Feb. 7,1916 Hardness and Toughness of Road-Building Rock 
907 
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 may probably prove sufficient, owing to the cushioning effect 
of the bituminous matrix. No maximum limit of toughness need, 
however, 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 
would perhaps appear advisable to set a minimum toughness of 6 or 7 
for light-traffic roads instead of 5, in order to insure against the possi¬ 
bility of the fragments of rock which have been coated with bitumen 
being fractured under the roller during consolidation, and of 12 or 13 for 
moderate and heavy traffic, instead 10 and 19, as in the case of water- 
bound macadam roads. ' 
For broken-stone roads which are to be maintained with dust palliatives, 
it would appear that the same limits of toughness should hold as for 
ordinary macadam. 
For easy reference the following limits of toughness are given in Table 
I, as representing facts developed in the foregoing discussion. It is, of 
course, quite probable that these limits will require modification as the 
correlation of laboratory tests to service results becomes more perfect. 
Table I.— Limits for toughness for rock used in the construction of broken-stone roads 
Type of road. 
Eight traffic. 
Moderate 
traffic. 
Heavy traffic. 
Mini¬ 
mum. 
Maxi¬ 
mum. 
Mini¬ 
mum. 
Maxi¬ 
mum. 
Mini¬ 
mum. 
Maxi¬ 
mum. 
Macadam. 
Macadam with dust palliative. 
} * 
9 
IO 
18 
19 
Macadam with bituminous mat. 
Bituminous broken stone with seal coat. 
Bituminous concrete with or without seal 
coat... 
} * 
7 
10 
13 
IO 
13 
/ 
