10 
BULLETIN 370, U. S. DEPARTMENT OF AGRICULTURE. 
group low results are frequently shown by laboratory tests for rocks 
which bind quite satisfactorily upon the road, provided traffic is suf- 
ficiently heavy to supply the requisite amount of fine material. Cer- 
tain granites, gneisses, and schists which are not suitable for use as 
binding material give good results in this test. In such cases it is 
usually found that the highly altered nature of the material reduces 
its toughness and resistance to wear to such an extent as to condemn 
it for use. 
Experience has shown that in general the following table of limit- 
ing values for the French coefficient of wear, toughness, and hardness 
may be used in determining the suitability of a rock for the con- 
struction of the wearing course of a macadam road : 
Table II. — Limiting values of physical tests of rock for water-bound macadam 
road construction. 
Character of traffic. 
Limits of tests. 
French coefficient of wear. 
Toughness. 
Hardness. 
Light 
5-9 
10-18 
Over 18 
10-17 
Over 14 
Over 17 
With relation to the limitations for hardness it may be noted that 
as a result of comparing hardness and toughness tests of some 3,000 
samples, the authors 1 have shown that when any given value for 
toughness falls within certain limits which define the suitability of 
the material for macadam road construction under given traffic 
conditions, the corresponding value for hardness will fall within 
similar limits for hardness. In this connection it will be seen, in 
Table II, that a maximum limit for hardness is only given in the 
case of light traffic. It has been found that the great majority of 
samples having a French coefficient of wear of from 5 to 8 and a 
hardness of over IT are granites, quartzites, and hard sandstones, 
which are unsuited for use in the wearing course of water-bound 
macadam roads due to their lack of binding power. 
BITUMINOUS ROADS. 
For broken-stone roads which are maintained with dust palliatives, 
the same limits for French coefficient of wear and toughness should 
hold as for ordinary macadam roads. 
In bituminous work observations indicate that in some cases it is 
advantageous to use a rock of relatively high absorption rather than 
one with low absorptive qualities, owing to a better adhesion of the 
bituminous material by a partial surface impregnation of the rock. 
1 Relation Between the Properties of Hardness and Toughness of Road-Building Rock, 
Journal of Agricultural Research, Vol. V, No. 19, D-3. 
