【文章內容簡介】 thetimeperiodbetween1978and1982.Tenofthesereportedlydisplayedbrittlefracturessuggestiveofenvironmentalcrackingphenomena.In1992,Ciolkosummarizedstructuralevaluationandfailureanalysisdatafrom.prestressedconcretepipelinefailures,estimatingbasedonindustryandwaterutilityrecordsthatmorethan60suchpressurepipelineshadfailedintheUS,principallyduetoacceleratedcorrosionofpoorlyprotectedprestressingwire.Commontoallasafactorcontributingtoperformanceimpairmentwastheheightenedsusceptibilityofprestressingwiretoacceleratedcorrosionphenomena.4CopyrightASCE2005 Structures2005StructuresCongress2005In1992,Podolnywarneddesignersofthedangerofplacencyresultingfromrelianceonthesomeofthefavorableconclusionsfromearlysurveysandreportsoftheconditionofprestressedstructures.Heconcludedthatalthoughthepopulationofprestressedstructureshasgreatlyincreasedinthelasttwodecades,itwasonlyrecentlythatwehavebeguntounderstandtherolethatenvironmentandthemanyformsofcorrosionhaveonreliabilityofprestressingreinforcement.In1998,PostonandWoutersreportedinanNCHRPstudyofdurabilityofprecastsegmental(posttensioned)bridges,thatnoevidenceofcorrosionorotherdurabilityproblemswiththesebridgesexisted,cautioninghowever,thatthegatheredinformationwasbasedprincipallyonvisualinspections.Thestudycitedthatlackofimprovednondestructiveevaluationoptionshindersevaluationofthesestructures.Podolny’scautionregardingplacencywaswarranted,aswerePoston’sconcernsforlackofdiagnosticnondestructivetoolswhichwouldhaveprovidedamorereliablemeansforjudgingconditionofposttensionedstructures.In1999,corrosionrelatedfailureofanexternal(unbonded)posttensioningtendoninFloridaDOT’s20yearoldNilesChannelBridgefocusedattentiononposttensionedbridges.ThreeotherFloridabridgesincludingtheMidBayBridgeinDestin,SunshineSkyway(St.Petersburg),andI75/I595SawgrassInterchangeinBrowardCountywerelaterinvestigated.Investigationsofthesestructureshighlightedthecriticalrolethattendongroutingandplementarycorrosionbarrierstakeinsuccessfulconstruction.Therootcausesofreinforcementdeteriorationinthesebridgeswereidentifiedasgroutingprocedureineffectivenessforcontrollingentrapmentofbleedwaterintendons,andpoortendonductdetailingcreatingaccessforaggressivesolutions.Theseincidentspromptedrigorousreviewofthedurabilityissuesrelatedtocorrosionofposttensionedconcretestructures.Corrosion–MechanismsandThreatsCorrosionisdefinedasthedegradationofametal’sintegrityandstrengthbyinteractionwithitsenvironment.Becausethemetalsusedinconstructionarerefinedfromtheirnaturallyoccurringoxides,therefinedmetalislessthermodynamicallystablethanitsoxide.Whencorrosionoccurs,therefinedmetal’sstructurerevertstoitsnaturallyoccurringstatethroughanelectrochemicalreaction. Initiationandsustenanceofthereactionrequireexistenceofanelectrolyticcell.Eachcorrosioncellrequiresthreeelements。? ananode(theregionofmetalwhichcorrodes,orrevertstoanoxide),? anelectrolyte(acorrosionenablingmediumorenvironmentwhichprovidesapathorenvironmentforelectronsorcurrenttoflow,whichmaybeinherentlyaggressiveorbenign),and? acathode(thelocationwhereelectronsareconsumedorabsorbed,andthemetalisprotectedfromcorrosion).5CopyrightASCE2005 Structures2005StructuresCongress2005Metalsinconcrete,includingmildsteelreinforcement,prestressingsteelencasedinhighqualityconcrete(pretensioned),andprestressingsteelinfullygroutedposttensioningductsareknowntoberesistanttocorrosionduetothebeneficialeffectsofthehighlyalkalineenvironmentthatcementitiousmaterialscanprovide.Whenthesealkalineconditionsareeffectivelysustainedinanunbroken,continuousstate,themetal’ssurface,whichwouldotherwisebeunstablethermodynamically,isknowntobepassivated,orprotectedbyathinironoxidefilm. Designofconcretestructuresforserviceabilityispredicatedonthisprinciple,andmorethanahundredthousandUShighwaystructureshaveperformedadequatelybasedonthisdesignphilosophy.However,ifthehighlyalkalineenvironmentprovidedbyconcreteorcementitiousgroutisabsent(atvoidsandotherdefects)。whereverthepassivationisinterruptedorbreached。orwhereverthepassivatingenvironmentisalteredchemically,corrosionwilloccur.Itisveryimportanttonoteinanydiscussiononthissubjectmatterthatcorrosionisneveruniformlydistributed.Thatkindofdeteriorationwouldideallyrequirethateachatomonthemetal’ssurfacebeequallysensitivetothecorrosionmedium(electrolyte)andthatthemediumhaveuniformaccesstoallsurfaceatoms.Thevariousformsandtypesofcorrosionobservedinreinforcedandprestressedconcretearethereforemanifestationsofthenonuniformitiesinherentinmaterials,manufacturingdefects,alterationoftheconcrete’schemistry,andmechanicaldamagearisinginmanufacture,constructionorduringservice.Initiatingmechanismsandparticularformsofcorrosionaffectingprestressedconcretestructuresaredescribedinthefollowingsections。considerationofeachasaninfluenceonoptimalservicelifeofaprestressedconcretebridgeisneededindesignandspecification,construction,andbridgemaintenancepractice.ChloridesanInitiator.Chlorideshavebeenintroducedintoconcretestructuresatthetimeofconstruction,althoughthatpracticeisto