Microstructure.docx

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