High Performance Concrete A Review of Book of P.C. Aïtcin Presented by Fahd Aslam
High Performance concreteIf concrete is considered in terms of its water/binder ratio, a high-performance concrete is a concrete having a low water/binder ratio.But how low?A value of about 0.40 is suggested as the boundary between usualconcretes and high performance concretes.This 0.40 value, which might be perceived as being totally arbitrary, isbased on the fact that it is very difficult, if not impossible, to make aworkable and place-able concrete with OPC , without the use of asuper-plasticizer,If the water/binder ratio is lower than 0.40.Moreover, this value is close to the theoretical value suggested byPowers to ensure full hydration of Portland cement.This value denotes concretes that are starting to present autogenousshrinkage.
High Performance concrete(cont…)Autogenous shrinkage (also called self-desiccation or chemicalshrinkage), which can develop when cement hydrates
High Performance concrete(cont…)It is evident that a 0.38 water/binder ratio concrete is not verymuch stronger and will not exhibit much better performance than a0.42 one.But if the water/binder ratio deviates significantly from the 0.40value, usual concretes and high performance concretes have notonly quite different compressive strengths but also quite differentmicrostructures (quite different shrinkage behavior) and quitedifferent overall performances.
Classes of HPCHPC can be divided into five classes. The division of high-performanceconcrete into five classes is not as arbitrary as it seems at firstglance, but derives rather from a combination of experience and thepresent state of the art. This classification might become codified in thenear future as progress takes place in our comprehension of thedifferent phenomena involved in making high performance concrete HPC Class-I Class-II Class-III Class-IV Class-IV (50Mpa) (75Mpa) (100Mpa) (125Mpa) (150Mpa)
Classes of HPC(Cont…)The high-strength range has been divided into five classescorresponding in each case to a 25 MPa increment. These compressive strengths correspond to average values obtained at 28 days on 100×200 mm cylindrical specimens cured under the standard conditions used to cure usual concrete. These are not specified strengths or design strengths, for which the standard deviation of the concrete production has to be taken into account.
Mix design and Purpose of mix designMix design is appropriate selection of different ingredients of concrete, to be designed for specific purpose.The objective of any mixture proportioning method is to Determine an appropriate and economical combination of concrete constituents that can be used for a first trial batch to produce a concrete that is close to that which can achieve a good balance between the Various desired properties of the concrete At the lowest possible cost.
Mix design It will always be difficult to develop a theoretical mix design method that can be used universally with any combination of Portland cement, supplementary cementitious materials, any aggregates and any admixture In spite of the fact that all the components of a concrete must fulfill some standardized acceptance criteria, these criteria are too broad; To a certain extent the same properties of fresh and hardened concrete can be achieved in different ways from the same materials.
Mix design(Cont…) This situation must be perceived as an advantage, because in two different locations the same concrete properties can be achieved differently using non-expensive locally available materials. A mixture proportioning method only provides a starting mix design that will have to be more or less modified to meet the desired concrete characteristics. In spite of the fact that mix proportioning is still something of an art, it is unquestionable that some essential scientific principles can be used as a base for mix calculations.
Limitation of present Mix design Methods It is interesting to see in the present literature a renewal of interest in mix design and mix proportioning. In fact this renewal of interest only traduces the limitations of the present mix proportioning methods that have been used for usual concrete without any problem for many years. As long as usual concrete was essentially a mixture of Portland cement, water, aggregates and sometimes entrained air, these methods could be used with a very good predictive value to design a concrete mixture having a given slump and 28 day compressive strength.
Limitation of present Mix design Methods(Cont…)This is no longer the case because: The water/cement or water/binder range of modern concretes has been drastically extended towards very low values due to the use of super-plasticizers. Modern concrete very often contains one or several supplementary cementitious materials that are in some cases replacing a significant amount of cement. Modern concrete sometimes contains silica fume which drastically changes the properties of fresh and hardened concrete. The slump can be adjusted by using a super-plasticizer instead of water without altering the water/cement or water/binder ratio.
Mix design methods for concrete ACI 211–1 Standard Practice For Selecting Proportions for normal, heavyweight and mass concrete ACI 363 R State-of-the-Art Report on High-Strength Concrete Proposed method from writer of Book De Larrard method (de Larrard, 1990) Mehta and Aïtcin simplified method
ACI 211–1 Standard Practice For Selecting Proportions for normal, heavyweight and mass concreteACI 211–1 Standard Practice for Selecting Proportions forNormal, Heavyweight and Mass Concrete offers a comprehensiveprocedure for proportioning normal weight concrete heaving Maximum specified compressive strength of 40MPa Maximum slump of 180mm.
ACI 211–1 Standard Practice For Selecting Proportions for normal, heavyweight and mass concrete(Cont…)The obtained mixture components do not include any supplementarycementitious materials or admixtures, except for an air-entrainingadmixture.NOTE:This procedure is applicable to aggregate with a wide range ofmineralogical and granulo-metric properties. It essentially assumesthat the W/C ratio and the amount of entrained air are the onlyparameters affecting strength, and that concrete slump is affected bythe maximum size of the coarse aggregate, the amount of mixingwater and the presence or absence of entrained air.
ACI 211–1 Standard Practice For Selecting Proportions for normal, heavyweight and mass concrete(Cont…)Data required to apply ACI 211-1 Fineness modulus of the fine aggregate Dry rodded unit weight of coarse aggregate Specific gravity of aggregates Specific gravity of Cement is 3.15 Moisture and absorption capacity of aggregate
ACI 211–1 Standard Practice For Selecting Proportions for normal, heavyweight and mass concrete(Cont…)Step by step procedure of mix design according to ACI211-1 Slump selection Determination of MSA Amount of mixing water air content Selection of W/C ratio Cement content Amount of coarse aggregate Calculation of the amount of sand Moisture adjustment TRIAL BATCH
ACI 211–1 Standard Practice For Selecting Proportions for normal, heavyweight and mass concrete(Cont…)MSA “Maximum size of coarse aggregate”For usual concrete it is economical to use a large MSA. The MSAshould not exceed one-fifth of the narrowest dimension between thesides of forms, one-third of the depth of slab, or three quarters of theminimum clear spacing between reinforcing bars, bundled bars ortendons.If we try to apply this procedure to high-performance concrete thereare several drawbacks:
ACI 211–1 Standard Practice For Selecting Proportions for normal, heavyweight and mass concrete(Cont…)Step 1: Slump selectionThe slump of a high-performance concrete is essentially dependent not only on the amountof mixing water but also on the amount of super-plasticizer used. The slump can be adjustedto the specific needs by playing with these two values.Step 2: MSAThe MSA is usually no longer dictated by geometrical considerations. It is also no longeradvantageous to select as coarse an aggregate as possible to , reduce the amount of mixingwater needed to meet a certain slump; rather it is advantageous to select the coarseaggregate as small as possible for placeability considerations and also for concrete strengthconsiderations.
ACI 211–1 Standard Practice For Selecting Proportions for normal, heavyweight and mass concrete(Cont…)Step 3: Estimation of mixing water and air contentIn a high-performance concrete the same slump can be achieved using different amounts ofmixing water and super-plasticizer. The final combination that is usually selected is the onethat gives a slump retention appropriate to field conditions. The suggested air content valuesgiven for ACI 211–1 are no longer appropriate to make high-performance concrete freeze-thaw resistant; it is rather the spacing factor that is the more relevant parameter in designinga freeze-thaw-resistant high-performance concrete.Step 4: Selection of the W/C ratioHigh-performance concrete most of the time contains one or several supplementarycementitious materials (fly ash, slag, silica fume), so the simple relationship linking the 28day compressive strength to the water/ cement ratio used in ACI 211–1 is in general nolonger valid and must be established in each particular case.
ACI 211–1 Standard Practice For Selecting Proportions for normal, heavyweight and mass concrete(Cont…)Step 5: Coarse aggregate contentThis is no longer influenced by the fineness modulus of the sand. The mixture is so rich inpaste that, whenever possible, it is better to use a coarse sand.
Requirements of HPCMoreover, unlike usual concrete, high-performance concrete can have several characteristicsthat need to be met simultaneously. Among these requirements are low permeability high durability high modulus of elasticity low shrinkage, low creep high strength high and lasting workabilityBecause of the large number of mixture components used in high-performance concreteand because of the various concrete requirements that can contradict one another, it is verydifficult to use a mix design method that gives mix proportions very close to that of the finalmixture
Required Compressive strengthMost proportioning methods are based on concrete compressivestrength; therefore, it is important to define the exact value ofconcrete compressive strength that has to be achieved before usingany mix proportioning method.Design strength or specified strength: f’c: this is the strength that hasbeen taken into account by the designer in his or her calculations.Required average strength: f’cr: this is the average strength requiredto meet the acceptance criteria; the f ’cr value depends on the designor specified value, but also on the level of control that is actuallyachieved in the field and the acceptance criteria.
Required Compressive strength(Cont…)ACI 318, Building Code Requirements for Reinforced Concrete and ACI322, Building Code Requirements for Structural Plain Concrete call fordesigning concrete using an average compressive strength of field testresults (f ’cr) that is higher than f ’c in order to reduce the occurrenceof strength values lower than f ’c. For usual concrete, the required f ’crshould be the larger value given by the following two equations:
Proposed method from writer of BookThe method that is discussed in the following is related to The calculation of the composition of non-air-entrained high- performance concrete. It can be used to design air-entrained high-performance concrete provided that the strength reduction due to the presence of the air bubble system is taken into account. The method itself is very simple: it follows the same approach as ACI 211–1 Standard Practice for Selecting Proportions for Normal, Heavyweight and Mass Concrete. It is a combination of empirical results and mathematical calculations based on the absolute volume method. The water contributed by the super-plasticizer is considered as part of the mixing water.
Proposed method from writer of Book(Cont…)The procedure is initiated by selecting five different mixcharacteristics or materials proportions in the following sequence:No. 1—the W/B ratio;No. 2—the water content;No. 3—the super-plasticizer dosage;No. 4—the coarse aggregate content;No. 5—the entrapped air content (assumed value).
Proposed method from writer of Book(Cont…)No. 1. Water/binder ratioThe suggested water/binder ratio can be found from following figurefor a given 28 day compressive strength (measured on 100×200 mmfaced cylinders). Owing to variations in the strength efficiency ofdifferent supplementary cementitious materials, the curve in Figure8.9 shows a broad range of water/binder values for a given strength.If the efficiency of the different supplementary cementitiousmaterials is not known from prior experience, the average curve canbe used to give an initial estimate of the mix proportions.
Proposed method from writer of Book(Cont…)No. 2. Water contentOne difficult thing when designing high-performance mixtures is todetermine the amount of water to be used to achieve a high-performance concrete with a 200 mm slump 1 hour after batchingbecause the workability of the mix is controlled by several factors: theamount of initial water, the ‘reactivity of the cement’, the amount ofsuper-plasticizer and its degree of compatibility with the particularcement.Therefore a 200 mm slump concrete can be achieved when batchingthe concrete with a low water dosage and a high super-plasticizerdosage or with a higher water dosage and a lower super-plasticizerdosage. From an economical point of view there is no great differencebetween the two options, but from a rheological point of view thedifference can be significant.
Proposed method from writer of Book(Cont…)No. 2. Water content(cont…)Depending upon the rheological ‘reactivity of the cement’ and theefficiency of the super-plasticizer. Owing to differences infineness, phase composition, phase reactivity, composition andsolubility of the calcium sulfate of the cements, the minimum amountof water required to achieve a high performance concrete with a 200mm slump varies to a large extent. If the amount of mixing waterselected is very low, the mix can rapidly become sticky and as a highamount of superplasticizer has to be used to achieve this highslump, some retardation can be expected.The best way to find the best combination of mixing water andsuperplasticizer dosage is by carrying out a factorial designexperiment. but this method is not always practical, therefore asimplified approach based on the concept of the saturation point isgiven in following figure.
Proposed method from writer of Book(Cont…)No. 2. Water content(cont…)To design a very safe mix, 5 l/m3 of water can be added to the valuespresented in following figure. If the saturation point of thesuperplasticizer is not known, it is suggested starting with a watercontent of 145 l/m3.No. 3. Super-plasticizer dosageThe super-plasticizer dosage can be deduced from the dosage at thesaturation point. If the saturation point is not known, it is suggestedstarting with a trial dosage of 1.0%.
Proposed method from writer of Book(Cont…)No. 4. Coarse aggregate contentThe coarse aggregate content can be found from Figure 8.11 as afunction of the typical particle shape. If there is any doubt about theshape of the coarse aggregate or if its shape is not known, a contentof 1000 kg/m3 of coarse aggregate can be used to start with.
Proposed method from writer of Book(Cont…)No. 5. Air contentFor high-performance concretes that are to be used in non-freezingenvironments, theoretically there is no need for entrained air, so theonly air that will be present in the mix is entrapped air, the volume ofwhich depends partly on the mix proportions. However, in order toimprove concrete handling place ability and finish ability the authorstrongly suggests use of an amount of entrained air. When makinghigh-performance concretes with very low water/ binder ratios it hasbeen observed that not every cement-super-plasticizer combinationentraps the same amount of air. Moreover, some concrete mixerstend to entrap more air than others. From experience it has beenfound that it is difficult to achieve less than 1% entrapped air andthat in the worst case the entrapped air contents can be as high as3%. Therefore, the author suggests using 1.5% as an initial estimateof entrapped air content, and then adjusting it on the basis of theresult obtained with the trial mix.
Limitations of proposed methodIn the present state of the art studies, the successful fabrication of high-performance concrete depends on a combination of Empirical rules derived from experience, Laboratory work Great dose of common sense and observation.The proposed method is a transition between an art and a science. Applying stepby step the materials selection criteria and the mix design method proposed in thisbook should help one to find the best, or at least the most acceptable, locallyavailable materials and to select their proportions in order to make the best (orclose to the best) high performance concrete in terms of rheology, strength andeconomy that can be made from the materials available in a given area to meet theexpectations of the designer. However, improvements to this proposed methodcan always be made.
Limitations of proposed method(Cont…) The most important factor in the mix design of a high- performance concrete, much more than in the case of usual concrete mixes, is the selection of materials. A blind application of the proposed mix design method (or of any other method) does not guarantee success because, in high- performance concretes, all concrete ingredients are working at, or near, their critical limits. Any high-performance concrete has a weak link within it, and when it is tested, failure always initiates at this weakest link and then propagates through the stronger parts. The key to success in designing a high-performance concrete mix is to find materials in which the weakest link is as strong as required to meet the performance requirements, while the stronger parts are not so strong that they lead to an unnecessary expenditure.
Limitations of proposed method(Cont…)The mix design method proposed in this chapter is based on experiencegained through the years. Like all mix design methods, it should be usedonly as a guide. In general, the calculated proportions should provide a mixhaving almost the desired characteristics. However, if the cement is not suitable, if the aggregates are not strong enough, if the superplasticizer is not efficient enough, if the selected cement/superplasticizer combination is not fully compatible or if some other unexpected factors intervene,the concrete may not reach the desired level of performance.
Limitations of proposed method(Cont…)For example, when the desired compressive strength is not obtained, and the fracture surface shows a number of intra-granular fractures, the aggregates can be blamed, and a ‘stronger’ aggregate has to be found. If the fracture surface shows considerable debonding between the coarse aggregate and the hydrated cement paste, the coarse aggregate used has too smooth a surface or is simply too dirty, and therefore a coarse aggregate with a rougher or cleaner surface must be used. If the fracture surface passes almost entirely through the hydrated cement paste around the aggregates, a stronger concrete can be made with the same aggregates by lowering the water/binder ratio further. If the compressive strength does not increase when the water/binder ratio decreases, this indicates that the aggregate strength or cement aggregate bonding controls the failure rather than the water/binder ratio, and so a stronger concrete may be possible using the same cement but a stronger aggregate.
Limitations of proposed method(Cont…) If the concrete does not have the desired slump, the superplasticizer dosage is not high enough and must be increased, or else the water dosage has to be increased as well as the cement content in order to keep the same water/binder ratio. If the concrete experiences a rapid slump loss, the cement is perhaps more reactive than expected and the water dosage has to be increased, or the superplasticizer is particularly inefficient with the selected cement, so its dosage has to be increased or another brand of superplasticizer has to be used. If the workability of the mix is inadequate, a poor shape or gradation of the coarse aggregate, or the incompatibility of the particular cement superplasticizer combination can be blamed. In the first case, the amount of coarse aggregate has to be decreased, and in the second case, either the cement or the superplasticizer (or both) should be changed.The method that has been presented is related to non-air-entrained high-performance concrete.
Method suggested in ACI 363 Committee on high- strength concreteACI 363 R State-of-the-Art Report on High-Strength Concrete alsospecifies that the required f ’cr (MPa) should be:
Method suggested in ACI 363 Committee on high- strength concreteThis procedure is consists of nine stepsStep 1: Slump and required strength selectionA table suggests slump values for concretes made with superplasticizers and for thosewithout superplasticizer. The first value of the slump is 25 to 50 mm for the concretebefore adding the superplasticizers in order to ensure that sufficient water is used inthe concrete.Step 2: Selection of the maximum size of the coarse aggregate (MSA)The method suggests using coarse aggregate with an MSA of 19 or 25 mm for concretemade with f ’c lower than 65 MPa and 10 or 13 mm for concrete made with f ’c greaterthan 85 MPa. The method allows the use of coarse aggregate with an MSA of 25 mmwith f ’c between 65 and 85 MPa when the aggregate is of a high quality. As in the caseof usual concrete, the MSA should not exceed one-fifth of the narrowest dimensionbetween the sides of forms, one-third of the depth of slab, or three-quarters of theminimum clear spacing between reinforcing bars, bundled bars or tendons.
Method suggested in ACI 363 Committee on high- strength concreteStep 3: Selection of coarse aggregate contentThis method suggests that the optimum content of coarse aggregate, expressed as apercentage of dry-rodded unit weight (DRUW), can be 0.65, 0.68, 0.72 and 0.75 for nominalsize aggregate of 10, 13, 20 and 25 mm, respectively. The DRUW is measured according toASTM Standard C29 Standard Test Method for Unit Weight and Voids in Aggregate. Thesevalues are given for concrete made with a sand of fineness modulus 2.5 to 3.2. The dry weightof coarse aggregate can then be calculated from the following formula: mass of coarseaggregate, dry = (% DRUW) × (DRUW).Step 4: Estimation of free water and air contentA table gives estimates for the required water content and resulting entrapped air contentfor concretes made with coarse aggregates of various nominal sizes. These estimated watercontents are given for a fine aggregate having a 35% void ratio. If this value is different from35%, then the water content obtained from the table should be adjusted by adding orsubtracting 4.8 kg/m3 for every 1% increase or decrease in sand air void.
Method suggested in ACI 363 Committee on high- strength concreteStep 3: Selection of coarse aggregate contentThis method suggests that the optimum content of coarse aggregate, expressed as apercentage of dry-rodded unit weight (DRUW), can be 0.65, 0.68, 0.72 and 0.75 for nominalsize aggregate of 10, 13, 20 and 25 mm, respectively. The DRUW is measured according toASTM Standard C29 Standard Test Method for Unit Weight and Voids in Aggregate. Thesevalues are given for concrete made with a sand of fineness modulus 2.5 to 3.2. The dry weightof coarse aggregate can then be calculated from the following formula: mass of coarseaggregate, dry = (% DRUW) × (DRUW).Step 5: Selection of W/B ratioTwo tables suggest W/B values for concretes made with and without superplasticizer,respectively, to meet the specified 28 and 56 day compressive strength. These values arebased upon the MSA and f ’c of the concrete.Step 6: Cement contentThe mass of cement is calculated by dividing the mass of the free water by the W/B ratio.
Method suggested in ACI 363 Committee on high- strength concreteStep 7: First trial mixture with cementThe first mixture to be evaluated can be batched using cement and no othercementitious materials. The sand content is then calculated using the absolute volumemethod described in the previous method.Step 8: Other trial mixtures with partial cement replacementsAt least two different cementitious material contents are suggested to replace some ofthe cement mass to produce other trial mixtures that can be batched and evaluated.Maximum cement replacement limits are suggested for fly ash and blast-furnace slag;no limits for silica fume are suggested because this method is valid for a maximum f ’cof 85 MPa. These limits are 15 to 25% of the mass of cement for Class F fly ash, 20 to35% of mass of cement for Class C fly ash, and 30 to 50% of the mass of cement forblast-furnace slag. Again, the unit mass of sand is calculated using the absolute volumemethod.Step 9: Trial batchesThe mass of aggregates, along with that of the mixing water, are adjusted for actualmoisture conditions, and trial batches are made using concretes made with no cementreplacement and others using fly ash or blast furnace slag. The concretes are thenadjusted to meet the desired physical and mechanical characteristics.
De Larrard method (de Larrard, 1990)This method is based on two semi-empirical mix design tools. The strength of the concrete ispredicted by a formula which is in fact an extension of the original Feret’s formula. Where alimited number of mix design parameters are to be used:wherefc = the compressive strength of concrete cylinders at 28 days;w, c, s = the mass of water, cement and silica fume for a unit volume of freshconcrete, respectively;Kg = a parameter depending on the aggregate type (a value of 4.91 applies to commonriver gravels);Rc = the strength of cement at 28 days (e.g. the strength of ISO mortar containing threeparts of sand for each part of cement and onehalf part of water).
De Larrard method (de Larrard, 1990)(Cont…)The workability is assumed to be closely related to the viscosity of the mix, which iscomputed using the Farris model. In a mix containing n classes of monodispersed grains ofsize di > di + 1, the viscosity of the suspension is assumed to be:whereΦi= the volume occupied by the i-class in a unit volume of mix;Φ0 = the volume of water;η0 = the viscosity of water;H = a function representing the variation of the relative viscosity of a monodispersedsuspension as a function of its solid concentration.
De Larrard method (de Larrard, 1990)(Cont…)The main idea of the method is to perform as many tests as possible on grouts forrheological tests and mortars for mechanical tests.The first step consists of proportioning a control concrete containing a largeamount of superplasticizer with the amount of cement corresponding to thelowest water demand possible. This amount of water is adjusted to obtain the rightworkability as measured with a dynamic apparatus.The second step consists of measuring the flow time of the paste of this controlconcrete using a Marsh cone.The third and fourth steps involve the adjustment of the binder composition andof the amount of superplasticizer until the flowing time does not decreaseanymore. This amount of superplasticizer is said to correspond to the saturationvalue.
De Larrard method (de Larrard, 1990)(Cont…)The fifth step involves the adjustment of the water content to obtain the sameflowability as in the control mix.The sixth step consists of following the variation of the flow time with time. If theflow time increases too much, a retarding agent should be added.The seventh step corresponds to the prediction of the strength of the high-performance concrete using Feret’s formula or the measurement of thecompressive strength of different mortars.In the eighth step, a high-performance concrete is made and its composition isslightly modified in order to get the targeted workability and strength values.
Mehta and Aïtcin simplified methodMehta and Aïtcin (1990) proposed a simplified mixture proportioning procedure that isapplicable for normal weight concrete with compressive strength values of between 60 and120 MPa. The method is suitable for coarse aggregates having a maximum size of between10 and 15 mm and slump values of between 200 and 250 mm.It assumes that non-airentrained high-performance concrete has an entrapped air volume of2% which can be increased to 5 to 6% when the concrete is air-entrained. The optimumvolume of aggregate is suggested to be 65% of the volume of the high-performance concrete.Step 1: Strength determinationA table lists five grades of concrete with average 28 day compressive strength ranging from 65to 120 MPa.Step 2: Water contentThe maximum size of the coarse aggregate and slump values are not considered here forselecting the water content since only 10 to 15 mm maximum size are considered andbecause the desired slump (200 to 250 mm) can be achieved by controlling the dosage ofsuperplasticizer. The water content is specified for different strength levels.
Mehta and Aïtcin simplified method(Cont…)Step 3: Selection of the binderThe volume of the binding paste is assumed to be 35% of the total concrete volume. Thevolumes of the air content (entrapped or entrained) and mixing water are subtracted fromthe total volume of the cement paste to calculate the remaining volume of the binder. Thebinder is then assumed to be one of the following three combinations:Option 1. 100% Portland cement to be used when absolutely necessary.Option 2. 75% Portland cement and 25% fly ash or blast-furnace slag by volume.Option 3. 75% Portland cement, 15% fly ash, and 10% silica fume by volume.A table lists the volume of each fraction of binder for each strength grade.Step 4: Selection of aggregate contentThe total aggregate volume is equal to 65% of the concrete volume. For strength gradesA, B, C, D and E the volume ratios of fine to coarse aggregates are suggested to be2.00:3.00, 1.95:3.05, 1.90:3.10, 1.85:3.15 and 1.80:3.20, respectively.
Mehta and Aïtcin simplified method(Cont…)Step 5: Batch weight calculationThe weights per unit volume of concrete can be calculated using the volume fractions of theconcrete and the specific gravity values of each of the concrete constituents. Usual specificgravity values for Portland cement, Type C fly ash, blast-furnace slag and silica fume are3.14, 2.5, 2.9 and 2.1, respectively. Those for natural siliceous sand, normal-weight gravel orcrushed rocks can be taken as 2.65 and 2.70, respectively. A table lists the calculated mixtureproportions of each concrete type and strength grade suggested in this method.Step 6: Super-plasticizer contentFor the first trial mixture, the use of a total of 1% super-plasticizer solid content of binder issuggested. The mass and volume of a Super-plasticizer solution are then calculated takinginto account the percentage of solids in the solution and the specific gravity of the Super-plasticizer (for naphthalene super-plasticizer a typical value of 1.2 is suggested).
Mehta and Aïtcin simplified method(Cont…)Step 7: Moisture adjustmentThe volume of the water included in the super-plasticizer is calculated and subtracted fromthe amount of initial mixing water. Similarly, the mass of aggregate and water are adjustedfor moisture conditions and the amount of mixing water adjusted accordingly.Step 8: Adjustment of trial batchBecause of the many assumptions made in selecting a mixture proportioning, usually the firsttrial mixture will have to be modified to meet the desired workability and strength criteria.The aggregate type, proportions of sand to aggregate, type and dosage ofsuperplasticizer, type and combination of supplementary cementitious materials, and the aircontent of the concrete can be adjusted in a series of trial batches to optimize the mixtureproportioning.
Use of HPC world wideThese 11 civil engineering projects are:1. Water Tower Place, built in 1970 in Chicago, Illinois, USA(60Mpa Concrete is used, lignosulfonate based water reducers were being used).ten different cements were tested to determine their rheological and mechanicalcharacteristics. Several commercially available water reducers were tested with the selectedcement in order to choose the most suitable additive. The objective of this testingprogramme was to produce a slump value of 100 mm at the job site without causingexcessive entrapment of air or excessive set retardation. In order to lower the amount ofmixing water necessary to obtain that 100 mm slump on the job site, approximately 15% ofa high-quality Class F fly ash was with a 60 MPa concrete before superplasticizers began tobe used in high performance concrete.
Use of HPC world wide2. Norway’s Gullfaks offshore platform, built in 1981.for the Gullfaks platform, the cement content was reduced to 400 kg/m3, the water contentwas reduced to 165 l/m3, and 61 of admixtures and 10 kg of silica fume were added toimprove the pumpability and cohesiveness of the high-workability concrete. The averageslump was 240 mm, and the average compressive strength 79 MPa. In this lattercase, uniform production quality has been achieved with a standard deviation of 3.4MPa, which corresponds to a coefficient of variation of 4.3%.3. Sylans and Glacières viaduct, built in France in 1986.it was necessary to adjust the 28 day compressive strength to 60MPa for the X and to50MPa for the deck, although the required design strength for the whole project was only40 MPa. No low-pressure steam curing was used. These high levels of strength wereachieved using 400 kg/m3 of cement and no silica fume. Enough melamine-basedsuperplasticizer was used to obtain a slump of 200 to 250 mm. The concrete had awater/cement (W/C) ratio of 0.37.
Use of HPC world wide4. Scotia Plaza, built in 1988 in Toronto, Canada. Water Cementatious materials Aggregates Admixture (kg/m3) (kg/m3) (l/m3) Cement Silica Slag Coarse Fine WR SP fume 145 315 36 135 1130 745 0.835 6.0This concrete was prepared in a dry-batch plant, meaning that the concrete was truck-mixed. A special loading sequence and procedure had to be developed in order to obtain areproducible mixture. The procedure developed was successful because the compressivestrength of the 142 loads of concrete that were tested averaged 93.6 MPa at 91 days with acoefficient of variation of 7.3%. This means that, if one test out of ten lower than thespecified strength is considered the minimum acceptance criterion, then the actual designstrength of the concrete was 85 MPa, which is well above the 70 MPa specified compressivestrength.
Use of HPC world wide5. Île de Ré bridge, built in 1988 in France.Île de Ré, a small island lying 2km off the coast of France near La Rochelle, is a popularvacation resort..While the design strength used in calculating the girders was 40 MPa(Virlogeux, 1990), they were actually built of a concrete with a characteristic compressivestrength of 59.5 MPa, well above the specification. The mean 28 day compressive strengthfor the 798 cylinders tested was 67.7 MPa with a standard deviation of 6.3 MPa. This level ofcompressive strength was selected because a 20MPa compressive strength was needed at 10to 12 hours of age to strip the box girders. The average slump of the concrete was 150 mm(Cadoret, 1987).6. Two Union Square, built in 1988 in Seattle, Washington, USA.The Two Union Square Building located in Seattle, Washington, offers a variety of interestingfeatures. In this particular case, the compatibility of the naphthalene superplasticizer withthe selected cement was excellent so that a water/cementitious ratio of 0.22 could be used.Such a strength level was achievable also because outstanding aggregates were available inSeattle. The selected coarse aggregate had a nominal size of 10 mm. It was a fluvioglacial peagravel, very strong, very clean and rough enough to develop a good mechanical bond withthe hydrated cement paste . The sand came from the same pit and had a rather coarsegradation corresponding to an average fineness modulus of 2.80. It consisted of sharpangular particles.
Use of HPC world wide7. Joigny bridge, built in 1989 in France.The Joigny bridge, located on the Yonne river 150 km southeast of Paris. The two piers wereconstructed with usual concrete so that high-performance concrete was used only for theconstruction of the bridge deck. The bridge deck was designed for a design strength of 60MPa instead of the usual 35 to 40 MPa design strength required by the French code.The average slump was 220 mm, with a minimum of 190 mm and a maximum of 250 mm.Compressive strength measured on 160×320 mm cylindrical specimens was:3 days = 26.2 MPa7 days = 53.6 MPa28 days = 78.0 MPa
Use of HPC world wide8. Montée St-Rémi bridge built near Montreal, Canada in 1993.The Montée St-Rémi overpass located near Montreal has two continuous 41 m spansmade of high-performance concrete having a specified 28 day compressive strength of 60MPa High-performance concrete for Montée St-Rémi bridge (Aïtcin and Lessard, 1994). Ingredient type Amount W/B (B= cement + Silica fume) 0.29 Water (l/m3) 90 Ice(kg/m3) 40 Blended silica fume cement 450 Coarse aggregate 1100 Fine aggregate 700 SP (Naphthalene type) 7.5 Air entering agent 325 Retarding agent 450
Use of HPC world wide9. The ‘Pont de Normandie’ bridge, completed in 1993.The ‘Pont de Normandie’ cable stay bridge was the longest in the world when it was builtin 1993.The mix design was developed in order to match not only the 60 MPa 28 day designstrength, but also the quite high early strength requirements.The the mix design composition is as follow. W/B Water Cement Aggregates Admixture (kg/m3) (kg/m3) (kg/m3) (l/m3) Coarse Fine SP 0.36 150-155 425 1065 770 10.6-11.7The fine aggregate was a 0.4 mm natural sand and the coarse aggregate, which was asemi-crushed gravel, had a maximum size of 20 mm. The super-plasticizer used was amodified melamine type. The cement was a blended silica fume cement containing 8%silica fume.
Use of HPC world wide10. Hibernia offshore platform, completed in Newfoundland, Canada in1996.The Hibernia offshore platform is a gravity-based structure (GBS) located in the Grand Banksarea, 315 km east of St John’s, Newfoundland, Canada.Different types of concrete were used to build the whole GBS, but the one that was mostwidely used was a so-called ‘modified normal weight’ high-performance concrete having awater/binder ratio of 0.31 for the splash zone and 0.33 for the submerged zone for a designcompressive strength of 69 MPaThe design of the ‘modified normal density’ mix had to be carefully adjusted to satisfydesign and placing constraints:• design constraints: its unit mass had to be between 2200 and 2250 kg/ m3 forbuoyancy, but at the same time its elastic modulus had to be greater than 32 GPa;• placing constraints: in many areas the concrete had to be placed around more than1000kg of steel reinforcement per cubic meter, and even much more than that in some verycongested areas. Moreover, the setting of the concrete had to be carefully adapted to theslip-forming rate. Finally, the concrete had to be fluid enough to be placed under vibrationin congested areas without any segregation.
Use of HPC world wide10. Hibernia offshore platform, completed in Newfoundland, Canada in1996.The Hibernia offshore platform is a gravity-based structure (GBS) located in the Grand Banksarea, 315 km east of St John’s, Newfoundland, Canada.In order to achieve such a low water/binder ratio, a silica fume blended cement, containing8.5% of silica fume and a naphthalene super-plasticizer were used. In order to obtain such alow unit mass, necessary to achieve the right buoyancy for the platform during its towingphase, half the volume of the coarse aggregate was replaced by a lightweight aggregate andsome air was entrained. All the aggregates were non-reactive toalkalis.
Use of HPC world wide11. Confederation bridge completed between Prince Edward Island and New Brunswick inCanada in 1997.This bridge is also known as the PEI bridge because it links Prince EdwardIsland to NewBrunswick in continental Canada (Tadros et al., 1996). The structure is approximately 13km long. The mix design used for different components are as under Ingredient type Marine Ice shield Thick girders sections W/B 0.30 0.25 0.31 Water (kg/m3) 145 142 142 Cement blended silica fume(kg/m3) 430 520 330 Fly ash class F(kg/m3) 45 60 130 Aggregate(kg/m3) Coarse 1030 1100 1050 Fine 0.18 0.35 0.19 Air entering agent 1.80 1.60 1.38 Water reducer - 0.58 0.30 Set retarder - 0.58 0.30 Super-plastisizer 3.20 6.00 1.80
Requirements of aggregatesFor 75MpaFinding aggregates that meet the minimumstandard requirements for usual concrete in the 20to 40 MPa range is fairly easy;However, whenTargeting 75 MPa, a number of problems arise.Performance can be• limited by certain aggregates, such as gravels that are too smooth and not clean enough, those containing too many soft and crumbly particles,• soft lime-stones and hard aggregates with a poor shape characterized by flat or elongated particles.
Requirements of aggregatesFor 75MpaFinding aggregates that meet the minimumstandard requirements for usual concrete in the 20to 40 MPa range is fairly easy;However, whenTargeting 75 MPa, a number of problems arise.Performance can be• limited by certain aggregates, such as gravels that are too smooth and not clean enough, those containing too many soft and crumbly particles,• soft lime-stones and hard aggregates with a poor shape characterized by flat or elongated particles.
Requirements of aggregatesThe higher the targeted compressive strength, the smaller the maximum size of thecoarse aggregate should be. While 75 MPa compressive strength concretes can easily beproduced with a good coarse aggregate of a maximum size ranging from 20 to 28mm, aggregate with a maximum size of 10 to 20mm should instead be used to produce100 MPa compressive strength concretes. Concretes with compressive strengths of over125 MPa have been produced to date, with coarse aggregate having a maximum size of10 to 14 mm.• Sand coarseness must increase proportionally to compressive strength and cementdosage; a fineness modulus in the 2.70 to 3.00 range is preferred if available.• Using supplementary cementitious materials, such as blast-furnace slag, fly ash andnatural pozzolans, not only reduces the production cost of concrete, but also providesanswers to the slump loss problem. The optimal substitution level is often determined bythe loss in 12 or 24 hour strength that is considered acceptable, given climatic conditionsor the minimum strength required.• While silica fume is usually not really necessary for compressive strengths under 75MPa, most cements require it in order to achieve 100 MPa. Given the materials availableto date, it is almost impossible to exceed the 100 MPa threshold without using silicafume.
Cost effectWhile the cost of silica fume varies according to regional and localmarket conditions, in some parts of the world specifying a compressivestrength of 100 MPa, instead of 90 MPa, can double the production cost ofa high-performance concrete, since silica fume can be 10 times moreexpensive than cement. When making a 90 MPa concrete, the produceroften does not need to use the 10% silica fume required to achieve a 100MPa compressive strength.
Selection of ingredients of HPCSelection of CementSelection of cement for HPC is a critical issue, class-I HPC can bemade with any of present OPC. Class-II HPC cannot be made with allcements, while only a few cements can be used to make class-IV andV HPC.The Reason is that all brands of cement does not behave in the samemanner to produce HPC. Some perfom very well in case of strengthwhile rheological properties are not good and vice versa.
Selection of ingredients of HPCSelection of SPThe selection of a good and efficient superplasticizer is also crucialwhen making high-performance concrete, because not allsuperplasticizer types and brands react in the same way with aparticular cement. Experience has shown that not all commercialsuperplasticizers have the same efficiency in dispersing the cementparticles within the mix, in reducing the amount of mixing water, andin controlling the rheology of very low water/binder ratio mixturesduring the first hour after the contact between the cement and thewater .
TYPES OF SPsThere are four main families of commercial super-plasticizer , SPmust comply with ASTM C494-921. Sul-fonated salts of poly-condensate of naphthalene and formaldehyde, usually referredto as poly-naphthalene sul-fonate or more simply as naphthalene super-plasticizers.2. Sul-fonated salts of poly-condensate of melamine and formaldehyde, usually referred toas poly-melamine sul-fonate or more simply as melamine super-plasticizers.3. Ligno-sulfonates with very low sugar and low surfactant contents.4. Polyacrylates.At present, the most widely used bases when making super-plasticizers are of the first twotypes, but in their formulations commercial super-plasticizers can contain a certain amountof normal water reducers, such as ligno-sulfonates and gluco-nates. Commercial super-plasticizers can be used in conjunction with water reducers, with set retarders or even withaccelerators.
TYPES OF SPsThere are four main families of commercial super-plasticizer , SPmust comply with ASTM C494-921. Sul-fonated salts of poly-condensate of naphthalene and formaldehyde, usually referredto as poly-naphthalene sul-fonate or more simply as naphthalene super-plasticizers.2. Sul-fonated salts of poly-condensate of melamine and formaldehyde, usually referred toas poly-melamine sul-fonate or more simply as melamine super-plasticizers.3. Ligno-sulfonates with very low sugar and low surfactant contents.4. Polyacrylates.
Cement SP compatibilityMaking a practical and economical high-performance concrete islinked to cement-super-plasticizer compatibility. In that respect, it isshown that present Portland cement characteristics, and particularlyits ‘gypsum’ content, are optimized through a standard procedurewhich can be misleading when this cement is used in conjunctionwith a super-plasticizer at a very low water/binder ratio.From experience, it is found that some commercial super-plasticizersand normal Portland cements are not satisfactory at this level in termsof rheological properties and compressive strength.For example, when used for high-performance applications, somesuper-plasticizer and cement combinations cannot maintain asufficient slump for even an hour to enable the placing of high-performance concrete to be as easy as that of usual concrete. Somecement-super-plasticizer combinations cannot reach 75 MPa evenwhen the cement content is increased or the water/binder ratio isreduced by adding more super-plasticizer.
Cement SP compatibilityAs at the present time it is still impossible to know by looking at thedata sheets of a particular cement and a particular super-plasticizerwhat kind of rheological behavior could be expected in low W/B ratiomixtures.There are two methods to determine cement SP compatability1. Marsh cone method2. Mini slump methodThe advantage of the mini-slump method is that it requires less material to beperformed, but the grout is evaluated in a rather ‘static’ behavior, while in the case of theMarsh cone method, more material is needed and the grout is tested in a more ‘dynamic’condition. The use of one of these two simplified methods is a matter of personalpreference. The simultaneous use of both methods is interesting because differentrheological parameters are predominant in both tests.
Selection of final cementitious systemFrom experience to date, it is known that high-performance concretes ofclasses I and II (50to 100 MPa) can be made using a great variety ofcementitious system: pure Portlandcement; Portland cement and fly ash;Portland cement and silica fume; Portlandcement, slag and silica fume; Portland cement, fly ash and silica fume. However, theliterature also shows that almost all class III high-performance concretes (100 to 125 MPa)that have been produced contain silica fume, except for a very few in the lower range of thisclass which were made with Portland cement only. Up to now, classes IV and V high-performance concretes have all beenproduced with silica fume. Moreover, it is observedthat as concrete compressive strength has increased, fewer and fewer types of cementitiouscombinations have been used. Thus from a practical point of view, whenmaking class I and II high-performance concrete the use of fly ash or slag should beseriously considered. These are less expensive than cement and usually require lesssuperplasticizer.
Dosages of SCMsSilica fume: optimum dosage ranges 3-10%,Fly ash: 15% is usual dosage general range is 10-30% generally preferred used is class-ISlag: Up to now, slag has always been used in conjunction with silica fume to make class I, IIand III high-performance concretes (50 to 125 MPa) and has never been used for class IVand V high-performance concretes (>125 MPa). This is probably because they were notinvestigated seriously for these applications, but there is no reason why slags should not beused in the future to make these classes of high-performance concrete.
Selection of aggregatethe selection of aggregates must be done carefully because, as thetargeted compressive strength increases, the aggregates can becomethe weakest link, where failure will be initiated under a high stress.Compared with usual concrete, a closer control of aggregate qualitywith respect to grading and maximum size is necessary, since aprimary consideration is to keep the water requirement as low aspossible. It should be obvious that only well-graded fine and coarseaggregates should be used.For fine aggregates finness modulus of 2.7-3.0 is recommended
Selection of aggregateThe selection of the coarse aggregate becomes more important asthe targeted compressive strength increases and will therefore bediscussed in more detail than the selection of the fine aggregate.Crushed hard and dense rocks, such as limestone, dolomite andigneous rocks of the plutonic type (granite, syenite, diorite, gabbroand diabase), have been successfully used as coarse aggregate inhigh performance concrete applications. It is not yet establishedwhether aggregates potentially reactive with the alkali in thecement can be used in high-performance concrete; therefore, it isbetter to be on the safe side.
Selection of aggregateThe shape of the coarse aggregate is also important from arheological point of view. During crushing, roughly equidimensionalparticles (also called cubic particles) should be generated, ratherthan flat and elongated ones. The latter are weak; they cansometimes be broken with the fingers and tend to produce harshmixes requiring additional water or superplasticizer to achieve therequired workability.With most natural aggregates, it seems that, for making high-performance concrete, 10 or 12 mm MSA is probably the safest inthe absence of any optimization testing. This does not mean that a20 mm aggregate cannot be used. When the parent rock (fromwhich the aggregate is derived) is sufficiently strong andhomogeneous, 20 or 25 mm MSA can be used without adverselyaffecting the workability and strength of the concrete.
Trial mixing batchesIn spite of the information presented in this chapter and theinformation on all successful high-performance concretecompositions that can be found in the literature, there comes atime when trial batches have to be made using the pre-selectedmaterials in order to be able to produce an economical optimizedhigh-performance concrete fulfilling all the specified requirements.