- GFRC (Glass Fiber Reinforced Concrete) is lighter, stronger, faster than normal concrete. GFRC pieces can be de-molded as early as 24 hours.
- This Expressions-LTD Basic GFRC Mix recipe works great for GFRC countertops, sinks, and furniture pieces.
GFRC (Glass Fiber Reinforced Concrete) Mix: (by Expressions LTD)
- Equipment Needed: 5 Gallon Buckets. Heavy Duty Electric Mixer and Paddle Mixing Blade. Scale. Measuring Cup(s). Hopper Sprayer and Air Compressor. Compaction Roller(s).
- Add about 3 quarts to a clean 5 gallon bucket (will need to adjust based on variables that can't be predicted- ie. some sand can absorb more water than others, etc.)
- Add 2.5 lbs. of Acrylic Polymer Curing Aid.
- Add 1 Fl. Ounce of Riteks Super Plasticizer.
- Mix in 23.5 lbs. of #30-#60 Silica Sand (#30-60 is the sand size/mesh, and available at most local hardware stores or masonry supply). Make sure sand is washed (no dust).
- Mix in 23.5 lbs. Portland Cement** (white or gray, available at your local hardware store)
- ** You may wish to substitute 6 lbs. Vitro Minerals Pozzolans (VCAS) in place of 6 lbs. of Portland Cement)
- Optional: Mix in any Integral Color, if desired.
- Add extra water if needed, in small amounts, until mix is creamy.
- Let mix 'set' (usually about 5 minutes) and then remix briefly.
- Spray Face Coat of this mix over the mold/form work using a Hopper Sprayer. Face coat should be @ 3/16" thick. (Clean sprayer immediately after use) See below for tips on spraying.
- Allow face coat to sit long enough to stiffen, but remain moist (Time varies, usually 15-30 minutes).
- Backer mix uses the same formula as above, with addition of fiber:
- Slowly mix in 1.6 lbs. of 3/4" Length AR Glass Fibers.
- Hand lay the fiber-rich backer mix over the surface of the mold/form at 3/8" to 1/2" thickness.
- Use Compaction Rollers to even out this layer, condense, and remove any air pockets.
- Place a second backer layer, again 3/8" - 1/2" thickness, and compact with rollers.
- Finished piece should be 3/4" thick at a minimum, 1" thick is better.
- Extra Reinforcing: Fiberglass Scrim Fabric can be placed between GFRC Backer layers.
- Fiberglass Scrim will help increase overall strength and flexibility. Ideal for reinforcing around integral concrete sinks.
- Try to keep Scrim 1/2" away from the Face Coat, to avoid ghosting (faint lines showing an image of something buried in the concrete).
- A second layer of Scrim can be laid down over the final coat of GFRC, and lightly rolled down into the cement surface.
- Face Coat Batch: Covers approx. 24 sq. ft. @ 3/16" thickness.
- Backer Layer Batch: Covers approx. 12 sq. ft. @ 3/8" thickness.
- At 3/4" thickness: 7.5 lbs per sq. ft.
- At 1" thickness: 11 lbs per sq. ft.
What is the strength of this GFRC recipe? This question is a tough one because of different cements, sand, admixtures and shop variables across the land that can’t be pre-controlled. That said, please review the data from the PCI Manual 128 “Glass Fiber Reinforced Concrete Panels”. Appendix J- Premix GFRC Table J.1. Typical Range of Premix Properties.
- Density (dry) …………………………………………110 to 130 (pcf) (most generic mix designs @ 1:1 cement/sand ratio around the 127 pcf)
- Compressive Strength…………………………..6000 to 9000 (psi)
- Flexural Yield (first crack)………………………700 to 1200 (psi)
- Ultimate (Flexural Ultimate)……………….. 1450 to 2000 (psi)
Again, depending on the materials used, W/C ratio of the mix, we can assume the following….
- 3 days…….40%
- 7 days…….60%
- 14 day……90%
These are guides only…each producer should make test boards and have the mix evaluated.
- The material (ie. cement mix) to spray is placed into the hopper, and gravity will make it flow down the chute and into the cavity on the metal spray gun part.
- Pulling the trigger then allows air to flow through that chamber, expelling the material in there.
- New material has to be pushed down from the hopper (again by gravity only) as fast, or faster than it is being blown out of that chamber.
- Thus, low PSI is ideal for the sprayer. Typically around 25 PSI. That allows the mix to refill the spray chamber.
- Low PSI also ensures that the mix doesn't come out too fast. If it gets ejected too fast, it will shoot the sand hard off the mold surface, causing it to separate from the cement and ricochet (usually to pile up in a corner of the form).
- If the mix is too dry, it won't flow into the chamber. If the mix is too wet, it will make a mess when it sprays out, and will also increase chances for cracking on the concrete surface from excessive shrinking once that excess water evaporates.
- Testing on a board or piece of cardboard and fine tuning the sprayer setting and mix design will ensure an even, consistent face coat in the casting.
- Watch small air compressors closely, as they can quickly run out of air and not produce pressure as fast as it's being used. Small compressors will require waiting periodically for them to refill. If PSI drops too low, the mix will not spray correctly, or at all.