22 BULLETIN 347, IT. S. DEPARTMENT OF AGRICULTURE. 
COMPUTATIONS. 
The number of blows required to destroy the specimen is called 
its cementing value. An average of ^.Ye determinations is taken. 
VALUE OF THE TEST. 
On account of the great number of variables entering into this 
test, some of which are very difficult to control, concordant results 
are not always obtainable. Also, certain types of rock, particularly 
some varieties of sandstone, give results that appear to be at variance 
with those obtained in actual practice. With this exception, how- 
ever, the test will, if properly carried out, give a very good pre- 
liminary idea of the binding power of the material under examination. 
Results of this test vary enormously, running from for pure quartz 
to as high as several thousand blows in the case of high-binding 
clays. 
COMPRESSION TEST. 
This test is not ordinarily made upon road-building rock. Since, 
however, it is included in certain specifications for rock used as paving 
block, it is made when requested in the following manner: 
A cylindrical test specimen 2 inches in diameter and 2 inches high 
is prepared by the same method as previously described under the 
toughness test, except that a 2-inch core drill is used. Both ends of 
the specimen which have been properly faced are then bedded in 
plaster of Paris, and the cylinder crushed in a 200,000-pound universal 
testing machine. A small 2-inch spherical bearing block is placed 
between the moving head of the machine and upper surface of the 
specimen. The average of at least two determinations are reported 
as the crushing strength calculated in pounds per square inch. 
APPENDIX A. 
Samples of road-building rock are tested free of charge by the 
Office of Public Koads and Rural Engineering for any citizen of the 
United States provided the samples are submitted strictly in accord- 
ance with full printed instructions, which are sent on request. The 
following forms are used by this laboratory in connection with the 
work of rock testing: 
Form 6— Rev. 
United States Department of Agriculture, 
Office of Public Roads and Rural Engineering, 
Washington, D. C. 
INSTRUCTIONS FOR SELECTING AND SHIPPING SAMPLES. 
In order to have road materials tested in the laboratory of the Office of Public Roada 
and Rural Engineering the instructions below must be carefully followed: 
1. All samples shall be selected to represent as nearly as possible an average of the 
material. 
