Mixes and Materials for GFRCMixes and Materials for GFRC
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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Typical GFRC Mix (Premixed)
- Chopped AR glass fibers-2 to 3% by weight for premixed; 4% to 6% for spray-up
- Acrylic polymer emulsion-5% acrylic solids by weight of cement
- Type I or II cement
- Sand:cement equals approximately 1:1
- Pozzolan (VCAS) at 10 to 25% cement replacement
- Admixtures: superplasticizer (high-range water reducer, such as a polycarboxylate) for face coat and pourable (self-consolidating) back coat
- Color-dry or liquid in face coat
- 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.
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