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 1£ 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. 



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. 



