18 DEPARTMENT BULLETIN 12 16, U. S. DEPT. OP AGRICULTURE 
B. TEST FOR GRAVEL CONTAINING CRUSHED PIECES s 
8. Gravel containing more than about 10 per cent of crushed pieces shall, for 
the purpose of this test, be considered as crushed gravel. In such cases the 
abrasion test shall be made on a representative sample of the whole, including 
the crushed pieces, following the procedure described in paragraphs 1 to 7. 
The percentage, by weight, of crushed pieces shall be determined and the 
permissible percentage of wear which shall govern for any given sample shall 
be calculated from the following formula : 
w _ AL+q00-A)L' 
100 
in which 
A = percentage of uncrushed pieces, 
100— A =percentage of crushed pieces, 
L=maximum percentage of wear permitted by the specifications for 
gravel containing no crushed pieces. 
Z/=maximum percentage of wear permitted by the specifications for 
gravel consisting entirely of crushed pieces, 
W =permissible percentage of wear. 
5. METHOD OF TEST FOR TOUGHNESS OF ROCK 
(A. S. T. M. standard method (4), serial designation D 3-18) 
1. Toughness, as applied to rock, is the resistance offered to fracture under 
impact, expresed as the final height of blow required of a standard hammer 
to cause fracture of a cylindrical test specimen of given dimensions. 
2. Any form of impact machine which will comply with the following essen- 
tials may be used in making the test : 
(a) A cast-iron anvil weighing not less than 50 kilograms, firmly fixed upon 
a solid foundation. 
(&) A hammer weighing 2 kilograms, arranged so as to fall freely betweeu 
suitable guides. 
(c) A plunger made of hardened steel and weighing 1 kilogram, arranged 
to slide freely in a vertical direction in a sleeve, the lower end of the plunger 
being spherical in shape with a radius of 1 centimeter. 
(d) Means for raisins the hammer and for dropping it upon the plunger 
from any specified height from 1 to nor less than 75 centimeters, and means 
for determining the height of fall to approximately 1 millimeter. 
(e) Means for holding the cylindrical test specimen securely on the anvil 
without rigid lateral support, and under the plunger in such a way that the 
center of its upper surface shall, throughout the test, be tangent to the spherical 
end of the plunger at its lowest point. 
3. Quarry samples of rock from which test specimens are to be prepared 
shall measure at least six 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. Samples should be taken from freshly quarried mate- 
rial, and only from pieces which show no evidences of incipient fracture due to 
blasting or other causes. The samples should preferably be split from large 
pieces by the use of plugs and feathers and not by sledging. Commercial 
stone-block samples from which test specimens are to be prepared shall measure 
at least 3 inches on each edge. 
4. Specimens for test shall be cylinders prepared as described in section 5, 
25 millimeters in height and from 24 to 25 millimeters in diameter. Three test 
specimens shall constitute a test set. The ends of the specimens shall be 
plane surfaces at right angles to the axis of the cylinder. 
5. One set of specimens shall be drilled perpendicular and another parallel to 
ihe 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 nor 
8 For the purpose of this test, a crashed gravel fragment may be considered as a 
fragment of gravel having at least one fractured face. 
