RESEARCH ARTICLE

Evaluating effect of chloride attack and concrete cover onthe probability of corrosion

Sanjeev Kumar VERMAa*, Sudhir Singh BHADAURIAb, Saleem AKHTARa

a Civil Engineering Department, University Institute of Technology, Rajiv Gandhi Technological University, BHOPAL Madhya

Pradesh 462036, Indiab S.G.S. Institute of Technology and Science, INDORE Madhya Pradesh 452003, India*Corresponding author. E-mail: sanjeev.apm@gmail.com

© Higher Education Press and Springer-Verlag Berlin Heidelberg 2013

ABSTRACT Corrosion of reinforced concrete (RC) structures is one of the significant causes of deterioration ofreinforced concrete (RC) structures. Chlorination is a major process governing the initiation and advancement of theinjurious corrosion of steel bars. Now, several researches on the chlorination of concrete structures have been ongoingaround the world. Present article reviews several recently performed chlorination studies, and from results of a fieldsurvey evaluates the effect of chloride content on the probability of corrosion and the influence of concrete compressivestrength on the chloride content and penetration, also evaluates the effect of concrete cover over the chloride content of theRC structures at rebar depth and on the probability of corrosion.

KEYWORDS concrete, chloride, reinforcement, corrosion, deterioration, cover

1 Introduction

Performance degradation of reinforced concrete (RC)structures due to corrosion initiated by chloride attack isa significant problem especially in marine regions. Thechloride ions penetrates in the concrete and results ininitiation of the corrosion. Corrosion of reinforcementleads to the formation of cracks on concrete cover, which,in the end results in decrease of residual life of RCstructures. Wardhana and Hadlipriono [1] defined failure ofRC structure as “incapacity of a constructed facility or itscomponents to perform as specified in the design andconstruction requirements.”Chloride ion can damage the protective passive film and

lead to corrosion causing loss of area of steel, reduction inload carrying capacity of structure and loss of durability Liet al. [2]. Durability of concrete structures exposed tochloride environment depends mainly on the resistance ofconcrete against chloride ingress (Costa and Appleton) [3].The chloride content in concrete structures at depth with

respect to age of concrete has been evaluated by most of

the researchers through relationship based on Fick’ssecond law of diffusion shown in Eq. (1), and found thatchloride ingress in the concrete structures depends on thechloride diffusion coefficient of concrete.

∂C∂t

¼ ∂∂x

D∂C∂x

� �(1)

where C is the chloride content at depth x and at time t;D isthe chloride diffusion coefficient.For improving the tensile strength of cement concrete,

reinforcing steels have been added to form a new materialreinforced cement concrete (RCC). During hydration ofcement in reinforced concrete structures a highly alkalinepore solution due to high pH (12.5–13.5) forms a passiveoxide film on the steel to protect it from corrosion. Thisprotective film is destroyed when sufficient amount ofchloride ions penetrates into concrete, and after theremoval of protective film reinforcing steel bars gets incontact with oxygen and water corrosion, thereafter,corrosion in the form of rust formation and loss in crosssection occurs. Hence, it has been recognized that thepresence of chloride ion is a significant factor causing theremoval of passive oxide coating. Small quantity ofArticle history: Received Mar. 20, 2013; Accepted Aug. 27, 2013

Front. Struct. Civ. Eng. 2013, 7(4): 379–390DOI 10.1007/s11709-013-0223-9

chloride will normally present in concrete structures, buthigh ingress of chloride ions form deicers or seawater candestroy the protective film. Several researchers considereddiffusion as the main transport mechanism of chloride ioninto concrete.Chloride ion penetration in the concrete structures is of

the main concern, for the durability of concrete structures.So, studies related to chloride attack on concrete structureshave been the subject of interest for the researchers in thelast few decades. In the present article several recentlyperformed chlorination studies have been reviewed, andfrom results of a field survey evaluates the effect ofchloride content on the probability of and the influence ofconcrete compressive strength on the chloride content andpenetration, also effect of concrete cover over the chloridecontent of the RC structures at rebar depth and on theprobability of corrosion has been evaluated.

2 Determination of chloride content

Chloride content in concrete as the percentage of concreteweight can be measured on concrete powder drilled out ofstructures on site, this method is in conform with ASTMC114. This concrete sample is then mixed into a 10ml ofextraction liquid and shaken for five minutes. Theextraction liquid removes disturbing ions, such as sulfideions, and extracts the chloride ions in the sample, and thensample is tested with electrode to evaluate chloride contentas the percentage of weight of concrete. Table 1 presentsvarious NDT methods for the evaluation of chloridecontent

3 Modeling chloride ingress in concretestructures

The significance of corrosion in service life of structureshas led to extensive researches to the deterioration of RCstructures. Deterioration of concrete structure is mainlygoverned by corrosion of steel, which is the result ofchlorination governed by diffusion coefficient and surfacechloride content (Eq. (2)). Most of the researchersconsidered chlorination for modeling life of RC structuresby dividing it in different phases. This method has beenfirst adopted by Tutti (1982), these models divide servicelife of RC structures in two periods. First period isinitiation (ti) which is the time required for chloride ionconcentration to reach the threshold value at the rebar leveland initiate corrosion, and second is propagation (tp) whichis the time between initiation and cracking, hence totalservice life of structures is ti + tp,. The two phase servicelife concept is shown in Fig. 1, it has been observed thatwith the increase in age of the structures the deteriorationlevel of structures increases and when deterioration levelreaches critical deterioration level it reflects end of theservice life.Following equations based on Fick’s second law of

diffusion (Eq. (1)) have been used by many researchers topredict corrosion initiation period by considering thefollowing boundary conditionsC(x> 0, t = 0) = 0 initial chloride content is zero at any

depth.C(x = 0, t> 0) = Cs surface chloride content.C(x = 1, t> 0) = 0 chloride content at large depths is

zero.

Table 1 methods for testing chloride content

method principle/procedure limitations

Quantab test Reaction between silver dichromate and chloride ionproduces white marks on the strips

It is expensive, hazardous and appropriate forlow thickness.

Potentiometric titration Using acid or water soluble methods, the final volumewill indicates chloride content.

Skilled personal are required.

Rapid Chloride Test Potential difference of unknown solution is comparedwith potential difference of solutions with knownchloride concentration.

Results are affected by the presence of certain materials.

Salt Ponding Test (resistance of concreteagainst penetration of chloride ion)

A 3% NaCl solution is ponded on the top surface ofthe 28 days dried specimen, bottom face is exposedto environment, and the chloride concentration of 0.5inch thick slice is measured.

Long-term test, complicated testing conditions, providesonly average value in place of actual

Bulk diffusion test (resistance of concreteagainst penetration of chloride ion)

Same as above with few changes, first is it samplesinitial moisture condition, specimen is saturated withlimewater instead of keeping dried.

Long-term test.

Rapid chloride permeability test Total charge passed is measured to evaluate ionicmovement.

Current passed indicates movement of all the ionsinstead of chloride ion.

Electrical resistance test Two cooper plate electrodes are connected on theopposite faces of specimen by thin wet sponges. Thenelectrodes are connected to resistivity meter.

Results are affected with change in temperature.

380 Front. Struct. Civ. Eng. 2013, 7(4): 379–390

Where Cs is the chloride content on the surface of theconcrete structures.The solution of Eq. (1) is

C x,tð Þ ¼ Cs 1 – erfx

2ffiffiffiffiffiDt

p� �� �

(2)

where erf is the error function

erf uð Þ ¼ 2ffiffiffiπ

p !u

0e – t

2

dt: (3)

So, corrosion initiation period (ti) which is the timerequired by chloride content at rebar depth to reachthreshold value, is given by

ti ¼C2

4Derf – 1 1 –

Cth

Cs

� �  35 – 224 (4)

where Cth is the threshold chloride content; C = concretecover. From Eq. (4) it has been observed that in RCstructures with high concrete cover the corrosion initiationperiod is longer than that for structures with low concretecover.

4 Review of few recent chloride studies

The deterioration of RC structures is mainly associatedwith the reinforcing steel corrosion. One of the main agentscausing corrosion of rebars is the chloride ion, whichdiffuses into concrete and causes chlorination of concrete.When chloride content at rebar depth reaches thresholdvalue, it initiates the corrosion of steel reinforcement.Fick’s laws of diffusion have been used many authors andresearchers for modeling the chloride ingress in concretestructures. Several recent chlorination studies conductedhave been reviewed in Table 2.

Fig. 1 Two phases of the service life (based on tuutti’s model,1982)

Table 2 Few recent chlorination studies

references study performed considerable research/ findings comments

Cusson et al. [4] Presented the development of probablisticmechanistic modeling approach supportedby durability monitoring to obtain improvedpredictions of service life. Developed amodel to predict service life of concretebridge decks exposed to chloride by analyz-ing surface chloride content, chloride diffu-sion coefficient, threshold chloride content,corrosion and deterioration rate.

Demonstrated that service lifepredictions using probabilistic modelscalibrated with selected monitored fielddata can provide more reliable assess-ments of the probabilities of reinforce-ment corrosion and corrosion-induceddamage, when compared to using deter-ministic models based on standard datafrom the literature. Such probabilisticmodels can improve life cycle perfor-mance of structures by extending itsservice life and reducing its lifecycle cost.

All the four parameters responsible forchloride ingress rate have been consideredand model is validated by experimentalresults.

Li et al. [2] Observed steel corrosion caused by chloridepenetration as the most significant problemrelated to durability of concrete structures.Performed experiments on stressed speci-mens exposed to salt solution, to study theeffect of stress on chloride ion penetrationresistance. Diffusion of chlorides in con-crete has been evaluated by Fick’s 2nd lawof diffusion.

Chloride contents in un-cracked conc-rete with different w/c ratio, states andlevel of stress and environmental condi-tions have been analyzed.

Normally chloride studies are performed onunstressed concrete. However, evaluatingthe effect of stress over the chloridepenetration provides more realistic results.

Balafas and Burgoyne[5]

observed that life of a bridge controlled bycorrosion has two phases, in first phasechloride penetrates to the depth of rebar andstarts corrosion, and in second phase rustproduced with higher volume puts pressureon the cover and led to spalling of concretecover. Developed a model to determine thetime span of two periods.

New formula is proposed for the rateof rust production, based on Faraday’slaw.Results are in good agreement withexisting experimental data on specimensunder uniform corrosion

Considered two phase service life andevaluated propagation time on the basis offracture mechanics.

Sanjeev Kumar VERMA et al. Evaluating effect of chloride attack and concrete cover on the probability of corrosion 381

(Continued)references study performed considerable research/ findings comments

Yuan et al. [6] Used multispecies model to describe thechloride transport in saturated concrete,which has been solved using FDM byinputting parameters such as porosity,density, chemical composition of poresolution, diffusion coefficient and chloridebinding isotherm. And used extension ofNernst-Planck equation to describe multi-species model.

Diffusion coefficient used in this modelwas depth dependent instead of fixed.

J ¼ –D½ð∂c=∂xÞ þ ðc=yÞð∂y=∂yÞþzcðF=RTÞð∂Eðx,tÞ=∂xÞ – cVðxÞ�

Where D = effective diffusion coeffi-cient; i = ionic concentration in poresolution; γ = chemical activity coeffi-cient; E = electric potential; F =Faraday constant; R = universalgas constant; z = valence.

In this model in place of constant diffusioncoefficient a depth-dependent diffusioncoefficient has been considered. This is agood approach as it has been observed bymany researchers that diffusion coefficientis a variable. And the results are verified byexperimental results.

Wang et al. [7] Chloride concentration and diffusion coeffi-cient decreases with the increase of com-pressive stress and increases with theincrease of flexural stress. Considering theabove statement a model for predicting thechloride ingress has been developed fordifferent loading conditions based on Fick’s2nd law of diffusion.

Results of performed experimentshows that diffusion coefficient haveinverse relationship with compressivestress and direct relationship withflexural stress, and apparent diffusioncoefficient decreases with the increasein compressive stress and increasedwith the increase of flexural stress.Developed model accounts for para-meters such as stress level, water/cement ratio, curing time, temperature,concrete age, humidity, and chloridediffusion coefficient.

Predicted values of chloride diffusion coef-ficient from the proposed model are com-parable with experimental results. Morevalidation of this model is required.

Chai et al. [8] Investigated corrosion of steel during theaccelerated corrosion test and critical chlor-ide ion concentration. Service life predictionequation of concrete structures has beenestablished through the experimentalresults. Defined concrete service life as theperiod from the initial use to depassivationor to the corrosion of steel. Two differenttypes of experiments were performed toevaluate critical chloride ion concentrationand concrete service life. Specimens pre-pared are having water cement ratio of 0.48.

Threshold value of chloride ionconcentration has been determined asfor different specimens as 0.485%and 0.461% (percent mass of concrete).Also observed that Mineral admixturesin concrete can preserve protectivepassive film over rebar and improvethe resistance to corrosion of steel rebar,also decrease the free chloride ion contentby absorbing large number of freechloride ions.

Considered threshold chloride concentrationas the criteria of the end of service life.Service life of concrete constructed byreplacing cement fly ash and slag is morethan a concrete constructed by ordinaryPortland cement.

Zhang and Ba [9] Conducted accelerated life test by chloridemigration equipment to save time andmoney, and found that the negative algo-rithm of chloride ion concentration has beenlinear with the electrochemical potential.

Presented a accelerated curve to predictthe Service life of concrete in naturaldiffusion test.The result shows that in chloride envir-onment service life of concrete structureswith 10 mm cover to rebar has beenbetween 11.89 and 12.45 years.

Service life obtained experimentally iscomparable with the results of Life-365models.

Lin et al. [10] Developed an integrated FE based numer-ical model for predicting service life of RCstructures exposed to chloride environ-ments, which accounts for the environmen-tal humidity, temperature fluctuations,chloride binding-diffusion and convection,as well as decay of concrete structuralperformance.

Results of numerical model were vali-dated by comparing with analyticalsolutions and experimental observa-tions, and its application for predict-ing its service life has been demon-strated on RC slab.

Chemical attack, environmental conditions,temperature and age of structures, combina-tion of all these parameters have beenrequired for performance evaluation ofconcrete structures.

Sun et al. [11] Proposed a service life prediction model forRC structures exposed to chloride environ-ment based on the analytical solution ofFick’s 2nd law of diffusion, also presentedtime and depth dependent chloride diffusioncoefficient obtained from the analyticalsolution of the nonlinear chloride diffusionequations.

Service life predicted by this model isfound to be comparable with wellknown Light Con model.

Considered time and depth diffusion coeffi-cient for better results, as now severalresearchers realized that diffusion coeffi-cient is not a constant, it depends on thequality of concrete and exposure conditions.To obtain more accurate results inspectionperiod must be longer. And to obtain morerealistic predictions different environmentalfactors can be considered.

382 Front. Struct. Civ. Eng. 2013, 7(4): 379–390

(Continued)references study performed considerable research/ findings comments

Andrade and Andrea[12]

Fick’s law has been used to calculatethe diffusion coefficient for predictingthe concentration of the aggressiveagents at a certain depth and atseveral periods of time. Electricalresistivity property has been used tocalculate both the initiation andpropagation periods, as well as forpredicting age of concrete related todurability and for measuring theefficiency of curing.

Observed resistivity as the propertybased on the concrete porous systemand its degree of moisture content.Concrete mix can be designed for atarget resistivity and this parametercan also be used as a performanceparameter (corrosion indicator).

Considered diffusion coefficient as thesignificant factor governing the chlorideingress and service life of the structures.

Wang et al. [13] Presented service life predictionmodel for chloride environmentbased on Fick’s 2nd law usingabove Eqs. (1 to 4), also developeda service life prediction programusing Monte-Carlo method.

This model is comparable with life-365 model.

Provides realistic results as it consideredenvironmental conditions with usage.

Conciatori et al. [14] Presented a numerical model“TransChlor” based on Finiteelement method (FEM) andFinite difference method (FDM).This model combines varioustransport modes such as thermaltransfer, hydrous transfer of vaporwater and liquid water by capillarysuction, CO2 diffusion, Chlorideion diffusion and chloride ionconvection by the hydrousmovement. Microscopic andmacroscopic models have beenoften used to model themovement of chloride ions.Microscopic models describechloride ion movements inconcrete and macroscopic modelsconsider the chemical conversionsand the thermal, hydrous and chlorideion variations by simulating overallchemical effects on transport.

Carbonation influences the capacityof concrete to capture chloride ion.

Provide results by combining various trans-port modes and ingress of harmful agents.

Song and Kwon [15] Proposed a neural network algorithmto determine chloride diffusion inhigh performance concrete (HPC)using micro pore structures.Electrically driven chloride penetrationtests for diffusion coefficient areperformed for the concretes withvarious parameters such as w/c ratioand various mineral admixtures.

Experimental data had been comparedwith numerical simulation results, andit has been found that developed tech-nique is applicable for different mix-ture design of HPC.

It has been observed that by applying neuralnetwork diffusion coefficient can be esti-mated successfully. However, by utilizingmore data from more number of test speci-mens a more effective model can bedeveloped.

Zhang and Lounis [16] Presented a performance-baseddurability design of concretestructures using simplifieddiffusion-based model based onFick’s law of diffusion. Numericalnonlinear relationship between thefour parameters governing thecorrosion initiation period ofreinforced concrete structuresincluding chloride diffusioncoefficient, chloride thresholdvalue of reinforcement, concretecover and surface chloride exposurecondition has been determined.

it has been observed that in aggres-sive chloride-laden environmentincreasing concrete cover is moreeffective than using corrosion resistantsteel, it is necessary to use both highperformance concrete and corrosion-resistant steel, a relative decrease in theconcrete cover has to be compensated bya much greater increase in the corrosionresistance of steel, and values of criticalchloride content and concrete cover aregoverned by chloride diffusion coeffi-cient.

This study considered all the significantparameters required for evaluating chlorideingress in concrete structures using Fick’slaw of diffusion.

Sanjeev Kumar VERMA et al. Evaluating effect of chloride attack and concrete cover on the probability of corrosion 383

(Continued)references study performed considerable research/ findings comments

Cheung et al. [17] Developed a 2-D FE coupled model toevaluate the chloride penetration process forpredicting the corrosion initiation time.Found that corrosion initiation time issignificantly governed by the speed ofchloride transfer and depassivation processwith in the structure. Variation in environ-mental conditions to which structure isexposed had very significant impact on thecorrosion process, therefore variation inmicroclimate on the concrete surface hasbeen investigated. The corrosion perfor-mance model is developed to considerchange in environmental conditions andsimulate the coupled diffusion process andcorrosion performance in time domain. AlsoProposed a set of realistic environmentalconditions based on material properties.

The parametric analysis results suggestthat the corrosion initiation time intropical/ subtropical regions dependsmainly on the annual mean relativehumidity (h), the source chlorideConcentration (C), concrete coversdepth (d) and w/c ration.Surface chloride concentration showsa quasi-linear increase with the nth rootof time and this increase is relatively fastand reaches a quasi-constant content inabout five years time.

This model considers the effect of environ-mental conditions, which have a verysignificant effect on the chlorination ofconcrete structures. Hence, it providesmore realistic results

Alizadeh et al. [18] Determined values of diffusion coefficientand surface chloride content in concretespecimens exposed to seawater in thePersian Gulf. Also effect of various curingregimes had been investigated on theestimation of time to corrosion initiation ofreinforced concrete structures during theDuraPGulf model.

Fick’s second law of diffusion (Eqs.(1) to (4)) has been used to evaluatechloride penetration rate as a functionof depth from the concrete surfaceand time.

Evaluated diffusion coefficient and surfacechloride content are not real diffusioncoefficient and surface chloride content,but only represents the regression coeffi-cients.

Shekarchi et al. [19] Presented the development of DuraPGulf(service life design model based on Fick’slaw) to predict the chloride induced corro-sion initiation of RC structures in the southof IRAN.

Model has been developed using theFE technique and user friendly soft-ware was programmed for practicalengineering applications.

Ingress of chloride ion and service life of aRC structure depends on the exposureconditions. Therefore, it is required todevelop local models based on local expo-sure conditions. Hence, it is good to developthis DuraPGulf as local model for thestructures in Gulf region.

Anoop and BalajiRao [20]

Demonstrated the use of data from fieldinspections for the assessment of remaininglife of corrosion affected RC bridge bydetermining the time taken for a givenperformance measure to deteriorate to atarget value using the concept of additivefuzzy logic. Corrosion initiation time hasbeen determined using Fick’s 2nd Law ofdiffusion. Uncertainities in the values of theparameters characterizing the enviromentand variables affecting the time to corrosioninitiation and corrosion propogation aretaken into account by treating them asfuzzy variables.

Usefulness of proposed methodologyhas been illustrated through a casestudy, by comparing the time to reachdifferent damage levels for a severlydistressed beam.

Utilizing field inspection data for modelingchloride ingress or other deteriorationmechanism provides the variation of theseparameters with the age of structures.

Evans and Richardson[21]

Analyzed the chloride diffusivity of IrishPortland cement concretes in chlorideenvironment, also influence of secondarycementitious materials has been investi-gated.

It has been observed that in concreteswith secondary cementitious materialsdiffusion coefficient is lower than theconcretes with Portland cement.

Results of this study can be used to designand develop concrete structures withincreased durability.

Polder and Rooij [22] Presented investigations series performedon six concrete structures between the agesof 18 to 41 years, most of them areconstructed using blast furnace slag cement.Interpretation is based on the Dura-Cretemodel developed using Fick’s 2nd law ofdiffusion for chloride ingress. Curve fittingof chloride profiles has been performed toevaluate chloride surface contents andapparent diffusion coefficients.

Comparison has been made with pre-viously published data on chlorideingress and electrical resistivity ofsimilar concretes.

Classifying structures according to their ageand condition is a good practice for short-term monitoring of structures.

384 Front. Struct. Civ. Eng. 2013, 7(4): 379–390

(Continued)references study performed considerable research/ findings comments

Khatri andSirivivatnan on [23]

Presented a model to determine service lifeof concrete structures in marine environ-ments, chloride ingress model based onFick’s 2nd law of diffusion has beenassumed. Effect of severity of environmentis also demonstrated. Service life of RCstructures found to be governed by coverdepth, diffusion coefficient, surface chlorideconcentration and critical chloride level.

It has been observed that cover depthis more important than diffusion coeffi-cient, and surface chloride concentra-tion affects service life more thancritical chloride value. Hence, to improveservice life it is better to provide sufficientcover depth.

Findings of this research is useful ofengineers and researchers, recommendationcan be used for improving the durability ofconcrete structures.

Martin-Perez and Lounis[24]

Presented an approach for predicting servicelife of RC structures exposed to chlorideenvironment, which combines a FE basedchloride transport model with a reliabilitybased approach to evaluate the damage.

The probabilistic distributions of thechloride penetration front and corro-sion initiation time are generated byusing Monte Carlo simulation.

Combining two different approaches forpredicting service life and chloride ingressusually provides good results.

Liang et al. [25] Examined mathematical service life predic-tion models for RC bridges in chloride ladenenvironment. The service model consists ofthree stages of corrosion initiation time (tc),depassivation time (tp) and propagationtime (tcorr). Hence, total service life ofexisting RC bridge is t = tc + tp + tcorr.Model is based on based on Fick’s 2nd lawof diffusion.

Solution of Fick’s law depends onthe initial chloride content and surfacechloride content.Degree of deterioration can be obta-ined by using the value of integrityof structures.

Here constant surface chloride content hasbeen used. However, to improve the resultstime and depth dependent surface chloridecontent can be used.

Cao andSirivivatnan on [26]

Presented a simple model to predict theservice life of RC structures based on thesolution derived from Fick’s 2nd law ofdiffusion. Defined service life as time afterconstruction until the chloride content at thereinforcement is high enough to initiate steelcorrosion.

Suggested that acceptable steel corro-sion rate can be used for predictingservice life of deteriorating concretestructures.

Diffusion coefficient and surface chloridecontent obtained are more than real value socorrection factors are required.

Scheremans and Gemert[27]

Chloride Ingress in the concrete is mainlygoverned by diffusion process and evalu-ated using Fick’s 2nd law of diffusion. Alsoconducted experiments on concrete struc-tures exposed to marine environment andobserved significant influence of depth onchloride diffusion coefficient, however, noeffect of time on diffusion coefficient havebeen observed.

Updated a probability based model forinterpretation of test results and predic-tion of the service life of RC structures.

From several researches it has beenobserved that diffusion coefficient is timeand depth dependent. So, better results canbe obtained by considering effect of time ondiffusion coefficients.

Costa and Appleton [28] Presented an experimental study for cali-brating the parameters in model based onFick’s second law of diffusion used topredict the chloride penetration, and con-cluded that both the concrete cover andconcrete quality affects the service life.

Studied time dependence of chloridediffusion coefficient and surfacechloride concentration for the variousmarine conditions. It has been foundfrom results that chloride diffusioncoefficient and surface chloride concen-tration depends on the time and depth.For a service life of more than 50 yearsconcrete cover must be more than 40mm.

This study is useful of researchers as itprovide values of chloride diffusion coeffi-cient and surface chloride concentration fordifferent regions and also provides time anddepth dependence of these parameters.

Ann et al. [29] Concluded from a literature review that inmarine environment chloride contentincreases with time and properties like W/C, cement content and binder governed thediffusivity of chloride ion.

Considered various significant factorsaffecting chloride ingress, whileevaluating chloride content in concretestructures.

Considered constant diffusion coefficient(2x10–12 m2/s), but it has been observed thatchloride diffusion coefficient depends ontime and depth.

Sharma and Mukherjee[30]

Studied progress of corrosion in chlorideand oxide conditions using Ultrasonicguided waves. Observed that corrosion rateis different in chloride and non- chlorideconditions.

Corrosion rate depends on the ingressof various agents responsible for initia-ting the corrosion, chloride contentincreases the corrosion rate afterinitiating the corrosion of rebars.

Application of Ultrasonic guided waves forevaluating the corrosion rate and for com-paring different environmental conditionshas been identified as good approach.

Sanjeev Kumar VERMA et al. Evaluating effect of chloride attack and concrete cover on the probability of corrosion 385

5 Effect of concrete cover and chloridecontent on corrosion of steel bars

Form the data obtained from a field survey conducted onseveral RC structures situated around the City of BhopalIndia, effect of concrete cover over the chloride content atrebar depth has been evaluated. Almost hundred structuresare identified and classified according to age of thestructures. Cover-meter, Rebound hammer, Ag/AgCl halfcell and rapid chloride tests have been performed toevaluate concrete cover, compressive strength, half cellpotential and chloride content respectively to performstudies presented in Sections 5.1, 5.2 and 5.3.Corrosion probability is indicated by the half cell

potential values, decrease in half cell values (increase innegative value) indicates higher probability of corrosion.Figure 2 presents the values of chloride content andconcrete cover of the surveyed structures. As the values ofchloride content are smaller in comparison with concretecover, so for plotting those in the same graph values ofchloride content are multiplied 100 times. Hence, threshold

value of chloride content (0.2%) becomes 20. Fall in valueof chloride content has been identified with the rise inconcrete cover also it has been observed that for most ofthe structures with concrete cover more than 40mmchloride content is lower than threshold value.

5.1 Effect of chloride content on probability of corrosion

Chloride attack is a major cause of corrosion in RCstructures. Significance of chloride attack in the corrosionprocess has evolved the threshold chloride contentconcept. Threshold value of chloride content is definedas the chloride content at rebar depth required to destroythe passive protective film and initiates the corrosion.Effect of chloride content at rebar depth on the half cellpotential has been presented in Fig. 3, which indicates thatincrease in chloride content increases the probability ofcorrosion. It has been also observed from Fig. 3 that whenvalue of chloride content at rebar depth is more thanthreshold value which is 0.2% of the weight of concrete,than the probability of corrosion is more than 90%.

(Continued)references study performed considerable research/ findings comments

Costa and Appleton [3] Performed study on three concrete mixes indifferent exposure conditions, and con-cluded from the results that diffusioncoefficient and surface chloride concentra-tion are time dependent.

Simple models based on Fick’s secondlaw of diffusion were used to predictthe chloride penetration. However, ithas been observed that these modelsare required to be calibrated usingexperimental results.

Evaluated values of diffusion coefficientand surface chloride concentration cannotbe used for long-term monitoring, as thesevalues are strongly time dependent.

Sengul and Tasdemir[31]

Investigated the effect of replacing cementwith supplementary materials on compres-sive strength and rapid chloride permeabil-ity of concrete. And it has been observedthat partial replacement (about 50%) ofPortland cement with ground fly ash andground granulated blast furnace slag sig-nificantly reduce the permeability of chlor-ide in concrete.

Considered three aspects minimumchloride permeability for moredurability, high compressive strengthfor safety and cost of concrete. It hasbeen required to estimate an optimizedmix percentage to satisfy all the aboveaspects (durability, safety, cost).

All the three aspects, durability, safety andcost, are significantly influence the utility ofa structure.Permeability is the main cause influencingthe ingress of harmful ions in the concretesurrounding the rebars, so this studyprovides useful results for reducing thepermeability of the concrete.

Fig. 2 concrete cover and values of chloride content (�100) of the surveyed structures

386 Front. Struct. Civ. Eng. 2013, 7(4): 379–390

5.2 Effect of compressive strength on chloride ingress ofconcrete

It has been observed by many authors that chloride ingressis slower in the high strength concrete. To evaluate thevariation of chloride content with the change in compres-sive strength, values of chloride content and compressivestrength obtained from field survey have been plotted andfitted in Fig. 4, and weak correlation is obtained because ofscattered values. It has been observed from Fig. 4, thatincrease in compressive strength reduces the chloridecontent at rebar depth, and for the concrete structures withlow compressive strength the chloride content at rebardepth is more than threshold value which indicates higherprobability of corrosion.

5.3 Effect of concrete cover on chloride ingress andprobability of corrosion

A graph (Fig. 5) is plotted between the evaluated values ofchloride content and concrete cover, then using Microsoftexcel curve fitting tool an exponential relation is obtained

between the plotted data shown in Eq. (5).

C ¼ 9:521e – 0:009Cc , (5)

where C = chloride content at rebar depth (% wt. ofconcrete), and Cc = concrete cover (mm). Negative signindicates inverse relationship between concrete cover andchloride content. It has been observed from Fig. 5, thatchloride content at rebar depth decreases with the increasein concrete cover of RC structures resulting in lessprobability of corrosion and further increase in the servicelife of the structure. The value of minimum concrete coverrequired to maintain chloride content lower than thresholdvalue has been obtained from above relation is almost40mm. From Fig. 6 it has been concluded that with the inconcrete cover half cell value increases (decrease innegative value), which indicates lesser probability ofcorrosion.

Data obtained from field survey has been classifiedconsidering chloride penetration period or age of structureand considering compressive strength of structure, classi-fication of structures in different groups has been shown inTables 3 and 4. Now, considering the following classifica-

Fig. 3 effect of chloride content at rebar depth on probability of corrosion

Fig. 4 Effect of compressive strength on the chloride content atrebar depth

Fig. 5 relation between chloride content and concrete cover

Sanjeev Kumar VERMA et al. Evaluating effect of chloride attack and concrete cover on the probability of corrosion 387

tions effect of concrete cover on the chloride content hasbeen analyzed through Fig. 7 to 13.From Fig. 7 to 10 effect of concrete cover over on the

chloride content has been evaluated, and it has beenobserved from Fig. 7 that for the structures of group A1even with low concrete cover chloride content is lower

than threshold value (0.2%). Figures 8 and 9 indicate thatwith the increase in chloride penetration period most of thestructures with low concrete cover are having chloridecontent more than threshold value. From Fig. 10, it has alsobeen observed that after the age of 50 years structures withhigh concrete cover are having chloride content more thanthreshold value. Therefore, it has been interpreted that bothconcrete cover and age of structures are affecting chloridecontent of the RC structures and probability of thecorrosion.

Fig. 6 Effect of concrete cover on the probability of corrosion

Fig. 7 Relation between concrete cover (Cc) and chloridecontent (C) for A1 group

Fig. 8 Relation between concrete cover (Cc) and chloridecontent (C) for A2 group

Fig. 9 Relation between concrete cover (Cc) and chloridecontent (C) for A3 group

Fig. 10 Relation between concrete cover (Cc) and chloridecontent (C) for A4 group

Table 3 Classification of data considering chloride penetration period

or age of the structures

group chloride penetration period/year No. of structures

A1 0 to 15 22

A2 16 to 30 33

A3 31 to 50 28

A4 51 to 60 13

Table 4 Classification of data considering compressive strength of the

structures

group compressive strength/MPa No. of structures

C1 0 to 15 44

C2 16 to 25 33

C4 more than 25 21

388 Front. Struct. Civ. Eng. 2013, 7(4): 379–390

Figures 11–13 represent the effect of concrete cover onthe chloride content over the structures of differentcompressive strengths. Figure 11, indicates that in thestructures with low compressive strength usually chlorideingress is more than threshold limit, and from Figs. 12 and13 it has been observed that with the increase incompressive strength chloride content remains under thethreshold limit for most of the structures.

6 Conclusions

In RC structures corrosion takes place when passiveprotective layer is destroyed by chloride ingress. Chloridepenetration in concrete is affected by variation intemperature, wet–dry cycles and change in other exposureconditions. There are numerous experimental and mathe-matical studies performed related to chloride ingress inconcrete structures.Several recent chloride studies have been reviewed and

it has been observed that most of the researchers usedFick’s laws of diffusion to model the chloride ingress.Chloride ingress in concrete structures mainly depends onthe diffusion coefficient. Increase in diffusion coefficientincreases the ingress of chloride ion [27] in concretestructures, and significantly reduces the service life of RCstructures. Increasing concrete cover depth is the mostappropriate technique for increasing chloride resistanceand further improving the service life of structures [16].Replacing cement by supplementary materials increases

the chloride resistance of concrete structures, which resultsin improvement in estimated service life of structures[8,31]. It has been found that use of pozzolanic materials ismore effective than decreasing theW/C ratio for improvingthe chloride resistance of concrete structures.Effect of chloride content on the probability of corrosion

has been evaluated, and it has been observed from Fig. 3that increase in chloride content increases the probabilityof corrosion. Effect of compressive strength on thechloride content has been investigated in Fig. 4, and ithas been observed that they are having inverse relationshipwith each other. Concrete structures with low compressivestrength have higher probability of corrosion.It has been observed from Fig. 5 that concrete cover and

chloride content at rebar depth are having inverserelationship. A minimum concrete cover of 40mm isrequired to keep the chloride content below the thresholdlimit. Figure 6 indicates decrease in probability ofcorrosion with the increase in concrete cover. Also, Fig.7 to 13 indicates the significance of age and compressivestrength on the chloride content of the structure.Several methods are available for measuring chloride

content and chloride ion resistance of concrete structures.However, additional research is required to develop morereliable and rapid methods.

Refrerences

1. Wardhana K, Hadipriono F C. Analysis of recent bridge failures in

the United States. Journal of Performance of Constructed Facilities,

2003, 17(3): 144–150

2. Li G P, Hu F J, Wu Y X. Chloride ion penetration in stressed

concrete. Journal of Materials in Civil Engineering, 2011, 23(8):

Fig. 11 Relation between concrete cover (Cc) and chloridecontent (C) for C1 group

Fig. 12 Relation between concrete cover (Cc) and chloridecontent (C) for C2 group

Fig. 13 Relation between concrete cover (Cc) and chloridecontent (C) for C3 group

Sanjeev Kumar VERMA et al. Evaluating effect of chloride attack and concrete cover on the probability of corrosion 389

1145–1153

3. Costa A, Appleton J. Chloride penetration in marine environ-

ment—part I: Main parameters affecting chloride penetration.

Materials and Structures, 1999, 32(4): 252–259

4. Cusson D, Lounis Z, Daigle L. Durability monitoring for improved

service life predictions of concrete bridge decks in corrosive

environments. Comp Aid Civil and Infr Eng, 2011, 26: 1–40

5. Balafas I, Burgoyne C J. Modeling the structural effects of rust in

concrete cover. Journal of Engineering Mechanics, 2011, 137(3):

175–185

6. Yuan Q, Shi C, De Schutter G, Deng D, He F. Numerical models for

chloride penetration into saturated concrete. Journal of Materials in

Civil Engineering, 2011, 23(3): 305–311

7. Wang H, Lu C, Jin W, Bai Y. Effect of external loads on chloride

transport in concrete. Journal of Materials in Civil Engineering,

2011, 23(7): 1043–1049

8. Chai W, Li W, Ba H. Experimental study on predicting service life

of concrete in the marine environment. Open Civil Eng J, 2011, 5:

93–99

9. Zhang W, Ba H. Accelerated life test of concrete in chloride

environment. Journal of Materials in Civil Engineering, 2011, 23(3):

330–334

10. Lin G, Liu Y, Xiang Z. Numerical modeling for predicting service

life of reinforced concrete structures exposed to chloride environ-

ments. Cement and Concrete Composites, 2010, 32(8): 571–579

11. Sun Y, Chang T, Liang M. Service life prediction for concrete

structures by time-depth dependent chloride diffusion coefficient.

Journal of Materials in Civil Engineering, 2010, 22(11): 1187–

1190

12. Andrade C, Andrea R. Electrical resistivity as microstructural

parameter for the modeling of service life of reinforced concrete

structures. In: Proceedings of the 2nd Int Sym on Ser Life Des Inf-

2010. Delft Netherlands, 2010, 379–388

13. Wang C J, Zhang G Z, Liu K X, Tu L Q, Li J H, Qin M Q. Research

on service life prediction model of concrete structure on sea crossing

bridge. J Wuh Univ Tech, 2010, 32(17): 141–146

14. Conciatori D, Laferriere F, Bruhwiler E. Comprehensive modeling

of chloride ion and water ingress into concrete considering thermal

and carbonation state for real time. Cement and Concrete Research,

2009, 40(1): 109–118

15. Song H, Kwon S. Evaluation of chloride penetration in high

performance concrete using neural network algorithm and micro

pore structure. Cement and Concrete Research, 2009, 39(9): 814–

824

16. Zhang J Y, Lounis Z. Nonlinear relationship between parameters of

simplified diffusion-based model for service life design of concrete

structures exposed to chlorides. Cement and Concrete Composites,

2009, 31(8): 591–600

17. Cheung M M S, Zhao J, Chan Y B. Service life prediction of RC

bridge structures exposed to chloride environments. Journal of

Bridge Engineering, 2009, 14(3): 164–178

18. Alizadeh R, Ghods P, Chini M, Hoseini M, Ghalibafian M,

Shekarchi M. Effect of curing conditions on the service life design

of RC structures in the persion gulf region. Journal of Materials in

Civil Engineering, 2008, 20(1): 2–8

19. Shekarchi M, Ghods P, Alizadeh R, Chini M, Hoseini M. Durapgulf

a local service model for the durability of concrete structures in the

south of Iran. Arab J Sci Eng, 2008, 33(1B): 77–88

20. Anoop M B, Balaji Rao K. Application of fuzzy sets for remaining

life assessment of corrosion affected reinforced concrete bridge

girders. Journal of Performance of Constructed Facilities, 2007, 21

(2): 166–171

21. Evans C, Richardson M G. Service life of chloride- contaminated

concrete structures. Concrete Res in Ireland Colloquium, 2005,

131–137

22. Polder R B. Rooij MRd. Durability of marine concrete structures-

field investigations and modeling. HERON, 2005, 50(3): 133–153

23. Khatri R P, Sirivivatnanon V. Characteristics service life for

concrete exposed to marine environments. Cement and Concrete

Research, 2004, 34(5): 745–752

24. Martin-Perez B, Lounis Z. Numerical modeling of service life of

reinforced concrete structures. Proceedings of 2nd Int. Rilem w/s on

Life Pred Ag. Mang 2003, Paris, France, 71–79

25. Liang M, Lin L, Liang C. Service life prediction of existing

reinforced concrete bridges exposed to chloride environment. J Infra

Sys, 2002, 8(3): 76–85

26. Cao H T, Sirivivatnanon V. Service life modelling of crack-freed

and cracked reinforced concrete members subjected to working

load. CIB world building congress, Wellington New Zealand, 2001,

1–11

27. Scheremans L, Gemert D V. Service prediction of reinforced

concrete structures, based on in-service, chloride penetration

profiles. Dur Build Mat Comp, 1999, 8: 83–93

28. Costa A, Appleton J. Chloride penetration in marine environment-

part II: prediction of long term chloride penetration. Materials and

Structures, 1999, 32(5): 354–359

29. Ann K Y, Ahn J H, Ryou J S. The importance of chloride content at

the concrete surface in assessing the time to corrosion of steel in

concrete structures. Construction & Building Materials, 2009, 23(1):

239–245

30. Sharma S, Mukherjee A. Monitoring corrosion in oxide and chloride

environments using ultrasonic guided waves. Journal of Materials in

Civil Engineering, 2011, 23(2): 207–2011

31. Sengul O, Tasdemir M A. Compressive strength and rapid chloride

permeability of concretes with ground fly ash and slag. Journal of

Materials in Civil Engineering, 2009, 21(9): 494–01

390 Front. Struct. Civ. Eng. 2013, 7(4): 379–390