Soft under-burned brick, such as are frequently used in filling in the interior of walls, will absorb from 30 to 35 per cent of their weight of water; some good dry-clay or pressed brick have an absorption of 15 to 20 per cent, while others run from 5 to 10 pei cent; and some vitrified brick absorb only 1 to 2 per cent.t Crushing Strength. The crushing strength of brick is valuable only in comparing different brands; and gives no idea of the strength of walls built of such bricks (see § 622). The crushing strength of brick is of relatively less importance than that of stone, since owing to the relatively smaller size of the brick and consequently the relatively larger proportion of mortar, the strength of brick masonry is more dependent upon the strength of the mortar than is stone masonry. The strength of the brick is of relatively small importance unless the mortar is nearly as strong as the brick (see § 623); and as this is never the case unless a rich portland-cement mortar is used, it follows that in ordinary brick masonry the crushing strength of the brick is of small importance provided a reasonably good quality is employed.
It has already been explained (§ 10-15) that the results for the crushing strength of stone vary greatly with the details of the experi ments; but this difference is even greater in the case of brick than in that of stone. In testing stone the practice is to test cubes (§ 11) whose faces are carefully dressed to parallel planes. In testing brick there is no established custom. (1) Some few experimenters test cubes; but nearly all of the tests that have been published have been made with some form of specimen other than the cube. With stone it is necessary to specially prepare a test specimen, and the cube is as easy to prepare as any form; but with brick it is not equally necessary to specially prepare a test specimen, and hence it has become the custom to use a half or a whole brick in making the compression test. (2) Some experimenters grind the pressed surfaces to exact planes, and some level up the surfaces by putting on a thin coat of plaster of paris, while others do nothing to prepare the bedding surfaces—particularly with pressed brick. (3) Sometimes brick are tested on end, sometimes on edge, and some times flatwise, the last being the more common practice with the testing machine at the U. S. Watertown Arsenal.
Soaking a brick in water decreases its compressive strength, apparently because the water acts as a lubricant on the plane of rupture. In a series of experiments with the U. S. testing machine at Watertown, of thirty tests upon ordinary building brick from ten localities, all but two showed a loss of strength due to immersion in water for one week; and the wet half of a brick gave an average strength of only 85 per cent of the strength of the dry half.
Some experiments with the testing machine at the U. S. Arsenal at Watertown, to determine the relative strength of hard burned face brick tested flatwise, edgewise, and endwise, gave aver ages for four tests each as follows:* The pressed surfaces were set in plaster of paris. Other tests with common brick gave approximately the same results.f According to the formula of § 17, a brick tested flatwise would be 32 per cent stronger than when tested as a cube.
Bricks sent to the World's Columbian Exposition at Chicago in 1893 from several States, and afterwards tested at the Watertown Arsenal flatwise with the pressed surfaces set in plaster of paris, gave the results in Table 7, page 42.
For bricks sent to the Louisiana Purchase Exposition at St. Louis in 1904, and afterwards tested at Watertown flatwise, the surfaces being set in neat portland-cement mortar, the average of five tests is as shown in Table 8.j Five samples each of fourteen lots of Hudson River brick gave an average crushing strength of 3,943 lb. per sq. in. for half bricks tested flatwise, the range for the averages of the several lots being from 2,701 to 5,416, and the range for the individual brick being from 1,607 to 8,944.* The highest known crushing strength of any brick is 38,446 lb. per sq. in., and as the brick was tested on end the result is exceed ingly remarkable.
According to experiments made with the U. S. testing machine at Watertown upon sixteen different grades of brick from six factories, the transverse strength is 13.5 per cent of the compressive strength of half brick tested flatwise4 From 1883 to 1905 there were made with the U. S. testing machine thirty-seven determinations of the transverse strength of brick from eleven factories, the lowest being 308 lb. per sq. in., the highest being 2,589, and the mean, 1,002.