In recent years, forms fabricated from glass-reinforced plastic have found increasing use because of their strength, light weight, and high number of reuses. Glass-reinforced plastic also produces high-quality concrete finishes. Glass-reinforced plastic forms are very flexible and can form complex or nonstandard shapes with little capital investment.
To fabricate glass-reinforced plastic forms, models of plaster, wood, or steel are prepared to the exact desired dimensions. The model is then waxed, polished, and sprayed with a parting agent to prevent sticking of the resin to the master pattern. Glass mat is then fitted over the model and thoroughly saturated with a brush
coat of polyester resin. When the resin has set and the heat dissipated, another layer of glass mat and polyester resin is added, and this process is repeated until the desired thickness of the fiberglass sheet is achieved.
Another method to build glass-reinforced plastic forms is through the use of a spray gun to apply the resin to chopped strands of fiberglass, which are used as the reinforcing material.
To increase the number of potential reuses with any of the methods of fabrication mentioned, an extra thickness of resin is molded into the contact surface or additional stiffening and supports are added by means of built-up ribs, wood struts, steel rods, or aluminum tubing.
The two major problems associated with glass-reinforced plastic forms are attack by alkalies in the concrete and form expansion because of exposure to hot sun or heat from hydration of cement.