In the last two pieces about glass fiber reinforced concrete I’ve discussed the basics of glass fiber reinforced concrete and the importance of fibers. To finish off this series on GFRC I’d like to move into a few of the technical aspects you will encounter when creating GFRC countertops including mix designs, casting and processing.
GFRC Mix Designs
If you’ve worked much with concrete you know that finding the right mix can be difficult and often requires years of experience. Many different factors impact the ideal composition for concrete, and GFRC is no different. Mix design isn’t a concept that can be tackled in one blog post, but here are some of the basic components in a good GFRC mix:
- Fine Sand- Sand used in GFRC should have an average size passing a #50 sieve to #30 sieve (0.3 mm to 0.6mm). Finer sand tends to inhibit flowability while coarser material tends to run off of vertical sections and bounce back when being sprayed.
- Cement- Typical proportions use equal parts by weight of sand and cement.
- Polymer- Acrylic polymer is typically preferred over EVA or SBR polymers for GFRC. Acrylic is non-rewettable, so once it dries out it won’t soften or dissolve, nor will it yellow from exposure to sunlight. Most acrylic polymers used in GFRC have solids content ranging from 46% to over 50%. Consider trying Smooth-On’s duoMatrix-C and Forton’s VF-774, two reliable acrylic polymer choices.
- Water- Common water to cement ratios range from .3 to .35. When determining how much water to use make sure to take the water content from your acrylic polymer into account. This can make calculating water to cement ratios difficult unless the solids content of the polymer is known. With a polymer solids content of 46%, 15 lbs of polymer plus 23 lbs of water are added for every 100 lbs of cement.
- Alkali Resistant Glass Fibers- Fibers are an essential component of GFRC. If you’re using the spray-up method for casting the fibers will be cut and added to the mix automatically by your sprayer at the time of application. If you’re using premix or the hybrid method for casting you’ll mix the fibers in yourself. Fiber content varies but is typically between 5% to 7% of the overall cementitious weight. Higher fiber content increases strength but decreases workability.
- Other Admixtures- Some other elements you may choose to include in your GFRC mix include silica fume, metakaolin, VCAS and superplasticizers.
I talked a bit about casting in the Introduction to GFRC article. There are a few different methods you can use for casting. Since I’ve already discussed the basic concepts behind casting (if you need a refresher check out Introduction to GFRC) I’d like to take a minute to review the pros and cons associated with each casting method.
A concrete mixture is sprayed into the forms using a special device that chops and sprays a separate stream of long fibers. The concrete and fibers mix when they hit the form surface.
- Pros: Allows for very high fiber loads using long fibers resulting in greatest possible strength.
- Cons: Requires expensive, specialized equipment (generally $20,000 or more).
Glass fibers are mixed directly into the fluid concrete. The mixture is then poured or sprayed into molds.
- Pros: Less expensive than spray-up, although a special spray gun and pump is required.
- Cons: Fiber orientation is more random than when using spray-up and fibers are shorter resulting in less strength.
The hybrid method for casting GFRC uses an inexpensive hopper gun (the same kind used with drywall) to spray a thin face coat into the forms. Once the face coat dries the fiber loaded backer mix is applied either by pouring or hand packing, just like ordinary concrete.
- Pros: Affordable way to get started with GFRC. A hopper and air compressor run about $400-$500, much less than the spray guns used for spray-up or premix.
- Cons: Since the face coat and backer mix are applied at different times careful attention is needed to ensure the mixes have a similar makeup to prevent curling.
The high polymer content of GFRC often means that long term moist curing is unnecessary. Cover a freshly cast piece with plastic overnight, but as soon as it has gained enough strength it can be uncovered and processed. Many GFRC pieces are stripped 16 to 24 hours after casting.
Your skill level, the composition of your mix and the method used will determine how much processing is needed once your GFRC countertop is removed from its molds. Grouting may be needed to fill in bug holes or surface imperfections. Any blowback (sand and concrete that doesn’t stick to the forms) needs to be cleaned or the concrete’s surface will be open and granular. Achieving a perfect piece right out of the mold is very difficult and requires great skill.
Common Questions About GFRC
- How Thick is a Typical GFRC Countertop?- Typical concrete countertops made with GFRC range from ¾” to 1” in thickness. This is the minimum thickness that a long, flat countertop can be made so it doesn’t break when handled or transported. Smaller wall tiles can be much thinner.
- Is GFRC Green?- GFRC is roughly on par with other forms of concrete countertops in terms of the “green-ness”. In comparing 1.5” thick concrete countertops to ¾” GFRC countertops, the same amount of cement is used, since GFRC tends to use about twice as much cement as ordinary concrete. This sets them equal to each other. The use of polymers and the need to truck them does make GFRC less green than using ordinary water, which could be recycled from shop use. Both traditional cast and GFRC can use recycled aggregates, and steel reinforcing is more green than AR glass fibers, since steel is the most recycled material, so its use in concrete of all forms boosts the concrete’s green-ness.
I hope you enjoyed this three-part series on GFRC. Before you go check out this short 7 ½ minute video featuring excerpts from my Comprehensive GFRC Self Study Course. Watching an actual GFRC countertop being constructed will help you better understand many of the topics I’ve covered in this series.