It is advisable 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 making cement concrete has to be studied from the following two aspects.
- Strength
- Corrosion of reinforcement
Strength
The following table shows the analysis of average sea water. It contains about 3.50 percent of dissolved salts. The approximate percentages of various salts are 78% of sodium chloride, 15% of magnesium chloride and magnesium sulphate and the rest 7% of calcium sulphate, potassium sulphate, etc.
| Name of constituents | Content in gm per litre |
| Calcium (ca) | 0.43 |
| Chloride (cl2) | 19.8 |
| Magnesium (Mg) | 1.33 |
| Potassium (K) | 0.40 |
| Sodium (Na) | 11.00 |
| Sulphate (SO4) | 2.76 |
| Total= | 35.72 |
Now all chlorides tends to accelerate the setting of cement and to improve the strength of concrete in early stages. On the other hand, the sulphates tends 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%. Hence the sea water can be used for making cement 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 structure is permissible.
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 cover over the reinforcement for concrete permanently under sea water 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.
What if the cement used is SRC type؟
If the collapsed condo contractor used either sea water and or beach sand to build the units, the corrosive aspect will be the end effect we witnessed on June 24, 2021 @ The Surfside condo. The corrosive effect of salts in sea water are very damaging to the steel reinforcing rods in the concrete mix. The reason only one area fell was because of a different mix of concrete in those units from the other.
can an admixture be used to counter effect?