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Of all the ingredients used to make concrete, water is the cheapest. But it is important that we pay sufficient attention to it.
Water is needed in the hydration of cement so that a binding matrix is formed to hold the inert aggregates in suspension until the matrix hardens. Workability is also contributed by the presence of water.
Generally cement requires about 30% of its weight of water for hydration. So, that makes the water-cement ratio 0.30. But at this level, the fresh concrete mix will be very hard to mix and place. Hence we need additional water to achieve the required workability but not too much as too much water will affect the strength.
The water-cement ration is determined, among others, by
Grade of concrete
Type of aggregates
Workability required
Durability
If too much water is added into the concrete mix, the excess water along will cement will rise to the surface through capillary action. This cement-water mixture forms a layer of chalky material known as laitance. It is this laitance that prevents bond formation between the successive layers of concrete. This laitance becomes a plane of weakness in the concrete.
Excess water may also leak through the joints of formwork resulting in honeycomb.
So, though important for hydration and workability, the amount of water should be calculated and added correctly to achieve strong and durable concrete.