Method of Testing Stone

METHOD OF TESTING STONE. There are two methods of determining the qualities of a stone for road-building purposes: (1) by using the stone in the road and keeping an account of the cost of repairs over a series of years, or (2) by laboratory experi ments. The first is uncertain owing to the variations in climatic conditions, and in the amount and nature of the traffic, etc., and would be very expensive and take a long time. In the second method of testing, it is difficult to duplicate in the laboratory the conditions of actual service; but nevertheless much valuable in formation may thus be obtained at a moderate expense and in a comparatively short time.

The tirit method of testing road-building stones will be eonsid ere(1 in .\ rt. 3—Maintenance.

Systematic laboratory tests of road metal are of comparatively recent origin, and may be said to have been started by the French governmental engineers about 1880, who have made extensive use of this method in determining the quality of the rock used in contract work and in selecting new quarries. Only a little such laboratory work has been done in England and Germany. From 1894 to 1899 the Massachusetts Highway Commission conducted a series of tests of road-making materials, and developed a new and important method of testing, and deduced much valuable information.

Abrasion Test.

This test determines the toughness of a stone, and is employed principally by the French engineers, who use the machine shown in Fig. 42.* "The specimens to be tested are sawed into rectangular prisms having a 4 cm. (1.6 inches) by 6 cm. (2.4 inches) base, and an 8 cm. (3.2 inches) height. These specimens are placed, two at a time, so that they rest on the upper surface of a circular grinding disk of cast iron, which can be rotated in a horizontal plane by a crank. They are held in clamps so arranged that the bases of the speci mens rest on opposite sides of the grinding surface, 26 cm. (10.4 inches) from the center. The specimens are weighted so that they press against the grinding disk with a pressure of 250 grammes (8.8 ounces) per square centimeter (0.4 inch). Sand is fed onto the disk from a funnel above. The sand used is of a standard quality and size, obtained by crushing quartzite rock and screen ing it to the standard size. The quantity of sand used in each test is one litre per specimen for each thousand turns of the grinding disk. The disk is rotated at the rate of 1,000 revolutions per half

hour, and a test is completed in 4,000 revolutions. The diminu tion in the height of the specimen is measured, and its loss in weight determined after each 1,000 turns of the disk. The per cent of loss undergone by each specimen after 4,000 revolutions of the grinding disk is set down as the result of the test, and serves for comparison." Impact Test. The French engineers test the ability of a stone to resist impact, by means of a machine which resembles very closely in principle the pile driver. The hammer is raised by a cord which passes over a pulley at the top of the guides and can be released at any desired height, from which it falls upon the specimen, which is held below by clamps. Two hammers are employed, one weighing 42 kilogrammes (92.4 pounds) and the other 20 kilogrammes (44 pounds), their respective falls being 100 cm. (40 inches) and 80 cm. (32 inches). The number of blows necessary to crack the specimen and also the number to produce its complete destruction are determined. The test is made upon 4-cm. cubes, at least three cubes being used for each specimen with each hammer.

This and the preceding test will not be further considered here, since the resistance to impact and abrasion are more easily deter mined by the method immediately following.

Abrasion and Impact Test.

This test consists in placing a known weight of fragments in a tight drum which rotates about an eccentric axis so that the bits roll over one another and also shift from end to end in the cylinder. This test was invented by Deval, a French engineer, and has been widely used. It determines at once the resistance to abrasion and also to impact, and is an important test for road metal. Fig. 43 shows the form of the Deval machine used in this test by the Massachusetts Highway Commission.* "The machine consists of four cylinders, each 20 cm. (7.9 inches) in diameter and 34 cm. (13.4 inches) in depth. Each of these cylinders is closed at one end and has a tightly fitting cover for the other. They are fastened to a shaft so that the axis of each cylinder is at an angle of 30° with the axis of rotation of the shaft. The shaft which holds the cylinders is supported by bearings; and at one of its ends is a pulley by which the cylinders are revolved, and at the other a revolution counter.