PHYSICAL PROPERTIES OF ROAD-BUILDING ROCK. 3 



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. 



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. 



Fig. 1.— Displacement method of determining specific gravity. 



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. 



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 



