VI. Pats of neat cement about 3 inches in diameter, one-half inch thick at the center, and tapering to a thin edge shall be kept in moist air for twenty-four hours.
(a) A pat is then kept in air at normal temperature, and observed at intervals for at least twenty eight days.
(b) Another pat is kept in water maintained as near 70 degrees F. as practicable, and observed at intervals for at least twenty-eight days.
(c) A third pat is exposed in any convenient way in an atmosphere of steam, above boiling water, in a loosely closed vessel for five hours.
VII. These pats, to satisfactorily pass the require ments, shall remain firm and hard and show no signs of distortion, checking, cracking, or disintegrating.
VIII. The cement shall not contain more than 1.75 Viii. The cement shall not contain more than 1.75 per cent of anhydrous sulphuric acid (SO,), nor more than three per cent of magnesia (Mg 0).
Only clean sand shall be used. It shall be coarse and sharp, with grains of various sizes, and also contain the smallest possible percentage of voids. Sand having over 3 per cent loam shall be rejected.
The aggregate shall be composed of broken stone or gravel. The broken stone shall be of a hard, close-grained quality, free from dust, and crushed so that its largest dimension shall pass through a ring one inch in diameter.
Gravel should be free from dirt, and should be in size ranging from that of a pea to an inch. Disintegrated stone or broken stone containing mica shall be rejected.
All concrete for slabs and beams shall be propor tioned of one part Portland Cement, two parts of sand, and four parts of broken stone or gravel.
All concrete for columns shall be proportioned of one part Portland cement, parts of sand, and 2 parts of broken stone or gravel.
All concrete shall be machine mixed, using either a batch or a continuous mixer of an approved design. Plenty of clean water should be used, so that the resultant mixture has the consistency of what may be known as a "wet" mixture. All materials shall be first thoroughly mixed dry, after which the proper amount of water is added, and the mixing continued until the concrete is uniform. A competent foreman must be in constant
attendance at the mixer, to give his approval of every batch which leaves the machine.
No reinforcing steel shall be considered that does not provide for shearing stresses as well as direct tension. These resisting members must be inclined to an angle of 45 degrees, pointing up and towards the supports of the structure. Shear members shall be rigidly attached to the main tension member. Sufficient steel shall be placed so that the concrete will be obliged to resist only direct compression, and shearing stress up to 50 pounds per square inch. No steel shall have, at any point, less than 1 inch of concrete covering. Steel bars shall not be painted. A slight film of rust will not be objectionable upon same, but any bar on which decided rust scale§ have formed shall be rejected. In no case shall steel of higher elastic limit than 45,000 lbs. be considered. The same shall have a tensile strength of from 60,000 to 70,000 lbs. per square inch, with an elongation of not less than 20 per cent in 8 inches. A bar should bend when cold, around its own diameter, through an angle of 180 degrees, and close down upon itself without cracking.
Steel shall be placed in exact accordance with detail drawin gs.
Concrete shall be placed as rapidly as possible after mixing, and shall bo thoroughly puddled immediately thereafter.
When concreting is once commenced, it should be car ried on vigorously to completion, if possible. If concret ing must be stopped before an entire floor is completed, the stop shall be made in the center of beams and center of floor slabs. The plane where concrete work is stopped must be at right angles to the direction of the beam or slab. In no event shall work be terminated in beams or floor slabs where future shearing action becomes great, as at their ends or directly under a heavily concentrated load.
In freezing weather, special precaution must be taken to protect the exposed surfaces.