Concrete. There is but little difference between concrete and coarse mortar. The mortar used in the masonry of castles and churches erected during the middle ages is in fact a concrete with small pebbles instead of the larger ones used in modern concrete. Any mixture of lime, sand, and water, with broken stones or bricks, bits of slag, gravel, or other hard material, is called a concrete. The hard lumps are termed the aggregate, and the mortar in which they are embedded is called the matrix. The mixture varies with the nature and quality of the materials, but it often consists of 1 part of quicklime, 2 of sand, and 5 of gravel. It is better to use such a material as broken stone rather than water-rolled gravel, which has often too smooth a surface. Lime concrete, as the kind above described may be termed, is used principally for foundations, that is, a thick bed of it is formed below the lowest course of stones or bricks in walls, in cases where the ground itself is not sufficiently firm and solid. Not unfrequently, in some countries, walls themselves are formed of concrete, by laying a foot or two of it in height at a time between boards, and giving it some time to harden. It then forms an artificial stone. Other methods of building walls of concrete are in use (see BUILDING).
Portland cement concrete is made either by mixing it with gravel alone, or more generally by using the cement along with sand and broken stones. The concrete used at Portland Breakwater Fort and at Cork Harbour have nearly the same composition. The cubic yard of it at the former place was made by using of Portland cement 5 cubic feet, of sand 10 cubic feet, and of broken stones (not more than 3½-inch gauge) 28 cubic feet, along with 23½ gallons of water. A concrete with these materials in much the same proportion has been used at other places, but different proportions are also employed. Portland cement being the binding material in this concrete, the question arises whether there is sufficient experience of its durability, especially where it is exposed to the action of sea-water, to warrant its employment in structural works of great or even of moderate size. See CEMENT.
Concrete made of the hydraulic lime from Teil in France, which contains 66 per cent. of silicate of lime, has been employed in the construction of breakwaters and similar works at Cherbourg, Marseilles, and other places. The Teil hydraulic lime is one of the strongest known, and the concrete made with it has resisted the action of sea-water for many years. Ordinary lime concrete does not set under water.
A concrete is made of broken stones and tar, about 12 gallons of the latter being used for every cubic yard of concrete. Bitumen or asphalt is better than gas-tar for this purpose, and either may be hardened by the introduction of dried and pounded lime, clay, or brick-dust. The materials should be heated before being mixed with the tar or bitumen. For backing armour-plates in forts a concrete of cast-iron turnings, asphalt, and pitch has been used. Gravel mixed with rather more than a thirtieth part, by bulk, of iron borings, was successfully used as a concrete at Stranraer pier.