56 DEPARTMENT BULLETIN 1 216, U. S. DEPT. OF AGRICULTURE 
0. Testing machines shall he calibrated frequently in order to determine 
their accuracy. 
7. Briquets that are manifestly faulty, or which give strengths differing more 
than 15 per cent from the average value of all test pieces made from the sample 
and broken at the same period, shall not be considered in determining the 
tensile strength. 
(B) COMPRESSION TEST 
8. A cylindrical test piece 2 inches in diameter and 4 inches in length shall 
be used in making the compression test. The molds (single or gang molds) 
shall be made of noncorroding metal. The ends of the mold shall be parallel. 
They shall be oiled before using. During the molding of the test piece, the 
mold shall rest on a clean, plane surface (preferably a piece of plate glass 
which is allowed to remain in place until the mold is removed.) 
9. The mortar shall be placed in the molds in four layers about 1 inch in 
thickness, each layer being thoroughly compacted, but not rammed by a steel 
tamper. The steel tamper shall have a total length of about 6y 2 inches. It 
shall be 1 inch in diameter tor 1*4 inches with the handle 5*4 inches long and 
five-eighths inch in diameter. It shall weigh approximately three-quarters of 
a pound. The surface of each layer shall be roughened before the addition of 
the next layer. In compacting the test pieces no mixing w T ater shall be forced 
out of the mortar. In finishing the test piece, mortar shall be heaped above 
the mold and smoothed off with a trowel. As soon as the test pieces from one 
sample are molded, the top of each mold shall be covered with a piece of* glass 
which shall be brought to a firm bearing on the fresh mortar. The cover 
glasses shall remain in place until the molds are removed. 
10. Tests of mortar cylinders shall be made in any testing machine which is 
adapted to meet the specified requirements. The test pieces shall be broken as 
soon as they are removed from the water. The ends of the test cylinders shall 
be smooth, plane surfaces. The metal bearing plates of the testing machine 
shall be placed in direct contact with the ends of the test piece. During the 
test a spherical bearing' block shall be used on top of the cylinder. In order 
to secure a uniform distribution of the load over the test cylinder, the spherical 
block must be accurately centered. The diameter of the spherical bearing 
block should be only a little greater than that of the test piece. The test piece 
shall be loaded continuously to failure. The moving head of the testing ma- 
chine shall travel at the rate of not lessi than 0.05 or more than 0.10 inch per 
minute. 
11. Testing machines should be frequently calibrated in order, to determine 
their accuracy. 
12. Cylinders that are manifestly faulty, or which give strengths differing 
moie than 15 per cent from the average value of all test pieces tested at the 
same period and made from the same sample, shall not be considered in deter- 
mining the compressive strength. 
(C) STORAGE OF TEST PIECES 
13. The moist closet may consist of a soapstone, slate, or concrete box, or 
a wooden box lined with metal. If a wooden box is used, the interior should be 
covered with felt or broad wieking kept wet. The bottom of the moist closet 
should be covered with water. The interior of the closet should be provided 
with nonabsorbent shelves on which to place the test pieces, the shelves being 
so arranged that they may be withdrawn readily. 
14. Unless otherwise specified, all test pieces, immediately after molding, shall 
be placed in the moist closet for from 20 to 24 hours. 
15. Cylinders or briquets shall be kept in molds on glass plates in the moist 
closet for at hast 20 hours. After 24 hours in moist air, the briquets or cylin- 
ders shall be immersed in clean water in storage tanks of noncorroding material. 
16. The air and water shall be maintained as nearly as practicable at a tem- 
perature of 21° C. (70° F.). 
Note. — It is recommended that the lahoratory method of proportioning fine aggre- 
gate, viz, by weight or by volume, for either the compression or tension test shall be 
(he same as the method used in proportioning concrete in the field. 
When the mortar is proportioned by volume the required quantity of fine 
aggregate and cement shall be determined from the unit weight of the material. 
