TESTS OF ROAD MATERIAL 



107 



stone used in the construction and maintenance of the national highways 

 of France. These results are said to agree well with those obtained in 

 actual practice. 



The apparatus used in the tests consists of a cast-iron cylinder 8 in. 

 in diameter and 13.6 in. in depth. At one end is an opening which can 

 be closed with a tightly fitting iron cover. This cylinder is mounted on 

 an axle at an angle of 30 with the axis of the cylinder, and is supported 

 on an iron frame. At one end of the axle is a pulley wheel by which the 

 cylinder is revolved; at the other is an instrument which records its 

 revolution. 



The stone to be tested is first broken into pieces, between 2^ in. 

 and i^in. in diameter, which are carefully washed, to remove any foreign 

 matter In the cylinder are placed 5 kilograms (i3^1bs) of this stone. 

 The top is then bolted on, and the cylinder is made to revolve for 5 

 hours at the rate of 2,000 revolutions an hour, making in all 10,000 revo- 

 lutions. By this process the stones are thrown from one end of the 

 cylinder to the other, and at the same time are rolled against the sides 

 of the vessel and against one another. When 10,000 revolutions are com- 

 pleted, the cover is removed, and the contents emptied into a tray. The 

 cylinder is then thoroughly washed, to remove the dust that adheres to 

 its sides. Each stone above i*^ in. in diameter is then washed under 

 the same water. This water is then filtered, and the filtrate when dry is 

 mixed with the detritus taken from the cylinder. The detritus is then 

 put into a sieve, by which it is separated automatically into seven sizes. 

 These seven sizes, together with the stones that have not been worn 

 below 3.18 cm. in diameter, are each carefully weighed, and their 

 weights recorded; 



The amount of detrition under r-16 in . is rarely less than 20 grams per 

 kilogram of stone usedfe %,)therefore 20 has been adopted as the standard, 

 and the coefficient of quality is obtained by the following formula : 



20 400 



u =; per cent 



u u 



in which u represents the weight in grams (15.43 grs.)of detritus per 

 kilogram (2 2-3 lbs) of stone. 



It seemed well, in beginning this work, to be guided as far as possible 

 by the experience of others, and for this reason the Deval test was 

 adopted, for it appeared to be the only practical method of testing road 

 metals yet devised. After a number of trials were completed with the 

 Deval apparatus, and their results studied, it was recognized that all the 

 valuable properties possessed by a good road metal were not embraced 

 in this test. The value of any good stone as a road metal is due to cer- 

 tain properties possessed by it. Among these there are three which 

 stand prominent — cementing value, toughness and hardness. It is evi- 

 dent that the Deval apparatus does not test the very important property 

 of cementing value in the different road metals. The commission, recog- 

 nizing this deficiency, accordingly directed its attention to devising some 

 means of supplying it. As no previous attempt has been made in this 

 direction, the commission had to invent its own method, which is as 

 follows : 



The stone to be tested is ground to a powder, and passed through a 

 sieve of 100 meshes to 1 in. The powder is then put in a slightly tap- 



