OIL-MIXED PORTLAND CEMENT CONCRETE. 7 
for three hours and remains hard and not disintegrated in any way, 
the indications are strongly in favor of the fitness of the sample. 
On work of any importance, the cement should be carefully 
sampled and tested by a testing laboratory equipped for that purpose. 
The character of the sand used in a concrete mixture has a marked 
effect on the strength of the concrete. The sand should be clean and 
coarse. It is not advisable to permit more than 5 per cent of silt or 
clay in the sand, since both of these materials tend to weaken a rich 
concrete mixture when present in large quantities. The sand grains 
should be coarse; that is, should be graded in size from one thirty- 
second up to one-eighth or one-fourth inch in diameter. Sand 
graded in size from small to large makes a denser and stronger mortar 
than sand of uniform size. Should fine sand be the only material 
available, it will be necessary to use an increased quantity of cement 
in order to obtain the same strength that would be obtained from 
the use of a coarser sand. 
The best rocks for concrete are, in general, the traps and granites, 
although some varieties of sandstone and limestone give very good 
results. Gravel which is clean makes an excellent material for use 
in concrete. The best results are usually obtained with stone graded 
in size from one-fourth inch up to 1J inches, but for reinforced 
work a maximum size of 1 inch is preferable. Whenever gravel is 
used, it should be screened through a one-fourth inch mesh screen 
and the finer particles should be later recombined with the coarser 
particles in the correct proportions. It is not a wise procedure to 
mix cement with the gravel as it comes from the bank, since the sand 
and larger pebbles are generally not proportioned correctly to obtain 
the densest and strongest concrete. 
WATER. 
The mixing water should be clean and free from all strong acids, 
alkalies, and vegetable matter. 
SPECIFICATIONS FOR OIL TO BE USED IN OIL-CEMENT CONCRETE. 
(Subject to revision.) 
(1) The oil shall be a fluid petroleum product and shall contain 
no admixture of fatty or vegetable oils. 
(2) It shall have a specific gravity not greater than 0.945 at a 
temperature of 25° C. 
(3) It shall show a flash point of not less than 150° C. by the closed- 
cup method. 
