PHYSICAL PROPERTIES OF ROAD-BUILDING ROCK. 
Hardness is the property a rock should possess in order to success- 
fully resist the abrasive action of traffic, especially iron-tired vehicles, 
which tend to grind to dust the individual fragments of rock forming 
the wearing course of a macadam road. 
Toughness is the property a rock should possess to successfully 
resist fracture under the impact of traffic. 
Binding power, or cementing value, as it is more frequently called, 
is the ability which the dust of a rock should possess or develop by 
contact with water, so as to bind or cement the larger rock fragments 
together and prevent their displacement under the shearing action of 
traffic. This property is especially valuable in water-bound macadam 
construction, since it 
is depended upon to 
maintain the integ- 
rity of the wearing 
course as the road 
surface is worn off by 
traffic. 
PHYSICAL TESTS OF 
ROAD MATERIALS. 
pmk^ 
Fig. 1.— Displacement method of determining specific gravity. 
A description of the 
various tests em- 
ployed to determine 
to what extent a rock 
possesses the qualities 
outlined above, in ad- 
dition to certain other properties of interest to highway engineers, is 
given in the following pages. 
SPECIFIC GRAVITY- WEIGHT PER CUBIC FOOT— WATER ABSORPTION. 
OBJECT. 
To determine the weight of a solid cubic foot of the material and the 
number of pounds of water which will be absorbed by a cubic foot in 
96 hours. 
EQUIPMENT. 
Beam balance weighing accurately to 0.01 gram and provided with 
suitable means for suspending the material in water while weighing. 
(See fig. 1.) 
Drying oven. 
Desiccator and calcium chloride. 
Thermometer. 
METHOD. 
A fragment of the rock weighing approximately 10 grams and rep- 
resenting as nearly as possible an average of the entire sample is 
dried for several hours to constant weight in a hot-air bath, cooled 
