外文翻译---腐蚀和预应力混凝土桥梁.doc

CORROSIONANDPRESTRESSEDCONCRETEBRIDGESAdrianT.Ciolko,P.E.1腐蚀和预应力混凝土桥梁阿德里安 T.Ciolko P.E.1AbstractTheimpactonbridgereliability,ofprestressingsteeldeteriorationandtendonrupturecreatedbycorrosionmechanisms,ismuchmorecriticalandrapidthanthatofanyothercomponentofaprestressedconcretebridge.Additionally,deteriorationofembeddedprestressingreinforcementinthesestructuresmaynotnecessarilybemadevisiblethroughmanifestationofexternaldistressintheConcrete.摘要腐蚀机制引起预应力钢筋的恶化和断裂，进而对桥梁的可靠性产生影响，这比对预应力混凝土桥的其他部分的影响更加严重，也更加迅速。 此外，在这些结构中，有粘结预应力钢筋的恶化也不必要通过混凝土外部糟糕的状况表现出来。Initiatingmechanismsandparticularformsofcorrosionaffectingprestressedconcretestructuresaredescribedbasedontheauthorsexperience.Thesedegradationmechanismsinclude:ChloridesandCorrosionConcreteCarbonationEffectsInfluenceofConcreteCrackingElectrochemical(Macrocell)andPittingCorrosionStressCorrosionCrackingandSteelEmbrittlementFrettingCorrosionCorrosionandFatigue关于腐蚀影响预应力混凝土结构的最初机制和特定形式的描述是基于作者的经验。这些退化机制包括： 氯化物和腐蚀 混凝土碳化影响 混凝土裂缝的影响 电化学 (小区) 和孔蚀 应力腐蚀开裂和钢的脆断 微动腐蚀 腐蚀和疲劳Recently,prestressed,post-tensionedconcretestructureshavebeenthoughttobethreatenedbycertainprematureperformanceimpairmentsworldwide,predominatelywhendefectswerepresent.Theseproblemsmaycontinuetomanifestthemselves.Therefore,corrosionpreventativemethodsandneedforscrutinyduringconstructionandmaintenanceneedswillbedescribed.Forexistingstructuresexperiencingevidenceofprematuredistress,theauthordiscussescase-studybasedinvestigativetechniquesandavailablerehabilitationStrategies.最近，世界各地的预应力、 后张法预应力混凝土结构已经被认为受到某些早期性能破坏的威胁，尤其是已经存在的缺陷。这些问题可能继续危害结构。因此，我将对预防腐蚀的方法和施工与维护期间的检测进行介绍。对于遭受早期破坏的现有结构，作者讨论案例研究是基于探测技术和有效的修复手段。1VicePresident,ConstructionTechnologyLaboratories,Inc.,5400OldOrchardRoad,Skokie,IL60077.1CopyrightASCE2005Structures2005StructuresCongress2005IntroductionThefirstprestressedconcretebridgeintheUSwasconstructedin1950;thepaceofacceptanceforthisconstructiontechnologyacceleratedrapidly;mostallconcretebridgesuperstructuresinexistencetodaywerebuiltinthatfashion.Assuringsuccessinnewconstructionandextendingservicelifeofprestressedandpost-tensionedconcretebridgesrequiresattentivenesstothedistinctive characteristicsandbehaviorsinherenttothisparticularconstructiontechnology.介绍美国的第一个预应力混凝土桥建于1950年；接受这种施工技术的步伐迅速加快;在这个时尚潮流中建造了当今大多数的混凝土桥梁上部结构。确保成功应用于新型结构和延长预应力和后张法预应力混凝土桥梁的使用寿命需要专注于这种特殊施工技术的显著特点和固有特征。Thispaperdiscussesconsiderationsnecessaryforachievingthepotentiallowmaintenanceservicelifethesebridgescanoffer.Theseobservationsarederivedfromtheauthorsexperienceininvestigatingandevaluatingprestressed,post-tensionedconcretebridges,anddevelopinginvestigativeandrehabilitationsolutionsattheCTLStructuralEngineeringLaboratorysince1988.Whilethedurabilityofprestressedconcretecanbeinfluencedbybehaviorsofboththeconcreteandreinforcement,recentconstructiontrendsandposttensionedconcretebridgeperformanceobservationshavefocusedmoreattentiononpreservingtheconditionofprestressingsteel.Consequencesofprestressingsteeldeteriorationaremostserious.Awarenessofthesefeaturesisakeyelementtoconstructionoflonglastingprestressedbridgesandextendinglifeofolder,serviceablestructures.本文讨论了实现这些桥梁潜在的低维护使用寿命所需的考虑。自1988 年以来，作者在 CTL 结构工程实验室调查和评估预应力、后张法预应力混凝土桥梁、开发检测和加固解决方案，这些观测结论就源于这期间的经验。混凝土和钢筋两者的性能都影响预应力混凝土的耐久性，但是当近的建设趋势和后张预应力混凝土桥梁性能检测都更多地集中于预应力钢筋的维护。预应力钢筋恶化的后果是最严重的。对这些特点的认识是对建设持久的预应力桥梁和延长旧的可用结构寿命的一个关键因素。FunctionofPrestressingReinforcementinBridgeDesignandConstructionMostconcretehighwaybridgesarecomprisedofprestressedconcreteintwopredominantfabrication/constructioncategories;namelypretensionedandpost-tensioned,withmorethan90%representingpretensioned.Thegeometriccrosssectionsanddesign/constructionproceduresusedrepresentabroadrangeofpossiblestructuraldesignconfigurations,bothprecastandcast-in-place;theinterestedreaderisencouragedtoobtaininformationfromorganizationssuchasthePrestressed/PrecastConcreteInstitute,thePostTensioningInstituteandtheAmericanSegmentalBridgeInstituteforin-depthdesignandconstructionguidanceforthistechnology,sinceexploringitscomplexityisbeyondthescopeofthispaper.预应力加固桥梁设计和施工中的作用最具体的公路桥梁是由组成的预应力混凝土在两个主要制造/建筑类别 ；即先张法和后张法，有 90%以上表示先张法预应力。几何截面和使用的设计/施工程序代表的范围广泛的可能结构设计配置、 预制和强制转换的地方 ；有兴趣的读者应该鼓励从预应力/预制混凝土研究所、 后张法研究所和美国节段桥梁研究所深入设计和施工指导对这一技术，等组织获得的信息，因为探索它的复杂性是超出了本文的范围。Prestressed(lesscommonlyknownaspretensioned)concreteismadebystressingthereinforcementbetweenfixedbulkheadsandcastingconcretearoundthesteelelements.Followingconcretecuring,thesteelstrandisreleasedatthebulkhead,transferringcompressivestressandaddedstiffnessintotheconcrete.Mostallpretensionedconcretebridgecomponentsareprecastinspecializedfabricationyards,thenshippedanderectedon-site.Post-tensionedconcreteforbridgespredominatelycontainsmultiplestrandorbartendons.Tendonsarestressedagainstbuilt-inmulti-pointanchorages;andtendonforcesareinternallyresistedbytheconcretemassincompression.Tendonsareinstalledinsheathingorductspreplacedintheformworkpriortocasting.Tensileforcesinstressedtendonsaretransferredtoconcretethroughspecializedanchoragehardware,creatingcompressionintheconcrete,makingthestructuralelementstiffer.Inbridges,bothbondedandunbondedtendonconstructionisused.Inbondedtendonconstruction,2CopyrightASCE2005Structures2005StructuresCongress2005cementitiousgroutisinjectedintotheductsafterstressing;protectingagainstcorrosionandenhancingthestrengthcapacityofthebridgeelement,sincegroutbondsthetensileelementtothesurroundingconcretemassalongitslength.BondedtendonsarepositionedwithinthethicknessofgirderwebsandflangesUnbondedtendonconstructionissometimesusedinconcretebridgeswhenexternaltendonsarechosenindesign;thesearepositionedoutsidethemassoftheconcretegirderelements,althoughthesearenearlyalwaysincorporatedwithintheinteriorsofcellularconstruction.Unbondedtendonsarealsogrouted,principallyforcorrosionprotectionCommonamongthesebridgecomponentsistheroleoftheprestressingreinforcement,whichcantaketheformofcold-drawnhighstrengthwire,orwirehelicallywoundtoform7-wirestrand,androlledhighstrengththreadedbars.Prestressingstrandforbridgeconstructionispresentlysuppliedwithatensilestrengthof1860MPa,andbarreinforcementin1030to1100Mpatensilestrength.Prestressingwire(parallelwiresystems)hasalsobeenusedforprestressedbridges,thoughnotinthiscountryformorethantwodecades.Inbridges,theprestressingreinforcementelementprovidestheconcretestructureameansforcounteractingtensilestressresultingfromserviceloads,andprovidestheneededcapacityforthestrengthlimitstate.Assuch,theprestressingreinforcementmustreliablyandcontinuouslysustain,overthelifeofthestructure,aminimumstresslevelontheorderof60%ofitsnominalstrength.Intherealmofengineeringdesign,thisdemandonmaterialperformanceisverysubstantial.Consequently,theprestressingreinforcementsqualityanditsprotectionagainstaginganddeteriorationprocessareparamounttoattainingfunctionalandsafetyrequirementsofaconcretebridgestructure.Designofprestressedmembersprovidesdirectlyforseveralformsofdurabilitymeasuressuchasdenseconcretecover,corrosionmitigatinggroutsystems,andinnovative,proprietaryfulllengthcorrosionbarriersintegratedwithanchoragesystemsforpost-tensionedstructures.Additionally,structuraldetailsandstandardshavebeendevelopedandintroducedbyAASHTOandvariousindustryorganizationstomanageeffectsofprestressingreinforcementsheightenedsusceptibilitytoknownagingmechanismssuchasfatigue,fretting,andtimedependent,inelasticchangesinconcretestructurecomponents.Bridgemanagementstrategypresentlyreliesonfixed-intervalinspectionsandcomputationofbridgeloadratings.Unfortunately,theconditioninspectionsarecomprisedexclusivelyofvisualobservationofthestructuralfeaturesvisibletotheunaidedeye,andfewwidelyacceptedpracticalmeansexistforquantitativelymeasuringconditionoftheprestressingandforintegratingresultsofsuchmeasurementsintothestrengthandserviceabilityratingcomputations.Themostrecent5-yearhistoryofpost-tensionedbridgeconstructionhasevolvedsignificantchangesinperceptionofthesebridgesperformance.AgingandDegradationMechanismsInfluencePracticesThehistoricperformancerecordofmorethan100,000prestressedconcretebridgestructuresintheUSisgenerallyconsideredsatisfactory;howeversomecorrosion3CopyrightASCE2005Structures2005StructuresCongress2005ofprestressingsteelhadbeenreportedinaminorityofstructures.FHWA-sponsoredsurveysin1987and1992of20majorin-servicestructuresrepresentativeoftheprestressedconcretecomponentofthenationshighwayinfrastructuresupportedandhighlightedneedfordesignforserviceability.Findingsofvisualinspection/corrosionsensing/materialsstudiesbyNovokshchenovandWhitingetal.forpretensionedandpost-tensionedsuperstructuresofvariousconfigurationsthroughoutthefullrangeofUSenvironmentalexposures,foundnoevidenceofseriousprestressingreinforcementcorrosionorothersystemicdegradation.Thesestudiesandothersimilarfindingsofthehighwaytransportationcommunityhaveledtorecommendationsrelatedprimarilytominimizationofthenumberofdeckjoints,moredurable,protectiveconcretesandcoatingsforanchoragecomponents,improveddeckandjointdrainagesystemstoredirectpassageofaggressivesolutionsfromcorrosion-susceptiblecomponentsneargirderends,andimplementationofmoreeffectivedeckjointmaintenanceprograms.Someprestressedconcretestructureshavedisplayedtroublesomeprematureperformanceimpairmenttrendsworldwide,discriminatingthisstructuralconceptfromotherswhenoneconsidersdevelopmentofarationaleforassessingandmaintainingbridgereliabilityandutilitybeyondthe50-yearagemilestone.Szilard,ina1969surveyperformedonbehalfofFIP,notedthatcorrosionofprestressingreinforcementwasthepredominantformofunexpecteddamagetostructures,whetheraccidentalordeterioration/agingbased.In1971,theAustralianWaterResourcesCouncilpublishedresultsofawide-rangingstudyoftheperformanceofprestressedconcretestructuresworldwideintendedtoassesstheperformanceofAustralianprestressedstructuresandcompareitwiththatofstructureselsewhere.Theworkwaspromptedbyprematuremultiplefailuresofprestressed,wire-woundpipelinesandtanks,andhelpedestablishimprovedspecificationforprestressingwireandconcrete/mortarandotherprotectivecoatings.Citingthecriticalityofcorrosionprotectionforprestressedconcretepressurevesselsfornuclearreactorcontainments,GriessandNausstudiedcorrosionbehaviorofhigh-strengthprestressingsteelsin1978,reviewingseveralincidentsofprestressingsteelfailuresinnuclearpressurevasselsintheU.S.,France,andUnitedKingdom.Schupackconductedaperformancesurveycoveringthetimeperiodof1950to1977,concludingthatthenotedfailureincidencerateof200tendonsoutofanestimatedworldwideprestressingsteelconsumptionof30,000,000tonswasnegligible.However,anNCHRPstudyperformedbyaforensicengineeringfirmin1982reportedthat50structureswithtendoncorrosionwerenotedinthetimeperiodbetween1978and1982.Tenofthesereportedlydisplayedbrittlefracturessuggestiveofenvironmentalcrackingphenomena.In1992,CiolkosummarizedstructuralevaluationandfailureanalysisdatafromU.S.prestressedconcretepipelinefailures,estimatingbasedonindustryandwaterutilityrecordsthatmorethan60suchpressurepipelineshadfailedintheUS,principallyduetoacceleratedcorrosionofpoorlyprotectedprestressingwire.Commontoallasafactorcontributingtoperformanceimpairmentwastheheightenedsusceptibilityofprestressingwiretoacceleratedcorrosionphenomena.4CopyrightASCE2005Structures2005StructuresCongress2005In1992,Podolnywarneddesignersofthedangerofcomplacencyresultingfromrelianceonthesomeofthefavorableconclusionsfromearlysurveysandreportsoftheconditionofprestressedstructures.Heconcludedthatalthoughthepopulationofprestressedstructureshasgreatlyincreasedinthelasttwodecades,itwasonlyrecentlythatwehavebeguntounderstandtherolethatenvironmentandthemanyformsofcorrosionhaveonreliabilityofprestressingreinforcement.In1998,PostonandWoutersreportedinanNCHRPstudyofdurabilityofprecastsegmental(post-tensioned)bridges,thatnoevidenceofcorrosionorotherdurabilityproblemswiththesebridgesexisted,cautioninghowever,thatthegatheredinformationwasbasedprincipallyonvisualinspections.Thestudycitedthatlackofimprovednondestructiveevaluationoptionshindersevaluationofthesestructures.Podolnyscautionregardingcomplacencywaswarranted,aswerePostonsconcernsforlackofdiagnosticnondestructivetoolswhichwouldhaveprovidedamorereliablemeansforjudgingconditionofposttensionedstructures.In1999,corrosion-relatedfailureofanexternal(unbonded)posttensioningtendoninFloridaDOTs20-yearoldNilesChannelBridgefocusedattentiononpost-tensionedbridges.ThreeotherFloridabridgesincludingtheMid-BayBridgeinDestin,SunshineSkyway(St.Petersburg),andI-75/I595SawgrassInterchangeinBrowardCountywerelaterinvestigated.Investigationsofthesestructureshighlightedthecriticalrolethattendongroutingandcomplementarycorrosionbarrierstakeinsuccessfulconstruction.Therootcausesofreinforcementdeteriorationinthesebridgeswereidentifiedasgroutingprocedureineffectivenessforcontrollingentrapmentofbleedwaterintendons,andpoortendonductdetailingcreatingaccessforaggressivesolutions.Theseincidentspromptedrigorousreviewofthedurabilityissuesrelatedtocorrosionofpost-tensionedconcretestructures.CorrosionMechanismsandThreatsCorrosionisdefinedasthedegradationofametalsintegrityandstrengthbyinteractionwithitsenvironment.Becausethemetalsusedinconstructionarerefinedfromtheirnaturally-occurringoxides,therefinedmetalislessthermodynamicallystablethanitsoxide.Whencorrosionoccurs,therefinedmetalsstructurerevertstoitsnaturally-occurringstatethroughanelectrochemicalreaction.Initiationandsustenanceofthereactionrequireexistenceofanelectrolyticcell.Eachcorrosioncellrequiresthreeelements;ananode(theregionofmetalwhichcorrodes,orrevertstoanoxide),anelectrolyte(acorrosion-enablingmediumorenvironmentwhichprovidesapathorenvironmentforelectronsorcurrenttoflow,whichmaybeinherentlyaggressiveorbenign),andacathode(thelocationwhereelectronsareconsumedorabsorbed,andthemetalisprotectedfromcorrosion).5CopyrightASCE2005Structures2005StructuresCongress2005Metalsinconcrete,includingmildsteelreinforcement,prestressingsteelencasedinhighqualityconcrete(pretensioned),andprestressingsteelinfullygroutedpost-tensioningductsareknowntoberesistanttocorrosionduetothebeneficialeffectsofthehighlyalkalineenvironmentthatcementitiousmaterialscanprovide.Whenthesealkalineconditionsareeffectivelysustainedinanunbroken,continuousstate,themetalssurface,whichwouldotherwisebeunstablethermodynamically,isknowntobepassivated,orprotectedbyathinironoxidefilm.Designofconcretestructuresforserviceabilityispredicatedonthisprinciple,andmorethanahundredthousandUShighwaystructureshaveperformedadequatelybasedonthisdesignphilosophy.However,ifthehighlyalkalineenvironmentprovidedbyconcreteorcementitiousgroutisabsent(atvoidsandotherdefects);whereverthepassivationisinterruptedorbreached;orwhereverthepassivatingenvironmentisalteredchemically,corrosionwilloccur.Itisveryimportanttonoteinanydiscussiononthissubjectmatterthatcorrosionisneveruniformlydistributed.Thatkindofdeteriorationwouldideallyrequirethateachatomonthemetalssurfacebeequallysensitivetothecorrosionmedium(electrolyte)andthatthemediumhaveuniformaccesstoallsurfaceatoms.Thevariousformsandtypesofcorrosionobservedinreinforcedandprestressedconcretearethereforemanifestationsofthenon-uniformitiesinherentinmaterials,manufacturingdefects,alterationoftheconcreteschemistry,andmechanicaldamagearisinginmanufacture,constructionorduringservice.Initiatingmechanismsandparticularformsofcorrosionaffectingprestressedconcretestructuresaredescribedinthefollowingsections;considerationofeachasaninflue