501. Concrete may be molded into blocks which are allowed to set and then are transported to the structure and laid as blocks of stone. This is the block system of construction. The adaptability of concrete to being built in place, however, is one of its chief merits, and consequently the monolithic method of construction is far more common Since it has been found that expansion and contraction, due to changes in temperature, affect concrete walls as they do any other walls of masonry, it has become customary to mold the concrete in sections, usually alternate sections of equal size and shape being built first, and the omitted sections built in later. This method of constructing a long wall is also called monolithic, since the blocks are of large size and are built in place.
502. When concrete is deposited either in air or in water, molds must be provided to keep the mass in the desired shape until it has lost its plasticity and acquired sufficient strength to stand alone. In foundations, the earth at the sides of the excavation may supply the place of a mold, and sometimes the mold forms a part of the permanent structure, as in the case of masonry piers with concrete hearting, and in steel cylinder piers filled with concrete.