Testing the Brick

The color of the outside of the brick is sometimes valueless owing to the sand employed to prevent sticking in the kiln, or to the effect of sulphur in the coal used in burning, or to salt glazing. Salt glazing is a trick occasionally employed to give a dark gloss to the outside which is very attractive to the superficial observer, but which is practically worthless, since it is only skin deep and soon wears off. Salt glazing makes it more difficult to detect soft brick, and should never be allowed on paving brick.

Specific Gravity.

In a general way, the more dense a brick the harder and stronger it is; and consequently early in the history of brick testing it was believed that a knowledge of the specific gravity would be of value in judging of the quality of a paving brick. It is now known that the specific gravity reveals nothing not determined by other tests; and further that the density depends upon the character of the clay, the kind of fuel, etc., and in no way measures the quality of the product. The specific gravity may be computed by the formula: in which W a represents the weight of the dry brick in air W s the weight of the saturated brick in air, W i the weight of the brick immersed in water. The specific gravity of shale brick ranges from 2.05 to 2.55, and usually from 2.20 to 2.40; and that of brick made from impure fire clay ranges from 1.95 to 2.30, and generally from 2.10 to 2.25.

Crushing Strength.

The results for the crushing strength vary more with the dethils of the method employed than any other test of paving brick. There is no standard method of making this test. (1) Some experimenters test cubes, others half brick, and still others whole brick; (2) some grind the pressed surfaces accurately to planes, some true the surface up by putting on a thin coat of plas ter of paris, while still others level up with blotting paper, card board, etc.; (3) some test the brick on end, some on edge, and others flatwise. For experimental data showing the marked effect of the different methods of testing, see the author's Treatise on Masonry Construction, page 41-45.

Tests on cubes cut from paving brick show that the best paving brick have a crushing strength of 10,000 to 20,000 pounds per square inch. This is the crushing strength when the load is applied uni formly over the surface of the test specimen; but if the pressure is applied to only a small part of the upper surface of a brick, the strength will be much greater.* Any brick that is likely to be accepted for paving purposes by any of the tests hereafter de scribed, is in no danger of being crushed by the pressure of the wheel of a vehicle. For example, the surface of contact between a wheel having a 1f-inch tire loaded with half a ton is about one half square inch, which gives a pressure on the brick of only about 2,000 pounds per square inch.

If the crushing strength could be easily and accurately found, it would be of value in determining the relative strength, and hence would be useful in comparing the quality of different brick; but owing to the difficulty of making the experiments and to the un certainty of the results, the test has been abandoned.

Absorption Test.

In the early days of the paving brick industry, many of the brick used were so porous and brittle that it was feared they would be disintegrated by the action of frost; and consequently the absorption test was employed to eliminate porous brick. Subsequent tests by repeatedly freezing and thawing paving bricks showed that any brick which was likely to be accepted for paving purposes would not be appreciably injured by the action of frost. There are probably two elements that prevent frost

from seriously injuring even a soft paving brick; viz.: (1) the cushioning effect of the air remaining in the pores of the brick, and (2) the strength of the brick may be greater than the disrupting effect of the frost. Alternate freezing and thawing might injure a non-vitrified brick, which is not only very porous but is also defi cient in strength; but such a brick would be rejected for paving purposes as the result of a casual inspection. The absorption test is no longer regarded of importance as measuring the ability of the brick to resist freezing and thawing.

The absorptive power of a brick is now regarded only as a measure of the porosity of the brick, i. e., of the degree of vitrifica tion. In the first stages of burning, free water is driven off with no perceptible effect upon the clay; in the second stage, the water of constitution of the clay is expelled, leaving the clay in a very porous and spongy condition. At this time the strength of the clay is at its lowest point, and the porosity is at its greatest devel opment; while the chemical combination between the clay and the other minerals has not yet begun. The partially burned brick can be easily crushed to a powder; and if wetted and frozen, it be comes a mass of gritty mud on thawing. But if the temperature of burning is increased, the clay decreases in porosity and increases in strength; and the walls of the pores fuse and close in on one another, gradually expelling the gases which filled them and oblit erating the original porous structure. As the cell walls collapse and tend to expel the gases from the pores, these gases encounter increas ing difficulty in finding their way out to the surface; and some of these gases become imprisoned in the clay, and as the latter becomes more and more fluid or viscous with the rising heat. these volumes of gases expand and create little spherical cavities or vesicles in the clay body. So it always happens that in burning a piece of clay-ware two opposing forces are at work at the conclusion of the burning. One is the obliteration of the pore structure, accom panied by the expulsion of the gases, which produces greater density and strength ; and the other is the expansion of imprisoned volumes of gases, which produce a cindery or vesicular structure, and de creases the density and strength. Therefore there is a point of maximum strength, and continuing the burning beyond this de creases the porosity and also the strength. In the early history of the paving-brick industry, it was held that the less the absorp tion the better the brick; but the above explanation shows that there is a degree of porosity corresponding to maximum strength. The relation between strength and porosity varies greatly with the character of the clay and with the method of molding. For example, a stiff-mud shale brick has an absorption of 4 per cent and disrupts under the action of frost; while a soft-mud brick made of surface clay will endure with an absorption of 10 or 12 per cent.* Different bricks vary widely in their rate of absorption. For example, one brick absorbed in one day 80 per cent of its total amount, while another absorbed only 8.7 per cent ; and two other specimens absorbed 71.8 and 19.5 per cent respectively in the same time.t The absorption of whole brick is slightly less than that of half brick, and the absorption of half brick is considerably less than that of small chips. For the above reasons and for other minor ones, results for the absorptive power are likely to be untrustworthy.