Characterizing material properties of cement-stabilized rammed earth to construct sustainable insulated walls

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Case

Study

Characterizing

material

properties

of

cement-stabilized

rammed

earth

to

construct

sustainable

insulated

walls

Rishi

Gupta

*

CivilandEnvironmentalEngineeringProgram,DepartmentofMechanicalEngineering,UniversityofVictoria,Victoria,BCV8W2Y2, Canada

1. Researchsigniﬁcance

Productionandtransportationofmanyengineeringconstructionmaterialsrequireshighamountsofenergyandhashigh levelsofGHG(greenhousegas)emissionsassociatedwithit.Thiscanhaveadetrimentalimpactontheenvironmentespecially withtherecentrealizationoftheseverityofclimatechangeandglobalwarming.Concreteisoneofthemostwidelyused constructionmaterialsandhasCO2emissionsassociatednotjustwiththemanufacturingprocessofcement,butalsotransport

ofingredientsoverlongdistances. Oneofthesolutionstoreduce theenvironmentalimpact ofconcreteistousemore environmentallyfriendlyingredientsandreducetheamountoftransportationrequiredinshippingtheseingredientsand/or theﬁnishedmaterial. Oneofthe buildingmaterials,RammedEarth(RE)alsoknownas‘‘Pise´ deterre’’orsimply‘‘Pise´’’ (Anderson,2000)isthematerialthatEcosolDesign&Construction(ED&C)LtdandthebuildermembersoftheNorthAmerican RammedEarthBuildersAssociation(NAREBA)havebeenusingforconstructioninNorthWesternWashingtonState,USA;and SouthernAlbertaandBritishColumbia,Canada.Thematerialtypicallyusedconsistsoflocallyavailablesand,soil,orgraveland isstabilizedusingnominalquantitiesofcement.TheauthorwasapproachedwasapproachedbytheCementAssociationof Canada(CAC)toundertakearesearchprojecttostudymechanicalpropertiesofRE.Intherecentyears,REwallsconstructionhas

ARTICLE INFO

Articlehistory:

Received6January2014

Receivedinrevisedform29March2014 Accepted7April2014

Availableonline24April2014 Keywords:

Rammedearth Insulatedconcretewalls

Mechanicalandstructuralproperties oframmedearth

Sustainableconstruction

ABSTRACT

Useoflocalmaterialscanreducethehaulingofconstructionmaterialsoverlongdistances, thusreducingthegreenhousegasemissionsassociatedwithtransportingsuchmaterials. Useoflocallyavailablesoils(earth)forconstructionofwallshasbeenusedinmanyparts oftheworld.Owingtothethermalmassofthesewallsandthepotentialtohaveinsulation embeddedinthewallsectionhasbroughtthisconstructionmaterial/technologyatthe forefrontinrecentyears.However,themechanicalpropertiesoftherammedearthandthe parametersrequiredfordesignofsteelreinforcedwallsarenotfullyunderstood.Inthis paper,theauthorpresentsacasestudywherefull-scalewallswereconstructedusing rammedearthtounderstandtheeffectoftwodifferenttypesofsheardetailingonthe structuralperformanceofthewalls.Themechanicalpropertiesofthematerialessential fordesignsuchascompressivestrengthofthematerialincludingeffectofcoringonthe strength,pulloutstrengthofdifferentrebardiameters,ﬂexuralperformanceand out-of-planebendingonwallswasstudied.Theseresultsarepresentedinthiscasestudy. ß2014TheAuthor.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCC

BY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/3.0/).

*Tel.:+12507217033. E-mailaddress:guptar@uvic.ca

ContentslistsavailableatScienceDirect

Case

Studies

in

Construction

Materials

j o urn a lhom e pa g e : ww w . e l se v i e r. c om / l oca t e / cs cm

http://dx.doi.org/10.1016/j.cscm.2014.04.002

2214-5095/ß2014TheAuthor.PublishedbyElsevierLtd.ThisisanopenaccessarticleundertheCCBY-NC-NDlicense(http://creativecommons.org/ licenses/by-nc-nd/3.0/).

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becomepopular.However,thestructuralperformanceofsuchcompositewallswhenbentout-of-planewasnotunderstood. ThiswasthemotivationbehindperformingmaterialandstructuraltestsonRE.Thisarticleoutlinesthebackground,methods, andproceduresusedtoconstructandstudythebehaviorofinsulatedREspecimens.

2. Background

ConstructionusingRammedEarth(RE)thatincludesuseoflocallyavailablesoilsstabilizedwithbinderssuchaslime datesbackmanycenturies.REstructuresincludingwallshavebeenbuiltinnumerouscountriessincethe1800s(Earth MaterialsGuidelines,1996).ResearchindicatesthattheUSAandAustraliahavebeenthepioneersinusingthissustainable materialin buildingconstruction(Nelson,1976).REstructuresutilizelocallyavailable materialswithlowerembodied energyandwastedmaterialsthantraditionalmethod(EarthMaterialsGuidelines,1996).ThesoilusedforREbuildingisa widelyavailableresourcewithlittleornosideeffectsassociatedwithharvestingforuseinconstruction.Thesoilsusedare typicallysubsoil,leavingtopsoilreadilyavailableforagriculturaluses.Oftensoilofreasonablequalitycanbefoundcloseto thelocationofconstruction,thusreducingthecostandenergyfortransportation.Signiﬁcantcostsavingscanbeachieved whenearth(aggregatesorsoil)isusedforconstructionsincethematerialisgenerallyinexpensiveandreadilyavailable.If theamountofcementusedinREiscarefullycontrolled,morecostsavingscanbeachieved.Todaymorethan30percentof theworld’spopulationusesearthasabuildingmaterial(Anderson,2000).Inaddition,REprovidesgoodthermalmass,with inherentgoodheatretentioninbuildingsandcost-savings.

OncetheingredientsforREhavebeenselected,compressionorrammingofthematerialcanbedonemanuallyusinga tamper (made of a heavy flat bottom plateconnected to a long vertical handle). However, RE construction without mechanicaltoolscanbeverytimeconsumingandlaborintensive.BuildingsconstructedusingREreducetheneedforlumber becausetheformworkisnormallyremovedandreused.Theformsareusuallymadeofform-plyandendpanelsreinforced andsecuredbyasystemofwhalers,strongbacksandintegratedscaffolding.Thefaceformworkissecuredtoendpanels.The spacingbetweentheendpanelsisdeterminedbythewalllength.Thespacingbetweenthefaceform-ply,whichformsthe facesofthewall,isdeterminedbythewallthickness.InREconstruction,onefaceofthewallisusuallyformedtothefull heightofthefinishedwall.Theotherface,thefaceofthewallatwhichmaterialwillbedelivered,isformeduptothefinal heightinsuccessive2000_to₆₀00_(500–1500_mm)_sections._The_wall_length_and_other_forming_details_govern_the_length_of_these

panels.Thisstepbystepprocessallowsfortheplacementofsoilin800₍₂₀₀_mm)_lifts._It_also_facilitates_the_placement_of

horizontal reinforcing, additional vertical reinforcing, insulation panels, and miscellaneous electrical, plumbing and mechanical elementsaswellasblocksoutsforarchitecturalcavities andmechanicalservices.Eachlooseliftofsoilis rammedwithpneumatictampersorhandtampersafterdeliveryintotheforms.

InaprojectinitiatedattheBritishColumbiaInstituteofTechnology(BCIT),REspecimenswereconstructedbyusingvery loww/cratiosandabout10%cementbymass.Thespecimenswereconstructedtosimulatefieldconditionsbyfieldexperts inthisindustry.Specimenswereconstructedtoevaluatecompressivestrength,pull-outstrength,flexuralstrength,and out-of-planebendingofRE.

3. Methodandtestset-up

Therammedearthspecimenswereconstructedbyusingtwolocallysourcedsoilsthatwereblendedina1:1ratio.Based ontheinformationprovidedbythesupplier,theﬁnenessmodulusofthesoilwas3.59.Theclaycontentwasapproximately 6.55% byweight.During constructionof thespecimens, formswithsecured scaffoldswereset-up.The mixcontained aggregatewithamaximumaggregatesizeof5/800₍₁₄_mm)_and_was_mixed_with_10%_cement_by_mass_(batched_by_volume).

Theamountofwaterrequiredinthebatchwasdeterminedbasedonthemixeroperator’sexperienceandhencetheexact amountofwatertocementratioineachbatchcouldnotbedetermined.Postconstruction,thespecimensweremoistcured bymistingfrequentlyforaminimumof28days.Thedetailsaboutpreparationofspecimenandconstructionisdescribedin thesubsequentsections.

3.1. ProductionofREspecimens

Assoonasthefirsttrialmixwaspreparedusingarotarydrummixer,atestcylinderwasmadetomakesurethemixmet expectationsintermsofcohesivenessandworkability.Fig.1showsthefirsttrialspecimenconstructedinitsfreshstate.The generalprinciplefollowedbythefieldrepresentativesforrammingwastousealiftof800₍₂₀₀_mm)_and_then_compact_the

materialtoaheightofapproximately600₍₁₅₀_mm).

3.2. Cylinders

Cylindersof600₍₁₅₀_mm)_diameter_and₁₂00₍₃₀₀_mm)_in_height_were_cast_using₂₀₀_mm_diameter_PVC_pipes._Cylinders

weremadeusingliftsof6–800_(150–200_mm),_which_were_compacted_down_to_about₄00₍₁₀₀_mm)._A_rectangular_block_of_RE

wasalsoconstructedfromwhichcylinderswerecoredbythird-partycontractorstocomparetheeffectofcoringonthe compressivestrengthofRE.Beforetesting,allcylinderswereweighedanddimensionsmeasuredtodeterminethedensityof RE.TheywerelatercappedusingsulphurcompoundaccordingtoCSA4.2.4.2requirements.Specimensweretestedusinga

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400kipForneymachineintheconcretelabasshowninFig.2.Thespecimenswereloadedatarateof50–80psi/s(0.35– 0.55MPa/s).Note:1kip(kilopound)=4459N.

3.3. Pull-out

Todeterminethepull-outstrengths,threerebardiameters((#3)10M,(#5)15M,and(#6)20M)wereused.Somewere orientedvertically(alongthedirectionoframming)andsomewereorientedhorizontally(perpendiculartocompaction direction).Thevariousspecimensalongwiththeirdesignationsareshowninthetablebelow.Also,Fig.3illustratesthe

Fig.1.Firstprototypetestcylinder.

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productionofpulloutsamples.TheactualmeasuredembedmentlengthsarereportedinTable1.Thespecimensweretested usingaBaldwinmachine(set-upasshowninFig.4).Therateofloadingwas0.56kips/min(2.5kN/min)approximately.

3.4. Flexuralspecimens(beams)

TodeterminetheﬂexuralcapacityofREbeams,twobeamsofsize800₁₀00₍₂₀₀_mm₃₀₀_mm)_and₆₀00₍₁₅₀₀_mm)_in

lengthwereconstructed;onewith2–#3(2–10M)rebarsandtheotherwith2–#5(2–15M)steelrebars2.500₍₆₄_mm)_above

thebottomedge.ThetwotypesofbeamsduringtheconstructionstageareshowninFig.5.

Fig.3.Rebarslayoutinverticalandhorizontaldirections. Table1

Pull-outtestspecimens.

Specimen Bardiameter(mm) Embeddedlength(in) Orientation

VPO10M_A 10 11.25 Vertical

VPO10M_B 10 11.25 Vertical

HPO10M_A 10 16 Horizontal

HPO10M_B 10 16 Horizontal

VPO20M_B 20 10.125 Vertical

Fig.4.Pull-outspecimenloadedinaBaldwinmachine.

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Thebeams wereloaded undera modiﬁed 3-pointloadingset-upasshown inFig.6.Toavoid large bearingstress concentrationsonthebeams,thebeamsweresupportedon300₍₇₅_mm)_wide_steel_plates_and_the_load_was_applied_through_a

200₍₅₀_mm)_wide_steel_plate._Note,_that_in_a_typical_3-point_loading_test,_load_is_applied_as_a_line_load_and_the_specimens_are

alsogenerallysupportedoverrollersupports.Themodiﬁedset-upresultedinaclearspanofthebeamof40₆00₍₁₄₂₀_mm).

Beam1with2–#3(2–15M)rebarswastestedinloadcontrolataspeedof1kN/min,sincetheultimateloadcapacityofthe beamwasnotknown.Thisspeedwasincreasedto0.45kips/min(2kN/min)afterthebeamreachedaloadof17kips(75kN).

3.5. Columns(out-of-planebending)

Twofull-scalecompositeREcolumns(representingasectionofawall)wereconstructedusingtwodifferenttypesof stirrupconﬁgurations.Therewerefour#6(20M)verticalrebarsrunningthefullheightofthecolumninbothspecimens,but

Fig.5.(A)Beam2with2–#3(2–10M)rebars,(B)Beam1with2–#5(2–15M)rebars.

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thedetailingofthestirrups(#3or10M)wasdifferent;eitherhorizontallylaidordiagonallyplaced.Theseconfigurationsare showninFig.7.Thefigurealsoshowshowtheinsulationwascuttoaccommodatethediagonalandhorizontalstirrups.Also seeninthefigureisthecoverdistanceoftherebarsfromtheformwork.Thestirrupsspannedacrosstheinsulationcoreand wereplacedinconjunctionwithhorizontal10Mbarsplacedevery600mminthefrontandbackofthewythes.

Aspecialtestset-upwasconstructedfortestingthecolumnsasshowninFig.8.Aspecialsteelseatwaspreparedtohold thecolumnsverticallyandtopreventmovementhorizontally.Thesteelseatsupportedthecolumnshorizontallythrougha 300₍₇₅_mm)_high_plate._At_the_top,_the_column_was_secured_using_another_special_steel_cap_arrangement,_which_also_supported

thecolumnthrougha300₍₇₅_mm)_high_collar._The_load_was_applied_along_the_entire₂0₍₆₀₀_mm)_width_of_the_column_(at

mid-height)usingaspeciallyconstructedchannelsection200₍₅₀_mm)_wide._Both_columns_were_tested_in_displacement_control.

Sincetheloadordeflectioncapacityofthecolumnswasnotexactlyknown,thefirstcolumnwasloadedatarateofonly 0.24in/s(6mm/s).Thisratewasgraduallyincreasedinstepsupto1mm/minastheendofthetestwasapproached.Also,the secondcolumnwithdiagonalstirrupconfigurationwastestedatrateof0.48in/s(12mm/s)tobeginwithandtheratewas

Fig.7.(a)Columnwithdiagonalstirrups,(b)columnwithhorizontalstirrups.

Fig.8.Set-uptostudybehaviorofREcolumn/wall.

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graduallyincreasedasitapproachedtotheendofthetest.Inadditiontoafewdialgauges,twoLVDTswereplacedalongthe columnwidthtorecordthemid-heightdeﬂectionofthecolumns(Fig.8).

4. Testresultsanddiscussion

4.1. Cylindercompressiontesting

CylinderswereweighedandtheirdimensionsmeasuredbeforetestingtocalculatethedensityofRE.Theaveragedensity of castand cored specimens after27 days was approximately158lb/ft3 _and ₁₄₈_lb/ft3 ₍₂₅₃₀_kg/m3 _and ₂₃₇₀_kg/m3₎

respectively.Cylinderscastinmoldsweretestedatanageof6,12,27,and58daystodeterminethecompressivestrengthin theREcylinderspecimens.Thetestingofsampleswasdividedintotwomaingroups.Theﬁrstgroupwerethecylinderscast usingthe600₍₁₅₀_mm)_diameter_pipe._The_other_type_were₆00₍₁₅₀_mm)_samples_cored_from_a_block_of_RE._The_results_of_these

sampleswerecomparedtothatofthecastspecimenstoinvestigatetheinﬂuenceofanyboundaryeffectsofthemolds.

Table2 summarizesresultsofthecompressiontestsof castsamples.In thetable, averagecompressive strengthsare presentedalongwiththeCoefﬁcientofVariation(COV).

InTable2,theaveragecompressivestrengthofthesamplesaftersixdayswas1741psi(12MPa).Thisincreasedby 480psi(4MPa)after12days.Theaveragecompressivestrengthunexpectedlyreducedafter27and55days.Manyfactors canbeattributedtothisreductionandthehighCOVinthetestresults(55%at58days).Itishypothesizedthatthevariability inthetestresultscomesfromthevariationin:compactioneffort,ﬂatnessofthetopsurfaceofcylindersandissueswith cappingsuchcylinders,REmaterialproperties,etc.Atotalofsixcoredsamplesweretestedeachat12and27days.The averagecompressivestrengthresultsofthesesamplesarecomparedinTable2.Thestrengthafter6daysincreasedfrom about2176psi(15MPa)toabout2618psi(18MPa)after27 days.AmaximumCOVofonly5%wasrecordedinthese specimens.

TheresultsofcastandcoredsamplescannotbeveryeasilycomparedduetothehighCOVintheresultsofcastspecimens. Inanycase, it seemsthat theaveragecompressive strengthof thecoredspecimens iscomparable tothatof thecast specimensandthatthecoredspecimensresultinslightlyhighercompressivestrengthwhencomparedtocastspecimensat 12and27days.

4.2. Pull-outtesting

Thepeakloadrequiredeitherforyielding,breaking,orpulling-outtherebarfromtheREspecimenwasrecordedforeach specimen.Thisvalueofloadwasusedtocalculatethebondstrength,basedonthesurfacearea(functionofbardiameterand embeddedlength)ofeachrebar.Fig.9presentsthesebondstrengthvaluesforvariousrebarsalongwiththemodeinwhich theyfailed.The#6(20M)rebarsrecordedthehighestbondstrengthbetween725and870psi(5and6MPa).Highvariability wasobservedwiththe#3(10M)rebarsembeddedvertically.The10Mrebarsembeddedhorizontallyhadabondstrength slightlylessthan363psi(2.5MPa).Somepull-outspecimens(includingoneofthe#5or15M)wereverybrittleandhence werelostduringhandlingandcouldnotbetested.The#5(15M)specimenthatwastestedresultedinaverylowvalueof bondstrength.FuandChung(1998)studiedtheeffectofvariousparametersincludingw/c,additionofadditives,surface treatmentofrebar,andtimeofcuringonbondstrength.Thetypicalbondstrengthobservedbytheauthorsrangedbetween6 and8MPaAscomparedtothesevaluesreportedbyFuandChung,thepull-outstrengthsreportedinFig.9arelower. TreatingVPO15M_Aasanoutlier,alltestedbarshadabondstrengthexceeding1.5MPaThe20mmbarshadabondstrength inexcessof5MPaandbothtestedrebarsofthisdiameteryieldedprovidingadequatebondstrength.

4.3. Beamtesting

Load,displacementofcross-head,anddeflectionsfromtwoLVDTsweremeasuredandrecordedatafrequencyof10Hz Tomaintainbrevity,theplotsarenotincludedinthisarticle.Noinitialflexuralcrackswereobservedandthebeamfailedin shearatapeakloadof17.5kips(78kN).Thedeflectionatthepeakloadwasapproximately0.22in(5.5mm).Theinitial portionofthisloadvs.deflectionplotcanbeusedtodeterminetheelasticmodulusofthebeam,whichcanbeveryusefulfor futureanalysisanddesignusingRE.Sincetheapproximateloadcapacityfromthefirstbeamwasnowknown,thesecond

Table2

Compressivestrengthofcastandcoredsamples.

Ageofcylinders(days) CompressivestrengthinMPa(%COV)

Castsamples Coredcylinders

6 12(22%) –

12 16(11%) 15.5(4%)

27 15.5(52%) 18(5%)

58 12.2(55%) –

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beamwithtwo#3(10M)rebarswasloadedat0.45lb/min(2kN/min).Firstcrackwasrecordedat8.5kips(38kN)and ultimatefailurewasabruptat13.5kips(60kN).

4.4. Columntesting(out-of-planebending)

Significanttimewasspentinsetting-upthesespecimens,asthisrequiredliftingofthespecimensandproperplacement onthetestframe.Thecolumnwiththehorizontalstirrupswastestedfirst.Thefinalresults(loadvs.displacement)ofthis testareshowninFig.10.

AsseeninFig.10,reasonableagreementbetweentheLVDTreadingsandthecross-head(position)displacementwas recorded.DuetothelimitonthedeﬂectionrangeontheLVDTs,nodatawasrecordedbeyondabout0.6in(15mm).Thetestwas continueduntilthecross-headdisplacementreachedavalueofmorethan1.2in(30mm).Theloadcorrespondingtothisvalue wasalittlelessthan13.5kips(60kN).Developmentofcracksandtheirpropagationwasalsorecordedduringthetest.

Fig.9.Bondstrength(load/surfacearea)forpull-outsamples(1MPa=145psi).

Fig.10.Loadvs.displacementforcolumn1withhorizontalstirrups(1lb=4.45N,1in=25.4mm).

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Theloadvs.deflectionplotofthesecondcolumn(withdiagonalstirrups)ispresentedinFig.11.Ascomparedtothefirst column,significantlyhigherload(36kipsor160kN)wasrecordedatacross-headdisplacementofabout1.2in(30mm).At thispoint,thecenterLVDTandsideLVDTdeflectedalmost100₍₂₅_mm)._In_general,_the_second_column_was_much_stiffer_than

theﬁrstone.LVDTsrecordedslightlylowerdisplacementforthesameloadascomparedtothecross-headdisplacement. SincesomenoisewasrecordedintheLVDTreadings,trendlinesarealsoincludedinFig.11foreasyinterpretation.Asinthe caseofcolumn1,locationofvariouscrackswasrecordedonthespecimens.

5. Concludingremarks

ThisarticlepresentstheﬁndingsofastudyinitiatedinCanadatodeterminethemechanicalpropertiesoframmedearth includingcompressivestrength,bondstrength,andﬂexuralstrength.

Thecompressivestrengthtestresultsindicatethatstrengthsinexcessof15MPacanbeeasilyachievedat28days.Cored specimensindicatedamuchlowerstandardvariability.Castspecimensontheotherhandcanbeusedtomeasurethe compressivestrength,butthehighvariability intestresultsneedstobeaccountedfor. Thisstudyalsopresentssome interestingfindingsonthepulloutstrengthofrebarembeddedverticallyorhorizontallyinRE.Rebarwith20mmdiameter embeddedverticallyhadthehighestpulloutstrengthwhencomparedtootherspecimenstestedinthisprogram.Theother specimensthatpulledouthighlighttheneedforlongerembedmentlengthforrebarembeddedinRE.Furtherresearchis neededinthisareatoconfirmthesefindingsincludingthoseoftheflexuraltestresultsanddevelopmodelsthatcanbeused todesign variousmembersusingRE.Twotypesofshearreinforcementdesignswerealsostudiedbyinvestigatingthe behavioroftwofull-scalewallpanelstestedinout-of-planeflexure.Furtherstudiesareunderwaytousethevariousfindings todevelopparametersthatcanbeusedtodesignstructuresusingthisinnovativeandsustainablematerial.

Acknowledgements

TheinvolvementofIrajManshadi(studentresearchassistant)onthisprojectisgreatlyappreciated.Acknowledgements areduetoCementAssociationofCanada(CAC)forsponsoringthisresearchproject.Theassistanceandinvolvementofthe manyindustrypartnersisacknowledged.TheassistanceofMeganChamber(AssistantInstructor,BCIT)duringtestinginthe concretelabisalsoacknowledged.

References

AndersonDW.Rammedearthconstruction.2000.RetrievedMay21,2009,fromAshlandctc:http://webs.ashlandctc.org/jnapora/hum-faculty/syllabi/trad.html. EarthMaterialsGuidelines(1996).Retrieved2009,fromgreenbuilder:http://www.greenbuilder.com/sourcebook/EarthGuidelines.html#Rammed. FuX,ChungDD.Effectsofwater-cementratio,curingage,silicafume,polymeradmixtures,steelsurfacetreatments,andcorrosiononbondbetweenconcreteand

steelreinforcingbars.ACIMaterJ1998;725–33.

NelsonW.Compressedearthblocks.1976.Retrieved2009,fromNetworkearth:http://www.networkearth.org/naturalbuilding/ceb.html. Fig.11.Loadvs.displacementforcolumn2withdiagonalstirrups(1lb=4.45N,1in=25.4mm).