CONCRETE, an artificial conglomerate or rubble masonry, consisting of a mixture of coarse pieces of stone, gravel, shingle, broken brick, or crushed slag with sand and Portland or other cement. It is employed for laying the foundations of bridges and of buildings on soft or wet ground, as also in the construction of moles and breakwaters, and of houses and churches ; for the backing of wharves, of the abutments of arches, and of masonry generally where heavy walls are required ; for the substance of fire-proof doors; for the making of sewer-pipes; and as a paving for streets and floors. It soon hardens after use, becoming a stony mass little permeated by moisture. In the shape of blocks, sometimes weighing very many tons, it has been found of great value for the formation of harbours and sea-walls in places to which stone could not have been transported. The foundations of the breakwater pier at Douglas, Isle of Man, were made by laying down concrete within frames resting on submarine rock. The quay walls of Stobcross Docks, Glasgow, are supported on triple groups of concrete cylinders, 27-i feet in length and resting on iron shoes. Each cylinder is formed by sinking in the soil a column of eleven rings of concrete ; this, after being cleared of the sand and gravel it contains, is filled with Portland cement concrete. Walls are made of concrete either by allowing it to harden in mass between two faces of boarding, or by making it into blocks and building as with bricks. Concrete was employed by the Romans, by whom the term signinum was applied to a kind of plaster composed of powdered tiles and mortar ; and Smeaton gained the idea of applying it to the construction of river-works from an inspection of the ruins of Corfe Castle in Dorset-shire, a Saxon structure. In the Middle Ages it was much used in the making of fortifications.

The composition of concrete necessarily depends to some extent upon the nature and qualities of the materials most available for making it. The baton agglomere of M. F, Coignet is composed of about 180 parts of sand, 44 of slaked line, 33 of Portland cement, and 20 of water. A mixture of the cement with the sand and lime is first made, with the addition of small quantities of water ; the mass is then incorporated with the requisite amount of water in a cylindrical machine, from the bottom of which it is delivered ready for compression in moulds. This composition can be formed into blocks of any desired bulk ; and these, after exposure to the weather for a few weeks, acquire a hardness equal to that of good building stone. A good concrete can be made from 60 parts of coarse pebbles, 25 of rough sand, and 15 of lime. Semple recommends a mixture of 8 parts of pebbles, 4 of sharp river sand, and 1 of lime. The proportions given by Treussart are unslaked hydraulic lime 30 parts by measure, trass of Andernach 30, sand 30, gravel 20, broken stone or hard limestone 40 parts ; and for another concrete, hydraulic lime 33 parts, pozzuolana 45, sand 22, broken stone and gravel 60 parts; the former is used as soon as made, the latter should be exposed about 12 hours after preparation. Burnt clay and pounded brick may be used in the same proportions as the trass, but are best not employed in sea-water. The quantity of natural or artificial pozzuolanas is increased, and that of the gravel or stone decreased, if rich limes are used (Burnell, Limes, &c). Excellent concrete is made from Thames or other river ballast mingled with -4th or -th its bulk of lime ; in setting it contracts by about one-fifth of its volume, a cubic yard of the concrete requiring 30 cubic feet of ballast and 3i cubic feet of lime. Hydraulic concrete must contain a sufficiency of mortar to aggregate the whole mass of rubbly material, and the lime or cement should be thoroughly slaked before the immersion of the concrete. For the breakwater at Dover Mr Lee employed 16 foot cubes of concrete made in moulds, composed of Portland cement, Portland stone chippings, sand, and shingle. The blocks at the mole, Marseilles, were formed from 5 parts of sand, 2 of broken stone, and 3 of Theil lime. The concrete used at the extension of the London decks consisted of 1 part of blue Lies lime to 6 of gravel and sand ; and that made by M. Vicat for the bridge at Souillac, on the Dordogne, contained, with 26 parts of hydraulic lime, 39 of granitic sand, and 66 of gravel. The composition for the Copenhagen sea-forts was 1 part of Portland cement, 4 of sand, and 16 of fragments of stone.

Austin's artificial stone is a concrete of sand and other materials cemented by lime. Ransonme's concrete stone is made by subjecting a mixture of sodium silicate and clean pit sand, to which between 5 and 10 per cent. of chalk has been added, to the action of a solution of calcium chloride, whereby insoluble calcium silicate and soluble sodium chloride, or common salt, are produced, the former acting as a cementing material for the particles of sand. The stone is made non-absorbent by giving the face of it a wash with sodium silicate, and then a second application of the calcium chloride. (See II. Reid, A Practical Treatise on Concrete, London, 1869.)