PLACING UNDER WATER.
Concrete can be placed under still water if proper precautions are taken. It should never be placed, while soft, in running water unless a form or cofferdam is used, as the cement will be washed out. When concrete is to be placed under water a form of tube or chute, known as a tremie ([Fig. 16]), may be used advantageously. The tube should be of sheet metal, about 8 inches in diameter, with a hopper on top, and means should be provided for quickly raising and lowering it without jolts, so that the concrete will feed out at the bottom without breaking the seal. The lower end of the tube should rest on the bottom or on the concrete as it is built up and a continuous flow of concrete, mixed somewhat soft so that it will flow easily, should be maintained.
Scum or laitance is likely to form on concrete when placed under water, and unless all of the concrete is! poured in one operation and brought to a little above the water surface, seams or planes of weakness will occur.
[CARE OF CONCRETE.]
After the concrete has been poured, care should be taken that it does not dry out too quickly, and in hot weather it must be protected from the sun. Exposed surfaces and objects made of dry concrete should be sprayed thoroughly with water twice or oftener each day for a week or 10 days. Sometimes surfaces are shielded with canvas, paper, boards, or layers of moist sand.