In placing concrete under water by tremie how does one ensure that the first concrete doesn't get pretty thoroughly mixed with the water? Is there any trick to getting the concrete to flow properly?
The standard practice is to plug the tremie pipe before it is lowered into position. After it has been filled with concrete the pipe is gently lifted, which causes the concrete to push the plug out the bottom. Concrete immediately flows around the end of the pipe and seals it. The pipe is then kept continuously immersed in concrete as it is moved horizontally or vertically. If the pipe is lifted out of the concrete the seal will be lost. The operation should be stopped, the tremie replugged, and the operation repeated as before. Placement should be continuous and fairly rapid to minimize formation of laitance.