Bridge research in Europe.docx

BridgeresearchinEuropeAbriefoutlineisgivenofthedevelopmentoftheEuropeanUnion,togetherwiththeresearchplatforminEurope.Thespecialcaseofpost-tensionedbridgesintheUKisdiscussed.InordertoillustratethetypeofEuropeanresearchbeingundertaken,anexampleisgivenfromtheUniversityofEdinburghportfolio:relatingtotheidentificationofvoidsinpost-tensionedconcretebridgesusingdigitalimpulseradar.IntroductionThechallengeinanyresearcharenaistoharnessthefindingsofdifferentresearchgroupstoidentifyacoherentmassofdata,whichenablesresearchandpracticetobebetterfocused.AparticularchallengeexistswithrespecttoEuropewherelanguagebarriersareinevitablyverysignificant.TheEuropeanCommunitywasformedinthe1960sbaseduponapoliticalwillwithincontinentalEuropetoavoidtheEuropeancivilwars,whichdevelopedintoWorldWar2from1939to1945.ThestrongpoliticalmotivationformedtheoriginalcommunityofwhichBritainwasnotamember.ManyofthecontinentalcountriessawBritain'sinterestasbeingpurelyeconomic.The1970ssawBritainjoiningwhatwasthentheEuropeanEconomicCommunity(EEC)andthe1990shasseenthewideningofthecommunitytoaEuropeanUnion,EU,withcertainpoliticalgoalstogetherwiththeobjectiveofacommonEuropeancurrency.Notwithstandingthesefinancialandpoliticaldevelopments,civilengineeringandbridgeengineeringinparticularhavefoundgreatdifficultyinforminganykindofcommonthread.IndeedtheeducationalsystemsforUniversitytrainingarequitedifferentbetweenBritainandtheEuropeancontinentalcountries.TheformationoftheEUfundingschemes一e.g.Socrates,BriteEuramandotherprogramshavehelpedsignificantly.TheSocratesschemeisbasedupontheexchangeofstudentsbetweenUniversitiesindifferentmemberstates.TheBriteEuramschemehasinvolvedtechnicalresearchgrantsgiventoconsortiaofacademicsandindustrialpartnerswithinanumberofthestates一aBriteEurambidwouldnormallybeledbyanindustrialist.Intermsofdisseminationofknowledge,twoquitedifferentstrandsappeartohaveemerged.TheUKandtheUSAhaveconcentratedprimarilyupondisseminatingbasicresearchinrefereedjournalpublications:ASCE,ICEandotherjournals.WhereasthecontinentalEuropeanshavefrequentlydisseminatedbasicresearchatconferenceswherethecirculationoftheproceedingsisrestricted.Additionally,languagebarriershaveprovedtobeverydifficulttobreakdown.IncountrieswhereEnglishisastrongsecondlanguagetherehasbeenenthusiasticparticipationininternationalconferencesbasedwithincontinentalEurope-e.g.Germany,Italy,Belgium,TheNetherlandsandSwitzerland.However,countrieswhereEnglishisnotastrongsecondlanguagehavebeenhesitantPartiCiPantS一e.g.France.EuropeanresearchExamplesofresearchrelatingtobridgesinEuropecanbedividedintothreetypesofstructure:MasonryarchbridgesBritainhasthelargeststockofmasonryarchbridges.IncertainregionsoftheUKupto60%oftheroadbridgesarehistoricstonemasonryarchbridgesoriginallyconstructedforhorsedrawntraffic.ThisislesscommoninotherpartsofEuropeasmanyofthesebridgesweredestroyedduringWorldWar2.ConcretebridgesAlargestockofconcretebridgeswasconstructedduringthe1950s,1960sand1970s.Atthetime,thesestructureswereseenasmaintenancefree.Europealsohasalargenumberofpost-tensionedconcretebridgeswithsteeltendonductspreventingradarinspection.ThisisaparticularprobleminFranceandtheUK.SteelbridgesSteelbridgeswentoutoffashionintheUKduetotheirneedformaintenanceasperceivedinthe1960sand1970s.However,theyhavebeenusedforlongspanandrailbridges,andtheyarenowreturningtofashionformotorwaywideningschemesintheUK.ResearchactivityinEuropeItgivesanindicationcertainareasofexpertiseandworkbeingundertakeninEurope,butisbynomeansexhaustive.InordertoillustratethetypeofEuropeanresearchbeingundertaken,anexampleisgivenfromtheUniversityofEdinburghportfolio.Theexamplerelatestotheidentificationofvoidsinpost-tensionedconcretebridges,usingdigitalimpulseradar.PosttensionedconcreterailbridgeanalysisOveArupandPartnerscarriedoutaninspectionandassessmentofthesuperstructureofa160mlongpost-tensioned,segmentalrailwaybridgeinManchestertodetermineitsload-carryingcapacitypriortoatransferofownership,foruseintheMetrolinklightrailsystem.Particularattentionwaspaidtotheintegrityofitspost-tensionedsteelelements.Physicalinspection,non-destructiveradartestingandotherexploratorymethodswereusedtoinvestigateforpossibleweaknessesinthebridge.SincethesuddencollapseofYnys-y-GwasBridgeinWales,UKin1985,therehasbeenconcernaboutthelong-termintegrityofsegmental,post-tensionedconcretebridgeswhichmaybeproneto'brittle5failurewithoutwarning.Thecorrosionprotectionofthepost-tensionedsteelcables,wheretheypassthroughjointsbetweenthesegments,hasbeenidentifiedasamajorfactoraffectingthelong-termdurabilityandconsequentstrengthofthistypeofbridge.Theidentificationofvoidsingroutedtendonductsatvulnerablepositionsisrecognizedasanimportantstepinthedetectionofsuchcorrosion.DescriptionofbridgeGeneralarrangementBesseso,th,BamBridgeisa160mlong,threespan,segmental,post-tensionedconcreterailwaybridgebuiltin1969.Themainspanof90mcrossesoverboththeM62motorwayandA665BurytoPrestwickRoad.Minimumheadroomis5.18mfromtheA665andtheM62isclearedbyapprox12.5m.Thesuperstructureconsistsofacentralhollowtrapezoidalconcreteboxsection6.7mhighand4mwide.Themajorityofthesouthandcentralspansareconstructedusing1.27mlongpre-castconcretetrapezoidalboxunits,post-tensionedtogether.Thisboxsectionsupportstheinsiteconcretetransversecantileverslabsatbottomflangelevel,whichcarrytherailtracksandballast.Thecenterandsouthspansectionsareofpost-tensionedconstruction.Thesepost-tensionedsectionshavefivetypesofpre-stressing:1. Longitudinaltendonsingroutedductswithinthetopandbottomflanges.2. Longitudinalinternaldrapedtendonslocatedalongsidethewebs.Thesearedeflectedatinternaldiaphragmpositionsandareencasedininsiteconcrete.3. Longitudinalmacalloybarsinthetransversecantileverslabsinthecentralspan.4. Verticalmacalloybarsinthe229mmwidewebstoenhanceshearcapacity.5. Transversemacalloybarsthroughthebottomflangetosupportthetransversecantileverslabs.SegmentalconstructionThepre-castsegmentalsystemofconstructionusedforthesouthandcenterspansectionswasanalternativemethodproposedbythecontractor.Currentthinkingsuggeststhatsuchaformofconstructioncanleadto4brittle,failureoftheentirestructurewithoutwarningduetocorrosionoftendonsacrossaconstructionjoint,Theoriginaldesignconcepthadbeenforinsiteconcreteconstruction.InspectionandassessmentInspectionInspectionworkwasundertakeninanumberofphasesandwaslinkedwiththetestingrequiredforthestructure.Theinitialinspectionsrecordedanumberofvisibleproblemsincluding:1、 Defectivewaterproofingontheexposedsurfaceofthetopflange.2、 Watertrappedintheinternalspaceofthehollowboxwithdepthsupto300mm.3、 Variousdrainageproblemsatjointsandabutments.4、 Longitudinalcrackingoftheexposedsoffitofthecentralspan.5、 Longitudinalcrackingonsidesofthetopflangeofthepre-stressedsections.6、 Widespreadsaplingonsomeinsiteconcretesurfaceswithexposedrustingreinforcement.AssessmentThesubjectofanearlierpaper,theobjectivesoftheassessmentwere:1、Estimatethepresentload-carryingcapacity.2Identifyanystructuraldeficienciesintheoriginaldesign.3、 Detenninereasonsforexistingproblemsidentifiedbytheinspection.ConclusiontotheinspectionandassessmentFollowingtheinspectionandtheanalyticalassessmentonemiijorelementofdoubtstillexisted.Thisconcernedtheconditionoftheembeddedpre-stressingwires,strands,cablesorbars.Forthepurposeofstructuralanalysistheseelements>hadbeenassumedtobesound.However,duetotheveryhighforcesinvolved,aarisktothestructure,causedbycorrosiontotheseprimaryelements,wasidentified.Theinitialrecommendationswhichcompletedthefirstphaseoftheassessmentwere:1. Carryoutdetailedmaterialtestingtodeterminetheconditionofhiddenstructuralelements,inparticularthegroutedpost-tensionedsteelcables.2. Conductconcretedurabilitytests.3. Undertakerepairstodefectivewaterproofingandsurfacedefectsinconcrete.TestingproceduresNon-destructieradartestingDuringthefirstphaseinvestigationatajointbetweenpre-castdecksegmentstheobservationofavoidinapost-tensionedcableductgaverisetoseriousconcernaboutcorrosionandtheintegrityofthepre-stress.However,theextentofthisproblemwasextremelydifficulttodetermine.Thebridgecontains93jointswithanaverageof24cablespassingthrougheachjoint,i.e.therewereapprox.2200positionswhereinvestigationscouldbecarriedout.Atypicalsectionthroughsuchajointisthatthe24drapedtendonswithinthespinedidnotgiverisetoconcernbecausethesewereprotectedbyinsiteconcretepouredWithoutjointsafterthecableshadbeenstressed.Asitwasclearlyimpracticaltoconsiderphysicallyexposingalltendon。Ointintersections,radarwasusedtoinvestigatealargenumbersoftendonsandhencelocateductvoidswithinamodesttimescale.Itwasfortunatethatthecorrugatedsteelductsaroundthetendonswerediscontinuousthroughthejointswhichallowedtheradartodetectthetendonsandvoids.Theproblem,however,wasstillhighlycomplexduetothehighdensityofothersteelelementswhichcouldinterferewiththeradarsignalsandthefactthattheareaofinterestwasatmost102mmwideandembeddedbetween150mmand800mmdeepinthickconcreteslabs.Trialradarinvestigations.Threecompanieswereinvitedtovisitthebridgeandconductatrialinvestigation.Onecompanydecidednottoproceed.Theremainingtwoweregiven2weekstomobilize,testandreport.Theirresultswerethencomparedwithphysicalexplorations.Tomakethecomparisons,observationholesweredrilledverticallydownwardsintotheductsataselectionof10locationswhichincludedseveralwherevoidswerepredictedandseveralwheretheductswerepredictedtobefullygrouted.A25-mmdiameterholewasrequiredinordertofacilitateuseofthechosenhoroscope.TheresultsfromtheUniversityofEdinburghyieldedanaccuracyofaround60%.Mainradarsurvey,horoscopeverificationofvoids.Havingcompletedaradarsurveyofthetotalstructure,abaroscopicwasthenusedtoinvestigateallpredictedvoidsandinmorethan60%ofcasesthisgaveaclearconfirmationoftheradarfindings.Inseveralothercasessomeevidenceofhoneycombingintheinsitestitchconcreteabovetheductwasfound.Whenviewingvoidsthroughthebaroscopic,however,itprovedimpossibletodeterminetheiractualsizeorhowfartheyextendedalongthetendonductsalthoughtheyonlyappearedtooccupylessthanthetop25%oftheductdiameter.Mostofthesevoids,infact,weresmallerthanthediameteroftheflexiblebaroscopicbeingused(approximately9mm)andwereseenbetweenthehorizontaltopsurfaceofthegroutandthecurvedupperlimitoftheduct.Inaveryfewcasesthetopsofthepre-stressingstrandswerevisibleabovethegroutbutnosignofanytrappedwaterwasseen.Itwasnotpossible,usingthebaroscopic,toseewhetherthosecableswerecorroded.DigitalradartestingThetestmethodinvolvedexcitingthejointsusingradiofrequencyradarantenna:1GHz,900MHzand500MHz.Thehighestfrequencygivesthehighestresolutionbuthasshallowdepthpenetrationintheconcrete.Thelowestfrequencygivesthegreatestdepthpenetrationbutyieldslowerresolution.ThedatacollectedontheradarsweepswererecordedonaGSSISIRSystem10.Thissysteminvolvesradarpulsingandrecording.Thedatafromtheantennaistransformedfromananaloguesignaltoadigitalsignalusinga16-bitanaloguedigitalconvertergivingaveryhighresolutionforsubsequentdataprocessing.Thedataisdisplayedonsiteonahigh-resolutioncolormonitor.Followingvisualinspectionitisthenstoreddigitallyona2.3-gigabytetapeforsubsequentanalysisandsignalprocessing.Thetapefirstofallrecordsatheader,notingthedigitalradarsettingstogetherwiththetracenumberpriortorecordingtheactualdata.Whenthedataisplayedback,oneisabletoclearlyidentifyalltherelevantsettingsmakingforaccurateandreliabledatareproduction.Atparticularlocationsalongthetraces,thetracewasmarkedusingamarkerswitchontherecordingunitortheantenna.AllthedigitalrecordsweresubsequentlydownloadedattheUniversity'sNDTlaboratoryontoamicro-computer.(Therawdatapriortoprocessingconsumed35megabytesofdigitaldata.)Post-processingwasundertakenusingsophisticatedsignalprocessingsoftware.Techniquesavailablefortheanalysisincludechangingthecolortransformandchangingthescalesfromlineartoaskeweddistributioninordertohighlight>突出Certainfeatures.Also,thecolortransformscouldbechangedtohighlightphasechanges.Inadditiontothesecolortransformfacilities,sophisticatedhorizontalandverticalfilteringproceduresareavailable.Usingalargescreenmonitoritispossibletodisplayinsplitscreenstherawdataandthetransformedprocesseddata.Thusoneisabletogetanaccurateindicationoftheprocessingwhichhastakenplace.Thecomputerscreendisplaysthetimedomaincalibrationsofthereflectedsignalsontheverticalaxis.Afurtherfacilityofthesoftwarewastheabilitytodisplaytheindividualradarpulsesastimedomainwiggleplots.Thiswasaparticularlyvaluablefeaturewhenlookingatindividualrecordsinthevicinityofthetendons.InterpretationoffindingsAfullanalysisoffindingsisgivenelsewhere,Essentiallythedigitizedradarplotsweretransformedtocolorlinescansandwheredoublephaseshiftswereidentifiedinthejoints,thenvoidingwasdiagnosed.Conclusions1. AnoutlineofthebridgeresearchplatforminEuropeisgiven.2. TheuseofimpulseradarhascontributedconsiderablytothelevelofconfidenceintheassessmentoftheBesseso,th,BamRailBridge.3. Theradarinvestigationsrevealedextensivevoidingwithinthepost-tensionedcableducts.However,nosignofcorrosiononthestressingwireshadbeenfoundexceptfortheveryfirstinvestigation.