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.
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.