Forms are required to hold the concrete in place until it has attained
sufficient strength to sustain itself and the initial loads to which it
may be subjected. Concrete is plastic and will assume the shape of the
form, thus any imperfection or impression on the face of the forms will
be reproduced.
Wood is commonly used for forms, as it can be easily worked into
different shapes, though various other materials sometimes are better
adapted to special conditions. Cast iron, for instance, is suitable for
casting small objects that are to be reproduced in quantities, such
as concrete block or tile; plaster of Paris, glue, or moist sand are
employed for casting ornaments or to produce a fine, smooth surface;
sheet metal is suitable when the forms can be used repeatedly or for such
circular structures as silos. When the sides of an excavation are not
likely to cave in the earth may serve as a form.