Water-Cement Ratio (W/C) of Concrete
Water-Cement Ratio (W/C) of Concrete
The water in concrete has to perform the following two functions:
(i) The water enters into chemical action with cement and this action causes the setting and hardening of concrete.
(ii) The water lubricates the aggregates and it facilitates the passage of cement through voids of aggregates. This means that water makes the concrete workable.
It is found theoretically that water required for these two functions is about 0.50 to 0.60 times the weight of cement. This ratio of the amount of water to the amount of cement by weight is termed as the water-cement ratio and the strength and quality of concrete primarily depend upon this ratio.
The quantity of water is usually expressed in litres per bag of cement and hence the water-cement ratio reduces to the quantity of water required in litres per kg of cement as 1 litre of water weighs 1 kg. For instance, if water required for,
1 bag of cement is 30 litres, the water-cement ratio is equal to 30/50= 0.60
The important points to be observed in connection with the water-cement ratio are as follows:
(i) The minimum quantity of water should be used to have reasonable degree of workability. The excess water occupies space in concrete and on evaporation. the voids are created in concrete. Thus the excess water affects considerably the strength and durability of concrete. In general, it may be stated that addition of one extra litre of water to the concrete of one bag of cement will reduce its strength by about 1.47 N/mm2 In other words, the strength of concrete is inversely proportional to the water-cement ratio.
(ii) The water-cement ratio for structures which are exposed to weather should be carefully decided. For instance, for structures which are regularly wetting and drying, the water-cement ratio by weight should be 0.45 and 0.55 for thin sections and mass concrete respectively. For structures which are continuously under water, the water-cement ratio by weight should be 0.55 and 0.65 for thin sections and mass concrete respectively.
(iii) Some rules-of-thumb are developed for deciding the quantity of water ii concrete. The two such rules are mentioned below. The rules are ordinary concrete and they assume that the materials are non-absorbent and dry.
(a) Weight of water = of the weight of the cement + 4% of the weight of total aggregate.
(b) Weight of water = of the weight of the cement + 5% of the weight of total aggregate.
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