PHYSICAL PROPERTIES OF ROAD-BUILDING ROCK. H 



grip carrying the specimen is then removed, brushed free from dust, 

 and accurately weighed. By means of the small metal washers, 

 any one or more of which may he slipped over the projecting rod of 

 the grip, the initial weight should be adjusted to exactly 1,250 grams. 

 The grip is then replaced in the same position as before and the 

 machine given 1,000 revolutions at the rate of 30 per minute, after 

 which it is again weighed. The test should be repeated with the 

 specimens reversed, in order to obtain the average hardness of the 

 two ends. 



The one-fourth inch opening in the funnel of the hardness machine 

 will allow 18.5 pounds of sand to pass through during a test. 



COMPUTATIONS. 



Calling the initial weight of grip plus specimen a and the final 

 weight after 1,000 revolutions h, 



the coefficient of hardness = 20 — - — =—^ • 



o 



The coefficient 20 was chosen as the standard of comparison to 



give about the same range of values as those obtained by the Deval 



abrasion test. The loss in weight i3 divided by 3 in order to avoid 



negative coefficients, since it was found that a specimen might lose 



as high as 60 grams in a single test. 



VALUE OF TEST. 



The test is of value in determining the resistance of a rock to the 

 grinding action of traffic. The coefficient of hardness for various 

 types of rock will run from 19.7 for the hardest varieties of quartzite 

 to for very soft limestones and sandstones. 



TOUGHNESS TEST. 



OBJECT. 



To determine the resistance which the material offers to fracture 

 due to impact. 



EQUIPMENT. 



Diamond core drill (same as used in hardness test). 



Diamond saw (same as used in hardness test). 



Grinding lap. 



The grinding lap used for preparing specimens for the toughness 

 test is shown in figure 6. It is 9 inches in diameter and is driven 

 by means of a small electric motor at a speed of 400 revolutions per 

 minute. Finely powdered carborundum is used as an abrasive agent. 



No. 120 carborundum. 



Page impact machine. 



In the Page impact machine (fig. 7), a 2-kilogram hammer (AQ 

 is guided by two vertical rods (J). The upper end of the hammer 



