle, will be assessed with regard to the influenceof carbonization effects. The reliability calculation forthe determination of the safety index β will be carriedout using the programme system COMREL.

The most important damaging effect on a concretebridge superstructure is the spread of corrosion on thereinforcement. Normally this is induced by carboniza-tion of the concrete or chloride penetration. Thesedamaging mechanisms can be investigated in two dif-ferent time-step life-cycle prognoses for the relevantlocations. The building member will be regarded asdamaged, as soon as the carbonization front or thecritical chloride concentration has reached the steelreinforcement. If this state is reached and no mainte-nance measures will be under-taken, it is unavoidablethat reinforcement corrosion occurs and on the longrun cracks and surface spallings will appear on theconcrete superstructure.

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Figure 5. Regarded bridge cross section and design point U1.

In order to explain the principal procedure, onlythe danger of carbonisation induced corrosion will bedealt with here. With regard to carbonization the bot-tom of the superstructure, which is protected from therain, is decisive. The cross section used for the life-cycle design and the decisive point for the load bearingcapacity in mid-span (U1) is given in figure 5.

According to the German “DIN Fachbericht 102”the exposure class XC 4 has to be used for the environ-mental influence. The nominal value for the concretecover is 45 mm, as the minimum values for the con-crete cover according to “DIN Fachbericht 102” areindependent from the decisive exposure class. Theminimum requirements (see parameter configurationPM1) are given in table 4.

The necessary additional requirements (e.g. for thelocal climate in the region of the structure and themagnitude of the environmental effects) are assumedaccording to the normal bridge design in Germany.It should be considered, that these basic conditionsnormally have to be specified individually for eachbridge.

The reliability index with regard to carbonization,depending on the lifetime of the construction and theparameter constellation according to the minimumrequirements (PM1) is given in Figure 6. After thedefinition of a socially accepted safety level, in anal-ogy to a load carrying design, the derived safety indexcan be compared with the necessary safety level. Thisverification will be carried out for the serviceabilitylimit state. If the reliability of the building member hasto be increased, the practical question: “Which mea-sures or alternatives have the maximum effects?”, hasto be answered. The options, available for the presentexample are:

– Use of a higher strength concrete with a higherdensity

– Increase of the concrete cover.

In order to clarify these influences, a life-cycledesign for the variants PM2 and PM3 was addition-ally carried out. As can be expected and is shown forthe design region U1, the use of a higher strength con-crete (PM2) or an increased concrete cover by 1 cm

Table 4. Compilation of the basic design parameters.

Parameter Concreteconfiguration Concrete cover cnom Remarks

PM1 C35/45 45 mm Minimum requirementaccording to “DINFachbericht 102”

PM2 C60/75 45 mm Use of a higherstrength concrete

PM3 C35/45 55 mm Use of an increasedconcrete cover

PM4 C35/45 35 mm Use of an reducedconcrete cover

PM5 C60/75 35 mm Use of an reducedconcrete cover anda higher strengthconcrete

Figure 6. Time-dependent limit state reliability of theinvestigated building member for corrosion induced bycarbonisation.

(PM3) can increase the life-cycle of the structure, forthe same target reliability, considerably, see Figure 6.But the results show too, that the descriptive procedureaccording to “DIN Fachbericht 102” (PM1) results in

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a sufficient reliability (reliability index between 1.3and 2.3 according to Joint Committee on StructuralSafety), but this requires a faultless construction.

The additional investigations (PM4 and PM5) showthe high influence of a reduced concrete cover on thetime-dependent development of the reliability index,which can occur due to inadequate construction.

It can also be shown, that by the use of a higherstrength concrete the target reliability according to“DIN Fachbericht 102”, despite of the reduced con-crete cover, can be reached.As far as corrosion inducedby carbonisation is concerned, the use of a higherstrength concrete can provide an additional safetyreserve in this respect.

This design example shows, that by the use of aprobabilistic life-cycle prognosis as a design instru-ment for securing the durability of a structural element,it is possible to obtain a very differentiated informationabout the building element under investigation.

Additionally, the reliability of the construction canbe increased by selective optimization measures, andthus can be secured for the relatively long bridgelife-cycle. Specific aspects, such as the environmen-tal exposure, as well as the economical interest of theowner and/or operating company can be incorporatedin such a “performance-based-design”.

6 SUMMARY

In the future the already existing bridges will requireenormous maintenance and repair measures. There-fore, for new constructions the design should befocussed on durable and low-maintenance bridge con-structions. In the scope of this paper considerationsconcerning the life-cycle of single building elementsand the relative future maintenance and repair effortsas well as a weak-point analysis was carried out. Onthis basis conclusions on the improvement of concretebridges can be drawn.

By the use of new building materials, e.g. highperformance concrete, it is possible to optimize themain influence parameters with regard to the durabil-ity, i.e. concrete density and concrete cover.The choiceof an appropriate cross-section offers the possibilityto reduce the surfaces exposed to the environment.Constructive measures, such as an increased con-crete cover, further increase the durability and are anapproach for a “performance-based-design” definedby the operating company.

All measures aimed at increasing the durabilityreflect more or less in the construction costs. Nev-ertheless, it is profitable to incorporate measuresfor increasing the durability with regard to “low-maintenance bridges” already in the design, as theinitial investments have to be compared with the higherlifetime and lower maintenance costs of the structure.

Additionally, the very often neglected economicalaspects, which result from construction sites andincreased pollution due to traffic hindrance, have tobe evaluated financially.

The influences of the proposed optimization mea-sures can be quantified by a life-cycle prognosis anddesign. This was shown with a simple example. Forthe time being there is a lack of systematic considera-tions and engineering design models for the proposedoptimization possibilities. With regard to an over-all durability optimization for bridge constructions,further research efforts are necessary.

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