- 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 6.5 lbs. water (about 3 quarts) to a clean 5 gallon bucket.
- Add 2.5 lbs. of Polyplex Curing Aid.
- Add 1 Fl. Ounce of SP 7000 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).
- 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.
- Material Cost:
- At 3/4" thickness: @$3.75 per sq. ft.
- At 1" thickness: @$5.00 per sq. ft.
- *Costs based on pricing for the following quantities:
- 5 Gallons Polyplex - $139.09
- 5 Gallons Riteks SP 7000 - $179.09
- 50 lbs. Sand - $4.00
- 94 lbs. Portland Cement - $11.40
- 44 lbs. Chopped AR 3/4" Fibers - $144.09
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.