Form Materials

Figure 7-26. Typical large footing form.

Engineering Aid 3 - Beginning Structural engineering guide book
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spaces enclosed by previously constructed FORMS. These forms are usually removed once the plastic concrete hardens into the shape outlined by the forms. Forms for concrete structures must be tight, rigid, and strong. If the forms are NOT tight, loss of water and paste may cause sand streaking as well as weakness to the concrete. The forms must be strong enough to resist the high pressure exerted by the concrete. Form Materials Undisturbed soil or clay, if sufficiently rigid and excavated to proper dimensions, maybe used as EARTH FORMS. Design, specifications, and construction methods, however, dictate what kind of form materials are to be used on certain structures. Wood, plywood, steel, fiber glass, and other approved materials are commonly used as form materials. Forms for concrete pavement and curves should be metal; surfaces exposed to view in the finished structure and those requiring special finishes should be wood, plywood, or other approved material. Figure 7-28.-Form for concrete column. Foundation Forms Foundation forms may include forms or parts of forms for column footings, pier footings, and wall footings. Whenever possible, the earth should be excavated and the hole used to contain the foundation of footing forms. In most cases, FOOTINGS are cast directly against the earth, and only the sides are molded in forms. In some cases where there is a firm natural earth surface that is capable of supporting and molding the concrete, parts of forms are often omitted. Figure 7-26 shows a typical large footing form. Figures 7-27 and 7-28 show typical footing forms for a concrete pier and a concrete column, respectively. Wall Forms Wall forms are made up of five basic parts. They are as follows: (1) sheathing, to shape and retain the concrete until it sets; (2) studs, to form a framework and support the sheathing; (3) wales, to keep the form aligned and support the studs; (4) braces, to hold the forms erect under lateral pressure; and (5) ties and spreaders or tie-spreader units, to hold the sides of the forms at the correct spacing (fig. 7-29). Wall forms may be built in place or pre- fabricated, depending on the shape and the desirability for reuse. Wall forms are usually reinforced against displacement by the use of TIES. Two types of Figure 7-29.-Parts of a typical wall form. 7-19

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