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When the concrete is to be placed underwater, special precautions need to be taken. Following are the recommendations of the Portland Cement Association in regard to the quality of concrete:
The concrete should be plastic and cohesive but should have good flowability. This requires a fairly high slump, commonly150 to180 mm.
A richer mix then generally used for placing under normal conditions is required; usually the cement requirement is not less than eight bags per cubic meter of concrete.
The proportions of fine and coarse aggregates should be adjusted to produce the desired workability with a somewhat higher proportion of fine aggregate than used for normal conditions. The excellence aggregate proportion can frequent be from45 to50% of the total aggregate, depending on the grading. It is also important that the aggregate contain sufficient fine material passing in the300 micron and150 micron sieves to produce a plastic and cohesive mixture. ASTM standard specifications for concrete aggregate require that not less than2% pass the150 micron sieve.
The fine aggregate should meet the minimum requirements and somewhat higher percentage of fines would be better in many cases. For most works, coarse aggregate should be graded up to20 mm or40 mm. The coarse aggregate should not contain loam which may cause laitance while being worked.
The formwork not only has to impart the required shape to the structure or to fits elements, it must also protect the concrete mix during placing until it matures, from the direct action of current and waves.
The formwork also serves as a temporary protective casing which during concreting prevents possible washing out of cement and the leakage of cement mortar from the concrete mix. After completion of concreting, that will protect the soft concrete from the impact and abrasive action of the water currents. The cofferdams may be constructed to reduce the velocity of flow.
