Process of underwater concrete construction.

Underwater Consrete Construction:

The comcrete should have a slump equivalent of very high degree of workability. The w-c ratio shall not exceed 0.6 and may need to be less depending upon the grade of concrete or type of chemical attack. For aggregate of 40mm maximum particle size, the cement content shall be atleast 350kg/cum of concrete. Coffer dams or forms shall be sufficiently tight to ensure still water if practicable, and in any case to reduce the flow of water to less than 3m/min through the space into which concrete is to be deposited. Coffer-dams or forms in still water shall be sufficiently tight to prevent loss of mortar through the walls. De-watering by pumping shall not be done while concrete is being placed or until 24 hour thereafter.

Concrete cast underwater should not fall freely through the water. Otherwise it may be leached and become segregated. Concrete shall be deposited continuously until it is brought to the required height. While depositing, the top surface shall be kept as nearly as possible and the formation of seams avoided. The methods to be used for depositing concrete under water shall be one of the following:

(i) Tremie: The concrete is placed through vertical pipes, the lower end of which remains inserted sufficiently deep into the concrete which has been placed previously but has not set. The concrete emerging from the pipe pushes the material that has already been placed to the side and upwards and thus does not come into direct contact with water.

When concrete is to be deposited under water by means of tremie, the top section of the tremie shall be a hopper large enough to hold one entire batch of the mix or the entire conctents of the transporting bucket, if any. The tremie pipe shall not be less than 200mm in diameter and shall be large enough to allow a free flow of concrete and strong enough to withstand the external pressure of water in which it is suspended, even if apartial vaccum develops inside the pipe. Preferably, flanged steel pipe of adequate strength for the job should be used. A separate lifting device shall be provided for each tremie pipe with its hopper at the upper end. Unless the lower end of the pipe is equipped with an approved automatic check value, the upper end of the pipe shall be plugged with a wadding of gunny sacking or other approve material before delivering the concrete to the tremie pipe through the hopper, so that when the concrete is forced down from the hopper to the pipe, it will force the plug (and along with it any water in the pipe) down the pipe and out of the bottom end, thus establishing a continuous stream of concrete. It will be necessary to raise slowly the tremie in order to cause a uniform flow of concrete, but the tremie shall not be emptied so that water enters the pipe. If the charge in the tremie is lost while depositing, the tremie shall be raised above the concrete surface, and unless sealed by a check valve, it shall be re-plugged at the top end, as at the beginning, before refilling for depositing concrete.

(ii) Direct placement with pumps: As the case of the tremie method, the vertical end piece of the pipe line is always inserted sufficiently deep into the previously cast concrete and should not move to the side during pumping. 

(iii) Drop bottom bucket: The top of the bucket shall be covered with a canvas flap. The bottom doors shall open freely downward and outward when tripped. The bucket shall be filled completely and lowered slowly to avoid backwash. The bottom doors shall not be open until the bucket rests on the surface upon which the concrete is to be deposited and when discharged, shall be withdrawn slowly well above the concrete.

(iv) Bags: Bags of atleast 0.028cum capacity of jute or other coarse cloth shall be filled about two third full of concrete, the spare end turned under so that bag is square ended and securely tied. They shall be placed carefully in header and stretcher courses so that the whole mass is interlocked. Bags used for this purpose shall be free from deleterious materials.

(v) Grouting: A series of round cages made from 50mm mesh of 6mm steel and extending over the full height to be concreted shall be prepared and laid vertically over the area to be concreted so that the distance between centres of the cages and also to the faces of the concrete shall not exceed one metre. Stone aggregate of not less than 50mm not more than 200mm size shall be deposited outside the steel cages over the full area and height to be concreted with due care to prevent displacement of cages.

A stable 1:2 cement -sand grout with a water-cement ratio of not less than 0.6 and not more than 0.8 shall be prepared in a mechanical mixer and sent down under pressure (about 0.2N/sqmm) through 38 to 50mm diameter pipes terminating into steel cages, about 50mm above the bottom of the concrete. As the grouting proceeds, the pipe shall be raised gradually up to a height of not more than 600mm above its starting level after which it may be withdrawn and placed into the next cage for further grouting by the same procedure.