MicrostructureWord格式文档下载.docx

MicrostructureWord格式文档下载.docx

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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;

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