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What is the purpose of keeping tremie pipe’s tip immersed in freshly-placed concrete for about 1m in underwater concreting?

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The size of tremie pipe is about 300mm with sections having flange couplings fitted with gasket to prevent water leakage. The tremie pipe should be closed initially to prevent water from entering the pipe. It should be designed with sufficient thickness and weight so that it would not be buoyant when empty inside water.

The placement of tremie concrete is commenced by putting the closed pipe underwater to the location for concreting, followed by partial filling of tremie pipe with concrete. In order to have tremie concrete flowed out of the pipe, it is necessary to fill the pipe with concrete of sufficient height to overcome the water pressure and frictional head. After that the tremie pipe is raised about 150mm to allow concrete to flow out. To enhance sufficient bonding, each succeeding layer of concrete should be placed before the preceding layer has reached the initial set. The tremie pipe should be kept full of concrete up to the bottom of hopper.

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The tip of tremie pipe should always be immersed in freshly-placed concrete for at least 1m to prevent inflow of water into the tremie pipe and to avoid contact of freshly placed concrete with water. This serves as the seal against water entry. The loss of seal may result in increased flow rate with fresh concrete affected by seawater. The distance that tremie concrete could be allowed to flow without excessive segregation is about 6-20m.

This question is taken from book named – A Closer Look at Prevailing Civil Engineering Practice – What, Why and How by Vincent T. H. CHU.

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