Page:Popular Science Monthly Volume 80.djvu/333

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water-bound broken-stone road, its physical characteristics and its behavior under certain conditions are important. Before the development of means of determining these characteristics in the laboratory, the only way in which they could be arrived at was from observation of the behavior of the particular rock in a road when exposed to travel for a considerable period of time.

The important characteristics of a rock which enables one to judge of its suitability for road construction are (1) its resistance to wear or abrasion by impact, (2) its hardness or resistance to the displacement of its particles by friction, (3) toughness or resistance to fracture by impact, (4) the cementing properties or value of the rock powder or dust produced by attrition, when moistened, (5) porosity or capacity for absorbing moisture, the latter being closely associated, for the same kind of rock, with the specific gravity and (6) the structure or size of the grain of the rock, the character of the minerals of which it is composed, and the extent to which these may have become altered by weathering, upon which all the other characteristics of the rock will depend.

The methods of determining these characteristics have been largely originated and developed in the Office of Public Roads in Washington, and are described in one of its bulletins, No. 31, as follows:

Percentage of wear represents the amount of material under 0.16 cm. in diameter lost by abrasion from a weighed quantity of rock fragments of definite size. It is determined in the following manner: The rock sample is broken into pieces that will pass through a 2.4-inch ring but not through a 1.2-inch ring, and after being thoroughly cleansed, dried and cooled, 5 kg. are weighted and placed in a cast-iron cylinder (34 cm. deep by 20 cm. in diameter) closed at one end and having a tight-fitting iron cover at the other. This cylinder is one of four attached to a shaft so that the axis of each is inclined at an angle of 30° with that of the shaft. These cylinders are revolved for five hours at the rate of 2,000 revolutions per hour, during which the stone fragments are thrown from one end of the cylinder to the other twice in each revolution. At the end of five hours the machine is stopped, the cylinders opened, and their contents poured into a basin, in which every stone is carefully washed to remove any adherent detritus. This abraded material is then thoroughly dried, and from the amount lost below 0.16 cm. the per cent, of wear is estimated.

Hardness is the resistance which a material offers to the displacement of its particles by friction, and varies inversely as the loss in weight by grinding with a standard abrasive agent. The test is made in the following manner: The test piece in the form of a cylinder about 3 inches in length by 1 inch in diameter is prepared by an annular core drill and placed in the grinding machine in such a manner that the base of the cylinder rests on the upper surface of a circular grinding disk of cast iron, which is rotated in a horizontal plane by a crank movement. The specimen is weighted so as to exert a pressure of 250 grams per square centimeter against the disk, which is fed from a funnel with sand of about 13 mm. in diameter. After 1,000 revolutions the loss in weight of the sample is determined and the coefficient of wear obtained by deducting one third of this loss from 20.