Water is measured by its volume and it is specified as no of litres per bag of cement. The further increase or decrease in water to be mixed, depends on the quantity of water present in sand and coarse aggregates, due to Hygroscopic action or any other reason.
[Note]
Due to Hygroscopic action fine and coarse aggregates absorbs water from its surrounding.
The water present in aggregates should be subtracted from the total quantity of water required for the mix to get the desired quantity of water needed. Similarly, if aggregates are dry and are likely to absorb water, extra water should be added to maintain the water content in the mix.
Water-Cement ratio
The ratio of the volume of water to the volume of cement used to form concrete is termed as water-cement ratio.
Optimum Water content
This is the most suitable amount of water required for a given proportion of materials to gain its maximum strength.
If the water content is below the optimum water content, the process of setting of cement will not be completed properly. Thus, there will be a decrease in the strength of concrete.
Similarly, if the water content is above the optimum water content, the workability of concrete will increase but the strength will decrease.
From studies and experiments, it is found that if the water is increased by 10% above the optimum water content the strength of concrete reduces by 15%, and the increase in 50% water content reduces the strength of concrete by 50%.
Abram's Law
Abram’s law states that the strength of concrete mix entirely depends on the water-cement ratio. The increase in water-cement ratio decreases the strength of the mix i.e. 'the strength of concrete is inversely proportional to the water-cement ratio'.
Abram’s Law is a special case of general rule formulated by FERET with his experience and experiments.
Where,
S = Compressive Strength of concrete
A & B are empirical constants
(w/c) is water-cement ratio
[Note]
Cement requires about 1/5 to 1/4 of its weight of water to hydrate completely.
