There are three methods for making concrete elements using GRC: traditional hand spray-up, vibration casting, and sprayed premix.
The traditional, and perhaps still the best, way to manufacture precast GRC elements is by hand spraying the GRC into a mold. This is how most precast GRC architectural cladding panels are made and also most ornamental precast GRC. With the direct spray-up method, you need a concentric chopper gun, which is fed by a spool of GRC roving pulled into the chopper gun and blended at the nozzle. This mix has a higher fiber content (4 to 6%) than can be achieved with premix and is the recommended method for larger panels. It does, however, require experienced workers, expensive equipment, and rigorous quality control.
Vibration casting uses premixed GRC poured into a mold and vibrated to achieve consolidation. This is a much simpler method, but requires water-tight molds and doesn't work well with rock molds.
Sprayed premixed GRC, with chopped fibers in the mix, requires a peristaltic pump and a special spray head. This method requires less expertise than the hand spray-up method and results in higher strengths than with vibration casting.
Most decorative GRC pieces, especially countertops, or fireplace surrounds are made using a two-layer approach. The facing layer is the thin decorative layer and the backup layer is thicker and contains the glass fibers.
The face coat is normally sprayed into the mold using a drywall hopper gun. This layer is about 1/8 to 3/16 inch thick.
"One square foot of countertop requires only about 2 pounds of concrete mixture for the face coat," said Mike Wellman, Concast Studios, Oceana, Calif. "It's pretty thin so with my mixer I'm able to do a 200 square foot job--about the biggest kitchen there is. This allows me to do the whole thing with one batch to insure color consistency."
"We let the face coat set to where it's moist but won't move-about ½ hour to 1 hour," said Wellman.
The GRC backer coat is then placed. Most decorative contractors either pour this layer or trowel it on by hand. The thickness of this layer is in the range of ¾ to 1 inch, depending on the size of the panel and the loads it will be carrying.
The GRC layer is typically placed in two layers of about 3/8 inch and compacted using rollers or a vibrating table.
Mixers for GRC need to provide a lot of shear at both low and high mixing speed-high for the low water-cement ratio concrete mix then low to prevent breakage when the glass fiber is added. Power-Sprays is a British company, represented in the U.S. by NEG America, that specializes in GRC equipment. They make an excellent upright mixer. You can also use a handheld mixer, such as those from Collomix or even a mixer blade on an electric drill. "The limitation for most guys is having a mixer that can mix enough volume and has the power to mix fiberglass in well," said Wellman.
With the polymer addition, GRC sets fairly quickly. Depending on conditions, panels can be stripped and polished within 24 hours, although Wellman waits 3 days for the concrete to gain nearly its full strength
