Cylinder Strength Versus Cube Strength

Standing Committee on Concrete Technology Annual Concrete Seminar 2013 (18 April 2013)

Cylinder Strength versus Cube Strength

Ir.HDWong,E/PWCL Standard&TestingDivision GeotechnicalEngineeringOffice CivilEngineeringandDevelopmen CivilEngineeringa ndDevelopmentDepartme tDepartment nt

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Outline 1 1

Background Background

2 2

Methodology

3 3

Analysisoftestresults Anal aly ysis of te test st re res sul ults ts

4 4

Discussion

5 5

Conclusion

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1.Background  Designofpublicworkscivilengineeringstructuresmainlymakes referencetoBritishStandards,suchas:  BS 5400: Bridges  BS8004:Foundations  BS8007:Waterretainingstructures  BS8110:Structuraluseofconcrete TheabovestandardsweresupersededbyEurocodes: BSEN1990:2002+A1:2005,BSEN199117:2006, BSEN199211:2004,BSEN19923:2006, BSEN19971:2004,etc.

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1.Background 

DEVBplannedtomigratetoEurocodesforthedesignofpublic  workscivilengineeringstructuresby2015. InBSEN199211:2004,Eurocode2:Designofconcrete  structures,characteristiccylinderstrength(fck)isusedinmostof thedesignformulae. InBSEN2061:2000:  Thecharacteristicstrengthat28daysof150mmdiameterby 300mmcylindersorthecharacteristicstrengthat28daysof 150mmcubesareusedforclassification.

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BSEN2061,Table7givesthefollowingcorrelation:

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InHK,100mmcubesarecommonlyusedfordetermining thecompressivestrengthofconcreteasthemaximum aggregatesizeusedinconcretemixesisnormally20mm. AccordingtoGS2006,Cl.16.59(6): Thesizeofthetestcubeshallbe100mmforconcretewith themaximumaggregatesizenotexceeding20mmand shallbe150mmwiththemaximumaggregatesize exceeding20mm.

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InordertoinvestigateiftheEurocoderelationshipbetween cylinderstrengthandcubestrengthalsoholdsforlocal concrete,DEVBproposedtocarryoutastudyinlate2012. Theaimistostudythecorrelationbetween150mmdia.by 300mmcylinderstrengthand100mmcubestrengthfor localconcrete.

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2.Methodology 

1. AstudybriefwaswrittenbyWSDinconsultationwithCEDD andProf.KwanofHKU. Originally,itwasintendedto produce50setsofcylinderandcubesamplesforeach gradesofconcreterangingfrom10MPato95MPa. 2. AfterdiscussionwithDEVB,Prof.KwanofHKUvolunteered toarrangethecastingofsamplesandcarryoutthetesting forthestudy.

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2.Methodology  Withaviewtoshortenthestudyperiodandreduce thenumberofsamples,Prof.Kwanmodifiedthe samplingrequirementsasfollowing: •

Threesetsofcylinderandcubesamplesfrom eachofthegradesofconcreterangingfrom 10MPato95MPa.

•

100mmCube:3samples,twospecimensfor eachsample.

•

150dia.x300mmcylinder:3samples,two specimensforeachsample.

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Testing  Betweenlate2012andJan.2013,atotalof126cubesand 126cylinderswerecastbyconcretesuppliersandtwo publicworkscontracts. ThePWCLwassubsequentlyrequestedtotakeoverthe testingofconcretesamplesduetosomedifficultiesinHKU inthetestingofcylinders. ByFeb.2013,PWCLcompletedallthetestsinaccordance withCS1.

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3.Analysisoftestresults  (1) Itisfoundthattheactualcubetestresultsrangedfromabout10MPa to130MPa. Theyweregroupedinto12strengthbandsrangingfrom1020MPato120130MPa.

(2)  AccordingtoGS2006: 1.

Thetestresultofasampleshallbetheaveragecompressive strengthsofthetwospecimenstestedat28days.

2.  Ifthedifferencebetweenthecompressivestrengthsoftwotest cubesmadefromonesampleofdesignedmixconcreteexceeds 15%ofthetestresult,thetestresultforthatsampleshallnotbe usedtocalculatethestandarddeviation.

AccordingtoBSEN2061:2000 Thetestresultshouldbediscardedincasetherangeofthetwo compressivestrengthsismorethan15%oftheaverageofthetwo compressivestrengths.

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3.Analysisoftestresults

3.Analysisoftestresults

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3.AnalysisoftestresultsbyHKU HKU carried out an analysis using regression technique based on the average values. The following best fitting curves were obtained: (i) 150 mm cylinder strength (y) to 100 mm cube strength (x) ratio is given by: y/x = 0.74 + 0.0008 x or (ii) 150 mm cylinder strength (y) to 100 mm cube strength (x) ratio is given by: y/x = 0.75 + 0.0007 x

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3.AnalysisoftestresultsbyPWCL  Duetothesmallnumberoftestspecimenswithineachstrength band,PWCLfoundthatthestandarddeviationwithineachstrength bandissometimesquitelargeespeciallyathigherstrength. A bettercurvefittingmaybeachievedifweseparatethedatainto2 zones,below80MPaandabove80MPa.

HKUsuggeststhat: Concretecubestrengthupto80MPa:afixedcoefficient0.78 Concretecubestrengthhigherthan80MPa:afixedcoefficient 0.80

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3.AnalysisoftestresultsbyPWCL PWCLobtainedsimilarresultsbasedontheaveragevaluesoftwospecimens

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3.AnalysisoftestresultsbyPWCL 

InEurocode,thedesignformulaemainlyusecharacteristiccylinderstrength. 

AccordingtoCl.16.05ofGSGuidanceNotesandBSEN206,characteristic strengthisthestrengththat5%ofsamplesfailtoreach.

Characteristicstrength=Meanstrength–1.64xstandarddeviation.

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3.AnalysisoftestresultsbyPWCL PWCLcarriedoutregressionanalysisusingonthecharacteristicvalueofstrengthgrade andfoundthefollowingrelationship. (TestresultswithS.D.largerthan5MPawerediscarded.)

3.AnalysisoftestresultsbyPWCL Table7ofBSEN2061

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3.AnalysisoftestresultsbyPWCL 

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3.AnalysisoftestresultsbyPWCL Relationship between 150 mm dia. cylinders, 150 mm cubes and 100 mm cubes strength

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3.AnalysisoftestresultsbyPWCL BestfittingcurvefromTable7ofBSEN2061modifiedfor100mmcubes

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4.Discussion  • 

Based on characteristic values, PWCL found an almost linear relationship of  y=0.788x between 150 mm dia. cylinders (y) and 100 mm cubes (x) strength for concrete of  grade 80 MPa or below.

• 

HKU proposed a linear relationship of  y=0.78x for concrete grade 80 MPa or below.

• 

Based on BS EN 206 Table 7, a linear relationship of  y=0.772x (modified for 100 mm cubes) is obtained for concrete of  grade 80 MPa or below.

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From the above, it is noted that the difference between the cylinder/cube ratio in Eurocode and the results obtained from this study is only by a negligible margin of  about 2%.

• 

However, the study is only based on 119 cylinders and 119 cubes. As the characteristic strength is affected by the standard deviation and this study is generally based on a set of  3 mean values obtained for each grade strength, PWCL proposed that the Eurocode relationship should be maintained for local concrete but modified for 100 mm cubes.

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5.Conclusion 

•

The results of  this study confirmed that the strength ratio (150 mm dia. cylinder vs. 100 mm cube) for local concrete is similar to that of  Eurocode. The difference is about 2%.

•  As this study is based on limited samples (119 cylinders and 119 cubes), it is proposed that the Eurocode relationship should be maintained for local concrete but modified for 100 mm cubes.

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Thecontributionfromthefollowing partiesfortheirsupporttothestudyis gratefullyacknowledged:  HKU  CPA  DSD  WSD

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Endofpresentation Thankyou!

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