Case
study
Collapse
of
prestressed
reinforced
concrete
jetties:
durability
and
faults
analysis
S.
Tattoni
1,
F.
Stochino
*
DepartmentofCivilandEnvironmentalEngineeringandArchitecture,UniversityofCagliari,ViaMarengo2,09123Cagliari,Italy
1. Introduction
Thedurabilityofreinforcedconcreteinmarinestructureshasalwaysbeenanimportanttopic.Inquitearecentwork[1], Songetal.investigatethefactorsaffectingcorrosionsandtheapproachesforimprovingdurabilityofmarinestructures.In thispapertherearediscussedenvironmentaleffects,constructionquality,coverthickness,characteristicsofconcreteand structuretype.Inaddition,theauthorspresentthepossibleapproachestoimprovethedurabilityofmarineR.C.structures basedonrealcasesandfieldsurveys.Aninterestingexperimentalwork[2]hasbeenrecentlydevelopedbyGiordanoetal. Theyinvestigatetheeffectofsimultaneouscorrosionandcyclicloadingonreinforcedconcreteelements.Thekeyroleofthe jointeffectofthosecausesofdegradationsishighlightedinthiswork.
Intheanalysisofprestressedreinforcedconcrete,thenumberoftheinvolvedvariablessharplyrisesalongwiththe uncertaintiesofthefactorsaffectingthem.Biondinietal.[3]developedafuzzymodeltosimulatetherealvaluesbymeansof bandsbondedbetweensuitableminimumandmaximumextremes.Probabilisticmethodsareusedalsoin[4],inwhichan optimizationofR.C.cross-sectionsinaggressiveenvironmentisdevelopedwithalifetimereliabilityapproach.Theauthors demonstratethattheamountandarrangementsofsteelreinforcementandthevalueoftheconcretecoverthicknessplayed acrucialroleintheprocess.
OneofthekeyparametersinlifetimeassessmentofmarineR.C.structuresisthechlorideconcentration.Asregardsits modeling,severalpapershavebeenwritten:in[5]ChatterjiproposesseverecriticismagainsttheuseofFlick’ssecondlawof
ARTICLE INFO Articlehistory:
Received17December2012 Receivedinrevisedform7May2013 Accepted10May2013
Availableonline24May2013 Keywords:
Prestressedreinforcedconcretejetty Durabilityandservicelife Corrosioninmarineenvironment Strutandtiemodel
Failureanalysis
* Correspondingauthor.Tel.:+390706755410;fax:+390706755418.
E-mailaddresses:stattoni@unica.it(S.Tattoni),fstochino@unica.it,fstochino@gmail.com(F.Stochino).
1
Tel.:+390706755409;fax:+390706755418.
ContentslistsavailableatSciVerseScienceDirect
Case
Studies
in
Engineering
Failure
Analysis
j ou rna l h ome pa ge : w ww . e l se v i e r. co m/ l oc a te / cse f a
2213-2902 ß2013ElsevierLtd.
http://dx.doi.org/10.1016/j.csefa.2013.05.006
diffusionregardingchlorideionmigrationthroughcementbasedmaterials.Hedevelopedthismodelonlyonanempirical basis,anditcannotbeacceptedfromatheoreticalpointofview.Nevertheless,thispopularmodelisstillusednowadays [6–8].Furthermore,thechloridedepositionratehasbeenconsideredabenchmarkfortheenvironmentalconditions.In[9], Meiraetal.studythedepositionofchloridesonwetcandledevices,anditsrelationwithchloridesaccumulatedinconcrete. Theauthorsclaim tousethechloridedepositionrateasanenvironmentalindicator,in ordertopredictservicelifeof constructionsand concretecover thicknessfora requiredservicelife.Asregardsthedurability ofreinforcedconcrete structures,the effects of carbonation and chloride penetration play a key role in it. In a recent work[10], Bertolini investigatesthisissueandproposespossibleapproachestothedesignofdurablereinforcedconcretestructures. 2. Background
Theprestressedconcretejettiesanalyzedinthispaperareapartofapicturesque,finemarinainItaly.Thestructurewas builtin1974andhasacapacityof1500berths.Itcanaccommodateanyboatupto50mlength.
ThejettiesconsistofprecastdecksandbeamssupportedbyR.C.piles.ReferringtoFig.1,thedeckshavea
p
-shaped cross-sectionofprestressedreinforcedconcrete.Itishinged ononeside andsimplysupportedon theother.Thegeometric characteristicsofthecross-sectionare:3.5mwidth,0.9mheight.Thelongitudinalspanis10m.Fig.1showsalsothereinforcements’distribution.Inthedeckslab,thereinforcementisadoublemeshwithastepsizeof 150mmandtherearealsobarsof4mmdiameter.Theauthorsfoundoutinthecalculationreportthattherearealsorebarsof 8mmdiameter.
Asregardsthetworibs,theonlyordinaryreinforcementsarethoseinthetoppartofthesection:two12mmdiameter barsthat,indeed,arestirrupssupportbars.Actually,thetensilereinforcementsofthiselementarethe40.6in.prestressed strandsinthebottompartofthecross-section.Inotherwords,thestabilityofthewholeelementdependsonthe pre-tensioning.
Itisimportanttohighlightthedesolidarizationofsomepartsoftheprestressingtendons.Thistechnique,frequentlyused inthistypeofconstruction,hasthefunctionofmodulatingtheprestressingforcealongthelongitudinalaxes,inorderto servetheactualstructuralneeds.Inthiscase,desolidarizationisobtainedbyinsertingpartsofthecableinaPVCcorrugated duct.Inthiswaytheconglomerate,notbeingabletocomeincontactwithsteel,doesnotsuffertheeffectofprestressing. TheratiobetweentheresistantbendingmomentandthedesignbendingmomentMR/MEmusthavebeengreaterthan
Fig.1.Cross(top)andlongitudinal(bottom)sectionsofthejetty. S.Tattoni,F.Stochino/CaseStudiesinEngineeringFailureAnalysis1(2013)131–138 132
1.3,accordingtothetechnicalstandardsofthattime[11](seeFig.2).ItisevidentfromanexaminationofFig.2,thatthisratio isalwaysobserved,butwithdifferentvalues.Thepointsofleastresistance(minimumvaluesoftheratioMR/ME)are2mfar
fromthesupportandnotinthemidspansection.
Fromtheoriginal designreport, theoriginalexperimental testand inspectionsin situ,it is possibletodeducethe followingmechanicalcharacteristics.Theconcreteprovedtobeaverygoodmaterialwithacharacteristicstrengthofatleast 30MPa.Theultimatecharacteristicstrengthofsteeltendonis1800MPawhileitsstrengthat0.1%strainis1540MPa.The concretecovervariesfrom20mmforthedeckslabto50mmfortheprotectionofthestrand.
Severalcrackshaveappearedalongdifferentjettiesinthemarinastartingfromthenineties.Thecollapse,asshownin Fig.3,occurredinautumn2011.
Afterthecollapse,thedamagedjettywasrecoveredandthiswasthestartingpointofthepost-failureanalysis.Itisclear (seeFig.3(b) and(c))thatthis isadurability problem,ormoreprecisely,acorrosionproblem. Theaggressive marine environmentandsomeexecutionflawcausedthefailure.
3. Durabilityanalysis
Thefirstanalysisconductedbytheauthorsregardsthedepassivationlimitstateforcarbonationinducedcorrosion.Fig.4 showsasketchedcarbonationparabolicmodel(seeEq.(1))adoptedbytheModelCode2010[6].Itisvisiblethatafter25–30 yearsthecarbonationdepthisgreaterthantheconcretecoverof20mm.
xcðtÞ¼
t0
t w
kpffiffit (1)
InEq.(1)(takenfrom[6,Eq.(7.8.2)]),xcrepresentsthecarbonationdepth,kisafactorreflectingaspectsliketheexecution
andbasicresistance oftheconcreteagainstingressofcarbonation;wistheweatherexposure(0<w<1,0forindoor conditionsand1forwetconditions);t0isthetimeofreferenceinyears.Someusefulindicationstoevaluatethevaluesoft0
andk canbefoundin[12,AnnexB].RegardingthegraphsshowninFig.4,theauthorsassumek=7.1102,t0=0.0767,
w=0.1062.
AsmentionedinSection1animportantissueisthechlorideattack(seeFig.5andEq.(2)).Theauthorsstudiedthe depassivationlimitstatesforchlorideinducedcorrosion.AsstatedbyModelCode2010[6],theingressofchloridesina marineenvironmentmaybemodeledbythemodifiedFick’ssecondlawofdiffusion.
C¼ðCDxðCDxCiÞÞ erf x 2pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiDappðtÞt ! " # (2) InEq.(2)(takenfrom[6,Eq.(7.8.11)],Crepresentsthecontentofchloridesintheconcreteatadepthx,CDxisthechloride
concentrationatconcretesurface[wt.%bindercontent],Ciistheinitialchloridecontentoftheconcrete[wt.%bindercontent]
(inFig.5itisassumedequalto0),Dapp(t)istheapparentcoefficientofchloridediffusionthroughconcrete[m/s2]attimet,its
expressioncanbefoundin[6,Eqs.(7.8)–(12)].
Accordingtothisapproach,Fig.5representsthecontentofchloridesinpercentageofbindercontent.Twoconcretecover depthswithdifferentCDxchlorideconcentrationattheconcretesurfacearebeingconsideredhere.Incaseof50mmthick
concretecover(Fig.5(a)),thecriticchlorideconcentrationof0.01%canbereachedforaconcentrationofchlorideatthe
Fig.3.Emptyspaceleftbythecollapsedjetty(a),degradationofconcrete(b),therecoveryofthecollapsedjetty(c).
Fig.4.DepassivationlimitstateforcarbonationcorrosionaccordingtoModelCode2010[6]. S.Tattoni,F.Stochino/CaseStudiesinEngineeringFailureAnalysis1(2013)131–138 134
concretesurfaceof3%after23yearsorinlesstime,ifCDxisgreaterthan3%.Thesituationchangesincaseofthetoppartof
thecross-sectionwheretheconcretecoveris20mmthick(seeFig.5(b)).Inthiscase,thecriticchloridecontentisreachedin differentscenariosafterafewyears.Takingintoaccountthattherealconcretecovermayvaryatdifferentpositionsofthe jetties,itislikelythatachlorideinducedcorrosioncouldhaveoccurred.
4. Rootcauseofthefailure
Tosumupthesituation:reinforcedconcretestructuresbuiltin1974showedfirstcracksafteralmost10–15years,and somecollapseoccurredafter20–25years.Probably,corrosion(duetocarbonationortochlorideconcentration)hasreduced reinforcementresistantcross-section.
Fig.6.Spallingoftheprotectivemortar(a)exposestheprestressedcableductstoweathering.In(b)theseawatercomesoutfromtheductoftherecovered collapsedjetty.
Fig.5.Depassivationlimitstateforchlorideinducedcorrosionwith50mmthickconcretecover(a)and20mmthickconcretecover(b),accordingtoModel Code2010[6].
Furthermore,someminordetailsandthelackofpreventivemeasuresmayhaveexacerbatedthesituation.Fig.6(b)shows theconditionofthecollapsedjettiesafteritsrecovery:theterminalsectionoftheductdidnothaveanadequateprotection. Actuallyonlyathincoverofmortar(almost10mm)protectedtheheadoftheribsinalmostalljetties(seeFig.6(a)).
Thespalling ofthe protectivemortardue to thecorrosionof thestirrupsexposed theprestressingcable ductsto weathering.Corrosionofthetendonscouldhavestartedimmediately.Inthiscasethedurabilityofaprestressedconcrete structureisjeopardizedbythelackofanadequateexternalprotectionofthestrands.
Fig.7.Mechanismofcollapse.
Fig.8.Strutandtiemodelofthejettiesbeforethetendoncollapse. S.Tattoni,F.Stochino/CaseStudiesinEngineeringFailureAnalysis1(2013)131–138 136
5. Descriptionofthemechanismofthefailure Itisinterestingtooutlinewhatprobablyhappened:
(1)Theordinaryreinforcementintheterminalcross-sections,characterizedbymoderateconcretecover(intheory30mm, inrealityevenlessthan20mm)wasaffectedbythefirstcorrosiveprocesses.Forthisreason,theformationofironoxide hydrates(Fe2O3nH2O)andthedevelopmentofcracksbegun.
(2)Thesea water couldhave penetrated thestructure, especially withinthedesolidarization duct. The situationwas aggravatedfromthecycleofimmersionandemersionduetothewavemotion,whichallowedthecontinuoussupplyof Cl,O
2andH2O.
(3)Thecorrosion started toaffect theprestressing reinforcement.Then theexpansion ofthe iron hydroxidescaused occurrenceofcracksparalleltothelongitudinalaxesofthebeam.Thereductionofthecross-sectionofthereinforcement tendonscontinueduntiltheyweregraduallybrokenup.
(4)Thecollapseofthejettiestookplaceinthesectionofleastresistance,atabout1/4ofthespanlengthdistancefromthe support.
(5)Thankstothenon-contemporeaneousfailureofthetworibs,thejettycouldhaveresistedbymeansofastrutandtie mechanism(seeFig.7).Itisobviouslyneitherreliablenorpermanent.Thejettyremainedinplacewithasettlement.The latterisclearlyvisibleinotherstillresistingjettieswherepresumablyaribisclosetocollapse,whiletheadjacentoneis stillresistant.
(6)Thefailureofthesteeldowel(seeFig.7)orofpartsofthejettyincontactwiththeadjacentelement,orothercauses(e.g. shock,highshotsmooring,andpiling-inducedmovementsofthewaves)suddenlymadeimpossiblethebalanceofthe element,whichfelldownintowater.
Fig.8showsthestrut andtiemodelthatcanrepresentaresistantmechanismofthejettyinordinarycondition.In particular,theribsonthesidesoftheelementcanbenoted.Thereareprestressingtendonsonthebottompartandsloping concretestrutonthetoppart.
Constraintsystemsimulatestheactualsupportingcondition:hingedontheleftandsimplysupportedontheright.An elasticlinearstructuralanalysiswasperformed.Itisimportanttoconsiderthevalueoftheforceactingontheties:142kN plustheprestressingforce(seeFig.8).Thisresultshowstheparamountrelevanceoftheprestressingstrands.Duringthe corrosionprocessthetendonsofoneribwerebeinggraduallybroken.
Fig.9presentsthestrutandtiemodelofthejettyafterthecollapseofthetendonofonerib.Theresultofthestructural analysishighlightsthattheforceactingonthelastprestressingtendonisequalto160kN.Therefore,inthisdamagedsystem, anincreaseofthe11%ofthestressontheprestressingstrandisexpected.Itseemsnotsuchabigrise,butthecorrosionofthe otherstrandhasreducedtheresistantcross-sectionofthelastreinforcements.Thus,thissystemisneitherreliablenor permanentandthetotalcollapseispending.
6. Conclusionsandrecommendations
Inconclusion,thisisanexemplarcasestudyregardingR.C.corrosioninmarineenvironment.Fromthefirstanalysisit seemsthatthedepassivationlimitstatecouldhavebeenreachedsincethefirstdecades.TheItalianstandardsofthetime oftheconstructiondidnotpaymuchattentiontodurabilityproblems,sotheoriginaldesignerarrangedthepreventive measuresonthebasisofhisexperience(forexample:theuseofaverygoodconcrete,concretecoversufficientforalmost all cross-sections,evenif in reality thisformula was not observed at all). Unfortunately, these measureswere not sufficient.Apartfromthereplacementofalltheremainingjetties,itseemsthatthesolutionfortherefurbishmentisnot connectedtotheresidualprestressing(inanycasedestinedtovanishintime),anditconsistsinbuildingsupportsor reinforcementsexternaltotheelements.Furtherdevelopmentsofthisresearchareexpectedwithregardtothecorrosion model,inordertobetterdefinewhenthedepassivationlimitstateoccurred.
Acknowledgement
FlavioStochinowishestoacknowledgethefinancialsupportreceivedfromFondazioneBancodiSardegna. References
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