Under Water concreting

 During the construction of bridge , dams or any other structure where the foundation part of the structure is most likely to lie underwater, our main objective is to create dry & water free environment for working in such a manner that the structural stability of the structure is not compromised.

Construction in water poses many difficulties especially in the places where there the depth is considerable very effort , should be concentrated on assuming continuity of placement between the construction joints . Puddling or any other types of physical disturbance should be carefully avoided once the concrete is in place , & every possible precaution should be taken to minimize segregation.

Method of placing concrete under water

1. Tremie method

2. Dewatered coffer dam or caissons

3. Placing in bags

4. Prepacked concrete

5. Drop bucket method

1. Tremie method

The most common method of handling concrete under water is by Tremie . The tremie concrete placement method uses a watertight pipe of about 250mm diameter through which concrete is placed below water level . Pipe has a funnel shaped hopper at its upper end and a loose end of pipe with a watertight seal at its tip of the pipe is kept immersed in fresh concrete so that the rising concrete form the bottom displaces the water without washing out the cement content.

           

                        fig. Tremie pipe

Before concreting air and water must be excluded keeping the pipe full of concrete all the time . For this the funnel and the pipe should have equal capacity . Firstly plug is inserted in the pipe with pressure of fresh concrete so that the air is displayed . The buckets are usually fitted with the drop-bottom or bottom-roller gates which open freely outward when tripped . The bucket is fully filled along  with the concrete and its top covered along with a canvas cloth or a gunny bag to avoid the disturbance of. concrete as the bucket is lowered within water . The tremie pipe is lowered into the position with the help of crane or powered hoist.

 Depending upon depth, the pipe is filled or partially filled with concrete . Then the pipe is gently raised about 6 inches to release the concrete letting it rise up around the tube. In proper tremie  placing, a closure is used to seal the pipe prior to placing the concrete . As the tremie is lowered to the bottom of the form , water pressure holds the plate tightly in position . Then the pipe and the hopper are filled. The plate retaining wire is released if it is single , or withdraw if it is double , and the tremie is raised about 4 to 6 inches from the bottom of the form , allowing the concrete to flow into place. An effective plug can be provided by forcing an inflatable rubber ball into the top of the pipe. The ball pushed downward by the weight of the concrete. Using a ball for the plug offers the advantage that it will pop to the surface on leaving the pipe.

As the hopper is filled , the pressure of the fresh concrete forces the plug down the pipe and the water in the tremie is displaced by the concrete . Since the end is blocked so water will not enter the pipe. Stoppages during tremie placing can cause considerable difficulty and every effort should be made to foresee and avoid them. The main causes of stoppage are : pipe dia too small as the mix gradually forms a bridge across the pipe walls ; delays in delivery because the mix inside the pipe begins to stiffen; mix tol harsh or too stiff making plastic flow impossible; poor aggregate grading and , in particular, insufficient fines; the lubrication coating on the pipe wall is broken and sticking follows. As the concreting proceeds the pipe is raised slightly, with the help of crane , and the concrete flow outwards. When large quantity of the concrete is to be placed continuously then it is preferable to use a series of tremie with spacing between them not exceeding 3.5m to 5m and the end tremie be 2.5m from the formwork.