PHYSICAL TESTS OF ROAD-BUILDING ROCK. 9 
SLAGS. 
Many slag varieties resemble in certain outward respects the com- 
mon road-building rocks. However, in general, they are more porous 
and glassy, and vary so greatly in physical properties that with ref- 
erence to their physical characteristics from the standpoint of road 
construction they can not well be considered as a single class with 
definite limits or general average numerical values. 
INTERPRETATIONS OF RESULTS OF PHYSICAL TESTS. 
The results of physical tests are only of value in predetermining 
the suitability of a rock for a given type of road under given condi- 
tions when the behavior of other rocks, having the same general 
physical characteristics, is known. Much investigation is still neces- 
sary to accurately correlate laboratory tests with service results, but 
in this connection certain facts have been determined from experi- 
ence, which may be briefly discussed under the different types of 
roads. 
As the amount of traffic to which a road is or will be subjected is 
a most important consideration, and as the terms light, moderate, 
and heavy are commonly used in describing the amount of traffic, 
such terms should be defined. For the purpose of comparison it has 
been assumed that traffic of less than 100 vehicles per day is light, 
between 100 and 250 moderate, and over 250 heavy. 
WATER-BOUND MACADAM ROADS. 
The ideal rock for the construction of a water-bound macadam 
road resists the wear of traffic to which it is subjected to just that 
extent which will supply a sufficient amount of cementitious rock 
dust to bind or hold the larger fragments in place. It is generally 
admitted that the ordinary macadam road is not well suited to any 
considerable amount of automobile traffic, because such traffic rap- 
idly removes the binder without producing fresh material to take its 
place. 
Cementing value is a necessary quality for rocks used in macadam 
road construction. As determined by test, cementing values below 
25 are called low; from 26 to 75, average, and above 75, high. In 
general, the cementing value should run above 25. For rocks which 
show a low French coefficient of wear, however, a relatively high 
cementing value is more necessary than for those which have a high 
French coefficient. Interpretation of results of the cementing value 
test is subject to a number of influencing considerations. For in- 
stance, it has been found that certain feldspathic varieties of sand- 
stone give excellent results in this test, while experience has shown 
that they do not bind well when used in the wearing course of 
macadam roads. In the case also of certain varieties of the trap 
