Reinforced Concrete - Wikipedia, The Free Encyclopedia

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ThenovelshapeofthePhilipsPavilionwasallowedbyreinforcedconcrete.

ReinforcedconcreteFromWikipedia,thefreeencyclopedia

Reinforcedconcrete(RC)isacompositematerialinwhichconcrete'srelativelylowtensilestrengthandductilityarecounteractedbytheinclusionofreinforcementhavinghighertensilestrengthand/orductility.Thereinforcementisusually,thoughnotnecessarily,steelreinforcingbars(rebar)andisusuallyembeddedpassivelyintheconcretebeforetheconcretesets.Reinforcingschemesaregenerallydesignedtoresisttensilestressesinparticularregionsoftheconcretethatmightcauseunacceptablecrackingand/orstructuralfailure.Modernreinforcedconcretecancontainvariedreinforcingmaterialsmadeofsteel,polymersoralternatecompositematerialinconjunctionwithrebarornot.Reinforcedconcretemayalsobepermanentlystressed(incompression),soastoimprovethebehaviourofthefinalstructureunderworkingloads.IntheUnitedStates,themostcommonmethodsofdoingthisareknownaspretensioningandposttensioning.

Forastrong,ductileanddurableconstructionthereinforcementneedstohavethefollowingpropertiesatleast:

HighrelativestrengthHightolerationoftensilestrainGoodbondtotheconcrete,irrespectiveofpH,moisture,andsimilarfactorsThermalcompatibility,notcausingunacceptablestressesinresponsetochangingtemperatures.Durabilityintheconcreteenvironment,irrespectiveofcorrosionorsustainedstressforexample.

Contents

1History2Useinconstruction3Behaviorofreinforcedconcrete

3.1Materials3.2Keycharacteristics3.3Mechanismofcompositeactionofreinforcementandconcrete3.4Anchorage(bond)inconcrete:Codesofspecifications3.5Anticorrosionmeasures

4Reinforcementandterminologyofbeams5Prestressedconcrete6Commonfailuremodesofsteelreinforcedconcrete

6.1Mechanicalfailure6.2Carbonation6.3Chlorides6.4Alkalisilicareaction6.5Conversionofhighaluminacement

6.6Sulphates

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6.6Sulphates7Steelplateconstruction8Fiberreinforcedconcrete9Nonsteelreinforcement10Seealso11References12Furtherreading13Externallinks

History

FranoisCoignetwasaFrenchindustrialistofthenineteenthcentury,apioneerinthedevelopmentofstructural,prefabricatedandreinforcedconcrete.[1]Coignetwasthefirsttouseironreinforcedconcreteasatechniqueforconstructingbuildingstructures.[2]In1853Coignetbuiltthefirstironreinforcedconcretestructure,afourstoryhouseat72rueCharlesMichelsinthesuburbsofParis.[2]Coignet'sdescriptionsofreinforcingconcretesuggeststhathedidnotdoitformeansofaddingstrengthtotheconcretebutforkeepingwallsinmonolithicconstructionfromoverturning.[3]In1854,anEnglishbuildercalledWilliamB.Wilkinson,reinforcedtheconcreteroofandfloorsinthetwostoreyhousehewasconstructing.Hispositioningofthereinforcementdemonstratedthat,unlikehispredecessors,hehadknowledgeoftensilestresses.[4][5][6]

JosephMonier,aFrenchgardenerandknowntobeoneoftheprincipalinventorsofreinforcedconcrete,wasgrantedapatentforreinforcedflowerpotsbymeansofmixingawiremeshtoamortarshell.In1877,Monierwasgrantedanotherpatentforamoreadvancedtechniqueofreinforcingconcretecolumnsandgirderswithironrodsplacedinagridpattern.ThoughMonierundoubtedlyknewreinforcingconcretewouldimproveitsinnercohesion,itislessknownifheevenknewhowmuchreinforcingactuallyimprovedconcrete'stensilestrength.[7]

Before1877theuseofconcreteconstruction,thoughdatingbacktotheRomanEmpireandreintroducedinthemidtolate1800s,wasnotyetaprovenscientifictechnology.AmericanNewYorkerThaddeusHyattpublishedareporttitledAnAccountofSomeExperimentswithPortlandCementConcreteCombinedwithIronasaBuildingMaterial,withReferencetoEconomyofMetalinConstructionandforSecurityagainstFireintheMakingofRoofs,Floors,andWalkingSurfaceswherehestatedhisexperimentsonthebehaviorofreinforcedconcrete.Hisworkplayedamajorroleintheevolutionofconcreteconstructionasaprovenandstudiedscience.WithoutHyatt'swork,moredangeroustrialanderrormethodswouldhavelargelybeendependedonfortheadvancementinthetechnology.[3][8]

G.A.Wayss,wasaGermancivilengineerandapioneeroftheironandsteelconcreteconstruction.In1879WayssboughttheGermanrightstoMonier'spatentsandin1884startedthefirstcommercialuseforreinforcedconcreteinhisfirmWayss&Freytag.Upuntilthe1890sWayssandhisfirmgreatlycontributedtotheadvancementofMonier'ssystemofreinforcingandestablisheditasawelldevelopedscientifictechnology.[9]

ErnestL.Ransome,wasanEnglishbornengineerandearlyinnovatorofthereinforcedconcretetechniquesintheendofthe19thcentury.Withtheknowledgeofreinforcedconcretedevelopedduringtheprevious50years,Ransomeinnovatednearlyallstylesandtechniquesofthepreviousknowninventorsofreinforced

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RebarsofSagradaFamlia'sroofinconstruction(2009)

concrete.Ransome'skeyinnovationwastotwistthereinforcingsteelbarimprovingbondingwiththeconcrete.[10]GainingincreasingfamefromhisconcreteconstructedbuildingsRansomewasabletobuildtwoofthefirstreinforcedconcretebridgesinNorthAmerica.[11]OneofthefirstconcretebuildingsconstructedintheUnitedStates,wasaprivatehome,designedbyWilliamWardin1871.Thehomewasdesignedtobefireproofforhiswife.Oneofthefirstskyscrapersmadewithreinforcedconcretewasthe16storey,IngallsBuildinginCincinnati,constructedin1904.[12]

Useinconstruction

Manydifferenttypesofstructuresandcomponentsofstructurescanbebuiltusingreinforcedconcreteincludingslabs,walls,beams,columns,foundations,framesandmore.

Reinforcedconcretecanbeclassifiedasprecastorcastinplaceconcrete.

Designingandimplementingthemostefficientfloorsystemiskeytocreatingoptimalbuildingstructures.Smallchangesinthedesignofafloorsystemcanhavesignificantimpactonmaterialcosts,constructionschedule,ultimatestrength,operatingcosts,occupancylevelsandenduseofabuilding.[13]

Withoutreinforcement,constructingmodernstructureswithconcretematerialwouldnotbepossible.

Behaviorofreinforcedconcrete

Materials

Concreteisamixtureofcoarse(stoneorbrickchips)andfine(generallysandorcrushedstone)aggregateswithapasteofbindermaterial(usuallyPortlandcement)andwater.Whencementismixedwithasmallamountofwater,ithydratestoformmicroscopicopaquecrystallatticesencapsulatingandlockingtheaggregateintoarigidstructure.Theaggregatesusedformakingconcreteshouldbefreefromharmfulsubstanceslikeorganicimpurities,silt,clay,ligniteetc.Typicalconcretemixeshavehighresistancetocompressivestresses(about4,000psi(28MPa))however,anyappreciabletension(e.g.,duetobending)willbreakthemicroscopicrigidlattice,resultingincrackingandseparationoftheconcrete.Forthisreason,typicalnonreinforcedconcretemustbewellsupportedtopreventthedevelopmentoftension.

Ifamaterialwithhighstrengthintension,suchassteel,isplacedinconcrete,thenthecompositematerial,reinforcedconcrete,resistsnotonlycompressionbutalsobendingandotherdirecttensileactions.Areinforcedconcretesectionwheretheconcreteresiststhecompressionandsteelresiststhetensioncanbe

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madeintoalmostanyshapeandsizefortheconstructionindustry.

Keycharacteristics

Threephysicalcharacteristicsgivereinforcedconcreteitsspecialproperties:

1. Thecoefficientofthermalexpansionofconcreteissimilartothatofsteel,eliminatinglargeinternalstressesduetodifferencesinthermalexpansionorcontraction.

2. Whenthecementpastewithintheconcretehardens,thisconformstothesurfacedetailsofthesteel,permittinganystresstobetransmittedefficientlybetweenthedifferentmaterials.Usuallysteelbarsareroughenedorcorrugatedtofurtherimprovethebondorcohesionbetweentheconcreteandsteel.

3. Thealkalinechemicalenvironmentprovidedbythealkalireserve(KOH,NaOH)andtheportlandite(calciumhydroxide)containedinthehardenedcementpastecausesapassivatingfilmtoformonthesurfaceofthesteel,makingitmuchmoreresistanttocorrosionthanitwouldbeinneutraloracidicconditions.WhenthecementpasteisexposedtotheairandmeteoricwaterreactswiththeatmosphericCO2,portlanditeandtheCalciumSilicateHydrate(CSH)ofthehardenedcementpastebecomeprogressivelycarbonatedandthehighpHgraduallydecreasesfrom13.512.5to8.5,thepHofwaterinequilibriumwithcalcite(calciumcarbonate)andthesteelisnolongerpassivated.

Asaruleofthumb,onlytogiveanideaonordersofmagnitude,steelisprotectedatpHabove~11butstartstocorrodebelow~10dependingonsteelcharacteristicsandlocalphysicochemicalconditionswhenconcretebecomescarbonated.Carbonatationofconcretealongwithchlorideingressareamongstthechiefreasonsforthefailureofreinforcementbarsinconcrete.[14]

Therelativecrosssectionalareaofsteelrequiredfortypicalreinforcedconcreteisusuallyquitesmallandvariesfrom1%formostbeamsandslabsto6%forsomecolumns.Reinforcingbarsarenormallyroundincrosssectionandvaryindiameter.Reinforcedconcretestructuressometimeshaveprovisionssuchasventilatedhollowcorestocontroltheirmoisture&humidity.

Distributionofconcrete(inspiteofreinforcement)strengthcharacteristicsalongthecrosssectionofverticalreinforcedconcreteelementsisinhomogeneous.[15]

Mechanismofcompositeactionofreinforcementandconcrete

ThereinforcementinaRCstructure,suchasasteelbar,hastoundergothesamestrainordeformationasthesurroundingconcreteinordertopreventdiscontinuity,sliporseparationofthetwomaterialsunderload.Maintainingcompositeactionrequirestransferofloadbetweentheconcreteandsteel.Thedirectstressistransferredfromtheconcretetothebarinterfacesoastochangethetensilestressinthereinforcingbaralongitslength,thisloadtransferisachievedbymeansofbond(anchorage)andisidealizedasacontinuousstressfieldthatdevelopsinthevicinityofthesteelconcreteinterface.

Anchorage(bond)inconcrete:Codesofspecifications

Becausetheactualbondstressvariesalongthelengthofabaranchoredinazoneoftension,currentinternationalcodesofspecificationsusetheconceptofdevelopmentlengthratherthanbondstress.Themainrequirementforsafetyagainstbondfailureistoprovideasufficientextensionofthelengthofthebarbeyondthepointwherethesteelisrequiredtodevelopitsyieldstressandthislengthmustbeatleastequaltoitsdevelopmentlength.However,iftheactualavailablelengthisinadequateforfulldevelopment,

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Aheavyreinforcedconcretecolumn,seenbeforeandaftertheconcretehasbeencastinplacearoundtherebarcage.

specialanchoragesmustbeprovided,suchascogsorhooksormechanicalendplates.Thesameconceptappliestolapsplicelengthmentionedinthecodeswheresplices(overlapping)providedbetweentwoadjacentbarsinordertomaintaintherequiredcontinuityofstressinthesplicezone.

Anticorrosionmeasures

Inwetandcoldclimates,reinforcedconcreteforroads,bridges,parkingstructuresandotherstructuresthatmaybeexposedtodeicingsaltmaybenefitfromuseofcorrosionresistantreinforcementsuchasuncoated,lowcarbon/chromium(microcomposite),epoxycoated,hotdipgalvanisedorstainlesssteelrebar.Gooddesignandawellchosenconcretemixwillprovideadditionalprotectionformanyapplications.Uncoated,lowcarbon/chromiumrebarlookssimilartostandardcarbonsteelrebarduetoitslackofacoatingitshighlycorrosionresistantfeaturesareinherentinthesteelmicrostructure.ItcanbeidentifiedbytheuniqueASTMspecifiedmillmarkingonitssmooth,darkcharcoalfinish.Epoxycoatedrebarcaneasilybeidentifiedbythelightgreencolourofitsepoxycoating.Hotdipgalvanizedrebarmaybebrightordullgreydependingonlengthofexposure,andstainlessrebarexhibitsatypicalwhitemetallicsheenthatisreadilydistinguishablefromcarbonsteelreinforcingbar.ReferenceASTMstandardspecificationsA1035/A1035MStandardSpecificationforDeformedandPlainLowcarbon,Chromium,SteelBarsforConcreteReinforcement,A767StandardSpecificationforHotDipGalvanisedReinforcingBars,A775StandardSpecificationforEpoxyCoatedSteelReinforcingBarsandA955StandardSpecificationforDeformedandPlainStainlessBarsforConcreteReinforcement.

Another,cheaperwayofprotectingrebarsiscoatingthemwithzincphosphate.[16]Zincphosphateslowlyreactswithcalciumcationsandthehydroxylanionspresentinthecementporewaterandformsastablehydroxyapatitelayer.

Penetratingsealantstypicallymustbeappliedsometimeaftercuring.Sealantsincludepaint,plasticfoams,filmsandaluminumfoil,feltsorfabricmatssealedwithtar,andlayersofbentoniteclay,sometimesusedtosealroadbeds.

Corrosioninhibitors,suchascalciumnitrite[Ca(NO2)2],canalsobeaddedtothewatermixbeforepouringconcrete.Generally,12wt.%of[Ca(NO2)2]withrespecttocementweightisneededtopreventcorrosion

oftherebars.Thenitriteanionisamildoxidizerthatoxidizesthesolubleandmobileferrousions(Fe2+)presentatthesurfaceofthecorrodingsteelandcausesthemtoprecipitateasaninsolubleferrichydroxide(Fe(OH)3).Thiscausesthepassivationofsteelattheanodicoxidationsites.Nitriteisamuchmoreactivecorrosioninhibitorthannitrate,whichisalesspowerfuloxidizerofthedivalentiron.

Reinforcementandterminologyofbeams

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Rebarconstructedtoproperlyreinforceabeamelement,awaitingconcretetobepoured.

Abeambendsunderbendingmoment,resultinginasmallcurvature.Attheouterface(tensileface)ofthecurvaturetheconcreteexperiencestensilestress,whileattheinnerface(compressiveface)itexperiencescompressivestress.

Asinglyreinforcedbeamisoneinwhichtheconcreteelementisonlyreinforcednearthetensilefaceandthereinforcement,calledtensionsteel,isdesignedtoresistthetension.

Adoublyreinforcedbeamisoneinwhichbesidesthetensilereinforcementtheconcreteelementisalsoreinforcednearthecompressivefacetohelptheconcreteresistcompression.Thelatterreinforcementiscalledcompressionsteel.Whenthecompressionzoneofaconcreteisinadequatetoresistthecompressivemoment(positivemoment),extrareinforcementhastobeprovidedifthearchitectlimitsthedimensionsofthesection.

Anunderreinforcedbeamisoneinwhichthetensioncapacityofthetensilereinforcementissmallerthanthecombinedcompressioncapacityoftheconcreteandthecompressionsteel(underreinforcedattensileface).Whenthereinforcedconcreteelementissubjecttoincreasingbendingmoment,thetensionsteelyieldswhiletheconcretedoesnotreachitsultimatefailurecondition.Asthetensionsteelyieldsandstretches,an"underreinforced"concretealsoyieldsinaductilemanner,exhibitingalargedeformationandwarningbeforeitsultimatefailure.Inthiscasetheyieldstressofthesteelgovernsthedesign.

Anoverreinforcedbeamisoneinwhichthetensioncapacityofthetensionsteelisgreaterthanthecombinedcompressioncapacityoftheconcreteandthecompressionsteel(overreinforcedattensileface).Sothe"overreinforcedconcrete"beamfailsbycrushingofthecompressivezoneconcreteandbeforethetensionzonesteelyields,whichdoesnotprovideanywarningbeforefailureasthefailureisinstantaneous.

Abalancedreinforcedbeamisoneinwhichboththecompressiveandtensilezonesreachyieldingatthesameimposedloadonthebeam,andtheconcretewillcrushandthetensilesteelwillyieldatthesametime.Thisdesigncriterionishoweverasriskyasoverreinforcedconcrete,becausefailureissuddenastheconcretecrushesatthesametimeofthetensilesteelyields,whichgivesaverylittlewarningofdistressintensionfailure.[17]

Steelreinforcedconcretemomentcarryingelementsshouldnormallybedesignedtobeunderreinforcedsothatusersofthestructurewillreceivewarningofimpendingcollapse.

Thecharacteristicstrengthisthestrengthofamaterialwherelessthan5%ofthespecimenshowslowerstrength.

Thedesignstrengthornominalstrengthisthestrengthofamaterial,includingamaterialsafetyfactor.Thevalueofthesafetyfactorgenerallyrangesfrom0.75to0.85inPermissiblestressdesign.

Theultimatelimitstateisthetheoreticalfailurepointwithacertainprobability.Itisstatedunderfactoredloadsandfactoredresistances.

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ReinforcedconcretestructuresarenormallydesignedaccordingtorulesandregulationsorrecommendationofacodesuchasACI318,CEB,CP110orthelike.WSD,USDorLRFDmethodsareusedindesignofRCstructuralmembers.AnalysisanddesignofRCmemberscanbecarriedoutbyusinglinearornonlinearapproaches.Whenapplyingsafetyfactors,buildingcodesnormallyproposelinearapproaches,butforsomecasesnonlinearapproaches.Toseetheexamplesofanonlinearnumericalsimulationandcalculationvisitthereferences:[18][19]

Prestressedconcrete

Prestressingconcreteisatechniquethatgreatlyincreasestheloadbearingstrengthofconcretebeams.Thereinforcingsteelinthebottompartofthebeam,whichwillbesubjectedtotensileforceswheninservice,isplacedintensionbeforetheconcreteispouredaroundit.Oncetheconcretehashardened,thetensiononthereinforcingsteelisreleased,placingabuiltincompressiveforceontheconcrete.Whenloadsareapplied,thereinforcingsteeltakesonmorestressandthecompressiveforceintheconcreteisreduced,butdoesnotbecomeatensileforce.Sincetheconcreteisalwaysundercompression,itislesssubjecttocrackingandfailure.[20]

Anotherwayistoinsertplastictubesintothebottomofthebeam.Rebarisinsertedintothesetubes.Oncetheconcretehascuredtherebarcanbetensionedandtheformworkremoved.Usuallythetensionisappliedusinghydraulicjacks.Theadvantageofthismethodisthatitiseasytomeasuretheappliedtension.Thenutsarethensnuggedupandthejobisdone.Itshouldbenotedthatallnutsandboltswhenmatedtogetherwillhaveahelicalgroovewhichisapotentialrusttrap.Aneasymethodofeliminatingthisrusttrapistoapplyrustpreventiveredoxidepainttothethreadsandtojointhenutsandboltswhilethispaintisstillwet.

Commonfailuremodesofsteelreinforcedconcrete

Reinforcedconcretecanfailduetoinadequatestrength,leadingtomechanicalfailure,orduetoareductioninitsdurability.Corrosionandfreeze/thawcyclesmaydamagepoorlydesignedorconstructedreinforcedconcrete.Whenrebarcorrodes,theoxidationproducts(rust)expandandtendstoflake,crackingtheconcreteandunbondingtherebarfromtheconcrete.Typicalmechanismsleadingtodurabilityproblemsarediscussedbelow.

Mechanicalfailure

Crackingoftheconcretesectionisnearlyimpossibletopreventhowever,thesizeandlocationofcrackscanbelimitedandcontrolledbyappropriatereinforcement,controljoints,curingmethodologyandconcretemixdesign.Crackingcanallowmoisturetopenetrateandcorrodethereinforcement.Thisisaserviceabilityfailureinlimitstatedesign.Crackingisnormallytheresultofaninadequatequantityofrebar,orrebarspacedattoogreatadistance.Theconcretethencrackseitherunderexcessloading,orduetointernaleffectssuchasearlythermalshrinkagewhileitcures.

Ultimatefailureleadingtocollapsecanbecausedbycrushingtheconcrete,whichoccurswhencompressivestressesexceeditsstrength,byyieldingorfailureoftherebarwhenbendingorshearstressesexceedthestrengthofthereinforcement,orbybondfailurebetweentheconcreteandtherebar.

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Concretewallcrackingassteelreinforcingcorrodesandswells.Rusthasalowerdensitythanmetal,soitexpandsasitforms,crackingthedecorativecladdingoffthewallaswellasdamagingthestructuralconcrete.Thebreakageofmaterialfromasurfaceiscalledspalling.

Detailedviewofspallingprobablycausedbyatoothinlayerofconcretebetweenthesteelandthesurface,accompaniedbycorrosionfromexternalexposure

Rebarforfoundationsandwallsofasewagepumpstation.

Carbonation

Carbonation,orneutralisation,isachemicalreactionbetweencarbondioxideintheairandcalciumhydroxideandhydratedcalciumsilicateintheconcrete.

Whenaconcretestructureisdesigned,itisusualtostatetheconcretecoverfortherebar(thedepthoftherebarwithintheobject).Theminimumconcretecoverisnormallyregulatedbydesignorbuildingcodes.Ifthereinforcementistooclosetothesurface,earlyfailureduetocorrosionmayoccur.Theconcretecoverdepthcanbemeasuredwithacovermeter.However,carbonatedconcreteincursadurabilityproblemonlywhenthereisalsosufficientmoistureandoxygentocauseelectropotentialcorrosionofthereinforcingsteel.

Onemethodoftestingastructureforcarbonatationistodrillafreshholeinthesurfaceandthentreatthecutsurfacewithphenolphthaleinindicatorsolution.Thissolutionturnspinkwhenincontactwithalkalineconcrete,makingitpossibletoseethedepthofcarbonation.Usinganexistingholedoesnotsufficebecausetheexposedsurfacewillalreadybecarbonated.

Chlorides

Chlorides,includingsodiumchloride,canpromotethecorrosionofembeddedsteelrebarifpresentinsufficientlyhighconcentration.Chlorideanionsinducebothlocalizedcorrosion(pittingcorrosion)andgeneralizedcorrosionofsteelreinforcements.Forthisreason,oneshouldonlyusefreshrawwaterorpotablewaterformixingconcrete,ensurethatthecoarseandfineaggregatesdonotcontainchlorides,ratherthanadmixtureswhichmightcontainchlorides.

Itwasoncecommonforcalciumchloridetobeusedasanadmixturetopromoterapidsetupoftheconcrete.Itwasalsomistakenlybelievedthatitwouldpreventfreezing.However,thispracticefellintodisfavoroncethedeleteriouseffectsofchloridesbecameknown.Itshouldbeavoided

wheneverpossible.

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ThePaulinsKillViaduct,Hainesburg,NewJersey,is115feet(35m)talland1,100feet(335m)long,andwasheraldedasthelargestreinforcedconcretestructureintheworldwhenitwascompletedin1910aspartoftheLackawannaCutOffraillineproject.TheLackawannaRailroadwasapioneerintheuseofreinforcedconcrete.

Theuseofdeicingsaltsonroadways,usedtolowerthefreezingpointofwater,isprobablyoneoftheprimarycausesofprematurefailureofreinforcedorprestressedconcretebridgedecks,roadways,andparkinggarages.Theuseofepoxycoatedreinforcingbarsandtheapplicationofcathodicprotectionhasmitigatedthisproblemtosomeextent.AlsoFRP(fiberreinforcedpolymer)rebarsareknowntobelesssusceptibletochlorides.Properlydesignedconcretemixturesthathavebeenallowedtocureproperlyareeffectivelyimpervioustotheeffectsofdeicers.

Anotherimportantsourceofchlorideionsisseawater.Seawatercontainsbyweightapproximately3.5wt.%salts.Thesesaltsincludesodiumchloride,magnesiumsulfate,calciumsulfate,andbicarbonates.Inwaterthesesaltsdissociateinfreeions(Na+,Mg2+,Cl,SO42,HCO3)andmigratewiththewaterintothecapillariesoftheconcrete.Chlorideions,whichmakeupabout50%oftheseions,areparticularlyaggressiveasacauseofcorrosionofcarbonsteelreinforcementbars.

Inthe1960sand1970sitwasalsorelativelycommonformagnesite,achloriderichcarbonatemineral,tobeusedasafloortoppingmaterial.Thiswasdoneprincipallyasalevellingandsoundattenuatinglayer.Howeveritisnowknownthatwhenthesematerialscomeintocontactwithmoisturetheyproduceaweaksolutionofhydrochloricacidduetothepresenceofchloridesinthemagnesite.Overaperiodoftime(typicallydecades)thesolutioncausescorrosionoftheembeddedsteelrebars.Thiswasmostcommonlyfoundinwetareasorareasrepeatedlyexposedtomoisture.

Alkalisilicareaction

Thisareactionofamorphoussilica(chalcedony,chert,siliceouslimestone)sometimespresentintheaggregateswiththehydroxylions(OH)fromthecementporesolution.Poorlycrystallizedsilica(SiO2)dissolvesanddissociatesathighpH(12.513.5)inalkalinewater.Thesolubledissociatedsilicicacidreactsintheporewaterwiththecalciumhydroxide(portlandite)presentinthecementpastetoformanexpansivecalciumsilicatehydrate(CSH).Thealkalisilicareaction(ASR)causeslocalisedswellingresponsiblefortensilestressandcracking.Theconditionsrequiredforalkalisilicareactionarethreefold:(1)aggregatecontaininganalkalireactiveconstituent(amorphoussilica),(2)sufficientavailabilityofhydroxylions(OH),and(3)sufficientmoisture,above75%relativehumidity(RH)withintheconcrete.[21][22]Thisphenomenonissometimespopularlyreferredtoas"concretecancer".Thisreactionoccursindependentlyofthepresenceofrebarsmassiveconcretestructuressuchasdamscanbeaffected.

Conversionofhighaluminacement

Resistanttoweakacidsandespeciallysulfates,thiscementcuresquicklyandhasveryhighdurabilityandstrength.ItwasfrequentlyusedafterWorldWarIItomakeprecastconcreteobjects.However,itcanlosestrengthwithheatortime(conversion),especiallywhennotproperlycured.Afterthecollapseofthree

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roofsmadeofprestressedconcretebeamsusinghighaluminacement,thiscementwasbannedintheUKin1976.Subsequentinquiriesintothemattershowedthatthebeamswereimproperlymanufactured,butthebanremained.[23]

Sulphates

Sulfates(SO4)inthesoiloringroundwater,insufficientconcentration,canreactwiththePortlandcementinconcretecausingtheformationofexpansiveproducts,e.g.,ettringiteorthaumasite,whichcanleadtoearlyfailureofthestructure.Themosttypicalattackofthistypeisonconcreteslabsandfoundationwallsatgradeswherethesulfateion,viaalternatewettinganddrying,canincreaseinconcentration.Astheconcentrationincreases,theattackonthePortlandcementcanbegin.Forburiedstructuressuchaspipe,thistypeofattackismuchrarer,especiallyintheeasternUnitedStates.Thesulfateionconcentrationincreasesmuchslowerinthesoilmassandisespeciallydependentupontheinitialamountofsulfatesinthenativesoil.Achemicalanalysisofsoilboringstocheckforthepresenceofsulfatesshouldbeundertakenduringthedesignphaseofanyprojectinvolvingconcreteincontactwiththenativesoil.Iftheconcentrationsarefoundtobeaggressive,variousprotectivecoatingscanbeapplied.Also,intheUSASTMC150Type5Portlandcementcanbeusedinthemix.Thistypeofcementisdesignedtobeparticularlyresistanttoasulfateattack.

Steelplateconstruction

Insteelplateconstruction,stringersjoinparallelsteelplates.Theplateassembliesarefabricatedoffsite,andweldedtogetheronsitetoformsteelwallsconnectedbystringers.Thewallsbecometheformintowhichconcreteispoured.Steelplateconstructionspeedsreinforcedconcreteconstructionbycuttingoutthetimeconsumingonsitemanualstepsoftyingrebarandbuildingforms.Themethodresultsinexcellentstrengthbecausethesteelisontheoutside,wheretensileforcesareoftengreatest.

Fiberreinforcedconcrete

Fiberreinforcementismainlyusedinshotcrete,butcanalsobeusedinnormalconcrete.Fiberreinforcednormalconcreteismostlyusedforongroundfloorsandpavements,butcanbeconsideredforawiderangeofconstructionparts(beams,pillars,foundations,etc.),eitheraloneorwithhandtiedrebars.

Concretereinforcedwithfibers(whichareusuallysteel,glass,orplasticfibers)islessexpensivethanhandtiedrebar,whilestillincreasingthetensilestrengthmanytimes.Theshape,dimension,andlengthofthefiberareimportant.Athinandshortfiber,forexampleshort,hairshapedglassfiber,isonlyeffectiveduringthefirsthoursafterpouringtheconcrete(itsfunctionistoreducecrackingwhiletheconcreteisstiffening),butitwillnotincreasetheconcretetensilestrength.AnormalsizefiberforEuropeanshotcrete(1mmdiameter,45mmlengthsteelorplastic)willincreasetheconcrete'stensilestrength.

Steelisthestrongestcommonlyavailablefiber,andcomesindifferentlengths(30to80mminEurope)andshapes(endhooks).Steelfiberscanonlybeusedonsurfacesthatcantolerateoravoidcorrosionandruststains.Insomecases,asteelfibersurfaceisfacedwithothermaterials.

Glassfiberisinexpensiveandcorrosionproof,butnotasductileassteel.Recently,spunbasaltfiber,longavailableinEasternEurope,hasbecomeavailableintheU.S.andWesternEurope.Basaltfibreisstrongerandlessexpensivethanglass,buthistoricallyhasnotresistedthealkalineenvironmentofportlandcement

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wellenoughtobeusedasdirectreinforcement.Newmaterialsuseplasticbinderstoisolatethebasaltfiberfromthecement.

Thepremiumfibersaregraphitereinforcedplasticfibers,whicharenearlyasstrongassteel,lighterinweight,andcorrosionproof.Someexperimentshavehadpromisingearlyresultswithcarbonnanotubes,butthematerialisstillfartooexpensiveforanybuilding.

Nonsteelreinforcement

Thereisconsiderableoverlapbetweenthesubjectsofnonsteelreinforcementandfiberreinforcementofconcrete.Theintroductionofnonsteelreinforcementofconcreteisrelativelyrecentittakestwomajorforms:nonmetallicrebarrods,andnonsteel(usuallyalsononmetallic)fibresincorporatedintothecementmatrix.Forexamplethereisincreasinginterestinglassfiberreinforcedconcrete(GFRC)andinvariousapplicationsofpolymerfibresincorporatedintoconcrete.Althoughcurrentlythereisnotmuchsuggestionthatsuchmaterialswillingeneralreplacemetalrebar,someofthemhavemajoradvantagesinspecificapplications,andtherealsoarenewapplicationsinwhichmetalrebarsimplyisnotanoption.However,thedesignandapplicationofnonsteelreinforcingisfraughtwithchallenges.Foronething,concreteisahighlyalkalineenvironment,inwhichmanymaterials,includingmostkindsofglass,haveapoorservicelife.Also,thebehaviourofsuchreinforcingmaterialsdiffersfromthebehaviourofmetals,forinstanceintermsofshearstrength,creepandelasticity.[24][25]

FibreReinforcedPolymer(FRP)(FibrereinforcedplasticorFRP)andGlassreinforcedplastic(GRP)consistoffibresofpolymer,glass,carbon,aramidorotherpolymersorhighstrengthfibressetinaresinmatrixtoformarebarrod,orgrid,orfibres.Theserebarsareinstalledinmuchthesamemannerassteelrebars.Thecostishigherbut,suitablyapplied,thestructureshaveadvantages,inparticularadramaticreductioninproblemsrelatedtocorrosion,eitherbyintrinsicconcretealkalinityorbyexternalcorrosivefluidsthatmightpenetratetheconcrete.Thesestructurescanbesignificantlylighterandusuallyhavealongerservicelife.Thecostofthesematerialshasdroppeddramaticallysincetheirwidespreadadoptionintheaerospaceindustryandbythemilitary.

InparticularFRProdsareusefulforstructureswherethepresenceofsteelwouldnotbeacceptable.Forexample,MRImachineshavehugemagnets,andaccordinglyrequirenonmagneticbuildings.Again,tollboothsthatreadradiotagsneedreinforcedconcretethatistransparenttoradiowaves.Also,wherethedesignlifeoftheconcretestructureismoreimportantthanitsinitialcosts,nonsteelreinforcingoftenhasitsadvantageswherecorrosionofreinforcingsteelisamajorcauseoffailure.Insuchsituationscorrosionproofreinforcingcanextendastructure'slifesubstantially,forexampleintheintertidalzone.FRProdsmayalsobeusefulinsituationswhereitislikelythattheconcretestructuremaybecompromisedinfutureyears,forexampletheedgesofbalconieswhenbalustradesarereplaced,andbathroomfloorsinmultistoryconstructionwheretheservicelifeofthefloorstructureislikelytobemanytimestheservicelifeofthewaterproofingbuildingmembrane.

Plasticreinforcementoftenisstronger,oratleasthasabetterstrengthtoweightratiothanreinforcingsteels.Also,becauseitresistscorrosion,itdoesnotneedaprotectiveconcretecoverasthickassteelreinforcementdoes(typically30to50mmormore).FRPreinforcedstructuresthereforecanbelighterandlastlonger.Accordingly,forsomeapplicationsthewholelifecostwillbepricecompetitivewithsteelreinforcedconcrete.

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ThematerialpropertiesofFRPorGRPbarsdiffermarkedlyfromsteel,sotherearedifferencesinthedesignconsiderations.FRPorGRPbarshaverelativelyhighertensilestrengthbutlowerstiffness,sothatdeflectionsarelikelytobehigherthanforequivalentsteelreinforcedunits.StructureswithinternalFRPreinforcementtypicallyhaveanelasticdeformabilitycomparabletotheplasticdeformability(ductility)ofsteelreinforcedstructures.Failureineithercaseismorelikelytooccurbycompressionoftheconcretethanbyruptureofthereinforcement.Deflectionisalwaysamajordesignconsiderationforreinforcedconcrete.Deflectionlimitsaresettoensurethatcrackwidthsinsteelreinforcedconcretearecontrolledtopreventwater,airorotheraggressivesubstancesreachingthesteelandcausingcorrosion.ForFRPreinforcedconcrete,aestheticsandpossiblywatertightnesswillbethelimitingcriteriaforcrackwidthcontrol.FRProdsalsohaverelativelylowercompressivestrengthsthansteelrebar,andaccordinglyrequiredifferentdesignapproachesforreinforcedconcretecolumns.

OnedrawbacktotheuseofFRPreinforcementistheirlimitedfireresistance.Wherefiresafetyisaconsideration,structuresemployingFRPhavetomaintaintheirstrengthandtheanchoringoftheforcesattemperaturestobeexpectedintheeventoffire.Forpurposesoffireproofinganadequatethicknessofcementconcretecoverorprotectivecladdingisnecessary.Theadditionof1kg/m3ofpolypropylenefiberstoconcretehasbeenshowntoreducespallingduringasimulatedfire.[26](Theimprovementisthoughttobeduetotheformationofpathwaysoutofthebulkoftheconcrete,allowingsteampressuretodissipate.[26])

Anotherproblemistheeffectivenessofshearreinforcement.FRPrebarstirrupsformedbybendingbeforehardeninggenerallyperformrelativelypoorlyincomparisontosteelstirrupsortostructureswithstraightfibres.Whenstrained,thezonebetweenthestraightandcurvedregionsaresubjecttostrongbending,shear,andlongitudinalstresses.Specialdesigntechniquesarenecessarytodealwithsuchproblems.

Thereisgrowinginterestinapplyingexternalreinforcementtoexistingstructuresusingadvancedmaterialssuchascomposite(fiberglass,basalt,carbon)rebar,whichcanimpartexceptionalstrength.Worldwidethereareanumberofbrandsofcompositerebarrecognizedbydifferentcountries,suchasAslan,DACOT,Vrod,andComBar.Thenumberofprojectsusingcompositerebarincreasesdaybydayaroundtheworld,incountriesrangingfromUSA,Russia,andSouthKoreatoGermany.

Seealso

References

ConcretecoverConcreteslabCoverMeterFalseworkFerrocementFormworkKahnSystemPrecastconcreteTypesofconcreteStructuralrobustness

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1. Day,p.2842. EncyclopdiaBritannicaFranoisCoignet(http://www.britannica.com/EBchecked/topic/124672/Francois

Coignet)3. Condit,CarlW.(January1968)."TechnologyandCulture"(http://www.jstor.org/stable/3102041?seq=4).The

FirstReinforcedConcreteSkyscraper:TheIngallsBuildinginCincinnatiandItsPlaceinStructuralHistory9(1).doi:10.2307/3102041(https://dx.doi.org/10.2307%2F3102041).Retrieved20February2014.

4. RichardW.Steiger(1995)."HistoryofConcrete"(http://www.theconcreteproducer.com/Images/The%20History%20of%20Concrete%2C%20Part%202_tcm771306954.pdf)(PDF).TheAberdeenGroup.Retrieved25April2015.

5. W.Morgan(1995)."ReinforcedConcrete"(http://www.jfccivilengineer.com/reinforced_concrete.htm).TheElementsofStructure.JohnF.Claydon(CivilEngineer).Retrieved25April2015.

6. DepartmentofCivilEngineering(2015)."HistoryofConcreteBuildingConstruction"(http://www.jfccivilengineer.com/reinforced_concrete.htm).UniversityofMemphis.Retrieved25April2015.

7. Mrsch,Emil(1909).ConcretesteelConstruction:(DerEisenbetonbau)(http://books.google.com/books?id=BZI1AAAAMAAJ&pg=PA205&lpg=PA205&dq#v=onepage&q&f=false).TheEngineeringNewsPublishingCompany.pp.204210.

8. Collins,Peter(19201981).Concrete:TheVisionofaNewArchitecture(http://books.google.com/books?id=7Zttxa_oHcEC&pg=PA58&lpg=PA58&dq=Thaddeus+Hyatt+concrete&source=bl&ots=k2wHydvwT8&sig=eosFTGCY_MzzyvHTUkc2EDRnvTU&hl=en&sa=X&ei=htYKU6bPGbbyQHv3oDoAw&ved=0CDgQ6AEwAg#v=onepage&q=Thaddeus%20Hyatt%20concrete&f=false).McGillQueen'sUniversityPress.pp.5860.ISBN0773525645.

9. Mrsch,Emil(1909).ConcretesteelConstruction:(DerEisenbetonbau)(http://books.google.com/books?id=BZI1AAAAMAAJ&pg=PA205&lpg=PA205&dq#v=onepage&q&f=false).TheEngineeringNewsPublishingCompany.pp.204205.

10. Mars,Roman."Episode81:RebarandtheAlvordLakeBridge"(http://99percentinvisible.org/episode/episode81rebarandthealvordlakebridge/).99%Invisible.Retrieved6August2014.

11. Collins,Peter(19201981).Concrete:TheVisionofaNewArchitecture(http://books.google.com/books?id=7Zttxa_oHcEC&pg=PA58&lpg=PA58&dq#v=onepage&q&f=false).McGillQueen'sUniversityPress.pp.6164.ISBN0773525645.

12. http://www.ce.memphis.edu/1101/notes/concrete/section_2_history.html13. "ReinforcingMeshForConcrete"(http://www.reinforcingsteelmesh.com).14. EmergingCorrosionControlTechnologiesforRepairandRehabilitationofConcreteStructures

(http://www.corrosionclinic.com/corrosion_resources/corrosion_control_concrete_structures_p1.htm)15. article"ConcreteInhomogeneityofVerticalCastInSituElementsInFrameTypeBuildings"

(http://www.concreteresearch.org/PDFsandsoon/Inhomog%20Denver.pdf).16. "Effectofzincphosphatechemicalconversioncoatingoncorrosionbehaviourofmildsteelinalkalinemedium:

protectionofrebarsinreinforcedconcrete"Sci.Technol.Adv.Mater.9(2008)045009(freedownload(http://www.iop.org/EJ/abstract/14686996/9/4/045009))

17. Nilson,Darwin,Dolan.DesignofConcreteStructures.theMacGrawHillEducation,2003.p.8090.18. http://121.183.206.200:8080/proto.board/abstractArticleContentView?

page=article&journal=sem&volume=53&num=4&ordernum=7&site=korsc19. http://ijce.iust.ac.ir/browse.php?a_id=563&sid=1&slc_lang=en20. http://www.ferrericon.com/Whatis.html21. "h2g2ConcreteCancer"(http://www.bbc.co.uk/dna/h2g2/A4014172).BBC.Retrieved20091014.22. 2006alertfordefectivecement

(https://web.archive.org/web/20061029112122/http://www.cementindustry.co.uk/main.asp?page=272)attheWaybackMachine(archivedOctober29,2006)

23. "HAC"(http://web.archive.org/web/20050911210803/http://www.questtech.co.uk/hac.htm).Web.archive.org.Archivedfromtheoriginal(http://www.questtech.co.uk/hac.htm)on20050911.Retrieved20091014.

24. BSEN1169:1999Precastconcreteproducts.Generalrulesforfactoryproductioncontrolofglassfibrereinforcedcement.BritishStandardsInstitute:http://shop.bsigroup.com/ISBN0580320529

25. BSEN11705:1998Precastconcreteproducts.Testmethodforglassfibrereinforcedcement.http://shop.bsigroup.com/ISBN0580292029

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26. ArthurW.Darby(2003),"Unknowntitle"(http://www.tunnels.mottmac.com/files/project/3372/Airside_Road_Tunnel.pdf)(PDF),www.tunnels.mottmac.com:645|chapter=ignored(help)

Furtherreading

ThrelfallA.,etal.Reynolds'sReinforcedConcreteDesigner'sHandbook11thed.(http://www.amazon.co.uk/dp/0419258302)ISBN9780419258308.NewbyF.,EarlyReinforcedConcrete,AshgateVariorum,2001,ISBN9780860787600.Kim,S.,Surek,JandJ.BakerJarvis."ElectromagneticMetrologyonConcreteandCorrosion."(http://nvlpubs.nist.gov/nistpubs/jres/116/3/V116.N03.A02.pdf)JournalofResearchoftheNationalInstituteofStandardsandTechnology,Vol.116,No.3(MayJune2011):655669.

Externallinks

TheConcreteReinforcingSteelInstitute(CRSI)isanationaltradeassociationthatstandsastheauthoritativeresourceforinformationrelatedtosteelreinforcedconcreteconstruction.(http://www.crsi.org)ConcreteResearch:http://www.concreteresearch.org(http://www.concreteresearch.org)Timelineofconcrete(http://www.auburn.edu/academic/architecture/bsc/classes/bsc314/timeline/timeline.htm)

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