When circumstances demand that concrete be placed under water, every effort should be concentrated on assuring continuity of placement between the construction joints. Puddling or any other type of physical disturbance should be carefully avoided once the concrete is in place, and every possible precaution should be taken to minimize segregation. The most common method of handling concrete under water is by tremie. A tremie consists essentially of a vertical steel pipeline, topped by a hopper and is long enough to reach from a working platform above water to the lowest point of the underwater formwork. By this technique the steel pipeline, with a watertight seal at its tip, is placed at the bottom of the formwork. 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 hopper are filled. The plate retaining wire is released if it is single, or withdrawn if it is double, and the tremie is raised about 4 to 6 inches from the bottom of the form, allowing the concrete to flow into place.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 is 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. 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 diameter 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 too harsh or too stiff making plastic flow impossible; poor aggregate grading and, in particular, insufficient fines; the lubrication coating on the pipe walls is broken and sticking follows.