20 



THE TESTING OF ROAD MATERIALS. 



THE DORRY TEST. 



The test for hardness is made with the Dorry machine. The speci- 

 mens to be tested are sawed into rectangular prisms S cm high 

 with a base 4 cm by 6 cm. 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 is rotated in a horizontal plane by a crank. They are 

 held in clamps so arranged that the bases of the specimens rest on 

 alternate sides of the grinding disk 26 cm from the center. The spec- 

 imens are weighted so that they press against the grinding disk with a 

 pressure of 250 grams per square centimeter. Sand of a standard 



FK,. -J. Dorry nmrhiiir for dctrniiiniiiK hunlnr^. 



(juality and si/e, obtained by crushing quart/ite rock and screening it, 

 is fed onto the disk from a funnel. The quantity of sand used in each 

 teal i< -1 liters. Thedi>k is rotated at the rate of y.ono revolutions per 

 hour for two hours. A drawing of this machine, slightly modified, is 

 reproduced in fig. 'l. 



After -2,000 revolutions the >pecimens are reversed to ascertain if 

 there is any difference in wear between the two end> and to make the 

 result approach more nearly a general average of the samples. The 

 diminution in the height of the specimen IB measured and its loss in 

 weight determined after each 1, OIK turns of the grinding disk. No 

 coefficient of wear has been established for this test, but the loss in 



