Case
Study
Monitoring
in
situ
performance
of
pervious
concrete
in
British
Columbia—A
pilot
study
§
Rishi
Gupta
*
Civil&EnvironmentalEngineeringProgram,DepartmentofMechanicalEngineering,UniversityofVictoria,Canada
1. Introduction
Stormwatermanagementhasbecomeaconcernforcitiesandmunicipalitiesduetoincreasedurbanizationofresidential andcommercialneighborhoods.Inabuiltenvironmentwithsignificantamountofimpervioussurfacesandintegrationof curbandguttersystemsinourpavements,stormwaterreachesthereceivingwaterbodiesmuchfaster,ingreatervolume and carries morepollutants. Citiesand municipalitiesalong withengineers,researchersand developers areexploring different ways to reduce the impervious surfaces and to deal with stormwater management in a sustainable and environmentfriendlymanner.Porouspavementisfoundtobeaneffectivemeasuretomitigatetheimpactofurbanization ontheenvironment.Withoutoccupyinganyadditionalspace,porouspavementonparkinglots,sidewalks,anddriveways providesmultiplebenefits,i.e.promotesinfiltration,reducespeakflowsandrunoffvolume,improveswaterquality,and reducesthermalpollution,thushelpingtomaintainourdelicateecologicalbalanceandtheenvironmentwelivein.Using materialsthatallowwatertopermeateintothegroundhelpscontributetothegroundwatertable.Onesuchmaterialthat canbeusedtoconstructporouspavementsandporousurbansurfacesis‘‘perviousconcrete.’’Thistypeofconcretehashigh permeabilityandallowsrainwatertopermeate.
AccordingtoSustainableConcreteCanada(2012),theperviousconcretesystemcanhavethefollowingimpactonthe environment:eliminatingtimeconsumingandcostlystormwaterdetentionfacilitiesandundergroundpipingsystems, allowingwater,airandnutrientstotreerootspromotinghealthytreegrowthwithoutdamagingyourpavementsurface, increasingthequantityofwaterwhichcanberetainedonyoursiteandinfiltrateintoaquifersthuspromotinghealthywater
A R T I C L E I N F O
Articlehistory:
Received25September2013
Accepted3October2013
Availableonline27October2013
Keywords:
Perviousconcretesystem
No-finesconcrete Raveling Stormwatermanagement Sustainability Asphaltreplacement A B S T R A C T
Moderndayinfrastructurecallsforuseofimpervioussurfacesandcurbandguttersystems onpavementstorapidlycollectandtransportrainrunoff.Duetothisstormwaterreaches thereceivingwaterbodiesrapidly,ingreatervolumeandcarriesmorepollutantsthan naturalconditions.Porouspavementonparkinglots,sidewalks,anddrivewaysprovidesa solutiontothisproblem.Onesuchmaterialthatcanbeusedtoproduceporoussurfacesis perviousconcrete.Eventhoughno-finesconcretemixhasbeenusedformanyyears,there arestillmanyoutstandingissuesrelatedtoitsstructuralperformanceandissueswith reducedpercolationcapacityovertimeespeciallywhenexposedtorealconditions.This paperpresentsacase study describingaproject in BritishColumbia, Canadawhere 1000ft2ofasphaltwasreplacedwithaperviousconcretesystem.Thedetailsoftheunique constructiontechniqueincludingdetailsofthematerialusedaredescribedinthispaper. On-goingteststomonitortheperformanceofthistestslabarealsodescribed.
ß2013TheAuthor.PublishedbyElsevierLtd.Allrightsreserved.
§
Thisisanopen-accessarticledistributedunderthetermsoftheCreativeCommonsAttribution-NonCommercial-NoDerivativeWorksLicense,which
permitsnon-commercialuse,distribution,andreproductioninanymedium,providedtheoriginalauthorandsourcearecredited.
* Tel.:+12507217033.
E-mailaddress:guptar@uvic.ca
ContentslistsavailableatScienceDirect
Case
Studies
in
Construction
Materials
j our na lh ome p a ge : w ww . e l se v i e r. co m/ l oc a te / c scm
2214-5095/$–seefrontmatterß 2013TheAuthor.PublishedbyElsevierLtd.Allrightsreserved.
debriscaneventuallyreducetheeffectivenessofthedrainageandpermeabilityoftheconcrete.Suchcloggingcouldthen leadtofloodingandtheconcretebeingsusceptibletoextensivefreeze–thawcycles(Deoetal.,2010).Oneissueassociated withthisistherequirementtomaintaintheslabsbyfrequentpowerwashingtounclogthepores.
1.2. Abrasionresistance
Asthebondinginperviousconcreteisaggregate-to-aggregateratherthantheaggregateembeddedinacementitious pastelikein regularconcrete,pervious concretehaspoorermechanicalproperties.Perviousconcreteis susceptibleto abrasion failure caused by the surface course being worn off or crushed under traffic loads (Wu et al., 2011). This phenomenonissometimesreferredtoas‘‘raveling.’’
1.3. Freezeandthaw
Whenperviousconcreteisexposedtocoldclimates,thereisapossibilitytheconcretewouldundergoextensivefreeze– thawcyclesiftheplacementwasfullysaturated.Thisleadstopressureonthethincementpastesurroundingtheaggregates andalossofdurabilityoftheconcrete(Kevernetal.,2010).
Tostudytheseissues,aprojectwasrecentlyinitiatedbytheauthoratBritishColumbiaInstituteofTechnology(BCIT)in Canada.Thisprojectinvolvedreplacingasectionoftheasphaltpavedsurfaceinaparkinglotwithperviousconcrete.Theaim ofthisprojectistodeterminethefeasibilityofusingperviousconcreteonalargerscale,especiallyasanalternativetousing asphaltforpaving.Thepilotslabisbeingexposedtorealenvironmentalconditionsandtraffic.Theobservationsandtest resultsfromthisstudywillhelpaddressabove-mentionedissuesanddeterminethefeasibilityofusinglargerplacementsin thefuture especiallywhenusinginregionsthatarepronetofreeze–thawcycles.Inthispaper,theprocedure usedto constructthisnon-traditionalsystemofperviousconcreteasapavementisdiscussedandtheon-goingteststomonitorthe performanceofthepavementsaredescribed.Someoftheinitialtestresultsarealsopresented.
2. Constructiondetails
ThesiteislocatedatthenorthernareaofParkingLotFattheBurnabycampusofBCIT,Canada(Fig.1a).Theplacementsize is24ft40ft,andcoversthreeparkingstalls(794,795,and796)andtheroadwayadjacenttoit.Thesitelocationwas specificallychosentostudytheeffectofstandingtraffic,movingtraffic,andturningvehicles.
Theconstructionoftheconcreteslabwascompletedinthreemajorstages:excavationandasphaltremoval,subbasefill, andtheconcreteplacementandcuring.Thedetailsofeacharedescribedbelow.
2.1. Excavation
Theexistingasphaltwassawcuttoformstraightedgesand12in.deepexcavationwasdone.Thesoilbelowtheasphalt pavementconsistedofsandysoilforthetop6in.,andsandyclayinthelower6in.Setsofperforatedpipeswereplacedbelow lanes795and796locatedatwestendofthetestslab(Fig.1b).Onesetwasplacedatthebottomoftheclearcrushandoneat thebottomofthe6in.thickperviousconcreteslab(Photos1and2).Separatepipeswereusedateachlevelunderlot795and 796tostudythereductioninpercolationcapacity(ifany)bynotmaintaining(powerwashing)onesectionofthepavement. Inthisstudy,lot795willbemaintainedandlot796willbeleftunmaintained.Asmallportionoftheditchnorthofthe placement(outsidethetestslab)wasalsoexcavatedtoaccommodateawatercollectionsystemfortestingpurposes(Photo 3).Theperforatedpipeswere3in.indiameter,7ftinlength,andmadefromPVC.
2.2. Subbasefill
Oncetheexcavationwascomplete,6in.offracturedclearcrushwithamaximumaggregatesizeof3/4in.wasdeposited
throughouttheplacement.Photo4showstheclearcrushbeingcompactedbythevibratorycompactor.Thebackgroundof
Photo4alsoshowsthesimultaneousexcavationofsoilandthetransportationofclearcrushfromanearbydepositedpile. Theclearcrushisnecessaryforperviousconcreteasitactsasastoragemediumandafiltrationsystemforwaterpassing throughtheperviousconcrete.Thecrushalsoactsasasubbaseforreceivingtheperviousconcretelayer.
2.3. Concreteplacementandcuring
Theconcreteplacementwasdividedintotwoequalbaysasthewidthoftheplacementwaslimitedbythelengthofthe rollerscreedthatwasapproximately12ft.Bay2wasplacedafterasevendaycureforthefirstbay.Fig.1billustratesthe divisionofthepavementintotwobays.Aproprietaryconcretemixwassuppliedbytheready-mixsupplier.Thetarget propertiesoftheconcretemixreportedbythesupplieraregiveninTable1.Inadditiontotheseproperties,accordingtothe supplier,thisproducthasaunitweightupto30%lessthanconventionalconcreteandisworkableforupto90min.The placementwassplitintotwobaystoaccommodatetherollerscreedthatwasusedforthisproject,astherollerscreedlength
Photo1.Perforatedpipesabovethesubgrade.
Fig.1.(a)SitelocationatBCIT’sBurnabycampus(inset–zoomedinviewofthetestslab).(b)Dimensionsofthetestslab.
of10ftapproximatelymatchedthewidthofhalftheplacement,oronebay.Thesitewaspreparedbyaddingminimal formworktosplittheplacementandtocreatestraightedgesalongthesides,ascracksoccurredintheexistingasphalt surfaceduringtheexcavationprocess.Photo5showsthecompactedsubbasepriortoreceivingtheconcreteplacement. Sixinches ofpervious concretewas placed on topof thecompacted clearcrush.The pervious concrete wasthen immediatelyleveledusingrakesandanaluminumrollerscreedforconsolidation.Photo6showsaconstructionworker levelingtheperviousconcretewiththerollerscreedintheforeground.Therollerscreedisessentiallyahollowtubethatis
Photo4.Compactionofclearcrushandsimultaneousexcavation.
Photo3.Downstreamendofpipesfeedingintoacollectionchamber.
Table1
Targetpropertiesofperviousconcrete.
Strength(MPa) Slump(mm) NominalMSA(mm) Voidcontent(%)
Flexural Compressive
filledwithwaterforadditionalweight.Therollerscreedwasapproximately50poundsempty,andapproximately100 poundswhenfilledwithwater.
Theestimatedspeedofrotationofthescreedwasapproximately250rpm.Asmallersecondrollerwasalsousedinthe transversedirectiontogetthedesiredfinishandcompaction.Photo7showsthesecondrollerinoperation.Theedgesofthe placementwerefurtherlightlycompactedbyusingaflatmetalplate.Thiswasdonetoformalevelsurfacebetweenthe existingasphaltandtheperviousconcrete.Thisprocesswasalsodonetocreatelevelsurfacesbetweenthetwoplacements oftheconcrete.Oncetheconcretewasplacedandconsolidated,theconcretewasmistsprayedwithwaterbeforebeing protectedbyathicksheetofpolyethylene.ThiswasdoneinaccordancewiththeCSAA23.1-09specificationsforcuring.
Photo 8 shows the finishedplacement withthe polyethylenecovering the pervious concrete.The second bayof the placementwasplacedsevendaysaftertheinitialplacement.Theinsideedgeoftheconcretewassawcuttoprovideasmooth
Photo5.Compactedcrushreadytoreceiveperviousconcrete.
Photo6.Rollerscreedandlevelingofperviousconcrete.
edgepriortothesecondplacement.Photo9showsthecuredcross-sectionoftheperviousconcreteinBay1(after7daysof concreteplacement).Thesecondplacementhadsimilarprocedurestothefirstplacement.Oncethefirstconcreteplacement wascuredfor15daysandthesecondcuredfor7days,sawcutswerecarriedout.Onesawcutwasmadetoreducethelength oftheplacementtohalf(20ft).
2.4. Concretesamplecollection
Therearenostandardtestmethodsthatdescribemoldingspecimensforcompressiontestingusingperviousconcrete, however,concretecylinderswereconstructedon-sitewithsimulatedcompactionforfurtheranalysisinthelab.Forboth placements,pervious concretesampleswerecollected incylindrical molds,4in.in diameterand8in. tall.A Marshall hammerusedinAsphalttestingwasusedtocompactconcreteasperthesuggestionoftheconcretesupplier.Eachcylinder wassubjectedtothreeblowswithadropofabout6–12in.
3. On-goingresearchandpreliminaryresults
Eventhoughtherecommendedcuringtimeis28daysforthisproduct,thetestslabwasopenedtotraffic7daysafterthe secondbaywasplacedand14daysafterthefirstplacement.Themotivationtodothiswastostudytheeffectofopening trafficatanearly-agewhenconcreteisnotfullycured.Performanceoftheperviousconcreteplacementiscurrentlybeing monitoredandsomepreliminaryresultsavailableatthistimehavebeendescribedbelow.
3.1. Materialdensity
Concrete densities were determined using the cylinders constructed during the two concrete placements. These measurementswereaveragedusingaminimumof3cylinders.Thefirstplacementconsistedoftwobatches,andtheaverage densityofthefirstbatchwas1720kg/m3,whilethesecondbatchwas1740kg/m3.Onthesecondplacementdate,the
averagedensityofthecastcylinderswasfoundtobe1720kg/m3.Thesemeasuredvaluesareveryconsistentandrepresenta
verylowbatchvariability.
Photo8.Finishedconcreteslab(Bay1)coveredwithpolyethylenesheet.
3.2. Compressivestrength
Specimenswerecaston-siteusing4in.concretecylindermolds andadrophammertosimulateapproximatelythe energyimpartedbytherollingscreedonthein-placeconcrete.Ideally,samplescanbecoredfromtheplacement,butthis wasnotanoptiononthisproject,asclosingoftheparkinglotforextractingsampleswasnotanoption.Thecylinders preparedonsitewerelefton-sitetoexposethemtothesameconditionsastherestoftheconcreteplacement.Theaverage compressivestrengthafter28daysforthesampleswasbetween3and4MPa.
3.3. Raveling
Theperviousconcreteplacementhasbeendividedintovariouszones:turning,driving,andparking.Visualobservations after40weeksofexposurehasindicatednoticeablewheeltravelpathsinthevehicleturningzone.Theturningzoneis expectedtoexperiencehighstressesandravelingforcesfromtheturningvehicles.Thepathsappeartohavelostbetween1 and3aggregatelayers.Intheregionofstraightvehiclemotionauniformlossofaggregatehasoccurredandtheparking regionshowslittleornosignofraveling.Photo10showsthecurrentstateoftheperviousconcreteplacement.Therateof ravelingobservedduringthefirsttwomonthssinceplacementseemstohavereducedovertime.Asdescribedearlier,oneof thefactorsthatmayhavecontributedtoearlyravelingisthefactthattheplacementwasopenedtotrafficwithin7daysand thatafull28daycurewasnotallowed.Researchison-goingtoquantifytheextentofravelingbyusingimageanalysisand othernon-destructivetechniques.Itshouldalsobenotedthattheextentofravelingintheturningzonesatthemomentis notsevereenoughtomakethedriveuncomfortable.
3.4. Percolationcapacity
In-placefiltrationratesweremeasuredaccordingtoASTMC1701/C1701M,‘‘StandardTestMethodforInfiltrationRateof inPlacePerviousConcrete’’.Thistestmethoddeterminesthefieldwaterinfiltrationrateofin-placeperviousconcrete. Maintenanceofthepilotslabwillonlybedoneonaspecificsideoftheslab(40ftlengthofstall796,WestsideofBay1),thus, filtrationratetestsresultswilldeterminetheextentofpostmaintenancerecoveryofpercolationcapacity.Theringusedfor thepercolationtestalongwiththeputtyduringatestisshowninPhoto11.Thediameterofthesteelringis300mmanda waterheadof10–15mmwasusedduringthetestasspecifiedbytheASTMstandard.Photo1(b)showsthelocationsofthe testsconductedsofar.Thelocationofeachofthetestswillremainthesameforanyfutureteststodeterminethechangeof
Photo10.Currentstateofravelinginperviousconcreteafter40weeksinservice.
infiltrationratewithtime.Frequenttestswilloccurforeachsideoftheslabtogatherresultsonboth maintainedand unmaintainedareasofthepilotslab.
Theaveragepercolationcapacityoftheplacementafter30weeksofserviceisstillquitehighat60,000mm/hformostof theslab.Theportionofthepavementdownstreamoftherestoftheasphaltparkinglotisindirectcontactofthesurface runoff(shownbyarrowsinPhoto12).Thisportionofthepavementreceiveshighsedimentloadanddeleterioussubstances. Duetothis,thepercolationcapacityinthissouthsideofthepavementis1500mm/hwhichrepresentsmorethan95% reductioninthepercolationcapacity.However,thisarearepresentsaverysmallportionofthetotalareaofthepavement. Moreover,asisevidentfromPhoto12duringameasured48mm/h15minstorm,thepavementwaseffectiveincapturing theentiresurfacerunoffandsurfacewaterwithin2ftofcomingincontactwiththepavement.
3.5. Qualityofinfiltratedwater
Intheditchnorthofthepilotslab,awatercollectionsystemwassetuptogathertheinfiltratedwaterthroughthefour setsofperforatedpipesembeddedinperviousconcrete.Twosetsarelocatedatthesubgradeoftheplacement,andtheother twosetsarelocatedabovethe6in.clearcrushasdescribedearlier.Theperforatedpipesareindistinctlocationstocollect waterdifferentiatedbythemaintainedand unmaintainedareasoftheslab.Thisis donetodeterminethequalityand quantityofwaterpercolatingthroughthepavement.Thecollectedwateriscontrolledbyvalvesfortheindividualsetof pipes.Waterfromeachofthepipescanthenbeheldthroughawatertankinapitwithintheditchnorthoftheconcrete placement.TheprocessofconductingwaterqualitytestsisshowninPhoto13(a)and(b).Initialtestsindicateaslight reductionintheCODoftheinfiltratedwaterwhencomparedtosurfacerunoffenteringthepavement.Moretestsneedtobe conductedtoconfirmthesefindings.
4. Concludingremarks
Acasestudyispresentedthatdescribesa1000ft2pilotplacementofperviousconcreteinaparkinglotthatisservingasa
non-conventionalpavingmaterialinanurbanenvironment.Anetworkofembeddedperforatedpipesisbeingusedto monitorthecapacityofthepavementtoabsorbanddetaintherainrunoffanditseffectonimprovingqualityofpermeated runoff.Theinnovativeconstructionprocedureiscomparabletoconventionalconstructionandadesiredfinish/texturefora parkinglotcanbeachieved.Thelackofanexistingtechniquetomanufacturemoldedspecimensforcompressiontesting mayhavepartlycontributedtothelowmeasuredcompressivestrength.Higherlevelsofravelingareobservedintheturning zonesascomparedtothedrivingandparkingzones.Therateofravelingseemstobeslowingovertimeandon-going researchinvolvesdevelopinganon-contacttechniquetoquantifyraveling.Percolationcapacityofthepavementiscurrently beingmonitoredandeventhoughsomepartsofthepavementhavereducedpercolationcapacityowingtoclogging,the overallcapacityofthepavementanditseffectivenessincapturingsurfacerunoffremainshigh.Watersamplescollected fromtheembeddedpipeswhencomparedtosurfacerunoffenteringthesystemhaveaslightlylowerCOD.Furthertestsare underwaytoconfirmthesefindings.
Acknowledgements
TheprojectdescribedherewasprimarilyfundedbytheGreenValueStrategiesFundawardedtotheauthorattheBritish ColumbiaInstituteofTechnology(BCIT).Theauthorwouldliketothankthevariousstudentsinvolvedinthisproject:Amos Kim,DustinErickson,andShaldonDutt.ThesupportofBCITParkingandSecurity,BCITDepartmentofCivilEngineering (includingDr.NazmunNahar),theReadymixsupplier,andthesubcontractorsinvolvedintheconstructionworkisgreatly appreciated.
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