Mixes and Materials for GFRC

GFRC can be sprayed directly onto the mold with the proper equipment. NEG America

This unique fireplace surround was created using GFRC. Absolute ConcreteWorks

Traditional spray-up GFRC is a low water-cement ratio mix. Most decorative GFRC products, other than artificial rocks, are made with a two-layer process with a very thin (1/8 to 3/16 inch) face coat and a thicker backing layer.

• Sand and cement are typically used at a ratio of about 1 to 1, although some mix designs call for slightly higher cementitious materials content (see "GRFC Mix Design," Concrete Décor, June/July 2008).
• With its high cement content and low water-cement ratio (0.33 to 0.38), GFRC can dry out quickly and not gain full strength. Traditionally, GFRC panels were cured in a moist-room for 7 days. Today, more commonly, this is overcome by using an acrylic polymer additive which serves as a curing compound to prevent the mix water from evaporating. The acrylic is typically in liquid form. NEG America's Mike Driver recommends using 5% acrylic solids by weight of cement, which he says will result in the same strength you would get from a 7-day wet cure.
• The acrylic also gives you concrete that gains strength rapidly. GFRC panels and countertops are ready for use within 3 days. Mike Wellman, Concast Studios in Oceana, Calif., uses 30% liquid acrylic emulsion and 70% water in his mixes.
• The fibers are added to the mix at about 2% to 3% for premixed GFRC or 4% to 6% by weight for spray-up mixes.
• Many GFRC experts will also use silica fume, metakaolin, or other pozzolans in their mix. This reduces the permeability of the concrete, making it more water-resistant and also reduces the alkalinity of the concrete, which means it doesn't affect the glass-both of these factors mean increased concrete durability. Vitro Minerals makes a pozzolan material they call VCAS 160 (for vitrified calcium alumino silicate). VCAS 160 is made primarily from waste E-glass, making it a "green" material, since it replaces cement with an industrial byproduct. Vitro Materials has shown that VCAS 160 (formerly called VCAS Micron HS) has 10% lower water demand than silica fume or metakaolin, can be used at cement replacement levels up to 30%, and is white in color. Research sponsored by NEG America demonstrates that the ideal replacement rate is 25% of the total cementitious materials-at that level strength gain is not delayed and all ASR is controlled.

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• With a two-coat system, the face-coat mix contains no fibers that would be visible when polished but does contain the integral color, so you only have to pay to color a small amount of concrete. Often a superplasticizer is added to this mix.
• The backing layer contains the glass fibers but no color. This layer is what provides the strength.
• The backing layer may also contain a high-range water reducer (superplasticizer), if it is to be poured into place. For sections that need to hold a vertical shape, such as sinks or drop edges in countertops, no plasticizer is used in order to keep the mix stiff.
• Keeping the water-cement ratio and polymer content about the same in the face-coat mix and the backing layer is important so that the shrinkage characteristics of both layers are similar and you don't get curling.

Find Manufacturers: GFRC Mixes

Find Local Suppliers: Decorative Concrete Stores

Clean Cast GFRC Mix Available in white or gray, in 50lb bags

Hopper Gun Large capacity hopper holds 2.5 gallons

Hand Held Power Mixer Professional grade, single paddle mixer.