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Additive strength

Concrete has become an integral part of the houses and buildings these days.

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Jagvir Goyal

Concrete has become an integral part of the houses and buildings these days. Major structural components such as foundations, columns, beams, roof slabs, lintel beams and projections are built in RCC only for their longer life and durability. Every house builder shall want these components to be stronger, durable and impregnable. Certain additives if added to concrete during its production help in achieving this goal. Let’s have a look at them: 

Basic ingredients

First of all, let’s have a look at the basic ingredients of concrete. We all know that these are cement, coarse aggregate called Bajri of 20 mm and 10 mm size, coarse sand and water. Among these, cement has to be fresh and good quality, coarse aggregate has to be well-graded and silt free, fine aggregate or sand has to be coarse with fineness modulus more than 2.6 and water has to be potable. 

Water-proofing additives

In order to make concrete water proof, certain additives are now available in the market. These additives if added to concrete during the mixing of ingredient materials in the concrete mixer, prove highly useful as these lend the property of water proofing to the concrete so produced. These water proofing additives are polymer emulsions that make concrete cohesive and impregnable. These additives are available in liquid form and addition of just 200 ml of them for every one bag of cement is sufficient. For adding these to concrete, a 200 ml container is used to add the quantity to the mixer while mixing other ingredients. These additives make concrete waterproof and help in preventing cracks in it. 

Water reducers

The lesser quantity of water in concrete makes it strong. Certain polymer emulsions that are now available, help in reducing the quantity of water in concrete. The addition of just 250 ml per bag of cement helps in reducing the quantity of water in concrete by 10 per cent and that is quite significant.

For further reduction of water, more quantity of these additives can be added to the concrete. However, the upper limit shall be about 400 ml per bag of cement for reduction of water by 15 to 20 per cent as a further increase will have contrary effects. Addition of these additives increases concrete strength and prevent corrosion of steel bars. 

Plasticisers

In order to increase the strength of concrete, if we  reduce the quantity of water in it too much then the concrete doesn’t remain workable and it is difficult to lay it well as it doesn’t flow well through the steel bars and a dense and homogeneous structure is not produced. To solve this problem, certain additives called plasticisers are available. Addition of these plasticisers make the concrete workable without increasing the quantity of water in it. 

Thus, mix design of concrete is finalised by keeping the water cement ratio as low and plasticisers are added to make it workable. The addition of plasticisers further increases the cohesiveness of concrete and avoids segregation of basic ingredients. Mostly, plasticisers are used in big concrete structures and their compatibility with cement is first tested in laboratory. 

Superplasticisers

Superplasticisers help in the reduction of water in concrete to a high range. If plasticisers help in reducing water in concrete by 15 per cent, superplasticisers can help in reducing water in concrete to even 30 per cent. Many superplasticisers are available in the market. Some superplasticisers help in producing and laying concrete when ambient temperature is low while some others help in concrete production when ambient temperature is high. However, among all, Poly Carboxylated ether copolymer superplasticisers prove the best ones as these continue to act well in concrete even if there is delay in handling and laying of concrete. The addition of just 75 ml to 150 ml of superplasticiser per bag of cement in concrete is sufficient. The exact quantity to be added to concrete is determined in the laboratory. 

Applicators

Certain chemicals, when mixed with water and applied on concrete surfaces with a brush, penetrate into the capillaries of concrete and block all pores and invisible micro cracks in its surface, making the surface waterproof. These resist permeation of water into concrete as their chemical reaction with concrete produces millions of needle like crystals which completely block the concrete surface. Therefore, their application in water tanks and swimming pools proves highly beneficial. 

All these additives and applicators don’t harm concrete in any way and help in enhancing its good properties at minimal cost. 

Water-cement ratio

Water cement ratio plays a major role in determining the strength and durability of concrete. More is the water cement ratio, less is the strength of concrete. Therefore, efforts are made to keep water-cement ratio as low as possible. Water cement ratio is determined from the quantity of water added for one bag of cement in concrete. Suppose 20 litres of water are added to the mixer for every one bag of cement weighing 50 kg then the water cement ratio is 20/50 i.e. 0.40. Higher water cement ratio also makes the concrete susceptible to penetration by water thus causing dampness and corrosion of steel embedded in concrete.

— The writer is former HoD and engineer-in-chief, Civil Engineering Department in a Punjab PSU 

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