In the convention of 1897 the Brick Manufacturers' Association adopted a method for this test consisting in rattling a given charge of bricks in a cylinder rotat ing about its axis, which is horizontal, and depending for its result upon the impact and abrasion of the bricks upon each other. In 190o, however, after more fully considering the matter, the test was modi fied, and a smaller charge of bricks, with the addition of a charge of cast-iron shot, was recommended as more nearly representing the conditions of practice and giving results more in accord with experience.
The method finally recommended by the Associa tion is as follows: "I. Dimensions of the Machine. The standard machine shall be 28 inches in diameter and 20 inches in length, measured inside the rattling-chamber.
"Other machines may be used varying in diameter between 26 and 3o inches, and in length from i8 to 24 inches; but if this is done, a record of it must be attached to official report. Long rattlers may be cut up into sections of suitable length by the insertion of an iron diaphragm at the proper point.
"II. Construction of the Machine. The barrel shall be supported on trunnions at either end; in no case shall a shaft pass through the rattling-chamber. The cross-section of the . barrel shall be a regular polygon having 14 sides. The heads and staves shall be composed of gray cast iron, not chilled or case-hardened. There shall be a space of one-fourth of an inch between the staves for the escape of dust and small pieces of waste. Other machines may be used having from 12 to i6 staves, with openings from one-eighth to three-eighths of an inch between staves; but if this is done, a record of it must attached to the official report of the test.
"III. Composition of the Charge. All tests must be executed on charges containing but one make of brick or block at a time. The charge shall consist of 9 paving-blocks or 12 paving-bricks, together with 30o pounds of shot made of ordinary machinery cast iron. This shot shall be of two sizes, as described below, and the shot-charge shall be composed of one fourth (75 pounds) of the larger size and three-fourths (225 pounds) of the smaller size.
"IV. Size of the Shot. The larger size shall weigh about 71 pounds and be about 2i inches square and 4i inches long, with slightly rounded edges. The smaller size shall be cubes of i inches on a side, with rounded edges. The individual shot shall be replaced by new ones when they have lost one-tenth of their original weight.
" V. Revolutions of the Charge. The number of revolutions of a standard test shall be i800, and the speed of rotation shall not fall below 28 nor exceed 30 per minute. The belt-power shall be sufficient to rotate the rattler at the same speed whether charged or empty.
" VI. Condition of the Charge. The bricks com posing the charge shall be dry and clean and as nearly as may be possible in the condition in which they were drawn from the kiln.
" V11. Calculation of the Result. The loss shall be calculated in the per cents of the weight of the dry brick composing the charge, and no result shall be considered as official unless it is the average of two distinct and complete tests made on separate charges of brick." The commission regard this as the most important test to be applied to paving-brick, and in fact it is the only one having their indorsement. It seems
reasonable to suppose that this test gives more nearly than the others a determination of the value of the brick for use in a pavement.
It is quite true that the action to which the brick is subjected in a test of thin character is different from the wear to which it is subjected when firmly held in the pavement, but the qualities necessary to resist wear in the two cases are very similar. We may form an idea of whether a material is suitable for the proposed use from such experiments, although no definite idea of the amount of wear that it will en dure can be obtained from them. It should also be pointed out that the method of estimating the loss of the brick, from abrasion tests made in this manner, as percentages of the total weight of brick, can, in the comparison of different bricks, only give correct re sults when the bricks compared are of the same size and shape. A brick with rounded edges evidently could not properly be compared with one with sharp edges by this method, and some engineers have divided the test into two periods for the purpose of separat ing the knocking off of the corners and preliminary rounding of the brick from the later abrasion upon the rounded surfaces which would be more nearly comparable for different specimens. If, in the test, the loss in the rattler during the first half-hour be separated from that during the second half-hour, the latter will be found to be much less affected by the form of the brick.
In comparing bricks of different sizes it should be noted that a small brick presents more surface for abrasion than a large one in proportion to its volume, and the results of such comparisons would be con siderably modified in some instances if the results be stated in terms of exposed surface instead of percent age of volume. With square-edged brick during the early period of the test, when corners are being chipped off, the loss is probably more nearly propor tional to length of edges than to surface or volume, which would be still more to the disadvantage of the small brick. Care is therefore necessary in drawing conclusions from such tests concerning the relative values of different materials that all the conditions which may affect such conclusions be fully under stood.
A rattler for testing paving-brick has been patented by Mr. Gomer Jones, formerly City Engineer of Geneva, N. Y., in which the brick are held in the circumference of the rattler, and a charge of 150 pounds of cast-iron cubes, 1 inches on the edge, is placed in the rattler and revolved. The machine is the ordinary rattler, 24 inches in diameter, with staves made to clamp the brick end to end, leaving the brick exposed about one inch in depth on the side. . The action is that of picking and abrading the surface by light blows, and is claimed to develop the defects and pick out soft spots in the brick much better than the ordinary method, and avoid breaking the brick by heavy blows. The treatment given by this apparatus is certainly more like the conditions under which the brick is used in the pavement than that given by the standard method.