U. S. A. D. Co-efficient is obtained as follows: Subtract 4,000 from the weight of the abraded material larger than 11 inches, and divide the difference by 10. If no wear takes place, the Co-efficient will be 100; and if there is 20 per cent of detritus smaller than 11 inches, the Co-efficient will be 0, and the material is considered unfit for road making. "This term was introduced to secure a result with more range to it and one that could be more easily understood by the average layman." The innovation is of doubt ful propriety.
"To make the test specimen, the dust of rock that is to be tested is passed through a screen having 40 meshes per cm. (100 per inch), and is obtained from the detritus of the abrasion and im pact test. The dust is made into briquettes of circular sections, 25 mm. (0.98 inch) in diameter and 25 mm. in height, by placing the dust in a metal die of the proper dimensions, with enough dis tilled water to moisten it (4 cc. or 0.24 cubic inches); a closely fitting plug is then inserted on top of the wet dust, and it is sub jected to a pressure of 100 kgs. per sq. cm. (1,422 pounds per square inch). The weight of the dust varies with the density and compressibility of the stone, but generally it requires about 25 grs. (0.9 ounce) of dust to make a briquette of the above dimen sions. Two weeks should be allowed for a briquette to dry, at the ordinary temperature of a room, after which it should be tested within a few days.
" Fig. 44, page 188, shows the machine employed in testing the specimen. The machine consists of a 1-kilogramme (2.2-pound) hammer, II, arranged like the hammer of a pile-driver on two ver tical guides, D. The hammer is raised by a screw, C, and dropped automatically from any desired height. It falls on a flat-end plunger, B, weighing 1 kilogramme, which is pressed upon the briquette, 0, by two light spiral springs held by the guide rods, F. The plunger, B, is bolted to a cross-head, G, which is guided by two vertical rods, F. A small lever, J, carrying a brass pencil, K, at its free end, is connected to the side of the cross-head by a link motion, arranged so that it gives a vertical movement to the pencil six times as great as the movement of the cross-head. The pencil is pressed against a drum, A, and its movement is recorded on a slip of paper fastened thereon. The drum is moved auto matically through a small angle at each stroke of the hammer; and thus a record is obtained of the movement of the hammer after each blow. The standard fall of the hammer for a test is 1 cm. (0.39 inch), and the blow is repeated until the bond of cementa tion of the material is destroyed. The final blow is easily ascer tained, for when the hammer falls on the plunger, if the material beneath it can withstand the blow, the plunger re-bounds; if not, the plunger stays at the point to which it is driven, which is clearly recorded on the slip of paper. The number of blows required to
break the bond of cementation is taken as representing the binding power of the stone." * Table 19, page 186, shows the results obtained under the aus pices of the Massachusetts Highway Commission; and Table 20, page 1S7, gives similar results obtained in the Road-material Lab oratory of the U. S. Department of Agriculture—in both cases under the immediate direction of Mr. Page, the inventor of this method of testing road metal. This test was not applied to as many specimens as the Impact and Abrasion Test.
Table 19, page 186, shows the results obtained by the Highway Commission, and Table 20, page 187, gives similar results by the U. S. Department of Agriculture.
The absorptive power of the stone is tested to determine the probable effect of frost upon the material. It is well known that water in freezing exerts a considerable expansive force; but this does not prove that freezing will seriously injure a road material that absorbs water even to a considerable amount. The absorptive power depends upon the porosity, and when a porous stone is immersed in water its pores are filled with air, which the water does not entirely drive out; and this air serves as a cushion to take up the expansion of the water in freezing. For this reason, freezing does little or no injury to stone or brick which are otherwise suitable for road purposes. A rock or brick so porous as to be damaged by freezing while wet, would probably be rejected by a casual inspection—certainly by the tests ordinarily applied.