Microwave self-healing technology as airfield porous asphalt friction course repair and maintenance system

Delft University of Technology

Microwave self-healing technology as airfield porous asphalt friction course repair and

maintenance system

Tabakovic, Amir; O’Prey, Declan; McKenna, Drew; Woodward, David

DOI

10.1016/j.cscm.2019.e00233

Publication date

2019

Document Version

Final published version

Published in

Case Studies in Construction Materials

Citation (APA)

Tabaković, A., O’Prey, D., McKenna, D., & Woodward, D. (2019). Microwave self-healing technology as

airfield porous asphalt friction course repair and maintenance system. Case Studies in Construction

Materials, 10, [e00233]. https://doi.org/10.1016/j.cscm.2019.e00233

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Case

study

Microwave

self-healing

technology

as

air

field

porous

asphalt

friction

course

repair

and

maintenance

system

Amir

Tabakovic

*

,

Declan

O

’Prey

,

Drew

McKenna

,

David

Woodward

a_{Research,EnterpriseandInnovation,TechnologicalUniversityDublin,Ireland} b_{MaterialsandEnvironment,DelftUniversityofTechnology,Netherlands} c

SchoolofCivilEngineering,UniversityCollegeDublin,Ireland

d

LaganBitumenLtd,BreedonGroup,Ireland

e

BelfastSchoolofArchitecture,ArtandtheBuiltEnvironment,UlsterUniversity,Ireland

ARTICLE INFO Articlehistory:

Received16October2018

Receivedinrevisedform25February2019 Accepted3March2019

Keywords:

Self-healingofasphaltpavements Microwaveheating

Porousasphalt PorousFrictionCourse

ABSTRACT

Aproblemincreasinglyfacedbyairportauthoritiesisthemaintenanceofrunways.Dueto theirlargeaircraft loadings associatedwith take-offand landingoperations,runways experiencesurfacedeterioration.Poorqualityrunwaysurfacescannotbetoleratedinsuch anenvironment.Maintenanceissuesmustbecarriedouttomaximisesafetyandminimise theriskofaircraftdamage.Arecentdevelopmenthasbeentheintroductionofself-healing technologies such as rejuvenatorencapsulation, inductionand microwave heatingto addresstheseissues.Thispapersummarisesalaboratoryinvestigationtodeterminethe effectivenessofmicrowaveself-healingforcrackrepairofPorousFrictionCourse(PFC) usedforairfields.Fourmixturescontainingvaryingpercentagesofconductivesteelfibre weretested.TheirrelativeperformancewasassessedusingtheIndirectTensileStiffness Modulus(ITSM)andIndirectTensileStrength(ITS)testmethods.Theresultsshowthatthe additionofconductive steelfibreincreasesinitialstiffnessandstrengthofthemix.A combinationofmicro-waveheatingandsteelfibreadditiontothemixindicatesthatitis possible to significantly improve asphalt performance by making it self-healing to structuralproblemssuchascracking.

©2019TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCC BY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/).

1.Introduction

Thetwomostimportantmaterialsusedintheconstructionofrunwaysurfacesaroundtheworldareeithercementor asphalt based.The EAPA [1] reportedthat out of 126runways surveyed,58 wereconstructed withasphalt,37 were constructed using concrete and 31 constructed using some other material. Irrespective of the construction method employed,runwaysneedtobeconstructedwithsufficientstrengthtocarrythemovingaircraft.Theirrunwaysmusthave adequatewetskidresistanceinviewoftheveryhighspeedsinvolved.Poorwetskidresistanceisacommonproblemfor agedconcreterunways.Onewayofmaintainingorrenewingoperationalwetskidresistanceintheseinstancesistooverlay theoldrunwaysurfacewithanewporousopen-gradedsurfacecourseknownasporousasphaltorfrictioncourse(EAPA, 2003).Theporousmaterialactsasadrainagelayertoreducesurfacewateradverselyaffectingaircrafttyregriponthe surfacinginwetweather.However,thehighairvoidscontentoftheseporousasphaltmaterialsi.e.typicallyaround20%,

*Correspondingauthorat:Research,EnterpriseandInnovation,TechnologicalUniversityDublin,Ireland. E-mailaddress:amir.tabakovic@dit.ie(A.Tabakovic).

https://doi.org/10.1016/j.cscm.2019.e00233

2214-5095/©2019TheAuthors.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/ licenses/by-nc-nd/4.0/).

ContentslistsavailableatScienceDirect

Case

Studies

in

Construction

Materials

allowwaterandairtopenetrateintothesurfacecourselayer.Thiscanaccelerateageingleadingtorapidhardening,reduced flexibilityandultimatelytoaggregatelossandfretting[2].

Self-healingtechnology[3]offersanalternativemethodforasphaltair_fieldrunwaymaintenance.Threemainmethods havebeendeveloped.Rejuvenationisanencapsulatedhealingagentintheformofacapsulethatisaddedintotheasphalt mixduringproductiontorestoretheoriginalbinderproperties[4–10].Whenmicrocracksareinitiatedwithintheasphalt layer,theyencounteracapsuleinthecrackpropagationpath.Thefractureenergyatthetipofthecrackopensthecapsule, releasingthehealingagentwhichthendiffuseswithintheasphaltbindertosealthemicrocrack.Ithasbeenreportedthat rejuvenationmaytakeapproximately20h[9].Inductionheatingandmicrowaveheatinginvolvetheadditionofelectrically conductivefillerssuchassteelfibresorsteelwooltotheasphaltmix[3,11–17]duringproduction.Inductionheatingcreates analternatingcurrentwhichflowsthroughacoilandproducesaninvisibleelectromagneticfieldaroundthecoil.Whenthis electromagneticfieldisinducedoveraconductiveasphaltpavement,itcausesthesteelfibreswithintheasphaltmixtoheat up.Thisheatstheagedbitumenandsoftensit,allowingittoflowandrepairthemicrocrackdamage.Microwaveheating createshighenergywavelengthswhichreactwiththeconductivefibresintheasphalt.Thiscausesthefibrestoheatup whichthenheatstheagedbitumenandsoftensit,allowingittoflowandrepairthemicrocrackdamage.Laboratorystudies haveshownboth methodscanrepair testspecimendamagewithin3minforinduction[18,19]and microwave[15,20] heating.Themicrowavemethodrequireslessconductive_fibrestoachievetherequiredtemperaturetohealthedamage. Thestudyreportedinthispaperinvestigatestheeffectivenessofmicrowaveself-healingforthecrackrepairofPorous FrictionCoursemixesthatwouldtypicallybeusedforairfields.Fourmixturescontainingvaryingpercentagesofconductive steel_fibrewereevaluatedusingatestingprogrammethatcrackedthetestspecimens,healedthesecracksandevaluatedthis processusingtheIndirectTensileStiffnessModulus(ITSM)andIndirectTensileStrength(ITS)testmethods.Theresults showedthatconductivesteelfibresincreaseinitialstiffnessandstrengthofthemix.Theadditionofsteelfibressignificantly improved asphalt healing performance. The results found that Porous Friction Course containing 5% steel fibres outperformedthecontrolmixwithoutfibresandallofthemixturescontaininghighercontentsofthefibre.

2.Materials

2.1.PorousFrictionCoursemix

ThePorousFrictionCourse(PFC)mixwaspreparedinaccordancewithMODSpecification040.Theaggregatewasahigh skidresistanceSiluriangreywackefromNorthernIreland.ThefillerwasacombinationofcrushedCarboniferouslimestone fillerandhydratedlimetoBSEN459-1.Thebitumenwasa160/220penasspecifiedbyMOD040.Fig.1showsthemix gradingcurve.

Table1summarisesthePFCmixconstituentsandshowstheirproportionsinthemix,bothwithandwithoutsteelfibres. Thefibreswereaddedinanamountof5%,10%and15%.Gonzalezetal[21]haddemonstratedthat5%isanoptimumfibre contentwithinanytypeofasphaltmixformicrowavehealingprocess.Liu[22]hadshowedthattheoptimumcontentof conductivefibreforinductionhealingofanPorousasphaltmixis10%.Theaggregategradingwaskeptconstant.Theaddition ofsteelfibreswasusedtoreplacebymasspartofthebitumencontent.

2.2.Steelfibres

ThesteelfibreusedwasGrade3coarse-grainedTrollullSteelWool.Fig.2illustratesthesteelwoolfibreused.Thefibre diameterwas90

m

ThemixingprocedurewasamendedtoavoidconglomerationofsteelfibreswithinthePFCmix.Thecoarseaggregate, sand,filler,limeandbitumenweremixedfirst.Thesteelfibreswerethenaddedslowlytothemix.Thisincreasedtheasphalt mixingperiodcausingthemixtostartcoolingandreduceditsworkability.ThePFCmixhadtobereheatedseveraltimesto 160
C.ThefinalmixingofthePFCwasperformedbyhandtoensurethatallofthemixconstituentswerefullyandevenly coatedbythebitumen.

Fig.1.PFCmixgrading.

2.3.Testspecimencompaction

ThetestspecimenswerecompactedinaccordancewithISEN12697-31:2007usingaSERVOPACgyratorycompactor. Eachtestspecimengot100gyrations.Theywere100mmindiameterandapproximately80mminthickness.Thetargetair voidcontentwas16%basedonamaximumdensityof2263kg/m3_{.Intotal16cylindricalspecimens,4testspecimensper} mix,wereproducedperPFCmix.Thespecimenswerecutto50mmthicknessusingamasonrycuttingsawfortesting. 3.Testingmethodology

3.1.PFCBinderDrainagetest

ThebinderdrainagetestwascarriedoutinaccordancewithEN12697-18usingtheSchellenbergMethod.Asampleof mixedPFCmaterialwasplacedintheglassjarandkeptat160
Cfor1h.Followingthisthecontentsofthejarwereemptied outandthejarreweighedtocalculatetheamountofbinderleftadheredtotheinsidesoftheglassjar.

3.2.IndirectTensileStiffnessModulustest

Thenon-destructiveIndirectTensileStiffnessModulus(ITSM)testwasconductedinaccordancewithEN12697-26:2012. ThisusedaCooperServo-pneumaticUniversalTestingMachinewithapneumaticcloseloopcontrolsystem.Thespecimens wereconditionedat10
Cforfourhourspriortotesting.TwoLinearVariableDifferentialTransformers(LVDT)wereusedto measurethehorizontaldeformation.Thestiffnessvaluewasrecordedontwodiametersorientatedat90
toeachotherand anaverageofthesetwovaluesreportedasthespecimenstiffness.

3.3.IndirectTensileStrengthtest

TheIndirectTensileStrength(ITS)testwasconductedinaccordancewithEN12697-23:2003.ThisusedaMarshall/ IndirectTensileCompressiontester.AfterITSMtesting,thespecimenswereconditionedinatemperaturecontrolchamberat 5
_{Cfor1h.TheITStestappliesaverticalcompressivestriploadataconstantloadingrate,inthiscase50mm/s,tothe}

cylindricalspecimen.Theloadisdistributedoverthethicknessofthespecimenthroughtwoloadingstripsatthetopand bottomofthetestspecimen.Thetestswereconductedat_{5
Ctoensurecrackinitiationandpropagationalongthetest}

specimencentralloadingline.Thespecimenswereloadeduntiltheloadvaluehadfallenbacktozeroorthespecimenhad fullysplitintotwo.UseoftheITStestcreatedtwohalvesofatestspecimenthatcouldthenberecombinedandsubjectedto micro-waveheating.

3.4.HealingefficiencyofPFCmixcontainingsteelfibres

Atestingprogrammewasdesignedtoinvestigatetheeffectoftheconductivesteelfibresonthemechanicalpropertiesof thePFCmixandevaluatethehealingefficiencyofthemicrowavehealingsystem.Theprogrammeisasfollows:

Table1

PFCmixcomposition.

MixConstituent(mm) Control 5%Steelfibre 10%Steelfibre 15%Steelfibre

14to10 3.0% 3.0% 3.0% 3.0% 10to6.3 50.0% 50.0% 50.0% 50.0% 6.3to2.0 25.0% 25.0% 25.0% 25.0% 2.0to0.063 17.5% 17.5% 17.5% 17.5% Filler 2.5% 2.5% 2.5% 2.5% Hydratedlime 2.0% 2.0% 2.0% 2.0% Bitumen(160/220) 5.5% 5.22% 4.95% 4.67% Steelfibre 0.0% 0.275% 0.55% 0.825%

1 Specimensarefirsttestedfornon-destructiveITSMat10
CanddestructiveITSat5
_C.

2 AftercompletionoftheITStestingthetwohalvesofthetestspecimenarerecombined.Itisthenplacedintoaspecially designedcylindricalplasticcollarwith100mminternaldiameterandconditionedfor1hatanambienttemperatureof 203
C.

3 Thecplasticcollarandrecombinedcrackedspecimenisthenplacedinamicro-waveovenandheatedfor3minusingthe Defrostsettingat300W.

4 Theplasticcollarisremovedandthespecimenisleftundisturbedtocooltoroomtemperature203
Cduringwhichthe healingprocesstakesplace.

5 Thistestingandhealingprocedureisrepeatedtwice.

Fig.3illustratesthemainelementsofthetestprogramme.ThemicrowaveovenusedwasaTescoSolo,ModelNo:MM08 with700Wpoweroutput.Thedefrostsettingof300Wfor3minwasfoundtobetheoptimumhealingconditionforthePFC mix.Somesparkswerevisibleduringthefirsthealingcycleprobablycausedbysomeofthefibresnotbeingfullycoatedwith bitumen.Nosparkswereobservedforthesecondhealingcycle.Thisisdifferenttothatfoundinliteraturewherethetest specimensarehealedbyheatingfor40sonthehighestmicrowavepowersettingof600W.Thestudysummarisedinthis paperobservedthatatthislaboratorymicrowavesettingintensesparksandsmokewereemittedasthehealingbegan. Subsequentattemptstoreducepowerhadlittleeffectonthesamples.

3.5.Thermalimaging

Thermalimagingwascarriedouttoobserveheatdistributionthroughoutthetestsampleduetothemicrowavebased healingprocess.ThisusedaMicroEpsilonThermoIMAGERTIM400.Recordingbeganimmediatelyaftertheplasticcollarwas removedfromthemicro-waveheatedspecimen.Fig.4showsathermalimageofatestspecimen60saftermicrowave healing.Theimageshowstemperaturedistributionacrossthesurfaceofthetestspecimen.Theaveragetemperatureis 61.8
Crangingfrom56
Cattheedgesofthetestspecimento66
Catitscentre.Fig.4showsthatthiselevatedtemperature isrelativelyevenlydistributedacrossthesurfacesuggestinggoodsteelfibredistributionthroughouttestspecimenandin turngoodasphaltmixhealing.

4.Results

4.1.PFCBinderDrainage

TheprinciplebehindtheBinderDrainagetestistoquantifytheamountofmateriallostbydrainagei.e.materialthathas adheredtothetruckormixerattheplant.Therefore,itisimportanttoverifywhateffecttheadditionofsteelfibreswould haveonBinderDrainageofthePFCmix.

TheequationusedforBinderDrainageBD¼100½W5_½W4W3_W3W6_  ð2Þ Where:BD=theDrainedmaterial(%);W3=massoftheemptybeaker(g);W4=massofthebeakerplusbatch(g);W5=mass ofthebeakerplusretainedmaterialafterupturning(g);W6=massofthedriedresidueretainedonthesieve(g).”

TheBinderDrainageresultsareshowninTable2.TheresultsshowthattheadditionofsteelfibrestothePFCmixin concentrationsofupto15%byweightofthebitumenhasno/negligibleeffectonthebitumenanditsBinderDrainage. 4.2.PFCmixstiffness

Fig.5plotstheITSMtestdata.ThisshowsasignificantimprovementinITSMbetweenthecontrolPFCmixcontainingno steelfibresandthePFCmixcontaining5%steelfibres.Theadditionof10%and15%steelfibrescausedareductiononITSM valuescomparedtothe5%PFCmixes.

Fig.6plotstheaverageITSMdatafortheinitialtestingandthenafter2rehealingperiodssimulating2periodsof simulated crack treatment. This shows a gradual linear stiffness decrease for the control mix with a significant

Fig.3.Microwavehealingtestsystemsetup.

deteriorationinstiffnessgivenbyanISTMratioof0.58.Theresultsshowthatthecontrolsampleswithnosteel_fibre havenostiffnessrecovery.ThePFCmixescontainingsteelfibresshowbetterinitialITSMcomparedtothecontrol.Asthe specimensarecrackedandhealedthereisareductioninstiffnessafterthefirstre-healingcycle.Thedatashowsthe ITSMtoeitherremainthesameortoslightlyimproveafterthesecondre-healingcycle.Afterthesecondre-healing cyclethePFCspecimenscontaining5%steelfibrehaveapproximatelytwicetheITSMofthecontrolPFCcontainingno steelfibre.Whilstthisisasimplelaboratoryinvestigationtheimplicationsofthisaresignificantformaintenanceofan airportrunway.

4.3.IndirectTensileStrength

Fig.7plotstheITSdata.AsummaryoftheaveragevaluesisgiveninTable3.TheITSdatashowsteelfibreadditionhas apositiveeffectonITS.ThecontrolPFCgroupcontainingnosteelfibreshadthelowestITSvaluesthroughouttesting. ThePFCmixeswith5%steelfibreshadthebestITSdatasimilartotheITSMtestdata.Increasingamountsofsteelfibre gaveITSvaluesgreater thanthe controlPFCcontainingnosteel_fibre.Thisisduetohowthe _fibresaredistributed throughoutthemix.

Fig.4.ExamplePFCtestspecimenthermalimage60safterremovalfrommicrowave.

Table2

BinderDrainageResults.

Mixture TargetTestTemperature(
C) ActualTemperature(
C) AverageBinderDrainage(%)

Control 160 160 0.1

5%Steelfibre 160 160 0.1

10%Steelfibre 160 159 0.1

15%Steelfibre 160 159 0.1

4.4.Thermalimaging

Fig.8plotstheaveragemaximumsurfacetemperaturereachedimmediatelyafterremovalfromthemicro-waveandafter 60scooling.ItshowsastrongpositivelinearcorrelationbetweentheamountofsteelfibresaddedtothePFCmixand temperatureaftermicro-waveheatingwithR2_{valuesof0.9832and0.9524respectively.Forexample,aftermicrowave} heatingPFCspecimens with0%steelfibresreachedanaverageof55
Cwhich after60scoolingdroppedto45
C.PFC specimenswith15%fibresreachedanaverageof81
C whichafter60scoolingdroppedto73
C.Fig.9illustratesthe temperaturedistributionacrossthesurfaceofselectedPFCspecimens60safterremovalfromthemicrowave.ThePFC

Fig.6.AveragedITSMdata.

Fig.7.AverageIndirectTensileStrengthbeforeandafterhealing.

Table3

AverageIndirectTensileStrengthbeforeandafterhealing.

SteelFibreContentinPFCMix(%) InitialIndirectTensileStrength(MPa) IndirectTensileStrengthafterhealing(MPa)

0 2.00 2.27

5 2.93 3.00

10 2.66 3.03

15 2.64 2.51

Fig.8.Averagesurfacetemperaturevssteelfibrecontent. 6 A.Tabakovicetal./CaseStudiesinConstructionMaterialsxxx(2019)e00233

specimenwithnosteelfibreisatanaveragetemperatureof40.8
C.IncontrastthePFCwith15%steelfibreis33
Chotter. PFCmixescontainingsteelfibreswerefoundtohavebettertemperatureretention,withthe5%mixretaining95%ofitsinitial temperatureafter60s.10%and15%mixesretained89%and90%oftheirinitialheatrespectively,whilethecontrolsample onlyretained82%ofitsinitialheat.Thisheatretentioncapabilityisdirectlyattributedtothethermalconductivityofthe steelfibres.

4.5.PFCmixmicrowavehealingefficiency

ThePFCmixcontaining15%_fibresdidnotperformaswellasthe5%and10%PFCmixes.Thisisprobablyduetothe clusteringofsteelfibressubsequentlysuperheatingthebitumenbeyonditsflashpoint.

Fig.10showsanexampleofsteelfibreclusteringandsubsequentsteelfibreoxidationinoneofthetestspecimens. Theyellowcirclesshowlocalisedareasofbitumenbinderdamageduetotheexcessiveheatingofsteelfibreclusters. Theredcircledepictsanoxidizedsteelfibrecluster.Largersteelfibreclusterswererecordedashavingaheatspikeof upto400
C.Temperaturesofthismagnitudeareabovetheflashpointofthebitumeni.e.approximately250
C,and wouldcauseittoinstantlyvaporize.Thisiswhatlikelycausedtheflashesandblackfumesthatwereemittedfromthe 15%PFCmixduringmicrowaveheating.Thismayhaveweakenedthetestspecimenstructureleadingtoitspoorertest data.

ThelinearlossinITSMforthecontrolPFCmixescontainingnosteelfibresuggestnothingtoassisttheheatingand softeningofthebitumentoinfillthecracksformedbyITStesting.Consequently,theinitialITScrackandsubsequentcracks didnotsubstantiallyhealduringmicrowaveheatingi.e.thetemperaturewasnotsufficienttobebeneficial.Thisrepeated stressinginevitablycontinuedtoweakenthePFCmix.ThisisshowninFig.11whichshowsPFCspecimen#4containing0% steel_fibres.TheimageontheleftisthetestspecimenafterthesecondITSphase.Theinducedcrackisvisible.Theimageon therightshowsthecrackstilltobevisibleindicatinginsufficienttemperatureduring healing.Thetestspecimenwas heatedto41
C,asshowninFig.9whichisjustslightlygreaterthanthebitumen’ssofteningpointof37
C.Although slightly greater it wasnot able tosuf_ficientlysoften thebitumen to_flow and repair thedamageclosing microand macrocracks.

ThePFCtestspecimenswith5%steelfibresincreasedstiffnessvalueafterthehealingcycles.Fig.12showsPFCsample#6 containing5%steelfibresbeforeandafterhealingcycle2.ComparedtoFig.11,thecrackiscompletelyhealed.Theincreasein heathasprobablyreducedbinderviscosityallowingittoflownotonlyintothevisiblecracksbutalsointoothermicrocracks withinthespecimen.Thisagreeswiththemajorhealingmechanismofinductionhealingbeingcapillaryflow[23]and diffusionoftheasphaltbinderathightemperatures.Fig.13showsPFCsample#15whichcontains15%steelfibres.Thecrack isfullyrepaired.

5.Conclusion

Thislaboratorystudyhasprovidedpositiveevidencethattheuseofsteelfibreandmicrowaveheatingisapotentialand viableoptionforairfieldasphaltrunwayrepairandmaintenance.Ithasshownthatadditionofsteelfibresand3minof microwavehealingcreatesarapidhealingprocessthatoffersscopeforbusyairportsfacedwithrealproblemsofrunway closure.PFCmixescontainingsteelfibresoutperformedthecontrolmixwithnosteelfibres.EventhoughthePFCmix withoutsteelfibreswasabletopartiallyrepairitscrackdamagethemixturescontainingfibresmanagedtoachievefullcrack closure. The addition of steel fibres reinforced the PFC mix structure improving its stiffness and tensile strength characteristics.Mixturescontaining10% and15%steelfibresinthemixhadlowerstiffnessand strengthrecoveryafter healingincomparisonwithmixturecontaining5%offibres.Thismaybeduetotheclusteringofthesteelfibre[24]causing hightemperatehotspotsinthetestspecimenduringthehealingprocess.Theselocalisedhotspotsmayhavecausedbitumen

Fig.10.PFCspecimen#16showinglocalisedissuesofoxidationandbinderdamage.

Fig.11.PFCcontrolspecimen#4:Left:beforehealingandRight:afterhealingshowingpartiallyclosedcrack.

Fig.12.PFCspecimen#6with5%steelfibre:Left:beforehealingandRight:afterhealingwithfullyrepairedcrack. 8 A.Tabakovicetal./CaseStudiesinConstructionMaterialsxxx(2019)e00233

evaporationthusweakeningstructuralintegrityofthetestspecimen.Thelaboratorytestingconcludesthattheoptimum steelfibrecontentforthePFCmixis5%agreeingwithpreviousstudies[8,15,21].Itbeenhasfoundthatalowerpower heatingof300WismoresuitableforcrackhealingofthePFCusedinthisinvestigationcomparedtothepossiblemaximum poweroutputof700Wforthemicrowaveused.Thisissignificantasitshowstheimportanceofthistypeoflaboratory investigationtooptimisenotonlytheamountofsteelfibreadditionbutalsotheamountofenergyrequiredtohealcracksin this PFCmix. It maybeconcludedthat this laboratory studyclearlyshowsthat theself-healing technologyusedhas significantpotentialinrunwaymaintenance.

Conflictofinterest

Theauthorsdeclarethattherearenoconflictsofinterstregardingthepublicationofthispaper. References

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