Concrete is an artificial compound, generally made by mixing lime or cement with sand, water, and some hard material, such as broken stone, gravel, burnt clay, bits of brick, slag, etc. These ingredients should be thoroughly mixed so as to form a sort of conglomerate. The lime, or cement, sand, and water, combine to form a lime or cement mortar in which the hard material is imbedded, so that the result is a species of very rough rubble masonry. The broken material is sometimes for convenience called the "aggregate," and the mortar in which it is encased the "matrix." The strength and other qualities of concrete depend chiefly upon the matrix. They are, however, influenced also by the aggregate.

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As to the matrix, the lime, or cement, sand, and water, should be so proportioned that the mortar resulting from their mixture is the best that can be made from the materials available. As a rule it should be better than the mortar used for walling, especially if the concrete is to be used in important positions. The reason for this is that, in concrete, the mortar receives less assistance, from the form and arrangement of the bodies it cements together, than it does in masonry or brickwork. In some cases the mortar is mixed separately, just as if it were to be used in building brickwork or masonry, and then added to the hard material. More generally, however, the ingredients are mixed together in a dry state.

The aggregate is generally composed of any hard material that can be procured near at hand, or in the most economical manner. Almost any hard substance may be used when broken up, e. g. broken stone, slag, bits of brick, of earthenware, burnt clay, breeze, and shingle. Preference should be given to fragments of a somewhat porous nature, such as pieces of brick or limestone, rather than to those with smooth surfaces, such as flints or shingle, as the former offer rough surfaces to which the cementing material will readily adhere. Any aggregate of a very absorbent nature should be thoroughly wetted, especially if it is used in connection with a slow-setting lime or cement, otherwise the aggregate will suck all the moisture out of the matrix, and greatly reduce its strength.

Many prefer aggregates composed of angular fragments rather than those consisting of rounded pieces, e. g. broken stone rather than shingle. The reason for this is that the angular fragments fit into one another, and slightly aid the coherence of the mortar or cement by forming a sort of " bond," while the round stones of the shingle are simply held together by the tenacity of the matrix. Moreover, the angular stones are cemented together by their sides, the rounded stones only at the spots where they touch one another.

The aggregate is generally broken so as to pass through a 1 1/2 - or 2-in. mesh. Very large blocks cause straight joints in the mass of the material, which should be avoided if the cement is to bear a transverse stress or to carry any considerable weight. Of the aggregates in common use, broken brick, breeze or coke from gasworks if clean, and burnt clay if almost vitrified throughout, all make very good concrete. Gravel and ballast are also good if angular and clean. Shingle is too round and smooth to be a perfect aggregate. Broken stone varies; some kinds are harder, rougher on the surface, and therefore better, than others. Flints are generally too round, or, when broken, smooth and splintery. Chalk is sometimes used, and the harder varieties make good concrete in positions where they are safe from moisture and frost. Slag from iron furnaces is sometimes too glassy to make good concrete, but when the surface is porous it is on of the best aggregates that can be used. It is hard, strong, and heavy, and the iron in it combines chemically with the matrix, making it much harder than it would otherwise be. as little mortar as possible.

The following Table shows the amount of voids in 1 cub. yd. of stone broken to different sizes, and in other materials : -

The size of the pieces of which the aggregate is formed influences the content of the void spaces between them, and therefore the quantity of lime and sand that must be used. Unless the mortar is of such a description that it will attain a greater hardness than the aggregate, the object should be for the concrete to contain as much broken material and

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1 Cub. Yd. contains Voids amounting to

Stone broken to 2 1/2-in. gauge........

10 cub. ft.

Do. 2 do. ............

10 2/3 do.

Do. 1 1/2 do. ............

11 1/3 do.

Shingle.........

9 do.

Thames ballast (which contains the necessary sand) ..

4 1/2 do.

A mixture of stones of different sizes reduces the amount of voids, and is often desirable. The contents of the voids in any aggregate may be ascertained by filling a watertight box of known dimensions with the material, and measuring the quantity of water poured in so as to fill up all the interstices, or by weighing 1 cub. ft. of the aggregate and comparing its weight with that of a cub. ft. of the solid stone from which it is broken.

In building walls, or other masses of concrete, large pieces of stone, old bricks, chalk, etc, are often packed in for the sake of economy. Care should be taken that the lumps thus inserted be at least 1 in. apart, and some distance clear of the face, so that they may be entirely surrounded by cementing matter. Where lumps of chalk or absorbent material are used, care must be taken that they are not exposed so as to absorb wet or moisture, otherwise they will be liable to the attacks of frost, and may become a source of destruction to the wall.

The proportion of each material is determined by custom, rule of thumb, or experience. A common mixture consists of 1 quicklime, 2 sand, 5 or 6 gravel, broken stone, or brick; or 1 quicklime, 7 Thames ballast (which contains sand and shingle). The same proportions are often blindly adhered to, whatever may be the nature of the materials used. The best proportions for the ingredients of 1 cub. yd. of concrete to be made with any given materials may, however, always be arrived at by ascertaining the contents of the voids in a cub. yd. of the aggregate (without sand), and adding to the latter such materials as will make mortar of the best quality and in sufficient quantity to perfectly fill those voids. If the aggregate contain sand (as in the case of gravel or ballast), the sand should be screened out of the sample before the voids are measured, and the amount of sand thus screened out will be deducted from that required for the mortar which is to form the matrix of the concrete. In practice, a little more mortar than is actually required to fill the voids is provided, in order to compensate for imperfect mixing. Drake recommends 1 Portland cement, 8 gravel, for walls of buildings; and 1 Portland cement, 6 gravel, for roofs, floors, etc.

On the Metropolitan Main Drainage Works the following proportions were adopted: - 1 Portland cement, 5 1/2 ballast, including sand, for sewers; and 1 Portland cement, 8 ballast, including sand, for backing walls and other work, except sewers.

Concrete is much used for paving, being made into slabs, and then laid like ordinary stone flags. For this purpose it is preferable to use an aggregate, such as shingle, much harder than the matrix, and to use very little sand in the latter. As the matrix becomes worn away, the pebbles of the aggregate project slightly, making the surface a little rough, and therefore less slippery, and at the same time the matrix is protected from further wear.