CHAPTER 3 DEFINITION AND THEORY3.1 INTRODUCTION The chapter gives the description about theory part of the project. The main factors related to the project are described here, such as HSC, workability, curing and its importance, gain in strength, Superplasticizers along its working mechanisms, PPC and its function. Concrete is classified as a Normal Strength Concrete (NSC), High Strength Concrete (HSC) and Ultra High Strength Concrete (UHSC). There is no clear cut boundary for the above classification. Indian Standard Recommended Methods of mix design denotes the boundary at 35 MPa between NSC and HSC.3.2 HIGH STRENGTH CONCRETEThe methods and technology for producing HSC are not basically different from those required for concrete of normal grade except that the emphasis on quality control is greater with HSC. HSC can be produced with all of the cements including Portland cement, sulfate-resisting Portland cement and combinations with pulverized fuel ash and ground granulated blast furnace, silica fume slag. There are special methods of making high strength concrete. They are given below a) Revibrationb) High speed slurry mixingc) Use of admixtured) Inhabition of crackse) Sulphur impregnationf) Use of cementitious aggregate.g) Seeding.High early strength cements should preferably be avoided as a rapid rise in hydration temperature may cause problems of (internal) cracks or micro-cracks due to the higher cementitious material content. HSC can be produced with a wide range of aggregates, but smooth and/or rounded aggregates may tend to exhibit aggregate bond failure at a relatively low strength. Crushed rock aggregates, of 10 to 20 mm size, which are not too angular and elongated, should preferably be used. However, it has been found that bond strength between smaller size aggregates is greater than between larger size aggregates and for that reason, smaller size aggregates (say 10 to 20 mm) tend to give better results. Fine sands should be avoided, particularly those with high absorption.Superplasticizer should be used to achieve maximum water reduction, although plasticizers may be adequate for lower strength HSC. The basic proportioning of an HSC mix follows the same method as for normal strength concrete, with the objective of producing a cohesive mix with minimum voids. This can be done by theoretical calculations or subjective laboratory trials. It is essential to ensure full compaction at these levels. A higher ultimate strength can be obtained by designing a mix with a low initial strength gain and cementitious additions. This is partially due to avoidance of micro-cracking associated with high thermal gradients. Increasing the cement content may not always produce higher strength. Above certain levels it may have little effect. An optimum amount of total cementitious material usually appears to be between 450 and 550 kg/m3. HSC mixes tend to be very cohesive and a concrete with a measured slump of 50 mm may be difficult to place. As HSC is likely to be used in heavily reinforced sections, a higher workability should be specified if honeycombing is to be avoided. When superplasticizer is used, concrete tends to lose workability rapidly. HSC containing such materials must therefore be transported, placed and finished before they lose their effect. Many modern superplasticizer can retain reasonable workability for a period of about 100 minutes, but care is still needed, particularly on projects where ready-mixed concrete delivery trucks have long journey times. Often, in order to avoid drastic decreases in slump and resultant difficulty in placing, superplasticizer are only partly mixed on batching, the balance being added on site prior to pouring. The same production and quality control techniques for normal strength concrete should also be applied to HSC. In general, production should include not only correct batching and mixing of ingredients, but also regular inspection and checking of the production equipment, e.g. the weighing and gauging equipment, mixers and control apparatus. With ready mix concrete supply, this control should extend to transport and delivery conditions as well. The main activities for controlling quality on site are placing, compaction, curing and surface finishing. Site experience indicates that more compaction is normally needed for high strength concrete with high workability than normal strength for similar slump. As the loss in workability is more rapid, prompt finishing also becomes essential. Particular attention needs to be given to vibration at boundaries. To avoid plastic shrinkage, the finishing concrete surface needs to be covered rapidly with water-retaining curing agents. As the quality of the structure with HSC is the main objective, it is essential that, in addition to the above, the accuracy of the formwork and the fixing details of the reinforcement and/or pre stressing steel should also be part of the control activities. It is also desirable to assess the in-situ strength of the concrete in the actual structure by some non-destructive method (such as hammer test or ultrasonic pulse velocity measurements) for comparison with compliance cube test results, to establish that no significant differences exist between the two sets of results. It should be kept in mind that factors which have only a second-order effect at lower strength levels may become of major importance at higher levels.3.3 ADMIXTUREChemical admixtures are the ingredients in concrete other than Portland cement, water, and aggregate that is added to the mix immediately before or during mixing. Producers use admixtures primarily to reduce the cost of concrete construction; to modify the properties of hardened concrete; to ensure the quality of concrete during mixing, transporting, placing, and curing and to overcome certain emergencies during concrete operations.Successful use of admixtures depends on the use of appropriate methods of batching and concreting. Most admixtures are supplied in ready-to-use liquid form and are added to the concrete at the plant or at the job site. Certain admixtures, such as pigments, expansive agents, and pumping aids are used only in extremely small amounts and are usually batched by hand from premeasured containers.The effectiveness of an admixture depends on several factors including: type and amount of cement, water content, mixing time, slump, and temperatures of the concrete and air. Sometimes, effects similar to those achieved through the addition of admixtures can be achieved by altering the concrete mixture-reducing the water-cement ratio, adding additional cement, using a different type of cement, or changing the aggregate and aggregate gradation. Admixtures being considered for use in concrete should meet specifications. Trial mixtures should be made with the admixture and the job material at temperatures and humidifies anticipated on the job. In this way the compatibility of the admixture with other admixtures and job materials, as well as the effects of the other admixture on the properties of the fresh and hardened concrete, can be observed. The amount of admixture recommended by the manufacturer or the optimum amount determined by laboratory test should be used. The major reasons used for using admixtures are.1. To reduce the cost of concrete construction.2. To achieve certain properties in concrete more effectively than by other means.3. To maintain the quality of concrete during the stages of mixing, transporting, placing, and curing in adverse weather conditions.4. To overcome certain emergencies during concreting operations. 3.3.1 SUPERPLASTICIZER Superplasticizers arewellknown chemical admixtures for concrete used in the reduction of water to cement ratio without affecting workability, and to avoid particle segregation in the concrete mixture. These are also known as high range water reducers (HRWR) and dispersants as these are capable of reducing water to cement ratio up to 40%. These chemical admixtures are added in the concrete just before the concrete is placed. These admixtures help to improve strength and flow characteristics of the concrete. Superplasticizer are essential sulfonic compounds attached to the polymer backbone at regular intervals. These can be added at a range of 0.15% to 3.0% of the weight of cement that is higher as compared to plasticizers. Flow characteristics and slump of concrete varies with type, dosage, and time of addition of concrete superplasticizer. Superplasticizer can be classified into four types such as, Sulfonated melamine-formaldehyde condensates (SMF), Sulfonated naphthalene-formaldehyde condensates (SNF), Modified lignosulfonates (MLS), and Polycarboxylate derivatives (PC). The selection of concrete superplasticizer is based on the type of concrete used, namely ready mix, precast, high strength, high performance, self-compacting, shotcrete, etc. The SMF, SNF, and MLS are very old but are still the highly consumed in todays concrete applications. The only drawback with these concrete superplasticizer is high slump loss and not preferable in cold weather conditions that can be overcome with polycarboxylates. Polycarboxylates, with superior performance characteristics, are the new generation superplasticizer, capable of water reduction up to 40%, and are also preferable in hot weather conditions.

3.3.2 EFFECT OF SUPERPLASTICIZER ON CONCRETEWhen cement mixes with water, cement particles always flocculate and agglomerate then electrostatic attractive forces are generated by the electric charge on particle surface as a results large amount of free water being trapped in flocs, leads to reduce the homogeneity of concrete. The water reducing agents or workability agents such as plasticizer and superplasticizer among which superplasticizer is more consistence and viscous even at low w/c ratio. Further, to achieve high filling ability, it is necessary to reduce inter-particle friction among solid particles in concrete by using superplasticizer and reducing coarse aggregate contents. The incorporation of a superplasticizer not only reduces the inter-particle friction but also maintain the deformation capacity and viscosity.3.4 WORKABILITYThe behavior of green or fresh concrete from mixing up to compaction depends mainly on the property called workability of concrete. Workability of concrete is a term which consists of the following four partial properties of concrete namely, Mix ability, Transportability, Mouldability and Compatibility. In general terms, workability represents the amount of work which is to be done to compact the compact the concrete in a given mould. The desired workability for a particular mix depends upon the type of compaction adopted. A workable mix should not segregate. 3.4.1 PARTIAL PROPERTIES OF WORKABILITY (i) Mixability: It is the ability of the mix to produce a homogeneous green concrete from the constituent materials of the batch, under the action of the mixing forces. A less mixable concrete mix requires more time of mixing to produce a homogeneous and uniform mix.(ii) Transportability: Transportability is the capacity of the concrete mix to avoid the homogeneous concrete mix from segregating during a limited time period of transportation of concrete, when forces due to handling operations of limited nature act.(iii) Mouldability: It is the ability of the fresh concrete mix to fill completely the forms or moulds without losing continuity or homogeneity under the available techniques of placing the concrete at a particular job/ this property is complex, since the behavior of concrete is to be considered under dynamic conditions.(iv) Compactibility: Compactibility is the ability of concrete mix to be compacted into a dense, compact concrete, with minimum voids, under the existing means of compaction at the site. The best mix from the point of view of compactibility should close the void an extent of 99% of the original voids present, when the concrete is placed in the mould.3.4.2 FACTORS AFFECTING WORKABILITYThe factors helping concrete to have more lubricating effect to reduce internal friction for helping easy compaction are:(i) Water Content: Water content in a given volume of concrete, will have significant influences on the workability. The higher the water content per cubic meter of concrete, the higher will be the fluidity of concrete, which is one of the important factors affecting workability. At the work site, supervisors who are not well versed with the practice of making good concrete resort to adding more water for increasing workability. This practice is often resorted because this is one of the easiest corrective measures that can be taken at the site. It should be noted that from the desirability point of view, increase of water content is the last recourse to be taken for improving the workability even in the case of uncontrolled concrete. For controlled concrete one cannot arbitrarily increase the water content. In case all other steps to improve workability fail, only as last recourse the addition of more water can be considered. More water can be added, provided a correspondingly higher quantity of cement is also added to keep the water/cement ratio constant, so that the strength remains the same.(ii) Mix Proportions: Aggregate/cement ratio is an important factor influencing workability. The higher the aggregate/cement ratio, the leaner is the concrete. In lean concrete, less quantity of paste is available for providing lubrication, per unit surface area of aggregate and hence the mobility of aggregate is restrained. On the other hand, in case of rich concrete with lower aggregate/cement ratio, more paste is available to make the mix cohesive and fatty to give better workability.(iii) Size of Aggregate: The bigger the size of the aggregate, the less the surface area and hence less amount of water is required for wetting the surface and less matrix or paste is required for lubricating the surface to reduce internal friction. For a given quantity of water and paste, bigger size of aggregates will give higher workability. The above of course will be true within certain limits.(iv) Shape of Aggregates: The shape of the aggregate influences the workability in good measure. Angular, elongated or flaky aggregate makes the concrete very harsh when compared to rounded aggregates or cubical shaped aggregates. Contribution to better workability to rounded aggregate will come from the fact that for the given volume or weight it will have less surface area and less voids than angular or flaky aggregate. Not only that, being round in shape, the frictional resistance is also greatly reduced. This explains the reason why river sand and gravel provide greater workability to concrete than crushed sand and aggregate. The importance of shape of the aggregate will be of great significance in the case of present day high strength and high performance concrete when we use very low w/c about 0.25.In years to come, natural sand will be exhausted or costly. One has to go for manufactured sand. Shape of crushed sand as available today is unsuitable but the modern crushers are designed to yield well shaped and well graded aggregates.(v) Surface Texture: The influence of surface texture on workability is again due to the fact that the total surface area of rough textured aggregate is more than the surface area of smooth rounded aggregate of same volume. From the earlier discussions it can be inferred that rough textured aggregate will show poor workability and smooth or glassy textured aggregate will give better workability. A reduction of inter particle frictional resistance offered by smooth aggregates also contributes to higher workability.(vi) Grading of Aggregates: This is one of the factors which will have maximum influence on workability. A well graded aggregate is the one which has least amount of voids in a given volume. Other factors being constant, when the total voids are less, excess paste is available to give better lubricating effect. With excess amount of paste, the mixture becomes cohesive and fatty which prevents segregation of particles. Aggregate particles will slide past each other with the least amount of compacting efforts. The better the grading, the less is the void content and higher the workability. (vii) Use of Admixtures: Of all the factors mentioned above, the most important factor which affects the workability is the use of admixtures. It is to be noted that initial slump of concrete mix or what is called slump of reference mix should be about 2 3 cm to enhance the slump many fold at a minimum doze. Without initial slump of 2-3 cm, the workability can be increased to higher level but it requires higher dosage hence uneconomical.3.5 GRADATION OF COARSE AGGREGATECoarse aggregate used in concrete contain various sizes. This particle size distribution of the coarse aggregates is termed as Gradation. The sieve analysis is conducted to determine this particle size distribution. Grading pattern is assessed by sieving a sample successively through all the sieves mounted one over the other in order of size, with larger sieve on top. The material retained on each sieve after shaking represents the fraction of aggregate are coarser than the sieve in lower and finer than the sieve above.Proper gradation of coarse aggregate is one of the most important factors in producing workable concrete. Proper gradation ensures that a sample of aggregates contains all standard fraction of aggregate in required proportion such that the sample contains minimum voids will require minimum paste to fill up the voids in the aggregates. Minimum paste means less quantity of cement and less quantity of water, leading to increased economy, higher strength, lower shrinkage and great durability. The workability is improved when there is an excess of paste above that required to fill the voids in the sand, and also an excess of mortar (sand plus cement) above that required to fill the voids in coarse aggregate because the material lubricate the larger particles.(i) Well Graded: Well-graded aggregate has a gradation of particle size that fairly evenly spans the size from the finest to the coarsest. (ii) Poor Graded: Poor-graded aggregate is characterized by small variation in size. It contains aggregate particles that are almost of same size. This means that the particles pack together, leaving relatively large voids in the concrete. It is also called uniform-graded.(iii) Gap Graded: Gap-graded aggregate consist of aggregate particle in which some intermediate size particle are missing. Poorly graded concrete generally require excessive cement paste to fill the voids making them uneconomical. Gap-graded concrete fall in between well graded and poorly graded in terms of performance and economy. Well graded aggregates are tricky in proportion. The goal of aggregate proportioning and sizing is to maximize the volume of aggregate in the concrete while preserving the strength, workability and finishing. Aggregate graded to maximum density gives a harsh concrete that is very difficult in ordinary concreting. So the proportioning should be based on the surface area to be wetted. Other things remaining same, it can be said that the concrete made from aggregate grading having least surface area will require least water which will consequently be the strongest. 3.6 PORTLAND POZZALANA CEMENT (PPC)3.6.1 COMPOSITION AND ACTION OF PPCCement is a material that can bind solid particles e.g. gravel, sand and aggregate etc. within a compact structure. A variety of materials may exhibit cementitious properties. In the concrete industry, hydraulic cements such as Portland cement have the ability to set and harden in the presence of water. They are usually manufactured from calcareous raw materials containing silicates, aluminates and iron oxides. Raw materials such as limestone and clay are heated in a kiln at 1400-1450C to form predominantly clinker, which is then finely ground together with additives such as gypsum to obtain Portland cement. Portland cement is the most common type of cement used in construction applications, but it is an expensive binder due to the high cost of production associated with the high energy requirements of the manufacturing process itself. Other cheap inorganic materials with cementitious properties such as natural pozzolana e.g. volcanic tuff and clay, and waste products from industrial plants e.g. slag, fly ash and silica fume can be used as a partial replacement for Portland cement i.e. blended cements. In addition, to reduce the cost of binder, there are potential technological benefits from the use of pozzolanic materials as those blended with Portland cement in concrete applications. These include increased workability, decreased permeability and increased resistance to sulphate attack, improved resistance to thermal cracking and increased ultimate strength and durability of concrete. Pozzolanic cement is a ground product of a mixture containing 20-40% natural pozzolana and 60-80% Portland cement clinker with the addition of a small amount of gypsum. Increase in the natural pozzolana content of cement would reduce the permeability of the paste with the implication of a high resistance to chemical attack, i.e. increase in durability. The addition of natural pozzolana (up to 20-30%) could also improve the compressive, splitting and flexural strengths of the concrete in the long term, for example, over a 365 day period. Mortar or concrete samples prepared from blended cement must produce 7- and 28-day compressive strengths of higher than 16 and 32.5 N/mm2, respectively. Pozzolana cannot develop hydraulic properties in the absence of hydrated lime. Hydrated lime or material that can release it during its hydration (e.g. Portland cement) is then required to activate the natural pozzolanas as a binding material. The activity of a natural pozzolana, which is essentially determined by the reactive silica content, is also closely controlled by its specific surface area, chemical and mineralogical composition. Reactive silica is readily dissolved in the matrix as Ca(OH)2 becomes available during the hydration process. These pozzolanic reactions lead to the formation of additional C-S-H with binding properties. Silicate minerals including feldspar, mica, hornblende, pyroxene and quartz or olivine present in volcanic rocks can easily undergo alteration to form secondary mineral phases such as clays, zeolites, calcite and various amphiboles. However, every natural pozzolana with a strong acidic character does not show pozzolanic activity, and hence the assessment of pozzolanic activity of a given natural pozzolana is a prerequisite for its use in the cement industry. Table 3.1 Chemical composition of natural raw material in cementCompositionContent (%)

Lime(CaO)62-67

Silica(SiO2)17-25

Alumina(Al2O3)3-8

Calcium sulphate(CaSO4)3-4

Iron oxide(Fe2O3)3-4

Magnesia(MgO)0.1-3

Sulphur(S)1-3

Alkalies0.2-1

3.6.2 SETTING ACTION OF CEMENT Following are important compounds formed during the setting action of cement:(1) Tricalcium aluminate (3Cao, Al2O3): This component is formed within about the 24 hours after addition of water to the cement. The main of this compound is to give the early strength to cement. It hydrates and hardens very quickly. It liberates a large amount of heat almost immediately and contributes somewhat to early strength. Therefore gypsum is added to cement to slow down the action of C3A(2) Tetra-calcium alumino-ferrite (4CaO, Al2O3, Fe2O3): This compound is also formed within about 24 hours after addition of water to the cement it provides the prolong strength to cement which plays an important role for gain in strength. The C4AF compound hydrates rapidly but contributes very little to strength. Its use allows lower kiln temperatures in Portland cement manufacturing. This compound is so acts as flux in clinker manufacture and imparts grey colour. (3) Tricalcium silicate (3CaO, SiO2): This component is formed within a week after addition of water to the cement and it is mainly responsible for imparting strength to the cement in early period of setting. (4) Dicalcium silicate (2CaO, SiO2): This component is formed very slowly and hence it is possible for giving progressive strength to the cement. 3.6.3 BENEFITS OF USING PPCPPC is produced when pozzolanas are used in the mixture. A pozzolanas is a cement extender improving the strength and durability of the cement or even reducing the costs of producing concrete. The term came from the root word pozzolana which is a form of volcanic ash. The introduction of pozzolana into a hydraulic cement like OPC, or any similar material, leads to a pozzolanic reaction. This, in turn, leads to a cementitious material that uses less cement but has the same or even greater material durability than without this addition. A pozzolanic material by itself has few, if any, cementitious properties by itself, but adding it into a cement mixture will result in the above-mentioned results (provided the cement has a greater volume in relation to the pozzolanic material added). PPC may take a longer time to settle than OPC, but it will eventually produce similar results given time. Though volcanic ash is the first form of pozzolana used, this now includes natural and artificial siliceous or siliceous, aluminous materials such as clay, slag, silica fume, fly ash, and shale. Note that some of these are effectively waste materials from other processes but are ideal to produce PPC.3.7 CURING3.7.1 DEFINITON OF CURINGCuring can be described as keeping the concrete moist and warm enough so that the hydration of cement can continue. More elaborately, it can be described as the process of maintaining a satisfactorymoisture content and a favorable temperature in concrete during the period immediately following placement, so that hydration of cement may continue until the desired properties are developed to a sufficient degree to meet the requirement of service. If curing is neglectedin the early period of hydration, the quality of concrete will experience a sort of irreparable loss. An efficient curing in the early period of hydration can be compared to a good and wholesome feeding given to a new born baby.A concrete element is expected to last a certain number of years. In order to meet this expected service life, it must be able to withstand structural loading, fatigue, weathering, abrasion, and chemical attack. The duration and type of curing plays a big role in determining the required materials necessary to achieve the high level of quality. Curing is the process in which the concrete is protected from loss ofmoistureand kept within a reasonable temperature range. The result of this process is increased strength and decreased permeability. Curing is also a key player in mitigating cracks in the concrete, which severely impacts durability. Cracks allow open access for harmful materials to bypass the low permeability concrete near the surface. Good curing can help mitigate the appearance of unplanned cracking.3.7.2 REASONS FOR CURING(i) Predictable strength gainLaboratory tests show that concrete in a dry environment can lose as much as 50 percent of its potential strength compared to similar concrete that is moist cured. Concrete placed under high temperature conditions will gain early strength quickly but later strength may be reduced. Concrete placed in cold weather will take longer to gain strength, delaying from removal and subsequent construction.(ii) Improved durabilityWell-cured concrete has better surface hardness and will better withstand surface wear and abrasion. Curing also makes concrete more water tight, with pavement moisture and water-borne chemicals from entering into concrete, thereby increasing durability and service life. (iii) Better requirements for curingA concrete slab that has been allowed to dry out too early will have a soft surface with poor resistance to wear and abrasion.3.7.3 WATER CURING This is by far the best method of curing as it satisfies all the requirements of curing, namely, promotion of hydration, elimination of shrinkage and absorption of the heat of hydration. It is pointed out that even if the membrane method is adopted, it is desirable that a certain extent of water curing is done before the concrete is covered with membranes. Water curing can be done in the following ways:1. Immersion.2. Ponding.3. Spraying or Fogging.4. Wet Covering.The precast concrete items are normally immersed in curing tanks for a certain duration. Pavement slabs, roof slab etc. are covered under water by making small ponds. Vertical retaining wall or plastered surfaces or concrete columns etc. are cured by spraying water. In some cases, wet coverings such as wet gunny bags, hessian cloth, jute matting, straw etc., are wrapped to vertical surface for keeping the concrete wet. For horizontal surfaces saw dust, earth or sand are used as wet covering to keep the concrete in wet condition for a longer time so that the concrete is not unduly dried to prevent hydration.

3.7.4 MEMBRANE CURINGSometimes, concrete works are carried out in places where there is acute shortage of water. The lavish application of water for water curing is not possible for reasons of economy. Curing does not mean only application of water; it means also creation of conditions for promotion of uninterrupted and progressive hydration. It is also pointed out that the quantity of water, normally mixed for making concrete is more than sufficient to hydrate the cement, provided this water is not allowed to go out from the body of concrete. For this reason, concrete could be covered with membrane which will effectively seal off the evaporation of water from concrete. Large numbers of sealing compounds have been developed in recent years. The idea is to obtain a continuous seal over the concrete surface by means of a firm impervious film to preventmoisturein concrete from escaping by evaporation. Some of the materials, which can be used for this purpose, are bituminous compounds, polyethylene or polyester film, waterproof paper, rubber compounds etc. When waterproofing paper or polyethylene film are used as membrane, care must be taken to see that these are not punctured anywhere and also see whether adequate lapping is given at the junction and this lap is effectively sealed.3.7.5 APPLICATION OF HEATThe development of strength of concrete is a function of not only time but also that of temperature. When concrete is subjected to higher temperature it accelerates the hydration process resulting in faster development of strength. Concrete cannot be subjected to dry heat to accelerate the hydration process as the presence ofmoistureis also an essential requisite. Therefore, subjecting the concrete to higher temperature and maintaining the required wetness can be achieved by subjecting the concrete to steam curing.A faster attainment of strength will contribute to many other advantages mentioned below. The exposure of concrete to higher temperature is done in the following manner:1. Steam curing at ordinary pressure2. Steam curing at high pressure3. Curing by Infra-redMany a time an engineer at site wonders, how early he should start curing by way of application of water. This problem arises, particularly, in case of hot weather concreting. In an arid region, concrete placed as a road slab or roof slab gets dried up in a very short time, say within 2 hours.Concrete should not be allowed to dry fast in any situation. Concrete that are liable to quick drying is required to be covered with wet gunny bag or wet hessian cloth properly squeezed, so that the water does not drip and at the same time, does not allow the concrete to dry. This condition should be maintained for 24 hoursor at least till the final setting time of cement at which duration the concrete will have assumed the final volume. Even if water is poured, after this time, it is not going to interfere with the water/cement ratio. However, the best practice is to keep the concrete under the wet gunny bag for 24 hours and then commence water curing by way of ponding or spraying. Of course, when curing compound is used immediately after bleeding water, if any, dries up, the question of when to start water curing does not arise at all.There is a wrong concept with common builders that commencement of curing should be done only on the following day after concreting. Even on the next day they make arrangements and build bunds with mud or lean mortar to retain water. This further delays the curing. Such practice is followed for concrete road construction by municipal corporations also. It is a bad practice. It is difficult to set time frame how early water curing can be started.It depends on, prevailing temperature, humidity, wind velocity, type of cement, fineness of cement, w/c used and size of member etc. The point to observe is that, the top surface of concrete should not be allowed to dry. Enough moisture must be present to promote hydration.Regarding how long to cure, it is again difficult to set a limit. Since all the desirable properties of concrete are improved by curing, the curing period should be as long as practical. For general guidance, concrete must be cured till it attains about 70% of specified strength. At lower temperature curing period must be increased. Since the rate of hydration is influenced by cement composition and fineness, the curing period should be prolonged for concretes made with cements of slow strength gain characteristics.Pozzolanic cement or concrete mixed with pozzolanic material is required to be cured for longer duration. Mass concrete, heavy footings, large piers, abutments, should be cured for at least 2 weeks.3.8 STRENGTH GAINStrength can be defined as ability to resist change. One of the most valuable properties of the concrete is its strength. Strength is most important parameter that gives the picture of overall quality of concrete. Strength of concrete usually directly related to cement paste. Many factors influence the rate at which the strength of concrete increases after mixing. Hardening is the process of growth of strength. This is often confused with 'setting' but setting and hardening are not the same. Setting is the stiffening of the concrete after it has been placed. Hardening may continue for weeks or months after the concrete has been mixed and placed.Voids in concrete can be filled with air or with water. Broadly speaking, the more porous the concrete, the weaker it will be. Probably the most important source of porosity in concrete is the ratio of water to cement in the mix, known as the 'water to cement' ratio.3.8.1 MEASUREMENT OF CONCRETE STRENGTHTraditionally, this is done by preparing concrete cubes or cylinders, then curing them for specified times. Common curing times are 3, 7, 28 and 90 days. The curing temperature is typically 20 degrees Centigrade. After reaching the required age for testing, the cubes/cylinders are crushed in a large press. The SI unit for concrete strength measurement is the Mega Pascal, although 'Newton per square millimeter' is still widely used as the numbers are more convenient. The table below shows the compressive strength gained by concrete after 1, 3, 7, 14 and 28 days with respect to the grade of concrete we use.

Table 3.2 Age-Strength relationAgeStrength %

1 Day16%

3 Days40%

7 Days65%

14 Days90%

28 Days99%

From above table, it is clear that concrete gains its strength rapidly in the initial days after casting, i.e. 90% in only 14 days. When, its strength have reached 99% in 28 days, still concrete continues to gain strength after that period, but that rate of gain in compressive strength is very less compared to that in 28 days. After 14 days of casting concrete, concrete gains only 9% in next 14 days. So, rate of gain of strength decreases. We have no clear idea up to when the concrete gains the strength, 1 year or 2 year, but it is assumed that concrete may gain its final strength after 1 year. So, since the concrete strength is 99% at 28 days, its almost close to its final strength, thus we rely upon the results of compressive strength test after 28 days and use this strength as the base for our design and evaluation.3.9 IS: 10262-2009 CONCRETE MIX PROPORTIONINGThis code provides the guidelines for proportioning concrete mixes as per the requirements using the concrete making materials including others supplementary material identified for this purpose. The proportioning is carried out to achieve the specified characteristics at specified age, workability of fresh concrete and durability required.3.9.1 DATA REQUIRED FOR MIX PROPORTIONThe following data are required for mix proportioning of a particular grade concrete a) Grade designationb) Type of cementc) Maximum nominal size of aggregated) Minimum Cement Contente) Maximum water-cement ratiof) Workabilityg) Exposure condition as per Table 4 and Table 5 of IS 456h) Maximum temperature of concrete at the time of placingi) Method of transport and placingj) Early age strength requirement, if requiredk) Type of aggregatel) Maximum cement contentm) Whether admixture shall or shall not be use and the type of admixture and the condition of use3.9.2 TARGET STRENGTH FOR MIX PROPORTIONING:As sufficient test results for a particular grade of concrete are not available, the value of standard deviation is taken from table 1 IS: 10262-2009In order that the specified proportion of test results are likely to fall below the characteristic strength, the concrete mix has to be proportioned for higher target mean compressive strength fck. The margin over characteristic strength is given by the following relation:fck = fck+ 1.65STable 3.3 Assumed Standard Deviation (IS: 10262 -2009)Sl No.Grade of concreteAssumed standard Deviation

a)M103.5

b)M15

c)M204.0

d)M25

e)M305

f)M35

g)M40

h)M45

i)M50

j)M55

3.9.3 SELECTION OF MIX PROPORTIONTable 3.4 Maximum W: C ratio for different exposure condition (IS: 456-2000)Exposure ConditionReinforced Concrete

Minimum Cement Content(Kg/m3)W:C ratio

Extreme3600.40

3.9.4 SELECTION OF WATER CONTENTWater content of concrete is influence by a number of factors, such as aggregate size, aggregate shape, aggregate texture, workability, water-cement ratio, cement and other supplementary cementious material type and content, chemical admixture and environmental condition. An increase in aggregate size, a reduction in water-cement ratio and slump, and use of rounded aggregate and water reducing admixture will reduce the water demand. On the other hand increase temperature, cement content slump, water-cement ratio, aggregate angularity and decrease in proportion of coarse aggregate to fine aggregate will increase water demand.The quantity of maximum mixing water per unit volume of concrete may be determined from Table 2.3 .The Table 2.3 is for 25 to 50 mm slump. For desired workability other than 25 to 50 mm slump range, the required water content may be increased by about 3 percent for every 25mm slump or by use of chemical admixtures conforming to IS 9103. Water reducing admixture or superplasticizing admixture usually decrease water content by 5 to 10 percent and 20 percent and above respectively at appropriate dose.Table 3.5 Maximum water content per cubic meter of concrete for nominal maximum size aggregate for 25 to 50mm slump (IS: 10262 - 2009)Sl No.Nominal Maximum size aggregateMaximum water content

a)10 mm208 kg/m3

b)20mm186 kg/m3

c)40mm165 kg/m3

3.9.5 ESTIMATION OF COARSE AGGREGATE PROPORTIONAggregate of essentially the same nominal maximum size, type and grading will produce concrete of satisfactory workability when a given volume of coarse aggregate per unit volume of total aggregate is used. Approximate value for this aggregate volume is given in Table 2.4 for water cement ratio of 0.5 which may be suitably adjusted for the other water-cement ratios. It can be seen that for equal workability, the volume of coarse aggregate in a unit volume of concrete is dependent only on is nominal maximum size grading zone of fine aggregate. Differences in the amount of mortar required for workability with different aggregate, due to difference in particle shape and grading, are compensated for automatically by difference in rodded void content.For more workable concrete mixes which is sometimes required when placed is by pump or when the concrete is required to be worked around congested reinforcing steel, it may be desirable to reduce the estimated coarse aggregate content determined using Table 2.4 up to 10 percent. However, caution shall be exercised to assure that the resulting slump, water-cement ratio and strength properties of the concrete are consistent with recommendation of IS 456 and met the project specification requirements as applicable.

Table 3.6 Volume of coarse aggregate per unit volume of total aggregate for different zones of fine aggregate (IS: 10262-2009)Sl No.Nominal maximum Size of Aggregate(mm)Proportion of CA with respect to the zone

Zone 1Zone 2Zone 3Zone 4

1100.500.480.460.44

2200.660.640.62 0.60

3400.750.730.71 0.69

3.9.6 COMBINATION OF DIFFERENT COARSE AGGREGATEThe coarse aggregate used shall conform to IS 383. Coarse aggregate of different size may be combined in suitable proportions so as to result in an overall grading conforming to Table 2 of IS 383 for particular nominal maximum size of aggregate.3.9.7 ESTIMATION OF FINE AGGREGATE PROPORTIONThese quantities are determined by finding out the absolute volume of the cementious material, water and chemical admixture; by dividing their mass by specific gravity, multiplying by 1/1000 and subtracting the result of their summation from unit volume. The values so obtained are divided into Coarse and Fine Aggregate fraction by volume in accordance with the coarse aggregate proportion. The coarse and fine aggregate content are then determined by multiplying with their respective specific gravities and multiplying by 1000.3.9.8 TRIAL MIXESThe calculated mix proportion shall be check by means of trial batches.Workability of the Trial Mix NO.1 shall be measured. The mix shall be carefully observed for freedom from segregation and bleeding and its finishing properties. If the measured workability of Trial Mix No.1 is different from the stipulated value, the water or admixture content shall be adjusted suitably. With this adjustment, the mix proportion shall be recalculated keeping the free water-cement ratio at the pre-selected value, which will comprise Trial Mix No.2 and varying the free water-cement ratio by 10 percent of the pre-selected value. More Mix No. will provide sufficient information, including the relationship between compressive strength and water-cement ratio, from which the mix proportions for field trial may be arrived at. The concrete for field trial is produced by methods of actual concrete production.28