It is invariably found that unscreened stone or the run of the crusher has a far less percentage of voids than screened stone, and it is therefore not only an extra expense, but also an injury to the con crete, to specify that broken stone shall be screened before being used in concrete, unless, as described later, it is intended to mix definite pro portions of several sizes of carefully screened broken stone. Since the proportion of large and small particles in the rim of the crusher depends considerably upon the character of the stone which is being broken up, and perhaps to some extent on the crusher itself, these proportions should be tested at frequent intervals during the prog ress of the work; and the amount of sand to be added to make a good concrete should be determined by trial tests, so that the resulting percentage of voids shall be as small as it is practicable to make it. It is usually found that the percentage of voids in crusher-run granite is a little larger than in limestone or gravel. This gives a slight ad vantage to the limestone and gravel, which tends to compensate for the weakness of the limestone and the rounded corners of the gravel.
S5. Broken stone is frequently sold by the ton, instead of by the cubic yard; but as its weight varies from 2,200 to 3,200 pounds per cubic yard, an engineer or contractor is uncertain as to how many cubic yards he is buying or how much it costs him per cubic yard, unless he is able to test the particular stone and obtain an average figure as to its weight per unit of volume.
SG. Cinders. Cinders for concrete should be free from coal or soot. Usually a better mixture can be obtained by screening the fine stuff from the cinders and then mixing in a larger proportion of sand, than by using unscreened material, although, if the fine stuff is uniformly distributed through the mass, it may be used without screening, and a less proportion of sand used.
As shown later, the strength of cinder concrete is far less than that of stone concrete; and on this account it cannot be used where high compressive values are necessary. But on account of its very low cost compared with broken stone, especially under sonic con ditions, it is used quite commonly for roofs, etc., on which the loads are comparatively small.
One possible objection to the use of cinders lies in the fact that they frequently contain sulphur and other chemicals which may produce corrosion of the reinforcing steel. In any structure where the strength of the concrete is a matter of importance, cinders should not be used without a thorough inspection, and even then the unit compressive v al es a owed' beat a 'very- .1 ow. figure.