TEST. 1 ( .> 



material below <>.!<' cm. the weight of which is recorded, is used for 

 the purpose of the test." 



At tirst :ist:uul;ird rock of superior wearing <|iiality \v:i> :d\\;i\ s placed 

 in one of the cylinders as a standard of comparison, and the propor- 

 tion of the weights of the dust under o. Hi cm from the standard rock 

 and from the rock to be tested was assumed to give their relative resist- 

 ance to abrasion. It was found, however, that only the brst varieties 

 of rock gave less than loo grams of powder under o. Id cm, i. e., 20 

 prams per kilogram of rock, or 2 per cent of their weight. The num- 

 ber L'O was therefore adopted as a standard of excellence, and the 

 u coefficient of wear" for any rock tested may be obtained by the 

 following formula: 



Coefficient of wear =20X= 



W W 



in which " W" is the weight in grams of the detritus under 0.10 cm 

 in size obtained per kilogram of rock used. a 



COMPRESSION TEST. 



In addition to the test made with the Deval machine the follow! no- 

 compression test was later adopted: This test is made on 25 mm cubes 

 of rock with a hydraulic compression machine. The cubes are sawed 

 at the laboratory from specimens carefully selected to represent the 

 average qualit}- of the rock. At least three cubes of each sample are 

 tested after desiccation either in the open air or at a temperature 

 of 40 C., or after being saturated with water. The resistance offered 

 by each cube is obtained in kilograms per square centimeter of bearing 

 surface, and the average of the results furnished by the different cubes 

 is use' 1 . 



Experience having shown that the hardest rocks rarely resist a load 

 greater than 2,000 kg per square-centimeter section, the commission 

 adopted the coefficient 20 for the materials having this degree of re-M- 

 ance. This corresponds approximately to the resistance to compres- 

 sion of wrought iron. The coefficient of any rock, therefore, may be 

 obtained by the following formula: 



=150 E. 



20 



U E" represents the breaking load in kilograms per square-centime- 

 ter section. To determine the density of rock the cubes used for the 

 compression test are measured and weighed. This is undoubtedly 

 the most accurate of any of the simple methods for making this 

 determination. 



"Results obtained from this test in the Road Material Laboratory are given in 

 Table II, page 26, and in Table A of the Appendix. 



