These ingredients should be thoroughly mixed so as to form a close conglomerate free from voids.
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, and it will be well to make a few remarks upon these two parts of the material separately before proceeding further.
The Matrix, as before stated, is the lime or cement mortar in which the hard broken material, or aggregate, is imbedded.
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, and sprinkled while they are being mixed.
For further remarks on the subject of mixing, see p. 214.
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. Among these may be mentioned broken stone, bits of brick, of earthenware, burnt clay, breeze, and shingle. If there is any choice, 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. When weight in the concrete is undesirable, a light porous material such as breeze1 may be used, but wben great weight is an advantage, as in the works of a breakwater or sea wall, the aggregate may be of the heaviest material that can be procured.
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 engineers 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 are supposed to 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, and angular stones are as a rule rougher and the cement adheres better to their surface.
The aggregate is generally broken so as to pass through a 11/2 or 2 inch 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 splinter)'. 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 one 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. Some slag, however, contains lime which may be dangerous (see p. 161).
The results of experiments as to the relative value of some of these aggregates are given at p. 222.
The materials for concrete may be broken by hand, except when large quantities are required, in which case a Blake's stone-crusher is generally employed.
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 as little mortar as possible.
The following Table shows the amount of voids in a cubic yard of stone broken to different sizes, and in other materials: -
1 Cubic Yard contains Voids amounting to
Stone broken to 21/2inch gauge..
10 cubic feet.
Do. 2 do. .
10 2/3 do.
Do. 1 1/2 do. .
11 1/3 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 aggregrate may be ascertained by filling a water-tight box of known dimensions, with the material thoroughly wetted so as not to absorb, and measuring the quantity of water poured in so as to fill up all the interstices; or by weighing a cubic foot of the aggregate and comparing its weight with that of a cubic foot of the solid stone from which it is broken.