Influence of steel reinforcement corrosion on the stiffness of simply supported concrete beamsO'FLAHERTY, Fin, MANGAT, Pal, LAMBERT, Paul and BROWN, E. H.Available fro !"effield Halla #niver$i%& Re$ear'" Ar'"ive (!H#RA) a%*"%%+*,,$"ura.$"u.a'.u-,./01,T"i$ do'uen% i$ %"e au%"or de+o$i%ed ver$ion.You are advi$ed %o 'on$ul% %"e +ubli$"er'$ ver$ion if &ou 2i$" %o 'i%e fro i%.Published versionO'FLAHERTY, Fin, MANGAT, Pal, LAMBERT, Paul and BROWN, E. H. (3454). 6nfluen'e of $%eel reinfor'een% 'orro$ion on %"e $%iffne$$ of $i+l& $u++or%ed 'on're%e bea$. 6n* !A#!E, Ri'"ard and FRANGOPOL, 7an, (ed$.) Brid8e ain%enan'e, $afe%& and ana8een% 9 6ABMA!'54. Brid8e, ain%enan'e, $afe%& and ana8een% . :R: +re$$.Repository use policy:o+&ri8"% ; and Moral Ri8"%$ for %"e +a+er$ on %"i$ $i%e are re%ained b& %"e individual au%"or$ and,or o%"er 'o+&ri8"% o2ner$. #$er$ a& do2nload and,or +rin% one 'o+& of an& ar%i'le($) in !H#RA %o fa'ili%a%e %"eir +riva%e $%ud& or for non9'oer'ial re$ear'". You a& no% en8a8e in fur%"er di$%ribu%ion of %"e a%erial or u$e i% for an& +rofi%9a-in8 a'%ivi%ie$ or an& 'oer'ial 8ain.Sheffield Hallam University Research Archive"%%+*,,$"ura.$"u.a'.u-1INTRODUCTION Theincreasingageofthehighwaybridgestock throughout Europe has led to increased deterioration inexistingstructures.Surveyshaveindicatedthat themainreasonsfordeterioration,besidesnormal wear and tear, are the increasing weights and volume oftrafficusingtheroadnetwork,andadverseenvi-ronmentconditionssuchasexposuretochlorides and freeze-thaw attack. The magnitude of the capital investment in European bridge stock requires that ef-fectivemaintenanceisprovidedtoensurethatthe bridges are kept in safe service at optimum cost. Re-liableassessmentmethodsareurgentlyrequiredto assist the bridge engineer to evaluate the residual ca-pacity of deteriorated elements. Whensteelreinforcementcorrodes,tensile stressesaregeneratedintheconcreteasaresultof the expansive corrosion products. Since concrete can enduremuchlesstensilestressthancompressive stress,tensilecracksare readily nucleated and prop-agatedasaresult.Thedevelopmentofcorrosion products along the bar surface may affect the failure mode and ultimate strength of flexural members due totwocauses:firstly,duetoareductioninthede-greeofbarconfinementcausedbyanopeningof longitudinalcracksalongthereinforcementand,se-condly,duetosignificantchangesatthesteel-concreteinterfacecausedbychangesinthesurface conditionsofthereinforcingsteel(Al-Sulaimaniet al.1990).Inallcorrosioncases,repairisnecessary to increase the service life of the member (Mangat & O'Flaherty1999,2000)anditisestimatedtocost approximately1.5bninEuropeeachyear(Davies 1996). 2BACKGROUND Inthisinvestigation,thesteelinreinforced concrete beamswassubjectedtoanacceleratedcorrosion techniqueinthelaboratoryusingoneoftheseveral methodsavailable.Thegalvanostaticmethodwas usedinthisstudytosimulatethefieldconditions. Themethodinvolvespassingadirectcurrent throughthereinforcementtoacceleratecorrosion. Thegalvanostaticcorrosioniscarriedoutwhilst the beam is unloaded, which is different from the corro-sion in actual structures. The corrosion by galvanos-taticmethodisgeneral,whereasactualstructures have some specific areas that are more prone to cor-rosion.Thus,inthelattercase,thereisalwaysthe possibilityofpittingcorrosionwherebythecross-sectional area of the reinforcing bars could be signif-icantly reduced, thus reducing the tensile strength of thereinforcingbars.However,inreality,localised pittingcorrosionextendstomanysitesresultingin extensiveandrelativelyuniformlevelsofcorrosion covering large areas of reinforcement (Mangat & El-garf 1999). In these tests, the aim was to subject the steel reinforcement to general corrosion. Concretestructureswhichhavesufferedfrom corrosion to the steel require maintenance to increase theirservicelife.However,itisdifficulttomakea Influence of steel reinforcement corrosion on the stiffness of simply supported concrete beams F. J. O'Flaherty, P. S. Mangat & P. Lambert Centre for Infrastructure Management, Sheffield Hallam University, Sheffield, S1 1WB, UK E. H. Browne Halcrow Group Limited, Birmingham, B16 8PE, UK ABSTRACT: The in-service performance of reinforced concrete beams can be severely affected through cor-rosion of the steel reinforcement when it becomes subjected to harsh corrosive environments containing chlo-ridesandcarbondioxide.Insuchinstances,corrosionislikelytooccurinthesteelreinforcement,withthe expansivenatureofthecorrosionproductslikelytoinducecrackingandspallingoftheconcrete.Alossof structural integrity (stiffness) will occur and this can severely influence the serviceability of the member. The purpose of this paper is to investigate the relationship between degree of corrosion and loss of stiffness in corrosion damaged under-reinforced concrete beams. Beams (100mm x 150mm cross section) were subjected toacceleratedcorrosioninthelaboratoryandsubsequentlytestedinflexuretofailure.Thepaperreportson the results of these tests and relates the degree of corrosion in the main steel to the percentage loss in stiffness in the concrete beams. Protected hanger bars (ca-thode) decision as to when repair is required without know-inghowmuchthecapacityhasbeenreduceddue to deterioration. Shear reinforcement normally corrodes firstasitisnearertotheconcretesurfacebutmain steel,too,canbeaffectedsimultaneouslyifthecor-rosioninducingelementspenetratedeepintothe concrete.Oneoftheobjectivesofthelaboratory workwastoevaluatetheinfluenceofcorrosionto the main and shear reinforcement on the stiffness of simply supported beams. 3EXPERIMENTAL PROCEDURE Atotaloftwentyreinforcedconcretebeamswere tested to examine the influence of reinforcement cor-rosion on the stiffness. Details of test specimens are given in Figure 1 and Table 1. Beams were 910 mm longwithacross-sectionof100mmx150mm deep.Spanwas750mm.Allspecimenswerede-tailedforflexuralfailure;sufficientlinkswerepro-vided to ensure adequate shear capacity at the antic-ipatedmaximumloadofthecorrodedbeam.Beams areidentifiedasfollows(Table1):e.g. 2T8/5+12D6/10givesthenumber,type,anddiame-ter of the main steel in mm/target percentage of cor-rosion+number,type,anddiameterofthelinksin mm/targetcorrosion.2T8/0+12D6/0weretested without corrosion to serve as control specimens (Ta-ble1).Coverthroughoutwas50mmtotheshear reinforcement and 56 mm to the main steel. Figure 1. Beam specimens. Table 1. Variables in test programme. Beam ID (main steel /shear steel) Target Corrosion (main and shear) No. ofspecimens (%) 2T8+12D6 02 56 106 156 Mainreinforcementconsistedofhighyield (ribbed)barswithanominalcharacteristicstrength of460N/mm2.Shearreinforcementwas6mmdi-ameterplainroundmildsteelbarswithanominal yieldstrengthof250N/mm2.Shearreinforcement wasspacedat65mmcentresfor56mmcoverto mainsteelreinforcement.Hangertopbarsforall beamsconsistedoftwo6mmdiameterplainround mildsteelbarswithayieldstrengthof250N/mm2. Thesteelreinforcementwasweighedbeforecasting to enable the actual percentage corrosion to be calcu-lated at a later stage. Corrosion of the top reinforce-ment(hangerbars)waspreventedbyproviding shrinkwraptubingatthepointsofcontactwiththe shearsteelreinforcementtobreaktheelectricalcir-cuit and hence prevent current flow during the acce-lerated corrosion process. Corrosion of the main and shearreinforcementwasconductedsimultaneously usingamultichannelpowersupply.Shrinkwrap tubingwasagainprovidedatthepointsofcontact between the shear and main steelto enable each bar tobecorrodedseparatelyusingthemultichannel power supply. Test specimens were cast in the laboratory using a concrete with target cube strength of 40 N/mm2. Mix proportionswere1:1.7:3.8ofPortlandcement:fine aggregate:coarseaggregate.Fineandcoarseaggre-gateswereovendriedat100Cfor24hours.Cal-cium chloride (CaCl2) was added to the mix (1% by weightofcement)inordertopromotecorrosionof thereinforcement.Thematerialwasplacedinsteel mouldsinthreelayers,eachlayerbeingcarefully compacted on a vibrating table. The specimens were thenplaced in the mist curing room (20C and 95% 5% Relative Humidity) for 24 hours. The samples weredemouldedafter1dayandcuredinwaterat 20Cforafurther27days(28 days in total). Speci-mens were then transferred to a tank filled with a sa-linesolutionforacceleratedcorrosionat28days age. A 3.5% CaCl2 solution was used as the electro-lyte.Thedirectionofthecurrentwasarrangedso thatthemainandshearreinforcingsteelservedas the anode and the hanger bars and additional copper, which was added to the tank, acted as the cathode. Aconstantcurrentdensityof1mA/cm2was passedthroughthemainandshearreinforcement. Thiscurrentdensitywasadoptedon the basis of pi-lotteststoprovidedesiredlevelsofcorrosionina reasonable time. The current supplied to each speci-menwascheckedonaregularbasisandanydrift was corrected. Thecontrolspecimens(zeropercentcorrosion) were tested at the age of 28 days but the deteriorated beams were tested at 42, 56 and 63 days for the 5, 10 and15%targetcorrosionrespectivelyduetothe timetakentoreachthedesiredlevelsofcorrosion. Allspecimensweretestedunderfourpointbending todeterminetheultimateflexuralstrength.Prema-tureshearfailurewaspreventedbysufficientshear reinforcement. The testing machine loading rate was set at 5 kN/min. Therebarswereretrievedfrom the concrete after thetestingprogramandreweighedtoestablishthe precise level of corrosion. Corroded length Corroded reinforcement 2T8 + 12D6 (anode) 4RESULTS AND DISCUSSION Theload-deflectioncurvesforthebeamsunderin-vestigation are given in Figure 2 and include 18 cor-rodedbeamsandtheaverageoftwocontrolbeams. Theeffectofthedegreeofcorrosionontheoverall stiffness of corroded flexural member is tabulated in Table 2 and is taken from the data presented in Fig-ure2.ReferringtoTable2,beamsareidentifiedin column1.Theactualpercentageofreinforcement corrosion,measuredasdescribedinSection3,is givenincolumn2.Column3,Table2,showsthe stiffness of the beams after testing in the laboratory. Thestiffnesswasdeterminedviathestraightline portion of the graph from the origin in Figure 2. The percentage difference in loss of stiffness between the control beam (2T8/0+12D6) and the corroded beams is given in column 4. To gain a better understanding of the performance ofthebeamswhenbothmainandshearreinforce-mentaresubjectedtocorrosion,Figure3showsthe effect of corrosion on the stiffness. Referring to Fig-ure 3, the beam stiffness, obtained from the slope of theload-deflectioncurvesinFigure2,isplotted againstthedegreeofmainreinforcementcorrosion only,asthisisconsideredthemainparameterwith respecttolossofstiffness(theshearreinforcement was also corroded in the test as this better represents actualconditions).Generally,anincreaseincorro-sion leads to a decrease in stiffness. At very high le-velsofcorrosion(>25%),themodeoffailure changes from flexure to shear. Table 2. Experimental data. 1234 Beam IDActual Main Bar and Shear Links Corrosion % Stiffness kN/mm % differ-ence in stiffness 2T8/0+12D6*0; 09.3 - 2T8/5+12D6/56.5; 4.39.5 2 2T8/5+12D6/52.9; 3.08.5 -9 2T8/10+12D6/54.8; 5.610.0 8 2T8/10+12D6/55.3; 6.59.1 -2 2T8/15+12D6/5**25.7; 24.74.5 -52 2T8/15+12D6/5**25.7; 27.64.3 -54 2T8/5+12D6/108.3; 6.36.1 -34 2T8/5+12D6/103.1; 3.87.7 -17 2T8/10+12D6/104.1; 3.98.9 -4 2T8/10+12D6/104.7; 8.28.1 -13 2T8/15+12D6/107.8; 4.56.8 -27 2T8/15+12D6/108.8; 9.16.8 -27 2T8/5+12D6/1510.7; 12.05.3 -43 2T8/5+12D6/1511.8; 14.26.8 -27 2T8/10+12D6/156.7; 9.411.4 23 2T8/10+12D6/157.9; 8.26.9 -26 2T8/15+12D6/156.9; 7.49.0 -3 2T8/15+12D6/156.9; 4.08.1 -13 * average of two specimens** failed in shear 4.1Influence of corrosion on stiffness ReferringtoTable2,col.4,thepercentagediffer-enceinstiffnessbetweenthecontrolbeam (2T8/0+12D6) and the corroded beams is given. This data is plotted against the degree of corrosion of the main steel in Figure 4 but excludes three positive da-tapointfromcolumn4whichareconsideredtobe erroneous-2T8/5+12D6/5,2T8/10+12D6/5and 2T8/10+12D6/15 and the two beams which failed in shear(2T8/15+12D6/5,2T8/15+12D6/5).Referring toFigure4,alinearcorrelationisevidentwith R2=0.58.Thedecreaseinstiffnessisequalto2.83 times the degree of corrosion of the main steel. The failure load of the control beam was 41.4 kN (Figure 2). However, if the service load is estimated as40%oftheultimate(control)loadtoaccountfor factorsofsafetyincludedinthedesign(deadload: 1.4;liveload:1.6;materials:1.5forconcreteand 1.05forsteel),theserviceloadwouldbeapprox-imately16.6kN.Therefore,thedeflectionofthis beamin service, assuming a stiffness of 9.3 kN/mm (Table 2) and a service load of 16.6 kN would be 1.8 mm(deflection=16.6kN/9.3kN/mm).However, theallowabledeflectionisspan/250(BritishStan-dardsInstitution),or3mmforthesebeamsmeaning only a further 1.2mm of deflection is required before thebeamisoutsideofallowabledeflectionlimits. Toestimatethepercentageofcorrosionrequiredto causethisincreaseindeflection,theserviceloadis assumedtoremainat16.6kNbutthedeflectionis assumedtoreachitsmaximumallowablelimitof 3mm. Therefore, since stiffness = service load / def-lection, stiffness = 16.6 kN / 3mm or 5.53 kN/mm. Thismeansthatthestiffnessofthebeamwould havetodecreaseto5.53kN/mmbeforeitexceeds serviceabilitylimits,adecreaseof40%((9.3- 5.53)/9.3). Applying this percentage loss of stiffness toFigure3,mainsteelcorrosionwouldhavetoex-ceed 14% before deflection criteria becomes an issue (40%/2.83). 5CONCLUSIONS Themainconclusionsfromtheresultsreportedin thispaperareasfollowsandapplywithinthelimit of the parameters covered by the test data in the pa-per. reinforced concrete beams show a loss in stiffness withincreasingcorrosionofthemainandshear steel reinforcement; anequationlinkingthedecreaseinstiffnessto main steel corrosion was determined as follows: %decreaseinstiffness=2.83xdegreeof main steel corrosion mainsteelcorrosionwouldhavetoexceed14% before deflection criteria are exceeded. 6ACKNOWLEDGEMENTS Theworkdescribedinthispaperformsapartofa researchprojectentitled'Residualstrengthofcor-rodedreinforcedconcretebeams'whichwascarried outatSheffieldHallamUniversity.Theassistance receivedfromthetechnicalstaffinthe laboratory is gratefullyacknowledged. The authors also acknowl-edgewiththanksthesupportofHalcrowGroupLi-mitedandCPIApsinprovidingthemulti-channel power supply for inducing accelerated corrosion. 7REFERENCES Al-Sulaimani, G. et al. 1990. Influence of Corrosion and Crack-ing on the Bond Behavior and Strength of Reinforced Con-crete Members. ACI Structural Journal 87 (2): 220-231. BritishStandardInstitution1997.Structuraluseofconcrete: Code of practice for, design and construction BS 8110-1. Davies,H.1996.Repairandmaintenanceofcorrosiondam-aged concrete:Materials strategies, Concrete Repair, Reha-bilitation and Protection. London: E &FN Spon. Mangat,P.S.,Elgarf,M.S.1999.Flexuralstrengthofconcrete beamswithcorrodingreinforcement,ACIStructuralJour-nal 96 (1): 149-158 Jan-Feb 1999 Mangat, P.S., OFlaherty, F.J. 1999. Long-term performance of highstiffnessrepairsinhighwaystructures,Magazineof Concrete Research 51 (5): 325-339. Mangat,P.S.,OFlaherty,F.J.2000.Influenceofelasticmod-ulusonstressredistributionandcrackingin repair patches. Cement and Concrete Research 30: 125-136.