SPECIFICATIONS FOR PORTLAND-CEMENT CONCRETE 1. Cement. The cement shall be portland, either American or foreign, which will meet the requirements of the standard specifications adopted by the American Society for Testing Materials. [See Appendix I.] S. Sand. The sand shall be clean, sharp, coarse, and of grains varying in size. It shall be free from sticks and other foreign matter, but it may contain clay or loam not to exceed five per cent. Crusher dust, screened to reject all particles over one quarter inch in diameter, may be used instead of sand, if approved by the engineer.
3. Stone. The stone shall be sound, hard and durable, crushed to sizes not exceeding two inches in any direction. For reinforced concrete, the sizes usually are not to exceed three quarter inch in any direction, but may be varied to suit the character of the reinforcing material.
4. Gravel. The gravel shall be composed of clean pebbles of hard and durable stone of sizes not exceeding two inches in diameter, free from clay and other impurities except sand. When containing sand in any considerable quantity, the amount per unit of volume of gravel shall be determined accurately to admit of the proper proportion of sand being maintained in the concrete mixture.
6. Water. The water shall be clean and reasonably clear, free from sul phuric acid or strong alkalies.
6. Mixing by Hand. (a) Tight platforms shall be provided of sufficient size to accommodate men and materials for the progressive and rapid mixing of at least two batches of concrete at the same time. Batches shall not exceed one cubic yard each, and smaller batches are preferable, based upon a multiple of the num ber of sacks to the barrel. .
(6) Spread the sand evenly upon the platform, then the cement upon the sand, and mix thoroughly until of an even color. Add all the water necessary to make a thin mortar, and spread again; add the gravel if used, and finally the broken stone, both of which, if dry, should first be thoroughly wet down. Turn the mass with shovels or hoes until thoroughly incorporated, and until all the gravel and stone is covered with mortar, which will probably require the mass to be turned four times.
(c) Another method, which may be permitted at the option of the engineer in charge, is to spread the sand, then the cement, and mix dry; then the gravel or broken stone, add water, and mix thoroughly as above.
7. Mixing by Machine. A machine mixer shall be used wherever the volume of work will justify the expense of installing the plant. The necessary require ments for the machine shall be that a precise and regular proportioning of materials can be controlled, and the product as delivered shall be of the required consistency and be thoroughly mixed.
8. Consistency. The concrete shall be of such consistency that when dumped in place it will not require much tamping. It shall be spaded down, and be tamped sufficiently to level it off, after which the water should rise freely to the surface.
9. Forms. (a) Forms shall be well built, substantial and unyielding, properly braced or tied together by means of wire or rods, and shall conform to the lines given.
(b) For all important work, the lumber used for face work shall be dressed on one side and both edges, and shall be sound and free from loose knots, secured to the studding or uprights in horizontal lines.
(c) For backing and other rough work, undressed lumber may be used.
(d) Where corners of the masonry and other projections liable to injury occur, suitable mouldings shall be placed in the angles of the forma to round or beifel them off.
(e) Lumber once used in forms shall be cleaned before being used again.
(f) The forms must not be removed within thirty-six hours after all the concrete in that section has been placed. In freezing weather, they must re main until the concrete has had a sufficient time to become thoroughly hardened.
(g) In dry but not freezing weather, the forms shall be drenched with water before the concrete is placed against them.