Case study
Effects of acidic water on hydro-mechanical properties of asphalt concrete
Shan Feng
a, Weibiao Wang
b,*, Hang Tan
c, Meng Xue
d, Kaare Höeg
eaInstituteofWaterResources&HydroelectricEngineering,Xi’anUniversityofTechnology,Xi’an,710048,PRChina
bInstituteofWaterResources&HydroelectricEngineering,Xi’anUniversityofTechnology,Xi’an,710048,China
cSouthwestElectricPowerDesignInstituteCo.,Ltd.OfChinaPowerEngineeringConsultingGroup,Chengdu,610066,PRChina
dShaanxiXixianNewAreaFengxiNewCityDevelopmentandConstruction(Group)Co.,Ltd.,Xianyang,712000,PRChina
eNorwegianGeotechnicalInstitute,NO-0806,Oslo,Norway
ARTICLE INFO Articlehistory:
Received6September2020
Receivedinrevisedform3October2020 Accepted7October2020
Keywords:
Acidicwater Asphaltmaterial Massloss
Degradationofproperties Perviousasphaltconcrete Imperviousasphaltconcrete Durability
Roadpavements Dams
Storageoftailings
ABSTRACT
Asphaltconcreteiswidelyusedinroadandairportpavementsandmorerecentlyalsointhe constructionofimperviousfacingsandcentralcoresindamsanddykes.Asphaltconcrete maybesubjectedtostronglyacidicwaterwhenusedinconnectionwithstorageofmine tailings(tailingsdams).Acidicwaterreactswithbitumenandmineralmaterialsandmay cause deteriorationofthebehavior ofasphaltconcrete withtime. Thepresentstudy investigatedeffectsofacidicwaterwithtimeonaggregate-bitumenadhesion,bitumen penetrationvalue,softeningpointandductility,masschangeofasphaltmaterials,and changeintensilestrengthforporousandimperviousasphaltconcrete.Thetestresults showthatimpervious,hydraulicasphaltconcreteisveryresistanttoacidicwaterandshow insignificant deteriorationof properties during the3-year testing periodused in the experiments.
©2020TheAuthor(s).PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCC BY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).
1.Introductionandscope
Asphaltconcreteiscommonlyusedinroadandairportpavements,andasdescribedintheliteraturereferredtobelow, researchhasbeendonetostudytheeffectsofacidrainonthepropertiesofasphaltconcreteusedforthatpurpose.In connectionwiththestorageofminetailings,asphaltconcreteisalsobeingusedinimperviousupstreamfacings(linings)or centralcoresofdams,wheretheasphaltconcreteisexposedtoacidicminedrainage(AMD).
Theasphaltconcreteusedforroadpavementsisusuallyrelativelypervious[1],whilethehydraulicasphaltconcreteused indamswithairporosityoflessthan3%isrelativelyimperviouswithpermeabilityabout10 10m/s [2–5].Verylittle researchhasbeendonetostudythedurabilityofasphaltconcreteusedfortailingsstoragefacilities[6]andthedamdesigner hassofarattemptedtocompensateforthepotentialmasslossanddegradationofhydro-mechanicalpropertieswithtimeby adding amore-or-lessarbitrary thicknesstothedamliningor core.Thescopeandpurposeofthepresentstudyis to investigatetheeffectsofacidicwateronthestrengthproperties,theresistancetocracking,durability,andwatertightnessof hydraulicasphaltconcrete.
*Correspondingauthor.
E-mailaddresses:[email protected](S.Feng),[email protected](W.Wang),[email protected](H.Tan),[email protected] (M.Xue),[email protected](K.Höeg).
https://doi.org/10.1016/j.cscm.2020.e00445
2214-5095/©2020TheAuthor(s).PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/
licenses/by-nc-nd/4.0/).
ContentslistsavailableatScienceDirect
Case Studies in Construction Materials
j o u r n al h o m e p a g e :w w w . el s e v i e r . c o m / l o c a t e / c s c m
2.Overviewofpreviousexperimentalinvestigations
Zhangetal.[7–9]performedacidicwatercorrosiontestsonasphaltconcreteappliedinroadpavementsusingwater solutionswithpH=1,3,and5.6bymixingH2SO4andHNO3withmoleratioof3:1tosimulatetheacidrain.Marshallasphalt specimenswith4differentmixeswereexposedto4cyclesof7days(6-dayimmersionplus1-daydrying).Thetestresults indicatedthatafter4cyclesthemasslossandvoidcontentofthespecimenshadincreased,whiletheMarshallstabilitywas reducedwiththereductionofpHvalue.
Wang[10]didasystematicinvestigationoftheeffectsofacidrainonbitumenandmineralmaterials.Hedescribedthe mechanismofacidicwaterreactingwithbitumenandmineralmaterialsanddetailsarepresentedbelowinthesectionon masschangeofasphaltmaterialscausedbyacidicwatercorrosion.Fengetal.[11]performedacidicwatercorrosiontests onbitumenspecimensusingacidicwatersolutionswithpH=2and4bymixingH2SO4andHNO3withmoleratioof9:1to simulatetheacidrain.Resultsfromtestsinneutralwater(pH=7)wereusedascomparison.Bitumenspecimenswere subjectedtocyclesof8days(7-dayimmersionplus1dayofdrying).Theacidityofthewatersolutionswaskeptconstant duringthetesting,andthebitumenpropertiesweremeasuredaftereverycycle.Thetestresultsindicatedthatafter5 cyclesthepenetrationvalueofthebitumenspecimensinthetwoacidicwatersolutionsincreasedbyonly2%.Theviscosity reducedby6.6%and4.4%,thesofteningpointincreasedby7.9%and6.5%,andtheductilitywasreducedby17.5%and15.8
%,in thepH=2and4 solutions,respectively. Thefour-componentanalysisindicated thatthe saturatedhydrocarbon content in the bitumen increased significantly, the aromatic hydrocarbon reduced slightly, the gel content reduced significantly,andtheasphaltenecontentincreasedslightly.Thefourcomponentsin thebitumenspecimenswerenot significantlychangedafterthespecimenshadundergone5cyclesintheneutralwater(pH=7).Theinfraredspectrum analysis indicated thatbefore andafter the specimens hadundergone 5 cyclesin the acidic water the locations of absorptionpeakswerethesameandtheshapesweresimilar.However,thestrengthsoftheabsorptionpeakschanged significantly,whichindicatedthatchemicalreactionhadtakenplace betweenthebitumen and acidicwater butnot produced a new functional group. The saturatedhydrocarbon content, aromatic functional group content, sulfoxide compounds,sulfiteorsulfatecontentincreasedslightlyintheacidicwaterwhiletheyreducedslightlyintheneutralwater (pH=7).
Zhangetal.[7–9]alsoinvestigatedthechemicalcorrosionmechanismanddidthermodynamicanalysisofacidrainon alkaliandacidicaggregates.Theyconcludedthatacidsubstancesinacidicwaterreactedwithacidicandalkalinesubstances intheasphaltmixtureandproducedwatersolublesalts.FranzoniandSassoni[12]investigatedthecorrelationbetween aggregatemicrostructuralcharacteristicsand materialdegradation(intermsofmass loss)in acidicwatersolutions to simulateacidrainwithpH=4,5,and5.6(cleanrain).Aftera14-dayimmersion,agoodcorrelationcoefficientr=0.91 0.93 (dependingonpHofsolution)wasfoundbetweenmasslossandtheproductofcarbonatecontentandspecificsurfaceareain thestartingmaterials.
Chandra [13] studied the hydrochloric acid attack on cement mortar specimens with dimensions of 40mm40mm160mm(cement:sandratio1:3).Afteraperiodofimmersionintheacidwater,thespecimens were takenoutandcutinslicesforanalyses.Aringofironwasnoticedtosurroundandprotecttheinsidemortarfromfurther deterioration. Liu etal. [14] putmarbleand redsandstonespecimens (ɸ50mm25mmand ɸ50mm100mm)in a hydrochloric acid water solution (pH=4) for 30 days. The mass losses of marble and red sandstone specimens (ɸ50mm25mm)were 8%and 4%, respectively.The compressive strengthsof marbleand red sandstone specimens (ɸ50mm100mm)reducedby52%and41%,respectively.Theindirecttensilestrength(Braziliansplittingtest)ofmarble andredsandstonespecimens(ɸ50mm25mm)reducedby15%and11%,respectively.Zhongetal.[15]putlimestone specimens(ɸ50mm100mm)innitricacidwatersolutions(pH=3and5)andinneutralwater(pH=7)for30days.The void contentsincreasedby 0.18and 0.03 percentagepoints forthespecimens in pH=3 and 5solutions, respectively.
Comparedwiththemechanicalbehaviorofthespecimensinneutralwater,thecompressivestrengthsreducedby25%and 10%whilethefrictionanglesandcohesionparametersreducedbyabout20%and11%,forthespecimensinthepH=3and5 solutions,respectively.
Xiaetal.[16]put7kindsofcarbonaterockspecimens(ɸ38mm8mm)withdifferentformationagesinAMD(pH=2.3) fromanabandonedmine.Threespecimensofeachofthe7kindswereimmersedin3000mLAMDfor5,10,15,25,30,and60 days,respectively.Massloss(erosion)wasmeasuredatthepredeterminedtimes.Scanningelectronmicroscopeandan energydispersivespectrometerwereusedafterthespecimenshadbeenimmersedintheAMDfor30days.Theerosionrate was intherange 22 3110 4 g/cm2/dayatthe5thdayand4 810 4 g/cm2/dayatthe60thdayforthe7kindsof specimens.Theerosionratewasreducedinthefirst30daysfromanaveragerateofabout2510 4g/cm2/dayatthe5thday toanaveragerateofabout1010 4g/cm2/dayatthe30thday.InadditiontotheincreaseofpHvalueinthe3000mLAMD duetothechemicalreactions,thecrystallizationrateofcalciumsulfateontherocksurfaceswasoneofthekeyfactors reducingtheerosionrateofcarbonatiteinthecarbonaterocks.Thedissolutionofmagnesiumcarbonate(Mg2+)wasan importantfactortoreducethecrystallizationrateofcalciumsulfate.
Melladoetal.[17]evaluatedtheresistancetoacidattackofordinaryPortlandcement(OPC)andalkali-activatedpastes.
ThetestresultsshowedthatOPCpastesufferedthemostdegradation,whilethealkaliactivatedpasteswiththelowest calciumcontenthadthebestperformance.
3.Materialsandmethodologies
Forthepresentstudy,thewidely-usedChinesebitumentypeB90fromtheKelamayirefinery(denotedbyK)andfromthe refineryofChinaNationalOffshoreOil Corporation(denoted byZ)wereselected.Commonlimestonefiller (lessthan 0.075mm),crushedlimestonesand(0.075mm–2.36mm)andaggregates(2.36mm–19mm)wereused.Testmethodsand proceduresfollowedtheChinesestandardandtestcodeforbitumenandasphaltconcreteforhighwayengineeringand imperviousfacingsandcores[18,19]withsomemodificationstosuitthespecificstudy.
3.1.Materials
ThepropertiesofthebitumenfromthetworefineriesareshowninTable1.Chemicalcomponentsoflimestoneinfiller, sandandaggregatesareshowninTable2.Theasphaltconcretemixusedforanimperviouscoreinanembankmentdambuilt inChinawasusedinthisstudyandisshowninTable3.Fig.1showstheaggregategradationcurve.
TheacidityofwatersolutionswithpH=2,4,and6wasadjustedbyusingH2SO4.Andneutralwater(pH=7)wasusedas reference.
3.2.Methodologies
(1) Testtodeterminebitumen-aggregateadhesion
The aggregate-bitumen adhesion (bonding) is known to have a very important effect on the hydro-mechanical propertiesofasphaltconcrete,forinstancethetensionandcrackingresistance.Wateris knowntohaveapotentially detrimentaleffectontheadhesion,andthesusceptibilitytomoistureisthereforeconsideredtobeagoodindicatorofthe bondingstrength.
OneofthemostusedcontrolmethodsistheTexasBoilingTest(ASTMD3625 2000),butthetestsconductedinthispaper followed the above-mentioned Chinesespecifications. Using the Chinesecode, one classifies thequality of aggregate adhesionintofivegrades.Fivecleanaggregateparticles,size13.2mm–19mm,areheatedandcoveredwithbitumen.The particlesarecooledandthenboiledinwaterfor3min.Thequalityofaggregateadhesionisclassifiedbytheobservationsof theconditionsofthebitumenfilm(membrane)leftontheparticlesafterboiling,asshowninTable4.
Inordertoinvestigatetheeffectofacidicwateronaggregate-bitumenadhesionthetestprocedurewasslightlymodified.
Limestoneaggregateparticles,size13.2mm–19mm,andbitumentypeB90fromtheKelamayirefinery(denotedbyK)were usedinthetest.Theinitialgradeoftheaggregate-bitumenadhesionwasGrade5.Fourglassbottleswithvolumeof750mL wereseparatelyfilledwith450mLacidicwaterwithdifferentpHvalues.Ineachbottletherewereput6bitumen-covered aggregateparticlessubmergedinthewater.Thebottlesweresealedandkeptat20C,andtheconditionsofthebitumen- coveredaggregateparticleswereobservedwithtime.Eachweekonebitumen-coveredparticlefromeachofthefourbottles wastakenoutandtheBoilingTestwasperformed.TheacidityofthewaterinthefourbottleswasmeasuredbypHmeter.
(2) Testofeffectsofacidicwateronbitumenproperties
Thepenetrationvalue,softeningpointand ductilityaretheimportantpropertiesofbitumenand thetestmethods followedtheabove-mentionedChinesespecifications.
Inorder toinvestigatetheeffects ofacidic wateron theproperties ofthe bitumenwith time,twenty-two (22) bitumenspecimens(B90,Kspecimen)300mmindiameterandabout3mmthickwereprepared.The3mmthicknessof bitumenspecimenswasdesignedbyreferringtothethinfilmoventest(TFOT).Inordertoincreasethetestaccuracy,the
Table1
PropertiesofthebitumenB90[4,20,21].
Testproperties Standard K Z
Penetration(25C,100g,5s)/(0.1mm) Ductility(15C,5cm/min)/(cm) Softeningpoint(TR&B)/(C) Density(25C)(g/cm3) TFOT
Lossbymass(%) Residualpenetration(%) Residualductility(cm) Fourcomponents(%) Saturatedhydrocarbon Aromatichydrocarbon Gel
Asphaltene
80 100
≧150 45-52 –
0.8%-+0.8%
≧70
≧100 – – – –
87
>150 46 0.982 0.01 86
>150 27.9 31.7 38.7 1.7
86
>150 46 1.006 0.10 72
>150 21.2 32.6 40.4 5.8
diameterofthebitumenspecimensinthisstudywaslargerthanthatinTFOT(300mmvs.140mm).Onegroupof11 bitumenspecimenswasputinacidicwaterwithpH=2andanothergroupof11bitumenspecimenswasputinneutral waterwithpH=7.Theacidityofthewaterwaskeptconstantandthetemperaturewas20C.Onespecimenofeach groupwastakenoutfortesting.
(3) Testofmasschangeofasphaltmaterialsbyacidicwatercorrosion
Thetest method was designed inthis study tomeasurethe mass change of differentcombinations of asphalt materials by acidic water corrosion. Bitumen type B90 from the tworefineries were used. Three kinds of asphalt material specimens were prepared, i.e., pure bitumen, asphalt mastic, and sandy asphalt mastic. Asphalt mastic consistedofbitumenandlimestonefiller(lessthan0.075mm),whilesandyasphaltmasticconsistedofbitumen,filler, andcrushedlimestonesand(0.075mm–2.36mm).Tenkindsofasphaltmaterialspecimensweretestedasshownin Table5.
Acid-resistantborosilicatedishesofinsidediameter100mmwereusedtoholdthespecimens.Eachdishwasweighed andlabeled.Thehotbitumenandasphaltmasticwereseparatelypouredintothedishes.Thethicknessofthethinbitumen andasphaltmasticspecimenswasdesignedtobeasthinas2mm.Thethicknessofthesandyasphaltmasticspecimenswas 5mmbecausethemaximumsizeofthecrushedsandparticleswas2.36mm.Aftertheasphaltmaterialscooleddown,each dishwiththespecimenwasweighed,andtheoriginalweightofeachspecimenwasobtainedbydeductingtheweightofthe dish.Allthedisheswithspecimenswereputinapolyethylene(PE)plasticcontainerthatwasfilledwithacidicwaterwith pH=2andkeptat20C.Onlythetopfaceofthebitumenandasphaltmasticspecimenswasincontactwiththeacidicwater, whileboththetopandbottomfacesofthesandyasphaltmasticspecimenscontactedtheacidicwaterbyusingaspecial supportunderthebottomfaceofthespecimens.Thecorrosionprocesslasted16monthswhiletheacidityofthewaterwith pH=2waskeptconstant.Atthepredeterminedtimesthedisheswiththespecimens,butwithoutthewater,wereweighed.
Afterweighing,thedisheswithspecimenswereputbackinthecontainer.Themasschangeofeachspecimenwascalculated inpercentbycomparingwiththeoriginalweight.
Table2
Chemicalcomponentsoflimestoneinfiller,sandandaggregates.
Component CaO SiO2 Fe2O3 Al2O3 MgO TiO2
Content(%) 27.08 8.82 0.14 1.54 19.28 0.07
Component MnO FeO K2O P2O5 Na2O Lossonignition
Content(%) 0.02 0.54 0.76 0.02 0.04 41.69
Table3
Asphaltmixdesignusedinthisstudy(%ofmineralweight).
Sievesize(mm) Bitumencontent
13.2 19 9.5 13.2 4.75 9.5 2.36 4.75 0.075 2.36 0 0.075
(Filler)
13 10 19 12 33 13 6.9
Fig.1.Aggregategradationcurveusedinthisstudy.
(4) Testofeffectofacidicwaterontensilestrengthofasphaltconcrete
Theindirecttensiletest(Braziliansplittingtest)wasusedtogiveaquantitativemeasureofthecrackingresistanceand tensilestrength.Thesizeofthecylindricalspecimenswas100mmindiameterand63.5mmhigh(Marshallspecimensize).
The test method followed the above-mentioned Chinese specifications. Indirect tensile tests were carried out at a displacementrateof 1mm/minat20C togetthetensilestrengthand thecorrespondingtensilestrainbefore acrack opened.
In ordertoinvestigatetheeffectofacidicwateronthebehaviorofasphaltconcrete,twokindsofasphaltconcrete specimensweretested,i.e.,porous(pervious)andimperviousasphaltconcretes.Theporousasphaltconcreteallowswaterto penetrateintotheasphaltporeswhiletheimperviousasphaltconcretepreventssignificantpenetration.Watersolutions withpH=2and7wereusedtocomparetheeffectsonthebehavioroftheporousasphaltconcrete,andwatersolutionswith pH=2,4,6,and7wereusedfortheimperviousasphaltconcrete.TheasphaltconcretemixisshowninTable3.Theporous asphaltspecimenswereobtainedbycompactingwith15blowsoneachfacetoyieldspecimenswithvoidcontent(air porosity)intherange3%–7%,thatistypicalfortheasphaltusedinroadandairfieldpavements.Theaveragevalueofthe porosityofalltheporousasphaltspecimenswas5%.Theimperviousasphaltspecimenswereobtainedbycompactingwith 35blowsoneachfacetoyieldspecimenswithporosityintherange1%–2%.Theaveragevalueoftheporosityofallthe imperviousasphaltspecimenswas1.5%.Thisistypicalfortheporosityoftheasphaltusedinthecoreofembankmentdams.
ThetestprogramfortheporousasphaltconcreteisshowninTable6.Table7showsthetestprogramfortheimpervious asphaltconcrete.
Forty-five(45)porousasphaltspecimensandninety-nine(99)imperviousasphaltspecimenswereseparatelyputin polyethylene(PE)plasticcontainersfilledwithacidicwaterwithdifferentpHvalues.Theacidityofthewaterwaskept constantat20C.
Atthepredeterminedtimes(every3monthsinthefirst15–24months)theasphaltspecimensweretakenout.Each specimenwasonlysubjectedtotheindirecttensiletestonce.
4.Testresultsandanalyses
4.1.Effectofacidicwateronaggregate-bitumenadhesion
ThetestresultsareshowninFig.2.Theresultswithneutralwater(pH=7)weretakenasreferencevalues.Afterthe6 bitumen-coveredaggregateparticles wereputintheacidic waterwithpH=2,chemical reactions withsmallbubbles occurredattheedgesandcornersoftheparticles.Thebubblesfirstgrew,butthentheirsizeandnumberdecreasedandthey disappearedwithtime.Someoftheaggregateedgesandcornersbecamebare,andthesurfacecolorbecamedarkbrowndue Table4
GradeofaggregateadhesiontobitumenaccordingtoChinesecode[19,22].
Conditionofbitumenfilmleftontheparticlesafter3-minboiling Grade
Completeaggregatesurfaceareastillcoveredbybitumen,andnofilmisremoved 5
Lessthan10%ofthebitumen-coveredareaisremoved,andthefilmisnotsignificantlydeformed 4 Lessthan30%ofthebitumen-coveredareaisremoved.Partofthefilmissignificantlydeformedbutstillstaysontheaggregatesurfaces 3 Morethan30%ofthebitumen-coveredareaisremoved.Asignificantpartofthefilmisdeformedbutstaysontheaggregatesurfaces 2 Theaggregatesaresubstantiallybare,andtherestofthebitumenfilmistotallydeformedorfloatsinwater 1
Table5
Tenkindsofasphaltmaterialspecimens.
Name Sample Massratio
(bitumen:filler:sand)
Thickness (mm)
Bitumen K1,K2
Z1,Z2
1:0:0 2
Asphaltmastic KF1 1,KF1 2
ZF1 1,ZF1 2
1:1:0 2
KF2 1,KF2 2 ZF2 1,ZF2 2
1:2:0 2
KF3 1,KF3 2 ZF3 1,ZF3 2
1:3:0 2
Sandyasphaltmastic KFS
ZFS
1:2:2 5
Note:KandZmeanthebitumenfromKelamayirefineryandfromtherefineryofChinaNationalOffshoreOilCorporation,respectively.
FandSmeanfillerandsand,respectively.
KF1 1meanssampleNo.1ofasphaltmasticmixNo.1usingKbitumen.
totheacidiccorrosion.Fig.2ashowsthattheGradeoftheaggregate-bitumenadhesionintheacidicwaterwiththeoriginal pH=2wasreducedfrom5to4inoneweek,andGrade4waskeptinthefollowing3weeks.Similarly,chemicalreactionsand smallbubblesoccurredatedgesandcornersoftheparticlesintheacidicwaterwiththeoriginalpH=4.However,thesize andnumberofbubblesweresmallerthaninthewaterwithpH=2.Fig.2ashowsthattheGradeoftheadhesionintheacidic waterwiththeoriginalpH=4wasreducedfrom5to4in2weeks,andGrade4waskeptinthefollowing2weeks.Therewere nobubblesdevelopedintheacidicwaterwiththeoriginalpH=6andneutralwaterwithpH=7,andthebitumenfilmonthe aggregateswasnotchanged.Grade5waskeptinthe4weeksoftesting.
Table6
Testprogramandnumberofporous(pervious)asphaltspecimensinthisstudy.
pH Time(Month)
0 3 6 9 12 15 25
2 3 3 3 3 3 3 6
7 – 3 3 3 3 3 6
Table7
Testprogramandnumberofimperviousasphaltspecimensinthisstudy.
pH Time(Month)
0 3 6 9 12 15 21 24 38
2 3 3 3 3 3 3 3 3 3
4 – 3 3 3 3 3 3 3 3
6 – 3 3 3 3 3 3 3 3
7 – 3 3 3 3 3 3 3 3
Fig.2. Gradeofaggregate-bitumenadhesionandacidityinthe450mLacidicwaterversustime.
Whenaggregateparticleswerecoveredwithbitumen,thebitumenfilmatsomeedgesandcornersoftheparticleswere thin.Acidicwaterwouldcontacttheparticlesthroughinterconnectedvoidsinthethinbitumenfilm.ReactionofH2SO4in thewatersolutionwithCaOandMgOinthelimestoneaggregateparticlesoccurredandproducedbubbles.Fig.2bshowsthe increaseintheacidityofthe450-mLwatersolutionswithtime.Theincreasingacidityofthewaterdocumentsthattheacidic waterreactedwiththeaggregatesbypenetrating(diffusing)throughthebitumenfilms.
Experimental results have documented that bitumen modified with anti-stripping agent increases the bitumen- aggregateadhesion[22].Polymermodifiedbitumenmayincreasetheresistanceofbitumentoacidicwater.
4.2.Effectsofacidicwateronbitumenproperties
Fig.3showsthetestresults.Fig.3ashowsthatthepenetrationvalueofthebitumenspecimensafterbeingimmersedin waterwasonlyveryslightlyreducedcomparedwiththeoriginalvalue.Thisreductionmaybecausedbythereheatingofthe specimens toformsamplesfortestingthepenetrationvalue.Fig.3b showsnochangeinthesofteningpoint between bitumenspecimensinacidicwaterandneutralwaterafter11months.Fig.3cshowsthattheductilityofbitumenspecimens inacidicwaterandneutralwaterafter11monthsdidnotsignificantlychange.Itshouldbenotedthatthetestmethodfor ductility unavoidably gives some scatter in the results, and themaximum range of the ductilimeter was 150cm. In conclusion,theacidityofwater(pH=2)didnotcausesignificantchangesinthebitumenpenetrationvalue,softeningpoint andductilityduringthe11-monthtestingperiod.
4.3.Masschangeofasphaltmaterialsbyacidicwatercorrosion
Fig.4showsthetopfaceofthespecimensafter16monthsintheacidicwaterwithpH=2.Fig.3ashowsthatthesurface brightnessofthetwokindsofbitumenspecimenshadbeenreducedafterbeingimmersedintheacidicwaterfor16months.
ThefaceoftheZbitumenspecimenlookedbrighterthantheKbitumenspecimen,whichmightimplythatthereactionor corrosionoftheacidicwaterwiththeZbitumenwasstrongerthanwiththeKbitumen.
Fig.4b,c,dshowthattinypinholeshaddevelopedinthefacesoftheasphaltspecimenswithdifferentratiosofbitumento filler,i.e.,1:1,1:2,and1:3byweight,respectively,afterbeingimmersedintheacidicwaterfor16months.Thesizeand numberofpinholesonthespecimenfacesincreasedwithtime.Thenumberofpinholesonthefaceoftheasphaltmastic specimenincreasedwithincreasingfillerratiofrom1:1(bitumen:filler)to1:2to1:3.
Fig.4eshowsthatpinholeshadalsodevelopedonthefacesofsandyasphaltmasticspecimens.Thesizeofsomepinholes wasaslargeas2 3mmindiameteraffectedbythemaximumsizeofthesandparticlesof2.36mm.
Theobservationsonthespecimenfacesindicatethatacidicwaterreactedwiththebitumenandthelimestonefillerand withthesandbypenetratingthethinbitumenfilmscoveringthemineralparticles.
Fig.5showsthemasschangeofthebitumen,asphaltmasticandsandymasticspecimenswithtimeintheacidicwater.
Masschangeisdefinedasspecimenweightafterimmersionminusoriginalweight,dividedbyoriginalweight.
Massincreaseorlossofbitumeninacidicwaterdependsonthemasschangesof thefourcomponentsofbitumen (Table1).AsdiscussedbyWang[10],whenbitumenisimmersedinacidicwater,esterificationandalkylationreactionstake place betweenbitumenandsulfuric acidandproduce isomericalkanes withlongerchainsthat causetheincrease of saturated hydrocarbon in bitumen. Oxidative condensation of thioether and mercaptan compounds takes place and produces sulfoxidecompounds andsulfuric dioxidethatcauses theincreaseof asphaltene.The increasesofsaturated hydrocarbonandasphalteneincreasethemassofbitumen.However,solutionandionizationofcarboxylicacids,phenols, andotheracidicsubstancesinbitumentakeplacetosomeextentinacidicwater,whichcausesthedecreaseofgelanda decreaseinthemassofbitumen(Wang2004).Totalmassincreaseorlossofbitumenisdeterminedbythesumofthe increasesinsaturatedhydrocarbonandasphalteneandthedecreaseingel.
Forlimestone,sulphuricacidreactswithcalciumcarbonateandotheralkalinesubstancesandproducessulphate,which causesmasslossoflimestone[10].
Fig.5showsthemassincreaseofpureKandZbitumenspecimenswithtimeinacidicwaterandthatthemassofK bitumenspecimensincreasedmorethanZbitumenspecimens.ThisisinagreementwiththeobservationinFig.4ainwhich thereactionoftheacidicwaterwiththeZbitumenseemedstrongerthanwiththeKbitumen.Fortheasphaltmasticand sandyasphaltmasticspecimenstheacidicwatercausedmassincreaseofbitumenbutcausedmassdecreaseoflimestone.
Themasschangesofasphaltmasticandsandyasphaltmasticspecimensdependonthevolumetricratioofbitumento limestone.Table8showsthevolumetricratioofbitumentolimestonebasedonthemassratioofbitumentolimestone(filler andsand)andthedensitiesofbitumenandlimestone.
BasedonthetestresultsshowninFig.5,themasschangesofthebitumen,asphaltmastic,andsandyasphaltmastic specimensversustimewereassumedtobeapproximatelylinear.Usinglinearregressionforallthetestresults,themass changerateofbitumenandlimestoneintheacidicwaterwas0.065%/monthfortheKbitumen,0.033%/monthfortheZ bitumen,and-0.115%/monthforthelimestoneintheasphaltandsandyasphaltmasticspecimens.
Thetestresultswithlinear“fitting”linesarecomparedandshowninFig.6.Itshouldbenotedthatitwasonlyonefaceof thebitumenandasphaltmasticspecimensincontactwiththeacidicwater,whileitwastwofacesforthesandyasphalt masticspecimens.Therefore,thecalculationsforthesandyasphaltmasticspecimensweremadeforonlyonefacetodirectly comparewiththebitumenandasphaltmasticspecimens.
4.4.Effectsofacidicwateronthetensilestrengthofasphaltconcrete
(1) Effectsonporous(pavement)asphaltconcrete
Theneutralwater(pH=7)hadinsignificanteffectontheasphalttensilestrengthwithtime,andthetestresultsforthe specimensintheneutralwaterweretakenasreferencevaluesforthespecimensintheacidicwater(pH=2).Fig.7showsthe strengthratiosofasphaltspecimenintheacidicandneutralwaterwithtime.Thestrengthratioisdefinedasthetensile strengthofthespecimenimmersedinacidicwaterdividedbythestrengthofthespecimeninneutralwater.Fig.7alsoshows thecorrespondingratiobetweentensilestrainacrossthespecimendiameteratfailurewhenthespecimenfailedintension (splitting).
Fig.3. Comparisonofpenetrationvalue,softeningpoint,andductilityofK90bitumenspecimensinacidicwaterwithpH=2andinneutralwaterwith pH=7.
Fig.4.Topfacesofbitumen,asphaltmastic,andsandyasphaltmasticspecimensafter16monthsintheacidicwaterwithpH=2at20C.Thelabelmeaning isshowninTable5.
Fig.7shows thatthe strengthratio wasreducedby about40% duringthefirst9monthsintheacidicwater (pH=2).Afterthattime thestrengthratiostayednearlyconstant.Thestrainratioshowedsomescatter,buttherewasanabout20%increaseafter9months.
Theacidicwaterpenetratesintothevoidsintheasphaltspecimenswithvoidporosityabout5%andthusreactswiththe bitumenand thelimestone.In additiontodegradingthepropertyofbitumenandlimestone,theacidreacts withthe suboxideinthelimestoneandproducesanemulsifyingagentcausingthebitumenfilmtopeeloffthelimestoneparticles [10].Thiswouldcausethedecreaseinstrengthratioduringthefirst9months.After9monthsthestrengthratiostayed nearlyconstant,probablybecausetheacidinthewaterinthevoidswouldbeconsumedwithtimeduetothechemical reactionwithbitumenandlimestone.Theacidityofthewaterinthespecimenvoidswouldthereforereducealthoughthe acidityintheacidicwaterinthecontainersurroundingthespecimenswaskeptconstant.Thewaterinthevoids with reducedacidityhadinsignificanteffectsontheasphaltconcretestrengthafterthefirst9months.Thereductionofacidityin acidicwaterisillustratedinFig.2bandisdiscussedinreference[16].
(2) Effectsonimpervious(hydraulic)asphaltconcrete
Fig.8showsthecorrespondingstrengthandstrainratiosfortheimperviousasphaltspecimensintheacidicwater(pH=2, 4,and6).
Fig.5.Masschangeofbitumen,asphaltmasticandsandymasticspecimenswithtimeinacidicwaterwithpH=2at20C.
Table8
Volumetricratioofasphaltmaterialspecimens.
Name Sample Massratio
(bitumen:filler:sand)
Volumetricratio (bitumen:limestone)
Bitumen K
Z
1:0:0 1:0
Asphaltmastic KF1
ZF1
1:1:0 0.74:0.26
0.73:0.27 KF2
ZF2
1:2:0 0.58:0.42
0.57:0.43 KF3
ZF3
1:3:0 0.48:0.52
0.48:0.52
Sandyasphaltmastic KFS
ZFS
1:2:2 0.41:0.59
0.40:0.60 Note:SamplemeaningisshowninTable5.
Fig.6.Comparisonoftestresultsandlinearfittinglinesformasschangesofbitumen,asphaltmastic,andsandymasticspecimenswithtimeinacidicwater withpH=2at20C.
Fig.8showsthattheacidityofthewaterhadnosignificanteffectonthecrackingresistanceandtensilestrength.After threeyearsofimmersioninstronglyacidicwater,thetensilestrengthwasaboutthesameasthatofasphaltconcreteinthe neutralwater(pH=7).Themainreasonforthatisthattheacidwaterdidnotpenetrateintothealmostimperviousasphalt specimensanddidnotdegradethebitumenandtheaggregate-bitumenadhesion(bonding).
5.Discussionofresultsandpracticalimplications
Theeffectsofacidicwateronasphaltconcretedependsontheacidityofthewater,typeofbitumen,typeofmineral materialsinaggregatesandfiller,asphaltmix,voidcontentofasphaltconcrete,temperatureandtime.Theacidicwater affectsasphaltconcreteinmainlythreeways.
Fig.7. Strengthandstrainratiosforporousasphaltspecimensintheacidic(pH=2)waterwithtimeat20C.
Fig.8.Strengthandstrainratiosfortheimperviousasphaltconcretespecimensintheacidic(pH=2,4,and6)waterwithtimeat20C.
(1)Acidicwaterreactswithbitumentodegradeitsproperties.Thetestresultsinthestudy showthattheacidicwater(pH=2) causedbitumenmass increase of 2-mmthickspecimens at arateof 0.78%/year (0.065%/month)forK90bitumenand0.40 %/year (0.033%/month)forZ90bitumen,respectively.If100%massincreaseisassumedtocorrodeallthebitumen,itwouldtakeabout hundredyearsfortheacidicwater(pH=2)tocorrodeallofthe2-mmthickbitumenspecimensinthetestprogram.
(2)Acidicwaterreactswiththemineralmaterialstodegradethepropertiesofthefillerandaggregatematerials.Therate ofdegradationdependsonthekindofmineralmaterials.Forthelimestoneusedinthestudy,theresultsshowthattheacidic water(pH=2)causedlimestonemasslossatarateof1.4%/year(0.115%/month).Itwouldtakeabout60yearsfortheacidic watertocorrodeallthelimestonematerialinthe5-mmthickspecimensinthetestprogram.
(3)Acidicwateraffectsasphaltconcretebyreactingwiththesuboxideinthelimestoneandproducinganemulsifying agenttocausethebitumenfilmtopeeloffthelimestoneparticles.Thislossofaggregate-bitumenadhesionacceleratesthe degradingprocessoftheasphaltconcrete.
Inroadandairfieldengineering,mostoftheasphaltpavementsaremadeofporousasphaltconcrete.Thedegradingof asphaltpavementsconsistsofmanyfactors,e.g.,vehicleloading,hydrodynamicpressures,abrasion,weathering,etc.Acid rainisonlyoneofthefactorsdegradingasphaltpavementsandmaynotbeoneofthecriticalissuesasthepavementsare designedforalifetimeofonly8–15years[20].
Forminetailingsdams,imperviousasphaltconcretemaybeusedasfacingorinteriorcore.Theasphaltmaybesubjectto stronglyacidicwater(AMD)erosion.Asphaltfacingsareusuallyabout10cmthickwhileasphaltcoresaremorethan50cm thick[2–4].Thebitumencontentisintherangeof6.5–8.0%,andthecorrespondingvolumetricratioofbitumentomineral materialsisintherange0.15:0.85to0.20:0.80.Thetestresultsreportedinthesectionofeffectsofacidicwateronthetensile strengthofasphaltconcreteshowthatthemasslossrate(corrosion)ofhydraulicasphaltconcreteisverylowevenwhen subjectedtoveryacidicwater.Themechanicaltestresultsshownosignsofpropertydegradationofimperviousasphalt concretein3yearsevenwhensubjectedtowaterwithpH=2.
6.Summaryandconclusions
Aggregate-bitumenadhesionwastested usingboilingtestsafterthebitumen-coveredaggregateparticleshadbeen immersedinacidicwaterwithpH=2,4,6,and7.Thepenetrationvalue,softeningpoint,andductilityofbitumenweretested afterthebitumenspecimenshadbeenimmersedinacidicwaterwithpH=2.Masschangesweremeasuredfortwokindsof bitumenspecimensandasphaltmasticspecimenswithdifferentmassratiosofbitumentofiller(1:1,1:2,1:3).Furthermore, sandyasphaltmasticspecimensweretestedafterhavingbeenimmersedinacidicwaterwithpH=2.Tensile(splitting)tests werecarriedoutonporousasphaltspecimens(roadasphalt)andonimperviousspecimens(hydraulicasphalt)afterthe specimenshadbeenimmersedinstronglyacidicwater.Thefollowingfindingsandconclusionsmaybepresented:
BitumenspecimensthathadbeenimmersedinacidicwaterofpH=2for11monthsshowedthattheacidicwaterhad insignificanteffectonthepenetrationvalue,softeningpoint, andductilityof thebitumen.Bitumenitselfis veryacid- resistance.
Whenbitumen-coveredlimestoneparticles wereimmersedin acidicwater for 4weeks, theacidreactedwiththe limestoneparticlestodegradetheaggregate-bitumenadhesion(bonding),butnotbyverymuch(Grade5→Grade4).
Asphaltmasticspecimenswithdifferentmassratiosofbitumentolimestonefiller,andsandyasphaltmasticspecimens wereimmersedinacidicwaterwithpH=2for16months.Theresultsshowedmasschangeratesofabout0.8%/yearforK90 bitumen,0.4%/yearforZ90bitumen,and-1.4%/yearforthelimestonematerial,respectively.
Porous(roadpavement)asphaltconcretewithvoidcontent(airporosity)about5%wasimmersedinacidicwaterwith pH=2.Thetensilestrengthofthespecimenintheacidicwaterwasreducedbyabout40%comparedtothatofthespecimen intheneutralwater(pH=7)inthefirst9monthsofimmersion.Afterthefirst9months,therewasnofurtherreductionin tensilestrength.Thisisprobablybecausetheacidityofthewaterinthevoidsintheasphaltconcretespecimenundergoesa reductionwithtimeasshownbyWang[10]andinFig.2b.
Impervious(hydraulic)asphaltconcretespecimenswithvoidcontentoflessthan2%wereimmersedinacidicwaterof pH=2,4,6,and7forthreeyears,andtensile(splitting)testswerecarriedoutquarterly.Therewasnosignofasphaltconcrete degradationwithtimecausingreductionincrackingresistanceandtensilestrength.
Basedontheabovetestresults,itmaybeconcludedthatthedegradationofimpervious(hydraulic)asphaltconcretein acidicwaterisextremelyslow.Hydraulicasphaltmaybeusedasimperviousfacingorcentralcoreintailingsdamsevenfor projectsrequiringstorageofveryacidicwater.
CRediTauthorshipcontributionstatement
ShanFeng:Datacuration,Investigation.WeibiaoWang:Methodology,Formalanalysis,Supervision,Writing-original draft.HangTan:Datacuration.MengXue:Datacuration.KaareHöeg:Writing-review&editing.
DeclarationofCompetingInterest
Theauthorsdeclarethattheyhavenoknowncompetingfinancialinterestsorpersonalrelationshipsthatcouldhave appearedtoinfluencetheworkreportedinthispaper.
Acknowledgments
The research work for the paper was partly supported by the NationalNatural Science Foundation of China(No.
51179155andNo.51779208).
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