Bond Breaking Agents

Bond breaking agents are one of the most important items of precast concrete construction. The most important requirement is that they 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 bond breaking agents commercially available. 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.
• Plywood, fiberboard, and metal effectively prevent a bond when oiled and can be used many times. Their initial cost is high and they leave joint marks 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. Do no use oil if the surface is to be painted, and never use crankcase oil.
• 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 ½ gallons of light oil or kerosene. The oil must be heated to dissolve the paraffin.
• Liquid soap requires special care to ensure it is not used in excess amounts or the surface of the cast panel will be sandy.

Apply these materials after the side forms are in place and the casting slab is clean but before placing any reinforcing steel. To ensure proper adhesion of the concrete, keep all bond breaking materials off the reinforcing steel.