Microstructure.docx
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Microstructure
Microstructure-BasedInherentAnisotropyofAsphaltMixtures
YuqingZhang1;RongLuo,Ph.D.,P.E.,M.ASCE2;
andRobertL.Lytton,Ph.D.,P.E.,F.ASCE3
Abstract:
asphaltmixtureshavebeendemonstratedtobeanisotropicmaterialsinbothlaboratoryandfieldtests.Theanisotropyofasphaltmixturesconsistsofinherentanisotropyandstress-inducedanisotropy.Inpreviouswork,theinherentanisotropyofasphaltmixtureswasquantifiedbyusingonlytheinclinationanglesofthecoarseaggregateparticlesintheasphaltmixtures.However,theinclinationoffineaggregatesalsohasacontributiontotheinherentanisotropy.Moreover,thecontributiontotheinherentanisotropyofeachaggregatemaynotbethesameasinthepreviousworkbutwilldependonthesize,orientation,andsphericityoftheaggregateparticle.Thispaperquantifiestheinternalmicrostructureoftheaggregatesinasphaltmixturesbyusinganaggregate-relatedgeometricparameter,thevectormagnitude.Theoriginalformulationofthevectormagnitude,whichaddressesonlytheorientationofcoarseaggregates,ismodifiedtoaccountfornotonlythecoarseaggregateorientation,butalsothesize,orientation,andsphericityofcoarseandfineaggregates.Thisformulationisappliedtocylindricallab-mixedlab-compactedasphaltmixturespecimensvaryinginasphaltbindertype,airvoidcontent,andagingperiod.Theverticalmodulusandthehorizontalmodulusarealsomeasuredbyusingnondestructivetests.Arelationshipbetweenthemodifiedvectormagnitudeandthemodulusratiooftheverticalmodulustothehorizontalmodulusisdevelopedtoquantifytheinfluenceoftheinherentmicrostructureoftheaggregatesontheanisotropyofthemixtures.Themodulusratioisfoundtodependsolelyontheaggregatecharacteristicsincludingtheinclinationangle,size,andsphericity,anditisindependentoftheasphaltbindertype,airvoidcontent,andagingperiod.Theinclinationangle,itself,provestobeinsufficienttoquantifytheinherentanisotropyoftheasphaltmixtures.
DOI:
10.1061/(ASCE)MT.1943-5533.0000325.©2011AmericanSocietyofCivilEngineers.
CEDatabasesubjectheadings:
Asphalts;Mixtures;Anisotropy;Microstructures;Aggregates;Fieldtests.
Authorkeywords:
Asphaltmixtures;Anisotropy;Microstructure;Aggregate;Vectormagnitude;Modulusratio.
Introduction
Theanisotropyofamaterialisthepropertyofbeingdirectionallydependent.Theanisotropyofanasphaltmixturecanbedefinedasthedifferenceinphysicalproperties,suchasmodulusandPoisson’sratio,whentheasphaltmixtureismeasuredindifferentdirections.Accordingtotheoriginsofanisotropy,granularmaterials,suchassoils,aggregatebase,andasphaltmixtures,consistoftwotypesofanisotropy:
(1)inherentanisotropy;and
(2)stressinducedanisotropy(Adu-Osei2000;Masadetal.2002;Kimetal.2005;Underwoodetal.2005).
Theinherentanisotropyofanasphaltmixtureisattributedtothepreferentialorientationofaggregates.Becausetheaggregatestendto“lieflat”duringthecompactionofasphaltmixtures,themajoraxis(i.e.,longestdiameter)oftheaggregatehasapreferentialdirectioninthehorizontalplane.Thestress-inducedanisotropyistheresultofcrackgrowthunderloadapplications.Theincreaseofthecracksurfacearealeadstothelossoftheintactmaterialarea,whichcausesthemodulusdegradation.Thecrackgrowthspeeddiffersindifferentdirections,whichresultsindifferentmodulusdegradationindifferentdirectionsandproducesthestress-inducedanisotropyintheasphaltmixture.Becausethetwotypesofanisotropyhavedifferentmechanisms,theinherentanisotropyandthestress-inducedanisotropymustbeinvestigatedseparately.Thefocusofthispaperisinvestigatingtheinherentanisotropyofasphaltmixtures.Toavoidtheinfluenceofthestress-inducedanisotropyandtoobtainthepureinherentanisotropy,allasphaltmixturespecimensweretestedwithinsmallstrainsinthisstudysothatnocracksgrewintheasphaltmixturespecimensduringthetest.
Theinherentanisotropyoftheasphaltmixturehasbeendemonstratedtobesignificantinbothlaboratoryandfieldtests.LevenbergandUzan(2004)conductedsmallstrain(i.e.,below150microstrains)hydrostaticcompressiontestsonlab-compactedcylindricalsamplesandfoundthattheasphaltmixturewas1.5timesstifferverticallythanhorizontally.MotolaandUzan(2007)conductedcompressivedynamicmodulustestsoneightfield-cutspecimensandillustratedthattheverticalmoduluswas40%greaterthanthehorizontalmodulus.Theysuggestedmodelingtheasphaltmixtureasacross-anisotropicmaterialwhosepropertiesaresymmetricabouttheverticaldirectionandareidenticalinthehorizontalplanethatisperpendiculartothecompactiondirection.Ramos-Aparicio(2004)andOhetal.(2006)backcalculated106groupsofgroundpenetratingradar(GPR)dataandfallingweightdeflectometer(FWD)fieldtestsofasphaltpavementandfoundtheanisotropicmodulusratio(i.e.,theratiooftheverticalmodulustothehorizontalmodulus)tobeanaverageof1.26.Asphaltmixturesexhibitsignificantinherentanisotropyonlyundercompression;undertensileloading,anasphaltmixturebehavesapproximatelyisotropically(Underwoodetal.2005;WagonerandBraham2008).Therefore,thispaperstudiestheinherentanisotropyofasphaltmixturesundercompressiveloadingonly.Theinherentanisotropyoftheasphaltmixtureneedstobeaccountedforduringtheperformanceanalysisofasphaltpavementsbecausebothfatiguecrackingandplasticdeformationmaybeunderestimatedifassumingthatasphaltmixturesareisotropicincompression.Wangetal.(2005)conductedtriaxialtestsoncubicfieldsamplesandshowedsignificantdifferencesinverticalandhorizontalstiffness.Theyfurtheranalyzedthepavementresponsesinafiniteelementpavementprogrambyusingtheanisotropicmoduliandtheisotropicmodulus,separately.Theyfoundlargertensileandshearstressesinthepavementwhenusingtheanisotropicmodulithanwhenusingtheisotropicmodulus.Ohetal.(2006)modeledtheasphaltpavementwithboththeanisotropicmoduliandtheisotropicmodulusfortheasphaltlayers.Theirmodelingresultsindicatedthatthepavementruttingpredictedbytheanisotropicmodulimatchedwellwiththemeasuredpavementrutting,whichexceededtheruttingpredictedbytheisotropicmodulus.
Toaddressthepreferredorientationofgeologicalstructures,aparameterofvectormagnitudewasfirstintroducedbyCurray(1956)forquantificationoftwodimensionalorientationdata.Subsequentresearchers(OdaandNakayama1989;Oda1993)addressedtheinherentanisotropyofsoilsfortheinclinationofsoilparticlesbyusingtheconceptofvectormagnitude,whichwasthenusedtoformulateamicrostructure-basedfabrictensortomodifytheeffectivestressinthesoilsduringacontinuumdamageanalysis.Thesimilarfabricanisotropyconceptswereemployedtodescribethegranularsandsandsoilspropertiessuchastheinherentanisotropy,thedirectionsofprincipalstresses,andtheanisotropicelasticdeformation(WongandArthur1985;HouqueandTatsuoka1998;Yoshimineetal.1998).Recently,bydirectapplicationofthefabrictensor,theanisotropicbehaviorofgranularsoilswassuccessfullysimulatedbyanumberofelastoplasticconstitutivemodels(LiandDafalias2002;Dafaliasetal.2004;LashkariandLatifi2007;
LoukidisandSalgado2009).
Thesameformulationsofthevectormagnitudeandthefabrictensorwereappliedtoasphaltmixtures(Masadetal.2002;MasadandButton2004;Tashmanetal.2005;Dessoukyetal.2006;Saadehetal.2007).ThepreferentialorientationofthecoarseaggregatesintheasphaltmixtureswasevaluatedwithX-raycomputedtomography(CT)andwasthenquantifiedbyusingthevectormagnitude.Subsequently,thefabrictensorwasformulatedonthebasisofthevectormagnitudetomodifytheeffectivestressintheasphaltmixtureduringacontinuumdamageanalysis.Thefabrictensorformulatedbyusingthevectormagnitudeprovedtobeaneffectiveindicatoroftheinherentanisotropyofasphaltmixtures.Accountingfortheinclinationofcoarseaggregatesinthecontinuumdamagemodelproducedmoreaccuratepredictionsofpavementrutting.
Theinherentanisotropyofasphaltmixturesisnotattributedonlytotheinclinationofcoarseaggregates.Theinclinationoffineaggregatesalsohasacontributiontotheinherentanisotropy.Moreover,thecontributiontotheinherentanisotropyofeachaggregatemaynotbethesamebutwilldependonthesize,sphericity,andorientationoftheaggregateparticle.Allthreeparametersmustbeaddressedwhenquantifyingtheinherentanisotropyofanasphaltmixture.Inotherwords,themicrostructure-basedfabrictensorshouldaddressnotonlytheinclinationofcoarseaggregatesbutthesize,orientation,andsphericityofbothcoarseandfineaggregateswhosesizeisbetween1.18and4.75mm.Tocharacterizethefineaggregates,animagingsystemwithhighresolutionisrequiredforscanningtheasphaltmixtures.TheimagesproducedbytheX-rayCTmaynothaveahighenoughqualityindotsperinch(DPI).Consequently,otherimagingmethodsthatcanprovidehigherqualityimagesneedtobeinvestigated.Inaddition,insteadofindirectlyaddressingtheinherentanisotropybyusingthefabrictensortomodifytheeffectivestressinthematerial,itwouldbedesirabletoestablishadirectrelationshipbetweentheinherentanisotropyandtheanisotropicmodulioftheasphaltmixtures.
Thispaperinvestigatestheinherentanisotropyofasphaltmixturesforthesize,theorientation,andthesphericityofaggregateparticles.Thesegeometric