Sea Water for Mixing Concrete

It is advisable, as stated above, to use clean water fit for drinking purposes for making cement concrete. However, at places where sea water is available in abundance and potable water is costly, the sea water can be used for making cement concrete. The problem of using sea water for mixing concrete has to be studied from the following two aspects:

(1) Strength 
(2) Corrosion of reinforcement.

(1) Strength:

Below table shows the analysis of average sea water. It contains about 3.50 per cent of dissolved salts. The approximate percentages of various salts are 78 per cent of sodium chloride, 15 per cent of magnesium chloride and magnesium sulphate and the rest 7 per cent of calcium sulphate, potassium sulphate, etc.

Now all chlorides tend to accelerate the setting of cement and to improve the strength of concrete in early stages. On the other hand, the sulphates tend to retard the setting of cement and to discourage the strength of concrete in early stages.

It is found that the net effect of these two contradictory actions is the fall in strength of concrete to the tune of about 8 to 20 per cent. Hence the sea water can be used for mixing cement concrete for structures where such fall in strength is permissible or where it is possible to correct the same by adjusting water- cement ratio, cement content in concrete, etc.

The sea water tends to develop dampness and efflorescence. Hence it can be adopted for concrete structures where finishing characteristics are not important or where persistent dampness of the surface is permissible.

(2) Corrosion of reinforcement:

It is found that the sea water does not lead to the corrosion of reinforcement, provided the concrete is dense and there is enough cover to the reinforcement.

The minimum cement content for concrete permanently under sea water should be 3 kN per m³and the minimum cover over the reinforcement should be 75 mm. However it is not advisable to take the risk of corrosion of reinforcement for prestressed concrete and hence the sea water should not be used for making prestressed concrete.