PHYSICAL TESTS OF EOAD-BUILDHSTG ROCK. 



11 



While the binding or cementing value of a rock is a most impor- 

 tant consideration from the standpoint of ordinary macadam con- 

 struction, the same is not true of broken-stone roads which are car- 

 peted or constructed with an adhesive bituminous material. The 

 French coefficient of wear is also of relatively less importance, ow- 

 ing to the fact that the fine mineral particles produced by the 

 abrasion of traffic combine, or should combine, with the bituminous 

 material to form a mastic which is held in place and protects the 

 underlying rock from abrasion so long as by proper maintenance it 

 is kept intact. The toughness of the rock is of more importance, as 

 the shock of impact is to a considerable extent transmitted through 

 the seal coat and may cause the underlying fragments to shatter. 

 It would, therefore, seem that the minimum toughness of a rock for 

 use in the construction of a bituminous broken-stone road or a 

 broken-stone road with a bituminous-mat surface should, for light 

 traffic, be no less than for ordinary macadam subjected to the same 

 class of traffic. For moderate and heavy traffic, however, the same 

 minimum toughness should prove sufficient, owing to the cushioning 

 effect of the bituminous matrix. No' maximum limit of toughness 

 need be considered for any traffic. 



In the case of bituminous concrete roads, where the broken stone 

 and bituminous material are mixed prior to laying and consolidation, 

 it generally appears advisable to set a minimum toughness of 6 or 7 

 for light-traffic roads, instead of 5, in order to insure that the frag- 

 ments of rock which have been coated with bitumen shall not be 

 fractured under the roller during consolidation; and 12 or 13 for 

 moderate and heavy traffic, instead of 10 and 19, as in the case of 

 water-bound macadam roads. 



Bearing in mind the fact that availability, cost, and various local 

 conditions may often modify the selection of proper limits. Table III 

 may be used as a general guide for minimum limits of French co- 

 efficient of wear and toughness in connection with bituminous broken- 

 stone roads. 



Table ITI. — Minmium limits of physical tests of rock for bituminous-road 



construction. 





Light to moderate traffic. 



Moderate to heavy traffic. 



Type of road. 



French coefficient of 

 wear. 



Toughness. 



French coefficient of 



wear. 



Toughness. 



Broken stone with bituminous 



carpet. 

 Bituminous broken stone with 



seal coat. 

 Bituminous concrete with or 



without seal coat. 



[5= (not over 8 per cent 

 1 wear). 



7= (not over 5.7 per 

 cent wear). 



} ^ 



7 



f7=(not over 5.7 per 

 \ cent wear). 



10= (not over 4 per 

 cent wear). 



} ■» 



13 



