Concrete shall be placed in freezing weather, only when same cannot possibly be prevented; and then espe cial precautions must be used to cover the work at once with at least five or six inches of sawdust or manure. The forms for such work should be left in place at least three weeks longer than customary, and under no circum stances shall they be removed unless the concrete has thoroughly set and the frost entirely disappeared. Con crete for slabs shall be laid immediately after pouring the beams. Under no circumstances shall the concrete in the beam boxes be permitted to set before the concrete for the slab is poured.
The centering must be true and rigid and properly braced, and of sufficient strength to carry easily the dead weight of the construction as a liquid, without deflection.
All joints must be fairly tight so as to prevent leakage of liquid masses. The design of the centering should be such that the sides of the beams can be taken down first, then the slab centering complete. Centering should never be removed until the concrete has thoroughly set, and has aged to give it sufficient strength to carry its own weight besides whatever live load is liable to come on the work during the course of construction. In no event shall the falsework be removed until approval is given by the architect Or engineer-in-charge. Beams shall remain supported for at least two weeks after all other falsework has been removed. Columns shall not be given their full loading in less than five weeks after concreting.
After the forms are removed, any small cavities or openings in the concrete shall be neatly filled with mortar if necessary. In order to obtain a very smooth finish, wash with a thin grout of mortar of the consistency of whitewash, mixed in a proportion of one part cement and three parts sand. This wash should be applied with a brush.
Floors shall be tested after the centering has been removed one month, to a uniformly distributed load equal to twice the safe live load. With this load there should not be a deflection exceeding 1-400th part of the span, and the floor should return to its normal position after the removal of the load.
The following stresses based on figuring full live and dead loads shall be used in the design of reinforced concrete work : For hooped concrete columns, 750 lbs. per sq. in. of concrete.
For latticed columns, 500 lbs. per sq. in.
For shearing stresses in concrete and adhesion of concrete to steel, 50 lbs. per sq. in.
Extreme fiber stress in compression for slab, beams, and girders, 750 lbs. per sq. in.
Tensile stress in steel, 16,000 lbs. per sq. in.
The ratio of moduli of elasticity of concrete and steel to be 1 to 15.
The tensile strength of concrete shall not be considered. When slab and beams are built continuous over their supports, the bending moment may be taken at 1-10 Wl. In cases of square floor panels, floor supported on all four sides and built continuous over supports, the bending moment may be taken at 1-20 Wl. In all such cases suffi cient reinforcement must be provided at the top of the slab to take care of the regular bending moment at the supports.
The cement finish for floors shall be proportioned not leaner than 1 to 2, using in all cases a specially sharp, clean, and gritty sand. It shall be troweled to a thor oughly smooth and even surface, and shall be cut in squares of not more than eight feet.
Cement finish when applied to a concrete base must be laid at the same time as the base, and shall not be less than 1/2 inch in thickness. When applied to tile surface, it shall be not less than two inches in thickness, special care being taken to clean the tile and secure a thorough bond between it and the cement.
Since the success of the structure depends so largely upon the steel and the manner of placing same, we quote an opinion by Mr. H. F. Porter, which he considers should be embodied in sets of specifications.
"All steel to be of best quality, made from original billets and in strict accordance with the regulation standard for structural steel. It shall be free from all scale and laminations and defects endangering the strength of the material. It shall also be free from all dirt, oil, and rust pocks, although a slight film of rust is permissible. The ultimate strength shall not be less than four times the safe unit-stress used, and the elastic limit less than two and a-half times. All reinforcement to be properly fabricated and supported in place, so that all parts shall remain in true position and alignment until locked in the setting concrete. All tensile stresses, theo retical and practical, that may occur in the structure and any of its parts, shall be fully and properly provided for by steel reinforcement. In general, the reinforcement shall run as continuously as possible throughout the struc. ture, and full development of every piece provided by suitable end-anchorage and lapping. Diagonal tensions in beams to be cared for completely by diagonal reinforce ment, either a part of or rigidly attached to the main reinforcement. In the slabs, these stresses and the nega tive stresses to be cared for simultaneously with the posi tive stresses, by elevating a sufficient number of the bars across the supports, providing in effect a catenary curve reinforcement.
"Vertical tension to be cared for by vertical loops of steel, passing around the lower horizontal bars and extending well up into the compression flange, in which they shall be well anchored. These loops or stirrups shall be spaced not farther apart than the effective depth of the member and at progressively diminishing intervals towards the supports. In case of T-beams, vertical stir rups are necessary to guard against possible separation of the rib from the slab along the plane of junction, and to insure the full and proper development of the T-action. All placing of reinforcement shall be done before under taking to concrete, so that in amount and position it may be easily and fully checked before any concrete is depos ited; conversely, no steel shall be left to be placed while the concreting is in progress. In slabs and walls, if bars are used, cross or shrinkage bars shall be provided at proper intervals, the intersections securely wired or clamped, and the bars properly and securely spaced and supported in position, prior to any concreting."