All methods of placing concrete under water are designed to prevent cement washout and the consequent formation of weakly cemented sand and gravel pockets. In the tremie process, concrete is placed through a vertical steel pipe with an open, funnel-shaped upper end. The lower end of the tremie is kept immersed in plastic concrete so that freshly placed concrete doesn't come into contact with the water.
CONCRETE MIX PROPORTIONING
Concrete to be placed by the tremie process must flow readily and yet be cohesive enough not to segregate. Pozzolans improve flow characteristics and are generally used in amounts up to about 15 percent of the cementitious material weight. A slump of 6 to 9 inches is commonly used, but a slightly higher range may be needed when reinforcing cages restrict the flow or when horizontal flow over long distances is required.
TREMIE EQUIPMENT AND PLACEMENT
Tremie pipe diameter usually ranges from 8 to 12 inches. End plates or plugs are used when a dry pipe technique is employed for starting the tremie pour. As the pipe is lowered to rest on the bottom, water pressure seals the gasket and the pipe is kept dry. In very deep placements, an open-ended pipe can be set and a go-devil or traveling plug inserted to keep water from penetrating the first concrete placed in the pipe. All vertical movements of the tremie pipe must be done slowly and carefully to prevent a loss of seal. If loss of seal does occur, the tremie must be brought back to the surface, the end plate must be replaced, and flow restarted. A go-devil must not be used when restarting a tremie after a loss of seal. Water pushed out by the go-devil will wash cement out of the previously placed concrete. Concrete placement should be as continuous as possible through each tremie. Longer delays must be treated by removing, resealing and restarting the tremie.