SHRPS359TechnicalAlertCriteriafortheCathodicProtectionofReinforcedCo - PDF document

1 SHRP-S-359TechnicalAlertCriteriafortheCa
SHRP-S-359TechnicalAlertCriteriafortheCathodicProtectionofReinforcedConcreteBridgeElementsJackBennettThomasTurkELTECHResearchCorporationFairportHarbor,OhioStrategicHighwayResearchProgramNationalResearchCouncilWashington,DC1994 SHRP°S-359ContractNo.C-102DISBN0-309-05615-2ProductNo.2034ProgramManager:DonM.HarriottProjectManager:H.M.(Marty)LaylorProgramAreaSecretary:CarinaS.HreibProductionEditor:KatharynL.BineFebruary1994keywords:cathodicprotectionStrategicHighwayResearchProgram2101ConstitutionAvenueN.W.Washington,DC20418(202)334-3774ThepublicationofthisreportdoesnotnecessarilyindicateapprovalorendorsementbytheNationalAcademyofSciences,theUnitedStatesGovernment,ortheAmericanAssociationofStateHighwayandTransportationOfficialsoritsmemberstatesofthefindings,opinions,conclusions,orrecommendationseitherinferredorspecif

2 icallyexpressedherein.©1994NationalAcade
icallyexpressedherein.©1994NationalAcademyofSciences1.5M/NAP/294 AcknowledgmentsTheresearchdescribedhereinwassupportedbytheStrategicHighwayResearchProgram(SHRP).SHRPisaunitoftheNationalResearchCouncilthatwasauthorizedbysection128oftheSurfaceTransportationandUniformRelocationAssistanceActof1987. ContentsAcknowledgments........................iiiIntroduction.............................1Organization............................2100-mVPolarizationDevelopment/Decay..........3CorrosionNullProbe.......................8ConstantCurrentCriterion...................13ListofFiguresFigure1.PolarizationDecayofSteelinConcrete.....5Figure2.MeasuringStaticHalf-CellPotentialsofEmbeddedSteel..................6Figure3.CorrosionNullProbeSchematic..........8Figure4.StaticPotentialSurveyMap.............9Figure5.LocatingEmbeddedSteelRe

3 inforcement....10Figure6.Saw-CuttingRein
inforcement....10Figure6.Saw-CuttingReinforcingSteel..........11Figure7.TypicalNullingCurve...............12Figure8.GraphforDeterminingRequiredCathodicProtectionCurrent...........14Figure9.TitrationforTotalChlorideDetermination...15iv IntroductionCathodicprotectionisrapidlybecomingacceptedasarepairoptionforsteel-reinforcedstructuresthatareundergoingcorrosioncausedbythepresenceofchlorides.Testsusedtoassesstheeffectivenessofcathodicprotectionsystemsareknownas"criteria".Severaldifferentcriteriahavebeensuggestedforusewithconcretestructures,andmanyareinusetoday.Mostofthesecriteriawereadaptedfromthoseusedintheundergroundcathodicprotectionindustryandareonlyrecentlybeingverifiedinconcrete.Thenumberoftests,togetherwiththecomplexityofsome,hasledtoconfusionanddisagreementregardingtheuseofsuchcriteria.TheStrategicHighwayRe

4 searchProgram(SHRP)issuedacontractoncath
searchProgram(SHRP)issuedacontractoncathodicprotectionofreinforcedconcretebridgeelements.Theobjectiveofpartofthatcontractwastoinvestigatethefeasibilityofimprovedandsimplifiedcontrolcriteria.Underthiscontract,corrosionratesofsteelinaconcreteenvironmentweremeasuredrelativetomanyfactors.Thesedatawereusedtoappraisecontrolcriteria.Thistechnicalalertpresentstheresultsandrecommendationsbasedonthatpartofthework. OrganizationThistechnicalalertisdividedintothreesections,eachofwhichdescribesadifferentcathodicprotectioncriterion.Section1discussesthe100-mVPolarizationDevelopment/Decaycriterion,whichisalreadyincommonuse.Section2describesanewcriterion,theCorrosionNullProbe.Section3describesanewConstantCurrentcriterion.Thesethreecriteriaarepresentedindecreasingorderofcomplexity,withthe100-mVPolarizationDevelopment/Decaycriteri

5 onbeingthemostcomplexandtheConstantCurre
onbeingthemostcomplexandtheConstantCurrentcriterionbeingtheeasiesttoapply.Othercriteriamaybeusefulfortheassessmentofcathodicprotectionsystems,buttheyarenotdiscussedinthistechnicalalert.TheElogIcriterion,forexample,isnotdiscussedsinceitisoftendifficulttoconductandinterpret.ThistestnormallyisconductedonlybyconsultantsorservicefirmsthatspecializeinusingtheElogIcriterion.Thecriteriadescribedinthistechnicalalertareintendedforusebyhighwayengineerswithoutextensivetrainingorexperience.Theyare,nevertheless,technicallyaccurateasshownbydatadevelopedinthisstudy. lO0-mVPolarizationDevelopment/DecayThe100-mVPolarizationDevelopment/DecaycriterionisoneofthreelistedasacceptableintheNationalAssociationofCorrosionEngineers(NACE)StandardRecommendedPracticeRPO290-90.Asurveythatcollecteddataon287cathodicprotectionsystemsintheUnitedS

6 tatesandCanadaidentifiedthisasthemostpop
tatesandCanadaidentifiedthisasthemostpopularcriterioncurrentlyusedforconcretestructures.Sincethiscriterionisalreadyincommonuse,thistechnicalalertdoesnotdescribeitingreatdetail,butofferscommentsaboutitsuseandinterpretationbasedonthestudyresults.The100-mVPolarizationDevelopment/Decaycriterionisbasedonthetheorythatpolarizationofcorrodingsteelinthecathodicdirectionwillinhibitanodic(corrosion)reactions.Thisideaisawell-establishedprincipleofelectrochemistry.Mostdisagreementaboutthiscriterionisfocusedontheamountofpolarizationneededtocontrolcorrosion.Thisfigurerangedfrom250mVto50mVorless.Basedonthesecorrosionratestudies,theamountoflong-termpolarizationneededisafunctionofchlorideconcentrationatthereinforcingsteel,aspresentedintable1.Directuseofthesedatarequiresdeterminationofthechlorideconcentrationintheconcreteattherei

7 nforcingsteel.Ifchlorideconcentrationsar
nforcingsteel.Ifchlorideconcentrationsarenotknown,100mVofpolarizationisareasonablecompromiseforarelativelybroadrangeofsaltconcentrations,although150mVshouldberequiredifconditionsareknowntobeverycorrosive.Polarizationisusuallyestimatedbymeasuringthepolarizationdecayofthesteelthatoccursaftertheprotectivecurrentisshutoff.Steelpotentialsversusasaturatedcalomelelectrode(SCE)maybeplottedagainsttimeasshowninfigure1.3 Table1.PolarizationRequirementasaFunctionofChlorideConcentrationattheSteelSurfaceChlorideConcentrationPolarizationNeeded*(lb/yd3concrete)(kg/m3concrete)(mV)01-20.6-I.2602-51.2-3.0805-103.0-6.010010-206.0-12.0150*Toachieveacorrosionrateofmil/year,or�.1 ;&#xTj 2; 0 ;&#xTD 0;20yearsuntildamageduetocorrosion1mil=0.0254mmThetimeperiodusedforthistestmaycausesomeproblems.Theinitialpotential,calledthe"inst

8 ant-off"or"IR-free"potential,shouldbetak
ant-off"or"IR-free"potential,shouldbetakenfromabout100to1,000msecaftershuttingoffthecurrent.Measuringthepotentialincorrectlycouldleadtosignificanterror.Anotherproblemisthatthepolarizationdecaytestistypicallyconductedovera4-hourperiod.Itwasfoundthataslittleas25%ofthetotalpolarizationmayberecordedinthefirst4hoursofthetest.Thisisespeciallytrueofverymaturesystemsorsystemsinwhichconcreteiswatersaturated.Recordingpolarizationdecayoverperiodslongerthan4hoursmaybebothimpracticalandmisleading.Ithasbeenshownthatchangesintheenvironment(temperature,forexample)canseriouslydistortthetestresultswhendataaretakenoveraverylongperiod.4 Figure1.PolarizationDecayofSteelinConcrete0-50-100-150r_rj-200-250--350-400"_-4500-500-_=IRfreepotential-550-600,I,I,I,I,I,I,I,I,I,I,[,i,020406080100'.20140160180200220240260Time,minutesThistestreq

9 uiresastablereferenceelectrode,whichisty
uiresastablereferenceelectrode,whichistypicallyembeddedintheconcreteofthestructure.Theelectrodeshouldbeinstalledinthemostanodic(corrosive)locationwithinthezonetobeprotected.Workhasunderscoredtheimportanceofproperlylocatingthereferenceelectrode.LocationisestablishedbyconductingapotentialsurveyaccordingtoASTMC876-91,"StandardTestMethodforHalf-CellPotentialsofUncoatedReinforcingSteelinConcrete."Figure2showsapotentialsurveybeingconducted.Insummary,the100-mVPolarizationDevelopment/Decaytestisareasonablyaccuratecriterionforcathodicallyprotectingsteelinconcrete,though100mVofpolarizationmaybeexcessivewhenverylittlechlorideispresent,andinadequatewhenthereisaveryhighchlorideconcentration.Also,completepolarizationdecaymaynotberecordedinthestandard4-hourtestperiod.5 Figure2.MeasuringStaticHalf-CellPotentialsofEmbeddedSteel

10 ".j'"._•....._..'.,f.6 CorrosionNullProb
".j'"._•....._..'.,f.6 CorrosionNullProbe1Thisworkfoundadirectcorrelationbetweenthecorrosioncurrentflowingtothereinforcingsteelandthecorrosionrate.Thedataalsoshowedthatthecathodicprotectioncurrentcontrolsbothmacrocellcorrosionandlocalcorrosion.Theseresultssuggestedanewcriterionbasedoncurrentflowtoorfromanelementofthenativelycorrodingsteel.ThisconcepthasbeentermedtheCorrosionNullProbe(CNP).TheconstructionoftheCNPisshowninfigure3.TheCNPcanbeestablishedbyfirstlocatingthemostanodic(corrosive)areainthezonetobeprotected.ThisisdonebyconductingapotentialsurveyaccordingtoASTMC876-91(seefigure2).Thisstepisveryimportantandmustbedonewithcare.Thesurveyshouldbedoneusinga4-ft(1.2-m)spacingbetweenmeasurements.Themostanodicareasthenshouldberesurveyedusing1-ft(0.3-m)centers,orelsethemostcorrosiveareasmaybemissed.Anexampleofastat

11 icpotentialsurveymapisshowninfigure4.Fig
icpotentialsurveymapisshowninfigure4.Figure3.CorrosionNullProbeSchematicIJUNC_ONBOX1CONCh,SURFACE,Ir._ADNII_--_I===========================1"•.::::::::::::::::::::::::::::::::::::::::::::::::::I"-";•.4°'%.?Z':':"i:_:-_o__:.:.:.:.:.:-_Ro_r.'::-.':':'Bartholomew,J.,etal.1993.CriteriaandMaterialsPerformancefortheCathodicProtectionofReinforcedConcreteBridgeElements.Reportno.SHRP-S-670.Washington,DC:SHRP,NationalResearchCouncil.7 Figure4.StaticPotentialSurveyMap-_334i290_.337!230\_OO]R_380,.z,,._.:,_'._,../300_tt32_1_?0/,4,?:1_"_:"_'_....._"-'-_I_¢,-',;_:.:_-_.,,.-.'4-._.?_--....,,.:...-_,-_;_,_:[_""__...:..._j...._.....a17:.a_-.aaoa3e387_e2,'1._8,CID._UI'VC..._'c,..:_..---_'._,_-PgY_:C.'_'_:_.--...."""_P::_':-_.'.._3/328337339_,':7_:'b.'--._&#x,000;_........../'.%__:".,-_-:_;373"'N_,r,.,,_.,_387:_..,._80368_,Ie.*,.

12 ..;.:._._'.i_.;_,_..:_,_-,\f:'._'_'_'_Ex
..;.:._._'.i_.;_,_..:_,_-,\f:'._'_'_'_Exposedrebar_",......_,""/_-:'-_.._a_37035b295329,37t/"........_..:_:_..•-......---¢__..'.&:;..............-_:.-:_*'r,'.,.l'_._:._;.._\_.-:......-:__//_..'::"......'..:-"e.._;:'Potentialsare-ragvsSCEOncethemostcorrosivespotisidentified,thelocationofthereinforcingsteelcanbedeterminedusingaconcretecovermeter(figure5).Ashortlengthofreinforcingbar,typically6in.(15cm)isisolatedfromtherestofthereinforcingbarbysaw-cutting(figure6).Theisolatedbarshouldbeinsoundconcrete,andcareshouldbetakennottodisturbtheconcretenearit.Awireisattachedtothebarasshowninfigure3.Thewireconnectionandthecutendsofthebararesealedwithepoxy.Thesawcutsarefilledwithmortarorgrout. Afterinstallation,theCNPshouldnotbeleftelectricallyisolatedfromtherestofthereinforcement,orthecorrosioncellwillbedisturbed.Therefore,

13 theCNPshouldbeconnectedtoelectricallycon
theCNPshouldbeconnectedtoelectricallycontinuousreinforcingsteelduringtheconstructionprocess.Theconnectioncanbemadewithatemporarywireifnecessary.Beforetesting,thewireattachedtotheCNPisconnectedtotherectifiernegativeterminalthrougha10-ohmresistor.Currentflowto,orfrom,theprobeismeasuredasavoltagedropacrosstheresistor.Whenthepositiveleadofa Figure6.Saw-CuttingReinforcingSteel_'....."...voltmeterisattachedtotheprobeandthenegativeleadisattachedtothesystemnegative,anegativereadingindicatesthattheprobesteeliscorroding.Whencathodicprotectioncurrentisslowlyapplied,thevoltageacrosstheresistorwilleventuallyreversepolarityandbecomepositive.Atthispoint,theCNPiscathodicandisnolongercorroding.Figure7illustratesaCNP'sresponsetoincreasingcathodicprotectioncurrent.WhentheCNPislocatedinthemostcorrosiveareainthezone,thatareaisprote

14 cted,andalltheothersteelinthezonewillbep
cted,andalltheothersteelinthezonewillbeprotectedaswell.Asmallsafetyfactorof0.25mA/ft2(2.5mA/m2)concreteisrecommended.Thissafetyfactorisnormallyadequatetocompensateforcurrentvariationscausedbyanoderesistance,concretecover,andchlorideandmoisture10 Figure7.TypicalNullingCurve1.00.9-0.8-0.7-0,6_,_0.50.40.30.2Nullpoint_0.0-0.1_-0,2--0.3-0.4--0.5,,,,,,___Iii__,,,,,I,,,,,,,,,I,,,,,,,,,I,,,,,,,,,o12345I_p,rnA/ft9steelcontent.Thetestshouldberepeatedafter1,3,and6months,andannuallythereafter.TheCNPcriterionistechnicallyaccurateandrelativelysimpletoapply.Itissitespecificandwillshowdiminishingcathodicprotectioncurrentrequirementwithtime.Installationissimple,andthetestiseasytoperform.Animportantfeatureisthatthecriteriondoesnotrelyonthelong-termstabilityofembeddedreferenceelectrodes.Atthistime,ithashadverylimiteduseinthefield

15 ,andadditionalfieldvalidationisneeded.11
,andadditionalfieldvalidationisneeded.11 ConstantCurrentCriterionCorrosionratestudieshavedemonstratedgoodcorrelationbetweentheconcentrationofthechlorideionatthesurfaceofthereinforcingsteelandtheamountofcathodicprotectioncurrentrequiredtoprotectthesteel.Thus,ifthemaximumchlorideionconcentrationatthesurfaceofthesteelcanbeapproximated,agoodestimateofthecurrentrequiredforprotectioncanbemade.Thedifficultywithestimatingthechlorideionconcentrationisthat,whencathodicprotectionisapplied,thechlorideionconcentrationatthesurfaceofthesteeldecreaseswithtime.Amathematicalmodelforpredictingthechlorideandhydroxideionconcentrationprofilesasafunctionofthetotalchargepassedbyacathodicprotectionsystemwasusedtocorrectforthisoccurrence.Theresult,showninfigure8,isagoodestimateoftherequiredcathodicprotectioncurrent.Analternativeapproach

16 forestimatingthechlorideconcentrationatt
forestimatingthechlorideconcentrationatthesurfaceofthesteelistousearandomsamplingprocedurethroughoutthezoneunderstudy.SHRPresearchhasdemonstratedthatasamplingof12pointswillgivearepresentativemeanandstandarddeviationforthezone.Thus,ifthemeanplustwostandarddeviationsisused,agoodestimateofthemaximumchlorideconcentrationatthesurfaceofthesteelcanbemade.Propercurrentforthezoneiscalculatedusingthefollowingformula:mA/fi2concrete=(mA/ft2topmatsteel)•(fi2topmatsteel/fi_concrete)(fordouble-matconstruction,dividemA/ft2concreteby0.7togetthetotalcurrentrequiredpersquarefootofconcrete.)12 Figure8.GraphforDeterminingRequiredCathodicProtectionCurrent2.5;I'I'I'I'I'I'1'2.0._1.5E3_1.00.50.0,I,I,I,I,I,_,I,0246810121416StartingCl-Concentration@SteelSurface,lbs/ydaThecurrent,asdeterminedfromthisgraphandcalculation,canbeexpectedtokeep

17 thesteelcorrosionratebelow0.1rail/year(0
thesteelcorrosionratebelow0.1rail/year(0.025mm/year)afterthefirsttwomonthsofoperation.Thisisanacceptableratetopreventcorrosiondamage.Thecorrosionratewillbeslightlyhigheratfirst,butnothighenoughtojustif,yaprogramofvaryingcurrentwithtime.Thegraphalsoincludesasafetyfactorof0.25mA/ft2concrete,whichisappropriateforawell-designedsystemandassuresthatallthesteelwillbeprotected,regardlessofitspositionwithinthezone.Tousethiscriterion,thehighestchlorideconcentrationatthelevelofthesteelmustbefound.ThiscanbedonebyfirstconductingapotentialsurveyaccordingtoASTMC876-91,"StandardTestMethodforHalf-CellPotentialsofUncoatedReinforcingSteelinConcrete."Afterthemost13 corrosivesiteshavebeenlocatedinthezone,concretesamplesaretakenandanalyzedaccordingtotheAmericanAssociationofStateHighwayandTransportationOfficials(AASHTO)T260-84,"Sampl

18 ingandTestingforTotalChlorideIoninConcre
ingandTestingforTotalChlorideIoninConcreteandConcreteRawMaterials,"oraccordingtoaSHRPguideline,"MethodforDeterminationofTotalChlorideContent"(figure9).1Thismethodprovidesaverysimple,butreasonablyaccuratecriterionforcathodicprotectionofsteelinconcrete.LiketheCorrosionNullProbe,itdoesnotrelyonthelong-termstabilityofembeddedreferenceelectrodes,anditsuseinfieldstructuresisverylimited.Additionalfieldvalidationisneeded.TheConstantCurrentcriterioncouldbethesimplestmethodforestablishingtheneededlevelofprotectionforthesteelwhileavoidingunnecessaryoverprotection.Figure9.TitrationforTotalChlorideDetermination110"":!'Cady,P.,etal.1992.ConditionEvaluationofConcreteBridgesRelativetoReinforcementCorrosion,Volume6:MethodforFieldDeterminationofTotalChlorideContent.Reportno.SHRP-S-328.WashingtonDC:SHRP,NationalResearchCouncil.14