SAMPLING AND TESTING HIGHWAY MATERIALS. 
IMPACT TEST FOR GRAVEL AGGREGATE. 
The material to be tested is accurately quartered until a sample 
is obtained which contains at least 50 pieces ranging in size from 
three-quarters inch to 2 inches in diameter. Care should be taken 
to see that the various sizes appear in about the same proportion 
as in the original sample. The pieces thus obtained are then 
tested individually by first determining the average diameter 
across the shortest dimension and then subjecting each to a single 
blow of the hammer using the machine described in a paper 
printed in the Proceedings of the American Society for Testing 
Materials for 1922 entitled "An Impact Test for Gravel," by F. H. 
Jackson, Bureau of Public Roads. 
Note. — It is suggested that the height of fall shall be that indi- 
cated in the following table : 
Diameter 
of particle. 
Height of fall. 
Sedimentary 
origin. 
Igneous or 
metarnorphie 
origin. 
Inches. 
1 
1 
Inches. 
2 
3 
4 
5 
Inches. 
3 
4 
5 
6 
TEST FOR PERCENTAGE OF SHALE IN GRAVEL. 
It is suggested that for the separation of shale and other pieces 
having low specific gravity from concrete aggregate, a solution of 
zinc chloride (ZnCl 2 ) or some other satisfactory liquid having 
a specific gravity of approximately 1.95 be used. A sample of 
the pebbles should be first dried to constant weight at not over 
110° C, then placed in a container partially filled with the solu- 
tion. Agitate for five minutes, skim off the lighter materials, 
and then pour the solution through a sieve which will retain the 
pebbles. Repeat the operation until the entire sample has been 
separated. Dry to constant weight, measure the volume of re- 
tained material, and compute the percentage of volume of shale 
or other soft material. 
TEST FOR QUANTITY OF CLAY AND SILT IN GRAVEL FOR 
HIGHWAY CONSTRUCTION. 
(A. S. T. M. standard method, serial designation D 72-21, slightly 
modified.) 
1. This is a test for the determination of the quantity of clay 
and silt in natural gravel to be used in highway construction. 
2. The sample as received shall be moistened and thoroughly Method of test. 
mixed, then dried to constant weight at a temperature between 
100° and 110° C. (212° and 230° F.). 
3. The pan or vessel to be used in the determination shall 
be 12 inches (30.5 centimeters in diameter by not less than 4 
inches (10.2 centimeters) deep, as nearly as may be obtained. 
4. (a) A representative portion of the dry material weighing 
not less than 50 times the weight of the largest stone in the 
sample shall be selected from the sample and placed in the pan 
which has been dried and accurately weighed. 
(b) Sufficient water shall be poured into the pan to cover the 
gravel. The gravel shall be agitated vigorously for 15 seconds 
with a trowel or stirring rod. After it has settled for 15 seconds 
the water shall be poured off into a tared evaporating dish, 
