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Bridge Management and Condition Monitoring

by

A Blankvoll, F Fluge, C K Larsen , I Markey- NPRAA Raharinaivo - LCPC

L Bevc, M Capuder, I. Peru - ZAG

Deliverable D8PL97-2220

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D8: Bridge Management and Condition Monitoring

BRIME PL97-2220 PAGEi

CONTENTS

Page

EXECUTIVE SUMMARY ............................................................ ....................................................... iii

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

1.1 OBJECTIVES OF THE RESEARCH ...................................................................................................2

2 CHLORIDE INGRESS MODELS ....................................................... ..................................... 3

2.1 FICKS SECOND LAW (REFERENCE MODEL)..................................................................................4 2.1.1 The physical condition of diffusion........................................................................................42.1.2 The mass balance of diffusion ...............................................................................................52.1.3 The diffusion coefficient ........................................................................................................62.1.4 The initial condition ..............................................................................................................72.1.5 The boundary condition.........................................................................................................72.1.6 Solutions to Ficks laws.........................................................................................................7

2.1.6.1 Steady-state diffusion the diffusion-cell technique .................... ..................... ..................... ... 72.1.6.2 Non steady-state diffusion the concentration-profile technique .................. ...................... ...... 8

2.2 LIGHTCON MODEL.......................................................................................................................9 2.2.1 Theoretical background.........................................................................................................92.2.2 Environmental load ....................................................... ...................................................... 102.2.3 Values for the -parameter .................................................... ............................................. 102.2.4 Diffusion coefficient................................................................ ............................................. 102.2.5 Effect of Maturity on Diffusion......................................................... ................................... 112.2.6 Values selected for LightCon calculations ........................................................... ............... 11

2.3 THE HETEK-MODEL.................................................................................................................11 2.3.1 Introduction ......................................................... ........................................................... .....112.3.2 Assumed available information ........................................................ ................................... 122.3.3 Assumptions of the HETEK-model ..................................................... ................................... 12

2.3.4 Mass balance of chloride in an element volume of concrete................................ ............... 132.3.5 Achieved chloride diffusion coefficient................................................................. ............... 132.3.6 Boundary condition ....................................................... ...................................................... 142.3.7 Plotting the chloride profiles........... ........................................................... ......................... 16

2.4 DIFFERENCE BETWEEN HETEK AND LIGHTCON MODELS .....................................................17 2.4.1 Introduction ......................................................... ........................................................... .....172.4.2 The LIGHTCON-model and the HETEK-model ........................................................ ............... 172.4.3 Comparison of solutions......................................................................................................182.4.4 Comparison of chloride profiles..........................................................................................182.4.5 Differences in the predicted initiation period of time..................................................... .....192.4.6 Example ..................................................... ........................................................... ............... 202.4.7 Summing up.........................................................................................................................22

2.5 CONDITIONAL AVERAGE ESTIMATOR - HYBRIDNEURALNETWORK.........................................22

3 CONDITION SURVEY DATA..................................................................................................25

3.1 INTRODUCTION..........................................................................................................................25 3.2 MEASUREMENT TECHNIQUES ....................................................................................................26

3.2.1 Norway ...................................................... ........................................................... ............... 263.2.1.1 Collecting the samples .................... ..................... ..................... ..................... ...................... .... 263.2.1.2 Determination of the chloride content.... ..................... ..................... ..................... ................... 27

3.2.2 Slovenia ..................................................... ........................................................... ............... 273.2.2.1 Collecting the samples .................... ..................... ..................... ..................... ...................... .... 273.2.2.2 Determination of the chloride content..................... ...................... ..................... ..................... . 28

3.2.3 France .................................................................................... ............................................. 293.3 DESCRIPTION OF BRIDGES..........................................................................................................29

3.3.1 French Bridge A11 PS 12-10 ........................................................ ................................... 293.3.2 Norwegian Bridges................ ........................................................... ................................... 303.3.2.1 Gimsystraumen Bridge....... ..................... ..................... ...................... ..................... ............... 30

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BRIME PL97-2220 PAGEii

3.3.2.2 Hadsel Bridge................ ..................... ..................... ...................... ..................... ..................... . 323.3.2.3 Sandhornya Bridge......... ...................... ..................... ..................... ..................... ................... 33

3.3.3 Slovenian Bridges................................................................................................................333.3.3.1 Viaduct kedenj 1 ................... ...................... ..................... ..................... ...................... ........... 343.3.3.2 Viaduct Preloge......... ...................... ..................... ..................... ..................... ...................... .... 353.3.3.3 Viaduct Slatina... ..................... ...................... ..................... ..................... ...................... ........... 373.3.3.4 Viaduct epina... ..................... ...................... ..................... ..................... ...................... ........... 373.3.3.5 Viaduct Ivanje Selo..................... ..................... ...................... ..................... ..................... ........ 39

4 CALCULATIONS AND RESULTS....................................................... ................................... 41

4.1 CALCULATION PARAMETERS .....................................................................................................41 4.2 RESULTS NORWEGIAN BRIDGES .............................................................................................42 4.3 RESULTS SLOVENIAN BRIDGES ...............................................................................................44 4.4 RESULTS FRENCH BRIDGE ......................................................................................................45 4.5 PREDICTION OF CURRENT CHLORIDE CONTENT..........................................................................46 4.6 DISCUSSION...............................................................................................................................47

5 PROBABILISTIC APPROACH TO SERVICE LIFE............................................................49

6 MONITORING AND BRIDGE MANAGEMENT..................................................................53

6.1 INTRODUCTION..........................................................................................................................53 6.2 GIMSYSTRAUMEN BRIDGE REPAIR PROJECT ...........................................................................53 6.3 WHY INSTALLA COMPUTERISED MONITORING SYSTEM?.........................................................54 6.4 WHAT SHOULD BE MONITORED?..............................................................................................55 6.5 WHERE TO INSTRUMENT? ................................................... ...................................................... 586.6 BMS, MONITORING AND CHLORIDE INGRESS MODELS .............................................................58

6.6.1 Routines for new and existing structures... ........................................................... ............... 596.6.1.1 Method of Inverse Cores.................... ..................... ...................... ..................... ..................... . 62

7 CONCLUSIONS AND RECOMMENDATIONS ................................................... ............... 65

8 FUTURE WORK ........................................................ ........................................................... .....67

9 REFERENCES............................................................................................................................69

Annexe A: Condition Survey Data

Annexe B: Results from the Chloride Ingress Models

Annexe C: Results from the Predicted Chloride Ingress

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BRIME PL97-2220 PAGEiii

EXECUTIVE SUMMARY

SCOPE

Europe has a large capital investment in the road network including bridges, which are

the most vulnerable element. As bridges age, deterioration caused by heavy traffic andan aggressive environment becomes increasingly significant resulting in a higherfrequency of repairs and possibly a reduced load carrying capacity.

The purpose of the BRIME (Bridge Management in Europe) project is to develop aframework for the management of bridges on the European road network that enables

bridges to be maintained at minimum overall cost i.e. taking all factors into accountincluding condition of the structure, load carrying capacity, rate of deterioration, effecton traffic, life of the repair and the residual life of the structure.

SUMMARY

This report presents the results of one of the eight work packages of the Europeanfunded project BRIME. In brief, the report deals with modelling chloride ingress inconcrete bridges and monitoring corrosion within a bridge management system. Furtherdetails concerning the project BRIME are given in the other project deliverables and a

brief overview is given on the TRL web-site (http://www.trl.co.uk/brime/index.htm).

Prediction of deterioration is an important aspect of bridge management for estimationof remaining service life and planning future maintenance tasks. The objective of the

work package was to consolidate and improve existing knowledge concerning themodelling and surveillance of chloride penetration into concrete. Chloride ions areconsidered the primary cause of corrosion in concrete bridges. The results of this work

package will help public authorities establish investigative procedures to monitor thedanger of and predict corrosion of their concrete structures. As such it will be animportant tool in:

increasing the durability of new concrete structures by allowing the identificationof and ranking in order of importance the predominate factors affecting corrosion;

deciding the optimal time to carry out preventative maintenance or repair; assisting in long term budget planning.

The work package had the following three main tasks:

1. Creation of a databank of condition parameters for several concrete bridgesincluding local exposure conditions (micro-climates). Some extra investigationswere performed to confirm/confute the predictions made by the models selected inTask 2.

2. Selection, use and assessment of several chloride ingress models. The modelsinvestigated were: Ficks 2nd law (used as reference model); Selmer - Poulsen

model (LightCon model) with improvements by Mejlbro (Hetek model). The useof neural network models was also evaluated.

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BRIME PL97-2220 PAGEiv

3. Investigate the requirements of a bridge management system that incorporatesprediction models, condition surveys and monitoring. Assessment of residualservice life and a probabilistic approach is also addressed.

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BRIME PL97-2220 PAGE1

BRIDGE MANAGEMENT AND CONDITION MONITORING

1 INTRODUCTION

The report presents the results of the Work Package 4 (WP4). The work package has limiteditself to dealing with modelling chloride ingress (initiation phase only) and monitoring corrosionwithin a bridge management system (BMS). This was due to the fact that the project had alimited time scale and that corrosion of reinforcement is the most serious and widespread typeof damage for bridges. As such the work package has not investigated other types of damagee.g. freeze/thaw, carbonation, sulphate attack which were treated to a certain extent in WorkPackage 3 [22]. Most deterioration types are described by one or more models [18] and themajority are under continuous development. The models differ in their accuracy and complexity

and frequently dont take more than one damage mechanism into account. In fact, quantificationof real damage to the structure or reduction to the bearing capacity is usually an extrapolation,

based on engineering judgement, of the results of models and investigations rather than a directresult of a model.

Figure 1.1 Bridge in exposed marine climate

The benefits of being able to predict the future condition of an element, or of the entirestructure, are discussed in Work Package 7 [26] and can be briefly summarised as:

allow the right maintenance/repair operation to be performed at the right moment in timethereby optimising the maintenance budget for that structure

optimise the long term budget of the bridge stock forecast the safety index of individual structures and of the bridge stock see the effect of non-optimal budget strategies

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As mentioned, this report does not cover all these topics. Similarly, where monitoring isconcerned, the report confines itself to the subject of corrosion monitoring of concrete bridgeswhere the corrosion is initiated by chloride ions. To ensure that the reader does not spendunnecessary time, the report does not treat:

corrosion propagation other damage types, or combination of other damage types with reinforcement corrosion deterioration of structure deterioration of bridge stock

1.1 Objectives of the research

Prediction of deterioration is an important aspect of bridge management for estimation ofremaining service life and planning future maintenance tasks. The objective of the work

package is to consolidate and improve existing knowledge concerning the modelling andsurveillance of chloride penetration in concrete. Chloride ions are considered the primarycause of corrosion in concrete bridges. The results of this work package will help publicauthorities establish investigative procedures to monitor the danger of and predict corrosionof their concrete structures. As such it will be an important tool in:

increasing the durability of new concrete structures by allowing the identification of andranking in order of importance the predominate factors affecting corrosion;

deciding the optimal time to carry out preventative maintenance or repair; assisting long term budget planning.

Work package 4 had the following three main tasks:

1. Creation of a databank of condition parameters for several concrete bridges includinglocal exposure conditions (micro-climates). Extra investigations are performed toconfirm/confute the predictions made by the models selected in Task 2.

2. Selection, use and assessment of several chloride ingress models. The models investigatedare: Ficks 2nd law (used as reference model); Selmer - Poulsen model (LightCon model)with improvements by Mejlbro (Hetek model). The use of neural network models is alsoevaluated.

3. Investigate the requirements of a bridge management system that incorporates predictionmodels, condition surveys and monitoring. Assessment of residual service life and a

probabilistic approach is also addressed.

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2 CHLORIDE INGRESS MODELS

Chloride penetration is mainly due to a combination of chemical and physical processes. Themost important processes are:

Diffusion; a process due to a gradient of chloride concentration in the concrete. Gradientmeans that the chloride concentration is higher at one location than at another. This isshown in figure 2.1 for the typical case where the concrete surface has a higherconcentration than in its core. As chloride is dissolved in water, diffusion process occursonly in pore solution inside concrete,

Capillary suction of chloride contaminated water; a process that takes place in empty orpartly filled concrete pores. It means that water (moisture) content and concrete porosityare the main parameters that influence capillary suction.

Some chloride binding reactions occur between cement components (chloroaluminates, etc.)

and chloride ions. These reactions are either physical adsorption, chemical reaction, or acombination of the two. Chloride binding is strongly influenced by climatic conditions. Allthe prediction models, which were applied in BRIME project, are based on the diffusion

process but they include several supplementary assumptions. The models are :

Fick's law describes a pure diffusion process. Any diffusion law is valid only in concretewhich is permanently saturated with water. It means that it is not valid in the concretesurface layer which sometimes can be dry. So, climatic conditions and concrete porositydetermine how thick this concrete layer is where the diffusion law does not apply.

LightCon and Hetek models are based on the diffusion process. However, boundaryconditions that are constant in Ficks law may be time dependent, e.g. chloride content on

concrete surface. Concrete porosity and cement type are important parameters in thismodel.

Conditional Average Estimator - Hybrid Neural Network (CAE-HNN). In this model,determination of the whole chloride profile at a certain location is based on a set ofmeasured data with similar features using neural networks. The ingress of the chlorides is

based on diffusion. For this reason a substitute diffusion coefficient is calculated betweenthe measured points. As there is currently not enough data available of chloride profiles atthe same locations at different times, Fick's law is used to make time prediction.

It should be noticed that the diffusion coefficient is determined for a given substance

(chloride ion, etc.) entering a given material. If this material changes, for example, afterageing, this coefficient also changes.

It was initially planned to use two other models: Vesikari and Steen. However, the analysistool for the Steen model was not obtained and the time necessary to develop an equivalenttool outweighed the possible benefits of its use. The Vesikari model is based on a feature ofdiffusion law, which states that a relationship exists between times t and depths L, for whichchloride content has a given value (t = K.L2). According to this model, factor K depends onconcrete water-cement ratio and on the environment. This model can be used in the design

phase for concrete bridge decks, but not with condition survey data. As such it was notpursued in this project.

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Extensive investigations on chloride ingress in concrete form the basis for the research onchloride induced rebar corrosion. In this project, observations are taken from real structuresmade of different concrete grades, and subjected to different environmental conditions andexposure times.

Cs

Ct

Cmax

xt

C :Computed chloride content

on the concrete surfaces

C :Maximum recorded

chloride content

max

C :Chloride threshold

value

t

Depth of Chloride Ingress

ChlorideContent

Figure 2.1 Simplified model for chloride ingress in concrete

The chloride profile, figure 2.1, is a simple illustration of some of the essential parameterswhen modelling chloride ingress in concrete and hence, also the service life of the structure.As a simplification the design service life is frequently taken as the initiation period and the

propagation period is conservatively neglected.

2.1 Ficks second law (reference model)

In 1855Adolf Eugen Fick (1829-1901) formulated the laws of diffusion by direct analogywith the equation of heat conduction presented by J. B. J. Fourier in 1822. However, the

application of Ficks laws to chloride ingress into concrete appears many years later whenpresented by Collepardi et al in the early seventies. Thus the study of chloride diffusionprocesses in concrete technology is still fairly young.

2.1.1 The physical condition of diffusion

A. E. Fickdefined the fundamental concept of the flux J(here for the transport of chloride inconcrete) as the transport (of chloride) through a unit area of a section (of the concrete) perunit of time, and he formulated the physical condition for diffusion in analogy withFourierswork. This condition, calledFicks first law of diffusion,says (for concrete) that - the flux (of

chloride) is proportional to the concentration gradient (of chloride) measured normally to thesection, i.e.:

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x

CDJ

= (1)

Where:

Jis the flux of chloride per unit area Cis the concentration of chloride at the abscissax [mol/m3] Dis the chloride diffusion coefficient [m2/s] (the factor D, which A. E. Fickcalled: the

constant depending on the nature of the substances).

The negative sign in Eq (1) arises because diffusion of chloride occurs in the directionopposite to that of increasing concentration of chloride. Eq (1) is the simplest possiblerelation between flux and gradient. In words the flux of chloride is proportional to thediffusion coefficientDand the gradient C/xof the chloride profile at the abscissa xand thechloride tends to diffuse against localities having less chloride concentration.

There are cases where such simple relation as Eq (1) should not be applied. The diffusionprocess may be irreversible or has a history-dependence. In such cases more complicatedrelations have to be applied. However, for normal chloride ingress into concrete no tests have

proved thatFicks first law of diffusionis not valid.

2.1.2 The mass balance of diffusion

When chloride diffuse into concrete a change of the chloride concentration Coccurs at anytime tin every pointxof the concrete, i.e. a non-steady state of diffusion.

Consider a small volume of the concrete. At time tthis volume contains a concentration ofdissolved chloride (in pore liquid) and bound chloride (chemically bound to the hydration

products of the cement and physically bound to the surface of the cement gel). During a smalltime interval the concentration of chloride will change, an amount of chloride will diffuseinto the volume and another (smaller) amount will diffuse out of the volume. Formulatingthat the change of (the total) chloride concentration of the volume is the difference betweenthe flow of chloride into the volume and the flow of chloride out of the volume the followingrelation is obtained:

dxxJdx

tC

=

(2)

wheret

C

is the rate of change in concentration of the solute [mol/m3s]

This equation of mass balance will be valid for all properties of concrete. Where Ficks firstlaw Eq (1) is valid, the relation

=

x

C

Dxt

C

(3)

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=

x

CCD

xt

C)( (7)

Constant chloride diffusion coefficient.The chloride diffusion coefficient of the concrete

is independent of location x, time t and chloride concentration C, i.e. D = D0. Thiscondition is e.g. used in Collepardisequation and when the chloride diffusion coefficientis determined by testing. In these cases,Ficks second law of diffusionyields:

2

2

0 t

CD

t

C

=

(8)

This form Eqn (8) of the diffusion equation is often erroneously called Ficks second lawof diffusion. However, Eqn (8) is only a special case ofFicks general law of diffusion,cf.Eqn (3).

2.1.4 The initial condition

The initial condition of the concrete is often taken as C(x,0) = Ci = a constant, which isequally distributed through the concrete. This may not be correct but it is convenient, since itmakes the solution simpler. The condition is not important and in many cases just Ci= 0 issubstituted.

2.1.5 The boundary condition

The boundary condition is of great importance to the solution, since it has a great influence onthe solution (the chloride profile). The main difference between the LIGHTCON-model and theHETEK-model is that the surface chloride content is constant when applying the LIGHTCON-model, while the surface chloride content is time dependent when applying the HETEK-model.Section 2.4 deals with the difference between HETEKand LIGHTCONin detail.

2.1.6 Solutions to Ficks laws

2.1.6.1 Steady-state diffusion the diffusion-cell technique

C = constant

l

x

CC

CCx =

12

1

Cxis the concentration of the diffusant at depthxC1is the concentration of the diffusant at the up-stream surface

C2is the concentration of the diffusant at the down-stream surface

C1

C2

x

0 l

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The diffusion coefficient may be determined by plotting against time how much diffusant, Qt,(e.g. chloride) diffuses through a sample of thickness l :

=

D6

21 lt

lCDQt

2.1.6.2 Non steady-state diffusion the concentration-profile technique

=

tD

x

CC

CC

s

tx,

4

erfc0

0 or

+=

tD

xCCCC stx,

4erfc)( 00

Cx,t is the concentration of the diffusant at depthxat time tC0 is the initial concentration of the diffusant in the sampleCs is the concentration of the diffusant at the surfaceerfc(..) is the error-function complement

1/6

Time lag

2

l

tD

lC

Qt1

Cs

x

Cx,t

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The diffusion coefficient (and the surface concentration) is determined by fitting theconcentration profile to the equation.

Necessary data for the model are:

Concentration profile, i.e. the concentration in the sample at various depths Exposure time Initial concentration in the sample, i.e. C0= Cx,0(often zero) Surface concentration (often not known, and estimated from the

concentration profile and the equation)

2.2 LightCon model

2.2.1 Theoretical background

It is assumed that chloride ingress into concrete obeys Fick's second law of diffusion for asemi-finite medium with constant exposure, and that there is a critical value of the chloridecontent in the concrete, C= Ccr, leading to the corrosion of steel.

The service lifetime tLTin this model is defined as the age of the structure when the chlorideconcentration Ca(c,tLT) at the depth of the rebar with cover c reaches the critical (threshold)chloride concentration Ccr for initiating corrosion. The development of the mathematicalmodel is presented in detailed in [14, 28]. The result of the development is the equation:

(1)

where tLT is the service lifetimetc is the time when the structure is inspectedc is the concrete cover

Dac is the diffusion coefficient calculated from the chloride profile in the structureat time t0

is a parameter expressing the exposure dependency of the diffusion coefficient

Da as given by equation 3. is a parameter given by the equation

(2)

where erfc is the error function in Fick's second law of diffusionCcr is the critical (threshold) chloride concentrationCi is the background chloride content

Csa is the chloride content on the exposed surface (the "load")

)Dtc(t=t

acc

-

cLT

))1/(2(

)C-C

C-C(2=isa

icrerfc 1-

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Based on this theoretical background, a number of different situations may be handled asdiscussed in [30].

2.2.2 Environmental load

The effect of the environment is represented by the Csa. This parameter identifies therepresentative chloride concentration at the concrete surface during the time of exposure. Thevalue of Csadepends both on the salinity of the water, the porosity of the surface layer (andthus the amount of saline pore water) and the length of wetting versus drying in the splashzone.

In calculating the value of Dsa from a measured chloride profile according to least-squarescurve fitting, the parameter Csa is represented as the chloride concentration at the surface.

The microclimate is of great importance. The parameter Csa depends very much on heightabove sea level and the wind direction (the windward/leeward-effect), most probable due towashing off chlorides by rain water on the exposed side.

2.2.3 Values for the -parameter

The achieved or in-situ chloride diffusivity for the skin concrete exposed to sea water, Da(t),is a time dependant parameter varying with concrete composition and environmentalconditions. It has been shown that Da(t)can be expressed as a function of the maturity age ofthe concrete t as:

(3)WhereDa0is the achieved chloride diffusion coefficient at the maturity age t0of the concrete,normally taken as the age when the exposure starts. Dacat concrete age tcis determined fromthe actual chloride profile on a drilled core from the structure according to APM 207 orequivalent. The parameter depends both on the material and the environment (reduced

permeability in the concrete skin due to beneficial interaction with the sea water).

The exponent represents the decrease in the achieved diffusion coefficient with age due tothe combined effect of hydration and all other mechanisms due to ion-exchange with theseawater that might reduce the porosity of the concrete skin.

2.2.4 Diffusion coefficient

The chloride diffusion coefficient,Da, is decreasing with age, represented by the -parameteraccording to equation 3. To compare Da for different concrete mixtures, the curing time

before exposure to seawater should be the same and Da should be calculated for a certainexposure time.

)tt(D=(t)D 0a0a

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The parameter is calculated according to least-squares curve fitting, based onDavalues fordifferent exposure times. When Da and are known for a certain mix, Da values may becalculated for any times. This technique is used to compareDafor different types of mixtures,curing times etc. The Da at 28 days, D28, is calculated based on Da and according toequation 3.

The parameter will normally be affected by the water-binder ratio and curing times. A shortcuring time before exposure to seawater will result in a somewhat higher compared to along curing time. This means that Da will be reduced faster with age compared to thesituation when the curing time is long. The consequence of this is that Dawill come closer toeach other after long time of exposure independent of curing time. However, the high Daatthe beginning of the exposure will result in higher chloride content in the concrete comparedto the situation when the curing time is long.

2.2.5 Effect of Maturity on Diffusion

The chloride diffusion coefficient varies with the maturity tof the concrete. Test results [15]fit well to a hypotheses supposing that the chloride diffusion coefficient is decreasing withincreasing curing time. In a double logarithmic system, this correlation is a straight withinclination expressed by the parameter . This means that a mathematical model may beexpressed as:

D(t) = Do(to/t) (4)

Here D(t) is the time dependent chloride diffusion coefficient, t is the maturity age of theconcrete, tois a reference maturity age typical of the concrete (e.g. 28 days), andDoand are

parameters to be determined by a regression analysis of test results.

2.2.6 Values selected for LightCon calculations

As mentioned above, and are calculation parameters that can be varied in the ingressmodels. For the purposes of this study and based upon the findings in [15], their values are

fixed for all calculations as follows:

= 0.6= 0.15

2.3 The HETEK-model

2.3.1 Introduction

A complete description and documentation of the HETEK-model Version 1997 is in [14]. Theentire series of HETEK-reports are recommended when a detailed study of the model is

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2.3.4 Mass balance of chloride in an element volume of concrete

The equation of mass balance (equilibrium) for chloride in concrete is expressed by Ficksgeneral law of diffusion:

=

x

CD

xt

C (1)

where C is the chloride content at the locality x at time t and D is the chloride diffusioncoefficient which may depend on the time t, the locality x, the chloride content C, and thecomposition and environment of the concrete. However, it is an assumption of the HETEK-model that the chloride diffusion coefficient D is independent of the locality x and thechloride content.

2.3.5 Achieved chloride diffusion coefficient

Diffusion coefficient after 1 year of exposure. The study of observations from theTrslvslge Marine exposure Station, cf. [15], has led to the conclusion that the chloridediffusion coefficient after (exactly) 1 year of exposure with suitable accuracy may beexpressed as (see [14], p. 114 Table 13.2.4:1):

{ }

=

Dw/beqv

10exp000,251 DkD mm2/yr (2)

where the equivalent water/binder-ratio (ratio by mass) with reference to diffusion is (see[14], p. 114 Table 13.2.4:2):

{ }SFFAPC

MW

++=

7eqv Dw/b (3)

Here

MW = concretes content of mixing water

PC = concretes content of portland cement FA = concretes content of fly ash SF = concretes content of silica fume.

The factor kD, cf. (3), depends on the concretes environment in the following way:

=

(SUB)seawaterinsubmergedconcretefor1.0

(SPL)zonesplashmarinetoexposedconcretefor0.6

(ATM)atmospheremarinetoexposedconcretefor0.4

Dk (4)

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{ }SFAPC

MW

+=

5.175.0eqv bw/b (10)

Here

MW = concretes content of mixing water PC = concretes content of portland cement FA = concretes content of fly ash SF = concretes content of silica fume.

The factor kb, cf. (3), depends on the concretes environment in the following way:

=

(SUB)seawaterinsubmergedconcretefor5.15

(SPL)zonesplashmarinetoexposedconcretefor3.70

(ATM)atmospheremarinetoexposedconcretefor2.20

bk (11)

NT Build 443. When testing according to NT Build 443 is carried out determining Csp at anexposure of 14 to 28 days, C1is proportional to Csp, cf. [14], p. 112. Therefore, it is possibleto write:

45.151spbCkC (12)

where kbis given by (11). Equation (12) may substitute or supplement equation (9). However,

if the testing is carried out at the time of inspection a correction is needed, cf. [28].

Method of inverse cores. If a determination of C1has been made according to the method ofinverse cores, cf. [27], this determination may substitute or supplement equation (12).

Development of the chloride content of the concrete surface.The achieved chloride content ofthe exposed concrete surface Csa versus time obey the following relation, cf. [14], p. 12,(3.2.2:3):

ppisa SCC += (13)

Here Ci is the initial (uniformly distributed) chloride content of the concrete. The timeparameter is defined as cf. [14], p. 12, (3.2.2:5):

=

=

t

t

t

t

t

t

t

t ex

exex

ex

1

1 (14)

The exponents andpand the factor Spdepend on the composition and environment of theconcrete.

The chloride content of the concrete surface at the time of inspection . By inspecting and

testing (determination of a representative chloride profile) at time of inspection (unit: years)

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aexexnn Dtux = 4 (20)

2.4 Difference between HETEK and LIGHTCON models

2.4.1 Introduction

The difference between the two models for predicting chloride ingress into concrete, theLIGHTCON-model, cf. [15] and [28], and the HETEK-model, cf. [14], lies mainly in how the

boundary condition is taken into account:

The LIGHTCON-model assumes that the achieved chloride content Csa of the exposedconcrete surface remains constant through the initiation period of time, except a period ofapprox. 2 years, cf. [28]

The HETEK-model assumes that the achieved chloride content Csaof the exposed concretesurface depends on time t. It is assumed that Csaincreases from the time of first exposureto chloride tex, cf. [15].

This chapter deals with the effect of this difference. It is assumed that the reader is familiarwith the notation of the HETEK-model, cf. [28].

2.4.2 The LIGHTCON-model and the HETEK-model

The LIGHTCON-model and the HETEK-model may be described mathematically by thefollowing equation:

( ) ( ) ( )ufCCCtxC isai +=, (1)

where

=+

=appliedisHandfor1where,

appliedisLandyears2henconstant wais

ETEK

IGHTCON0

exex

ex

ppi

satt

t

t

t

tSC

tC

C (2)

As a consequence of these boundary conditions the functionf(u) yields

=appliedisHwhen,)(

appliedisLwhen,)erfc()(

ETEK

IGHTCON

u

uuf

p

(3)

where

aexexDt

xu

=

5.0 (4)

For identical composition of two concretes exposed to the same type of environment the otherparameters, i.e. , texandDaexof the LIGHTCON-model and the HETEK-model are the same.

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2.4.3 Comparison of solutions

Under identical conditions the difference between the chloride content of concrete at the levelof the steel reinforcement bars with a concrete cover cyields:

( ) ( ) )()erfc(, 0 cpp

pci uSuCCtcC = (5)

where

aexex

cDt

cu

=

5.0 (6)

The chloride content of the exposed concrete surface C0 has to be chosen when theLIGHTCON-model is applied. Magne Maage et al, cf. [15], recommends that C0is chosen so

that C0 is greater than or equal to the achieved chloride content of the exposed concretesurface during a required time less than the initiation period of time, i.e. tobs= tcr. If so, wehave

obsexp

pi tttpSCC > and0allfor0 (7)

Since

0alland0allfor)()erfc( >>> upuu p (8)

cf. [28, 29], it is seen that C(c, t) > 0 for allp> 0 and all texttobs. This means that thechloride content at the level of the reinforcement is greater when applying the LIGHTCON-

model compared with the HETEK-model, all other parameters kept equal.

However, in most cases Csafor the LIGHTCON-model is chosen equal to the value found by aninspection carried out when the concrete has reached an age of about 10 to 20 years. In suchcases it is not possible to say if the chloride content of the concrete next to the reinforcementis greater or smaller when found by means of the LIGHTCON-model compared with theHETEK-model.

2.4.4 Comparison of chloride profiles

The shape of the graph of the error function complement erfc(u) and the shape of the graph ofthe p(u) function are different for p> 0. In fact the error function complement is identicalwith the pfunction forp= 0, i.e.

0allfor)()erfc( 0 = uuu (9)

Chloride profiles determined by the LIGHTCON-model and the HETEK-model for p> 0 yieldslightly different shapes.

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2.4.5 Differences in the predicted initiation period of time

The LightCon-model

The initiation period of time tcr according to the LIGHTCON-model may be determined from

the equation:

( )aexex

iiDt

cCCCC

cr

0cr

5.0erfc

+= (10)

Introducing the term

i

iL CC

CC

=0

crerfcinv2 (11)

Equation (10) may be written as2

cr

=

aexexL Dt

c (12)

Since

=

1

cr

cr

crcr 1

ex

ex

ex t

t

t

t

t

t (13)

when tcr>> tex(which normally is the case) Equation (12) may be written as

=

1

2

cr

aexexL

exDt

ctt (14)

The Hetek-model

The initiation period of time tcraccording to the HETEK-model may be determined from theequation, introducing the p-function, cf. [16]:

+=

+=

aexex

p

p

aexex

pi

aexex

pp

piDt

c

Dt

cSC

Dt

cSCC

cr

2

cr

cr

5.05.05.0 (15)

Introducing the term

=

p

aexex

p

iH c

Dt

S

CC2

cr

5.0inv2 p (16)

Equation (15) may be written as

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2

cr

5.0

=

aexexH Dt

c (17)

Since

=

1

cr

cr

crcr 1

ex

ex

ex t

t

t

t

t

t (18)

when tcr>> tex(which is normally the case) Equation (17) may be written as

=

1

2

cr

5.0

aexexHex Dt

c

tt (19)

Initiation period

The threshold value of chloride content in concrete Ccrapplied in Equations (11) and (16)may be found by the following equation:

{ }( )w/ceqv2.4expcrcr =kC unit: % mass binder (20)

where the equivalent w/c ratio with respect to the threshold value of chloride in concreteyields:

{ }SFC

W

5.3eqv

=w/c (21)

Here W, C, F and S denote mixing water, Portland cement, fly ash and silica fumerespectively. The environmental factor kcris found to be:

=(SUB)seawaterinsubmergedisconcretewhen5.2

(SPL)splashmarinetoexposedisconcretewhen2.0

(ATM)atmospheremarineatoexposedisconcretewhen2.0

crk (22)

2.4.6 Example

In [28] and [29] examples of predicting the chloride ingress were calculated for a specifiedconcrete exposed to an environment of marine splash. The data were:

Concrete

The binder of the concrete is ordinary Portland cement The water/binder-ratio is w/c= 0.48 by mass

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The chloride content of the concrete in unit % mass binder is 6.11 times the chloridecontent in unit % mass concrete

The concrete is exposed to chloride for the first time at tex= 0.027 year

Inspection

The concrete has the age of tin= 18 years on the inspection Achieved diffusion coefficientDain= 32.9 mm2/yr was determined at time tin= 18 years Achieved chloride content of the chloride exposed concrete surface was determined by

testing to Csin= 0.333 % by mass concrete at time tin = 18 years. This corresponds to achloride content of 0.333 6.11 = 2.035 % by mass binder

Initial chloride content of the concrete Ci= 0.008 % mass concrete = 0.008 6.11 = 0.049% mass binder was determined

Environment

The exposure is marine splash.

The chloride profiles at 2 yr, 10 yr, 20 yr, 50 yr and 100 yr are calculated and their graphs areshown in Figure 2, cf. [28] and [29].

Initiation period

The threshold value of chloride content in the concrete yields, cf. (20):

{ }( ) ( ) bindermass%0.6348.04.2exp0.2eqv2.4expcrcr === w/ckC

since eqv{w/c} = w/c= 0.48 because the binder is 100 % Portland cement. Applying the

LIGHTCON-model L, cf. (13), is determined as:

509.1049.0035.2

049.063.0erfcinv2 =

=L

Hence, the initiation period yields, cf. (14):

years701131027.0509.1

50027.0

386.4

cr =

=t

Applying the HETEK-model H, cf. (13), is determined as:

463.1505.0

1131027.0

459.1

049.063.0inv2

208.0

1.0 =

=H

Hence, the initiation period yields, cf. (14):

years80

1131027.0463.1

50027.0

386.4

cr =

=t

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2.4.7 Summing up

The initiation period and the chloride ingress into concrete exposed to the marine splash orsubmerged in seawater may be predicted by the HETEK-model as well as the LIGHTCON-

model. At first the LIGHTCON-model looks simpler than the HETEK-model. However, while allthe parameters of the HETEK-model are functions of the eqv{w/c} and the environment, anapplication of the LIGHTCON-model involves a subjective estimate of the chloride content ofthe near-to-surface layer of the exposed concrete surface. This does not make the LIGHTCON-model unambiguous to apply.

When applying the HETEK-model and LIGHTCON-model through a spreadsheet like Excel 5 ora computer programme like the AECs program HETEKDESthe numerical work and the timeconsumed are of the same order of magnitude.

2.5 Conditional Average Estimator - Hybrid Neural Network

(CAE - HNN)

A hybrid neural network-like approach (CAE - HNN) was developed by ZAG and involvesan empirical treatment of the phenomena. This is very suitable for problems where models are

based on the experimental data. It was shown elsewhere [7] that such an approachcorresponds to the use of the intelligent systems.

We assume, that the complete phenomenon, in our case in-depth chloride ion penetration, ischaracterised by a sample of the measurements onNtesting specimens that are described by afinite set of so called model vectors:

{ }X X X1 2, , ..., N ... /Eq. 1/.

Such a finite set of model vectors will be called a database in the subsequent text.

In formulating the modeler of the phenomenon Cl-= Cl-(x, h, o, wt, c) we further assume thatone particular observation of a phenomenon can be described by a number of variables, whichare treated as components of a vector:

= Clcwtohx ,,,,,X ... /Eq. 2/,

where x is depth, h height above sea level, o orientation, wt wetting, c variable whichdescribes concrete cracks and Cl-chloride ion concentration at depthx.

Vector Xcan be composed of two truncated vectors:

{ }#;,,,, cwtohx=P and = Cl;#R ... /Eq. 3/,

where # denotes the missing portion. Vector Pis complementary to vector Rand thereforetheir concatenation yields the complete data vector X. The problem now is how an unknown

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complementary vector Rcan be estimated from a given truncated vector Pand sample vectors{X1, X2, ..., XN}. By using the conditional probability function the optimal estimator for thegiven problem can be expressed as [7-9]:

r A rk k nk n

N

= =

1

... /Eq. 4/

where

A a

ak

n

jj

N=

=

1

and( )

a

p p

wn

i nii

L

=

=

exp

2

122

... /Eq. 5/.

rk is the k-th output variable (e.g. Cl-; k is equal to 1 in a given problem), rnk is the same

output variable corresponding to the n-th model vector in the data base, Nis the number ofmodel vectors in the data base, pni is the i-th input variable of the n-th model vector in thedata base (e.g. x, h, o, wt, c), piis the i-th input variable corresponding to the model vectorunder consideration, andLis the number of input variables. wdescribes the average distance

between the specimens in the sample space and is called smoothing parameter.

A general application of the method does not include any prior information about thephenomenon. Because in some cases there is still lack of data, a prioriinformation is neededto better fit a particular phenomenon. By a relatively simple improvement [10], the methodcan be effectively used for the modelling of many problems in civil engineering. Furthermore,

CAE (conditional average estimator) stems from a probabilistic approach and phenomena arenot treated just deterministic.

For the application of the CAE-HNN a database is needed. It consists of model vectors, whatcan be presented in general case in matrix form as:

mv1 = p11 p12 ... p1L r1mv2 = p21 p22 ... p2L r2... ... ...

... ... ...

mvN = pN1 pN2 ... pNL rN ... /Eq. 6/.

The main task in the first step is therefore to represent the measured data and, if necessary, aprioriknowledge about the phenomenon in vector or matrix form. Finally, in the second stepthe choice of appropriate value of smoothing parameter is needed. The parametric study hasshown that the appropriate value for modelling chloride ion penetration into concrete is w=0.15. Due to the lack of experimental data on time dependence of chloride ion penetration,Ficks 2ndlaw is used for time prediction.

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3 CONDITION SURVEY DATA

3.1 Introduction

Data from Slovenia, Norway and France were collected and analysed. Even though chlorideanalysis has been performed on a large proportion of all concrete bridges, only a limitednumber of structures could be retained for further analysis. This was done to limit the numberof calculations but also because there is only a limited number of bridges where there is acomplete set of data. The project was also primarily interested in structures with goodchloride profiles (accurate and measurements at several depths), taken from several locationson the structure and taken at several ages at the same location.

All the Slovenian structures included in the case studies are placed on highways. The bridgesare situated in a continental climate environment with hot summers and cold winters. During

the winter, de-icing salts are used to provide suitable traffic conditions. The French bridge islocated about 50 km south-west of Paris and has similar conditions. The Norwegian structuresare all coastal bridges which cross a fjord or a sound. They all have piers placed in the sea.

The Slovenian bridges are: kedenj 1, Preloge, Slatina, epina and Ivanje Selo. The Norwegianbridges are :Gimsystraumen (bridge chosen for the method of inverse cores), Hadsel andSandhornya bridge. The French bridge is: A11 PS12-10.

Table 1:Example of chloride data. Note that in this case three profiles were determined forthe same location and age.

Norway Chlorides at Gimsystraumen Bridge

Height Location Depth Cl-weight Factor Cl

-weight Age Location

above code mm % of cement mconc/mcem % of concrete at time of

sea level inspection

m

3.6 Column 3 7.5 2.05 6.4 0.3200 12 West side

(Cl.3.07) 22.5 2.05 6.4 0.3200

40.0 1.38 6.4 0.215062.5 0.49 6.4 0.0760

87.5 0.25 6.4 0.0390

3.6 Column 3 7.5 1.47 6.4 0.2300 12 West side

(Cl.3.08) 22.5 1.54 6.4 0.2400

40.0 0.93 6.4 0.1450

62.5 0.51 6.4 0.0800

87.5 0.27 6.4 0.0420

3.6 Column 3 7.5 1.34 6.4 0.2100 12 West side

(Cl.3.09) 22.5 0.93 6.4 0.1450

40.0 0.58 6.4 0.0900

62.5 0.22 6.4 0.034087.5 0.14 6.4 0.0220

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In addition to chloride data, see Table 1, certain information about the structures was alsocollected. This was based on a Norwegian database [19] and categorised as follows:

General information about the structurecounty, commune, name, number, length, etc

Description of superstructuretype of element, specified cover, specified concrete quality, w/c ratio, quantity of cement,entrained air, curing regime, etc

Description of columns/piersmethod of construction (sliding, climbing, ...)

In-situ measurementslocation, structural element, axis or span, shape (circular, square, ..), distance from land,height above water, orientation (north, south, ..), micro-climatechloride measurements, depth of carbonation, cover, electro chemical potential, relativehumidity

Damage/rating in measurement locationstype of damage, degree of damage, chiselled concrete away to inspect rebar (y/n)

It should be noted that it was not possible to complete all the above fields for the all bridges.

3.2 Measurement techniques

Provided water and oxygen are freely available, the chloride content of a concrete element is

the most important parameter when evaluating the risk of reinforcement corrosion. There areseveral different methods available to determine this chloride content and the results areusually presented as a chloride profile (variation of chloride concentration with depth). Inaddition, the chloride concentration is usually reported as a percentage concentration ofcement weight or of concrete weight.

3.2.1 Norway

Generally speaking, there are two common techniques in use to determine the chloride

content of concrete powder. To collect the powder, two methods are used. Which method andtechnique that are used, depends of the accuracy of the investigation.

3.2.1.1 Collecting the samples

For detailed investigations (special inspections), concrete cores are taken from the structureand sent to the laboratory. The cores are typically between 50 and 100 mm in diameter and upto 100 mm long. They are wrapped in plastic before they are sent to the laboratory. In thelaboratory, a profile-grinder is used to produce powder in small intervals (typically 1-2 mm)in depths between 25 and 50 mm. For less accurate investigations, drill powder is collectedon-site by using an ordinary drill. Dependent on the diameter of the bore (typically 20-25

mm), three or four holes are drilled per sample. Typical depths are 25-50 mm in 5-mmincrements.

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3.2.1.2 Determination of the chloride content

Potentiometric titrationThis method is the standard method in the laboratory. 5 gram dried powder is dissolved in

10 ml distilled water, allowed to boil for one minute, and then 10 ml 10% nitric acid is added.The solution is boiled and stirred for another minute. 10 ml distilled water is the added, andthe solution is allowed to cool down. The solution is filtrated and distilled water is added upto 100 ml. This extract is then titrated using automatic potentiometric titration with 0.01molar AgNO3and a chloride sensitive electrode.

RCT Rapid Chloride TestThis method is also used in some of the county laboratories, and as a field-method on site.The RCT is commercially available (German Instruments, Denmark), and comes with a

complete suitcase with acid, calibration solutions, beakers, voltmeter, etc. A small amount ofpowder (about 1.5 gram) is placed in a special beaker, compacted, and transferred into aplastic flask with a weak acid (ready to use). The flask is shaken for about 5 minutes, andallowed to stand for another 5 minutes. A chloride sensitive electrode is then placed in thesolution, and the potential is noted after it has stabilised. The chloride concentration isdetermined by using a calibration-curve.

3.2.2 Slovenia

The procedures described here those used at National Building and Civil Engineering Institute

(ZAG), Slovenia.

Chloride content in the concrete is determined in the laboratory either by the potencio-titration method or by the photometric method. The latter is also suitable for use on site forthe quick determination of the chloride content by weight of concrete. The chloride profiles,which were used within WP4 were determined in the laboratory by the photometric method.The LASA-Chlorid-System equipment of the German manufacturer Dr.Lange was used. The

procedure to determine the chloride content has several steps, which are briefly described inthe following sections.

3.2.2.1 Collecting the samples

During detailed inspection of a bridge, 50 mm diameter cylinders are taken from the concretestructural elements by core bore drilling. During the boring, a constant flow of the water issupplied for cooling the drilling cylinder. The length of the cores is up to 200 mm, but mainlyaround 100 mm. Each cylinder is put into a plastic bag, marked and delivered to thelaboratory the same day as it is taken.

During next few days the concrete powder is obtained from the extracted cores by drillingholes with a 6mm diameter drill at a certain depths (fig. 3.1). The depth, at which the dust for

the determination of the chloride content is obtained, depends on the cylinder length, itsintegrity and on the depth of the reinforcement.

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3.2.2.2 Determination of the chloride content

Photometric measurement

Procedure for the determination of the chloride content is basically executed at the roomtemperature around 20C. In a small plastic or glass beaker, which is laid on a very precise

balance, exactly 2.0 g of the concrete powder is weighted into the beaker.

In the next step, the weighted powder is transferred into a 100 ml plastic or glass beaker. Witha dispenser, exactly 25 ml (five times of 5 ml) of 18% nitrogen acid is poured over the

powdered concrete sample in the beaker. Immediately afterwards the beaker is shaken for afew times and then left to stand for about ten minutes.

In the next step, about 5 to 6 ml of the acid dissolved solution of concrete powder sample is

sucked from the beaker by the plastic syringe. After the suction a membrane filter is put onthe syringe end and the solution is filtered into another plastic or glass beaker.

With the automatic pipette 1 ml of the filtered solution is sucked from the beaker and pouredinto the kivette with prepared solution, which reacts with the dissolved solution of thesample. The kivette is closed with the screw cap, shaken few times and left to stand for 3minutes. After 3 minutes the reference kivette is put into the photometer (zero reading)followed by the kivette which contains the dissolved solution of the sample. The displayshows the concentration of the chlorides by weight of concrete. If the reading is outside thefirst range (0 to 0.08% by weight of the concrete), than the additional steps must be taken.

With a new pipette, 0.2 ml of the filtered solution is taken and poured into a new kivette. Thekivette is closed with the screw cap, shaken a few times and left to stand for 3 minutes. After3 minutes, the same reference kivette is put into the photometer (zero reading) followed thekivette with the dissolved solution of the sample. The display shows the concentration of thechlorides in the range of 0.08 to 0.30% by weight of the concrete. If the reading is outside thisrange, an additional measurement is performed by the potencio-titration method.

The two methods mentioned were compared several times against each other using the samesample specimens. Comparisons were also made on test powder, which was obtained fromconcrete cubes prepared with the known weight of salt. The difference between the two

methods was from 0.01 to 0.03% by weight of concrete.

After each procedure the beakers and syringe are thoroughly cleaned with distilled water. Thepipettes and filter membranes are discarded (one use only).

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50 mm

5 15 20 40 mm

100 mm

Figure 3.1 Scheme of drilling the core to obtain the dust for determination of chlorideprofiles where the thickness of the concrete cover is about 20 mm

3.2.3 France

The procedures used in France are similar to those used in Norway.

3.3 Description of bridges

3.3.1 French Bridge A11 PS 12-10

The PS 12-10 bridge was built in 1972 and crosses Highway A11 about 50 km south-west ofParis, France. It has four spans of reinforced concrete and its columns measure4.50x0.50x0.60 m. Every winter, de-icing salt are spread on the highway pavement and carwheels made it splash onto the bridge columns. Under such aggressive conditions, somecracks and spalling of concrete cover were observed after only a few years of service.

According to the results of rebar half-cell potential, steel was corroding on the lower parts(about 1.90 m high) of columns, where de-icing salt solution was splashed. Carbonation depthranged between 9 and 23 mm and the concrete cover was about 30 mm thick. Some chloride

profiles were determined. The results were used for comparative calculations.

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The conditions of these piles were suitable for a chloride removal treatment, by applying amethod with galvanic anode. However, before this treatment, repairs were made for replacingspalled concrete (with patches) and filling (by injection) cracks in the concrete.

Figure 3.2 Columns of A11 PS 12-10 bridge. Chloride removal treatment was performedduring the summer of 1997.

3.3.2 Norwegian Bridges

3.3.2.1 Gimsystraumen Bridge

The Gimsystraumen Bridge was constructed between 1979 - 1981, the bridge is a post-

tensioned, balanced cantilevered box construction. The nine span bridge has a total length of840 m. The box height varies from 2.2 m to 7.4 m. The designed concrete strength is gradeC 35 for foundations and columns and grade C 40 for the superstructure. Concrete cover wasspecified to 50 mm and 30 mm for columns and superstructure respectively. The bridge isexposed to severe climatic conditions with strong winds and salt spray from the sea. Theenvironmental exposure changes along the bridge as the bridge deck level varies from 4 m to36 m above sea level.

An extensive condition survey including visual inspection, analysis of chloride profiles,potential mapping and rebar inspection was carried out. More than 4500 chloride analyseswere performed and concrete cover was measured at more than 900 locations. In addition,condition survey and material testing has been performed. The extensive condition surveydetected that the most serious reinforcement corrosion had occurred in the lowest part of thesuperstructure where the chloride load is very high and the concrete cover is low. Thespecified concrete cover was only 30 mm. In addition, fixing bars used to assemble thereinforcement cage were allowed in the concrete cover. No serious corrosion had started onthe columns where the specified concrete cover was 50 mm. Consequently, the condition ofthe bridge was very much influenced by the prevailing requirements at the time the bridgewas built. The bridge is illustrated in figures 3.3 and 3.4.

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Figure 3.3 The Gimsystraumen Bridge.

Figure 3.4 Photo of the eastern part of Gimsystraumen Bridge (axis 1 - 6).

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3.3.2.2 Hadsel Bridge

The Hadsel Bridge was constructed between 1976 - 1978. The bridge has a total length of1010 m composed of a main section of 300 m and two side sections of 355 m each. The main

section consists of a main span which is 150 m and two side spans of 75 m. The main sectionis build after the balanced cantilevered method with a post-tensioned box construction. The

box height varies from 1.85 m to 7.90 m. The two side sections are divided into 12 spanseach. The bridge is illustrated in figure 3.5.

The designed concrete strength is grade C 30 for foundations and columns and grade C 40 forthe superstructure. Concrete cover was specified to 50 mm and 30 mm for columns andsuperstructure respectively.

The bridge is exposed to severe climatic conditions with strong winds and salt spray from the

sea. The condition of the bridge is strongly effected of the windward / leeward effect. Thismeans that the leeward side of structure parts which are close to the sea level are exposed tovery high chloride loading. The bridge type with side sections with short spans and more than20 columns standing in the has resulted in huge areas of concrete exposed to extreme highchloride loads. This has caused heavy corrosion in the lower part of the columns.

Figure 3.5 Photo of the main section (balanced cantilever) and side spans of Hadsel Bridge.

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3.3.2.3 Sandhornya Bridge

The Sandnornya Bridge was constructed during 1988 - 1989. The bridge is a post-tensioned,balanced cantilevered box construction. The bridge has a total length of 374 m divided into

three spans. The main span is 154 m and both side spans are 110 m. The box height variesfrom 2.3 m to 7.7 m. The designed concrete strength is grade C 35 for foundations, gradeC 45 for columns, C 45 for the main span (superstructure) and LC 55 for the side spans(superstructure). Concrete cover was specified to 50 mm and 40 mm for columns andsuperstructure respectively. The bridge is exposed to coastal climate with winds and salt sprayfrom the sea. No serious corrosion has been detected on any part of the structure.

The bridge is illustrated in figure 3.6.

Figure 3.6 Photo of the main span of Sandhornya Bridge.

3.3.3 Slovenian Bridges

Data of chloride profiles from the Slovenian bridge structures were obtained during in-depthinspections, which were carried out between 1994 and 1996. All structures included in the

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The average concrete cover depth over the stirrups of pre-cast longitudinal beams is 22 mmand over the main reinforcement 35 mm. The average concrete cover depth of the box girderstructure is 35 mm and main reinforcement 51 mm.

Figure 3.8 Deterioration of the column concrete surface due to damaged expansion - joint

The main source of concrete deterioration was due to water leakage in the presence of de-icing salts through damaged cast-in-situ joints on cantilever parts of the deck. The samesource of deterioration was present on the first and third unit. On the second unit the mainsource of deterioration was wetting of the concrete surface around the outlets of the drainage

pipes. Due to the damaged sealing of the expansion joint between the first and second unit apier had suffered a severe deterioration of the concrete surface. In the area of heavy wettingthe corrosion of the ordinary reinforcement was observed. In some places of the box girderstructure a light corrosion of a few tendons was also detected.

3.3.3.2 Viaduct Preloge

Viaduct Preloge was built in 1976. It is 558 m long 14 span simply supported structuredivided into three units (5+4+5 spans). Each span is 40 m long. The superstructure is totallymade of pre-cast elements. Four longitudinal girders of height 2.20 m are linked up with two

transverse end-beams and one transverse mid-span beam. The deck is made of pre-castelements, which are interconnected with other superstructure elements by cast-in-situ joints ofwidth 0.20 m.The designed concrete strength was 40 MPa. The average concrete strength established oncylinder cores at the time of an in-depth inspection was 58.7 MPa.

The shape of piers is an octagonal hollow cross section. The height of the piers varies from 8to 26 m.

Average concrete cover depth on the bottom side of the deck is 11 mm. The pre-cast

longitudinal beams have 27 mm average concrete cover depth over stirrups and 38 mm overthe main longitudinal reinforcement.

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The main source of concrete deterioration is water leakage in the presence of de-icing saltsthrough damaged cast-in-situ joints in the cantilever parts of the superstructure's deck. The

piers under damaged expansion joints between the units have severe deterioration of theconcrete surface. Some joints between the segments of the pre-cast girders were open on the

bottom side of the bottom flange and superficial corrosion of the tendon wires was observed.Some outer girders had in the area of heavy wetting up to 20% corroded tendons.

Figure 3.9 Longitudinal view on the viaduct Preloge

Figure 3.10 Wetting of the concrete surface of the edge beam under the damaged joint of thedeck's pre-cast elements

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3.3.3.3 Viaduct Slatina

Viaduct Slatina was built in 1976. It is 440 m long eleven span simply supported structuredivided into two units (6+5 spans). Each span is 40 m long. the superstructure is totally madeof pre-cast elements. Four longitudinal girders of height 2.20 m are linked up with two

transverse end-beams and one transverse mid-span beam. The deck is made of pre-castelements, which are interconnected with other superstructure elements by cast-in-situ joints ofwidth 0.20 m. The outer girders are prestressed with 12 tendons of 167 mm wires andmiddle girders with 10 tendons of 167 mm wires. The designed concrete strength was40 MPa. The average concrete strength established on cylinder cores at the time of an in-depth inspection was 67.1 MPa.

The shape of piers is an octagonal hollow cross section. The height of the piers varies from 13to 38 m.

Figure 3.11 Longitudinal view of the viaduct Slatina

The pre-cast longitudinal girders have 27 mm average concrete cover depth over stirrups and39 mm over the main longitudinal reinforcement.

The main source of deterioration of the concrete is water leakage in the presence of de-icingsalts through damaged cast-in-situ joints on the cantilever parts of the deck cross section. The

pier under the damaged expansion joint between the units has severe deterioration of theconcrete surface. A few outer girders have in the area of heavy wetting up to 15% corrodedtendons.

3.3.3.4 Viaduct epina

Viaduct epina was built in 1975. It is 140 m long five span simply supported structure. Eachspan is 28 m long. The superstructure is made of five pre-cast post-tensioned girders and cast-in-situ deck. Longitudinal girders of height 1.80 m are linked up with two transverse end-

beams and three transverse beams situated at a quarter of the span length. Outer girders areprestressed with 10 tendons of 127 mm wires. The designed concrete strength was 40 MPa.

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The average concrete strength established on cylinder cores at the time of an in-depthinspection was 48.9 MPa.

The piers are multi column type structure. The height of the piers is about 5 m.

Figure 3.12 View on the Zepina viaduct

The girders have 35 mm average concrete cover depth over stirrups and 43 mm over the mainlongitudinal reinforcement in the web and bottom flange.

Figure 3.13 Corroded tendon with broken wires

The main source of deterioration of the concrete is water leakage in the presence of de-icingsalts around damaged drain pipes and outlets. A few outer girders had up to 20% corrodedtendons in areas of heavy wetting.

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Figure 3.14 Heavily wetted surface of the outer girder

Figure 3.15 Source of the wetting - the outlet of the drainage pipe does not lead surfacedrainage water away from the structure

3.3.3.5 Viaduct Ivanje Selo

Viaduct Ivanje Selo was built in 1972. It consists of two parallel structures, one for eachdirection of traffic flow. It is 225 m long nine span simply supported structure divided intothree units (3+3+3 spans). Each span is 25 m long. The superstructure is made of seven pre-cast post-tensioned girders and cast-in-situ deck. Longitudinal girders of height 1.60 m arelinked up with two transverse end-beams and three transverse beams situated at quarter andmid-span. Outer girders are prestressed with 9 tendons of 127 mm wires and the rest of thegirders with 8 tendons of 127 mm wires. The designed concrete strength was 50 MPa. Theaverage concrete strength established on cylinder cores at the time of an in-depth inspectionwas 70.6 MPa.

The shape of piers is rectangular solid cross section. The height of the piers varies from 5.5 to13 m.

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Figure 3.16 View of the Ivanje Selo viaduct

The girders have 35 mm average concrete cover depth over the main longitudinalreinforcement in the web and bottom flange.

The main source of deterioration of the concrete is water leakage in the presence of de-icingsalts around the drainage outlets and damaged drain pipes. A few outer girders had in areas ofheavy wetting up to 44% corroded tendons.

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4 CALCULATIONS AND RESULTS

4.1 Calculation parameters

The models are to calculate the time to corrosion initiation for three different cover depthsand three different threshold values (critical chloride content). These are:

Cover depths: 25, 30 and 50 mmThreshold values: 0.4, 0.7 and 1.0 % of cement weight.

The results for one location on one Norwegian, one Slovenian and one French bridge are givenin sections 4.2, 4.3 and 4.4, respectively. It should be noted that the Hetek model is the onlymodel of the four that can take into account two or more chloride profiles. All the other modelshave based their calculations on the last (most recent) profile. For the nine bridges, a total of 63

chloride profiles, representing 44 locations, were collected and judged fit for furtherinvestigation. Predicted chloride ingress was calculated at all 45 locations with all the models.The results of these calculations, 44 pages, are presented in annexe B in a manner similar toFigs 4.2- 4.4.

When interpreting the results, it should be remembered that chloride content is normallydetermined to an accuracy of 0.01 % of concrete weight (approx. 0.07 % of cement weight).

To illustrate the effect that errors in raw data can have on the predicted time to corrosioninitiation, a simple example is taken. Using chloride concentrations from a real bridge, anoptimistic and pessimistic profile are made by varying the chloride concentration by 0.01 %of concrete weight and depth by 2.5 7 mm. Fick's 2ndlaw is then used to calculate the timeto corrosion for a critical chloride content of 0.1% by concrete weight (0.7 % by cementweight) at 30 mm depth. As can be seen from Fig. 4.1, there is a factor greater than four

between the times to corrosion initiation for the pessimistic and optimistic profiles.

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Figure 4.1 Influence of errors in the determination of chloride profiles on time to corrosion

4.2 Results Norwegian bridges

From the three Norwegian coastal bridges, 16 locations with a total of 36 chloride profiles

have been investigated. The estimated time to corrosion, taking all the Norwegian results intoaccount, varied from about two days to more than 1000 years. This indicates that there is alarge variation in chloride exposure, a large variation in concrete quality, a large variation inhow the various models handle different sets of data, or a combination of the three factors.

Fig. 4.2 shows an example from Gimsystraumen Bridge. The Norwegian results are given inAnnexe B, pages B-3 To B-18.

Measured chloride profile after 27 yearsDepth (mm) 2.5 10 15 30 45 Optimistic profileCl (% conc.) 0.45 0.23 0.05 0 0Depth (mm) 5 15 22.5 37.5 52.5 Measured profile

Cl (% conc.) 0.46 0.24 0.06 0.01 0Depth (mm) 7.5 20 30 45 60 Pessimistic profileCl (% conc.) 0.47 0.25 0.07 0.02 0.01

Optimistic Measured PessimisticTime to corrosion initiation(years) 105 49 25

Calculated D (mm2/yr) 2 5 9Calculated Cs(% concr.) 0.55 0.61 0.65

Cover depth (mm) = 30 mmCritical chloride content = 0.7 (% cement)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 10 20 30 40 50 60 70 80 90

Depth from concrete surface (mm)

Totalchloridecontent(%concr.)

Cl-crit

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Gimsystraumen bridge; Column 5 West; Heigth 3.8 m

Measured chloride profile

Depth (mm) 7.5 22.5 40 62.5 87.5

Cl (% conc.) 0.278 0.207 0.039 0.01 0.012 12 years

Depth (mm) 5 15 25 40 62.5

Cl (% conc.) 0.342 0.286 0.216 0.087 0.024 18 years

Time to corrosion initiation (years)Cover depth

(mm)Cl_crit

(% cement) Ficks 2. HETEK LightCon CAE_HNN

25 0.4 4 4.7 0.5 1.5

25 0.7 7 10.4 1.7 2.2

25 1 11 18.9 5 3.5

30 0.4 5 7.9 1.3 3.2

30 0.7 10 16.9 3 4.6

30 1 16 29.8 13 7.5

50 0.4 17.5 36.2 17 19

50 0.7 28 70 52 27.9

50 1 43 114.4 160 45.4

Calculated D (mm2/yr) 33 25.5 29

Calculated Cs(% concr.) 0.44 0.485 0.44

0.1

1

10

100

1000

0 10 20 30 40 50 60Concrete cover (mm)

Timetocorrosioninitiation(yr) 1%

0.7%

0.4%

Ficks 2. Law - erf

0.00.10.20.30.40.5

0.60.70.80.91.0

0 10 20 30 40 50 60 70 80 90


Totalchloridecontent(%concr.)


0.1

1

10

100

1000



0.7%

0.4%

HETEK

0.1

1

10

100

1000



0.7%

0.4%

Li htCon

0.1

1

10

100

1000



0.7%

0.4%

CAE HNN

Figure 4.2 Example of result page for a Norwegian bridge from chloride ingress models

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4.3 Results Slovenian bridges

From the five Slovenian bridges, 26 locations with a total of 28 chloride profiles have beeninvestigated. The estimated time to corrosion, taking all the Slovenian results into account,varied from less that a month to more than 1000 years.

Fig. 4.3 shows an example from Zepina Bridge. The Slovenian results are given in Annexe B,pages B-19 To B-43.

Zepina, profile 1


Depth (mm) 5 20 40 80

Cl (% conc.) 0.21 0.1 0.05 0.01 22 years exp.

Depth (mm)

Cl (% conc.)


(mm)

Cl_crit

(% cement) Ficks 2. HETEK LightCon CAE_HNN25 0.4 10 13 0.6 15

25 0.7 23 29 4.2 44

25 1 62 67 50 480

30 0.4 10 18 1.3 24

30 0.7 33 42 11 70

30 1 90 93 125 766

50 0.4 41 55 18 44

50 0.7 92 118 130 127

50 1 230 240 1600 1385

Calculated D (mm2/yr) 24 38.7 15 -

Calculated Cs(% concr.) 0.24 0.26 0.24 -

0.1

1

10

100

1000



0.7%

0.4%

Ficks 2. Law - erf

0.00.10.20.30.40.50.60.70.80.91.0

0 10 20 30 40 50 60 70 80 90


Totalchloridecontent(%concr

.)Measured chloride profile

0

1

10

100

1000



0.7%

0.4%

HETEK

0.1

1

10

100

1000


Timetoc

orrosioninitiation(yr) 1%

0.7%

0.4%

LightCon

0

1

10

100

1000


Timetoc


0.7%

0.4%

CAE_HNN

Figure 4.3 Example of result page for a Slovenian bridge from chloride ingress models

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4.4 Results French bridge

From the French bridge, 3 locations with a total of 3 chloride profiles have been investigated.The estimated time to corrosion, taking all the French results into account, varied from aboutthree years to more than 1000 years.

Fig. 4.4 shows an example from the PS12-10 Bridge. The French results are given in AnnexeB, pages B-44 To B-46.

A11 PS12-10, Col. 1


Depth (mm) 2.5 7.5 12.5 17.5 22.5 27.5

Cl (% conc.) 0.198 0.17 0.108 0.123 0.083 0.065 24 years

Depth (mm)

Cl (% conc.)


(mm)

Cl_crit

(% cement) Ficks 2. HETEK LightCon CAE_HNN25 0.4 18 21.8 2.7 29.9

25 0.7 45 56.6 28 69.9

25 1 155 210 710 314.8

30 0.4 25 31.4 6.6 40

30 0.7 68 81.5 70 93.6

30 1 230 303 1800 421.8

50 0.4 72 87.4 86 124.1

50 0.7 170 227 890 290

50 1 690 841 10000 1000

Calculated D (mm2/yr) 16 13 10 -

Calculated Cs(% concr.) 0.21 0.21 0.21 -

0.1

1

10

100

1000



0.7%

0.4%

Ficks 2. Law - erf

0.00.10.20.30.40.50.60.70.80.91.0

0 10 20 30 40 50 60 70 80 90


Totalchloridecontent(%concr

.)Measured chloride profile

0.1

1

10

100

1000



0.7%

0.4%

HETEK

0.1

1

10

100

1000


Timetoc


0.7%

0.4%

LightCon

0.1

1

10

100

1000


Timetoc


0.7%

0.4%

CAE_HNN

Figure 4.4 Example of result page for the French bridge from chloride ingress models

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4.6 Discussion

It is difficult to say which of the models gives the best prediction of future chloride ingress. Thisis primarily due to the lack of data from the same location over an extended time scale withwhich to calibrate and verify the various models. In addition to the predictions given by the

various models, the time, data required and complexity involved in using them is also taken intoaccount in their evaluation.

It must be pointed out that chloride ingress in concrete is not a well-understood phenomenon.Concrete by its nature is a heterogeneous material and chloride ingress in laboratory samples, inwell-controlled conditions, will vary for apparently identical test specimens. Thus it should beno surprise to find that chloride ingress in real constructions will vary for every metre, either inthe horizontal or vertical direction. This is due on one hand to

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