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Abituminoussurfacetreatment•Abituminoussurfacetreatment(BST),alsoknownasasealcoatorchipseal,isathinprotectivewearingsurfacethatisappliedtoapavementorbasecourse.BSTscanprovideallofthefollowing:•Awaterprooflayertoprotecttheunderlyingpavement.•Increasedskidresistance.•Afillerforexistingcracksorraveledsurfaces.•Ananti-glaresurfaceduringwetweatherandanincreasedreflectivesurfacefornightdriving.•Historically,BSTshavebeeninusesincethe1920s,primarilyonlowvolumegravelroads.BSTsareincreasinglyinuseasapreventativemaintenanceprocedureonflexiblepavementsofgoodstructure.•Purpose•ABSTofferspreventivemaintenancefromtheeffectsofsunandwater,bothofwhichmaydeterioratethepavementstructure.BSTscreateanewwearingcourse,aswellasawaterproofcoveringfortheexistingpavement.ABSTmakesitmoredifficultforwatertoenterthebasematerial,andpreventingfreezethawdamageforthoselocationswithbelowfreezingtemperatures.•BSTsalsoincreasethesurfacefrictionofthepavement,duetotheadditionofthecoveraggregate.Thiscombatstheeffectsofraveling,whichcanmakethepavementslipperyandstoppingdifficult.ABSTgivesgood,grippingtexturetothepavementsurface.•BSTsshouldbeappliedtoadistress-freetomoderatelydistressedpavementsurface.Atwotofouryearservicelifeiscommon,whilefiveyearservicelivesareachievable.•Materials•BSTsarecreatedusingtwomainmaterials:asphaltandacoveraggregate.•Asphalt•Asphalt(asasphaltbinder,cutbackasphaltorasphaltemulsion)andaggregate(uniformlygraded).Theasphaltistypicallyanemulsion.Caremustbetakenwiththeweatheronthedayofconstruction–ideally,awarmdaywithlowhumidityispreferred.•BSTsshouldneverbeconstructedonrainydaysorwhenrainispredicted.Raincandilutetheasphaltbinderifithasnotyetcured,bringingthebindertothetopofthecoveraggregate;afterthewaterevaporates,tirescanpickupthelooseaggregateortrackbinderacrossthesurface.•CutbackAsphalt•WhilecutbackasphaltishistoricallyanoptionforBSTs,thesolventused(usuallygasolineorkerosene)isexpensiveandpotentiallydangerous.Acutbackisasphaltdissolvedinasolvent,allowingtheasphalttobepumpedandsprayedwithoutheatingittohightemperatures.•Thesolventevaporatesintothesurroundingair,leavingtheasphaltbinder.Oncethesolventhascompletelyevaporated,thecutbackhascured.Fastsettingtimecutbacksusegasoline,whilekeroseneisusedforlongercuringtimes.Cutbackasphaltusagehasdeclinedinrecentyearsduetopollutionandhealthconcerns.•AsphaltEmulsion•Today,asphaltemulsionsaremorecommonlyused.Anemulsionconsistsofanoilsuspendedinwater.Inthiscase,theasphaltcementistheoilcomponent.Asurfactant(alsocalledanemulsifyingagent)needstobeaddedinordertomaketheoilandwatermix.HavingtheasphaltsuspendedinwaterallowstheplacingtemperatureforaBSTtobesignificantlylowerthanforahotmixasphalt.•Asurfactantofferstwobenefits–one,itcausesasphalttoformtinydropletsthatwillsuspendinwaterbyloweringthesurfacetensionbetweentheasphaltandthewater.Two,asurfactantdeterminestheelectricalchargeoftheemulsion.•Aggregatewillhaveanelectricalcharge,usuallynegative.Sinceoppositechargeswillattract,itisimportanttochooseanemulsionwiththeoppositecharge,whichwillenhancethebindingoftheasphalttotheaggregate.Cationic(havingapositivecharge)emulsionsaretypicallyused.•Thenextlargest(byvolume)ingredientinanasphaltemulsionisthewater.Waterformsapproximatelyone-thirdofthevolumeoftheemulsion.Asphaltparticleswill,withthehelpoftheemulsifyingagent,suspendinthewater.•Itisimportanttonotethatiftheemulsionbreaks(whentheasphaltandwaterseparatefromoneanother)theemulsionwillchangefrombrowntoblack.Theaggregatemustbeappliedandrolledbeforetheemulsionbreaks.Whentheemulsionbreaks,thewaterevaporates,andtheasphaltremainsontheroadway.•TheasphaltcementistheprimaryingredientinaBST.Thisasphaltismuchliketheasphaltusedinhotmixpaving.Sometimes,alatexorpolymermodifiedasphaltmaybeusedtoimproveearlychipretentionorBSTdurability.•Aggregate•Aggregatestypicallyusedincludenaturalgravelorcrushedstone.Thesemustbecleananddustfree,hard,anduniform.Thisistoprovideadurablesurfacefortraffic.ItiscommontolimitthepercentofmaterialpassingtheNo.200sieve(0.075mm)toabout1percentorlesssinceexcessivedustcanbeaseriousadhesionissueforBSTs.•AggregateusedinconstructionofaBSTshouldbeplacedonlyonelayerthick.TheexceptionstothisiswhenaddingchokestoneorasecondBSTlayer(atwolayerBSTisoftenreferredtoasa“doubleshot”treatment).•Placingtoomuchaggregatewillcauseaggregatepickup,wherebythewellplacedstonesaredislodged,andmaycauseautomobilewindshielddamage.Asmallamountofexcessaggregate,morethan5%butlessthan10%,maybeplacedinturningandstoppingzones.ThiswillreducetirescuffinginthefreshlylaidBST.•Design•DesignofBSTsmusttakeintoconsiderationallofthepointsmentionedintheAsphaltandAggregatesectionsofthisarticle.Materialmustbeofgoodqualityandthecorrectproperties.•Thedesignmustalsotakeintoconsiderationtheamountofbinderandcoveraggregatetoapply.Inordertopreventexcessivechiploss,about70percentoftheaggregate(andaminimumof50percent)mustbeembeddedintheresidualasphalt–theasphaltleftafterthewaterorcutbackhasevaporated.•Withanasphaltemulsion,thebindermustriseneartothetopoftheaggregate,otherwisetheresidualasphaltwillbe