Concrete Construction: Methods and Costs

by Halbert Powers Gillette; Charles Shattuck Hill

Give the forms a final inspection before concreting to check line and level, to close open joints and to tighten up clamps and wedges. Finally clean each form and wet it down thoroughly before placing the concrete—do this just before placing the concrete.

REMOVING FORMS.—Good judgment and extreme care are essential in removing centers. It goes without saying that forms should never be removed until the concrete has set and hardened to such strength that it will sustain its own dead weight and such live load as may come upon it during construction. The determination of this condition is the matter that calls for knowledge and judgment. Some cements set and harden more rapidly than others, and concrete hardens more and more slowly as the temperature falls. These and other circumstances must all be taken into account in deciding upon the safe time for removal. Many large contractors mold a cube of concrete for each day's work and leave it standing on the finished floor exposed to the same conditions as the concrete in the forms; examination of this sample block gives a line on the condition of the concrete in the work and on the probable safety of removing the forms at any time. In all cases it should be the superintendent's duty to determine when to remove forms, and he should satisfy himself by personal inspection that the concrete is in condition to stand without support. It is also wise at least as a matter of precaution for the contractor to secure the engineer's or the architect's approval before removing any formwork.

Care in removing forms is essential for the reason that green concrete is particularly susceptible to injury from shock or sudden strain. It is well, therefore, to have a separate gang always doing the work. These men will in a few days become trained under an experienced foreman so that they will not only do the work with greater safety but also more rapidly. This gang should, furthermore, be required to follow a regular system in its work; a system which may not be departed from without direct orders from the superintendent. An example of such a system is outlined below.

The time of beginning this work of removal shall be given by the superintendent. In warm, dry weather, with other conditions favorable, removal may be begun after seven days. Then the following schedule may be followed: At the end of seven days remove the sides of the column forms. This gives an opportunity to determine the soundness of the column casting and also serves the further desirable purpose of baring the concrete to the curing and hardening action of the air. At the end of 14 days loosen the wedges of the posts supporting the slab centers and drop these centers a couple of inches: leave the centers in this position for another day, meanwhile examining the tops of the slabs to note their condition. Then remove the sides of the beam molds and the slab centers, replacing the latter with temporary uprights supporting a plank bearing against the underside of the slab. This precaution is often neglected and with very little reason considering the importance of the safeguard thus secured. Ordinarily the shores need not be left in place more than a week, so that the amount of lumber thus tied up is small. At the end of three weeks remove the uprights under the beam and girder molds and strip the bottom plank. In this schedule it is assumed that the floor is free from any great load and that no unusual loading is put upon it; if a load of any consequence is to come on the floor the shores and uprights should be left in place longer. No schedule of removal can be blindly followed, and that given above is certain only when the conditions are right and as stated.

FABRICATION AND PLACING OF REINFORCEMENT.

The amount of reinforcing steel used varies from 50 lbs. to 275 lbs. per cu. yd. of concrete; the highest figure will be had only in very heavy work and where very heavily reinforced raft foundations are employed, and the lowest only in one-story buildings consisting of walls and roof. A fair average is perhaps 150 lbs. per cu. yd. The cost of fabricating and placing reinforcement will run from ⅓ ct. to 1½ cts. per pound, but the last figure is exceedingly high; ¾ ct. per pound for fabricating and placing is a reasonable labor charge.

Contractors frequently have their choice whether the steel shall be fabricated into frames and placed as units or whether it shall be placed in separate bars. For girders and columns the difference in cost of the two methods is not so very great for steel in place when the fabrication is done in the field. The unit frames cost considerably more than separate bars to fabricate, but the cost of handling and placing them in the forms is materially less; on an average the differences balance each other. Where the frames are made up in regular mills unit frames generally cost less to fabricate and place than do separate bars. The use of unit frames in wall and floor slab reinforcement is generally more expensive than the use of separate bars. The chief gain that comes from the use of unit frames is the gain due to the certainty that the reinforcing bars, stirrups, etc., are all there and are properly spaced and placed.

FABRICATION.—Fabrication includes all the work necessary to prepare the reinforcement ready to place in the forms. It amounts to very little where separate bar types of reinforcement are used. Plain bending and shearing operations comprise the whole task. Where the beam or column reinforcement has to be made up into complete frames which can be handled and placed as units this task is more complex and considerable apparatus is essential to rapid and economical work. For this reason it is wise usually to contract with some metal working shop to assemble and connect up the various units and to furnish them ready for installation. In many cases these unit frame types of reinforcement are patented and the proprietors contract to fabricate and furnish them complete according to the plans of the engineer or architect. Even where the frame construction is not so controlled it will be economy generally to have the fabrication done at regular shops where the necessary tools and skilled workmen are had. In any case the bars should be ordered cut to length at the mill so far as possible.

Fig. 207.—Rack for Storing Reinforcing Bars.