Glass-fiber reinforced concrete, commonly known as GFRC, is a composite concrete product fabricated by many precast concrete manufacturers. It consists of a portland-cement-based composite that is reinforced with alkali-resistant glass fibers, which are randomly dispersed through the material. The fibers serve as reinforcement to enhance the concrete’s flexural, tensile and impact strength.
The major benefit provided by GFRC is its light weight, which provides substantial economy resulting from reduced costs of product handling, transportation and erection. It also can lower seismic loads, requiring a lighter support system than typical precast concrete components would require. By casting it into a mold, GFRC also can create highly detailed, ornamental pieces.
The manufacture of GFRC products requires a greater degree of craftsmanship than other precast concrete products. Many combinations of shapes, sizes, colors and textures are demanded of this product. Typically, the fibers in a GFRC component make up at least 4 percent of the total weight, with a minimum thickness of 1/2 inch for the piece. The fibers are specially designed for use in these components, and no others should be used.
With GFRC, any change in face-mix materials or proportions will affect the surface appearance. If the face-mix surface is progressively removed by sandblasting, retarders or other means, the color becomes increasingly dependent on the fine and coarse aggregates. A change in aggregate proportions, color or gradation will affect the uniformity of the finish, particularly where the aggregate is exposed.
Different cements have different color characteristics that affect the desired GFRC face mix. The cement color exerts a considerable influence on the color of the finished product.
Normal production variables, such as changes in water content, curing, cycles, temperature, humidity and exposure to climatic conditions at varying strength levels, all will cause color variations. Color variations also will be greater in a gray-cement matrix than in white-cement matrices. If a gray color is desired, consideration should be given to using white cement with a black pigment or a blend of white and gray cement.
Rib formers may be used to produce supports that provide structural rigidity and stiffness for the GFRC panels. Expanded polystyrene foam and polyurethane foam are the most common materials used. Tests should be run to determine the allowable loads for any insert molded into the GFRC skin. Many inserts used in GFRC panels have been designed and tested by the insert manufacturer.
GFRC components often are created by spraying the material into a mold to create a lightweight but highly detailed decorative piece. The appearance of the finished panel surface in these pieces is directly related to the choice of mold material and the quality of the mold. The in-service life of a mold also is a function of the mold materials.
Molds can be made of a range of materials, including plywood, concrete, steel, plastics, polyester resins reinforced with glass fibers and GFRC. For complicated details, molds of plaster, rubber, foam plastic or sculptured sand may be used.
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