【正文】 corrosionispresent.Underthesecircumstances,fatiguestresseswillcauseashortertimetofailurethanastaticstresslevelofthesamemagnitude. Thismechanismwillmanifestitselfmorefrequentlyinunbonded(external)tendonsthanbondedsystems.CorrosionArrest,MonitoringandControlWhenaposttensionedstructurehasdeterioratedasaresultofcorrosionprocesses,orifdefectspermittingcorrosiontoinitiateandprogressarediscovered,threedifferent,generalapproacheshavebeenconsideredformitigatingfuturedamage.Theseinclude:? Providingorrestoringconditionswhichpreventcorrosionofsusceptiblesteelsurfaces(passivation)? Preventingressofaggressivesolutionswithbarriers(sheathing,coatings,sealantsandsealers)? Controllingtheelectrochemicalbalancewithinthereinforcedconcreteenvironmenttohaltmetalloss.Cathodicprotectionfunctionsinthismanner.Accurateandreliabledefinitionofdeteriorationmechanismsandextentisrequiredinallcircumstances. Experienceofthelast5yearshascontributedsignificantlytoadvancementofpracticalmethods,largelyduetobridgerehabilitationeffortsinitiatedasaresultofdiscoveryoftendonvoidsandotherdefectsinFloridaandotherstates.Whenvoidsanddefectsarediscoveredingroutedposttensioningducts,regroutingisapracticalsolutionforassuringthatdesignlifeofthestructurewillnotbepromised,aslongasthedamageaccumulatedtothepointoftendondefectdiscoveryismanagable.Recentinnovationsinnondestructivetesttechnicuesandvacuumgroutingpermitaccurateidentificationandrepairofvoids.Thesemethodscanbeusedin8CopyrightASCE2005 Structures2005StructuresCongress2005。allrightsreserved.conjunctionwithtechniquesofthesecondapproachwhichpreventinfiltrationofaggressiveagentsfromthestructuresenvironment. Considerationsrelatedtoinstallationofcathodicprotectionsystemsarealsopresentedinfollowingsections.NDTMethods.Thebinationofthetwomethodsispresentlyusedforthelocationofvoidsinductsforgroutedposttensionedstrandsandbars. ThenondestructiveImpulseRadarandImpactEcho(IE)methodsarethemostpromisingforthedetectionandquantificationofvoidsingroutedinternallyposttensionedtendons. ThesemethodsaredescribedintheAmericanConcreteInstituteReportACI,“NondestructiveTestMethodsforEvaluationofConcrete in Structures.” Also, Sansalone and Streett (ImpactEcho:nondestructiveevaluationofconcreteandmasonry,BulbrierPress,Ithaca,NY,1997)describetheuseofImpactEchotestingforthedetectionofgroutedorungroutedtendonducts.ImpulseRadarisusedtolocatethelongitudinalalignmentanddepthtotheductsofinterest,beforeIEtesting. Itisessentialthattheductsbelocatedaccuratelybeforeanystresswavetestingisperformed,eveninprecastunits.Inaddition,posttensioneddecksandpiersusuallycontainaconsiderablequantityofstandardreinforcingsteel,whichhastobeavoidedduringIEtesting.Ferromagneticdevicessuchascovermetersarenotadequateforthispurpose,becauseoftheconcentrationofdifferenttypesofsteelreinforcementandthedepthofductcoverusuallyencountered.ThelocationofallductsinthedeckorpiershouldbeclearlymarkedontheconcretesurfacetofacilitatesubsequentIEtesting.TheIEpressionwavevelocityoftheconcretecanbetested,forexample,atdrainageopeningsintheouterdeckflangewheretheflangethicknesscanbeconfirmed,orattheedgesofpiers.ThreeIEtestsshouldbemadeateachlocation,andtheindividualresultsstoredonputer.Inaddition,anaveragetestresultispoundedfromthethreetests,andprocessedaccordingtothemethodoutlinedinAbraham,M.,C.L233。onard,P.C244。teandB.Piwakowski(TimeFrequency Analysis of ImpactEcho Signals: Numerical Modeling andExperimentalValidation,ACIMaterialsJournal,V.97,No.6,.2000,pp.645657).ThisprocesseliminatestheinfluenceoftheRayleighwaveontheresult,andalsominimizesanyeffectfrombackgroundnoise.FloridaDOT’sCentralStructuresOfficesoffersadetailedreportonitswebsitedocumentingtheselectionandparativeevaluationofvariousNDTsystems for prestressed construction (Manuals/).ThestudywasinitiatedasadefinitiveparisonbetweenvariousavailabletechnologiesandsystemsresultingfromFloridaDOT’sneedtoevaluateexistingposttensionedbridges.VacuumGrouting.Vacuumgroutingtechniquesareknowntohavebeenrefinedtothepointthatprehensiverestorationofgrout’scontributiontostrengthanddurabilityofposttensionedconstructioncanbeassured.Reliabletechniqueshavebeendevelopedbyposttensioningcontractorsfortheprocess.Thevolumeoftendonvoidsismeasuredbycreatingandmonitoringavacuumatvarious9CopyrightASCE2005 Structures2005StructuresCongress2005。allrightsreserved.positionsalongatendon.Thevacuumactstoalsodrawsgroutintothevoid.CombinedwithNDTmethods,aparisonbetweenthemeasuredvolumeofvoidandtheamountofgroutconsumedprovidesacontrolmeasureonthesuccessoftheoperation.InpreparationforrepairoftheMidBayBridgeinFlorida,contractor’sresultsfromfullscalemockuptestingprovedthepotentialofvacuumgrouting.Hundredsofvoidswerevacuumgrouted.Sincerefinementofvacuumgroutingtechniquestotheirpresentstateofadvancement,morethentenmajorbridgesaroundthecountryhavebeenrestoredwiththeprocess.CathodicProtectionConsiderations.Cathodicprotectiontechnology’spromiseissometimessoughtforprestressedbridgestruct