26 



The Department coefficient of wear is obtained by subtracting 4,000 

 from the weight of the unabraded fragments of rock and dividing the 

 difference by 10. This allows a possible range in results from zero to 

 100, i. e., if 1,000 grams of the material is abraded from the original 5 

 kilograms the result will be zero, and the material is considered unfit 

 for road making. If no dust is worn from the original 5 kilograms 

 the coefficient will be 100. 



The French coefficient of wear is obtained by the following formula: 



20 400 

 Coefficient of wear = 20 x = 



liV IT 



where "w" is the weight in grams of dust under 0.16 Dm. (one-six- 

 teenth of an inch) in size, obtained per kilogram (2.2 pounds) of 

 rock used, 20 being the standard of excellence. 



The percentage result includes all sizes of material worn from the 

 original 5 kilograms. The results of 8 abrasion tests can be seen in 

 the exhibit case. 



This test was designed by M. Deval, and was first exhibited in Paris 

 in 1878. It is, as far as the writer has been able to ascertain, the 

 first machine especially constructed for testing road materials. 



ABSORPTIVENESS AND DENSITY OF ROCK. 



As soon as the abrasion test is completed one of the smoothly worn 

 fragments, from 20 to 40 grams in weight, is used for determining the 

 absorptiveness of the sample, which is done in the following manner: 

 This smoothly worn stone is weighed in air after it has been brought 

 to a constant weight in a hot-air bath. It is then immersed in water 

 and immediately re weighed in water. After ninety-six hours of 

 immersion it is again weighed in water. 1 The absorption is obtained 

 by the following formula: 



Number of pounds of water absorbed by a cubic foot of rock 



C^ T? 



A _ B X62.5, in which "A" is equal to the weight in air, "B" the 



weight immediately after immersion in water, " C " the weight after 

 absorption for ninety-six hours, and 62.5 the weight of a cubic foot 

 of water. 



This method has no particular advantages over others except that 

 it requires less manipulation and time and the result is a little more 

 practical. It is not intended to give the porosity of rock, but merely 

 to obtain from a small specimen the number of pounds of water 

 absorbed by a cubic foot of rock in ninety-six hours. It can be 

 readily seen that the specific gravity and weight of a cubic foot of the 

 rock can be determined from the above data. Two such trials are 

 always made for each sample, and three or more when necessary. 



1 It was found by experiment with a number of stones that absorption practically 

 ceases after sixty-two hours, so ninety-six hours was given as a safe allowance. 



