Tremie concrete is the name given to the method of placing concrete under water by means of a pipe called a tremie pipe, it has been found from many years' experience that concrete of high slump, 7 to 8 inches, is necessary in order to achieve high-quality concrete with this method. Therefore, concrete is "poured" in its literal as well as its figurative sense. The tremie method is used whenever concrete must be placed under water, and if proper care and careful control are exercised, concrete placed using this technique will be of the highest quality. The trick in tremie concreting is to introduce concrete below the surface of the water and then to continue to introduce new concrete below the previously placed fresh concrete in a continuous operation, causing an outward and upward flow. As long as this flow is smooth and continuous and the surface is not disturbed, an extremely high quality concrete slab will result. There are many advantages of tremie concrete. Among these are: (1) it is unnecessary to de-water the caisson or cofferdam. (2) it is possible to place large volumes of concrete very quickly at great depths. (3) the curing conditions are perfect. (4) Voids and honeycombs are eliminated provided the tremie seal is not broken. Some engineers avoid using tremie concrete, usually because they lack experience with the technique. But tremie work does have its disadvantages: (1) It is necessary to use a high slump, 7 to 8 inches, since it is not practical to vibrate tremie concrete. (2) In many cases it is difficult or impossible to inspect the results of the job. (3) The quality and strength of tremie concrete is questionable, especially at the outer edges of the job, unless the work is done by experienced workmen under proper supervision and control. (4) It is necessary to add extra cement to tremie concrete for safety. This results in increased costs and greater internal heat development. However, heat development is generally dissipated fairly quickly because of the temperature of the water above the concrete. (5) There can be an excess of laitance unless a proper mix design is used.