high strength concrete - UnivAQ · High strength concrete first started to be ... Self compacting...
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High strength / high performance High strength / high performance concrete concrete A historical perspective A historical perspective
Transcript of high strength concrete - UnivAQ · High strength concrete first started to be ... Self compacting...
High strength / high performance High strength / high performance concreteconcrete
A historical perspectiveA historical perspective
1960. At that time most designers were 1960. At that time most designers were satisfied to design structures based on satisfied to design structures based on the the 15 to 30 MPa15 to 30 MPa concretes that we re concretes that we re well understood, economi cal and safe.well understood, economi cal and safe.
High strength concrete fi rst started to be High strength concrete fi rst started to be used in Chicago area in used in Chicago area in the early the early 1960s1960s ..
Even though the strength of the fi rst high Even though the strength of the fi rst high strength concretes seem qui te modest strength concretes seem qui te modest ((≈≈50MPa) by current standards, i t should 50MPa) by current standards, i t should remembered that, at that ti me, most remembered that, at that ti me, most commercial cements were groun d much commercial cements were groun d much coarser than at present, and the coarser than at present, and the commercial water reducers we re mostly commercial water reducers we re mostly lignosulphonatelignosulphonate--basedbased ..
The compressive strength, over a peri od of The compressive strength, over a peri od of about 10 years, slowly and progressively about 10 years, slowly and progressively raised from raised from 1515--30MPa to 4530MPa to 45 --65MPa65MPa ..
This was the state of devel opment of highThis was the state of devel opment of high--performance concrete technol ogy in the performance concrete technol ogy in the early 1970s when early 1970s when superplasticizers superplasticizers based on polycondensates of based on polycondensates of naphtalene sulphonatenaphtalene sulphonate were first were first introduced to the concrete market (1977).introduced to the concrete market (1977).
It should be mentioned that the fi rst It should be mentioned that the fi rst applications of superplasticizers were applications of superplasticizers were as as fluidifiersfluidifiers , rather than as , rather than as water water reducersreducers ..
By using large enough dosages of By using large enough dosages of superplasticizers it was found possi ble superplasticizers it was found possi ble to lower the W/C ratio of concrete down to lower the W/C ratio of concrete down to to 0.300.30 and still get an initial slump of and still get an initial slump of 200mm.200mm.
It was possible to decrease W/C rati o to It was possible to decrease W/C rati o to 0.30, then to 0.27, then to 0.25 and 0.30, then to 0.27, then to 0.25 and recently down to 0.23 to obtain a recently down to 0.23 to obtain a compressive strength of 130MPa.compressive strength of 130MPa.
It was It was onlyonly in the in the latelate 1970s 1970s thatthat silicasilicafumefume startedstarted toto bebe usedused asas a a supplementarysupplementary cementitiouscementitious material in material in concrete.concrete.
The particular advantage of usi ng silica The particular advantage of usi ng silica fume as a very fi ne and reactive fume as a very fi ne and reactive pozzolanpozzolanfor use in high performance concrete was for use in high performance concrete was rapidly recognized.rapidly recognized.
By using silica fume, it has been shown that By using silica fume, it has been shown that it is possible to make workable concretes it is possible to make workable concretes with compressive strengths in the 100 to with compressive strengths in the 100 to 150MPa range.150MPa range.
High performance concrete i s not a High performance concrete i s not a super super materialmaterial , without any weaknesses and , without any weaknesses and drawbacks. But as we l earn more through drawbacks. But as we l earn more through its growing use, research, successful its growing use, research, successful experiments and fai lures, it will be more experiments and fai lures, it will be more effectively utilized to the advantage of us effectively utilized to the advantage of us all.all.
High performance concrete makes High performance concrete makes concrete a better performi ng material, concrete a better performi ng material, allowing designers to use i t efficiently in allowing designers to use i t efficiently in increasingly slender structures.increasingly slender structures.
Concrete with high strengthConcrete with high strength
The way in The way in which concrete which concrete transfers i ts transfers i ts forces can be forces can be schematical ly be schematical ly be imagined as imagined as shown in the shown in the figure.figure.
The forces in the concrete go from The forces in the concrete go from aggregate to aggregate. Furthermore aggregate to aggregate. Furthermore transversal forcestransversal forces are transmitted by the are transmitted by the binder material between the aggregates.binder material between the aggregates.
The concrete can be l oaded The concrete can be l oaded uniaxiallyuniaxially in in compression until the binder material fails. compression until the binder material fails. Therefore the cracks i n concrete loaded i n Therefore the cracks i n concrete loaded i n compression develop in the direction of the compression develop in the direction of the compressive forces.compressive forces.
It is well known that the transversal forces It is well known that the transversal forces inside the concrete can be mi nimized by inside the concrete can be mi nimized by realizing a realizing a dense packing of t he dense packing of t he aggregatesaggregates . A second well known method . A second well known method in order to increase the strength i s in order to increase the strength i s decreasing the W/C ratio.decreasing the W/C ratio.
The silica fume, a waste product of the The silica fume, a waste product of the siliciumsilicium production, has a significant effect production, has a significant effect of the of the force transmissionforce transmission in the concrete.in the concrete.
The silica fume grains are a factor The silica fume grains are a factor 100 100 smallersmaller than the cement grai ns. This fact than the cement grai ns. This fact can be described as a can be described as a filler effectfiller effect ..
The small grains fit in to the space The small grains fit in to the space between the cement grai ns and thus between the cement grai ns and thus increase the densi ty of the paste, increase the densi ty of the paste, furthermore they are reactive.furthermore they are reactive.
The amorphous si lica fume converse the The amorphous si lica fume converse the weaker CH crystals in the stronger Cweaker CH crystals in the stronger C --SH SH gel. The binder becomes much stronger gel. The binder becomes much stronger and the transi tion zone between and the transi tion zone between aggregate and paste i s improved. The aggregate and paste i s improved. The YoungYoung ’’s modulus of the binder in much s modulus of the binder in much higher than before, thus the concrete i s higher than before, thus the concrete i s more homogeneous.more homogeneous.
An analysis of the cost for usi ng high An analysis of the cost for usi ng high strength concrete l eads to a surpri sing strength concrete l eads to a surpri sing result. The price for high strength result. The price for high strength concrete is about concrete is about twicetwice as much as for a as much as for a conventional concrete. But consi derable conventional concrete. But consi derable advantages have been found i n contrast advantages have been found i n contrast to the price of the concrete i tself.to the price of the concrete i tself.
Because of the smal ler surface of the Because of the smal ler surface of the crosscross--section of the structural elements, section of the structural elements, less concreteless concrete is necessary. Due t o the is necessary. Due t o the fast hardening of the concrete, fast hardening of the concrete, the the construction time can be short enedconstruction time can be short ened. . Because of the dense materi al structure Because of the dense materi al structure of the high strength concrete, the of the high strength concrete, the durability is improveddurability is improved and the and the maintenance costsmaintenance costs are expected to be are expected to be lessless..
Realizing this, it is more Realizing this, it is more and more talking about high and more talking about high performance concrete performance concrete instead of high strength instead of high strength concrete.concrete.
Mix design and proportionsMix design and proportions(according Federal Highway Administration(according Federal Highway Administration’’s)s)HPC helps to optimize the constructi on HPC helps to optimize the constructi on process, but onl y after optimizing the process, but onl y after optimizing the concrete mixture for speci fic concrete mixture for speci fic desiderabledesiderableproperties.properties.The mix designer may require higher The mix designer may require higher compressive strength, or speci fic compressive strength, or speci fic requirements on shrinkage, creep or requirements on shrinkage, creep or modulus of elasticity. modulus of elasticity.
Mix design and proportionsMix design and proportions(according Federal Highway Administration(according Federal Highway Administration’’s)s)Exposure condi tions may dictate concrete Exposure condi tions may dictate concrete that has speci fic levels of resistance to that has speci fic levels of resistance to sulphate attach, abrasi on resistance, sulphate attach, abrasi on resistance, resistance to alkaliresistance to alkali--silica reaction or frost silica reaction or frost damage. damage.
Mix design and proportionsMix design and proportions(according Federal Highway Administration(according Federal Highway Administration’’s)s)Mixture proportioning Mixture proportioning -- cementcementTypically use only Typically use only TipeTipe I or I or TipeTipe II cement.II cement.Look for medium range fineness in the Look for medium range fineness in the cement, if cement is too fine, the concrete cement, if cement is too fine, the concrete will produce excess heat duri ng hydration. will produce excess heat duri ng hydration. The cements produced under the same The cements produced under the same specification may not perform the same.specification may not perform the same.
Mix design and proportionsMix design and proportions(according Federal Highway Administration(according Federal Highway Administration’’s)s)Mixture proportioning Mixture proportioning -- cementcementThe mix designer will need to trial batch The mix designer will need to trial batch concrete with a specific cement and then concrete with a specific cement and then continue to use this cement for the continue to use this cement for the duration of project.duration of project.
Mix design and proportionsMix design and proportions(according Federal Highway Administration(according Federal Highway Administration’’s)s)Mixture proportioning Mixture proportioning -- cementcementHPC requires an understandi ng of the HPC requires an understandi ng of the interaction of Portland Cement wi th Fly interaction of Portland Cement wi th Fly Ash or Silica Fume.Ash or Silica Fume.Usually the combination of Fly Ashes that Usually the combination of Fly Ashes that are low in calcium and cement del ays the are low in calcium and cement del ays the setting time and thus the shortsetting time and thus the short --term term strength development.strength development.
Mix design and proportionsMix design and proportions(according Federal Highway Administration(according Federal Highway Administration’’s)s)Mixture proportioning Mixture proportioning -- cementcementSilica Fume has an extremel y small Silica Fume has an extremel y small surface area wi th the ability to react with surface area wi th the ability to react with calcium hydroxide to form a very hard and calcium hydroxide to form a very hard and impermeable concrete.impermeable concrete.However one short comi ng of this material However one short comi ng of this material is its propensity for shrinkage cracking is its propensity for shrinkage cracking (bleeding retarded).(bleeding retarded).
Mix design and proportionsMix design and proportions(according Federal Highway Administration(according Federal Highway Administration’’s)s)Mixture proportioning Mixture proportioning -- aggregatesaggregatesSmaller maximum size aggregates are Smaller maximum size aggregates are typically needed to ensure a high mortar to typically needed to ensure a high mortar to aggregate bond.aggregate bond.A general guideline should suggest that as A general guideline should suggest that as the compressive strength i ncreases, use the compressive strength i ncreases, use 25mm to 10mm maxi mum size aggregate.25mm to 10mm maxi mum size aggregate.
Mix design and proportionsMix design and proportions(according Federal Highway Administration(according Federal Highway Administration’’s)s)Mixture proportioning Mixture proportioning -- aggregatesaggregatesCrushed aggregates are better than Crushed aggregates are better than smooth, because of the angul ar surfaces smooth, because of the angul ar surfaces that are formed as a resul t of the crushing that are formed as a resul t of the crushing process.process.The rough angular surfaces forms a strong The rough angular surfaces forms a strong bond at the aggregate/cement paste bond at the aggregate/cement paste interface.interface.
Mix design and proportionsMix design and proportions(according Federal Highway Administration(according Federal Highway Administration’’s)s)Mixture proportioning Mixture proportioning -- aggregatesaggregatesFine aggregate is al so a very important Fine aggregate is al so a very important part of the mixture.part of the mixture.In general HPC can be produced b y using In general HPC can be produced b y using natural or uncrushed rounded sands, wi th natural or uncrushed rounded sands, wi th a fineness modulus between 2.6 a nd 3.1.a fineness modulus between 2.6 a nd 3.1.When designing HPC, the ratio of the fine When designing HPC, the ratio of the fine to coarse aggregate i s reduced.to coarse aggregate i s reduced.
Mix design and proportionsMix design and proportions(according Federal Highway Administration(according Federal Highway Administration’’s)s)Mixture proportioning Mixture proportioning -- aggregatesaggregatesThe workability of concrete is assisted by The workability of concrete is assisted by the use of high range water reducing the use of high range water reducing admixtures.admixtures.
Mix design and proportionsMix design and proportions(according Federal Highway Administration(according Federal Highway Administration’’s)s)Mixture proportioning Mixture proportioning -- admixturesadmixturesThe purpose of hi gh range water reducing The purpose of hi gh range water reducing admixtures (HRWR) i s to give additional admixtures (HRWR) i s to give additional time to place concrete and to del ay the time to place concrete and to del ay the setting time.setting time.HRWR will normally reduce early strength HRWR will normally reduce early strength at 24 hours, but after thi s initial delay, at 24 hours, but after thi s initial delay, typical strength gain occurs. typical strength gain occurs.
Mix design and proportionsMix design and proportions(according Federal Highway Administration(according Federal Highway Administration’’s)s)Mixture proportioning Mixture proportioning -- admixturesadmixturesThe dosage rate of HRWR will typically be The dosage rate of HRWR will typically be higher than normal concrete mixtures.higher than normal concrete mixtures.If a newer generation of HRWR are uti lized If a newer generation of HRWR are uti lized (admixture designed for flowing and SCC) (admixture designed for flowing and SCC) the HRWR usag e rates will be defined by the HRWR usag e rates will be defined by trial batching the concrete and i dentifying trial batching the concrete and i dentifying the plastic properties of the mixtures.the plastic properties of the mixtures.
Mix design and proportionsMix design and proportions(according Federal Highway Administration(according Federal Highway Administration’’s)s)Mixture proportioning Mixture proportioning -- admixturesadmixturesIf the concrete wi ll be exposed to cycl es of If the concrete wi ll be exposed to cycl es of freezing and thawing, air entrainment freezing and thawing, air entrainment admixture will be needed to al low the admixture will be needed to al low the concrete to expand and contract. concrete to expand and contract.
Mix design and proportionsMix design and proportions(according Federal Highway Administration(according Federal Highway Administration’’s)s)Mixture proportioning Mixture proportioning -- admixturesadmixturesA typical amount of ai r entrainment is A typical amount of ai r entrainment is approximately 3 to 4%. However air approximately 3 to 4%. However air entrainment will reduce the strength of entrainment will reduce the strength of concrete (5concrete (5 --7%). 7%).
Early crackingEarly crackingHigh performance concretes have the reputation High performance concretes have the reputation of being prone to develop early cracking.of being prone to develop early cracking.HPC have a low water/cement or water/binder HPC have a low water/cement or water/binder ratio and this makes a great difference from a ratio and this makes a great difference from a volumetric stability point of view.volumetric stability point of view.Basically HPC have a Basically HPC have a w/cw/c or or w/bw/b ratio lower than ratio lower than 0.4.0.4.The lower The lower w/cw/c or or w/bw/b the earlier and greater the the earlier and greater the development of development of autogenousautogenous shrinkage. shrinkage.
TimeTime--developmentdevelopment of of autogenousautogenous and and dryingdrying shrinkageshrinkagein in normalnormal and and highhigh--performanceperformance concreteconcrete
Early crackingEarly crackingMoreover, low Moreover, low w/cw/c or or w/bw/b concretes after their concretes after their placing do not bleed, therefore there are prone to placing do not bleed, therefore there are prone to develop plastic shrinkage if not appropriately develop plastic shrinkage if not appropriately cured.cured.
How to minimize early crackingHow to minimize early crackingFrom a practical point of view HPC having From a practical point of view HPC having w/cw/c or or w/bw/b ratio close to or slightly higher than 0.36 and ratio close to or slightly higher than 0.36 and cured in the presence of external source of water cured in the presence of external source of water are rather robust towards early cracking.are rather robust towards early cracking.In order to make HPC structures durable, plastic In order to make HPC structures durable, plastic shrinkage and shrinkage and autogenousautogenous shrinkage must shrinkage must absolutely be eradicated during the first hours absolutely be eradicated during the first hours following its following its placing.placing.
How to minimize early crackingHow to minimize early crackingFog spraying or the application of an evaporation Fog spraying or the application of an evaporation retarderretarder must be carried out to prevent a plastic must be carried out to prevent a plastic shrinkage.shrinkage.As the As the autogenousautogenous shrinkage is linked to cement shrinkage is linked to cement hydration, water curing must start as soon as hydration, water curing must start as soon as hydration hydration starts.starts.
Mechanical propertiesMechanical propertiesStressStress--strain curve in compressionstrain curve in compressionThe stressThe stress--strain curve for HSC is different than strain curve for HSC is different than that for normal strength concrete.that for normal strength concrete.As the concrete strength increases, the concrete As the concrete strength increases, the concrete stressstress--strain curves exhibit increased initial strain curves exhibit increased initial stiffness and greater linearity.stiffness and greater linearity.In HSC In HSC microcracksmicrocracks occur only at higher load occur only at higher load levels than in concrete of normal strength, thus levels than in concrete of normal strength, thus the stressthe stress--strain curves show a linear elastic strain curves show a linear elastic shape over a large shape over a large area.area.
Mechanical propertiesMechanical propertiesStressStress--strain curve in compressionstrain curve in compressionThe strain under maximum load grows with The strain under maximum load grows with increasing concret e strength.increasing concret e strength.After the maximum load has been exceeded the After the maximum load has been exceeded the stresses drop steeply.stresses drop steeply.The portion of the energy consumption in the post The portion of the energy consumption in the post peak area is lower when compared to concrete of peak area is lower when compared to concrete of normal normal strength.strength.
VariationVariation of of compressivecompressive stressstress--strainstrain curvescurveswithwith increasingincreasing compressivecompressive strengthstrength
StressStress--strainstrain diagramdiagram forfor uniaxialuniaxial compressioncompression
TangentTangent modulesmodules EEcici, E, Ec1c1, , εεc1c1 and and εεcc,,limlim forfor variousvariousconcrete concrete gradesgrades
StressStress--strainstrain diagramsdiagrams forfor differentdifferent concrete concrete strengthsstrengths
Mechanical propertiesMechanical propertiesTensile strengthTensile strengthA significant effect of the increase in compressive A significant effect of the increase in compressive strength is the decreasing ratio of the direct strength is the decreasing ratio of the direct tensile strength versus the result of the standard tensile strength versus the result of the standard splitting test.splitting test.So the conventional determination of tensile So the conventional determination of tensile strength may strength may overstimateoverstimate the real tensile the real tensile properties or cracking limit of properties or cracking limit of concrete.concrete.
Mechanical propertiesMechanical propertiesLong term evaluation Long term evaluation otot tensile strengthtensile strengthIt was noted a systematic loss in tensile strength It was noted a systematic loss in tensile strength statistically related to compressive strength, statistically related to compressive strength, reaching about reaching about --10% for 80MPa compressive 10% for 80MPa compressive strength and strength and --20% for 120MPa.20% for 120MPa.It seems reasonable to relate this loss to It seems reasonable to relate this loss to autogenousautogenous shrinkage, which increases with shrinkage, which increases with compressive strength and leads to internal compressive strength and leads to internal cracking due to the stiffness of cracking due to the stiffness of aggregates.aggregates.
Durability performance of HPCDurability performance of HPCA large amount of experimental data indicate that A large amount of experimental data indicate that as the compressive strength increases from as the compressive strength increases from 25MPa to 100MPa the porosity accessible to 25MPa to 100MPa the porosity accessible to water is reduced up to about 50%, the chloride water is reduced up to about 50%, the chloride diffusion factor is significantly reduced, diffusion factor is significantly reduced, carbonated dept after 28 days, significant for carbonated dept after 28 days, significant for normal strength concrete, is not detectable also normal strength concrete, is not detectable also after 90 days after 90 days for HPC (>50MPa).for HPC (>50MPa).
PorePore sizesize distributiondistribution of UHPC, HPC and of UHPC, HPC and NormalNormalStrengthStrength Concrete Concrete
ChlorideChloride diffusiondiffusion valuesvalues of UHPC, HPC and of UHPC, HPC and NormalNormal StrengthStrength ConcreteConcrete
ScalingScaling of UHPC under of UHPC under freezefreeze--saltsalt attackattackcomparedcompared withwith air air entrainedentrained concretesconcretes
Self compacting concreteSelf compacting concrete
A large disadvantage of conventi onal A large disadvantage of conventi onal concrete is that it has to be mechani cally concrete is that it has to be mechani cally compacted. Thi s compacting is an compacted. Thi s compacting is an unpleasant job. Also in this case, a smal l unpleasant job. Also in this case, a smal l change in the mix composi tion, can lead change in the mix composi tion, can lead to a significant improvement. The i dea is to a significant improvement. The i dea is rather simple. One adds a rather simple. One adds a little more little more binderbinder than is necessary for a goo d than is necessary for a goo d performance of the concrete i n hardened performance of the concrete i n hardened state.state.
By this small amount of addi tional cement By this small amount of addi tional cement paste, small layers of paste, small layers of lubrificantlubrificant develop develop around each grain. The grai ns start to around each grain. The grai ns start to float. The inner friction during the flow of float. The inner friction during the flow of the concrete is further reduced due to the the concrete is further reduced due to the presence of presence of fly ashfly ash or or limestone limestone powderpowder as an extra component. as an extra component.
These materials have a grain diameter These materials have a grain diameter which roughly fits between sands and which roughly fits between sands and cement. A three phase parti cle skeleton cement. A three phase parti cle skeleton with high density is formed. The grai ns with high density is formed. The grai ns move and rol l easily over and along each move and rol l easily over and along each other, which makes the materi al other, which makes the materi al very very flowableflowable..Due to the dense grai n packing, the Due to the dense grai n packing, the strength is automatically relatively high.strength is automatically relatively high.
On the other hand the mi xtures are rather On the other hand the mi xtures are rather sensitive for smal l changes in the sensitive for smal l changes in the properties of the ingredients.properties of the ingredients.It is therefore not surpri sing that the It is therefore not surpri sing that the application of self compacti ng concrete application of self compacti ng concrete mainly develops in the prefab i ndustry.mainly develops in the prefab i ndustry.The circumstances in a factory are The circumstances in a factory are relatively constant and acti ons are relatively constant and acti ons are repeated.repeated.
It is true that the sel f compacted concrete It is true that the sel f compacted concrete is a little more expensive due to the extra is a little more expensive due to the extra costs for costs for superplasticizersuperplasticizer and filling and filling agents. But considerabl e savings agents. But considerabl e savings compensate for these costs due to compensate for these costs due to decrease of energ y consumption, decrease of energ y consumption, decrease in maintenance costs, l onger decrease in maintenance costs, l onger life time of the moulds.life time of the moulds.
