VII. THE CALCULATION OF THE RESULTS.
The loss shall be calculated in per cents of the weight of the dry brick com posing 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 abandonment of cast-iron shot as a feature of the rattler test was not in accord with the experience of others, and many engineers felt that it was a mistake. The results of tests made independently of the Paving Brick Commission pointed to this conclusion. The use of high grade brick only in the N. B. M. A. investigation of this new form of test was itself an element of weakness and a very bad feature as it proved to be.
Among experiments which threw some light on the discussion which came up about the efficacy of the new test were those conducted at the University of Illinois from 1895 to 1899 under the direction of the writer to determine the best composition of the rattling material. The investigation showed that shot composed of small pieces gave an effect which was almost wholly abrasive and that the heavier cast-iron shot pro duced a spalling and breaking effect which was altogether too severe. It was felt that the rattler test should include the effect of both abrasion and impact, and a series of tests were made to determine what mixture of two sizes of shot would give the best combined effect of impact and abrasion, such as would approximate to the wear of brick in service in the street. The tests were conducted principally with a rattler 24-in. in diameter and 36-in. long. The small shot were with rounded edges and weighed about 1 pound each. The large shot were with edges rounded to radius, and weighed about 8 pounds each. From the results of the experiments it was con cluded that for the 24x36-in. rattler, 150 pounds of 8-pound shot and 150 pounds of 1-pound shot gave results with a satisfactory proportion of abrasion and impact. When a rattler 18-in. long was used, one-half of this charge was selected. The speed was about twenty revolutions per minute. Twelve brick were used in the full rattler and six in the half.
The test was conducted for 1800 revolutions. These tests were reported to the Illinois Society of Engineers and Surveyors, and were described in an article on standard methods of tests of paving brick printed in The Technograph,* and reprinted in a number of technical journals. The tests brought out the facts that a combination of large and small shot give a test which will provide both impact and abrasive effects to any degree and that such a test will distinguish soft from hard brick to a fair degree.
The investigations by the writer also called attention to the fact that the test then adopted by the National Brick Manufacturers Association, using brick alone in the rattler, was defective in that it failed to dis tinguish in any marked degree between hard brick and soft brick. Objections were also made in various quarters. In some tests reported at that time, brick called by the maker as entirely too soft for paving purposes gave a smaller loss than the selected paving brick of the same manufacturer. In another test, three makes of brick of the same gen eral quality made practically the same showing by other methods of test ing, while by the National Brick Manufacturers Association, one brick lost less than two-thirds of that lost by either of the other two. It was also stated that in some instances the test gave as good standing to an inferior brick as to a superior paving brick. Soft brick soon broke in the rattler, and thereafter the loss was lighter, so that the final results were likely to be lower than would be expected from the apparent quality of the brick. In general, the test was not very efficient in measuring the toughness of brick. It seems that in the investigations conducted by Professor Orton the use of only one quality of brick, and that a high grade paver, (lid not permit the real deficiencies of the test to be discovered. The discussion of this test created wide-spread interest. Finally, as a result of a paper presented at the meeting of the Na tional Brick Manufacturers Association in 1899, the association asked Professor Orton to make a further investigation of the subject.