Concrete

an artificial stone. Builders' concrete of the present day is made of one part of Portland or hydraulic cement, three parts of sand, and six parts of crushed stone. The sharper, more angular, and clean the sand and stone, the better. These materials are mixed thoroughly with water, deposited either in molds to form slabs and blocks for curbing and paving, or else laid in the foundation or wall, and allowed to harden where the concrete is to remain. The ancient Babylonian, Egyptian, Greek, and Roman builders were familiar with this kind of material. The Romans, especially, employed concrete in the construction of foundations, piers, aqueducts, roadways, and bridges, portions of which still remain. In the early part of the nineteenth century the French constructed a breakwater to protect the harbor of Algiers. For want of quarry stone, blocks of concrete, weighing twenty-two tons each, were dropped into water fifty feet deep until they formed a long, irregular mole or protective wall. Concrete is used extensively in making a footing for brick or stone walls, especially in muddy, soft excavations where it is not practicable to reach bed rock. The foundation walls of the Chicago postoffice are supported in this way. Of late concrete slabs of superior quality have come into favor for sidewalks, and the cheapening of Portland cement has suggested the use of concrete for entire buildings. It is not unusual to use concrete blocks in place of cut stone for door sills, window sills, window caps, and the like. See Cement