SAMPLING AND TESTING HIGHWAY MATERIALS. 5 
lower end of the specimen rests on the steel disk. The funnel shall be rilled with 
sand and the machine run until the lower end of the specimen has been worn down 
to the plane of the disk. The grip carrying the specimen shall then be removed, 
brushed free of dust, and accurately weighed. By means of small metal washers, 
any one or more of which may be slipped over the projecting rod of the grip, the 
initial weight shall be adjusted to exactly 1,250 grams. The grip shall then be replaced 
in the same position as before and the machine given 1,000 revolutions at the rate of 
30 per minute, after which the grip and specimen shall be weighed again. The 
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test shall be repeated with the specimens reversed, in order to obtain the average 
hardness of the two ends. 
(2) The sand used as the abrasive agent shall be a crushed quartz, screened to pass 
a standard sieve having 30 meshes per linear inch and to be retained on a standard 
sieve having 40 meshes per linear inch. Since it is almost impossible to obtain 
such a sand commercially, it is customary to specify a sand not more than 5 per cent 
of which will be retained on a No. 30 sieve and not more than 25 per cent of 
which will pass a No. 40 sieve. Sand known to the trade as No. 2\ quartz will usually 
fulfill these requirements. The |-inch opening in the funnel of the hardness machine 
will allow about 18 pounds of sand to pass through during a test. 
(3) Calling the initial weight of grip plus specimen a, the final weight after 1,000 
revolutions b, 
the coefficient of hardness = 20— ±-= — -• 
