Bond behaviour of corroded reinforcing steel bars in concrete

6 (2006) 1931–1938
Cement and Concrete Research 3
Bond behaviour of corroded reinforcing steel bars in concrete

Congqi Fang a,⁎, Karin Lundgren b, Mario Plos b, Kent Gylltoft b

a Department of Civil Engineering, Shanghai Jiaotong University, Shanghai 200240, Chinab Department of Civil and Environmental Engineering, Structural Engineering, Chalmers University of Technology, SE-412 96, Göteborg, Sweden

Received 29 July 2005; accepted 16 May 2006

Abstract

The effect of steel corrosion on bond between steel bars and the surrounding concrete was investigated for different corrosion levels. Bothpullout tests and finite element analysis were used and the results from the two were compared. An electrolyte corrosion technique was used toaccelerate steel corrosion. For confined deformed bars, a medium level (around 4%) of corrosion had no substantial influence on the bondstrength, but substantial reduction in bond took place when corrosion increased thereafter to a higher level of around 6%. It is demonstrated thatthe confinement supplied an effective way to counteract bond loss for corroded steel bars of a medium (around 4% to 6%) corrosion level. Theresults of finite element analyses, where it was assumed that rust behaved like a granular material, showed a reasonably good agreement with theexperiments regarding bond strength and bond stiffness.© 2006 Elsevier Ltd. All rights reserved.

Keywords: Bond strength; Reinforcement; Corrosion; Reinforced concrete

1. Introduction

As the interaction between reinforcing steel and thesurrounding concrete, bond is fundamental for all reinforcedconcrete structures. For those concrete structures located in anaggressive environment bond may be weakened by corrosion ofthe reinforcing steel, affecting the serviceability and ultimatestrength of concrete elements within the structure. While someresearchers think corrosion of a limited degree improves thebond between steel and concrete [1], corrosion is considered oneof the main causes for the limited durability of steel-reinforcedconcrete [2]. The basic problem associated with the deteriorationof reinforced concrete due to corrosion is not that the reinforcingsteel itself is reduced in mechanical strength, but rather that thecorrosion products due to the volume increase cause splitting [3].Studies conducted by Auyeung confirmed that the loss of bondstrength for an unconfined corroded reinforcing steel bar is muchmore critical than the cross section loss, since a 2% diameter losscould lead to 80% bond reduction [4].

⁎ Corresponding author. Tel./fax: +86 21 54744558.E-mail address: [email protected] (C. Fang).

0008-8846/$ - see front matter © 2006 Elsevier Ltd. All rights reserved.doi:10.1016/j.cemconres.2006.05.008

The interaction of reinforcing steel bars with concrete is acomplex phenomenon that has important effects on the responsecharacteristics of reinforced concrete elements and structures. Alarge number of research programmes have been carried out inthe area of steel–concrete bond behaviour, such as those byTepfers [5,6]. Also, many researchers have conducted studies onthe mechanism of bond failure. For example, Cairns and Jonesproposed an analysis of the bond strength of ribbed reinforcingbars in which bond failure was considered as bearing failure ofthe ribs on the concrete. Cairns and Jones's analysis suggestedthat bond strength was only partially dependent on the splittingresistance of the beam section [7,8]; Balázs developed a theoryfor crack formation in reinforced concrete based on an analysisof slip, bond stresses and steel stresses, and steel stress waspresented. The problem was treated as an initial value problemboth for initial crack formation when the cracks wereindependent, and for stabilized crack formation when the crackswere independent, and for stabilized crack formation when thezones of bond stresses reached each other [9].

The deterioration in bond is of particular concern for concretestructures when the embedded steel reinforcing bars are cor-roded. Many studies have been conducted on the influence ofcorrosion on bond between steel and concrete [3,4,10–20]. Due

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Fig. 1. Specimen geometry. All dimensions in mm.

1932 C. Fang et al. / Cement and Concrete Research 36 (2006) 1931–1938

to the nonuniformity in bond stress, experiments have been theprimary method for studying how corrosion affects the bondmechanism. Experiments provide a simple, effective way ofevaluating the effects of different parameters on bond behaviour.However, due to the difficulty of obtaining the exact expectedlevels of reinforcement corrosion from experiments, numericalmodeling is becoming increasingly important as an alternative orsupplementary method for studying the effect of corrosion onbond behaviour [21–24]. A disadvantage of this approach is thata bond–slip relation is predefined and some circumstances haveto be known in advance, includingwhether the concrete will splitor the reinforcement will yield [25].

In this study the effect of reinforcement corrosion on bond isinvestigated using comparisons between the results of analysisand those from the tests carried out. The research program wasconducted for reinforcing bars of different corrosion levels.Specimens were first investigated under pullout loads, and werethen modelled with nonlinear finite element analysis.

2. Experiment

A total of 24 pullout specimens were included within thecurrent investigation. The test specimens were divided into twomajor groups: Confined and unconfined specimens. Each majorgroup was divided into 4 small groups according to the targetsteel corrosion from 0% (non-corrosion) to 6%. Three replicatespecimens were cast within each small group.

Hot-rolled deformed steel bars, 20 mm in diameter and420 mm in length, were used as the main reinforcing steel in allspecimens. The embedment length was four times the bardiameter, e.g., 80 mm, as shown in Fig. 1. For those specimenswith stirrups supplying lateral confinement, round steel with adiameter of 6 mm was selected for rectangular closed stirrupswith a spacing of 40 mm, as shown in Fig. 1, which shows thegeometry of the specimens. The main reinforcing steel had ayield point of 351 MPa and a tensile strength of 521 MPa.

The concrete cast had a target compressive strength of 50 to55 MPa. Ordinary (ASTM C150 Type I) Portland cement, fineaggregate (medium sized natural sand), and coarse aggregate(crushed limestone) in the ratio of cement/sand/coarse aggre-gate=410:658 :1120 at a water/cement ratio of 0.44 were used toprepare the concrete mixture. Concrete cubes with dimensions of100×100×100 mm3 were also cast for the concrete compressivestrength. After casting, the concrete specimens, both test speci-mens and cubes for compressive strength, were kept in themouldsand cover with damp polyethylene for 3 days. They were thendemoulded and kept in a curing roommaintained at 20±3 °C and90% or greater relative humidity. The 28-day cured specimenshad an average measured strength of 52.1 MPa.

An electrolyte corrosion technique was used to accelerate steelcorrosion. The 28-day cured specimens were fully immersed in a5% NaCl solution in a plastic tank. Electric current and the dura-tion of exposure were used for the theoretically estimated massloss of reinforcing steel due to corrosion according to Faraday'sLaw. Power supplies with adjustable voltage and direct current(from 0 to 2 A) were chosen for the electrolyte corrosion process.The direct current was impressed on the main steel bar embeddedin the concrete, using an integrated system incorporating a rec-tifier with a built-in ammeter to monitor the current, and a po-tentiometer to control the current intensity. The direction of theelectric current was such that the reinforced steel bar served as theanode while a stainless steel plate counter-electrode was posi-tioned in the tank to act as a cathode. The stainless steel plate thatserved as the cathode consumed the excess electrons given off bythe reinforcement during the corrosion process. The theoreticallyestimated mass loss of steel due to corrosion can be expressed bythe following equation [4]:

mt ¼ T � I � 55:8472� 96487

ð1Þ

where T is the duration of exposure and Ī the average magnitudeof electrical current.

Eq. (1) is based on Faraday's Law, which states: (1) The massof substance formed or consumed in electrolysis is proportional tothe amount of charge passing through it; (2) The mass of asubstance formed or consumed in electrolysis is also proportionalto its molar mass; (3) The mass of a substance formed or con-sumed in electrolysis is inversely proportional to number of elec-trons per mole needed to cause the indicated change in theoxidation state. The quantity of applied charge of any electrolysisis given by the product of applied current and the duration ofexposure. For the corrosion process, for each mole of iron oxi-dized, 2 mol of electrons are given out, consuming a charge of2×96487 coulombs (C). The mass loss is then calculated bymultiplying the applied charge by the molar mass (55.847 g/molfor iron) and dividing by charge needed per mole. This techniquewas successfully adopted by many researchers [4,10].

The mass loss of reinforcing steel due to corrosion was usedto select the density of electric current and the duration ofexposure according to Eq. (1). The actual degree of corrosion, orcorrosion level was, however, measured as the loss in weight ofthe steel bar relative to the weight of the bond length before

Fig. 2. Test setup.

Fig. 4. The chosen function.

1933C. Fang et al. / Cement and Concrete Research 36 (2006) 1931–1938

corroding, thereby representing an average corrosion level alongthe bond length; see Eq. (2). The actual corrosion level wasdetermined with the following equation:

CR ¼ G0−Gg0l

� 100% ð2Þ

whereG0 is the initial weight of the steel bar before corrosion,Gthe final weight for the steel bar after removal of the corrosionproducts, g0 the weight per unit length of the steel bar, and l thebond length.

The pullout tests were performed using a universal testingmachine that had a capacity of 50 kN. A specially designedloading frame was used for the tests under pullout loading, asshown in Fig. 2. In order to measure the slip more accurately, onecrack opening displacement (COD) gauge was used duringloading when the slip was less than 2.0 mm, while a linearvariable differential transducer (LVDT) (not shown) was usedduring the entire loading cycle. The outputs from the COD and

Fig. 3. Physical interpretation of the v

the LVDT as well as the loads were connected to a computer. Allthe loads were applied through the computer, using displace-ment-control. For all the tests, the maximum strain in the steelbars was less than 30 MPa and yielding of the steel bars was notobserved.

3. FE analysis

The modelling method used is specially suited for detailedfinite element analyses, where both the concrete and the rein-forcement are modelled with solid elements. Special interfaceelements were used at the surface between the reinforcement barsand the concrete to describe a relation between the traction t andthe relative displacement u in the interface. The physical inter-pretations of the variables are as described in Eq. (3).

t ¼ tntt

� �¼ normal stress

bond stress

� �u ¼ un

ut

� �

¼ relative normal displacementslip

� �ð3Þ

The corrosion model and the bond model developed in[22,26] were used to describe the effect of corrosion and bond.The corrosion model and the bond model can be viewed as twoseparate layers around a reinforcement bar. However, to reducethe number of nodes required to model a structure, they areintegrated in one interface element. Due to equilibriumbetween the two layers, the traction t is the same in the bond

ariables in the corrosion model.

Fig. 5. The resulting function.

Fig. 7. Bond stress–slip curve in confined non-corroded steel bar.


and in the corrosion layer. The deformations are related as inEqs. (4) and (5).

un ¼ uncor þ unbond ð4Þut ¼ utbond; utcor ¼ 0 ð5Þ

where the index cor means for the corrosion layer, and theindex bond means for the bond layer.

All input parameters for both the corrosion and the bondmodel were chosen as described in [22,26]. No adjustmentswhatsoever was done to fit the results of the experiments des-cribed here. The corrosion and bond model are briefly describedhere, for details see [22,26].

In the corrosion model, the swelling effect of the rust wasincluded, and rust was assumed to behave like a granular ma-terial; i.e. its stiffness increased with the stress level. Fig. 3

Fig. 6. Mesh for FE analysis of pullout test.

shows the physical interpretation of the variables in the cor-rosion model.

The volume of the rust relative to the uncorroded steel, andthe corrosion penetration as a function of the radius, i.e. howmuch the radius would increase if the normal stress was zerowas calculated from Eq. (6):

a ¼ −r þffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffir2 þ ðv−1Þd ð2rx−x2Þ

pð6Þ

where r is the radius of the reinforcement bar, and the othervariables are defined in Fig. 3. The variable v is the volume of therust relative to the uncorroded steel. This is known to varydepending on what corrosion product that forms. Here, it wasassumed to be 2.0, as the other model parameters were calibratedfor that value in [26]. It can be noted that the same value was usedalso in the work of for example [27,28]. The real increase of theradius is uncor, corresponding to a strain in the rust:

ecor ¼ uncor−axþ a

ð7Þ

From Eq. (7) strain in the rust, the normal stresses in the layerwere determined.

Fig. 8. Bond stress–slip curve in confined steel bar of 3.8% corrosion.

Fig. 10. Bond stress–slip curve in unconfined non-corroded steel bar.


As mentioned before, the corrosion model was combinedwith a model of the bond mechanism [22]. Corrosion of thereinforcement was assumed to influence the coefficient of fric-tion; for small corrosion penetration, a slight increase in thefriction was assumed, while for larger corrosion penetration, thefriction was decreased. This was done by introducing a functionk(x/r), and letting the coefficient of friction be calculated as inEq. (8).

lðjÞ ¼ kðx=rÞd l0ðjÞ; but lðjÞz0:4 ð8Þwhere μ0(κ) is the function chosen for the coefficient of frictionfor uncorroded reinforcement. The function k(x/r) was chosenas shown in Fig. 4. The resulting function μ(κ) for some cor-rosion levels are plotted in Fig. 5. The lower limit of thecoefficient of friction comes from an assumption that this cor-responds to the friction between concrete and concrete. Forpullout failure, the failure mode is assumed to be shear crackingbetween the ribs, and therefore it appears reasonable that thefriction at large slips does not depend on the level of corrosion.

The corrosion and bond interface elements were used betweensolid elements describing the steel and the concrete in the FEanalyses. Fig. 6 shows the mesh for FE analysis of the pullout testspecimen used in this study. These types of analyses can be donefor arbitrary geometries, simply by changing the meshes. Somemore examples can be found in [26]. In some examples there,shrinkage of the concrete was included. The concrete was mod-elled with a constitutive model based on nonlinear fracture mech-anics. In analyses using the smeared crack concept, a rotatingcrackmodel based on total strain was used. Axisymmetric modelswere used where four discrete radial cracks were assumed.

4. The results and discussion

Typical tested specimens were modelled, and typicalcalculation results were compared with the test results. Thenumerical predictions were generally in reasonably good agree-ment with the observed maximum bond strengths and stiffnessesof the ascending stage on bond stress–slip curves. Figs. 7–9

Fig. 9. Bond stress–slip curve in confined steel bar of 6.1% corrosion.

show FE analyses compared with the tests for confined bars. Itcan be noted that there is a discrepancy in stiffnesses for smallslips; with less stiffness obtained in the analyses compared to thetests. One reason for this discrepancy can be that the bond modelused is mainly calibrated for large slips. Another reason can bethat the rib geometry is not exactly similar in these tests as inthose the model was calibrated for.

Fig. 7 shows the comparison for confined control specimen(non-corrosion specimen). The maximum bond strengthreached was close to the prediction. A sharp drop in bondstress after the maximum value was observed in bothprediction and test. The drop in bond stress was caused bycracking in the cover concrete. Fig. 8 shows bond stress–slipcurves from FE analysis compared with the tests for a confinedbar with a medium corrosion level of 3.8%. The maximumbond strength as well as bond stiffness of the ascending stageof the tests were close to the calculated curve. Larger slip thanthe control specimen was observed after the maximum bondstress was reached. In Figs. 8 and 7, it can be seen from bothexperiment and analysis results that the bond strength of theconfined steel bar of 3.8% corrosion was only 10% lower than

Fig. 11. Bond stress–slip curve in unconfined steel bar of 4.0% corrosion.

Fig. 12. Bond stress–slip curve in unconfined steel bar of 6.1% corrosion.


that of the confined control specimen (no corrosion). Thisindicated that for confined deformed bars, corrosion of amedium level (around 4%) had no substantial influence on thebond strength. A possible explanation may be that, forcorroded reinforcing steel bars with a low to medium levelof corrosion, although corrosion had caused fine cracks in thespecimen, the stirrups provided enough confinement. When apullout load was applied to the specimen, the bearing action ofthe ribs of the deformed bars against the concrete causedhorizontal bearing stresses as well as hoop stresses. Due to thecorrosion cracks, the tension in the specimen was reduced andtherefore the confinement was reduced and the slip increasedwhich finally resulted in bond failure of the specimen. For thecorrosion levels in this range, bond failure usually was theresult of splitting of the specimen along the corrosion cracks.For this type of failure, the corrosion-induced cracks hadalready weakened the concrete.

Fig. 9 shows bond stress–slip curves from FE analysis com-pared with the tests for a confined bar with a corrosion level of6.1%. No apparent failure was observed until the slip reached1.5 mm. This can be considered as a post-peak plateau or residualbond strength.

Figs. 10–12 show bond stress–slip curves from FE analysiscompared with the tests for unconfined specimens of acorrosion level of 0% (control specimen), 4.0% and 6.1%,respectively. In Fig. 10, similar to the confined controlspecimen, a sharp drop in bond stress after the maximumvalue was also observed in both analysis prediction andexperiment. For both experiment and analysis results, thedrop in bond stress was caused by splitting in the coverconcrete. In Fig. 11, the bond stiffness of both experiment andanalysis results was greater than that of control specimen.

In Fig. 10, the bond strengths of the unconfined steel bar ofthe control specimen (no corrosion), 22 and 23.5 MParespective of test and analysis, were not substantially lowerthan that of the confined steel bar of the control specimen (nocorrosion), 22 and 25.8 MPa, as shown in Fig. 7. In Fig. 11, thebond strengths of the unconfined steel bar of 4.0% corrosion,12 MPa from both experiment and analysis, was 30–35% lower

than that of confined steel bar with a close corrosion level of3.8%, as shown in Fig. 8. Also, in Fig. 12, the bond strengths ofthe unconfined steel bar of 6.1% corrosion, 6–6.3 MPa fromanalysis and tests, were 65% lower than that of the confinedsteel bar with the same corrosion level, as shown in Fig. 9.These findings demonstrated that confinement supplied an ef-fective way to counteract bond loss for corroded steel bars of amedium corrosion level (around 4% to 6%). The possible ex-planation may be: The presence of the normal stress is acondition for transferring bond stress after the chemical ad-hesion between reinforcing bars and concrete is lost. If thenormal stresses are lost, bond stresses cannot be transferred.This occurs if the concrete around the reinforcement bar ispenetrated by longitudinal splitting cracks, and if there is noconfining reinforcement that can continue to carry the forces.This type of failure is splitting failure. When the concretesurrounding the reinforcement bar is well-confined, it canwithstand the normal splitting stresses, and a pullout type failuretakes place. Under these conditions, the failure is characterizedby shear cracking between the adjacent ribs.

After conducting the pullout text, the specimens werebroken and the weight loss due to corrosion of the bondportion of steel bar was determined by cleaning it withClark's solution (ASTM G1-76). Similarly, the degradation inthe rib profile was determined by measuring the rib heightbefore applying the impressed current and after cleaning itwith Clark's solution. From the results it was observed thatthe actual corrosion levels measured as the weight loss of thesteel bars were generally lower than the correspondingtheoretically estimated ones. The difference between the twoindicates that other factors such as the permeability ofconcrete may had played a role in the corrosion. Surfacetreatment may also be a factor which had influenced thecorrosion [29]. Another factor that may have affected thecorrosion is the heterogeneity of the coarse aggregate withinthe specimen. In the current study, however, these factorswere not included in the theoretically estimated corrosionlevels when Faraday's Law was used. In the current study, themaximum difference between measured and theoreticallyestimated corrosion level was 20%. The maximum differencein bond strength between replicate specimens was 22%.Despite the data scatter, reasonably good agreement wasachieved between tests and FE analysis results with respect tobond strength and bond failure type. It should be noted thatthe differences between the theoretically estimated andactually measured corrosion levels had no influence on theconclusions, since only the measured corrosion levels wereused for the analyses of the test results.

5. Conclusions

(1) For confined deformed bars, a medium level (around 4%) ofcorrosion had no substantial influence on the bond strength.But substantial reduction in bond took place when corrosionincreased thereafter to a higher level of around 6%.

(2) For non-corrosion bars, the bond strengths of unconfinedbars were not significantly lower than that of confined


bars. For corroded bars of a corrosion level of around 4–6%, however, both experiment and analysis resultsshowed that the bond strengths of unconfined steel barswere 30–65% lower than that of confined steel bars of asimilar corrosion level. It is thus concluded that confi-nement supplied an effective way to counteract bond lossfor corroded steel bars of a medium corrosion level(around 4% to 6%).

(3) The results of finite element analyses, where it wasassumed that rust behaved like a granular material,showed a reasonably good agreement with the experi-ments regarding maximum bond strength and bondstiffness of the ascending stage on the bond stress–slipcurve.

(4) The above findings and conclusions are preliminary, andthe first author plans to repeat some of these experimentsin future research.

Notationmt theoretically estimated mass loss of reinforcing steel

due to corrosionT duration of exposure during accelerated corrosionĪ average current over the duration of exposure which

the reinforcement was exposed toCR corrosion levelG0 the initial weight of the reinforcing steel before corrosionG the weight of reinforcing steel after removal of the

corrosion productsg0 the weight per unit length of the reinforcing barl bond lengtha free increase of the radius due to corrosionr radius of the reinforcing steelv volume of rust/uncorroded volume of steelx corrosion penetrationεcor strain in the rustucor normal deformation in corrosion layer; i.e. increase of

the radius due to corrosiont tractionu relative displacementtn normal stresstt bond stressun relative normal deformationut slipuncor normal deformation of the corrosion layer when the

cover cracksunbond normal deformation in the bond layerutbond slip in the bond layerutcor slip in the corrosion layeru(κ) coefficient of frictionk(x/r) introduced functionu0(κ) function chosen for the coefficient of friction for

uncorroded reinforcing steel

Acknowledgements

The first author gratefully acknowledge the support of ChinaState Scholarship and Swedish Institute (SI) Scholarship, which

he received for exchange program between the two govern-ments, for a 1-year visit as a guest professor in the Departmentof Civil and Environmental Engineering, Structural Engineer-ing, at Chalmers University of Technology.

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Bond behaviour of corroded reinforcing steel bars in concrete