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.