TESTS FOR BRICK. The tests usually applied to determine the quality of brick are those for: (1) form, (2) texture, (3) absorptive power, (4) crushing strength, (5) transverse strength. Brick are so common, and the requisites for good building brick are so obvious, and it is so easy to determine whether any particular lot of brick has the desirable qualities, that there is not much need of laboratory tests of building brick. However, the several tests enumerated above will be briefly considered.
A good brick should have plane faces, parallel sides, and sharp edges and angles. In regularity of form re-pressed brick ranks first, dry-clay brick next, then stiff-mud brick, and soft-mud brick last. Regularity of form depends largely upon the quality of the clay and the method of burning. A good brick should not have depressions or kiln marks on its edges caused by the pressure of the brick above it in the kiln.
A good brick should have a fine, compact, uniform texture; and should contain no fissures, air bubbles, pebbles, or lumps of lime. It should give a clear ringing sound when struck a sharp blow with a hammer or another brick. A brick .which gives a clear ringing sound is strong and durable enough for any ordinary work.
The compactness and uniformity of texture, which greatly influence the durability of brick, depend mainly upon the method of moulding. As a general rule, hand-moulded bricks are best in this respect, since the clay in them is more uniformly tempered before being moulded; but this advantage is partially neutralized by the presence of sand seams
69). Machine-moulded soft-mud bricks rank next in com pactness and uniformity of texture. Then come machine-moulded stiff-mud bricks, which vary greatly in durability with the kind of machine used in their manufacture. By some of the machines, the brick is moulded in layers (parallel to any face, according to the kind of machine), which are not thoroughly cemented, and which separate under the action of the frost. In compactness, the dry-clay brick comes last. However, the relative value of the products made by the different processes varies with the nature of the clay used.
Formerly, it was believed that the absorptive power of a building brick had an important effect upon its ability to resist destruction by frost; but experiments and a more careful study of experience have shown that the absorptive power of a brick has little or nothing to do with its durability. Apparently there are two reasons for this: (1) the pores of the brick are not entirely filled with water, and consequently the expansion of the water in freezing is cushioned by the air in the pores; and (2) with the more porous bricks, the water freezes in the pores without any destructive effect much as water freezes in a large-necked bottle, and with the more dense bricks the strength of the burned clay is greater than the expansive force of the water. The absorptive power varies with the chemical composition of the clay, and there seems to be no close relation between the absorptive power and the strength of a brick or the loss of strength by freezing.* There are different methods in use for determining the amount of water taken up by a brick, and these lead to slightly different results. Some experimenters dry the bricks in a hot-air chamber, while some dry them simply by exposing them in a dry room; some experimenters immerse the bricks in water in the open air, while others immerse them under the receiver of an air-pump; some immerse whole brick, and some use small pieces; and, again, some dry the surface with bibulous paper, while others allow the surface to dry by evaporation. Air-drying represents the conditions of actual exposure in masonry structures, since water not expelled in that way is so diffused as to do no harm in freezing. Immersion in the open air more nearly represents actual practice than immersion in a vacuum. The conditions of actual practice are best represented by testing whole brick, since some kinds have a more or less imper vious skin. Drying the surface by evaporation is more accurate than drying it with paper; however, neither process is capable of giving mathematical accuracy.