Concrete is a heterogeneous composite material consisting of cement, water, fine aggregates and coarse aggregates. Aggregates occupy about 60 to 80 percent of the volume of concrete. The paste which is formed from cement and water constitutes 20 to 40 percent of the total volume. Concrete is one of the most frequently used building materials. The quality of the concrete is greatly depending upon the quality of paste, which in turn, is dependent upon the ratio of water to cement content used. To get quality concrete due attention should be paid in choosing the constituents, in mixing them in correct proportions, in mixing the concrete in correct manner and finally in using it properly followed by proper curing.
The ingredient of concrete can be classified in to two groups active group such as cement and water, inactive group such as fine and coarse aggregate.
Fine Aggregate
The particle that passes through 4.75 mm sieve and retain on 0.075 mm sieve is known as fine aggregate. The surface area of fine aggregates is higher. The voids between the coarse aggregate are filled up by fine aggregate. It reduces the cost of the concrete and increase the workability of concrete. The main characteristics of fine aggregate which affect in the properties of concrete is bulking. The phenomenon of increase in sand volume due to the increase of moisture content i.e. called Bulking of sand. The main causes of bulking of sand are the moisture content in the sand and it makes thin films around sand particles. Hence, each particle exerts pressure. Thus they move away from each other causing increasing in volume.
The bulking of the aggregates are dependent on two factors; the fineness of the aggregates and percentage moisture content. A fully saturated fine aggregate does not show any bulking. Thus when the sand contains sufficient moisture 12-20%, it occupies the same volume as when it was dry. The percentage of bulking is inversely proportional to the size of the fine aggregates. Hence, finer the sand more is bulking.
Coarse Aggregate
The particles that are retained on the 4.75 mm sieve are called coarse aggregate. Use of the largest maximum size of coarse aggregate permits a reduction in cement and water requirements. Using aggregates larger than the maximum size of coarse aggregates permitted can result in interlock and form arches or obstructions within a concrete form. That allows the area below to become a void, or at best, to become filled with finer particles of sand and cement only and results in a weakened area. Size of aggregate shall depend upon the type of work and the reinforcement. Size of aggregate should be less than the distance between two consecutive steel bars in RCC.
The main characteristics of fine aggregate which affect in the properties of concrete is crushing strength of aggregate. The crushing strength of aggregate defines as the resistance of aggregate to the compressive load. The compressive strength of concrete is depending on the strength of aggregate.
Cement
Cement is the most important ingredient of concrete act as a binding material having both adhesive and cohesive properties. Cement binds the coarse and fine aggregate by filling the voids and chemically reacting with water. It contains about 10% of the volume of concrete mix. The type of cement to be selected for concrete is based on the factors like type of work, site condition, transportation time etc. The different types of cement are ordinary portland cement (OPC), portland slag cement, portland pozzolana cement, white cement, sulphate resisting cement, low heat cement, rapid hardening cement, quick setting cement, blast furnace slag cement, high alumina cement, colored cement, expansive cement etc.
Water
Water is an important ingredient of concrete without which concrete cannot be manufactured. Water in concrete making is used for mixing, washing aggregate and curing. In general, water which is acceptable for drinking purpose is suitable for making the concrete. The main harmful substance in water for concrete is salt which is present in sea water. The salts present in sea water reduce the strength of concrete, but sometimes it has to be used when there is no alternative. Sea water contains up to 3.5% salt and has tendency to decrease the strength 10% - 20% of the concrete. There is more chance of causes of dampness and surface efflorescence in building.
Admixtures
Admixtures are optional in concrete. This is used for the special purposes like to increase or decrease the initial or final setting time of the concrete, workability of concrete etc. Admixtures are used in special cases only. Type of admixtures used in concrete includes accelerators, retarders, water-reducing agents, super plasticizers, air entraining agents etc. The proportion of all the ingredients of concrete is considered as per grade of the concrete or as per mix design of concrete.
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