26 



THE TESTING OF ROAD MATERIALS. 



pounds). The difference obtained is the weight of detritus under 0.16 

 cm ( ,',; inch) worn off in the test, from which the French coefficient of 

 wear is determined by the formula already given. Besides the French 

 coefficient, the percentage of material under 0.16 cm in size is also 

 reported. For ;i time another coefficient, called the Department coeffi- 

 cient, was used, which was obtained by subtracting 4,000 grams from 

 the weight of the remaining fragments over 3 cm (1.2 inches) in size 

 and dividing the difference by 10. This allows a possible range in 

 results from to 100, i. e., if 1,000 grams or 20 per cent of the material 

 is abraded from the original 5 kg, the result will be 0, and the 

 material is considered unfit for road making; if no dust is worn from 

 the original 5 kg the result will be 100. 



Although both the French and the Department coefficients are arbi- 

 trary, they have a distinct value. The French coefficient has been in 

 use for many years, and is familiar to road builders throughout the 

 East. At present both the French coefficient and the per cent of wear 

 are reported, and the Department coefficient will also be given on 

 application. In Table II are given the maximum and minimum per- 

 centages of wear and the French coefficient for all varieties of rock 

 that have been tested in this laboratory, and in Table A of the Appen- 

 dix the results on each sample tested are given. 



TABLE II. Abrasion text. 



