Moisture Susceptibility of Hot mix Asphalt Mixes: Identification of Problem and Recommended...

8/9/2019 Moisture Susceptibility of Hot mix Asphalt Mixes: Identification of Problem and Recommended Solutions

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MOISTURESUSCEPTIBILITYOF HMAMIXES:IDENTIFICATIONOFPROBLEMANDRECOMMENDEDSOLUTIONS

By

PrithviS.Kandhal

PublishedasNationalAsphaltPavementAssociation,QualityImprovementProgram(QIP)Series119,December1992

277TechnologyParkway Auburn,AL36830


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MOISTURESUSCEPTIBILITYOFHMAMIXES:IDENTIFICATIONOFPROBLEMANDRECOMMENDEDSOLUTIONS

By

PrithviS.Kandhal

AssociateDirectorNationalCenterforAsphaltTechnology

AuburnUniversity,Alabama

PublishedasNationalAsphaltPavementAssociation,QualityImprovementProgram(QIP)Series119,December1992


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DISCLAIMER

Thecontentsofthisreportreflecttheviewsoftheauthorswhoaresolelyresponsibleforthefactsandtheaccuracyofthedatapresentedherein.ThecontentsdonotnecessarilyreflecttheofficialviewsandpoliciesoftheNationalCenterforAsphaltTechnologyofAuburnUniversity.Thisreportdoesnotconstituteastandard,specification,orregulation.

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ABSTRACT

Strippingofhotmixasphalt(HMA)pavementsappearstohavebecomeamajorproblemin

recentyears.Moreandmorestatesarespecifyingtheuseofantistripping(AS)agents.Thereisaneedtoidentifytheproblemproperlysothatdecisionsarenotmadebasedonvisualobservationsofsomeisolateddistressedareas.

Externalfactorsand/orin-placepropertiesoftheHMApavementscaninducepremature

strippinginHMApavements.Thispaperdescribesthesefactorssuchasinadequatepavementdrainage,inadequatecompactionofHMApavement,excessivedustcoatingonaggregate,inadequatedryingofaggregates,andoverlaysonconcretepavements.Suggestionsforalleviatingtheproblemsassociatedwiththesefactorshavebeengiven.

Aninvestigativemethodologybasedonforensicexperiencehasbeenrecommendedforuseby

thespecifyingagenciesandindustrytoestablishstrippingasaproblemonaspecificprojectorstatewide.

ThecurrentpracticesofspecifyingmoisturesusceptibilitytestsacrosstheUnitedStateshavebeenreviewed.AASHTOT283(ModifiedLottman)testmethodhasbeenrecommendedtodeterminemoisturesusceptibilityofHMAmixesuntilmoresuitableandreliabletestsaredevelopedandvalidatedbySHRPorotheragencies.

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MOISTURESUSCEPTIBILITYOFHMAMIXES:IDENTIFICATIONOFPROBLEM

ANDRECOMMENDEDSOLUTIONS

PrithviS.Kandhal

INTRODUCTION

Inrecentyearsstrippingofhotmixasphalt(HMA)pavementsappearstohavebecomeamajorproblem.Everyyearmoreandmorestatesarespecifyingtheuseofantistripping(AS)agents.Thereisaneedtoidentifytheproblemproperlysothatdecisionsarenotmadebasedonvisualobservationsofsomeisolateddistressedareas.Prematurestrippingcanresultfrompoorsubsurfacedrainage(causingexcessivemoistureinthepavementstructurallayers),useofweakandfriableaggregates(fracturingduringconstructionandsubsequentlyinserviceexposinguncoatedsurfaces),excessivedustcoatingaroundtheaggregates,andverypoorcompactionoftheHMAmatduringconstruction.

AmongthestateswhichhavestartedtospecifyASagentstheproliferationofspecificationsand

testmethodsislarge.Differenttestmethodssuchasimmersion-compression,boilingwater,Texaspedestal,Lottman,modifiedLottman,andTunnicliff-Rootarespecifiedusuallywithsomevariations.Differentacceptancecriteriaareusedforthesametestmethod.

OBJECTIVES

Thisstudywasundertakentoachievethefollowingobjectives:

1.ListanddiscussthefactorswhichcaninduceprematurestrippinginHMApavements.

2.Recommendaninvestigativemethodologywhichcanbeusedbythespecifyingagencies/industrytoestablishstrippingasaproblemonaspecificprojectorstatewide.

3.ReviewthecurrentpracticeofspecifyingASagents,testmethodsandacceptancecriteria.Makerecommendations;foraviablecommonstrategyonspecificationsandtestmethods.

FACTORSRESPONSIBLEFORINDUCINGPREMATURESTRIPPING

Figure1showstheestimatedpercentageofHMApavementsexperiencingmoisturerelateddistressintheUnitedStatesaccordingtoa1989surveyofstatedepartmentsoftransportation(1).ResearchconductedattheNationalCenterforAsphaltTechnology(NCAT)undertheSHRPA-003BProjecthasshownthatthephysicochemicalsurfacepropertiesofmineralaggregatearemoreimportantformoistureinducedstrippingcomparedtothepropertiesofasphaltcementbinder.Somemineralaggregatesareinherentlyverysusceptibletostripping.However,inmanycasesexternalfactorsand/orin-placepropertiesofHMApavementsinduceprematurestrippingin

HMApavements.Aproperknowledgeofthesefactorsisessentialinidentifyingandsolvingthestrippingproblem.Adiscussionofthesefactorsfollows.

InadequatePavementDrainage

Inadequatesurfaceand/orsubsurfacedrainageprovideswaterormoisturevaporwhichisthenecessaryingredientforinducingstripping.IfexcessivewaterormoistureispresentinthepavementsystemtheHMApavementcanstripprematurely.Kandhaletal.(2)havereportedcasehistorieswherethestrippingwasnotageneralphenomenonoccurringontheentireproject

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Figure1.EstimatedPercentageofPavementsExperiencingMoistureRelatedDistress( 1)

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butratheralocalizedphenomenoninareasoftheprojectover-saturatedwithwaterand/orwater

vaporduetoinadequatesubsurfacedrainageconditions.

WatercanentertheHMApavementlayersindifferentways.Itcanenterasrun-offthroughthe

roadsurface,primarilythroughsurfacecracks.Itcanenterfromthesidesandbottomasseepagefromditchesandhighwatertableinthecutareas.

Themostcommonwatermovementisupwardbycapillarityunderapavement.Abovethe

capillaryfringewatermovesasavapor.Manysubbasesorsubgradesintheexistinghighwaysystemlackthedesiredpermeability,and,therefore,aresaturatedwiththecapillarymoisture.Theconstructionofmultilanehighways(orwidening)togreaterwidths,gentlerslopesandmildercurvesinallkindsofterrainhascompoundedthesubsurfacedrainageproblem.Doublingtheroadwidth,forexample,makesdrainageaboutfourtimesasdifficultasbefore(3).Quiteoften,afour-lanehighwayisrehabilitatedbypavingthemedianandshoulderswithHMAresultinginafullypavedwidthof72-78feetwhichisequivalenttoasix-lanehighwaywithoutanyincreaseinthesubsurfacedrainagecapability(2).

ExtensiveresearchhasbeenconductedonthemechanismofasphaltstrippingattheUniversityofIdaho(4).Ithasbeenreportedthat"airvoidsinasphaltconcretemaybecomesaturatedwithwaterevenfromvaporcondensationduetowaterinthesubgradeorsubbase.Atemperatureriseafterthissaturationcancauseexpansionofthewatertrappedinthemixturevoidsresultinginsignificantvoidpressurewhenthevoidsaresaturated.Itwasfoundthatvoidwaterpressuremaydevelopto20psiunderdifferentialthermalexpansionofthecompactedasphaltmixtureandcouldexceedtheadhesivestrengthofthebinderaggregatesurface.Ifasphaltconcreteispermeable,watercouldflowoutofthevoidspacesunderthepressuredevelopedbythetemperatureriseand,intime,relievethepressuredeveloped.Ifnot,thenthetensilestressresultingfromthepressuremaybreakadhesivebondsandthewatercouldflowaroundtheaggregatescausingstripping.Thestrippingdamageduetovoidwaterpressureandexternalcyclicstress(bytraffic)mechanismisinternalinthespecimens,theexteriorsidesofthe

specimensdonotshowstrippingdamageunlessopenedupforvisualexamination."

MajidzadehandBrovold(5)havealsostatedthattheporepressurefromstressesinducedby

trafficcausethefailureofthebinder-aggregatebond.Initially,thetrafficstressesmayfurthercompactthemixtureandtraporgreatlyreducetheinternalwaterdrainage.Therefore,theinternalwaterisinfrequentmotion(cyclic)andconsiderableporepressureisbuiltupunderthetrafficaction.Hallberg(6)hasreportedthat"therequiredinternalwaterpressurecausinganasphalticmixturetohaveadhesiveorinterfacialtensionfailure(stripping)isinverselyproportionaltothediameterofthepores."Bindercoursemixturesgenerallystripmorethanthewearingcoursemixturespossiblyduetolargediameterporesinthebindercourse.Moreover,thewearingcourseisexposedtorepeatedhightemperaturedryingperiodswhenthepavementheals.Theasphaltfilms

whichdebondfromtheaggregateattachthemselvesagainandthemixregainsitsstrengthandwaterresistance.Thehumidperiodsarelongerintheunderlyingbindercourseand,therefore,theselfhealingforcesduringwarmperiodshavemuchlessinfluence.

LoveringandCedergren(7)havereportedthat"withinsufficientdrainage,watermayfloodthe

baseandrisethroughthepavement.Manydrainageproblemsanddeterioratedpavementscanbeattributedtowaterthatentersthestructuralsectionfrombelow."Apparentlythedeteriorationiscausedbyprematurestrippinginmanycases.

TelltalesignsofwaterdamagetoHMAoverlays(overconcretepavements)havebeendescribed

byKandhaletal.(2).TheyobservedwetspotsontheHMAoverlaysurfacescatteredthroughouttheproject.Usuallyatthesewetspotswateroozedoutduringhotafternoons.Someofthewet

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spotscontainedfinessuspendedinthewaterwhichweretrackedonthepavementbythetraffic

andappearedaswhitespots.Mostwhitespotsturnedintofattyareas(resultingfromasphaltstrippingandmigratingtothesurface)whichusuallyprecededtheformationofpotholes.Figures2and3showallthreestages:whitespots,fattyareas,andpotholesonafour-lanehighway.Figure4

showsseverelystrippedaggregateparticlesinapothole.

Figure2.ThreeStagesofStripping:WhiteSpots,FattyArea,andPothole(a Closeup)

Figure3.SlowTrafficLaneShowingThreeStagesofStripping4


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Figure4.Close-UpofPotholeShowingSeverelyStrippedAggregate

Smallandlargeblisterswerealsoobservedduetoentrappedmoisture.AveryseverecaseofblisteringfrommoisturevaporpressureatEmporiaAirport,VirginiahasbeendescribedbyAcottandCrawford(8)andisshowninFigure5.However,blisterscanoccurwithoutanyasphalticglobulesatthesurface.

Figure5.MoistureVaporBlistersWithinStrippedAsphalticGlobules( 8)

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Usuallystrippinginafour-lanehighwayfacilityoccursfirstintheslowtrafficlaneasevidentin

Figure3becauseitcarriesmoreandheaviertrafficcomparedtothepassinglane.Typically,strippingstartsatthebottomofHMAlayerandprogressesupwards.

ItisevidentfromtheprecedingdiscussionthatinadequatesubsurfacedrainageisoneoftheprimaryfactorsinducingprematurestrippinginHMApavements.

Subsurfacedrainageproblemscanbealleviatedindifferentwaysdependingonthelocal

conditions.Kandhaletal.(2)havereportedsomecasehistoriesindetailwhereithasbeendone.Thesearedescribedbrieflyhere.Figures6aand7ashowtypicalmedianandcutsectionsoftheEast-WestPennsylvaniaTurnpike,respectively.Thissectionreceiveda4-inchHMAoverlayonthemainlinein1977anditsmedianwasalsopavedforthefirsttimewitha3-inchHMAbinderandwearingcourse.Theworkalsoincludedtheinstallationofnewpipeinthemedian.However,thenewsubbaseabovethepipewasalmostimpermeable.Strippingwasobservedinthispavementduringthesummerof1978whensmallpotholesstartedtodevelopmainlyintheinsidewheeltrackoftheslowtrafficlane.Itwasobservedfromextensivetrenchingand

samplingthatwaterand/orwatervaporwasgettingintothepavementstructuralsystemfromunderneathprimarilythroughthelongitudinalandtransversejoints,cracksintheconcretepavementandthedisintegratedconcreteitselfatsomeplaces.TherewasalsoevidencethatmoisturewasbeingdrawnfromthesubbaseunderthepavedmedianintotheHMAoverlaylayersprobablyintheformofwatervaporduringtheheatoftheday(Figure6a).WatervaporwhichaccumulatedinthepavementlayersduringthedaycondensedduringthenightuntiltheHMApavementlayersbecomesaturatedwithwater.Withsaturationtheporewaterpressuredevelopedbydifferentialthermalexpansionandcyclicstressesfromthetrafficrupturedtheasphalt-aggregatebondcausingstripping.

Figure6.TypicalMedianSectionofEast-WestPennsylvaniaTurnpike( 2)

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nearthecenterline.MostofthisNorth-EastExtensionsectionismountainousandis

predominantlybuiltincutareas.

Figure8.TypicalCutSectionofNorth-EastPennsylvaniaTurnpike( 2)

Thesubsurfacedrainagecanbeimprovedinthisinstancebyincreasingthedepthofthetwolongitudinalunderdrainsattheshoulderedgeincutareas.TheproposedimprovementasshowninFigure8bwillalsodrainthenewshouldersubbase,whichissandwichedbetweentwoimpermeablelayersandiscausingasphaltstrippingintheoverlyingnewbindercourse.

Inadequatecompaction

InadequatecompactionofHMAmatisprobablythemostcommonconstructionrelatedfactorresponsibleforprematurestripping.Studieshaveshownthatatlessthan4-5%airvoidcontentin

theHMAthevoidsaregenerallynotinterconnectedandthusalmostimpervioustowater.MostHMAmixesaredesignedtohave3to5%airvoidcontents.Whenconstructed,amaximumairvoidcontentof8%(atleast92%ofthetheoreticalmaximumspecificgravity)isspecifiedbymostagencies.Itisassumedthatthepavementwillgetdensifiedtothedesignairvoidcontentunder2-3yearstraffic.However,someagenciesdonotexercisegoodcompactioncontrolresultinginairvoidscontenthigherthan8%atthetimeofconstruction.Thiscancauseprematuresurfaceravelingbecausethemixdoesnotpossessadequatecohesion.TherelationshipbetweenairvoidcontentandextentofravellingobtainedfromeightpavingprojectsisshowninFigure9(11).Quiteoften,strippingisblamedforthistypeofprematureravellingwithoutcloselyexaminingthemixture.However,iftheHMApavementremainsperviousforanextendedperiodoftime,strippingislikelytooccurduetoingressofwaterandhydraulicporepressuresinducedbythetraffic.

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Figure9.AirVoidContentversusExtentofRaveling(11)

TerrelandShute(12)haveadvancedtheconceptof"Pessimum"voidcontentforstripping.Figure10showsthegeneralrelationshipbetweenairvoidsandrelativestrengthofHMAmixturesfollowingwaterconditioning.Theamountofstrengthlossdependsupontheamount

andnatureofthevoids.AsshowninFigure10,atlessthan4percentvoids,themixtureisvirtuallyimpermeabletowater,soisessentiallyunaffected.Unfortunately,regionBtoCiswheremanypavementsgetconstructed.AsthevoidsincreasetoDandbeyond,themixstrengthbecomeslessaffectedbywaterbecausethemixtureisnowfreedraining.TheregionBtoCinFigure10canbecalled"Pessimum"voidcontentbecauseitrepresentstheoppositeofoptimum.Theobjectiveistostayoutofthe"Pessimum"voidrangetominimizestrippingproblem.Thiscanbedonethroughpropermixdesignandcompactioncontrolprocedures.

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Figure10.AirVoidContentversusRetainedMixStrength-Regionof PessimumVoids(12)

ExcessiveDustCoatingonAggregate

Thepresenceofdustandclaycoatingsontheaggregatecaninhibitanintimatecontactbetweentheasphaltcementandaggregateandprovidechannelsforpenetratingwater(13).Theasphaltcementcoatsthedustcoatingandisnotincontactwiththeaggregatesurface.Ithasalsobeenhypothesizedthatsomeveryfineclayeymaterialmaycausestrippingbyemulsifyingtheasphaltcementbinderinpresenceofwater,butthisappearstobeaninsignificantanduncommonfactor.

Theauthorisawareofoneprojectonwhichstrippingoccurredbythemechanismofhydraulic

scouringwhichisapplicableonlytosurfacecourses.Unliketypicalstripping,suchastripping

startsatthesurfaceandprogressesdownward.Hydraulicscouringresultsfromtheactionofvehicletiresonasaturatedpavementsurface.Thewatergetspresseddownintothepavementinfrontofthetireandimmediatelysuckedawayfromthepavementbehindthetire.Thiscompression-tensioncyclecontributestothestrippingoftheasphaltfilmfromtheaggregate(14).Theaggregateusedonthatprojecthadexcessiveamountsofaveryfinedustcoating.Whentheaggregatewaswashedinthequarryandusedagaintheproblemwentaway.Laboratorystudies(15)havealsoshownimprovedadhesioncharacteristicsofsomedustcontaminatedcoarseaggregateswhenwashed.

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UseofOpen-GradedAsphaltFrictionCourse

SeveralstatesinthesoutheasternUnitedStatesexperiencedstrippingintheHMAcourseunderlyingopengradedasphaltfrictioncourse(OGFC)duringthelate1970s.Ithasbeen

hypothesizedthattheOGFCretainsmoistureforalongertimeanddoesnotdryoutafterrainasfastasaconventionaldensegradedHMAsurface.ThewaterinOGFCisalsopressedintotheunderlyingcoursebythetrucktiresinitiatingthestrippingactionwhichcancauseflushing,ruttingorshovingatthesurface.SeveralstatessuspendedtheuseofOGFCinearly1980s.InSouthCarolinathestatewideaveragestrippingfrequencywasdeterminedtobe18.7%underOGFCcomparedwithastatewideaverageof8.5%forallpavementlayers(16).SomestudieshavealsoshownthatthestrippinginthelayersunderlyingOGFCresultedfromtheirhighairvoidcontent(lackofadequatecompaction).Evidently,itisallthemoredesirabletohaveanimperviousHMAcoursebelowtheOGFCtominimizestripping.ItisrecommendedthattheairvoidcontentoftheunderlyingHMAcourseshouldnotexceed4-5percentwhentheOGFCisplacedtominimizestrippingintheunderlyingcourse.Quiteoften,theairvoidcontentintheHMAcoursecanbeasmuchas8percentjustafterconstruction.TheconstructionofOGFCin

suchcasesshouldbedelayeduntilthetrafficdensifiestheHMAcoursetoanairvoidcontentof4-5percent.

InadequateDryingofAggregates

Laboratorystudies(17)haveshownthathighresidualmoisturecontentinthemineralaggregatepriortomixingwithasphaltcementbinderincreasesthepotentialforstripping.WhendrummixfacilitieswereintroducedforHMAproductioninthe1970s,lowmixingtemperatures(andhighmoisturecontentintheHMA)wereencouragedtofacilitatecompaction.Itishypothesizednowthatthismighthavecausedsomeofthestrippingproblems.However,moststateshavenowincreasedthemixtemperaturerequirementsfordrummixfacilitiestothoserequiredforbatchmixfacilities.Undoubtedly,adryaggregatesurfacewillhaveincreasedadhesionwiththeasphaltcementcomparedtoamoistorwetsurface.

WeakandFriableAggregate

IfweakandfriableaggregatesareusedintheHMAmix,degradationtakesplaceduringrollingandsubsequentlyunderheavytraffic.Degradationordelaminationexposesnewuncoatedaggregatesurfaceswhichcanreadilyabsorbwaterandinitiatethestrippingphenomenoninthemix.Also,ifnotobservedcarefully,theseuncoatedaggregatesurfacescanmistakenlybedeemedasstrippedaggregateparticles.Obviously,useofsoundanddurableaggregateintheHMAisrecommended.

OverlaysonDeterioratedConcretePavements

Manyconcretepavementsofinterstateandprimaryhighwaysaredeterioratingbeforethedesign

life.RecentyearshaveseenincreasedHMAoverlaysovertheseexistingconcretepavementssomeofwhichhavefaulted,spalled,cracked,andwater-pumpingslabs.Densegradedsubbasematerialunderconcretepavementscanholdconsiderableamountsofwaterwhichescapethroughcracks,longitudinalandtransversejoints(Figure11).OncetheconcretepavementisoverlaidwithanimperviousHMAcoursethewateristrappedunderneath.Excessiveporepressureisbuiltunderthetrafficinitiatingstrippingandsubsequentlypotholingatworstspots(Figure12).Wheneveraconcretepavementisduetobeoverlaidforthefirsttime,itisnecessarytoevaluatetheexistingdrainageconditions.Ifnecessary,theprojectmustincludeinstallationofapositivedrainagesystemespeciallyintheworstspotslikeshowninFigure11.Unlessthisisdone,theproblemofstrippingandpotholingwillpersistforever.Usuallytheedgedrainsarenotefficienttodraintheentireroadwaywidth.Therefore,transverse(lateral)drainsarenecessary

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Figure11.WaterPumpingfromTransverseJointofConcretePavement

Figure12.PatchedPotholesinHMAOverlayonEitherSideofTransverseJointof UnderlyingConcretePavement

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especiallyonsteepgradeswherewaterwilltendtoflowlongitudinallyratherthantowardsthe

edgedrain.Suchlateraldrainscanbeinstalledatorneartheexistingtransversejointsofconcretepavementspriortooverlay,andconnectedtotheedgedrain.

Iftheexistingconcretepavementisbadlydeteriorated,crackedandpumpingwaterduetoinadequatesubsurfacedrainage,itisrecommendedtoprovidea4-inchdrainagelayerofopen-gradedATPMdirectlyaboveitpriortoplacingthedensegradedHMAoverlay.Thisdrainagelayershouldbeconnectedtotheedgedrain(s).TheATPMwillnotonlydrainthewaterveryefficiently,itwillpreventanymoisturevaporbuildupinthepavementsystem.Atypicalroadcross-sectionshowingsuchusageofATPMisshowninFigure13.TheATPMhasbeenusedsuccessfullyinsuchapplications.Itwillalsohelptominimizereflectioncrackingemanatingfromtheconcretepavement.Ifrequired,theATPMcanalsobeplacedoverconcretepavementswhichhavebeensubjectedtocrackandseat,breakandseat,andrubblizingoperations.References9and10givedetailsonthedesignanduseofATPM.

WaterproofingMembranesandSealCoats

Ifthesourceofmoistureisfrombeneaththepavement,whichisusuallythecase,thensealingoftheroadsurfacecanbedetrimental.Useofsomewaterproofingmembranes(suchasstressabsorbingmembranestominimizereflectioncracking)andsealcoatsbetweenthepavementcoursesoratthesurfaceactslikeavaporsealoravaporbarrier.McKesson(18)hasmadesomeinterestingobservations.Heobservedthat"groundwaterandwaterenteringtheroadbedfromtheshoulders,ditchesandothersurfacesources,iscarriedupwardbycapillarityunderapavement.Abovethecapillaryfringewatermovesasavaporand,ifunimpededatthesurface,itpassestotheatmosphere.ThismethodofreductionofmoisturehasbeentermedDrainagebyEvaporation,anditistheconsideredopinionofthiswriterthattheDrainagebyEvaporationisusuallyasimportantasdrainagedownwardbygravitation.Ifthepavementorsealcoatconstitutesavaporsealoravaporbarrier,themoistureduringcoolnightsandincoolweathercondensesbeneaththesurface.Whenthepavementabsorbssolarheat,thewaterisagain

vaporizedand,ifnotfreetoescape,substantialvaporpressureresultsbecausewaterasvaporhasmorethanathousandtimesthevolumeofwaterinliquidform.Vaporpressureforcesthemoistureupintothepavementandthroughthesurface.Blisteringinbituminouspavementsisawellknownexampleoftheeffectofentrappedmoistureandmoisturevapor."

Manyasphaltpavingtechnologistshaveexperiencedtheprecedingphenomenonwhichinduced

strippinginthepavementlayersunderlyingwaterproofingmembranesandsealcoats.Thepotentialforstrippingshould,therefore,beconsideredwheneversuchsystemsareused.

INVESTIGATIVEMETHODOLOGY

AninvestigativemethodologybasedonforensicexperiencewithHMApavementsisneededtoestablishifstrippingisaproblemonaspecificprojectorstatewide.Merevisualobservationsof

theroadsurfaceisoftenmisleadingbecausetheHMAsurfacedistressessuchasravelling,flushingandruttingcanbecausedbyfactorsotherthanstripping.Thefollowingmethodologyissuggested.

Sampling

Inspectthewholeprojectandselecta500ftlongsectionwhichrepresentsthe"distressedarea."Mostprojectswillalsohaverelativelybetterareaswithminimalornodistress.Selectanother500ftlongsectionfromthesameprojectwhichcanbetermedrelatively"goodarea."Documenttheobserveddistress(suchasravelling,flushing,ruttingandpotholing)inbothareas.

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Obtainatleastseven4-inchdiametercoresatrandomlocationsineacharea.Aminimumsample

sizeof7foreachareaisnecessaryforreasonablestatisticalanalysisofthedataandtorepresentthesampledpopulationwithanacceptabledegreeofconfidence.Ifitisa4-lanehighway,obtainallcoresintheinsidewheeltrackoftheslowtraffic(outside)lane.Ifitisa2-lanehighway

obtainallcoresfromtheoutsidewheeltrackofthelane.Accordingtoauthor'sexperiencestrippingusuallyoccursfirstattheselocationsacrosstheroadwaypavement.Four-inchdiametercoreshavebeensuggestedsothattheindirecttensiletestcanbeconducted.Anadditionaleighthcorecanalsobeobtainediftheagedasphaltcementbinderistoberecoveredandtestedforpenetrationand/orviscosity.

Itisnecessarytodrillthesecoreswithoutusingwaterasacoolantsothatthein-situmoisture

contentscanbedetermined.CompressedairandC02areintroducedunderpressuretocooltheinsideofthecoredrill.Theadvancerateofthegas-cooledcoredrillisusuallyslowerthanthatofthewatercooledcoredrillbutthevaluableinformationofmoisturecontentcannotbeobtainedfromwetcoring.SimilarprocedureshavebeenusedbyChevronResearchCompanyinstudiesofasphaltemulsionmixturesinCalifornia(19)andbytheSouthCarolinaDepartmentof

HighwaysandTransportationininvestigationofstrippingofHMAinSouthCarolina(16).Coresshouldbesealedinair-tightcontainersfordeterminingthein-situmoisturecontentinthelaboratorylater.Seasonalvariationsofthein-situmoisturecontentinHMAlayersmustbetakenintoaccount.

Ifdrycoringcannotbedonethenadditionalpavementlayersamplesshouldbeobtainedadjacent

tothewetcoringsitesusingajackhammer.TheHMAchunksamplesloosenedbythejackhammerfromeachlayershouldalsobesealedinair-tightcontainerssothatthein-situmoisturecontentcanbedeterminedinthelaboratorylater.Kandhaletal.(2)usedjackhammerininvestigatingstrippedpavementsasshowninFigure14.

Figure14.UsingJackHammertoObtainSampleforMoistureContent

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Testing

TherecommendedtestingplanisshowninFigure15.Thein-situmoisturecontentshouldbedeterminedbyweighingthecoresbeforeandafterdryingtoconstantweight.Itispreferableto

drythecoresatambienttemperatureswithafan.Measurethethicknessofalllayersinthecore.Observetheconditionofthecoreespeciallyanyevidenceofstrippinginthelayer(s)orattheinterfacebetweenthelayers.Itisnotalwayspossibletoseethestrippingontheoutsideofcores.

SawthecorestoseparatetheHMAlayerssothattheindividuallayer(s)canbetested.Measure

theaveragethicknessofeachlayerspecimenaftersawing.

Determinethebulkspecificgravityofallspecimens(AASHTOT166).Determinetheindirect

tensilestrengthofthedryspecimensat77FusingAASHTOT283(Sections10and11)orASTMD4867(Sections8and9).

Figure15.TestingPlan

Examinethesplitexposedsurfacesofthetestedcorespecimensforstripping.Disregardthe

fracturedandcrushedaggregateparticles.Heatthespecimenjustenoughtopushitapartbyhandandobservetheextentofstripping.Avisualratingofthestrippingontheexposedsurfaceshouldbemadeanddocumented.AratingsystemdevelopedbytheGeorgiaDepartmentofTransportationandusedbytheSouthCarolinaDepartmentofHighwaysandPublicTransportation(SCDHPT)intheirstatewidestrippingsurvey(16)isrecommended.Thisvisualstrippingratingisbasedonbroad,easilyassessedrangeestimatesofstripping.Theratingsystemconsidersthestrippingofthefineaggregatematrixandthecoarseaggregatefractionseparately.Strippingofthefineaggregatematrixisconsideredtobemorecriticalthanacomparable

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percentageofstrippinginthecoarseaggregatefraction.Theprocedure,however,doesrequire

sometrainingforconsistentinterpretationofobservations.

TheGeorgiaDOTstrippingrating,S,iscalculatedbyassigningvaluestoCandFinthe

expressionS=(C+F)/2whereCandFare:

ValuesofC

C=CoarseAggregateStripping

1=lessthan10%2=10-40%3=morethan40%

ValuesofF

F=FineAggregateStripping

1=lessthan10%2=10-25%3=morethan25%

Ifpossible,haveatleastthreeevaluatorsnotethestripingineachcoreandthencalculatetheaveragestrippingrating.

Anaveragestrippingratingof2.5and3.0wereusedbySCDHPTtoidentifypavementsforwhichstrippingwasconsideredsevere.

Afterallsevencoresfromanareahavebeenratedforstripping,determinethemaximum

theoreticalspecificgravity(AASHTOT209)ofthepavingmixturesfrom3cores(Cores1,4and7arerecommendedtoencompassmostoftherepresentativearea).

Conductextractiontest(AASHTOT164)andgradationofextractedaggregate(AASHTOT30)

onallsevencorestodeterminethemixcomposition(asphaltcontentandgradation).

CalculationsandTabulation

Figure16showstheflowdiagramforcalculations.TheeffectivespecificgravityofaggregatesinCores1,4and7shouldbecalculatedusingtheirmaximumtheoreticalspecificgravityvaluesandtheirrespectiveasphaltcontentvalues.Calculatetheaverageeffectivespecificgravityoftheaggregatefromthesethreevalues.Calculatethemaximumtheoreticalspecificgravityvaluesfor

Figure16.CalculationofAirVoidContent

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eachofthesevencoresusingthisaverageeffectivespecificgravityandtheirrespectiveasphalt

contentsobtainedbyextraction.Calculatetheairvoidcontentineachcorefromitsbulkspecificgravityanditsmaximumtheoreticalspecificgravity.

Calculatethepercentageofin-situwatersaturationbythefollowingformula:Tabulateallcalculatedandobserveddataseparatelyfor"good"and"distressed"areas.Calculatethemean,standarddeviation,and95%confidencelimitsforeachparameter.Ahighstandarddeviationwouldindicatelackofuniformity(orconsistency)forthattestparameter.

Comparethemeanandstandarddeviationofeachtestparameterobtainedin"good"and

"distressed"areastoidentifythedifferences,ifany.Inamajorityofcases,thedeficienciesinthe

"distressed"areawillstandoutbythiscomparison.Example

Tables1and2showsomehypotheticaldatafromathree-yearolddistressedproject.Table1representstestdataobtainedbythisinvestigativemethodologyfroma"good"areawhereasTable2hasdatafromarepresentative"distressed"areaoftheproject.ThehypotheticaldatainTable2hasbeenpresentedpurposelytoillustratemostoftheHMArelatedfactors(ordeficiencies)whicharelikelytoinducestripping.Therefore,thiscanbeconsideredastheworstscenario.This"distressed"areahasthefollowingproblems:

1.Veryhighandinconsistentairvoidcontent;2.Deficientandinconsistentasphaltcontent;3.Excessiveandinconsistentminus200material;and

4.Veryhighin-situmoisturecontentsorsaturationlevels.

TheaboveproblemscanbeidentifiedeasilybycomparingthedatafromTable2withthatof

Table1.Inthisexample,severestrippingwasobservedinthe"distressed"area,whichisalsoindicatedbylowertensilestrengthscomparedtogoodareas.

WhendatalikeinTable2isobtained,oneshouldnotstartspecifyinganantistrippingagentasa

curebuttakeremedialmeasurestoremovethecause(s).Inthisexample,thefollowingneedswouldbeindicated:

1.Adequatecompactionlevelatthetimeofconstruction.Anaverageairvoidcontentof8.9percentafter3years'serviceisunacceptable.TheHMApavementshouldhaveachieveditsdesignairvoidcontent(3-5%)bynow.

2.Qualitycontrolofmixcomposition.Theaverageasphaltcontentof6.4percentis

deficientby0.5percentfromthejob-mixformula,andalsothestandarddeviationof0.45percentistoohigh.Theaverageminus200contentisexcessiveby1.9percentfromthejob-mixformulaandisalsoveryvariablebasedonthestandarddeviationof1.97percent.

3.Positivedrainagesystem.Theprojecthaswaterdrainageprobleminthedistressedareawithsaturationashighas100percent.

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Table1.CoreTestData-GoodArea

Test Job-Mix CoreNo.Formula Std. 95%

1 2 3 4 5 6 7 Dev. ConfidenceLimits

BulkSpecific 2.290 2.286 2.287 2.285 2.271 2.256 2.293 2.260 2.277 0.0145 2.248-2.306GravityMax.Specific 2.385 2.394 2.380 2.398 2.371 2.380 2.389 2.394 2.386 0.0098 ---Gravity%Voids 4.0 4.5 3.9 4.7 4.2 5.2 4.0 5.6 5.6 0.63 3.3-5.9

TensileStrength,psi --- 118 130 110 128 98 121 90 90 15.1 84-144

%AsphaltContent 6.9 6.7 7.0 6.6 7.2 7.0 6.8 6.7 6.7 0.21 6.4-7.2

%Minus200 5.2 5.8 6.1 5.3 4.3 4.8 6.0 4.5 4.5 0.74 2.6-8.0

%in-situMoisturein --- 0.3 0.2 0.3 0.2 0.3 0.2 0.4 0.4 0.076 0.1-0.4Core%in-situSaturation --- 15.2 11.7 14.6 10.8 13.0 11.5 16.1 16.1 2.05 9.2-17.4

StrippingRating --- 1.0 1.0 1.0 1.0 1.5 1.0 1.5 1.5 --- ---

Table2.CoreTestData-DistressedArea

Test Job-Mix CoreNo.Formula Std. 95%

1 2 3 4 5 6 7 Dev. ConfidenceLimits

BulkSpecific 2.290 2.154 2.213 2.213 2.212 2.135 2.211 2.205 2.192 0.0329 2.126-2.258GravityMax.Specific 2.385 2.385 2.411 2.380 2.407 2.429 2.385 2.407 2.408 0.0202 ---Gravity%Voids 4.0 11.5 8.2 7.0 8.1 12.1 7.3 8.4 8.9 2.02 4.9-12.9

TensileStrength,psi --- 76 52 107 83 72 97 56 78 20.1 38-118%AsphaltContent 6.9 5.8 6.3 7.0 6.4 5.9 6.9 6.4 6.4 0.45 5.5-7.3%

Minus200 5.2 4.5 7.2 9.6 9.2 7.1 4.7 7.3 7.1 1.97 3.2-11.0

%in-situMoisturein --- 5.2 4.5 0.8 3.5 5.1 1.1 5.8 3.7 2.02 0.3-7.7Core%in-situSaturation --- 97.4 121.4 25.3 95.6 90.0 33.3 152.2 87.9 45.30 0-178.5*

* *StrippingRating --- 2.5 3.0 2.0 2.5 2.5 2.0 3.0 2.5 --- ---*Calculatedsaturationcanexceed100%becausepartofthewaterhasbeenabsorbedbythestrippedaggregateparticles.

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0

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IftestdatalikeinTable1isobtainedthroughoutaprojectandthereisevidenceofstripping,the

HMAmixismostlikelysensitivetomoisturedamage.Insuchcases,asuitableantistrippingagentshouldbeconsidered.

StatewideSurveyBeforespecifyingantistrippingagentsand/ormoisturesusceptibilitytestmethodsstatewide,itisprudenttofirstestablishifstrippingisastatewideproblemorjustisolatedoccurrences.BothGeorgiaandSouthCarolinacompletedastatewidesurveyandevaluationoftheproblemthroughanextensivecoringprogram.Forexample,SouthCarolinasampled500milesofpavementsbycoring1,324coresandtested4,503pavementlayers(16).Arandomsample,consistsoftwopavementcores,wastakenfromeverytwo-milesegmentforeachhighwaysectionsampled.Bothtwo-laneandmulti-lanehighways,andHMApavementswithandwithoutopen-gradedfrictioncourses(OGFC)weresampled.Asimilarunbiasedstatewidetestingprogramisrecommended.However,itissuggestedtoobtainatleastthreefour-inchdiametercoresrandomlyfromeachprojecttoobtainpreliminarydataonin-situmoisturecontent,airvoid

content,mixcomposition,tensilestrength,andextentofstripping,ifany.If100projectsareselectedacrossthestate,testingof300coresdoesnotappearunreasonabletoestablishifstrippingisastatewideproblemornot.

Thedatafrom100projectswillnotonlyassessthestatewideaveragefrequencyforsevere

stripping(thatis,visualratingsof2.5and3.0),itwillalsoindicateiftherearesomeotherstatewideproblemstobeaddressedsuchasinadequatecompaction,lackofHMAproductionqualitycontrol,andinefficientsubsurfacedrainagesystems.

Someselectedprojectscanberevisited,sampled,andtestedeveryyeartoassessincreasing

moisture-induceddamage,ifany.GeorgiaDOThasasimilarsuccessfulprogram.

Sincethematerials,mixdesign,constructionpractices,maintenanceproceduresand

climatologicalconditionsvaryfromstatetostate,itisveryessentialthateachstateconductitsownstatewidesurveytoassessandquantifythe"stripping"problemasrecommended.Specifyingantistrippingagentsasan"insurance"withoutestablishingtheextentandcause(s)oftheproblemisnotjustified.Notonlyisituneconomical,itcanalsobeineffectiveiftheunderlyingcausesresponsibleforstrippinghavenotbeenaddressedproperly.

CURRENTPRACTICESFORMINIMIZINGSTRIPPING

TestMethodsNumeroustestmethodshavebeendevelopedandusedinthepasttopredictthemoisturesusceptibilityofHMAmixes.However,notesthasanywideacceptance.Thisisduetotheirlowreliabilityandlackofsatisfactoryrelationshipbetweenlaboratoryandfieldconditions.Only

selectedtestmethodswhicharecommonlyusedbysomeagencieswillbediscussedbriefly.AnoutlineofeachtestisgiveninTables3through7whichhavebeenpreparedbyHicks(1).Thetablesalsosummarizetheadvantagesanddisadvantages(somemodifiedbytheauthor)associatedwitheachtestprocedure.

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Specimens

Compaction

AirVoids(%)

Procedure

DamageAnalysis

Advantages

Disadvantages

Table3.BoilingWaterTests-ASTMD3625(1)

Fieldmixturerepresentation@designAC

NoneNone

-Placeabout950mlofdistilledwaterin1500-2000mlbeaker-Heattoboil,thenaddmixture-Bringmixbacktoboilandholdfor1min-Decantasphaltfromvesselandrefillwithcoldwater

-Visualassessment-


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Table4.NCHRP246-IndirectTensileTestand/orModulusTestWithLottman

Conditioning(1)*

Specimens 9samplesdividedinto3groupsSize:4-in.diameterby2.5-in.height

Compaction ASTMMethods:D1559orD1561orD3387

AirVoids(%) Normally3to5

GroupI: -Waterbathfor5hr-Test**(Unconditioned)

GroupII&III: [email protected](Conditioned) -AtmosphericPressure,submerged,for30min

GroupII: -Testtemperaturewaterbathfor3hr-Test**(Conditioned)

GroupIII: -Freeze@0Ffor15hr(Conditioned) -Waterbath@140Ffor24hr

-Testtemperaturewaterbathfor3hr-Test**

DamageAnalysis Ratios:DiametralResilientModulusTestDiametralTensileStrengthTestGroupII ShortTermGroupIII LongTermGroupI (saturation)GroupI (accelerated)

Advantages

Disadvantages

-Conductedonlabmixes,fieldmixes,orcoresamples-Severetest-Candifferentiatebetweenadditivelevels-Faircorrelationwithfieldperformance-Doesnotgivebiasedresultstowardlimeorliquidadditive

-Timeconsuming-Amountandtypeofequipmentrequiredisnotalwaysreadilyavailable

*Thereareaumberofmodificationstothistestmethod. **Testcanberun@55For73F.

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Table5.ASTMD4867-IndirectTensileTestwithTunnicliffandRootConditioning( 1)

Specimens

CompactionAirVoids(%)

Procedure

DamageAnalysis

Advantages

Disadvantages

6samples-2groupsof3Size:4-in.diameterby2.5-in.height(foraggregate#1in.)

ASTMMethods:D1559orD1561orD33876to8%orexpectedfieldlevel

Sortintogroupssoaverageairvoidsareapproximatelyequal

GroupI:(unconditioned)storedryatroomtemperature

GroupII:(conditioned)soak20min@77F-Test

-Obtaina55%to80%saturationlevel(20in.Hgforabout5minin

distilledwater)-Rejectifsaturationis>80%-Soak24hr@140F-Soak1hr@77F

-Test-DiametralTensileStrength(ASTMD4123)-Visual

-Canuselab,plant,orfieldmixes;alsocoresfromexistingpavements-Mixtureswithorwithoutadditives-Timerequiredismoderate-Initialindicationsshowgoodcorrelation(basedon80%retainedstrength)

-Mayrequiretrialspecimenstoobtainairvoidlevelordegreeofsaturation

-Maynotbesevereenough(majorlimitation)

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Specimens

CompactionAirVoids(%)

Procedure

DamageAnalysis

Advantages

Disadvantages

Table6.AASHTOT283-IndirectTensileTest( 1)

6samples/setofmixconditionsSize:4-in.diameterby2.5-in.height

ASTMMethods:D1559orD1561orD33876to8%orexpectedfieldlevel

Sortspecimensintotwosubsetsofthreespecimens

GroupI:(unconditioned)store@roomtemperature-Placeinwaterbath@77Ffor2hrpriortotesting

GroupII:(conditioned)partialvacuum(20in.Hg)for5minthensoakfor30minoruntilthedegreeofsaturationis55-80%

-Freeze@0Ffor16hrfollowedbysoakingina140Fbathfor24hr-Placeinwater@77Ffor2hrpriortotesting

-DiametralTensileStrength(ASTMD4123)-Visual

-Conductedonlabmixes,fieldmixes,orcoresamples-Severetest-Candifferentiatebetweenadditivelevels-Goodcorrelationwithfieldperformance-Doesnotgivebiasedresultstowardlimeorliquidadditive

-Timeconsuming-Amountandtypeofequipmentrequiredisnotalwaysreadilyavailable

Table7.Immersion-CompressionTests-AASHTOT165orASTMD1075( 1)Specimens

Compaction

AirVoids(%)

ProcedureDamageAnalysis

AdvantagesDisadvantages

6samples-2groupsof3Size:4-in.diameterby4-in.height

Doubleplunger-finalpressure3000psifor2min(ASTM)

Varies

GroupI:Aircured@77F-Test@77F

GroupII:Watercured@120Ffor4daysor140Ffor1day-Test@77F

[email protected]./min

-Usesactualmix-Timerequiredis4daysplus-Poorreproducibility-Airvoidlevelplayssignificantrole-Waterquality(ionsandsalts)canaffectmoisturesensitivity-Equipmentmaynotbereadilyavailable

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QualitativeorSubjectiveTests

1.BoilingWaterTest(ASTMD3625oravariation):LooseHMAmixisaddedto

boilingwater.AlthoughthecurrentASTMD3625-83specifiesone-minuteboiling,

mostagenciesusea10-minuteboilingperiod.Thepercentageofthetotalvisibleareaoftheaggregatethatretainsitsoriginalcoatingafterboilingisestimatedasaboveorbelow95%.ThistestcanbeusedforinitialscreeningofHMAmixes.Someagenciesuseitforqualitycontrolduringproductiontodeterminethepresenceofantistrippingagent.Thistestmethoddoesnotinvolveanystrengthanalysis.Also,determiningthestrippingoffineaggregateisverydifficult.

2.Static-ImmersionTest(AASHTOT182):AsampleofHMAmixisimmersedindistilledwaterat77Ffor16to18hours.Thesampleisthenobservedthroughwatertoestimatethepercentageoftotalvisibleareaoftheaggregatewhichremainscoatedasaboveorbelow95percent.Again,thismethoddoesnotinvolveanystrengthtest.

QuantitativeStrengthTests

1.LottmanTest(NCHRP246):ThismethodwasdevelopedbyLottman(20)undertheNationalCooperativeHighwayResearchProgram246.Ninespecimens(4"diameterand21/2"high)arecompactedtoexpectedfieldairvoidcontent.Specimensaredividedinto3groupsof3specimenseach.GroupIistreatedascontrolwithoutanyconditioning.Group2specimensarevacuumsaturated(26inchesHg)withwaterfor30minutes.Group3specimensarevacuumsaturatedlikeGroup2andthensubjectedtoafreeze(0Ffor15hours)andathaw(140Ffor24hours)cycle.All9specimensaretestedforresilientmodulus(MR)and/orindirecttensilestrength(ITS)at55For73F.Aloadingrateof0.065inch/minuteisusedfortheITStest.

Group2reflectsfieldperformanceupto4years.Group3reflectsfieldperformancefrom4to12years.Retainedtensilestrength(TSR)iscalculatedforGroup2andGroup3specimensasfollows:

AminimumTSRof0.70isrecommendedbyLottmanandMaupin( 20,21)whoreportedvaluesbetween0.70and0.75differentiatedbetweenstrippingandnonstrippingHMAmixtures.IthasbeenarguedthattheLottmanprocedureistooseverebecausethewarmwatersoakofthevacuumsaturatedandfrozenspecimencandevelopinternalwaterpressure.However,Stuart(22)andParkerandGharaybeh(23)generallyfoundagoodcorrelationbetweenthelaboratoryandfieldresults.Oregonhassuccessfullyusedthistestwithmodulusratioinlieuoftensilestrengthratio(TSR).

2.TunnicliffandRootConditioning(NCHRP274):Thismethodwasproposedby

TunnicliffandRootundertheNCHRPProject274(24).Theyproposedsixspecimenstobecompactedto6-8%airvoidcontentanddividedintotwogroupsofthreespecimenseach.Group1istreatedascontrolwithoutanyconditioning.Group2specimensarevacuumsaturated(20inchesHgforabout5minutes)withwatertoattainasaturationlevelof55to80percent.Specimenssaturatedmorethan80percentarediscarded.Thesaturatedspecimensarethensoakedinwaterat140Ffor24hours.AllspecimensaretestedforITSat77Fusingaloadingrateof2inches/minute.AminimumTSRof0.7to0.8isusuallyspecified.Evidently,theuseofafreeze-thawcycleisnotincorporatedintoASTMD4867-88whichisbasedonthismethod.Thefreeze-thawcycleisoptional.Theprimaryemphasisisonsaturation

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ofthespecimenwhichforashortdurationofabout24hourshasbeenreportedtobeinsufficienttoinducemoisturerelateddamage( 25).

3.ModifiedLottmanTest(AASHTOT283):ThismethodwasproposedbyKandhal

andwasadoptedbyAASHTOin1985(26).ItcombinesthegoodfeaturesofLottmantest(NCHRP246)andTunnicliffandRoottest(NCHRP274).Sixspecimensarecompactedto6-8%airvoidcontent.Group1of3specimensisusedasacontrol.Group2specimensarevacuumsaturated(55to80%saturation)withwater,andthensubjectedtoonefreezeandonethawcycleasproposedbyLottman.AllspecimensaretestedforITSat77Fusingaloadingrateof2inches/minute,andtheTSRisdetermined.AminimumTSRof0.7isusuallyspecified.Thismethodisgainingacceptancebythespecifyingagencies.

4.Immersion-CompressionTest(AASHTOT165):Sixspecimens(4"diameterx4"

high)arecompactedwithadoubleplungerwithapressureof3,000psifor2minutestoabout6%airvoidcontent.Group1ofthreespecimensistreatedascontrol.Group

2specimensareplacedinwaterat120Ffor4daysorat140Ffor1day.Allspecimensaretestedforunconfinedcompressivestrengthat77Fusinga0.2inch/minuteloadingrate.Theretainedcompressivestrengthisdetermined.Manyagenciesspecifyatleast70%retainedstrength.Thistesthasproducedretainedstrengthsnear100%evenwhenstrippingisevident.Stuart(13)hasattributedthistotheinternalporewaterpressureandtheinsensitivityofthecompressiontesttoproperlymeasurethemoistureinduceddamage.Lackofsatisfactoryprecisionhasbeenamajorproblemwiththistest.

5.OtherTests:Moisture-vaporsusceptibility,swelltest,andafilmstrippingtestare

usedbyCaliforniaDOT.RetainedMarshallstabilityisusedinPuertoRicoandsomeotherstates.

SurveyofTestMethodsUsedAsurveyoftestmethodsusedintheUnitedStatesandtheireffectivenessinpredictingthemoisturesusceptibilitywasconductedin1989byHicksforNCHRPTopic19-09(1).Figure17showstherelativeeffectivenessofdifferenttestmethodsona0to9scaleaccordingtothissurvey.0meansnoteffectiveand9means100%effective.Briefly,theresultsareasfollows:

No.of AverageRating

TestMethod Agencies

BoilingWater

Static-Immersion(AASHTOT182)Lottman(NCHRP246)

TunnicliffandRoot(ASTMD4867)

ModifiedLotman(AASHTOT283)

Immersion-Compression(AASHTOT165)

Using

9

33

9

9

11

Number

5

47.5

5

7.5

5

DescriptionofEffectiveness

slighttomoderate

slighthigh

slighttomoderate

high

slighttomoderate

AlthoughtheTunnicliffandRootprocedureisusedbynineagencies,onlyfourrateditseffectiveness(rangeof2to8withanaveragevalueof5)apparentlyfromlackofsufficientexperience.

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Figure17.RelativeEffectivenessofMixtureTestProcedurestoIdentifyMoisture-Related

Problems(1)

Evidently,awidevarietyoftestmethodsarebeingusedbyvariousagencies.However,notesthasproventobe"superior"andcancorrectlyidentifyamoisturesusceptiblemixinallcases.KiggunduaridRoberts(27)quantifiedthesuccessrateofsometests,basedontestdataavailablefromvariousresearchreportsandpapers,asfollows:

TestMethod

ModifiedLotman(AASHTOT283)

TunnicliffandRoot(ASTMD4867)

10-MinuteBoilTest

Immersion-Compression(AASHTOT165)

MinimumTestCriteria

TSR=70%TSR=80%

TSR=70%

TSR=80%TSR=70-80%

RetainedCoating85-90%

RetainedStrength75%

%Success

6776

60

6767

58

47

ThedataonsuccessratesindicatesthatmanyHMAmixeswhichmightotherwiseperformsatisfactorilyinthefield,arelikelytoberenderedunacceptableifthesetestsandcriteriaareused.Theuseofthesetestshassimplyencouragedtheincreaseduseofantistrippingagentsinmanystates.

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Therearestillmanyconcernsandrequirementsrelatedtothetestmethodswhichneedtobe

addressed:1.Proliferationoftestproceduresandcriteria.2.Reproducibilityofmosttestmethodsisnotsatisfactory.Forexample,small

variationsinairvoidcontentofthespecimenscansignificantlyaffecttheTSRresults.

3.Needtoconsiderminimumwetstrength(ifthedesiredvaluecanbeestablished)oftheconditionedspecimensratherthanrelyingsolelyontheTSRvalue.Forexample,someadditivesincreasebothdryandwetstrengthsbutmighthavealowTSRvalue.

4.Lackofsatisfactorycorrelationbetweenlaboratoryandfieldperformance.

However,basedontheprecedingdiscussionitappearsthattheModifiedLottmanTest

(AASHTOT283)isthemostappropriatetestmethodavailableatthepresenttimetodetectmoisturedamageinHMAmixes.AminimumTSRof0.70isrecommendedwhenusingthistestmethod.Thiscriterionshouldbeappliedtothefieldproducedratherthanlaboratoryproducedmixes.

StrategicHighwayResearchProgram(SHRP)hastworesearchcontractsdealingwithmoisturesusceptibilityofHMAmixes.SHRPprojectA-003A"PerformanceRelatedTestingandMeasuringofAsphalt-AggregateInteractionsandMixtures"isdevelopinganimprovedtestmethodtoevaluatemoisturesusceptibility.SHRPprojectA-003B"FundamentalPropertiesofAsphalt-AggregateInteractionsIncludingAdhesionandAdsorption"studiedthefundamentalaspectsofasphalt-aggregatebond.ANetAdsorptionTest(NAT)wasdevelopedinSHRPA-003BcompletedbytheNationalCenterforAsphaltTechnology.Thisisapreliminaryscreeningtestformatchingmineralaggregatesandasphaltcement.ConsiderableworkwillberequiredtovalidateSHRPdevelopedtestsinthefield.

AntistrippingAgents

AntistrippingagentsmightbeneededifithasbeenestablishedthataHMAmixisinherentlypronetostrippingbasedontheresultsofthemethodologicalinvestigationsandmoisturesusceptibilitytestsdiscussedearlier.

LiquidAntistrippingAdditives

Mostoftheliquidantistripping(AS)agentsaresurfaceactiveagentswhichwhenmixedwith

asphaltcementreducesurfacetensionand,therefore,promoteincreasedadhesiontoaggregate.ThechemicalcompositionofmostcommerciallyproducedASagentsisproprietary.However,themajorityofASagentscurrentlyinusearechemicalcompoundsthatcontainamines(28).TheseASagentsmustbe"heatstable,"thatis,theyshouldnotlosetheireffectivenesswhenthemodifiedasphaltcementisstoredathightemperaturesforaprolongedperiodoftime.

ThesimplestandmosteconomicalwayistomixtheASagentwiththeasphaltcementinaliquidstatepriortomixingtheasphaltcementwiththeaggregate.Althoughthismethodismostcommonlyused,itisinefficientbecauseonlyaportionoftheASagentreachestheaggregate-asphaltcementinterface.DirectapplicationoftheASagenttotheaggregatesurfaceisundoubtedlythemostefficientandpossiblythemosteffective(1).However,auniformdispersionisnotpossiblebecauseverysmallamountsofASagents(forexample0.5%byweightofasphaltcement)arenormallyused,andtheHMAmixcontainssubstantialamountoffines.

TheamountofASagenttobeusedinimportant.Toolittlemaynotbeeffectiveandtoomuch

maybedetrimentaltotheHMAmix.ThelongrangeeffectivenessofliquidASagentsduringtheservicelifeoftheHMApavementshasnotbeenestablished.

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SomeagenciesmaintainanapprovedlistofASagentsandrequirethecontractorstouseanyAS

agentinallHMAmixeswithoutconductinganymoisture-susceptibilitytest.Thispracticehasmanyseriousdisadvantages.SomeHMAmixesdonotneedanyASagentand,therefore,itisuneconomical(andsometimesdetrimental)tousetheseagents.SomeASagentsareasphalt

cementandaggregatespecificand,thus,arenoteffectiveinallHMAmixesunlessverifiedbytests.ItshouldbelefttothecontractortoselectasuitableASagentanditsdosagetomeetthetestcriteriaofthespecifiedmoisturesusceptibilitytest.SuchcriteriashouldbeconstantlymodifiedtoreflecttechnologicaladvancementsandproductdevelopmentsfromthesuppliersofASagents.

LimeAdditives

UnlikeliquidASagentswhichareusuallyaddedtotheasphaltcement,limeisaddedtotheaggregatepriortomixingwithasphaltcement.Manystudiesindicatethatlimeisaveryeffectiveantistrippingagent.However,itsantistrippingmechanismisnotwellunderstood.Variousmechanismshavebeenpostulated:(a)limeinteractswithacidsintheasphaltcementthatare

readilyadsorbedontheaggregatesurface,(b)limeprovidescalciumionswhichcanreplacehydrogen,sodium,potassiumandothercationsontheaggregatesurface,and(c)limereactswithmostsilicateaggregatestoformacalciumsilicatecrustwhichhasastrongbondtotheaggregateandhassufficientporositytoallowpenetrationoftheasphaltcementtoformanotherstrongbond(1).BothhydratedlimeCa(OH)2andquicklimeCaOareeffective,althoughtheformerismostcommonlyused.Dolomiticlimes(bothTypeSandN)havealsobeenusedasantistrippingadditives.However,asacarbonateCaC03limeisnotaseffective.Generally,1to1%oflimebyweightofdryaggregateisused.Fineraggregatesmayrequirehigherpercentagesbecauseofincreasedaggregatesurfacearea.

Aggregateshavebeentreatedwithlimebythefollowingfourmethods(1):

1.Dryhydratedlime:Themainprobleminusingdrylimeistomaintainitscoatingontheaggregatesurfaceuntilitiscoatedwithasphaltcement.Itismorecriticalindrummixerswhichtendtopickupsomeofthelimeintheexhaustgasflow.However,GeorgiaDOThassuccessfullyuseddryhydratedlimeindrummixersbyinjectinglimeintothedrumjustaheadofasphaltcement.Thepickupoflimebythegasstreamispreventedbymodificationsoftheflightsandprovidingsuitablebafflesinsidethedrum(29).DryhydratedlimecanbeaddedtotheaggregateatdifferentpointsinbatchanddrummixfacilitiesasshowninTable8whichalsoliststheadvantagesanddisadvantages(30).SomeasphaltpavingtechnologistsbelievethatuseofdrylimeisnotconsistentlyeffectivealthoughGeorgiaDOThashadverysatisfactoryresultswithdrylime.

2.Hydratedlimeslurry:ThismethodrequiredadditionalwatertobeaddedtotheaggregateswhichresultsinincreasedfuelcostsandreducedHMAproductionrates.

ThecommonlyusedtechniquesofintroducinglimeasaslurryaregiveninTable9(30).Additionalmixingequipmentisalsoneeded.

3.Dryhydratedlimetowetaggregate:Inthismethoddryhydratedlimeisaddedtowetaggregate,usuallycontaining3-5%water,andthenmixedinapugmillortumblemixertoobtainahomogeneousmix.Drylimecanalsobeaddedtodryormoistaggregateandthensprayedwithwater.Unlessthewatercontentislow,increasedfuelcostsandreducedHMAproductionrateswillresult.

4.Hot(Quicklime)slurry:Theuseofquicklime(CaO)hasatleasttwoadvantages:(a)itscostisequaltothatofhydratedlimebutwhenslakedthehydratedlimeyieldis25%greater,and(b)theheatfromslakingresultsinanelevatedtemperaturewhichhelpsintheevaporationoftheaddedmoisture.Itshouldbehandledwithcautionbecauseitcancauseburnsonhumans.

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Table8.MethodsofIntroducingDryLime(30)

Methods

OntheColdFeed

PremixingPugmill

Advantages

a)BatchMixPlants

-Scalpingscreenandbeltchangescanimprovemixing

-Maximizescoatingoftheaggregate-Minimizeslossesduetodusting

Disadvantages

-Mayproducedustingandsomelimeloss-Mixingandcoatingofaggregatesisminimized

-Somelimelossduetodusting

-Somelimemaybelostinthe

asphaltcement

PugmillPriorto -Maximizesmixingandcoating-SomelimemaybelostintheStockpiling

PriortoStockpilingOntheColdFeed

PremixingPugmill

PriortoStockpiling

PriortoAdding

Asphalt

oftheaggregate-Minimizeslossesdueto

dusting-Limemaybeaddedpriortostockpiling

b)DrumMixPlants


-Maximizescoatingofthe

aggregates-Minimizeslossesduetodusting

-Allowsaggregatedrainage

-Dustlossisminimized

asphaltcement

-Maximizeschanceofcarbonationoccurring-Somelimemaybelostduetoconstruction-Mayproducedustingandsomelimeloss-Mixingandcoatingofaggregatesisminimized

-Somelimelossduetodusting

-Somelimemaybelostinthe

asphaltcement

-Maximizeschanceofcarbonationoccurring-Onlycertainaggregatesmaybetreated

-Notrecommendedwithoutspecialequipment

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Table9.MethodsofIntroducingLimeSlurry(30)

Methods

OntheColdFeed

PremixingPugmill

PriortoStockpilingOntheColdFeed

PremixingPugmill

PriortoStockpiling

OnaSlingerBelt

*Addedbytheauthor.

Advantages

a)BatchMixPlants


-Betteraggregatecoverageandallowsfordrainage-Minimizeslossesduetodusting


b)DrumMixPlants


-Betteraggregatecoverageandallowsfordrainage

-Minimizeslossesduetodusting


-Minimizestheamountofmixing

Disadvantages

-Onlycertainaggregatesmaybetreated-Addinglimeateachcoldfeedbinmayberequired-Somedustlossmayoccurduringdyring

-Highcost*

-Maximizeschanceof

carbonationoccurring-Onlycertainaggregatesmaybetreated-Onlycertainaggregatesmaybetreated-Addinglimeateachcoldfeedbinmayberequired-Somedustlossmayoccurduringdrying

-Highcost*

-Maximizeschanceofcarbonationoccurring-Onlycertainaggregatesmaybetreated

-Maximizestheamountofmoisturetoberemoved

Therelativeeffectivenessoftheprecedingfourtreatmentsbasedona1989surveyisshowninFigure18takenfromReference1.However,comparativelaboratoryandfieldstudieshavebeengenerallyinconclusiveand,therefore,increasedfuelandequipmentcostsanddecreasedHMAproductionratesassociatedwiththewetprocessmaynotbejustifiedatthepresenttime.

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Figure18.RelativeEffectivenessofLimeTreatmentofAggregatebyMethodofLime Addition(1)

SUMMARY,CONCLUSIONSANDRECOMMENDATIONSStrippingofhotmixasphalt(HMA)pavementsappearstohavebecomeamajorprobleminrecentyears.Moreandmorestatesarespecifyingtheuseofantistripping(AS)agents.MoisturesusceptibilityofHMAmixeshasbeenreviewedinthispaperintermsofidentificationoftheproblemandrecommendedsolutions.Thefollowingconclusionsandrecommendationsarewarranted:

1.Externalfactorsand/orin-placepropertiesoftheHMApavementscaninduceprematurestrippinginHMApavements.Aproperknowledgeofthesefactorsisessentialinidentifyingandsolvingthestrippingproblem.Someofthesefactors

whichhavebeendiscussedindetailare:inadequatepavementdrainage(especiallysubsurfacedrainage);inadequatecompactionofHMApavement;excessivedustcoatingonaggregate;inadequatedryingofaggregatespriortomixingwithasphaltcement;useofweakandfriableaggregatesinHMA;overlaysondeterioratedconcretepavements;useofwaterproofinglayersandsealcoatswhenthesourceofthemoistureisfrombeneaththepavement;andpossiblytheuseofopen-gradedasphaltfrictioncourses.Suggestionsforalleviatingtheproblemsassociatedwiththesefactorshavebeengiveninthepaper.

2.Aninvestigativemethodologybasedonforensicexperiencehasbeenrecommendedforusebythespecifyingagenciesandindustrytoestablishstrippingasaproblemonaspecificprojectorstatewide.Detailsofsampling,testing,andinterpretationoftestresults(alongwithexamples)areincluded.Thismethodologywillhelpdeterminethe

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Kandhal 3.

4.5.

6.

cause(s)ofstripping(ifpresent),takeremedialmeasurestoremovethecause(s),andspecifyantistrippingagentsonlywhenabsolutelynecessary.Thecurrentpracticesofspecifyingmoisturesusceptibilitytestprocedures(andacceptancecriteria)andantistrippingagentshavebeenreviewed.Untilmoresuitable

testproceduresaredevelopedandvalidatedbytheSHRP,ModifiedLottmantest(AASHTOT283)hasbeenrecommendedtodeterminepotentialmoisturesusceptibilityofHMAmixes.AminimumTSRof0.70isrecommendedwhenusingthistest.ThiscriterionshouldbeappliedtothefieldproducedratherthanthelaboratoryproducedHMAmixes.Antistripping(AS)agents(bothliquidandlimeadditives)shouldnotbespecifiedacrosstheboardinallHMAmixesand/orfromanapprovedlistofsourcesasan"insurance."Someagentsareaggregateandasphaltspecificand,therefore,maynotbeeffective(andcouldbedetrimentalattimes)inallmixes.ThispracticeisalsouneconomicalbecausesomeHMAmixesareinherentlyresistanttomoisturedamageanddonotneedanyASagent.Variouslaboratoryandfieldstudiesindicatethatlimeisaveryeffectiveantistripping

agentformostaggregates.Limecanbeaddedtotheaggregateindryformorasalimeslurry.Itisgenerallybelievedthatthewetprocessismoreeffectivethanthedryprocess.However,comparativelaboratoryandfieldstudieshavebeengenerallyinconclusiveand,therefore,increasedfuelandequipmentcostsanddecreasedHMAproductionratesassociatedwiththewetprocessmaynotbejustifiedatthepresenttime.Athoroughandfundamentalunderstandingofmechanisms(especiallyasphaltcementaggregateinteractions)involvedinmoistureinduceddamageisnecessarytodevelopimprovedandmorereliablelaboratorytestmethodsandcriteriatopredictmoisturesusceptibilityofHMAmixes.SuchmethodswhicharebeingdevelopedbySHRPwillthenneedtobecorrelatedandvalidatedwithfieldperformance.

ACKNOWLEDGMENTS

Theopinions,findings,andconclusionsexpressedherearethoseoftheauthorandnotnecessarilythoseoftheNCATandAuburnUniversity.

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