CHAPTER XVIII FORMS FOR CONCRETE 1. Principal Requirements. All forms should be constructed true to LINE and GRADE; they must also be sufficiently strong and rigid to support without sagging or bulging, the weight, or pressure, of the plastic concrete, and the additional loads incident to construction. Furthermore, forms should be sufficiently tight to avoid any appreciable leakage of mortar. As the cosT OF FORMS is always a very large item in concrete buildings, the profit or loss upon the entire job may depend upon the degree of skill employed in their construction. The dimensions of the component parts must be chosen to carry safely a liberal estimate of the imposed loads, and economy is gained by SIMPLICITY and UNIFORMITY of construction rather than by skimping the design. The size of wallpanels, and other individual pieces of form-work, vary with conditions and the means available for placing. In general, there should be as FEW UNITS as practicable, and as FEW DIFFERENT PIECES in each unit, but the individual sections of form-work should never be larger, or heavier, than can be economically handled with the means available. The construction should also be as light as consistent with strength. All joints should be arranged so as to be either horizontal or vertical when the forms are in place. STOCK SIZES and lengths of lumber are used whenever practicable, planning the work so that panels, and similar units, are a number of WHOLE BOARDS in width. Finally, it must be constantly borne in mind that FACILITY IN STRIPPING IS A MOST VITAL CONSIDERATION, and the procedure to be followed in this operation governs the simplest details of construction. 2. Form Lumber. Any GOOD, SOUND LUMBER, free from KNOTS, SHAKES or DECAY, can be used for the construction of forms. SPRUCE, because of its general availability, and the fact that it is reliable, and stands up well under alternate wetting and drying, has been probably as widely used in the past as any other material. At present, in the Eastern States, SHORT-LEAF-PINE is being substituted as a cheaper material, although it is not quite as satisfactory. HEMLOCK can be employed if better wood is unavailable, but it is not to be recommended, as it is unreliable when used as posts, and warps when exposed to wet concrete. For floor sheathing and wall panels, tongue-and-groove North Carolina ROOFERS are generally used. Lumber should be AIR-DRIED rather than kiln-dried, as the latter is more likely to absorb water and cause the forms to bulge. Green lumber is difficult to work and likely to SHRINK and CHECK. All faces of lumber placed in contact with concrete should be planed at least one side and two edges, except where fillets are used, in order that the forms may be TRUE and TIGHT. In order to |