20 DEPARTMENT BULLETIN 1216, U. S. DEPT. OF AGRICULTURE 
8. METHOD OF TEST FOR QUANTITY OF SOFT PEBBLES 
IN GRAVEL 11 
1. The method consists in determining ilie resistance of the pebbles to uniform 
Static pressure. 
2. A sample of at least 1.000 grams shall be separated into three sizes and 
subjected to uniform pressure without impact, as follows : 
Pounds 
0.185 inch to 0.375 inch 35 
o.:!7r> inch to 0.750 inch 60 
0.750 inch to maximum size 75 
3. Pebbles crushing under these loads shall be classed as soft pebbles; and 
^'hall be reported as a percentage of the original sample by weight. 
4. -Disintegrated rocks or other rocks which will obviously break up into fine 
material during the screening or mixing operation, shall not be considered in 
this test. 
9. METHOD OF TEST FOR SOUNDNESS OF COARSE 
AGGREGATE 
(Sodium-sulphate soundness test) 
1. Immerse 10 small pieces (total weight about 1,000 grams) of the rock in 
a saturated solution at 70° P. of sodium sulphate (Na 2 S04) for 20 hours, after 
which place them for 4 hours in a drying oven maintained at 100° C. Repeat 
the treatment — times. The operation of immersing, heating, reimmersing, etc., 
shall be continuous. Note the condition of the rock as to soundness at the 
end of the test. 
2. Samples which exhibit marked disintegration shall be considered to have 
failed in this test. 12 
10. METHOD OF TEST FOR PERCENTAGE OF SHALE 
IN AGGREGATE 
It is suggested that for the separation of shale and other pieces having low 
specific gravity from concrete aggregate, a solution of zinc chloride (ZnCb) 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 solution. Agi- 
tate 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 retained material, and compute the percentage of volume of shale or other 
soft material. 
Note. — Attention is called to the fact that this method of test is not applicable when 
the specific gravity of the shale exceeds the specific gravity of the solution. In cases 
where a solution or very high density is required, a mixture of bromoform and benzol may 
be employed. 
M A satisfactory instrument for classifying soft stone by this method was devised by 
late W. H. Douglass of the Iowa Highway Commission. A description of this 
apparatus follows: 
The Douglass stone meter consists of a jack acting through the specimen to compress 
., calibrated spring. The jack, which is of the rack and pinion type, is located in the 
base of the machine and is actuated by means of a hand lever attached to the shaft of 
the pinion. To the base are attached two vertical standards which are attached to each 
other by two cross members, one at the top and one near the center. The coil compres- 
sion spring is located between these two cross members. The top of the spring bears 
against the top cross bar and the bottom against a collar on the movable plunger which 
passes through both cross members and the spring. To the lower end of the plunger 
is screwed a mechanic's jack which allows adjustment for the various sizes of stones 
and also carries a ball and socket head. The top end of the plunger is graduated to read 
the desired pressures as it passes through the top cross member. 
U A correct, interpretation of the results of the sodium-sulphate soundness test will be 
greatly assisted by a visual examination of the ledge from which the sample was obtained. 
Failure in the test, when accompanied by evidence of weathering of the exposed surface, 
may be considered sufficient ground for rejection. 
