22 BULLETIN 347, U. S. DEPARTMENT OF AGRICULTURE. 



COMPUTATION*. 



The number of blows required to destroy the specimen is called 

 its cementing value. An average of five 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 inehes 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 Roads 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 Roads 

 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. 



