PHYSICAL TESTS OF EOAD-BUILDING 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 III. 
-Minimum 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. 
1 5= (not over 8 per cent 
| wear). 
7= (not over 5.7 per 
cent wear). 
} * 
7 
(7= (not over 5.7 per 
\ cent wear). 
10= (not over 4 per 
cent wear). 
} " 
13 
