CONCRETE. An artificial stone composed of hydraulic cement (q.v.), sand, and broken stone or gravel, or other hard material in small frag ments. The mixture of sand and cement is commonly called the matrix, and the broken stone or other material is similarly called the aggregate. The matrix may be either lime and sand or cement and sand mortar, but is more usually the latter. The aggregate may be peb bles, gravel, broken stone, broken bricks, shells, slag, coke, etc., but the most commonly used aggregates are broken stone and gravel. Broken stone gives a stronger concrete than gravel, other Things being equal. The proper proportions of the several constituents composing concrete is considered to be attained when the cement paste exactly fills the voids in the sand. and the matrix exactly fills the voids in the aggregate; less than enough mortar to fill the voids in the aggregate results in a weaker and more porous concrete, and more than enough adds to the cost of the concrete without increasing its strength.
It is evident from this statement that the relative proportions will vary with the char acter of the sand and aggregate employed. A fair range of proportions for most engineering works is cement, one part; sand, one to three parts; aggregate, four to six parts. There is considerable diversity of opinion as to the amount of water to be used in making concrete. According to one extreme view, the amount of water should be such that the concrete will quake when tamped; according to the other extreme, the mixture should he made so dry that water will barely flush to the surface when the concrete is tamped. Current practice varies all the way between these two extremes:. The manufacture of concrete consists simply in mix ing the water, cement, sand, and aggregate of which it is composed. To obtain the best results this mixture should be exceedingly thorough; the ideal mixture is attained when every grain of sand is covered with a film of cement paste and every fragment of aggregate is covered with a coating of mortar. Both hand mixing and ma chine mixing are employed in practice. In hand mixing the proper proportions of cement and sand are deposited on a timber platform and mixed dry by repeated turnings with a shovel. The proper quantity of water is then added, preferably with a spray, and the mixture then turned and returned with shovels until the water is thoroughly and evenly incorporated with the cement and sand. The aggregate is then thor oughly wetted and is mixed with the mortar by similarly repeated turnings with shovels.
A variety of concrete-mixing machines arc em ployed, some being intermittent and some being continuous in operation, the latter sometimes automatically measuring the proportions of cement, sand, aggregate, and water. Perhaps the most common form of intermittent mixer is a cubical iron box hung on trunnions at diagonally opposite corners; the cement, sand, and aggregate in the proper proportions are placed in the box through a suitable door in one side which can be closed and fastened; the water is admitted through the hollow trunnions, and the box is put in revolution by an engine or other motive power. After from ten to twenty turns the box is brought to rest, its contents of thoroughly mixed concrete dumped out into barrows or cars and a new charge of cement, sand, and aggregate introduced for mixing. A common form of con tinuous concrete-mixer consists of a trough or cylinder in which a spiral or bladed screw shaft revolves; the raw materials are introduced con tinuously at one end, and a continuous discharge of mixed concrete takes place at the opposite end. There are numerous other forms of con
crete-mixing machines.
The value of concrete as a structural material consists in its property of changing from a plas tic condition into a hard, rigid, artificial stone by the setting and hardening of the cement paste. (See CEMENT.) Concrete composed of one part cement, two parts sand, and six parts broken stone has a compressive strength of from ten to twenty tons per square foot at the age of one year. The method of laying the concrete after mixing depends upon the position in which it is placed and upon the form in which it is to be used in the structure. \\Then used in the form of blocks, the blocks are made by placing the plastic mix ture in suitable molds in thin layers and tamp ing each layer thoroughly with wooden or iron rammers before placing the succeeding layers. The mixture remains in the molds until it is hard, when the block is removed and laid in the struc ture just as a corresponding block of stone would be laid. The more common method of using con erete is to place it in the structure in its plastic condition, and let it harden in place. When the work is in the air the mixture is laid and rammed in layers, just as is done in making concrete blocks, but when it is laid under water it has to be deposited in buckets, which open when the bottom is reached and discharge their contents, or it is run through circular chutes, which reach from the surface to the bottom. Sometimes con crete is placed under water by inclosing it in paper bags, which are slid down a chute; the bags become wet and the concrete bursts them open, thus allowing the succeeding bagfuls to unite into a solid mass. Sometimes, also, coarse open cloth bags are used, the cement oozing through the meshes sufficiently to unite the whole into a single mass. The chief care to be exercised in placing concrete under water is to prevent the cement, sand, and aggregate from becoming sepa rated. Concrete weighs when hard from 130 to 160 pounds per cubic foot, and costs, laid, from *2 to $10 per cubic yard.
The uses of concrete are excellently summarized in A Treatise on Masonry Construction, by Prof. 1. 0. Baker, as follows: "Concrete is admirably adapted to a variety of most important uses. For foundations in damp and yielding soils, and for subterranean and submarine masonry, under almost every combination of circumstances likely to be met with in practice, it is superior to brick masonry in strength, hard ness. and durability: it is more economical: and in some cases is a safe substitute for the best natural stone, while it is almost always preferable to the poorer varieties. For submarine masonry, concrete possesses the advan tage that it can be laid, under certain precautions, without exhausting the water, and without the use of a diving-bell or submarine armor. On account of its continuity and impermeability to water, it is an excellent material to form a substratum in soils infested with springs; for sewers and conduits; for basement and sustain ing walls; for columns, piers. and abutments: for the pointing and backing of walls faced with brick, rubble, or ashlar work; for pavements in areas, basements, sidewalks, and cellars; for the walls and floors of cisterns, vaults, etc. Oroined and vaulted arches, and even entire bridges (see Bl;mEs), dwelling-houses, and factories in single monolithic masses, with suitable ornamentation, have been constructed of this material alone." Consult Baker. Treatise on Heronry Construction (New York. 1!1001. See :MORTAR.