PHYSICAL PROPERTIES OF ROAD-BUILDING ROCK. 15 



A core of the rock which is to be tested is first drilled, as described 

 under the hardness test. In many cases the core which has already- 

 been subjected to the hardness test may be used. A cylinder a little 

 over 25 millimeters long is next cut from the rock core by means of 

 the diamond saw. The ends are then ground plane on the grinding 

 lap, making the length of the specimen when completed exactly 25 

 millimeters. Although it is not necessary that the two ends of the 

 specimen be exactly parallel, one surface at least should be as nearly 

 perpendicular to the axis of the cylinder as it is possible to make it. 

 After drying, the specimen is placed on the anvil (W) of the Page 

 impact machine, taking extreme care that when in position the 

 specimen rests with a perfect bearing on the anvil and that the 

 spherical end of the plunger touches the specimen exactly in the 

 center of its upper surface. The small thumbscrew used for sepa- 

 rating the clips which hold the specimen in place should be so adjusted 

 that the clips hold the specimen firmly without any undue pressure 

 on the spring. Attention to these details is very important if accu- 

 rate results are to be obtained. When the specimen is in position 

 the current is turned on the magnet and the upper crosshead is 

 lowered until the rod projecting from it just comes in contact with 

 the rod projecting up from the lower crosshead (M). This occurs 

 at the instant the pilot lamp, in series with the magnet circuit, flashes 

 out. The dial (Q) is then adjusted to (0), the upper crosshead 

 raised 1 centimeter, and the motor started. After each blow the 

 fall of the hammer is increased 1 centimeter until failure of the test 

 piece occurs. 



COMPUTATIONS. 



The height in centimeters of the blow at failure is recorded as the 

 toughness of the material. The average of two determinations is 

 taken. 



VALUE OF TEST. 



This test is useful in determining the comparative resistance of 

 rock to the impact of traffic produced by the action of horses' hoofs, 

 etc., on the stones forming the wearing course of a macadam road. 

 Toughness in rock varies through a wide range of values from as low 

 as 2 or 3 in the case of some limestones and sandstones to as high 

 as 60 in rare instances. 



This test was standardized in 1908 by the American Society for 

 Testing Materials. 



CEMENTING VALUE TEST. 



OBJECT. 



To determine the ability which the rock powder, when wet, pos- 

 sesses of binding the larger fragments together. 



