FORM CONSTRUCTION A Field for Invention. Concrete, being a plastic material and requiring time to set, must needs be confined, during the chemical process of setting, within the bounds and in the shape and form the completed structure or member is to possess. The form or temporary support that so holds the material during this setting process, must be rigid. The materials commonly used are wood and iron. In building construction, the use of wooden forms may be said to have been hitherto the almost universal practice, though several types of metal forms have been invented and are now on the market.
There is a great field for research and inven tive genius in the production of cheap, durable forms sufficiently flexible for many different uses, as the cost of this part of concrete work is at present one of the greatest handicaps in the way of a reduction of cost and a more general use of concrete construction.
The writer has used metal forms to some ex tent, and has observed their use by contractors. If sheet metal is placed on a wooden back or on a metal stiffening frame, there is danger of its becoming dented, bent or otherwise defaced so as to give an imperfect surface to the concrete; and if the metal covering is sufficiently thick and strong to resist damage, it is likely to be too heavy and expensive for general use. Moreover, such forms are not flexible so that they can be used for various purposes. Wood has thus far, therefore, proved to be the most economical and flexible in the way of being changed from one use to another of anything which the writer has seen, and his study has been concentrated on the most effective and economical use of lumber. When a sheet metal form becomes dented, it is usually cheaper to throw the covering away and start new than to straighten and re-use that which is bent.
The cost of construction as a whole is as likely to be governed by the cost of the forms as the cost of the concrete. For illustration, in the cost of columns, although they are made so as to be reduced in size as easily as possible, this is somewhat expensive in labor; and after they are reduced, the girders and beams which meet at a column are too short and have to be spliced out, which adds to the cost, and these costs are likely to exceed that which can be saved in concrete. It is therefore more economical to run columns one size through the full height of a low build ing, or to reduce the size only twice, or at most three times, in the height of a high building.
For instance, the actual cost of labor (without regard to wastage of lumber) in reducing col umns from 16 inches to 12 inches on a certain job amounted to $5.70, whereas the cost of the concrete saved by the reduction was only $2.30. In the writer's office a set of plans in pamphlet form has been compiled for standard forms for all kinds of work; and where special framing is required, plans are made for these special cases. But in spite of this care and study, the above fig ures represent actual experience. Therefore, in designing and handling form work, the cost of labor is the principal item to be considered.
The opinion was expressed by Mr. Larned that the forms could be cheapened by the use of common and rough lumber instead of a fairly good grade of dressed lumber. The fact that it is almost universal to use a good grade of dressed stock would seem to disprove the above state ment. The cost of dressing varies, according to the mill, from $1.50 to $4.00 per thousand feet for planing four sides. The price of second grade hemlock and spruce differs but little from that which has small, sound knots and is free from wind-shakes or large season cracks; and such lumber works so much easier as to cut down the labor cost more than the equivalent of the difference in cost of material.
Some builders use stock; others, inch; and some, 2 inches thick. The thicker lum ber will stand the and tear longer than the light, and can therefore be used so many more times than the thin that it is more economical in the long run if the work in hand is large enough so that the forms can be used several times with out delaying the rapid progress of the work. With planed stock, tighter joints can be obtained between boards, which prevents leakage of the fine materials and weakening and roughening of the surface; and the boards are of even thickness, so that a fairly good surface is obtained which needs little treatment after the forms are removed, except where an ornamental appearance is especially desired. If rough lumber is used, in order to get even a passable surface finish, considerable labor must be spent upon dressing the con crete after the forms are removed, and this must be done by mechanics.