6 BULLETIN U9, U. S. DEPARTMENT OF AGRICULTURE. 



(4) The coefficient 20 is 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 is 

 divided by 3 in order to avoid negative coefficients, since it is found that a specimen 

 may lose as high as 60 grams in a single test. 



4. TEST FOR TOUGHNESS OF ROCK. 



(A. S. T. M. Standard Method, Serial Designation: D 3-18.) 



(1) Toughness, as applied to rock, is the resistance offered to fracture by impact, 

 expressed as the height of the final blow of a standard hammer required to cause frac- 

 ture of a cylindrical test specimen of given dimensions. 



(2) Quarry samples of rock from which test specimens are to be prepared shall meas- 

 ure at least 6 inches on a side and at least 4 inches in thickness, and when possible 

 shall have the plane of structural weakness of the rock plainly marked thereon. Sam- 

 ples shall be taken from freshly quarried material, and only from pieces which show 

 no evidences of incipient fracture due to blasting or other causes. The samples shall 

 preferably be split from large pieces by the use of plugs and feathers and not by sledg- 



Fig. 4. — Details of diamond saw. 



ing. Commercial stone-block samples from which test specimens are to be prepared, 

 shall measure at least 3 inches on each edge. 



(3) Specimens for test shall be cylinders prepared as described in paragraph 4, 

 25 mm. in height and from 24 to 25 mm. in diameter. Three test specimens shall 

 constitute a test set. The ends of the specimen shall be plane surfaces at right angles 

 to the axis of the cylinder. 



(4) One set of specimens shall be drilled perpendicular and another parallel to the 

 plane of structural weakness of the rock, if such plane is apparent. If a plane of 

 structural weakness is not apparent, one set of specimens shall be drilled at random. 

 Specimens shall be drilled in a manner which will not subject the material to undue 

 stresses and which will insure the specified dimensions. The ends of the cylinders 

 may be sawed by means of a band or diamond saw, 1 or in any other way which will 

 not induce incipient fracture, but shall not be chipped or broken off with a hammer. 

 After sawing, the ends of the specimens shall be ground plane with water and carbo- 

 rundum or emery on a cast-iron lap (see fig. 5) until the cylinders are 25 mm. in length. 



(5) Any form of impact machine which will comply with the following essentials 

 may be used in making the test: 



1 Satisfactory forms of diamond drill and diamond saw are shown in figs. 3 and 4. 



