MANUFACTURING GFRC PIECES
Larger architectural elements are created by directly spraying the premixed GFRC into a mold. NEGAmerica
There are three methods for making concrete elements using GFRC: traditional hand spray-up, vibration casting, and sprayed premix.
- The traditional, and perhaps still the best, way to manufacture precast GFRC elements is by hand spraying the GFRC into a mold. This is how most precast GFRC architectural cladding panels are made and also most ornamental precast GFRC. With the direct spray-up method, you need a concentric chopper gun, which is fed by a spool of GFRC 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 GFRC 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 GFRC, 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 GFRC 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 GFRC 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 GFRC layer is typically placed in two layers of about 3/8 inch and compacted using rollers or a vibrating table.
- Mixers for GFRC 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 GFRC 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, GFRC 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
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