BEHAVIOUR OF REINFORCEDCONCRETE SLABS MADE WITH HIGH4STRENGTH CONCRETE
by
©
Amgad Ahme dHussein,B.Se. (E ng. )A thesis sub m it t ed tothe Schoolof Graduate Studies in partial fulflllm entof the
requirementsfor the degreeof Master of Engineering
Facultyof~ngineeringand Applied Science Memorial UniversityofNewfoundland
August199 0
St.John's Newfound la nd Cana da
Nalional libraf)' of Canada
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ABS T RACT
The presentresearch investigation deals wit h the structural behaviourof two-way slabsmadewit h high-strengthconcretesubjected topunching shear.
Ahigh-str e ngth concretemix,suitable for offshoreapplications,was developed using conventionalcementand aggregates available in Newfoundland.The incor- poration of silica fume and high-rangewater reducing agent madeitpossibleto achieve high strengths atearly ages. A compressivestrengthof 70 MPa at 28 days was achieved for concretemix incorporating12%class Fflyash,8%condensedsilo ica fume and a high-range waterreducing agentof napht halene forma ldehyde base.
The relevantrheologicalandmechanicalproperties of the mix wereexami ned. In addition,an expe rimentalprogram was carriedoutto studythe effect of coldocean water,simula te d underlabora tory condit ions,onthemechanic al prope rti es of green high-st rengthconcretecontainingsilicafumeand fly ash.
Seventeenslabs were testedin thestructural laborato ry atM.U.N. Theeffect of thereinforcementra tio, concretestrength,slab dept handcolumn sizeonthe behaviour oftheslabs wasinvestigated. The struct uralbehaviourofthe tested slabswithregard to deformations, strains, ultimatecapacit.y,andmodesoffailure was examined.Test resultsrevealed tha tthepresentNorth Amer ican Codesare unsafe forhigh-str engthconcreteslabs,since they overestimatetheinfluenceofthe concretestrengt h,asafactor, onthe ultimate ca pacit yoftwo-way slabs.
Basedon thetest results, a mechanical model was adopted.The formu lation takes into accounttheactualbehaviourofthe high-st rengthconc reteandsteel.
The proposed model gave afairly goodagreemen t betweenthe predictedand ex- perimentalpunc hingloads.
A CKNOWLEDGEMENTS
This thesiswas completed atMemorial Uni versity of Newfound landaspart ofa project funded by the Natural Sciences and EngineeringResearchCouncil of Canada.Funding intheform of graduatefellowshipand graduatesupplement from MemorialUniversityisgratefullyacknowledged.
Iamgreatlyindebted toDr.H.Merzouk,Associa teProfessor ofCivil Engineer- ing,underwhose guidanceand supervision the projectwas carriedout. Tha nksare dueto Dr.T.R.Chari, Acscciat eDeanof Engineering ,for hisencouragementand the facilities provided.
Sincere thanksare due to theTechnical Staff whomadetheirservic esavailableat everystageofthisproject,specially Mesers C. Ward.A.Bursey and R. O'Driecoll.
Finally,I take this chanceto express my profound gra titudetoallmyfam ily membersfortheir continuingencourage mentand affect ion .
iii
Contents
AB ST R AC T ACKNOWLEDGEMENTS
ListofFigu res List ofTables Listof Symbols 1 Int eodu efi on
l.l Scope 1.2 Objectives . 1.3 Thesis Outline.
2 Revie w ofLiterat ure
2.1 ConcreteforMarine andOffshore Structu res. 2.1.1 Durability ofConcreteinMarine Environment.
2.1.2 Selection ofMateriab. 2.2 Punching ShearStrengthofConcreteSlabs..
2.2.1 Introduction.
iii viii
xi
xiii
14
"
2.2.2 Em pirical Studies. 2.2.3 Rati onal Studies 3 Mat erialInvest iga t ion
3.1 Introduct ion . 3.2 Selecti onof Ma.terials.•.
"
18
31 . . . ... 31 31
3.2.2 Aggregat es .
3.2.3 ChemicalAdmixtures. 3.3 MixDesign . , 3.4 AdoptedMix . .. .
3.2.1
3.4.1 3.4.2
Ceme ntitiouaMeteriele .
MixDesignProportions
MixingProcedure. ... . .. . . . ...• .. 32 33 3"
37 oil oil oil 3.4.3 Prcpertjesof FreshConcrete. . . ... ... 42
3..1.4 Properties ofHardenedConcrete ... 42
3.5 EffectofLo....Tempeeet ure cnthePeope,tiesoflIigh-Str engthConcrete 51 3.5.1 TestSpecimens ... ...•..•.•• . ... •... . 51
3.5.2 Com pressiveStre ngt h SI
3.5.3 Elas ticit y . . .
3.5.4 Stress-St rain Relationship . . . .. . .•. . • ... . 59 3.5.5 ~ffedofLowTempera turesonthe Cement Hydrates of High-
StrengthConcrete 4 Expe r im en t alProg r am
·1.1 Introd uction .
59 64 601
4.2 TestSlabs 4.3 SlabTest Set-up
4.4 Instrumentationand Measurements 4.4.1 Loading System. 4.4.2 Deflectlcne. 4.4.3 Steel Strains. 4.4.4 ConcreteStrains 4.5 PreparationofTestSlabs...
4.5.1 Fabricationof theModel . 4.5.2 Casting and Curing.
4.6 TestProcedure ... .. . .... .
5 TestResult s an d Discus sion 5.1 Introduction .
5.2 Load-deflectionCharacteristics .. 5.3 DuctilityandEnergyAbsorption 5.4 SlabRotation... 5.5 ConcreteStra ins • 5.6 StedStrains . 5.7 CrackingCharacteristics 5.8 Post PunchingBehaviou r.
5.9 ModesofFailure .
5.10 Test Resultsversus Codes Predictions.
6 PROPOSED MECHA NI CALMODEL
.4
67 69 69 71 71 71
78 81 82 82 82 87 89 89 93 96 104 105 105 H1
6.1 Introduction. ._ . 6.2 Punching FailureMechan ism. 6.3 Stress-Strai nRelationships.. . . 6.-1 ForcesAct ing onaRadialSegment . , •. _ .
6.4.1 Stee l Forces..
6.4.2 ConcreteForces .
6.4.3 DowelForces _.. ...
6..i Calculation of the Ultimate Load 6.6 Failure Criteria
III III II>
IU
us
119 121 122 122
6.7 Numerical Proced ure . 123
6.8 Compvisonof TestResultswith theAssum edTheoreticalModel 124
1 Con cl us ion s 127
7.1 Materiallnvest igat ion 128
7.2 Structu ralInvestigationon the Two-waySlabs ... 129
Refer ences 132
ACherniealand Physica lAn al ysi s ofPort la n d Ceme ntand Sup ;:;!e-
mentaryCernentitious Mat erials 140
BCompar iso nof Theoreti calResul t s wit h Othert'TeetResults 143
vii
List of Figures
2.1 Deterioration ofII.concretestructure in sea. water[Mehta19SO]. 2.2 Punching modeladopted by Kinnunen and Nylander 19 2.3 The mechanical model proposed by Reiman n.. ,., . 21 2.4 Yieldline mechanism usedbyGesundetat.!35,36J :H 2.5 Failuremechanism ofNielsonetal.[371• ..•.. . 26 2.6 Failurecriterionand yieldlineloc us forconcre te. 27
2.7 Andre's trussmodel. 28
2.8 The trussmodelas proposedbyAlexande rand Simmonds (411 . 30
3.1 Gradingofaggregates. 36
.
3.2 Failure surfaceof ahlgb-str engt h concretecylinder 44 3.3 Modulusofelast icity versusCompressive strength 48 3.4 Splittingtensilestrengthversus compressivestrength 49 3.5 Mod ulu s ofruptureversuscompressivestrength .'50 3.6 Watertanksandcooling system usedinthestudy.. .. 52 3.7 Compressive strengthversustemperatureafter exposureof3, 7,14,
~~~91d~ ~
3.8 Compressivestrengthversusexposuretimeat 20and100
e
.;6 3.9 Compressivestrengthversus exposure time at differenttemperatures 57viii
63 3.10Modulus of elasticityversus exposuretime... ... 58 3.11Stress-straincurvesat different tempe ratu resafter 91daysof exposure 60 3.12Concret e specim ensaft er 91daysof exposuretoocean waterattern-
peraturesof20andlOPC. •.
3.13Surface deterioration of concrete specimens after 91 daysof exposure tooceanwaterattempera t ures of-5and-lOPC ... ... .... .. 63 4.1 Detail sof a typi cal test specimen.
4.2 Testsetlip.• . • . •. . . • . .. . • 4.3 Supportingreinforced concret eframe .. 4.4 Dial gaugelocations. ... 4.5 Steelstraingaugelocations.
4.6 Concretestrain gaugelccetio n.
4.7 Digitalstrainindicator.•.
65 68 70 72 73 75 . . . ••• . .• 76 4.8 Stress-straincurve foratjt'i~1reinforcing bar.
4.9 Formworkused.
5.1 Typical load-deflection cherecterisfics atcentre spanof test slabs [Serlea IandII)..
5.2 Typical load-deflectioncharacteristics atcentrespan oftestslabs (SeriesIIIand VI)
5.3 Typicalload-rotation characteristicsin the lateraldirectionfor test slabs(SeriesIandIf).
5.4 Typicalload-rotationcharact eristics inthelateral directionfortest slabs(SeriesIIIend VI) ....
ix
79 80
84
85
91
92
5.5 Observed distributionof the concretestrain,iO',at thetoooftese slabs"54 and "S8atvari ous valuesoCthe1O&d P (kN).• .. . .. 9-1 5.6 Applied load versusconcretestrain aroundthecolumn periphery.. 95 fj.7 Observeddist ri butionofthe straininthe flexuralreinfcreement,i.,
atCortest slabs HS3 andHS9 at vario'ls values oftheloadP(kN). 97 5.8 Typical load-tensionsteelstrainbehaviouratthe column periphery
for testslab3 (SeriesIandII) ..• . . ..
5.9 Typical load-te nsionsteelstr ain beh aviouratthe colum n peripher y
Cortestslabs(SeriesIIIandVI) 99
5.10 Failurepattern s oftestslabsNS1.HS1,HS2and"53... lOll 5.11 Failurepatternscf testslabs U57,H54, NS2andIIS6 . 101 5.12Failure patt ernsof test slabs U58,HS9,HSI0and US 1:.. 100l 5.13 Failure patternsofle'lt slabs8512,8513,85 14 and 85 15 103
5.14Yield linemechanism 107
6.1 ModifiedKinnun enand Nylander punchingmodel. 113 6.2 Failure crackandassumedshear crack..• ••.. .. 114 6.3 Relation between deformationsandstrains.. ... 114 6.4 Idealizedstress-straincurves forsteel end concrete . 116
6.5 Punching Cailuremodel andforces. 117
6.6 Bear ingstressfailure atthe colum nface. . . 1211
List o f Ta bles
3.1 Gradingofaggregates. 35
3.2 Physic al propert iesofaggregates 35
3.3 Mixproport ionsofonecubicmeter forthe trial batches. 39 3.4 Propertiesof fresh concrete and compr essivestrength for thetrial
batches.
3.5 Streng thproperti esofthe adopted mix
3.6 Rat ioof compressive strengthatvarious tempe ratu res an d exposures to theone at 2QoC after3days •. . . .
3.7 Ratio of modulusof elastici tyat variou stempera t ur.andexposures to the one at20°Cafter 3days
4.1 Deta ils-o fthe test slabs .
4.2 Mix proporti onsof onecubicmeterof concrete usedin thetwo-way slab invest igation. .... . ...
S.l Deflect ioncharacteristicsoftest slabs . 5.2 Observed,f,;'"tilityand etlffness 5.1 Deformati oncha racterist icsoftest slabs 5.4 Comparisonof code predictions withtestresults . . .
40 47
53
53
66
77
86 88 90 109
6.1 Compar ison oftheoretic al result swithtestresults.. A.l Chemicalanalysisofportlandcementandsupplementa rycemenri-
tiousmaterials
A.2 Physical analysis of portla ndcement and supplementarycementi- tiousma terials
8.1Comparison of theoret icalresultswithElstnerandHognestad'a test results
8.2Compa risonof theoreticalresultswith Kinnunenand Nylander'stest results
8.3 Compar isonof theoretica l results with Kinn unen,Nylanderand'Iolf's test results.
xii
125
HI
J·l'.!
145
145
List of Symbols
A. area of steel
A... areaof thesteel in the radial direction A. , area ofthe steelin the tangentialdirection E. modulusof elasticity of concrete E. modulus of elasticity of steel F.r radial concreteforce F<l tangential concre teforce
Fe rol totalconcreteforcesin a radial direction
F.. radial steelforce F.r tange ntialsteel force F.loj totalst ee l forces in a radial direction
!( coefficient
Pood. ultima teshearcapacity
p/1•z ultimatef1.exural load capacity as predic tedby yieldline P'.' l ulti matetestload
T obliquecompressiveforcein the imagenary conical shellassumed byKinnunen and Nylander
sidedim en sionbe t wee n sup p or-tsof a squareslab perimeter oftheloaded area
side dim ension of a squarecolumn alab effectivedepth 1b~ limiti ng bearing strength /; compressivecylinderstrengthof concrete
xiii
feu cu beconcretestrength
f:
modulusofrupt uref:,
tensilesplitti ngstrength ofconcret eII
conc retetensilestrength fv yield strengthofreinforcin gsteelslaboveralldepth un itmomentcapacity
m.. positive ultimate moment per unitlength
T" radius ofcolumn orloadedarea.
r, radiustotheloa ding
T", radius ofpunch
TV radiuswithinwhich all flexuralreinforcemen t yield T3 radius ofa slab
sidedim ension ofslabspeci men c, nominalshear capacity
neutr al axisdepth leverarm
engle of inclinati onofthe concret eforce at the columnface concre te strain
~cl 0.85f~/4700{ii.
~~ concret e tangentialstrain
~cu concreteulti matestrain fot steel tan genti al strain f'
lI yieldst rainof reinforcin g steel
xiv
reinforcement rat io(A./bd) P.. ratio of radialreinforcement PI ratioor tangential reinforcement
e,
P,./PII.~~W small sectorialangle of a radialsegm ent rot at ion of the slabportion outsidethe shear crack 1/J, rota tion at failure
Chapter 1 Introduction
In recent years,considerable attentionhas been given to theuse of silica fume as a partialreplacementfor cementto producehigh-strengt hconcrete. High -strength concret eis use dfor offshore platforms, marine structu res,bridg es.tall buildings and long span bridges.Recently,a gravitybased structure utilizing high-strength concrete bas been recommendedfortheHibernia developmentoff the eastern coast of Newfoundlan d.High-strengthconcrete,containing mineral admixturessuch as ailicaIume, is relativelyimpermeable.Hence,itoffers greatpromiseforthe dura- bility problem associated withmarine and offshore structuressituated in thehars h NorthAtlantic.waters. Furthermore, the comp ressivestrength is thefundamen- tal basisfordesign and quality eseessme nv,andhas a. major im pact onthe cost effectivenessofeachplatform.
In spiteof the wideuse ofhigh-str engthconcrete,verylittle informat ionis avail- able on thematerial and struct ural beh aviourofthis mat erial.Inthelese decade, some research has beenconductedonthe struct ura l behaviour of high-strength beams.However, to thebest ofthe author's knowledge, no investigation has been reportedon the two way slab structural behaviourandpunchingcharacteristicsof
high-strengthconcrete.
The presentbuildingcode specificationsforshearstrengthofreinforced concrete slabs are based on the test resultsof slabsma dewithrelatively lowcompressive strengths, varyingmostlyfrom 14 to 40MPa. These design provisions maynot applytositua t ionsir-volving parametersdifferentthan those upon whichthe em- pirical equationsarebased.He nce,it is necessary tore-examine the present shear design metho dsas theyapplyto high-strengthconcrete .
Offshore concrete platformshave proven theirwort hin theharsh NorthSea environmentfo r thepastdecade. Theynowface the add itionalchallenges posedby theNorth Atlanticandthe Arct icOffshorefron t iers.Offshorestructures frequently have a concret eperimeterwallwhich is norma llydesignedtoresist the impact of ice onthestruct ure. Both flat and curved exte rior walls have been used.Thus,the concreteplateand shellpanelsreprese nt the most predominant structural element
ed inthewalls ofconcreteoffshoreplatforms.
The presentrese-crcn, triggeredbythe potentialdevelopment ofoffshore oil explorationfor Hibern ia,includes a partofaresearchprogrammeat Memorial university,inwhich th e use of high-stre ngth concrete, as a structural mate rialfor potentia larcticand su b-arcticstructures,is bei ng investigated.
1.1 Scope
Themain objecti veofthema terialinvestigat ion isto determinethebasic me- chanical propertiesof high-stre ngth concretecontainingsilicafume andBy ash.A high-strengthconcret e mix was develo ped usinglocal aggregatesandconvent ional cement. The mix ha.dto be suitab le for offshoreapplications.The effect ofcold
ocean wateronthe mtchankalpropertiesorhigh-stren&thconcrete containing.i1ica fume andfiyashwas,150studied.
The presentresea.rchinvestigat ion addrosesth estruc t ur albeh aviourortwo way-slab systemsmadewith high-stre ngth concrete. Filt ee n high-st rength con- crete slabsand twonorma lstrengt h concre te slahs ,weretes ted.The slabs wert' sym metricaland theywereloadedthrough a colum nstubby a hydraulic actuator in displac...ment co n trol.Different measurements (de£ormations, stra ins,etc.]were collec tedduringthecourse ortesti ng.Thebehavi our or theslabs were obs erved and thetestresullaandobservationwereanalyzed.
Based onthestrainmeasuremen ts andtestresults,amechanica lmodelWIL!J .ievelopedto predicttheultimate capacityof high-strength two-wayslabs. The modelusesthest ra in compatibilityandequ ilibriumequationsof an assumedIxllure crit eria.
1.2 O :' je c t ive s
Themainobjecti vesofthisresearchinves tiga tioncanbesu m marizedas{olio"..,:
1.To developahigb- stren~thconcretemix,ror offshoreapplication s,using con- vent ionalce men t andaggregates availab lein Newfoundland.
2.To studythe mechanical propertiesofhigh~strengthco ncrete containingsilica fumeandflyash.
3.To investigatetheeffect orcoldoceanwater,simulatedunde rlaboratory cond itions, onthemechanicalpropert ies or greenhigh-s tre ngthconcre te con- tainingsilica IumeandlIyash.
4. To exam ine thestruct uralbehaviouroftwo-waysla bs.
i.Todeterminethe deformat ioncharact eris tics(deflect ions, rotati ons, andstrains) oftwoway slabs.
6.To correlatebetweenflexural stre ngthandpunchingstress resistan ce oftwo- way slabs.
7.Tostudy theeffect ofthedifferentparamet ers (concretestrength.flex ural reinfor cement,loa ding area/span ratio, etc.]on thepunching shear capacity of two way slabs.
8. To developarat ionalmechanicalmodelfor the predictionofpunching shear of high-strengt hsimplysupportedtwo-w-,y slabs.
1.3 Thesis Outline
Chapter 2isdividedintotwo parts .The first part reviews theuseofconcretestruc- turesin an oceanenv i ronment and add ressesrecommend an o.isforadurable mix.
Theeeccn-t partrevie wspre vio us resear ch condu cted ontwo-wayslabsincluding both empiricalandrationalapproac hes.
Chapte r 3contains thematerial investig atio n phase of theresear ch.Itdealswith the developmen tof high-strengt h concr etemix, suita blefor offshor e applica t ions.
Italsopresentsthe mechanical propert ies of thehigh-strengthconcrete contai ning fly ashandsilica fume, and the effectoflowtemperat ures onthesemechanical prope rties.
Chapter4describ esthe two-way slab experimentalinvestiga tion. Deta ils of experimentalfacilities ,testproceduresandinstrument ation are presented.
Chapte r5presentsthetestresultsandobservations obtai ned from theexper i- mental investigation,theanalysisoftheseresults,and comparisonofultimate1000d!l obteinedfrom thetested slabs withdifferentcodespred ictions.
Chapter6 dealswiththe theoreticalformulationanddevelopmentofthe adopted mechanicalmodel.
Finally,asummaryoftheinvestigation andtheconclusionsreachedare given inChapter 7.
Chapter 2
Review of Literature
2. 1 Concrete for Mari ne a n d Offshor e Struct ures
Concretehas beensuccessfullyused in ma.rineenvironmentfor centuries. Ithas been employedinthe construction of piers,bridgefounda.tions,retainingwallsand breakwater s.In1971, co n creteW4lIfirstintr oduced on8.lar gescaleto oil industry withthe construction of Ekofsk 1,thefirstconcreteGravity Based Structure(GDS) installedin theNorthSea. Itwas constructed in70 m deep water and containe d 80000m3ofOOHOa!strength concretewith a specified 28-day compressivestrength of45MPa.Since that time, overtwenty other concr etegravityplatforms havebeen constructedin the NorthSea, the Baltic Sea and offshore Brasil [1).
The main desirable characteristicsof the concre teused for platformconstruction are[2J: (a) strength,(b) durability,and (c) comtructibilty.Balled onthesecriteria, high-strength concreteoffersgreatpromisefer offshorestructures.High-s t rength concretes (50 to70MP a ) , containingwater reducing andminera l admixtures,are relatively impermeableandoffer an excellentsolut ion to theproble mof durability of concreteina seawaterenvironment.Furthermore,the compressivestrength is the fundamental basis fordesign andquality assessment, and has a majorim pact
on thecost-effectivenessof110plat form.
2.1.1 Dur a bilit y ofConcretein Marine Envi r onment
Severalresearchworkers[3,4,5, 61stud iedthe casehist o riesofdet erioratedport- lai.dcement concretesexposed to seawater,in bothmildand cold climates,for differentexposureages up to 67 years. It was concluded that the permeabilityof concretewasthe mostimp ortant characteristicdeterminin gitsdu rability.Mehta [,I) showedthat, dependingonthetidallines,theindividualprocess ofdeterioration tend tolimititself todifferent parts of a structu re,From this standpoint, a marine struct urecan be dividedinto threezones as illustrat ed inFig 2.1.The uppermost flart (atmosphericzone),which is abovethehigh-tideline,i8 not directly exposed to seawater.However,it is exposedtoatmospher ic air, winds carryingsalts and frost action. Conseque ntly,cra ckingdueto corrosion of thereinforcement and/or freezingand thawing ofconcrete arethepredominant phenomena -:oausing deteri- oration inthis zone.Theconcreteinthesplashzone,whichisbetweenthe high andlow-tide marks, lsnot onlyvulnera bleto crack ingandspallingofconcretedue towettin g and drying,frost actio n, and corrosionof reinfo rceme nt , butalso to loss of materialduetochemical deco mposit ionofhydrationproduct s ofcement,and impact of waves conta in ingice ,sand,andgravel.The lo werpa rtofthestructure (submergedzone),whi chisalways subme rgedinseawater,is vulnerab le to strength retrogression andloss ofmaterialasa result of the chemicalreactionbetween sea waterandhydration products of cement.
Generallyspeaking, for long-t ime durability ofcoast al andoffshore structures, the concretemustshow resistanc e to; thermalcrack;" ~,frostaction,abrasion/ erosion
"_o~"'~
l;OIlROSIO~Dl' SlU~- CRAI;lW<G!lLf: Tl) FIlEUING..,OTHIomNG
...'I.IC.ll.. P...$lQHMTO W_VEACTKlN. 5 ANOAItI;l GR....EL.NOFUlATlNGICI
-:JEIl'CZllI<E
;:; -,- ~ -c--j- ~ ....
nOl:t.r,::,·,~
i rrl}:f;·_~·;1:~~L
%0"£~~~~ 1 1 ~·q
I
SlJ••IlQGlZOIC=::1Il.--ll.--.i 1 ~
Figure2.1:Deter ioration of a concrete st r ucture in sea water(Meht a1980]
lossandexpansivechemicaland electr ochemical phenomena(such as elkah-eggregete reaction and corrosionofreinforcing steel). The permea bilityof-cncrete is the most importantfactorin alltheth reezonesbecause it influencesall physicalandchemical phenomenacausing concret edeteri oration.
2.1.2 SelectionofMateria ls
IntheUnitedStatesand Canada, theACIComm ittee357 CACI357 R-84)[i}
reportisused for thedesig nand construction of fixed and prestres sed concrete structures forservicein a marineen vironment.In EuropeandAsia, forthedesign and constructi onofconcrete insea stru ctures it is customary tousetherecommen- dationsof theIntern ationalFedera.tion ofPrestressedConcrete Struct ures(FIP.
4t h edition1984)[8]. Althoughboth theACIandFIPrecomm endat io ns werepub- lishedquiterecently(1985 ) ,itis not surprisingthatthe recom mend edpractice lagsbehindtile curr ent(198 6-1989 )fieldpractice at theNo-thSea . Asufficient amount ofpublisheddata isnow availablefromfield experien ce inthe North Sea where twentyoffshoreconcre te platformshavebeen constructedduringthe period 1912-1987 19.1O}.
2.1. 2.1Cement
The ACI357 R-84 maximum limitof10%C3Acontent is based onthe aseum-stlcn thatcements withhigherthan 10% C3A are suspectibleto sulphate attack. Some researchershave quest ioned thevalidityof such an assumption;Mehta {4Jcited threecase studies where no9ulr,l.a.teattack wasobservedinlong tim e(46 to67 years]sea waterexposureof concretecontainin g 14to I7%C3Aandhe attributed
10
that tothe low-per meabilityoftheserichmixes,Mather(5J ccefirm ed thatconcret e prismsI adewithahigh C:sA{12.5%jAST MTypeIIIportland cementdid not show any lackofche micaldurability after more than30yearsof exposure at Treat Island,Maine. Asfor the current field of practic eat the Nort hSea.the heal ofhydrati onandpermeabilityconsiderat ionrequ ire that,respectively, the useof ASTMTypeIIIandType V portland cementsmay be discouragedandtheuse of portl and-pozzolana cementsandportl and-blast furnaceslagcementaareto be givenspecialconsideration.Ingeneral, I\fehta.[111 recom mended that theuse of any portland cement ha.ving6·12%C3Aconte ntfor concretesea st ructu res should he satisfact oryprovidedthatthe cement is compatib lewiththeadmixtu resto be used .
Both theFIPand ACIrecommendations requirea.minimumcementcontent of360 kgfm3jhowevertheformer recomm endsaminimum of400 kg/ rn'J cement for the splashzoneconcretemixture. Theuse of high cement content in massive struct uresfrequentlyleadsto thermalcracking,whichhas theeffectofincrea:sing thepermeab ility and reducing the durability.In order to avoidthisproblem,the NorwegianOffshore concrete practice has dev elopeda high-strengthcementwith moderate heatof hydration. In general,adequatemeasures shouldbetakento controlthetempera turegradientsinconcrete in ordertoprevent therm alcracking.
The maximumallowablpwatercement(w/c)rat iorecommendedby ACIfor marinestr uct ures is0.45 forthesubmergedzone,and 0.40for the splas hzone and the atmo spher iczone.FIP requiresa maximum
wlc
cemei.;-atio of 0.45 but prefersthe use or 0.40.Inthe North Sea fieldpracti ce, awlc
ratio of 0.45 wasused inthe BerylAconcrete platformand was redu cedto0.38in Gullfaks C concret eII
platform.
2.1.2.2Ad m ixtu r es
The FIPrecommendation pro- .dee thathigh-qualitypoazolanic materials, such as silica fume may be added to produce improvedstrength, durability and workability.
However,contrary to the sufficientevidence fromboth laboratory and field practice, the ACI 357R-84 does not appear to takea clearly supportivestand in favour ofthe use of pozzolanic and cementitiousadmixtures inconcretesea structures. Somerecent publicationsand researchundert aken by CanadaCentre for Mineral and EnergyTechnology (CANMET) provide a wealth of useful informationon the composition and properties of these concrete admixtures [121. These pub lications revealtheadvantages of using pozzolanlc admixtures; ther~pozzolanic admixlures helpin improving the workability,reduci ng theheat of hydration, and increasing the strength of the transition zone betweenaggrega teand cement paste. The fine par ticlesof a mineral additive are also ableto enhance thehomogeneity in the hardened concret emicrostructure,thusim proving the ability of concrete to resist microcracking, which is importantfor maintaining the impermeability during service.The amountof a mineraladditiveneededfor thispurposevarieswith its particlesizeand chem icalcomposition.Severalstudies[13, 141recommendeda.fly R3hreplacement of 12 to 25%by weight of totaleementit iousmaterials.Whereas, 5 to10%condensed silica fumeis adequate due to its exceed inglysmall particlesize.
Similarly, to enhance thehomogeneity in the hardenedconcretemicrostr u ct ure,i~
is essential touse a low water-cementrati o andto obtain a prope rdispersion oftill...
cementiticusmaterials in a fresh concrete mixture.Water-reducing ad mixt ures are
12
commonlyusedfor thispurpose. In earlier work(BerylA. 1974),a Iignosulfate- type water reducingagent wasemployed .It containedcertainimpurities which caused considera bleairentrainmentandexcessiveretarda tion infresh conc rete.
High-pu ritynapht halene ormelam inesulpho natetype wate r reducingadmixtures shouldbe usedin concretemixtures when resista ncetointernal crackingdueto freezing andthawingcycles is desired.Pi!.-n etal.(15J reportedfromlabor at ory tes ts on concrete containing9%condensedsilica fume and 0.3 wat er / cementrat io (w/c),thatenough freezab lewate ris present in theselow w/ c ratioconcre teto causedamageinfreezing andthawingtests, unlessair entr ainmentwas used.
Thedesired aircontent recom mended formarine struct ures isnorm allyinthe ran ge of4-6%.Recently, the airentra inmentis specifiedaa an air-voidsystem inste adofthetotal airvolume. Aminim umspacing factor 0.25mm ,aspecific surfaceareaof 25 mm2/mm3and an air void-conte nt ofat least 3%are generally recomme nded bythe Norwegian Offshore concretepract ice[10].
DothACIandFIPwarn that whentwoor moreadmixtures are used, their comp atibility with the cementandaggregatetypesshouldbe examined.Also to protect reinforcingand prestr essing steel fromcorr osion, noCaCf2or admixtures containing chloridesshouldL"used .
2.1.2 .3Aggregates
ACI permitstheuseofanynatural sandand gravel orcrushed rock thatconform to AST MC33specificati ons for concrete aggregate s. FIP recommendsthat aggre- gates likelytoundergophysical or chem icalchangesshould beavoide d.The North Sea fieldpractice is the only specificatio nswhich requires the useof high-quality
aggregates. The maximum size of aggregateusedin theconstruct ionof concrete structuresin the North Seaislimitedto20 mrn,while the recommendedpractice in bothACIandFIPmakesno attemptto suggesta relat ionship between themax- imumslz.,IJfaggregateand the permeability ofconcrete.Theaggregategrading used in theNorthSeais achievedbya strictlycontrolledprocess. TwodilTl'rent sizes areused to classifytheaggregates;0-5 mmfor thefineaggregates and 5-20mm for the coarseaggregates. Both ACI and FIPdonot require anyspecial grading for theconcreteusedin mar ine environment.
The incorporationof a highlyreactivepoazolanic material,such ascondensedsil- ica fumeimprovesconsiderably the aggregate-ceme nt pastebondand consequently, the abrasion/erosio n resistance of theconcret e {161.
2.1. 2 .4 CompressiveStrength
Concrete qualit y is generallydescribedby itscompressive strengt h.Inthedesign processthispropertyis also the keyparamet eralthoughotherstrengthpropert ies mustbeconsidered. The minimumspecifiedstrength requiredby ACI is 35 MPa for allzones and 42MPawhereseveresurfacedegradationis likely. The modern concreteoffshorestruct ures inthe North Seaarebuiltwith high-strengthconcrete, the minimum,beinga60MPaconcrete[101. Forinstance , the specifiedstrength (56 MPa ) forGullfaksC(1986-S7) concreteis50% higher thanthestrengt h ofBeryl A(1973· 75)concrete(36 MPa).Theactual 28-daycompressivestrengthof the Gullfaksconcretecoresampleswas foundto be approximat ely70 MPa.This indi- cates thatthe compressivestrengthspecified bythe Nort hAmericanspecifications is laggingbehindtheNort hSeafield ofpractic e byalmost 15 years.
14
2.2 Punching Shear Strength of Concrete Slabs
2.2.1 IntroductionResea rchon pun ching shearhas yieldedanumberof methods by whichtheulti- mateshea ringstrengthof slabscan he predicte d.In genera l,the varioussuggested approaches can be described as eithertheresults of an empiricalstudy, inwhich a statisticalanalysisofthe available test results isused toestablish a relat ionship between the loadorstress at failureand pa ramet ers of the slab,or the result of a rationa lstudy,in which the strength of theslab materials and themechanismof f~ilureare ideal izedand describedmath ematically.
2.2.2 Empirical Studies
In aninves tigationofwall andcolumnfootings,Talbot (17]proposed an empirical equation fordesign against punching expressedinterms ofthe nominal shearstress ata.critical perimeter at a distance d fromthecolumnface:
v
v=4(c
+
2d)jd (2.1)wherevis the ult imate shearstress,V is the ulti mateshear force,cis theside dimensionof a squarecolumn,disthe effect ivedepth oftheslabandjdis thelever arm of theinte rnal resisting moment of theslab.According toTalbot,punching shearfailureis mainlycue to diagonaltensionand hence thelimitfor v should be proport ional toItandconsequently proportional tof~.This equation contr adicts theexperimenta lfindingssince,as thereinforcement ratioincreases,thepredicted punchingsheardecrease".
In193bGraf[18J reported the resultsof his experimenta lstudyonslabssub-
15 [ectedto concentrated loads and this was followedbythe work ofForsell andHolem- berg [19) in 1946.Their formulat ionwas similarto that of Talbot'sexcept for the location of thecriticalsection.According to GraC,the criticalsection was atthe columnperiphery,whileForsellandHolemberglocatedthe critical section at a distanceh/2fromthe column face. In the lattercase, the shearstressdistribution over the slab thickness is assumedto be parabo lic;
~
(Geaf)4{C:
~)h
(Forsell andHolemberg)(2.2)
(2.3) I-Iognest ad[201. in1953,wasthe firstto propos ea design equa tion in whichthe influenceof flexuralstrengthon theultimate shearingstress wasrecc gnleed . After a re-evaluationof Richart's [21]tests on columnfootings,Hognestad showed that thedesignmethod sof the time (based on Eq.2.1)didnotgive a consisten t factor of saIetywithrespectto shearing failures.Hesuggestedthat ultimate st rengt h of slabs failinginshear is mainlydependentuponthe following var iables:
1.Propertiesof the ma.terialsusedin thesla.b :
(a) qualityof concreteasexpressed by the cylinde rstrength
t;
(b)amount, type, and quality of tensionand compressionreinforcement . 2. Size andshape ofthe loade dareaascomparedtothe slabthickness.
3.Span, support conditionand edge restraintsof theslabs.
16 Ilognest edfou n dtha t the ult ima teshe ar ing stressofa varietyofsla bs canbe expresse d by the empiricalequation;
lJ
=
(0.035+ O~7 )I;+
130 psi (2.4)where<flois the ratiooftheultim ateshearingcapacity oftheslab to the ultimate flexural capacity ofthe slabifithad not fai'edinshear. The ultimate flexural ca pacit yof thesla bwas computedusing yieldlinetheoryandwas dependent on theprop ert.iesofthesla b and thesize andposition of theloaded area.Hogn est ad conc eded th a tEq u at ion2.4 might notbe validifused for slabs withdimensional ratios or concretestrengthsoutside the rangesofthose of Richart's slabs.
In 1953 ElstnerandHognesr ad[22]re porte d sheartest sof afur t her24slabs.
Eq uat io n 2.4wasre viewed inthe ligh t of these tests and those reporte dby Forsell andHolemb erg [19J andRichartandKluge123J. Satisfactoryagreementshc·· ,....een observed and predictedshearingfailu reloa dswer efou ndto exist.
After carryingoutadditionaltests inorde r toextendthe ra nges of theslab variablesofprevio ustest program mes ,Elstne rand Hognestad[24], in 1956,re por t ed th at Eq.2.4ga;eunsafe estima te s of theultimateshe arstre ngt h ofslabs ofhigh- stre ngt h concrete.The following equationwas found to beinbetteragreement with the testresu lt s.
v=
O~:6
/;+
333 psi (2.5)Itwas also repor t ed th at ne it her a concentrat ionoftensile rei nfor ceme ntdire ctl y beneaththeloaded areanorthe presence of compression reinfor cementha d any appreciable effectonthe ultimateshearingstrengt hsof theconcreteslabstested .
(2.6) 17 Moe /25} conductedan experimentalinvestigationconsisting offive different series of testslaband studyingthe effectof;
LCasting holes in the slab in the columnvicinity.
2. Concentratingthe tensile reinforcementover the colu mn.
3. Including special typesof shear reinforcement.
4,Extreme colu mnsize.
5.Eccentric columnloads or transferof moment.
Moe then,basedonhis experimental program,develope d11.semi -empirica ltype equatio nto calculate theultimateshearstrength;
v -
11 _ ~5( 1
-'MJf)Ji
si"-cd -
l +u~~.'0 P
InconclusionMoe stated thatthe criticalsectionof11.slab,subjectedto11.
concent ratedload,was atthecolumnperimete rand that theshear strengthisto some extent dependentupon the flexuralstrength.
In thereport ofACI· ASCEcommittee326126]of 1962,therecommendatio ns of the existing buildingcode werereviewed in thelight ofall the researchcarriedout at thattime.Itwas suggestedthatMoe'sequation(Eq.2.6)was thebestequat ion to datefor th...pred ictionof the failure load ofslabstestedunder laboratory condi- tions. Forpracticaldesign,~owas considered to heanunimportan t variableand, becauseit shouldalwaysbe equalto unity, it was eliminatedfrom Moe's equatio n by assumingthatitwasequal to unity.Following Moe's suggestionthattheshear- ingstrengt h is afunction of the squa rerootoftheconcretecompressivestrength ,
18
theCommitteerecommended thatthe followingdesignequation for the calculation oftheultima teshea r load :
v=
vbd (2.7)wherev:54.0!E andbis thelengthofthe"pseudocrit ica lsect ion," takenasthe perimeter ata distance ofd/2from the periphery oftheloaded area.The position oftheassumedcrit ical sectionwas chosen so thatEq. 2.7incorp orates allowance forthecldratio.
2.2.3 Ratio n alStudie s
Basedon observat ions of a number oftestsofcircularslabs with centralcolumns, Kinnunenand Nyland er [27]conceivedan idealised modelof a slabatpunching failure. It was assumed thattheslab portionoutside theshearcrack.which is bcundeaJy thiscrack,by radial cracks.and by thecircumf erenceofthe slab , can be regardedalla rigidbodywhichis turned under theact ion of the load arounda centreof rotationlocated atthe rootof theshear crack.Themodelisillustra ted inFig 2.2.
The criterion of failure inthe mathematical modelwas thecollapse of the conical shell which.occurredwhen the tangentialst rain on the surfaceof theslabin the vicini tyofthe root oftheshear crackreachedan empirical criticalvalue. The characteristic tangentialstrain at failure wasdetermined from tests onslabswith ringreinforcementonly. The pun chingload wascalcu latedby assumingadimension oftheconica lshell and thenfollowingaconvergent iterat ive process.
KinnunenandNyla nderfound thattheir theoryga vevaluesfor thepunch- ingload which werein satisfactoryagreement withresultsof theirowntests as
0)
"
19
Figure 2.2: Punching model adopted by Kinnunen and Nylander
20
wellasthoseofEistner and Hognesta d (24].They attributedlowcalculated val- ues, inthecaseoftwo wayreinforcement ,to dowelandmembrane effects,and suggested that they shouldbe taken intoaccountbymultiplyin gthecalculate d ul- tima teloadby1.1.
Kinnunen!2Sl1aterpublishedapaperin whichhe presented aprimp.ly qual- itative study ofdowel andmembranep.~ tionin slabswithtwo-way reinforceme nt , andconcludedthatthe punchingloadisincreasedby about20 percentby dowel andmem br a ne effect s.
Thetheory of Kinnunenand Nylander hasbeencriticizedbyLong{291and Reima nn 130).Theirmajor commonobjection wasthat the assumptiontha t the compre ssive strengthin the conicalshell was approxim atelyconstant throughout , wasmadein neglectof probable shearingstresses onthe shellsurface. This criticism im pliesthat the existenceof the assumed conical shellshouldhavebeen proven.
Ratherthanverifyingitsexistence, Kinnune nandNyland erhavejustifiedits use, along withthatoftheircrite rionoffailure,by showingconsistentsatisfactory agree- mentbetweencalcula tedandtestpunchingloads.
Based onthetest observationsof Kinnunenand Nylander127],Reim an n[30]
proposeda.simple idealisedmodel ofslabatpunchingfailure from which the punch.
ing load can hecalculate directly.The theoreticalmodelis madeupof a punched cone of concrete, an outerannularslab anda joint,which was idealised as a hinge bridgedby aspring, betweenthe inner and outerregion sat the peripheryofthe col- umn. The hinge was assumedtocoincide with thecent re ofrot ati on ofthe annular slab.The model isillustratedin Fig2.3.Reimannappliedhismethodofanalysis toresultsoftest sby Kinnunen and Nylanderand others. He foundreasonable
"
Fi«ure2.3:The mechanical model proposedbyReimann
22
agreement betweencalculat ed andtestfa ilureloads althoughthe average values ofthe ratioofthe actualIailureload to hiscalculat ed failure loads wereusually greater thanunity.Reiman n ignored anydowel and tensile membraneeffects and thiscould accountfor hissomewhatlowcalculated values.
In1967.Long [29Jand Long and Bond[31] reporteda theoreticalmethodfor thecalculati on ofthepunchingload of aslab wit htwo-way reinforcement. As- suminga linear distrib ut ionofst ress,thestresses inthe shear-compression acne were calculated usingthinplate theory. An octahedral shearst resscriterionof failurewas usedto find the stresses at failure and from that anunco rrectedl~ad wascalculated. Theuncorrectedload was then adjust edtogive the punching load byapplying corrections forsurrounding slab and supportcond itions and for dowel andtensile membrane effects.
LongandBondshowedthat theirtheory gave punchingloads in good agree- ment withtestresultsof,amongothers,Richart[21J,Elstner andHognestad {22J, Moe[25Jand Kinnu nenandNylander[27J.
In thediscussionwhich followedthiswork,the relevanceof the equationsof elasticit y toslabs nearfailure andthe suggest ed mechanismof punchingfailure was questioned. Theassumption of Longand Bondthat theloadsu pported by a failed slab approximatesthe effect of dowel and tensilemembrane actionon the failure load is alsoquestionable.
Masterson and Long [32]later developedthis approach andproposeda simpll- fiedfiniteelement modelforlocalslab conditions at ultimatefailure. Theiridealized representationof the slab-colum nconnection was equivalent to thetheory of devel- cpment oflocal plasLicityatthe column periphery.By relat ingthe appliedload to
:!3
the intern al momentat iailureandbymaking anappropriate allowancefordowel ar;d membra neeffects,thepunchingstrength was foundtobe well predicte d for themajorityof chosenrealistic slab-columnspecimenstested by som e researchers.
Long[33j lat er formulateda two-phase designprocedure in which thepunch- ingstrengthwaspredictedas th"lesser ofeither a flexureorshearcriterion of failure.This approach hascertainlimitat ions, as itcouldnothand le slabswith high-stre ngth concrete and slabswithlowlevel ofreinforce ment .
In1987,~.log[34]extendedhiswork by usingamore rationaltreatmentofthe flexuralmode ofpunchi ng failure.Longused ananalytically basedlinearinterpo- lat ion momentfactor to relatethe ultimateflexuralcapacity tothe yieldmoment.
This factor dependedon theslab ductilitywhichwas consideredto controlthede- greeofyieldinginthe slab atfailure. Thecltimate shear capacitywas based on anempirical relatio nshipforvertical shearstresson a criti cal sectiondose tothe column perimeter .
In1970,Gesundand Dikshit(351usedyieldline theoryof Johanse n(1962)to analyze punchingfailure sinslabs. Theirworkwasbased onthe researchca rried outby Gesund andKaushik (36)whoconcludedthattherat ioPIle,,!P'.'lfor 106 alleged punching shear failureaveraged1.015with a standar d deviat ion of0.248.
Theyassumed a yield linefan mechanismaround the column,as showninFig 2.4.
The ultimateloadcalculatedforthatmechanismwas consideredtobe the ultimate punching loadforthe case ofan interiorcirc ularcolumn.Thismethodoverestimates the punching strengthunlessthe flexural modeof failure is thedomina ntone.
Auot her application of plastic theoryto estimat ethepunchingresistanceof axisymmetri cconcrete slabs withoutshear reinforcementwas presented by Nielsen,
Aslob
24
Figure2.4:Yieldlinemechanism usedbyGeec edttal.[35,361
25 Braestrup et al.[371in1978. This theo ryisin contrast withCeaund's method,a.~
thepunchingmechanismadopted(Fig2.5) is totallyindependent oftheflexural propertiesof the slab.The mechanismisoneofthe punchingout of asolid of revolu tionattachedto the column ,whiletherestofthestab remains rigid. Using minimizationof worktogether withthe modified Coulombcriterionand the norma l flow rule (Fig 2.6),theminimu m upper boundsolutionfor the ultimatepunching loadwasobtained. Thistheory contradictsexperimentalfindingsasitneglects the effect ofthe Bexural reinforcemen tonthe ultimatepunching capacityof theslab.
In1982 Andra[38] presented a theoreticalmodel for thepunching shearof a circularslab withringreinforcement.A finite dementanalysiswas usedto derive theproposed model.As inKin nunenandNylander, this model considersthe rigid bodyrotationof radialsegmentsarounda centerof rotatio n located at thefaceof the columnand the neutralaxis,as showninFig 2.7.Each segmentis analyzed usingatruss model(Fig 2.7)beyondthe shearcrack, with45~tension and com- pressionelements representingthe behaviouroftheuncracked web . Superimposed on thislattice thereare addit ionalcom pressionsradiatingfromthecolumnface belowthecrack.
Fig 2.7shows the failurephilosophyassumed by Andra.Hedescribes thefailure of the concretestrut nearthecolumnface asbeingarest ricted crushing ofconcrete , while,in the part of thestr u t far fromthe columnface,its cha racter is of an unrestrictedsplitting.
Shehata{39]. in1985, followed by Shehata and Regan(401 in1989. presented amodelwhich they claimedto bean improvementoverthat ofKinnu nen and Nyla n der. In thatmodel.theeffect ofdowelandmembrane wasdlreetly calculated
0)
f Oisplocrnmt
I
t~
26
bJ
Figure 2.5:Failure mechanism of Nielson etal.131J
T
, I M odified C oulomb hilureeriterioo
~
f,
(I.O )
(K,-II blYield I" " in plm ' lu in
Figure2.6: Failure criterion and yield linelocusforconcrete.
:.!'j
form lronlnentialnfl'i:JefS
~ I :::::~ " r- ,=j
I~
'$I
<1., ;1. "",1",
,"" ""r 10'". !
Figure 2.7:Andra's truss model.
28
29
fromthemodel and thefailurecriterion wasmodi fied. This modelwillbediscussed indeta ilsinchapter 6together withitsdrawbacks.
In 1987,Alexande r andSimmonds{411presen tedtheirtruss model. The model prop osed consi sted ofd.three dimensional spac e truss composedofconcreteCOIll- pressionstruts andst eeltension ties. The reinforcing st eelandcon cretecom pression fields were brokendown into individual bar-st rutunits (Fig2.8).
Thetruss model included twotypes of compression struts:(1) those parallel to theplane ofslab(anchoringties) and,(2) thoseatsome anglecto the plane of the slab(shearstru ts) .The modelpredicts onlytwopossible failuremodesfor a.shearstru t;eitherthesteelyields and the angle ofshearstrutQreaches some critica lvalue, or theconcrete failsincompr ess ion prior toyielding ofsteel. Thi s im pliesthat the tra.ditiona lconcepts of shea rand flexur edoes notapply, andthe two possiblemodes of slabfail ure sho uldbeclassifiedaslocal connection failures as opposedto overallslabcollapse.
In concl u sion, Alexanderand Sim mondsstatedthatthe evaluati on ofthe angle Qneededfu rther invest igat ion , asitwas based on an empiricalequation obtained fromthe testresults.
fnterestingly,all ofthe reviewedrationalapproache shave been shownto provide satisfad orily accur ateest imates oftheultima te punchingload ,ellenthoughin each casefundamentalobjections tosom eofthe assumpt ions of the respectiveauthor s have been raised.
i l l
o
I - f
VJ
30
Chapter 3
M aterial Investigation
3.1 I nt r o d uct ion
The mater ials phas e of thecurrentresearchworkwillbe presentedinthis chapte r.
Thedevelopmentof thehigh-stren gthconcrete mixdesign,(or offshoreapplica- tions,willbe desc ribed. Atthe outset,thediffer ent mater ials used intheconcrete product ion arebrie fly discussed.Thetrialhatchesrequ ired toidentif ytheopt imum mixingproport ions and mixingprocedure are described.Themechan icalproperties of the recommendedmix werethenexamin edtoensure thesuitability foroffshore applicat ions.Fi na lly,the eITedoflowtempera t uresonthe mechanical properties of high-strengthconcreteisexamined .
3.2 Se lection of Materia ls
Theproductionofhigh-strengthconcretethat meet s the requirem ent for worka- bility and strength development demand smore stri ngent requirements on material selectio nthan{or lower-strengthconcretes. Inthis study, localmat erialsare uti- lized.Theselect io nof thesemater ialsWa5based on the recommendationsof earlier researcher sand testresul tsfrom experiment s cond uct edatthe concr ete laboratory
31
of Memorial University.
3.2.1 CementitiousMateria ls
Theterm"cement itio us materi als"referstothe comb ined total weightofportland cementand pozzolanicmat erials(fly ash andsilica. fume)tor theproductionof high-strengthconcrete.
3.2.1. 1Cement
Ordinaryportlandcement (Type10)CSA3·A SSwithmodifiedC3Acontentof about6%asproduced in Newfoundlandwasused. Asdiscussedin chapte r2,this C3Acont entlieswit hin the acceptable limitsof6-12% forcon creteapplicationin marineenvironmen t.Chemicalandphysical analysisoftheportlandcement used are given in TablesAt andTableA2 of Appendix A,reapecflvely.
3.2.1.2Fly Ash
ClassFlignanflyash (romNova Scotiawas used.Chemicalandphysicalanalysis ofthe fly ash are giveninTables Al and TableA2 ofAppendix A,respect ively.
CIMsFfly ashisnormallyproduced from burning&D.thraciteorbituminouscoal and has pozzolanlc propertiesbutlittleornocementitious properties.Generally, Ii)'ash improvestheworkability,reta rdsthetime o(setting ofconcret e,lowers the heatof hydration,imp roves resistanceto ettcekby sulphatesoilsand may reduce thecostofcementiti ousmaterials.
3.2.1. 3 SilicaFume
A typical chemicalandphysicalanalysisforthesilicafume, whichwas supplied fromthe only Canadian sourcein Quebec,are given in TableA I and Table A2 of AppendixA.respect ively. Silica. fumeisa by-productinthemanufactureof Ier- rosilicon and silicon met al. Because ofits extremefineness and highglasscontent, silicafume is a veryefficientpczaclenicmaterial. Therefore,it is able to react very efficiently withthe products ofhydration ofportlandcementto createsecondary cementing mate rialsin hydratingconcrete.Inasilica.fume concret esystem,the calcium hydroxide prod ucedbythehydrat ingpo..tlend cement is largelyconsumed inthe ensuing pozzolan icreactions.Thisresults in a productwithvery lowperme- abilityandabsorption,thus enhancing the resistanceto deterioratio nin aggressive environments.
3.2.2 Aggregat e s
3.2.2 .1 Coa rse Aggregate
Local aggregateswere used; thecoarseaggregatewasmostl y crushed quart zite sand- stonewith aminorper centage ofsiltstoneandshale, andwitha maximum nominal size of 20 mm. The use of crushed aggregates, ratherthan round aggregates,is recommended for theproductionofhighstrengthconcrete. Carrasquillo{42)re- ported that concretemadeusinground gravelaggrega tedoes not produceashigh compressivestrengt has concr etesusingcrushedaggregates;thiswas attributedto the reducedaggregat e-mortarinterfacebondstrengthofnat uralgravel aggregates.
34
3.2.2 .2Fine Aggregate
Theoptimumgrading of thefineaggregatesforhigh-str en gth conc ret eisde termined morebyits effectonth ewaterrequirementsthanon phys ical packin g {43].Blick [44J stat edthatasand wit ha finenessmodu lus(FM)below2.5gavetheconcretea sticky consiste ncy, ma king itdifficult to compact.Howe ver,san d withan FM of about3.0providedthebest workability and com pressivestren gth. Inthepresent investigation, thefine aggregat ehadidenticalcom positio n as thecoarse aggregate, mainly quartzite sandstone, with a FMof3.1.
Sieveanalysisof theaggregateswas conductedaccord ingto ASTM C 135.Tests fordeter minat ionof sp ecific gravity andabsorp tionperce nt age wer e done according toAST M C 127andASTM C128,resp ectively.Theres ultsofsieveana lysis are plotte d togeth er with thecurvesindicati ngthelimite sp e cifiedin ASTM C 33 for fine and coarse aggregatesas sho wnin Fig3.1. Thegrad i ng andthephysicalprop- erties ofboththe fineand coa rseaggregat esare given inTable3.1andTable3.2, respective ly.
3.2.3 ChemicalAdmixtures
Allchem icaladmixt u resmetth erequire mentofAST MC 494.Thecom pat ibility oftheadmixtu reswiththechoiceof cement is a very import antconsideration so thatnoslumploss orany undes irableeffects inthe concr etearecreated.
3.2.3.1AirEntrain ingAdmixture
The use ofair entr ainme ntismandat ory toassurethe durability of high-strength concrete whenitis su b jected tofreezingand thawingas bas been discussedin
Table 3.1:Gradingof aggrega tes
Ccerseaggregate Ftne agrregete
Sieve Cumula t ive Sieve Cumulative
size percentage size perce ntage
passing passing
19.0 mm 98.23 4.75mm(No.4) 99.86
12.7mm 45.87 2.36mm(No.8) 80.67
l.18 mm 17.2 1 1.18mm(No.16) 41 .85
4.15mm 1.120 600 pm (No. 30) 29.28
300 pm (No. 50) 14.66 150 pm(No.lOO) S.18
80 pm 1.43
Pan 0
Table 3.2:Physicalpropertiesofaggregates
Coarse aggregate Fineagrregate
Bulkspecificgravity,SSD 2.603 2.6 71
Apparentspecificgravity 2.626 2.695
Absorption,percentage 0.56 0.52
35
" , r---
---
Ir-, <, ---- , I ----
. r- - .- .::::: ~ r-,
~.
-- . <,
1\
<, ',,-
.~
~ I"'~
~ \
I '" r'\. <, <, r-, ,
"
r-, ~
<,
r-. ,.-\
I'-... I~
-, \
\
0-
o
'"
oo 36
oo
chapter1. Inthisinvest iga.tion ,a neutralizedvinsolresinair-en t rainingagcn~
conformingtoASTMC 260wasut ilized.
3.2.3 .2Ret ard er
High-strengthconcretemixdesigns incorporate highcementFactorswhichare not common to norm al concre t e. A retarderisbeneficial in controllingearly hydration.
The addition ofwater to retemperthe mixture will resultinmarked strengthre- duction. In thetwo-waysla'"'testing program,a nonchloride water reducingAgent ofpolyhyclroxycarboxylicbase, conformingtoASTM C 494Type BandD,was used.
3.2 .3.3High-ran ge WaterReducer s
The use of highrangewaterred ucin gagents,also knownassuperplasticizers,in high-strengthconcretemay serve thepurposeof inc reasingst .rengthatslump ortn- creasingslump.Througho u t the whole experimenta lprogram,a superplasticizerof sulphonatednaphthalene for malde hydebase,conforming toASTM C 494 Type F, was employed.
3.3 Mix Design
The main objectivesofthetrial mixtures were
•To achieve a 70 MPa. class co ncrete,usinglocally availab le materials inthe provinceof Newfound land.
• To find thebestmix ingpropo rtionstogetherwith best mixing procedurein orde rto achievea be t t erprope rtiesof fresh concrete ,and a highe rcompressive
38
strengthof hardened concrete.
Previousdesignsof thishigh-strengthconcretemix conductedby CANMET[13, 141wereused as a preliminaryguide. In those studies,theoptimu msilicafume contentwas found to be inthe range of5%to 10%onthebasi s of weight, and a 12.5%repl aceme nt of cementby flyash was recomm ended .Atotalof12batches werecond uctedin thepresent investigation. Formostofthe mixturesportland cement togetherwithflyash and silicafumewere kept constantat550kg/m3 ,as
WiI..'Ithe wat er-to-cementrat io at 0.27. The loss of workabilitydue to the useof
silica fumewascom pensatedfor by the use ofsuperp last icizer.
Thefollowingseriesof trialmixeswere conducted:
•Usinga melamine basedeuper ples ticizer . However ,afterfe w trialbatches the melami ne basedsuperplast icizerdidnot givea satisfactoryresultsandit was repla ced witha naphthalenebased superplasticizer.
•Using differe nt replacementratiosofflyash andsilicafume.
•Usingdifferentfineto coarse aggregatera tios.
•Using differe ntwatercement ratios.
•Using a reta rde rinsteadoftile waterred ucingagent.
The conc retemixproport ionsare givenin Table3.3.Table3.4pre sentsthe propertiesoffreshconcret e; thatisslump,air conte ntanddensity,and the com- pressive strengthofhardened concreteat7, 14and28days.
Slump losswas the majorproblemraced during the trialmixes.Evenwhena superplas t.icizer and a waterreducingagent were used, themix lostitsworkabili t y
Table 3.3:Mixproportionsof onecubic meter for thetrialbatches
cemen t By ash eilicaIume w w fine
00='
admixturesnn. c c+f+.s egg. a",. air super- water retarder
ent.ago plast. red . ago
kg % kg % kg kg ml ml ml 1111
1 400 60 12 40 8 0,46 0.36
50.
1100 350 85.J 045-
2 44. 66 12 44 8 0.36 0.29 600 1100 35. 6600 345
3 44. 66 12 55
I. o.
37o.
29 65. 1100 350 7500 345-
4 44. 55 10 55
I.
0.36 0.2950.
1100 350 7500 3455 44. 66 12 44 8 0.34
o.
27 65. 11eo
35. 7500 345-
6 44. 66 12 44 8 0.34 0.27 750 1100 35. 7500 345
-
7
44.
66 12 44 8 0.34 0.27 750 HOD 35. 7500 3458 44. 66 12 44 8 0.34 0.27 650 1100 35. 7500 345
9 44. 66 12 44 8 0.32 0.25 650 1100 35. 7500 345
-
10 44. 66 12 55 10 0.32 0.25 65. HOD 350 7500 345
-
11 400 60 12 40 8 0.33 0.27 65. HOD 350 7500
-
75.12 400 6{1 12 40 8 0.33 0.27 600 llOO 35. 7500
-
75.f3
Table 3.4:Propertiesoffresh concreteand compressivestrength forthe trialbatches
Mix Prop. offresh concrete Camp.streutil.
No. slum p air density 7 14 28
cant. days days days
mm % k
1 m
3 MPa MPa MPa1 25 3.85 2403 36 41 45
2 13 3.75 2407 51 48 54
3 50 4.00 2405 41 51 60
4 76 4.00 2401 '6 51 62
5 25 3.95 2407 50 51 68
6 76 3.80 2408 '0 42 46
7 76 3.80 2408 51
8 9{J 3.85 2407 44 54 71
9 13 3.70 2407 40 48 61
10 14 .170 2407 38 45 68
11 3.3 2405 51 55 68
12 3.3 2411 41 56 10
•Flowingconcrete
40
after15 - 20 minutes. In thetwo-way slab testing program, a retarder hadto be employed since morethan one ba.tch wurequired and the concrete badtobe80win,!l for the castingrequ ireme nts.Itshouldbenoted that differentmixing pecced ures wereemp loyed inorde rto find outthe optimu mmix design. The mixingprocedure recomm endedby CAN!\IET(131didnotgive asatisfacto ry results , when local materialswereused, andthemix wasveryhanh . A newmixingprocedur ewas reccm rnendedandit is described inthe following sectio ns.
3.4 A d opted M ix
3.4.1 Mix De si gnProportionsBased on the trial mixes ,anairentrai nedhighstre ngth-concrete can be produced usingconventional cement and aggregatesfromNewfound land. Theincorporat ion ofsilicafumeandhigh·range wat erreducers makesitpossibleto achievehigh str engths atearly agea. Compressivestr engthof70MPaat28dayswu&ehieved for the concrete containingJ2%classFfiy ash,8%conde nsedsilica fumeanda high-rangewaxerreducingagent ofnaphthalene form aldehyde base.
3.4.2 Mixin gPro ced ure
TJ-efollowing mixing procedur e wasdevelopedfortheproductionof a workable high-stren gth mix usinglocal Newfoundlandmaterials:
•Charge lOa%ofcoarse aggregate;
•Batch100%ofcement;
•Batch100%offlyash;
42
•Batch100%of sand:
•Mixfora-.5minutes afteraddingM%of estimated water withwater reducing agent;
• Prep a re a slurryofsilica fume, together with 25%of grosseuperplaetlcieer doseand 20%of wate r;
•Mixfor5 minutes;
•Add30%of mix water togetherwith air entrain ingadmixture;
•Retempe rwiththerest of euperplastlclaerdoseto targetslump.
3.4. 3 Propertiesof Fresh Concrete
Freshunitweight ofthe concretemix was almost con st an t wit han averagevalue of2400kg{m3.The air contentin the majoritycf the mixtureslie withina0.5% toleranceof thetarget4.0%value. Slumpvalues weregenerally at orbelow the 100 mm targetforthosemixesincorporatinga super plasticizerandlow-rangewater reducing agent, whileforthose mixturesincluding a euperpleeticiseranda retarder, flowingconcretewas attai ned.
3.4.4 Properties of HardenedConcrete
Propert iesofconcretesuchas stress-strainrelationship, modulus ofelas ticity and tensilestrengt harefrequently expressed interms of uniaxial compressivestrengt h or150x300mm cylinders. In thepast, theexpressions available in litera turewere basedonexperimentaldata less than 41 MP&.. Recently, new expressions have been proposed forhigh-strengt h concrete. One ofthe objectivesofthisresearch
istoselectoneof the available expressions forhigh-str ength concret e and check it against the experiment alvalues.Thisexpressionis tobe adopted forthematerial modellingth roughoutthestudy.
Three batches were cast, each batchhad eightee n150 x300cylinders andnine 75x 75 x 300 prisms.Testspecimens were castin threelayers andcompactedby means ofastandardrod.After casting,themoulded specimens were coveredwith water saturatedburlap and leftinthecastingroom at 20"for24hrs.They were thendemouldedand transferredto the watertank untilrequired for tcating . 3.4.4.1Compressi veStre ngth an d Modulus ofElasticity
The compressive strengthandmodu lusof elasticitywereobtained(romtellting 150x300mm cylindersusing;,2670 kN(600 kips) Soiltestcompressiontesting machine.The testcylinderswere cappedwithahigh-str engthsulphurcompound onboth endsand testedin accordance wit hAST MC39 for compressivestrength andC 496 for modulus of elast icity at7, 28 and91 days. Thestrainsweremeasured usinganLVDT mountedon thepiston ofthetestingmachineandbymeans ofa com presecmete r.Theloaddefor mation curveswereplotted automaticallybymeans of aHP plotte r.
Failureof thetest specimensoccurredsuddenly in an inclined,nearlyflat plane passingthroughtheaggregateand the morta r without bias as shown inFig 3.2.
The failure mode ofhigh-strengthconcretein compressionWalltypicalof that ofa homogeneousmaterial.
Themodulusofelasticitywas calculated at astress levelwhich corres ponds to about 0.45
t :
The descendingpartof thecurve was not recordedaccurately44
Figure 3.2: Failure surface of a high-strength concrete cylinder
15
because of thespecimen-t esti ngsysteminteraction,since thereisnoclosed-loop testing r-echin eforaxialcompressiontestingatM.U.N.However , the descending partof thecurvewas monitoredfor some ofthe test specimens.The maximum recordedstrainwas foundto be 0.0036 for the 150 x 300 mm cylinders. Thestress straincurvewas steeper and more linearto a higherapplied stress to strength ratio than for lowerstrengt hconcrete.
The strengthpropertiesof theadopted mix aregiven in Table 3.5. A comparison of theexpe rimenta llyde terminedvalues of the modulus of elasticitywit hthose predictedbythe expressionsrecom mended by ACIComm ittee363 (Eq.3.1)is given in Fig 3.3.
E.=;3320/f.
+
6900 MPa where:Eo Modulus ofelasticityof concrete
t;
Compressive strengt h of concrete 3.4.4 .2Tensil eStrength(3.1)
The tens ilestrengthof concretecanbeexperiment allydeterminedin three different methods.These three methods are: (1)uniaxialtens iletest, (2) splitcylindertest, and (3)beamtestin flexure.The firstmethod of obt aini ng thetensile strength mayberefer red to as "direct",andthesecondandthirdmet hods maybe refer red to as indirect. Inthedirect test for tensilestrength,thespecimen is grippedat its ends and pulled apar tintension. Inthesplitt ing ten sion test,a cylinderis loaded incom pression ontwo diamet ricallyopposit eside s, and thespecimenfails in tensio n on the plan betweenthe loaded sides. Inthebeam flexuretest (modu lus
46
of rupture test),a.rectangularbeam is loaded at the centreorthirdpoints and the beamfails in bending; the computed tensile stressat failurelead is called the modulus of rupture.
In the present study, the flexure strength was obtainedexperimentally,carrying outtests on two different sets of specimens. The first set, used to obtain the modulus of rupture,was conductedon 30 x 30 x 355 mm beamsloaded at midpoint on a300mm span. The testswere carried out using a dosed loop MTS testing machine.The tests were conformingto the procedure outlined inAST M. The second set, usedto obtain thetensilesplittingstrength, was conductedon 150 x 300mmcylinders loaded through a 2670 kN (600 kips) Soiltest compressiontesting machine according to ASTM procedure.
Acomparisonof theexpertrnenralfydetermined values of thesplitting tensile strength and modulusofru pt ure, with those predicted by the expressionsrecom- mended by ACI Committee 363 [Eqa. 3.2and3.3) aregivenin Fig 3.4 andFig 3.5, respectively. The e:.perimentallyobtainedresultswere found to be in good agree- ment ...iththe empiricalexpression adopted by ACI Committee 363 fordetermi ning the modulusofrupt ur e andtensile splittingstrength.
where:
I:,
~0.59Ii
MP.I; =
0.94Ii
MP.s ;
Tensile splittingstrengthofconcrete ,; Mod ulus ofrupt ure(3.2) (3.3)
