Category: Concrete

When concrete is placed, it is in plastic state. It requires to be supported by temporary supports and casings of the desired shape till it becomes sufficiently strong to support its own weight. This temporary casing is known as the formwork. Requirements of a good formwork Steel formwork Steel is used for formwork when it…

Concrete derives its strength by the hydration of cement particles. The quality of the product of hydration and consequently the amount of gel formed depend on the extent of hydration. Theoretically, water–cement ratio of 0.38 is required to hydrate all the particles of the cement and to occupy the space in the gel pores. In…

Compaction of concrete is the method adopted for expelling the entrapped air from the concrete. If the air is not removed fully, the concrete loses strength considerably. Figure 16.3 shows the relation between loss of strength and air voids left due to lack of compaction. It can be seen from Figure 16.3 that 5 per cent voids reduce the…

Soon after mixing, the concrete has to be transported and placed in the moulds. A maximum time limit of 1½ hours is allowed between the moment of actual mixing and placing of concrete. It should never be disturbed once the setting has commenced. While transporting and placing of concrete, care should be taken to avoid…

Thorough mixing of the materials is necessary for the production of uniform concrete. The mixing should ensure that the mass becomes homogeneous, uniform in colour and consistency. There are two methods adopted for mixing of concrete. Hand mixing Hand mixing is practised for small-scale unimportant concrete works. As the mixing cannot be thorough and efficient,…

Cement concrete possesses the following important properties: Strength Concrete is to be strong enough to withstand the stresses caused on it with a required factor of safety. The strength of the concrete is measured in N/mm2 as said earlier and it is the ultimate compressive strength of 15 cm cubes after 28 days of curing (sometimes…

Cement and water are the only two chemically active elements in concrete. By their combination they form a glue-like binder paste, which surrounds and coats the particles of the inert mineral aggregates, sets and upon hardening binds the entire product into a composite mass. Next only to cement, water is the most important element in…

Sand or the fine aggregates form an important constituent of concrete. It helps to increase the bulk or volume of concrete, which results in the reduction of cost. It helps in the adjustment of the strength of concrete by variation of its proportion with cement. It also increases the resistance to crushing. The aggregates reduce…

 Fine aggregate Fine aggregate is the inert or chemically inactive material, most of which passes through a 4.75 mm IS sieve and contains not more than 5 per cent coarser material. They may be classified as follows: The fine aggregates serve the purpose of filling all the open spaces in between the coarse particles. Thus,…

The process of selection of relative proportions of cement, sand, coarse aggregate and water to obtain a concrete of desired quality is known as proportioning the concrete. There are various methods for determining the volumetric proportions of various components, like the arbitrary method, fineness modulus method, minimum voids method and maximum density method. The recommended…