PHYSICAL TESTS OF ROAD-BUILDING ROCK. 3 
constructed is a most important consideration, and in general the se- 
lection of rock should be based upon the character and volume of 
traffic as related to the type of road in which it is to be used. 
The more common types of road in which stone is used are : 
1. Water-bound broken-stone roads, as macadam, maintained as 
such. 
2. Water-bound macadam roads maintained with dust palliatives. 
3. Water-bound macadam roads with bituminous carpet. 
4. Bituminous broken-stone roads with a seal coat of bituminous 
material constructed according to the penetration method. 
5. Bituminous concrete roads with a seal coat of bituminous 
material. 
6. Bituminous concrete roads without a seal coat of bituminous 
material. 
7. Portland cement concrete roads with a coarse aggregate of 
broken stone. 
8. Stone-block pavements. 
The destructive effect of traffic, both with respect to character and 
volume, varies to a considerable extent for the different types of 
road. 
PHYSICAL PROPERTIES OF ROAD-BUILDING ROCK. 
The success or failure of a rock for road building depends largely 
upon the extent to which it will resist the destructive influences of 
traffic. The three most important physical properties are hardness, 
toughness, and binding power. Hardness is the resistance which 
the rock offers to the displacement of its surface particles by abra- 
sion; toughness is the resistance which it offers to fracture under 
impact; and binding power is the ability which the dust from the 
rock possesses, or develops by contact with water, of binding the 
large rock fragments together. In order to approximate as closely 
as possible in the laboratory the destructive effects produced by the 
various agencies which have been mentioned, certain physical tests 
have been developed. Brief descriptions of these tests are as follows : 
HARDNESS TEST. 
Hardness is determined by subjecting a cylindrical rock core 25 
mm. in diameter, drilled from the specimen to be examined, to the 
abrasive action of quartz sand fed upon a revolving steel disk. The 
end of the specimen is worn away in inverse ratio to its hardness 
and the amount of loss is expressed in the form of a coefficient as 
follows : 
Coefficient of hardness = 20—1/3 w, where w equals the loss in 
weight after 1,000 revolutions of the disk. 
