256 ACI Materials Journal/July-August 2005

ACI MATERIALS JOURNAL TECHNICAL PAPER

ACI Materials Journal, V. 102, No. 4, July-August 2005.MS No. 04-217 received July 9, 2004, and reviewed under Institute publication

policies. Copyright © 2005, American Concrete Institute. All rights reserved, includingthe making of copies unless permission is obtained from the copyright proprietors.Pertinent discussion including authors’ closure, if any, will be published in the May-June 2006 ACI Materials Journal if the discussion is received by February 1, 2006.

Corrosion of embedded reinforcement is the most prevalent form ofdegradation of reinforced concrete structures, and may impairstructural capacity through loss of bar section, loss of bond betweenreinforcement and concrete as a result of longitudinal cracking, orloss of concrete cross section. The effect of corrosion attack onmechanical properties of reinforcement is investigated throughphysical tests on bars with simulated and real corrosion damageand through a simple numerical model. Bars subjected to local orpitting attack may suffer a relatively modest loss of strength but asignificant loss of ductility, and this is related principally to thevariability of attack along the length of the bar. The numericalmodel supplements experimental work through a parametric studyon the influence of steel characteristics. Finally, guidelines onassessment are suggested that are derived from results reported inthe paper and from elsewhere in the published literature.

Keywords: corrosion; ductility; reinforcement.

INTRODUCTIONCorrosion of embedded reinforcement is the principal

cause of deterioration of structural concrete and a majoreconomic cost for maintenance of national infrastructures.1

The effect of this deterioration on residual capacity is there-fore a matter of concern to those charged with ensuring safeoperation of concrete structures. It is clear, however, thatmany reinforced concrete structures remain in service oncereinforcement has started to corrode and cover concrete overthe bars has begun to spall; there is extensive evidence thatmodest amounts of corrosion do not pose an appreciablethreat to structural stability. Nonetheless, it is essential thatresponsible engineers have at their disposal the means toverify that the affected structures retain an acceptable marginof safety. However, data and methods from which a reliableassessment of residual capacity of corrosion-damagedstructures can be made are scarce.

Corrosion may affect residual capacity through severalmechanisms, including loss of bar section, loss of concretesection as a result of longitudinal cracking and spalling, anda reduction in the interaction, or bond, between reinforcementand concrete.2,3 This study focuses on the first of thesemechanisms, namely the changes to mechanical propertiesof reinforcement as a consequence of corrosion.

Corrosion attack may be broadly classified as either: 1)uniform; or 2) localized, sometimes referred to as pitting attack.The consequences of these two forms of attack differ markedly.Uniform attack can be addressed simply by using the residualcross section of the bar with essentially unchanged mechanicalproperties; only in the case of substantial loss of section on rein-forcement manufactured through the quenched and temperedprocess might an adjustment be necessary to account for thevariation in material properties through the bar section.

Pitting attack is a more insidious form of attack for tworeasons. First, the localized nature of attack together with aless expansive form of oxidation products from the corrosion

reactions means that substantial section loss may occur priorto warning signs of longitudinal cracking becoming visibleon the surface of the member. Local corrosion sites are,however, readily detectable by the half-cell method, wherethey appear as a zone of strongly negative potential surroundedby a high potential gradient. Second, pitting attack affectsnot only strength, but also ductility. The normal design andassessment rules established in codified procedures are basedon an assurance of adequate ductility that may no longerbe applicable under pitting attack, and could thus be unsafe.

This paper reports a study into the effects of corrosion onresidual mechanical properties of steel reinforcementconducted through numerical modeling, physical tests inwhich corrosion damage was simulated by machined defects,and physical tests in which corrosion was accelerated bymeans of anodic polarization. This report is confined toperformance under static loading only.

RESEARCH SIGNIFICANCEThe paper describes the behavior of reinforcing bars

suffering irregular loss of cross section as a consequence ofcorrosion and identifies the factors that will affect theresidual mechanical behavior of the bars. Although themathematical modeling techniques deployed are unlikely tofind direct use in practice, the understanding developedthrough their use will guide engineers to be responsible forsafe operation of corrosion-affected concrete structures inthe application of empirical knowledge.

NUMERICAL MODELINGA simple nonlinear numerical model was constructed to

assess the influence of various parameters on residualstrength and ductility of corroded reinforcement, includingthe effects of pitting attack. The model was implementedthrough a spreadsheet.

A stress-strain relationship for an undamaged bar was firstobtained through testing (or, in the case of the parametricstudy, assumed). Steel properties were assumed uniformthroughout the volume of the bar and to be unaffected bycorrosion. There is evidence from other studies, includingPalsson and Mirza,4 to support the latter assumption. Inanalyses reported herein, a simple trilateral approximation tothe measured stress-strain relationship has been consideredadequate (Fig. 1, Plot A).

The bar was divided into short incremental lengths foranalysis. The variation of the cross-sectional area along a barwas then measured, or an assumed variation was derived.The elongation of each increment of bar length was then

Title no. 102-M29

Mechanical Properties of Corrosion-Damaged Reinforcementby John Cairns, Giovanni A. Plizzari, Yingang Du, David W. Law, and Chiara Franzoni

257ACI Materials Journal/July-August 2005

calculated under successive increments of load using theaverage stress-strain relationship for the undamaged bar andthe residual cross-sectional area for that increment of length.The model ignores stress concentration and eccentric loadeffects, which test results show to have no appreciable effectfor an embedded bar under static loading. Average strain wascalculated by taking a summation of incremental elongationsand dividing by the gauge length. Fracture is taken to occurwhen the most highly strained increment reaches εu, thestrain at maximum load measured in the undamaged bar.Average strain at fracture and ultimate load were calculatedat this point. Yield stress was also determined as 0.002 proofstress, based on average elongation and stress.

Figure 1 shows the application of the model to a simplecase where the cross section of the bar is locally reduced by10% over a length equal to one bar diameter. The gaugelength is taken as 20 bar diameters. Plot A shows the assumedstress-strain relationship for the reference undamaged bar. Theplot shows an initial linear-elastic portion followed by a shortyield plateau and a strain-hardening phase. The ultimate tensilestrength of the bar is taken to be 1.15 times the yield strength.Plot B shows results for the damaged case. Stress is based onthe reference cross section. A comparison of Plots A and Bshows no change in the apparent yield strength of the bar asa result of local damage, but the ultimate tensile strength isreduced by 10%. As code strength procedures are based onyield rather than ultimate tensile strength, however, thisreduction would have little effect on calculated sectionstrength. The major change in mechanical characteristics isin ductility—a property of particular significance for plastic

analysis and seismic resistance—which is reduced byapproximately 50% in this illustration.

Despite the local damage, apparent yield strength ismaintained because the ultimate tensile strength of the steelcan be exploited where the cross section is locally reduced.The corresponding ultimate strains at the locally reducedsection develop only over the short length of the pit, andhence have little effect on overall elongation. Strain capacityat the reduced section is exhausted before appreciableyielding can develop in the remainder of the bar and thusoverall elongation at fracture is reduced.

MECHANICAL TESTS ON BARS WITHSIMULATED PITTING DAMAGE

A series of tests to investigate the influence of localdamage on mechanical characteristics of reinforcement wereconducted in the Structural Engineering Laboratories at theUniversity of Brescia. In this part of the study, corrosiondamage was simulated by removing a section of bar using amultifluted, hemispherical end mill with a cylindrical shank(Fig. 2). This enabled a realistic simulation of the pittingcorrosion. The tests were carried out on deformed B500Bbars having diameters of 12, 16, 20, and 24 mm. The steelsused in the tests conformed to draft European regulationrequirements.5 Several degrees of section reduction werecreated using mills of 4, 6, 8, and 10 mm radii. The proportionof the cross section removed at the most damaged section (onthe axis of the milled defect) were 5, 10, 20, 30, 40, and 50%of the nominal area of the bar section.

Reference undamaged and damaged bars were subjectedto tensile testing. The deformation of the bar in the damagedarea was measured by two linear variable differential trans-formers (LVDTs) with gauge lengths equal to five times thediameter of the bar. One transducer was placed in front of themachined defect, while the other was placed behind it on theintact side of the bar. Elongations plotted herein wereobtained from the average of the two measurements. Loadand displacement data were logged by a data acquisitionsystem at a frequency of 1 Hz.

Sample results from the tensile tests are presented in Fig. 3,which shows load-displacement plots obtained from a 12 mm-diameter bar with various degrees of damage machined usingan end mill of 8 mm radius. The reduction of the maximumload is proportionate to the damaged area, while the reductionin the force at yield is slightly less-than-proportional to thesection loss. The main change in mechanical performance,

ACI member John Cairns is a senior lecturer in the School of the Built Environment,Heriot-Watt University, Edinburgh, UK.

ACI member Giovanni A. Plizzari is a professor of structural engineering in theDepartment of Engineering Design and Technology, the University of Bergamo, Italy.His research interests include material properties and structural applications of high-performance concrete, fiber-reinforced concrete, concrete pavements, fatigue andfracture of concrete, and steel-to-concrete interaction in reinforced concrete structures.

Yingang Du is a lecturer of civil and structural engineering at Telford College,Edinburgh. He received his BEng and MEng from Xian University of Architecture andTechnology, China, and his PhD from The University of Birmingham, UK. Hisresearch interests include the safety and durability of concrete structures undercorrosion, earthquakes, fire, and high temperature.

David W. Law is employed by the Advanced Materials Group for Maunsell Australia,Melbourne, Australia. His research interests include corrosion monitoring usingnondestructive electrochemical techniques, durability assessment, predictive modeling,and life-cycle management of reinforced concrete structures.

Chiara Franzoni is a professional engineer. She received her degree in civil engineeringfrom the University of Brescia. Her research interests include the durability of concrete,corrosion of reinforcing bars, and zinc-coated reinforcing bars.

Fig. 1—Analysis of bar containing single defect. Fig. 2—Schematic of machined defect geometry.

258 ACI Materials Journal/July-August 2005

however, is the significant reduction in bar ductility causedby the absence of yielding out with the damaged area.

The marked reductions in ductility measured on 12 mm-diameter bars are summarized in Fig. 4, which shows thevariation in displacement corresponding to a postpeak loadequal to 99% of the maximum load as a function of residualcross section at the defect; ductility of bars with 5 and 50%of section loss at the damage is reduced by 30 to 40% and byapproximately 80%, respectively. The diameter of theartificially induced defect does not exert a significantinfluence on mechanical properties. The other bar diameterstested show similar results.

MECHANICAL TESTS ON BARS WITH ACCELERATED CORROSION DAMAGE

Although simulated damage provides a convenient andcontrolled way of investigating the effects of section loss, itdoes not adequately represent the true nature of corrosionattack. This section of the study reports tests on barssubjected to corrosion attack. To obtain results within areasonable timescale, corrosion was accelerated under animpressed current. Two types of specimens were tested: oneset at Heriot-Watt University, Edinburgh (designated as theHW series), and a second at the University of Brescia(designated as the UB series). The HW specimens used plainround bars of 16 mm diameter, while the UB specimens used20 mm-diameter high-yield ribbed bars. Test bars were castin concrete before being subjected to corrosion acceleratedby anodic polarization. This produced a mixture of general

and pitting attacks of varying intensities throughout thelength of the bar, and although it cannot be claimed that theprocedure produced damage that exactly simulated long-term field corrosion, it is more representative than single-machined defects.

Bars in Series HW specimens were cast in the corners ofcubic beam end-type bond specimens. Specimens wereconditioned under a cyclic wetting/drying regime of 1 day ofwetting by 3% salt solution followed by 6 days at a relativehumidity of approximately 70%. Stainless steel anglesections were placed on the test specimens and connected tothe negative terminal of a controlled-current power supply.Test bars were connected to the positive terminal of thesupply. A conductive foam was placed between cathodicplates and concrete to ensure good connectivity. Currentdensities of between 0.01 and 0.05 mA/cm2 were appliedduring the no-spray part of the conditioning cycle. A total of25 bars were tested.

In the six specimens in Series UB, bars were cast inconcrete cylinders, and after demolding, were conditionedunder an impressed anodic potential while partly immersedin a 5% salt solution. At the end of the conditioning period,bars were broken out of concrete and cleaned in an acid bath.The bars were subjected to varying degrees of corrosion.Some had only superficial corrosion with no significant lossof section, while others underwent severe corrosion with upto 72% of the section lost at the most severely damaged section.

Measurements of corrosion attackBecause of the different levels of corrosion attack induced

in the two sets of specimens, different methods were used tomeasure section loss. In the less heavily corroded HW series,bars were weighed to determine the average loss of the crosssection and average corrosion penetration was calculated.Bars were also examined in detail, and dimensions of the largestpits were measured by a graduated magnifier and micrometer.The movable anvil of the micrometer was a conical shapewith a point radius of 1 mm to allow it to reach to the bottomof the larger pits. The area lost at the pit was calculated as π/4times the depth and breadth of the pit. Test bars had meansection loss (based on weight loss measurements) of up to 4%,with a maximum reduction at an individual pit of 8%.

Residual cross sections of the more heavily corroded UBseries bars were measured using a liquid displacementtechnique. The test bar was placed in a narrow calibratedmeasuring cylinder, and a known quantity of liquid wasadded to the cylinder. The volume of the test bar could thenbe determined from the incremental rise in liquid level. Theprocedure was repeated to obtain a profile of the crosssection along the entire gauge length.

Corrosion topographyMeasurements on the relatively lightly corroded HW

series bars showed that the breadth of pits (that is, thecircumferential dimension) averaged slightly less than twicethe depth, confirming that the assumption that pits arecircular in cross section is reasonable. Section loss at a pitthus increases approximately in proportion to the square ofits depth (or width).

The maximum loss of section at a pit averaged approximatelytwice the average loss of section (Fig. 5), although there wasa wide scatter. The ratio of maximum pit depth to meanpenetration was generally in the range of 10 to 50, with atendency for the ratio of maximum pit depth to mean

Fig. 3—Plots of load versus elongation from tensile tests on12 mm-diameter bars with 8 mm-radius machined defects ofvarious depths.

Fig. 4—Displacement at peak load for various damagelevels and geometries, 12 mm-diameter bars.

ACI Materials Journal/July-August 2005 259

penetration to reduce as corrosion progresses (Fig. 6). Inpitting corrosion, a relatively large cathode drives corrosionat a concentrated anode. As the size of the pit/anodeincreases, the ratio of anodic area to cathodic area willreduce, hence reducing the rate of depletion at the pit. It istherefore to be expected that the rate of penetration at the pitwould slow in relation to the mean rate of penetration ascorrosion progresses. The ratio lies above that recommendedin the CONTECVET Manual,2 probably due to the higherdegree of corrosion covered in supporting investigations.

Two of the corroded test bars from Series UB are shownin Fig. 7(a). One bar suffered only light surface corrosion,and section loss did not exceed 2% in any length increment.The second bar was severely corroded with approximately72% of the cross section lost to corrosion at the mostseverely damaged section. The variation in diameter and thein cross-sectional area along the second bar are plotted inFig. 7(b); it illustrates the highly nonuniform nature oflocalized corrosion attack. No meaningful conclusions canbe drawn about the relationship between local and mean attackpenetrations from these relatively short bar specimens, however.

Residual mechanical characteristicsof corroded bars: HW series

Tests for mechanical properties were conducted followingsimilar procedures to those used for bars containing machineddefects, with the exception that a gauge length of 12.5 timesbar diameter was used for HW series bars. Figure 8(a) showsthe stress-strain diagram for Bar F3, a representativereference (uncorroded) bar from the HW series. The plot ischaracteristic of a mild steel, with a clearly defined yieldpoint and yield plateau, followed by a strain-hardeningportion. The descending tail on the plot marks the unloadingof the bar shortly after peak stress when the test was haltedto avoid damage to the strain sensor if the test bar was tofracture. Reference uncorroded test bars had a yield strengthof 311 N/mm2, an ultimate-to-yield strength ratio of 1.46, andan elongation at fracture of 29%, exceeding the requirementsof the relevant standard.6 Mean strain at maximum force wasnearly 20%.

Even after corrosion, all test bars met the requirements ofthe standard, although two of the bars came close to the limitwith an elongation at fracture of 22.5%, compared with thespecified minimum of 22%. Figure 8(b) plots yield andultimate tensile strengths of bars, calculated on the residualcross section, against section loss at the largest pit. Figure 8(b)

shows there to be no loss in yield strength when strength iscalculated in this way, while ultimate tensile strength showsa slight increase with increasing section loss. By the use oflinear regression analysis, it was found that the ultimatetensile strength increased by approximately 5.7% for a 7.0%loss in section at a pit. In effect, this means that the ultimateforce developed in the bar is reduced by only approximately1% for a 7% loss in section. If results are plotted using meaninstead of maximum section loss, the change in peak strengthwith corrosion is not found to be significant.

The apparent increase in the ultimate tensile strength at apit is at variance with what would be expected from both asimple analysis and from the numerical model. A possibleexplanation was put forward by Castel, Francois, andAirliguie.7 If a bar does not have a completely uniform crosssection and material composition throughout its length, thenit would clearly be expected to fracture at the point where thematerial is weakest. If the position of the pit does not coincidewith the location where the steel is weakest, an apparentincrease in strength (where this is based on the minimumcross-sectional area) will be measured.

Corrosion initiates where micro-differences in environmentor materials’ composition allow micro-electropotential differ-ences to develop. Steel strength is also influenced by minordifferences in alloy composition. If the differences that result inone section being of stronger composition than the next alsocaused corrosion to develop preferentially at the same location,

Fig. 5—Relationship between mean section loss and loss ofcross section at pit measured in HW series.

Fig. 6—Decreasing rate of penetration at pit with increasingcorrosion.

Fig. 7—Test bars from Series UB: (a) lightly corroded (Test 1)and heavily corroded (Test 2) test bars; and (b) variation incross section and diameter along heavily corroded bar.

(a)

(b)

260 ACI Materials Journal/July-August 2005

then it is apparent that bar strength would be reduced by a lesseramount than the bar section. Given that pure iron generallycorrodes less but is weaker than a mild steel alloy, one can spec-ulate that corrosion pits are more likely to develop where themetallic structure is composed of a stronger alloy.

Figure 8(c) shows the influence of corrosion on ductility.Two sets of data are represented. Open squares representstrain at fracture and are plotted against the left-hand axis.Filled triangles represent elongation at fracture (elongationmeasured over the fracture on a gauge length of five timesthe bar diameter), and are plotted against the right-hand axis.Both measures show a reduction in ductility with increasingcorrosion. The reduction in ductility is evidently moremarked than the reduction in strength shown in Fig. 8(b).

Comparison with results from numericalmodel: HW series tests

The numerical model was also applied to the test bars eventhough the variation in cross section along test bars was notmeasured in full detail. To simplify input data, it was assumedthat the mean loss of section occurred at all incrementallengths, except for one increment where the area losscorresponded to the area of the largest measured pit. Analysisshowed that results were not sensitive to the presence ofsmaller pits. For a maximum section loss of 8%, correspondingto the highest section loss plotted in Fig. 8(b) and (c), nosignificant loss of yield strength was measured, and theaverage strain at maximum load decreased from 0.20 to 0.15(Fig. 8(d)). The numerical model results lie within the rangeof the experimental scatter shown in Fig. 8(b) and (c).

Residual mechanical characteristics of corroded bars: UB series

Plots of stress (based on original cross section) againstelongation measured for the two bars from the UB series

shown in Fig. 7(a) are presented in Fig. 9(a). The bar in Test1 had suffered only a 2% loss of section, while the Test 2 barsuffered a maximum section loss of around 70%. The reductionin both strength and ductility is clearly evident.

Plots obtained from the numerical model are alsopresented in Fig. 9(a). In this case, the bar cross section wastaken from the incrementally-measured variations throughoutthe gauge length. The plot shows that the model provides areasonable representation of behavior, with estimatedreductions in both strength and ductility corresponding wellto measured values. In Fig. 9(b), the variations in strainalong the length of the bar calculated by the model are alsoshown, and can be related to the variation in the cross sectionpresented in Fig. 7(b). The very high strains calculated at themost severely attacked sections are evident, with fracturedeemed to occur when the local peak reaches a limitingvalue. Overall elongation, however, is related to the integralof strain along the bar, that is, to the area under the plot. Itcan be seen that the contribution of the highly localized peakstrain does not provide a correspondingly large contributionto overall elongation.

PARAMETRIC STUDY USING MODELFollowing validation against measured data, the numerical

model described previously has been used to investigate theinfluence of corrosion attack and mechanical properties ofundamaged bars on residual mechanical performance.Figure 10(a) shows the trilinear stress-strain relationshipsused in analyses. Two lines are shown that represent the twotypes of steel considered. The solid line HR represents a hot-rolled mild steel, and the dashed line HRCW represents ahot-rolled, cold-worked bar. The only difference betweenthe two is the value of stress assumed at a strain of ε1 at theend of the second component of the diagram. The influence

Fig. 8—Results from Series HW bars.

ACI Materials Journal/July-August 2005 261

of the ratio of ultimate to yield strength fu/fy, of strain at peakstress εu, and of the type of bar, whether hot-rolled or cold-worked, represented by the shape of the stress-strainrelationship and the ratio f1/fy have been investigated.Default values in the analysis were taken as Es = 205 kN/mm2;fy = 460 N/mm2; fu = 552 N/mm2 (fu/fy = 1.2); εu = 6%; ε1 =4 × fy/Es; average section loss = 8.6%; and f1/fy = 1.0005(where f1 is the stress corresponding to strain ε1), repre-senting a hot-rolled bar with a clearly defined yield plateau.

Loss of section was represented by assuming that sectionloss from corrosion followed a random normal distributionalong the bar. The validity of this assumption has not beenchecked. The same randomized distribution was used in allanalyses. Different levels of corrosion were represented byfactoring this randomized distribution (Eq. (1)). This step isquestionable as the rate of penetration at pits in relation to theaverage penetration is expected to reduce as the pits increasein area. There is, however, no more accurate information onwhich to proceed at this time. For these reasons, the parametricanalysis should be considered to provide only qualitative results

(1)

where A0 and Ares are the original and residual cross-sectional areas, respectively, for the incremental length of barconsidered; {Random(Alos)} is a normally distributed randomvariable representing the cross-sectional area lost to corrosion;and UF and PF are coefficients representing mean and localsection loss, respectively, used in the parametric analysis.

The variation in mechanical characteristics of the bar withincreasing amounts of corrosion predicted by the model isshown in Fig. 10(b). The variation in cross section along thebar was obtained by varying coefficient PF in Eq. (1) whilemaintaining coefficient UF = 1.0. Ultimate tensile strengthand yield strength are plotted on the left axis; strains areplotted on the right. Bar stress is based on the averageresidual cross section. Figure 10(b) shows ultimate tensilestress fu to reduce with mean section loss, that is, withcorrosion. Bar tensile strength is determined by minimumcross section, and thus reduces more rapidly than an averageloss of bar section. Nominal yield strength fy also reduceswith increasing corrosion, although the reduction is some-what less marked. At an average section loss of 10%, forexample, fu and fy were calculated to reduce by approximately20 and 10%, respectively. The calculated reduction inductility is substantially greater, however, at around 75%. Ata mean section loss of 20%, the strain at fracture is reducedclose to the yield strain, and any semblance of overall plasticbehavior is lost. As stated previously, only a qualitativeassessment can be made because of the assumptions made,particularly in respect to corrosion topography, and thevalues given herein are intended only to demonstrate that barstrength may reduce more rapidly than a mean loss of sectionalone would predict, and that the effect on ductility will besubstantially greater than the effect on strength.

Uniformity of corrosion attack also influences residualmechanical properties (Fig. 10(c)). Herein, the uniformity ofcorrosion is adjusted by varying coefficient PF in Eq. (1) andadjusting coefficient UF to maintain a constant averagesection loss. Lower values on the horizontal axis thus representmore marked pitting attack, whereas a ratio of 100% representsuniform corrosion. Stress and strain are again plotted againstleft- and right-hand axes, respectively. Effective strength

Ares UF A0 1 P– F Random Alos( ){ }( )⋅=

and ductility (based on residual mean cross section) bothreduce as pitting becomes more marked, but the effect onductility is much greater. Note that if the plot were based ona minimum residual cross section, the ultimate bar strengthwould remain constant and the yield strength would increasewith increasing corrosion.

The influence of a change in ultimate tensile strength whileyield strength remains constant is examined in Fig. 10(d). Stressand strain are again plotted against left- and right-hand axes,respectively. Ductility is strongly impaired for bars with alow ratio of fu/fy, but the effect on yield strength is small.Similarly, ductility is strongly impaired for bars with a lowfracture strain εu (Fig. 10(e)).

Figure 10(f) shows stress-strain plots for two differenttypes of bar in both a corroded and noncorroded state. Plotsare offset from the origin to differentiate each set. The left-hand pair of plots represent a bar in which cold-working hasremoved a clearly defined yield point. The right-hand pair ofplots represent a hot-rolled bar that has a clearly definedyield point and plateau before strain hardening begins. Ineach pair, the left-hand plot represents the noncorroded bar,and the right-hand plot represents the corroded bar. Thedifference between the two is solely the value of f1, which isgreater in the case of the cold-worked bar. The two types ofbars have similar residual ultimate and yield strengths, butthe corroded cold-worked bar has appreciably poorerductility than its hot-rolled companion.

RESULTS FROM OTHER INVESTIGATIONSAlthough the numerical model outlined previously shows

a satisfactory correlation with test data and is useful forunderstanding of the factors that influence residual mechanicalcharacteristics of reinforcement, it is not suitable for practicaluse because of the lack of information on corrosion topography.Practical models for residual strength and ductility are, atpresent, thus confined to empirical correlations with section

Fig. 9—Results from Series UB bars.

262 ACI Materials Journal/July-August 2005

loss. Such relationships may be represented by the expressionsgiven in Eq. (2) to (4)

(2)

(3)

(4)

where fy, fu, and εu represent yield strength, ultimate tensilestrength, and elongation corresponding to ultimate strengthafter time t, based on the original cross section, respectively; fy0,fu0, and ε0 represent yield strength, ultimate tensile strength, andelongation of the noncorroded bar, respectively; Qcorr isaverage section loss, expressed as a percentage of original crosssection; and αy, αu, and αl are empirical coefficients.

A summary of α coefficients determined from variousinvestigations, mainly derived from the work of Du,8 aresummarized in Table 1. An αy or αu value of 0.01 representsa uniform corrosion attack where loss of strength is directlyproportional to the average loss of section. Values of αy orαu in excess of 0.01 represent the effects of nonuniformcorrosion attack.

All but two investigators report α values in excess of 0.01,indicating a more rapid reduction in strength properties thanin mean cross-sectional area. The relatively low weight loss oftest samples in the other two studies by Maslehuddin et al.9

and by Allam et al.10 may have obscured any variations.Castel, Francois, and Airliguie7 reported a 10% increase inultimate strength at the most heavily attacked section, butherein, stress was derived from the dimensions of theminimum cross section. The majority of results indicate thatthe reduction in the force at which a bar yields and its ultimatetensile strength is between 20 and 70% greater than would be

fy 1.0 αy Qcorr⋅–( )fy0=

fu 1.0 αu Qcorr⋅–( )fu0=

εu 1.0 α1 Qcorr⋅–( )ε0=

Fig. 10—Results from parametric study.

263ACI Materials Journal/July-August 2005

estimated on the basis of average loss of cross section. Thereduction in elongation is always greater than the reductionin strength. Changes are attributable to the nonuniformnature of corrosion attack as discussed previously.

α-coefficients determined from the HW series reportedherein tend to be less than in the majority of studies. Theplain round mild steel bars used in the HW series tests hadhigh ductilities and high ultimate-to-yield strength ratios(fu/fy). The parametric analysis reported previouslydemonstrated that such bars would be less susceptible tothe consequences of pitting attack.

Several other points are worth noting from the studiesreviewed in Table 1, although cross-study conclusions regardingthe effect of other parameters must be highly tentative due tothe several differences in exposure/conditioning of testsamples, test procedures, steel properties, bar diameters, andconcrete qualities between studies. • Morinaga’s study,11 in which bars were corroded in

service, produced larger reductions in strength andelongation than accelerated tests. This may indicatethat accelerated corrosion produces more uniformsection loss than service conditions.

• Zhang, Lu, and Li’s test bars, in which corrosionresulted from carbonation, tend to show lesser reductionsthan studies in which corrosion was chloride-induced.12

This is to be expected, as carbonation-induced corrosionis more uniform than chloride-induced.

For the present, therefore, models must remain semi-empirical, and their predictions must be treated with caution.

EFFECT ON STRUCTURAL PERFORMANCEIn considering the significance of these results, it should

be noted that the full ultimate tensile strength of reinforce-ment is not normally usable in concrete construction as thestrain capacity of concrete limits the reinforcement strainsthat can be developed. Design calculations are normallybased on yield strength, ignoring any potential enhancementfrom strain hardening. Yield strength and ductility are thusthe more important properties on which to focus.

Reinforcement strength is explicitly considered in designcodes. The effect of reinforcement section loss on section

strength could reasonably be estimated from original materialproperties and mean section loss modified by a residualstrength function based on Eq. (2) to (4) together with anappropriate choice of an α coefficient.

Reinforcement ductility is not explicitly considered in themajority of national design codes. Instead, design rulesassume that a minimum ductility, based on standard specifi-cations, is provided with detailed design rules derived usingthis assumption. As long as the residual ductility of thecorroded reinforcement satisfies standard requirements, thenthere is no need to adjust design procedures in respect ofductility. If residual ductility reduces below the standardspecification, due allowance must be made if it is intended toexploit any form of plastic behavior (including redistributionof elastic moments) in an assessment, and special procedureswould have to be followed.

It is emphasized, however, that the loss of a bar section isonly one of several mechanisms through which corrosion ofreinforcement may affect residual strength and performanceof concrete structures, and that the effects of loss of concretecross section and of loss of bond must also be considered inan assessment.

SUMMARY AND CONCLUSIONSMechanical properties of reinforcing bars may be affected

by corrosion. The effect is attributable to the uneven natureof corrosion attack along the length of the bar. Results froma simple nonlinear numerical model of the effects of variationsin cross section along a bar were validated against physicaltests on corroded bars, and satisfactory agreement was obtained.

For the plain mild steel bars subjected to light amounts (upto 7%) of corrosion, the reduction in yield strength is notmarkedly greater than the reduction in the cross-sectionalarea. For heavily corroded high-yield bars, the reduction inyield strength was less than the maximum reduction in thecross section. The reduction in ductility was appreciablygreater than the reduction in yield strength in both cases; forexample, a bar with a maximum reduction in cross section of8% lost approximately 20% of its ductility. The numericalmodel has also been used to show that, for a given corrosiontopography, residual mechanical properties of bars with a

Table 1—Empirical coefficients for strength and ductility reduction of reinforcement

Authors Exposure Qcorr, % αy αu α1

Palsson and Mirza4 Concrete Service, chlorides 0 to 80* 0.0 0.0 NS

Castel, Francois, and Airliguie7 Concrete Chlorides, 0.0 mA/cm2 0 to 20 0.0 NS 0.035

Du8Bare Accelerated, 0.5 to 2.0 mA/cm 2 0 to 25 0.014 0.014 0.029

Concrete Accelerated, 1.0 mA/cm 2 0 to 18 0.015 0.015 0.039

Maslehuddin et al.9 Bare Service, marine 0 to 1 0 0 0

Allam et al.10 Bare Service Arabian coast 0 to 1 0 0 0

Morinaga11 Concrete Service, chlorides 0 to 25 0.017 0.018 0.06

Zhang, Lu, and Li12 Concrete Service, carbonation 0 to 67 0.01 0.01 0

Andrade et al.13 Bare Accelerated, 1.0 mA/cm 2 0 to 11 0.015 0.013 0.017

Clark and Saifullah14 Concrete Accelerated, 0.5 mA/cm 2 0 to 28 0.013, 0.012

0.017, 0.014 NS

Lee, Tomosawa, and Noguchi15 Concrete Accelerated, 13.0 mA/cm 2 0 to 25 0.012 NS NS

Present study Concrete Accelerated, 0.01 to 0.05 mA/cm 2 0 to 3 0.012 0.011 0.03

*Based on minimum, not average, residual section.Note: NS = not supplied.

264 ACI Materials Journal/July-August 2005

clearly defined yield plateau and with a high ratio of ultimatetensile strength to yield strength are less vulnerable to corrosion.

Guidance on assessment is provided for carbonation andchloride-induced attack. The nature of corrosion attack instructures in service will depend on conditions of environ-mental exposure, however, and the simulated damage andlaboratory-induced corrosion in test bars herein may not berepresentative of field conditions. Application of the numericalmodel to field exposure conditions using corrosion topographymeasured under field exposures is recommended.

It is not feasible to routinely measure the precise variationsin cross section along the length of a corroded bar requiredfor the numerical model in an assessment of a real structure.Alternative approaches for practical application are thereforeoutlined, based either (conservatively) on the minimum barcross section at any point or on mean section loss togetherwith empirical coefficients.

ACKNOWLEDGMENTSThis project was partly funded by British Energy Generation (UK) Ltd. and

BNFL plc through the Industry Management Committee Programme in associ-ation with the UK Health & Safety Executive. The views expressed herein arethose of the authors and do not necessarily represent those of British Energy,BNFL, or the HSE. Reinforcement bars with a special heat treatment wereprovided by Ferriera Valsabbia (Odolo, BS, Italy) to the University of Brescia.

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