Walls in mass work, 1 to 3 days, or until the concrete will bear pressure of the thumb without indentation.
Thin walls, in summer, 2 days; in cold weather, 5 days.
Slabs up to 6-ft. span, in summer, 6 days; in cold weather, 2 weeks.
Beams and girders and long span slabs, in summer, 10 days or 2 weeks; in cold weather, 3 weeks to 1 month. If shores are left without disturbing them, the time of removal of the sheeting in summer may be reduced to 1 week.
Column forms, in summer, 2 days; in cold weather, 4 days, provided girders are shored to prevent appreciable weight reaching columns.
Conduits, 2 or 3 days, provided there is not a heavy fill upon them.
Arches, of small size, 1 week; for large arches with heavy dead load, 1 month.
The method of removing forms will vary in detail with the character of the structure. With proper design and lubrication of forms they will ordinarily come away from the concrete with a moderate amount of sledge and bar work. If the work will warrant it, have a special gang under a competent foreman for removing forms. The organization of this gang and the procedure it should follow will vary with the nature of the form work, and they are considered in succeeding chapters for each kind of work.
ESTIMATING AND COST OF FORM WORK.—It is common practice to record the cost of forms in cents per cubic yard of concrete, giving separately the cost of lumber and labor. This should be done, but the process of analysis should be carried further. The records should be so kept as to show the first cost per 1,000 ft. B. M. of lumber, the number of times the lumber is used, the labor cost of framing, the labor cost of erecting and the labor cost of taking down, all expressed in M. ft. B. M. In this way only is it possible to compare the cost of forms on different kinds of concrete work, and thus only can accurate predictions be made of the cost of forms for concrete work having dimensions differing from work previously done. It is well, also, to make a note of the number of square feet of exposed concrete surface to which the forms are applied.
Some of the items mentioned demand brief explanation. Framing and erecting costs are kept separate for the reason that the framing is done only once, whereas the erecting occurs two or more times. The lumber cost, where the material is used more than once, can be computed in two ways. An example will illustrate the two modes of procedure. In one of the buildings described in Chapter XIX the lumber cost $30 per M. ft. B. M. and was used three times. As 34,000 ft. B. M. were required to encase the 200 cu. yds. of concrete in one floor, including columns, it would have required 34,000 ÷ 200 = 170 ft. B. M. of lumber at $30 per M. per cubic yard of concrete if it had been used only once. But since it was used three times we may call it 170 ft. B. M. at $10 per M. per cubic yard of concrete, or we may call it 170 ÷ 3 = 57 ft. B. M. at $30 per M. per cubic yard of concrete. The authors prefer the first method, due to the fact that it is 170 ft. B. M. that is handled and taken down each time and it is more consistent to have the lumber cost on the same basis thus: