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FORMS The material most commonly used for edge forms is 2-by lumber. The lumber must be occasionally replaced, but the steel or aluminum angles and charnels may be reused many times. The tops of the forms must be in the same plane so that they maybe used for screeds. They must also be well braced to remain in good alignment. Edge forms should have holes in them for rebar or for expansion/contraction dowels to protrude. These holes should be 1/4 inch larger in diameter than the bars. At times, the forms are spliced at the line of these bars to make removal easier. The forms, or rough bucks, for doors, windows, air-conditioning ducts, and so forth, are set before the steel is placed and should be on the same plane as the edge forms. BOND-BREAKING AGENTS Bond-breaking agents are one of the most important items of precast concrete construction. The most important requirement is that they must break the bond between the casting surface and the cast panel. Bond-breaking agents must also be economical, fast drying, easily applied, easily removed, or leave a paintable surface on the cast panel, if desired. They are broken into two general types: sheet materials and liquids. There are many commercially available bond-breaking agents available. You should obtain the type best suited for the project and follow the manufacturer's application instructions. If commercial bond-breaking agents are not available, several alternatives can be used. Paper and felt effectively prevent a bond with a casting surface, but usually stick to the cast panels and may cause asphalt stains on the concrete. When oiled, plywood, fiberboard, and metal effectively prevent a bond and can be used many times. The initial cost, however, is high and joint marks are left on the cast panels. Canvas gives a very pleasing texture and is used where cast panels are lifted at an early stage. It should be either dusted with cement or sprinkled with water just before placing the concrete. Oil gives good results when properly used, but is expensive. The casting slab must be dry when the oil is applied, and the oil must be allowed to absorb before the concrete is placed. Oil should not be used if the surface is to be painted, and crankcase oil should never be used. Waxes, such as spirit wax (paraffin) and ordinary floor wax, give good-to-excellent results. One mixture that may be used is 5 pounds of paraffin mixed with 1 1/2 gallons of light oil or kerosene. The oil must be heated to dissolve the paraffin. Liquid soap requires special care to ensure that an excess amount is not used or the surface of the cast panel will be sandy. Materials should be applied after the side forms are in place and the casting slab is clean but before any reinforcing steel is placed. To ensure proper adhesion of the concrete, keep all bond-breaking materials off the reinforcing steel. |
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