What is the purpose of keeping tremie pipe’s tip immersed in
freshly-placed concrete for about 1m in underwater concreting?
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.
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.
The reason to immerse the tremie pipe lower end is to prevent intermixing of both concrete and water.
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