EffectofRiceHuskAshonCementMortarandConcrete

SudishtMishra,FacultyCivilEngineer.DepttNERIST,Itanagar,Prof(Dr.)S.V.Deodhar,Principal,SSVPSBSDCollegeofEngineering,Dhule.Introduction

Workability, strength, and durability are three basics properties ofconcrete. Amount of useful internal work necessary to overcome theinternal friction to produce full compaction is termed asWorkability.Size,shape,surfacetextureandgradingofaggregates,watercementratio, use of admixtures and mix proportion are important factorsaffecting workability. Strength is to bear the desired stresses withinthe permissible factor of safety in expected exposure condition. Thefactor influencing the strength are: quality of cement, watercementratio, grading of aggregates, degree of compaction, efficiency ofcuring, curing temperature, age at the time of testing, impact andfatigue. Durability is sustenance of shape, size and strength resistance to exposure conditions,disintegrationandwearingunderadverseconditions.Variationinconcreteproduction,loadingconditionsinservice life and subsequent attack by the environment factors aremain deteriorating factor of concrete.ProperlycompactedandcuredconcreteusedinRCCcontinuestobesubstantiallywatertightanddurabletillcapillaryporesandmicrocracksintheinteriorareinterconnectedtoformpathwaysuptosurface.

Durability is mainly influenced by environmental exposure condition, freezing thawing, contact toaggressivechemicals,typeandqualityofconstituentmaterials,watercementratio,workability,shapeandsize of themember, degreeof compaction, efficiencyof curing, effectiveness of cover concrete, porosityand permeability. During service life of structures, penetration of water and aggressive chemicals,carbonation, chloride ingress, leaching, sulphate attack, alkalisilica reaction and freezingthawing areresultingdeterioration.Loadingandweatheringinterlinkvoidsandmicrocrackspresentintransitionzoneand network of samemicro cracks gets connected to cracks on concrete surfacewhich provides primarymechanismofthefluidtransporttointeriorofconcrete.Subsequentincreaseofpenetrabilityleadstoeasyingressofwater,oxygen,carbondioxideandacidic ionsetc intoconcreteresultingcracking,spalling, lossatmass,strengthandstiffness.

Low permeability is key to durability and it is controlled by factors like watercement ratio, degree ofhydration, curing, entrapped air voids, micro cracks due to loading and cyclic exposure to thermalvariations.Admixtureimprovesworkability,compactibility,strength,impermeability,resistancetochemicalattack, corrosion of reinforcement and freezing thawing etc. and in turn to durability. For this studydurability is interpreted in terms of porosity, moisture movement, surface strength, ultra sound pulsevelocityandelasticitymodulusof concrete.OptimumuseofRiceHuskAsh (RHA),obtainedbyopen fieldburningmethod,isdecidedforimprovingworkability,strengthanddurabilityofconcrete.RiceHuskAsh

RHA,producedafterburningofRicehusks(RH)hashighreactivityandpozzolanicproperty.IndianStandardcodeofpractice forplainandreinforcedconcrete, IS4562000, recommendsuseofRHA in concretebutdoes not specify quantities. Chemical compositions of RHA are affected due to burning process andtemperature. Silica content in the ash increases with higher the burning temperature. As per study byHouston,D.F.(1972)RHAproducedbyburningricehuskbetween600and700Ctemperaturesfor2hours,contains9095%SiO ,13%K Oand

hydrated cement paste through pozzolanic reaction. RHA minimizes alkaliaggregate reaction, reducesexpansion, refinesporestructureandhindersdiffusionofalkali ions to thesurfaceofaggregatebymicroporousstructure.

Portlandcementcontains60to65%CaOand,uponhydration,aconsiderableportionoflimeisreleasedasfree Ca(OH) , which is primarily responsible for the poor performance of Portland cement concretes inacidic environments.Silicapresent in theRHAcombineswith the calciumhydroxideand results excellentresistance of the material to acidic environments. RHA replacing 10% Portland cement resists chloridepenetration,improvescapillarysuctionandacceleratedchloridediffusivity.

Pozzolanic reaction of RHA consumes Ca(OH) present in a hydrated Portland cement paste, reducessusceptible to acid attack and improves resistance to chloride penetration. This reduces large pores andporosity resulting very low permeability. The pozzolanic and cementitious reaction associated with RHAreduces the free lime present in the cement paste, decreases the permeability of the system, improvesoverall resistance to CO attack and enhances resistance to corrosion of steel in concrete. Highly microporous structure RHA mixed concrete provides escape paths for the freezing water inside the concrete,relievinginternalstresses,reducingmicrocrackingandimprovingfreezethawresistance.NonDestructiveTests(NDT)

NDT,systemsrequiredforassessingstrengthinservicefeature,isdefinedasatestwhichdoesnotimpairtheintendedperformanceoftheelementormemberunderinvestigation,carriedoutonsite,withabilitytodeterminethestrengthanddurabilityofcriticalconstructionswithoutdamaging.

For conducting ND Tests, Rebound hammer, Protimetermoisturemeasurement system, Porositester andpulseUltrasonicNonDestructiveIndicatingTester(PUNDIT)equipmentareused.

Reboundhammertest isconductedtoassestherelativestrengthofconcretebasedonthehardnessatornear its exposed surface. Concrete Rebound Test Hammer is a traditional instrument used for the nondestructive testing of hardened concrete. This provides a quick and simple test method for obtaining animmediate indication of concretestrength in various parts of a structure. Knob of this instrument is keptperpendiculartothesurface(i.e.90degreetothesurface)formeasurementandispushpressedfromthebottomtowardsthesurfaceoftheconcrete,hammerlikesoundisproduced.Thebuttonnearthebottomoftheinstrumentispressedtolocktheindicatorandreadingistaken.

The surface moisture is generally not seen for cleaning and restoring the structure after damage fromstorms, floods or fires. For exposed or unseen structural damage, the undetected moisture damagesstrength,durabilityandreliability.TheProtimeterMoistureMeasurementSystemisapowerfulandversatileinstrument for measuring and diagnosing surfacemoisture in buildings. This instrument directly displaysmoisturecontent(%)alongwith threeconditionsofmaterial likeDRY,WETandRISKcondition.ForusingProtimeteroperationalmodesare selectedand information ispresentedona large,back lit liquid crystaldisplay.Theradiofrequencysensorispositionedsothatlargenumberofmoisturereadingsistakenquicklyandeasily.

Porositester contains of 3 glass tube used to measure the water penetration in concrete at atmosphericpressure. Tubes are fixedwith vacuumplate andvacuumpumpwithout any subsequent cleaning.Rubberseal inserted in the tube provides a defined contact surface of dia 25 mm vertical or horizontal.Compressiblesealsonsuctionplateandtubepermitsecurefixingalsotounevensurfaces.Currentsupplyisprovidedfroma12Vrechargeablebattery.Vacuumplateispressedagainstthefaadewithawetsponge,motorisplacedonadhesionoftheplateischecked.Testtubesareplacedundertheclipandsecuredfirmlywithscrews.Testtubesarefilledwithwateruptothezeromarkandagainrefilledafterthedescentofthewater levelby1or2mland readingsofquantityofpenetratedwaterarenoteddown for15minute testtime. As per the manual of equipment, water absorption coefficient A is calculated by using followingformula:

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2

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Figure1:Porositester

Where,

X=Amount(level)ofwaterpenetratedinml.

d=Dia.oftesttubeinmm.

t=Timeofpenetrationinminutes.

Figure 1 shows the Porositester mentioned above Pundit is a highly trustworthyequipment for ultrasonic pulse velocity teshon concrete. Pundit plus is used todeterminepulsevelocity,(UPV)modulusofelasticity,cavitiesandcrackspresent inthe concrete. Equipment is ruggedly built for onsite reliability for simple, speedyoperationwith integralRS232 interface, automemory store for readings, large LCDdisplaywithexternalorbatterypowersupply.Thesetupparametersaredefinedforthepreferredmodeofoperationandrespectivevaluesaredisplayed.Workplan

Objective of thiswork is to study the effects of RiceHusk Ash as an admixture onworkability,strength,durabilityofcementconcreteandcementmortar.Basedontheabove, optimum dose of RHA is determined to enhance the desired properties ofconcretewithoutcausinganyadverseresultonotherproperties.

Rice Husk from local Rice Mills was burnt completely in open field condition andsieved with 150micron IS sieve. Rice Husk Ash percentage was gradual increasedfrom7.5%,10.0%,12.5%,15.0%and17.5%.M gradenominalmix concrete (1 : 1.5 : 3) and cementmortarofproportion(1:4).Coarseaggregateof20mmgradednominalsize,riversandzoneIIItypeand53gradesPozzolanaPortlandCement (PPC)wereused for thiswork.Forslumpvalues15mmto35mmand compaction factor 0.85 to 0.90,water cement ratio for plain and RHAmixed concretewas 0.50 and0.575respectively.Forcastingconcreteandmortarcubes,150mmsteelcubemouldsand70.7mmsteelcubemouldswereused.EachsetContainedSixsamplesofplainconcreteandsixofRHAmixed.After24hoursofcasting,sampleswereopenedandkeptundertapwatercuringfor28days.Laterdestructiveandnondestructive tests were carried out on set of three separate cubes and average values taken for thisstudy.TestResultsCompressiveStrength

Compressive strength for RHAmixed concrete samples increased upto 12.5% of RHA and decreased forhigher%ofRHA.Higheststrengthwas found30.3N/mm followedby30.07N/mm forRHAcomposition10.00% and 12.5% respectively. In comparison to normal M mix samples, compressive strengthdecreasedby12.94%and19.17% for 15.00%and17.5%ofRHAmixed concrete samples.BetweenRHAcompositions 10% and 12.5%, compressive strength increased very marginally (0.80%) whereas samevaluewashighest(3.08%)betweenRHAcompositions7.50%and10.00%.

Compressivestrengthofnormalmortarcubeswas10.39N/mm andsameincreasedto16.43N/mm and17.44N/mm forRHAcomposition7.5%and10.00%respectively.ForhigherproportionofRHA,12.50%,15.00% and 17.50%, compressive strength decreased to 12.74 N/mm , 10.73 N/mm and 7.71 N/mmrespectively.Maximum increase in strengthwas67.85% followedby58.13% for10.00%and7.50%RHAcompositionrespectively.

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2 2

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2 2

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ReboundHammerTest

Surface strength of M grade concrete cube was 20.05 N/mm . For RHA mixed samples, same valuesincreasedtoamaximumof28.00N/mm andminimum23.00N/mm (Table1) for10%and17.5%ricehusk respectively. For RHA mixed mortar samples, surface strength increased marginally from 16.67N/mm to17.33N/mm (3.96%)whichfurthershowedadecreasedvalueof16.67N/mm formortarcubeswith17.5%RHA. For concrete samples, strength increasedupto 10%RHA and later it started decreasingbut for the mortar samples same trend continued upto 15% RHA. Interpreted from the graph that thestrength is increased rapidly from 7.5% to 15% and it starts decreasing with increase in percentage ofadmixture.ThereferencemortarcubeishavingstrengthofN/mm .

SurfaceMoistureTestResults

Surfacemoisture for concrete cubes was 17.31%which increased to 17.97% for 7.5% RHA content. Forhigher%ofRHA,itshowedadecreasedtrendwithminimumvalue17.2%for17.5%RHA.ForRHAmixedmortar cubes,maximumvaluewas17.32%with10%RHAwhich furtherdecreased to15.89% for17.5%RHA.

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Figure2(a):VariationofNDTPropertiesofMortarCubes

PulseVelocityTest

Pulse velocity was observed 3258 m/sec. in normal concrete cubes which increased to maximum 3736m/sec. in 15.00%RHAmixed cubes. Increase in pulse velocitywas6.78%,10.25%,12.68%,14.67%and13.51%forcorrespondingRHA7.5%,10%,12.5%,15%,and17.5%respectively.

Elasticmodulus increased from 3.83GN/m to amaximum 5.97GN/m (55.87%) for 12.5% RHAmixedsamples.

Figure2(b):VariationinCompressiveStrength,ReboundHammer

StrengthandSurfaceMoisture

Porosity

FornominalmixM grade concrete cubes,water absorption coefficientwas found1.19Kg/m /min. InRHAmixed concrete samples, water absorption coefficients exhibited decreasing trend to a minimum of1.34 (29.84%) for 12.5% of RHA but the same increased with higher percentage of RHA. FormaximumproportionofRHA(17.5%)itwasfound1.63Kg/m /min,15.18%higherthantheminimumvalue.

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Figure2(c):VariationinCompressiveStrength,ReboundHammer

StrengthandSurfaceMoisture

Conclusion

InnominalmixM gradeconcreteand1:4cementmortarRHAwasaddedasanadmixturefrom7.50%to17.50%withanuniformvariationof2.5%.Duringdestructive test,compressivestrengthofmortarcubesandreboundhammerstrengthofconcretesamplesfoundincreasedwithmaximumvariationof67.85%and39.65%for10%RHA.Maximumvariationsofelasticmoduluswere55.87%followedby27.94%for12.50%and10%RHAmixedsamples.Compressivestrengthofconcretesamplesshowedmaximumincrease3.08%betweenRHA7.50%to10.00%whichdecreasedfurtherforhigherpercentageofRHA.

Reductioninwaterabsorption,fromresultsobtainedfrom6testsconcreteand3testsonmortarsamples,itisobservedthatupto10%RHAwithconcreteandmortarenhancesallproperties(Figures2atoc)anditisobservedthat12.5%ofRiceHuskAshbymassofcementastheoptimumdosestobeaddedinconcreteproductionofM particularlywhenthehuskisburntunderfieldconditiontoutilizetheeasilyavailableandlowcost resources forbettermentof concrete structurewith respect toeconomy,durabilityandstrength.So best applicable percentage of rice husk ash as per field condition 10.00% for optimal strength anddurability.References

BronzeoakLtd,RiceHuskAshMarketStudy,ETSUU/00/00061/REPDTI/PubURN03/668,2003.

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Satish Chandra, Waste materials used in concrete manufacturing, William Andrew Inc. Norwich, NY13815,2002.

Hwang,C.L.,andWu,D.S.,PropertiesofCementPasteContainingRiceHuskAsh,ACISP114,1989.

Anderson,L.L.andTillmanD.A.,Fuelsfromwaste.AcademicPressInc.,NewYork,U.S.A,1978.

E. B. Oyetola and M. Abdullahi, The Use of Rice Husk Ash in LowCost Sandcrete Block Production,Department of Civil Engineering, Federal University of Technology, P.M.B. 65, Minna, Nigeria, June2006.

NBMCWOctober2010