PHYSICAL TESTS OF KOAD-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. 



