Conductivity-of-carbon-fiber-reinforced-cement-bas

Conductivityofcarbonfiberreinforcedcement-basedcompositesBingChena,*,KeruWub,WuYaobaSchoolofCivilEngineeringandMechanics,ShanghaiJiaotongUniversity,Shanghai200240,ChinabStateKeyLaboratoryofConcreteMaterialsResearch,TongjiUniversity,Shanghai200092,ChinaReceived7June2002;accepted13November2002AbstractTheconductivebehaviorofcarbonfibercement-basedcompositesispresented.Theinfluenceofcarbonfibervolume,size,ce-ment-basedmatrix,relativehumidityandcuringageonthecharacteristicofsystemwerestudied.Therelationshipbetweencon-ductivityandvolumefractionofcarbonfiberindicatedthatthestatisticalpercolationtheoryissuitableandapplicableforthechangeruleofconductivityofsystemwiththevolumeofcarbonfiber.Basedontheclassicpercolationtheory,thepercolationthresholdofcarbonfibercement-basedcompositeswasdeterminedas/¼/c2andtheconductivemechanismchangesfromelectrontunnelingconductiontoohmiccontactingconduction.Thestudieshaveofferedbasictheoryforsmartcement-basedcomposites.
2003ElsevierLtd.Allrightsreserved.Keywords:Carbonfiber;Cement-basedcomposites;Percolationtheory;Conductiveproperties1.IntroductionConcrete,consistingprimarilyofPortlandcement,fineandcoarseaggregate,hasbeenusedasaprimarybuildingmaterialformanyyearsowingtoitsexcellentengineeringpropertiesanddurability.Normalconcrete,however,isapoorelectricalconductor,especiallyunderdryconditions.Thepoorelectricalconductivitylimitsitsapplicationasafunctionalmaterialwhichmaybere-quiredofinmodernbuildings,forexampleelectro-magneticinterferenceshielding,electrostaticdischargeandothers[1–3].Therefore,theconductivebehaviourofcement-basedcompositesisnowreceivingconsiderableattention.Normally,inordertoproduceconductiveconcrete,conductivephases,eitherconductiveparticlesorcon-ductivefibers,areintroducedintothecementmatrix.Concretecontainingconductivefibershavesuperiorflexuralstrengthandtoughnessduetotheeffectoffiberreinforcement[4–6].Asanexample,concretecontaining1%carbonfiberinvolumeshowsatwo-foldimprove-mentintensileandflexuralstrength,aswellasa30-foldincreaseinspecificfractureenergyovertheunreinforcedmatrix.Althougheffortshavebeenmadeatinvestigat-ingthemechanicalpropertiesofcarbonfiberreinforcedconcrete,thereisinsufficientandconflictinginformationregardingitselectricalconductivity.Acarbonfiberre-inforcedcement-basedcompositesisacomplexmulti-phasesystem,andmanyfactorsinfluencetheelectricalconductivityofthesystem;forexample,themicro-structureofthematrix,carbonfibervolumefraction,andrelativehumidityofthespecimen.Inthispaper,somescientificprinciplesfortheconductivityofsuchasystemarereported.Thesesincludetheeffectofcontentandlengthofcarbonfiber,water/cementratioandsand/cementratioontheelectricalconductivityofcement-basedcompositesystems.TestsarealsoconductedtodeterminetheeffectofrelativehumidityandspecimenageontheelectricalpropertiesofCFRC(carbonfiberreinforcedcement-basedcomposite).Basedontheclas-sicpercolationtheory,howtodeterminethepercolationthresholdofcarbonfibercement-basedcompositeareanalyzedanddiscussed.2.Experimentaldetails2.1.MaterialsandspecimensThecarbonfiberswereisotropicpitch-basedandun-sized.ThefiberpropertiesareshowninTable1andthelengthisvariableaccordingtotheexperimentrequire-ment.TypeIIIPortlandcementwasusedthroughout.*Correspondingauthor.Tel.:[phone].E-mailaddress:[e-mail](B.Chen).0958-9465/$-seefrontmatter
2003ElsevierLtd.Allrightsreserved.doi:10.1016/S0958-9465(02)00138-5Cement&ConcreteComposites26(2004)291–297(ElkemMaterials)wasusedintheamountof15%bymassofcement.ThesandusedmetISOstandard.Water/cement(w=c)ratioswere0.25,0.35,0.45and0.50(bymass).Sand/cement(s=c)ratioswere0,1.0and2.0(bymass).Themortarmixtureswerepreparedinalaboratorymortarmixer.AHobartmixerwithaflatbeaterwasusedformixing.Themixture,sandcement,waterandcarbonfibersweremixedinthemixerfor5min.Rect-angularspecimens,40mm40mm160mmwerecastinplexiglassmolds.Twocopperelectrodes,40mm45mm0:2mm,wereembeddedinthefreshmix(seeFig.1).After24h,thespecimenswereremovedfromthemoldsandtransferredtoamoist-curingroom.2.2.ElectricalconductivitymeasurementGenerally,therearetwobasictypesofelectricalconductioninmoistspecimens:electronicandelectro-lytic[7].Theformeristhroughthemotionoffreeelec-tronsintheconductivephases,e.g.carbonfibers,andthelatteristhroughthemotionofionsintheporesolution.Inthisinvestigation,theprincipalcontributiontotheelectricalconductionisexpectedtobeelectronic.Theelectricalresistivitymeasurement,therefore,requirestheeliminationoftheeffectofelectrolyticconduction.Inordertoavoidtheproblemsofpolarization,al-ternatingcurrentsareoftenusedfordeterminingtheresistivityofelectrolytesandhenceofcementsandconcretes.Ithasbeensuggestedthatthepolarizationeffectsduringthepassageofanalternatingcurrentarenoteliminatedbutrathermanifestedinadifferentform.Thepolarizationisintheformofintroducingacapac-itorinseriesorinparallelwiththeresistance.Inanycase,foraalternatingcurrent,thereexists:Z¼VAð1Þwhere,ZisthesystemimpedanceinOhms,Visthemeasuredvoltage,andAisthemeasuredcurrent.Theimpedance,Z,mayberelatedintotheresistance