FusionBonded Epoxy Coatings a Technology for Rebar Corrosion Prevention.docx
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FusionBonded Epoxy Coatings a Technology for Rebar Corrosion Prevention.docx
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FusionBondedEpoxyCoatingsaTechnologyforRebarCorrosionPrevention
Fusion-BondedEpoxyCoatings:
aTechnologyforRebarCorrosionPrevention
J.AlanKehr,3MCorrosionProtectionDepartment,USA
FikryF.Barouky,ConsultingServicesDept.,SaudiAramco,SaudiArabia
Introduction
CorrosionofsteelinconcretehasbecomeacostlyproblemintheUnitedStatesoverthelasttwenty-fiveyears.ApproximatelyhalfofthenearlysixhundredthousandbridgesintheUSFederalAidHighwaysystemhavestructuraldeficienciesorarefunctionallyoutmoded.AccordingtoUSFederalHighwayAdministration(FHWA)estimates,aquarterofUSbridgedecksarebadlydeteriorated.Sincethebeginningofroad-saltapplication,expensiverepairsareoftenrequiredwithinfivetotenyears.
It’saworldwideproblem.ResearchindicatesthattheservicelifeofbuildingsintheArabianGulfmaybefivetofifteenyears.ReinforcedconcretebridgesneartheseashoreinJapanshowrapiddeteriorationwithintenyearsofconstruction.
Theproblemiscausedprimarilybyinorganic-saltinducedcorrosionofsteelinconcrete.Thesalt,primarilychloride,penetratestheconcretefromsourcessuchasroad-deicingsaltsorseaexposure.Itcanalsobebuiltinthroughtheuseofsalt-contaminatedaggregate,seawaterintheconcrete,orchloride-basedadmixtures.
Thechlorideioninitiatesandcatalyzesthecorrosionreaction.Theironcorrosionproductsresultingfromthereactionoccupyamuchgreatervolumethanironandcausetremendouspressureontheconcrete.Thepressurecausestheconcretetocrackandspall,allowingevengreateraccessofcorrodentstothesteelandaccelerateddeteriorationofthestructure.2
Whilemostattentionispaidtorebar,allsteelcomponentsareaffectedaswellbridgedecks,piers,pilings,andguardrails.1Thispaperaddressestheprotectionofthoseelementsbydescribingavailablealternativesandprovidinganoverviewoffusion-bondedepoxycoatingmaterials,casehistories,trendsintheindustry,andspecificconcernsabouttheuseoffusion-bonded-epoxycoatedrebar(FBECR).
MechanismofReinforcingSteelCorrosioninConcrete
Thetraditionalviewofthereinforcedconcretestructureisthattheconcreteisprotectivetothereinforcingsteelbarsthroughthecombinedeffectsofthechemicalreactionsbetweenthesteelandthecementhydrationproductsandtheenvironmentalbarrierprovidedbytheconcretecover.Iftheseconditionsaremaintainedwithintheconcretemass,thesteelbarsdonotcorrodeandthestructureshouldhavetheexpectedtrouble-freelifespan.
Poorqualityreinforcedconcretestructurecontributestoafasterdeteriorationofthesteelreinforcingbars.Lowdegreeofcompaction,excesswaterintheconcretemix,andthehydrationprocessareconsideredthemainfactorstocreatevoidswithintheconcreteandmaketheconcretestructureporous.
Porosityofconcreteallowspenetrationandingressofaggressiveelements(e.g.,chloride,oxygen,carbondioxide,andothermaterialsthatvaryfromonelocationtoanother)totheembeddedsteelrebarandtoinitiatecorrosion.
Theprimaryfactorscontrollingtheinitiationofthesteelcorrosionanditsmechanisminconcretearesummarizedinthefollowingpoints:
∙Therateofsteeldepassivation
∙Theinitiationofthemacrocellsduetothedifferentialaerationandchlorideabsorption
∙Thelowresistivityattributedbytheconcreteporewater
∙Thepresenceofoxygentoacceleratethecorrosionprocess
Thecorrosionofsteelinconcreteisanelectrochemicalprocess,whichresultsintheformationofacorrosioncell.Thefollowingcorrosionmechanismisthemostlikelyforsteelrebarembeddedintheconcretewhensignificantvariationsexistinthesurfacecharacteristicsofthesteel.Thesteelsurfaceinitiatescathodesandanodeselectricallyconnectedthroughthebodyofthesteelbar.The“halfcellreaction”takesplace,byinducinganelectromotiveforceknownasstandardredoxpotentialwhenthemetalisconnectedtoahydrogenelectrode–seeEquation1.
Equation1
Foriron:
Fe--Fe+2+2e-(Anode)
TheelectronsliberatedattheanodemigratetothecathodeandreactinvariouswaysdependantuponthepHvalueandtheavailabilityofoxygen.SeeEquation2,Equation3,andEquation4.
Equation2
2e+2H+½O2------H2O
Equation3
2e+H2O+½O2-----H2O
Equation4
2e+2H----------------H2
Theanodicandcathodicreactionsareautocatalyticandresultinthetransformationofmetalliciron(Fe)torust.Therustformationisaccompaniedbyasignificantincreaseinthevolume,suggestedaslargeasseventimesthatoftheoriginalFevolume.Thevolumeincreasecausesconcretecrackingandspalling.
EffectofChlorideIons
Whenthesteelisplacedinahighlyalkalinesolution(pH>11.5),eveninthepresenceofoxygen,corrosionwillnotbeinitiated.Infact,slightlyrustedbarswillbedissipatedwhenplacedinstrongalkali.Thatisthereasonwhy,duringconstruction,slightlyrustedsteelbarsdonotcreateaconcern.
ThechlorideionsingressdoesnotlowerthepHintheconcrete.However,itdestroysthepassivelayeronthesteelbars.Thedepassivatedsteelbarsdonotcorrodeinthepresenceofthechlorideionsonly.ThecorrosionoccursafterthepresenceofthecarbondioxidelowersthepHbelow11,thuscontributingtocorrosioninitiation.
Sourcesofchlorideareeitherintheconcretemix,mainlyfromthesand,aggregates,orthewaterused,oraschlorideingressfromtheenvironment,suchasinthemarineatmosphericenvironment.
EffectofCarbonation
Carbonationisthealkalinitylossintheconcretemass.Theproductofthereactionbetweencarbondioxideinthenormaloutsideairandthealkalineproducts,mainlythecalciumhydroxides,iscalciumcarbonate.Incaseofhighwater/concreteratio,carbonationcontinuestothedepthwherethereinforcingsteelbarisembedded.
Whencarbondioxidepenetratesthroughtheconcretecoverinthepresenceofwaterinthepores,itdrivesthepHtolowervalueswhichdepassivatesthesteel
Otherhydrationproductsinthecementcangothroughthesamereactionwithcarbondioxidecausingasignificantqualitylossofthecementandfasterdeteriorationoftheconcretemix.
EffectofotherElements
Sulfidecanbefoundinthecementasacontaminant(morethan0.2%).Thesulfideionhasbeenfoundmoredestructivetothesteelrebarembeddedintheconcreteifitgoeshigherthantheregulatedpercentageshown.Regardlessofthesulfideionsource,ithasbeenthecauseofseveralcasesofhydrogenembrittlement–particularlyinpre-stressedrebar.
MechanismofFBEcoatedsteelcorrosioninconcrete
InthewakeoftheprematurefailureofFBEcoatedsteelrebarintheFloridaKeysbridgesubstructures,manyresearchworksbyusersandacademiainvestigatedtheperformanceofFBEcoatedsteelinvariousenvironmentsandserviceconditions.MostofthelaboratorytestresultsconfirmedthattheFBEmaterial,appliedundercontrolledconditions,passedsuccessfullyallqualificationandservicesimulatedtests.However,inafewcases,fieldsamplesshowedpooradhesiontotheextentofdelaminationanddisbondingoftheFBErebar.Oftentimes,thatdelaminationwasusedasthedefinitionfora‘coatingfailure,’ratherthancorrosionorconcretedistress.InordertounderstandwhatwentwrongtocauseprematurefailureoftheFBErebar,numerousdatawerecollectedfromvariousfieldsforinvestigationandassessment.
CorrosioncontroloftheFBEcoatingisafunctionofthecoating’sabilitytoprovideabarrieragainstwater,oxygen,chloride,andotheraggressiveelementsthatpreventspermeationthroughthecoatingfilmtoattackthemetalsubstrate.TherearecriticalpropertiesrequiredforcorrosionprotectionFBEcoatingsthatincludeadhesionandwettingabilitytotherebar.
Reductioninadhesivestrengthwillincreasethedelaminationprocessrate.,.AninvestigationintodelaminationofFBEcoatingsinasimulatedporesolutionenvironmentsuggestedthefollowingdelaminationmechanism:
∙Delaytimebeforeinitiationofobservabledelaminationprocessesmaybeafunctionofwaterpenetrationthroughthecoatingtotheinterfacialorinterphasialcoating/substrateregion.
∙DelaminationofFBEcoatingsfromsteelsubstratesispredominantlycausedbyhydroxylions.
∙RateofFBEdelaminationiscontrolledbytransportprocessesfromaporeinthecoatingandalongthedelaminatedcoating/substrateinterfacetothedisbondmentfront.
∙Thelocalityoffailureofcoatingadhesionisintheinterfacialorinterphasialcoating/substrateregion.
∙TherateofFBEdelaminationinnear-passiveconditionsiscontrolledbyhydroxylionmigrationfromthebulkexternalsolutiontothecoating/substratedisbondmentfront.
∙TherateofFBEdelaminationintheconditionofunderfilmcorrosioniscontrolledbyhydratedcationmovementtothecathodesite.
DesignofFBEcoatingpowderforsteelrebarcoating
NewtechnologiesareundercontinualdevelopmenttooptimizethepropertiesoftheFBEcoatingtoimprovecoatingutility.Thestoiciometricratiomustbecontrolledbytheequilibriumbetweenthecuringgroupandtheepoxygroup.Forexample,increasingthelevelofcuringagentmayreducethecross-linkdensityandincreaseflexibility,whiledecreasingchemicalresistance.
Impactresistanceorhardnessisafunctionofthecross-linkdensity.Higherdensitiescanbeachievedusinglowmolecularweightcuringagentsthatshowtightlycross-linkedstructures.Addingnon-reactivediluentscaninterferewiththisstructure,providingtheendproductwithmoreflexibilitybutlesstoughness.
Mechanicaladhesionisthegrippingforcethatresultsfromthero
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