Tag Archives: Sand

What is concrete? Part-I

Think of a modern construction and you are sure to end up equating it with concrete. As construction technologies evolved, concrete usurped the numero-uno position among all the building materials. But construction material expert L.H. Rao says that there are many myths surrounding concrete.

For one, he says that there is an excessive emphasis about the strength of concrete and there is very little deliberation about the durability of the structure. “Concrete as a product was devised to mimic natural rock.

The primary objective of creating concrete was to make a substance that can be moulded as per our requirements, but once set in that mould, it can become a very strong compound,” he says.

While of late there was much technological advancement in the field of enhancing the durability of the concrete, Indian builders are still stuck with the strength aspect only, Dr. Rao, a Technical Adviser at JSW Cement, observes.

“As is commonly known, concrete is obtained by mixing cement, water, sand and rock pieces. But what is not very widely known is that the properties of concrete can vary widely when the amount and quality of its ingredients are tinkered with,” he points out.

Ideally, a good mix can be obtained by using cement, sand and rock pieces in a proportion of 1:1.5:3 respectively.

For better results, sand grains should be of even size and care should be taken so that it does not contain silt or clay in it. Apart from the sand and cement, size of rock pieces also plays an important role in determining the strength of the concrete. “Smaller the rock pieces, greater is the strength of the concrete,” Dr. Rao says.

Next important ingredient in concrete is water and people often make mistakes by using excessive amount of water resulting in weaker concrete structures. “A good concrete mix can be set by using very little amount of water, just 20 percent of the total volume of the cement.

Since it is difficult to mould concrete by using that much water, masons usually add water up to about 50 percent of the total volume of cement used. But some add water indiscriminately to the detriment of concrete’s health,” he explains.

It is a common practice for people to add more water when they find concrete mix too hard to spread and add sand if they find it too slurry. A judicious mix of all ingredients is a must for making healthy concrete, he suggests.

But, does the argument put forward by masons and builders that water eventually evaporates and hence there is not much of a problem if they use more water, stand to the scientific evaluation? Dr. Rao says that water, if used in excess, can cause irreparable damage to the building.

“Excess water usually occupies smaller spaces between the ingredients during construction.

These globules evaporate in due course of time leaving behind pores in concrete reducing its strength. Porosity also encourages atmospheric gases and elements to speed up the effects of corrosion,” he explains.

For a quick check on field, Dr. Rao gives a simple thumb rule: a concrete mix can be deemed as a perfect mix if one can roll a perfect ball out of it.

If the ball is too rough resulting in lack of proper cohesion we need to add more water. On the other hand if the mixture slips off ones hand and is difficult to role into a ball there is excessive amount of water in it.

A plain mix of cement, sand and rock pieces is called Plain Cement Concrete (PCC) and is used as a strong filler material for making floors, ramps and even roads.

But by introducing steel rods into it the structure its tensile strength increases and the resultant is known as Reinforced Cement Concrete (RCC).

“RCC was designed after observing the flexibility of a tree. By adding steel rods the strength of the building is increased.

But in effect, the role of concrete in RCC changes from being just a strong compound to that of a protective layer around the steel rods to shield them from corrosion,” Dr. Rao explains.