Question

Concrete is a composite material. From microscopic point of view, it can be considered
as a three-phase composite with aggregate, cement paste and interface zone between
these two components.

1. For concrete with 28-day compressive strength of 35 Mpa (5076psi) made with
ordinary Portland cement and crushed granite aggregate, which phase of the
concrete is generally the weakest?
2. If the granite aggregate for the concrete mentioned in (i) is replaced by lightweight
aggregate. How would this affect the strength of concrete?
3. If we would like to increase the strength further, what approach shall we take?

Answer 1

1. The concrete with any characteristic strength gains its strength with the hydration and indirectly with time. As the age of concrete increases the strength of concrete also increases. The concrete made up of crushed granite aggregate gains strength with the formation of hydrations compounds which are the functions of time. The main compounds which provide strength to the concrete are di-calcium silicate and tri-calcium silicate which are produced at the age of seven days.

Thus, the concrete is generally weakest till seven days (fresh state of concrete) and this phase is very sensitive to hydration and proper curing is required so as to increase the strength to the desired characteristic compressive state specified in the mix design.

2. The strength of concrete is drastically affected with the replacing of granite aggregate with the light weight aggregate. The flexural, compressive and ultimate strength drastically reduces with the use of light weight aggregate. This will impact on the characteristic compressive strength of mix design as the replacement will not provide the required strength parameters and reduces the load bearing capacity of the concrete.

But with the use of light weight concrete the serviceability parameters of the structural component can be sustained as the deflection and cracking are functions of self weight and live loads. In the case of light weight concrete the self weight of structure is reduced due to its low density and the load bearing is constant because of which the load carrying capacity is equivalent to the normal concrete.

3. If the strength is to be future increased we can reduce the water cement ratio of the concrete by using plasticizers and super plasticizers. The strength is inversely proportional to the water cement ratio, as the water cement ratio increases the strength decreases and vice-versa. If we want to increase the strength of light weight concrete we can replace water with plasticizers which impart workability without compromising with the strength parameters.

The other option to increase the strength further is use of high strength cement and increased grade of cement and also the strength can be increased with increase in cement paste in co-ordination with aggregate. The use of rounded or cubical aggregates give higher compressive strength to the concrete.

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