TESTS FOR PAVING-BRICK.
To determine the probable durability of brick designed for use in paving, mechanical tests may be applied which will show the relative rank of different samples in their most important characteristics. It is, however, a matter of considerable difficulty to set a standard to which the brick should be required to con form, or to determine, from the behavior of the bricks under test, the relative value of various samples which it may be desired to compare.
The tests ordinarily proposed or used for this purpose are those of crushing strength, transverse strength, abrasion and impact, absorption, and specific gravity. The relative importance of these tests and the weight which should be given to their results is a matter concerning which considerable difference of opinion has been developed amongst engineers, and practice va ries considerably. The National Brick Manufacturers' Association have considered the matter, and in 1895 appointed a committee which in 1897 reported a set of rules for making the tests, with resolutions expressing their views as to the relative importance and reliability of each. These rules, which were somewhat modified in 190o, are very commonly followed and furnish a standard method of testing.
The recommendations of the commission for con ducting this test are as follows: "I. The crushing test should be made on half bricks, loaded edgewise, or as they are laid in the street. If the machine used is unable to crush a full half-brick, the area may be reduced by chipping off, keeping the form of the piece to be tested as nearly prismatic as possible. A machine of at least 100,000 pounds capacity should be used, and the specimen should not be reduced below 4 square inches of area in cross-section at right angles to direction of load.
"II. The upper and lower surfaces should prefer ably be ground to true and parallel planes. If this is
not done, they should be bedded in plaster of Paris while in the testing-machine, which should be allowed to harden ten minutes under the weight of the crush ing planes only before the load is applied.
"III. The load should be applied at a uniform rate of increase to the point of rupture.
"IV. Not less than an average obtained from 5 tests, on 5 different bricks, shall constitute a stand ard test." The result of a compressive test of stone or brick depends very largely upon how it is made, and the results of tests are only properly comparable with others made in the same manner and with equal care. The use of plaster beds as suggested above, it is thought, conduces greatly to regularity of result in the work of different men, as it tends to reduce the effect of differences in the accuracy of dressing the surfaces of contact. The size of the test-piece is also impor tant, the strength usually increasing as the size in creases. Small pieces, i- or 2-inch cubes, are often employed because of the large force necessary to crush a whole or half brick, although where machinery exists capable of doing it the larger tests entail much less work in preparing specimens and also yield much more satisfactory results. Where small specimens are used it is to be observed that the unit strength will not be the same as for larger ones, and must be judged by a different standard. In the preparation of speci mens it is better, when possible, to saw than to break them by chipping, in order not to injure the block by the shock of the blows.
The commission in their discussion concluded that no connection has been shown between high strength and the qualities necessary for a good paving material, and adopted the following resolution: