FORMS FOR CONCRETE.
}reshly mixed concrete is plastic until the process of setting begins, and hence must be confined within the limits set for the finished structure until the mass hardens. This is usually accom plished by the use of wooden moulds or forms which are built so that their inside dimensions are exactly the shape of the finished structure In this article it is proposed to describe the more important principles involved in designing and constructing forms for the most simple concrete structure; but the more complicated problems encountered in the construction of forms for the more elaborate concrete structures will be considered in connection with the discussion of the structures themselves—particularly reinforced-concrete buildings, see Art. 3, Chapter VIII.
The forms should be designed at the same time that the outlines of the structure are fixed, and instructions and sketches should be prepared for the guidance of the carpenter who is to build the forms, in order that they may be sufficiently strong and not needlessly extravagant, and also that the carpenter may not waste time in the field in studying the masonry plan. The forms should be designed with a view to saving time and material in taking down as well as to economy of material and time in the construction and the erection. ORDINARY CONSTRUCTION. The forms are usually made by setting up studdings and nailing horizontal planks to them. The forms must be built strong enough to hold the plastic mass in place during tamping and hardening; and should be tight enough to prevent serious leakage of the more fluid portion of the concrete. The forms should have a smooth even inside surface so as to give a smooth finish to the completed structure. If the forms spring out of place, the concrete may flow out and be wasted; and at best any springing of the forms will injure the appearance of the surface of the finished structure.
In concrete buildings, where the cost of the forms is a large part of the entire cost of the structure, and where a failure of the forms may cause serious loss of property and possible loss of life, it is of the utmost importance to carefully study every detail of the design and construction of the forms to secure safety and prevent extrava gance; but in the simpler concrete structures where the concrete is usually deposited in layers, it is not possible to secure great accuracy in the design of the forms. Either 1-inch or 2-inch plank may be
used, and the studding may vary from 2- by 4-inch to 4- by 6-inch, depending upon their distance apart, the height of the forms, and the amount of bracing. Experience seems to prove that with 1-inch plank the studding may be 2 feet apart, with 1F-inch plank 31 or 4 feet, and with 2-inch plank 5 feet.
Inclined Braces. The studding may be braced by inclined braces whose lower ends are nailed to or abut against a stake and whose upper ends are nailed to the studding or abut against a block nailed to the studding—see Fig. 17. It is important that the stake, or better the post, be substantial enough to give the required resist ance; and for that reason, unless the ground is quite firm, it is wise to drive a second stake behind the first one and brace the top of the inside stake from the bottom of the outside one, somewhat as shown in Fig. 17. For convenience in setting the braces, it is wise to place, say, a 2- by 6-inch scantling against a row of stakes for the braces to abut against and insert a folding wedge between the foot of the brace and the scantling, as at E in Fig. 17. The upper end of the brace is sometimes simply nailed against the side of the vertical posts as shown at C and D, Fig. 17; but this is wise only when the brace is a 1- by 6-inch plank. When the brace is a 2- by 4-inch scantling, the upper end is sometimes beveled and toe-nailed against the outside of the vertical post, as shown at B; but the most substantial construction is to nail a block on the post and cut the brace to a bevel so it will fit against the post and also against the block, as shown in A.