Concrete

Concrete comprises of cement, sand, aggregate and water mixed together in specified proportions. In case steel reinforcement is incorporated to increase the tensile strength of concrete, the same is referred to as reinforced cement concrete (RCC). Concrete without reinforcement bars is referred to as plain cement concrete (PCC).

In RCC, since reinforcement steel needs to be protected from harmful effects corrosion causing agents present in the environment, I.S 456-2000 recommends a minimum grade of M20 concrete (volumetric mix proportions - 1:1.5:3 with maximum water content is 27.5 liters per bag i.e. 50 kg of cement) for structures exposed to mild environment.

Suggestions for making good concrete

Concrete strength and durability depends upon the quality of its ingredients like cement, sand, aggregate and water. Usually, the ingredients are volumetrically proportioned relative to one cement bag (50kgs by weight or 35 lit by volume). In order to correctly proportion the volume of the sand & aggregates, measuring boxes, having dimensions 35x25x40cms should be used.

The shuttering should be sufficiently strong to withstand the weight of concrete, reinforcement steel, weight of persons (involved in placing and compacting concrete) and impact loads(due to concrete placement and vibrations of vibrator).

For footings, columns, beams, slabs etc., the minimum grade of concrete, exposed to mild environment, as per IS 456-2000, is M20 for which the volumetric mix proportion is 1:1.5:3.

Concrete strength and durability depends mainly on the water-cement ratio used in the concrete mix. It is recommended that the mixing water should be properly measured while preparing the concrete mix. The maximum water that can be added per bag (50 kg) of cement is restricted to 27.5 liters.

Only that quantity of concrete mix should be prepared that can be consumed within 30-40 minutes. If concrete is not deposited & compacted in forms within the stipulated time, the initial setting of the cement takes place and results in reduced workability of concrete.

Good concrete is homogeneous and is prepared by intimate & thorough mixing of all ingredients. To ensure homogeneity of concrete mix, it should be prepared in mechanical mixers and mixed for at least 2.5 minutes. Under inevitable circumstances, if the concrete is to be made by hand mixing, it should be done on a clean & water tight platform. Cement content of concrete should be increased by 10%, whenever hand mixing is done.

The concrete mix should be placed in the forms gently and ensuring that it is not dropped from a height of more than 1.5m.

Freshly prepared concrete contains entrapped air, and the same should be removed by compaction either by vibration (with help of needle or plate vibrators) or rodding (with help of a 16mm bullet nosed rod of 2' length). Proper compaction of concrete is extremely important because for every 1% voids in concrete, its strength reduces by 5%.