Introduction to GFRC (Glass Fiber Reinforced Concrete) and its Benefits
If you aren’t yet familiar with glass fiber reinforced concrete (GFRC), you should be. GFRC is a specialized form of concrete. It’s a cement-based composite material reinforced with alkali-resistant glass fibers.
The fibers serve a purpose similar to the reinforcing steel in reinforced concrete, and they also add flexural, tensile and impact strength. As a result, GFRC can be used to produce strong, lightweight architectural concrete products such as building panels.
It can also be used to create decorative concrete products such as façade wall panels, fireplace surrounds, vanity tops and concrete countertops due to its unique properties and tensile strength. Most concrete countertop professionals use GFRC as their technique of choice due to its versatility, strength and lighter weight.
One of the best ways to truly understand the benefits of GFRC is to take a deeper look into this unique compound.
What is GFRC?
GFRC is similar to chopped fiberglass (the kind used to form boat hulls and other complex three-dimensional shapes), although much weaker. It’s made by combining a mixture of fine sand, cement, polymer (usually an acrylic polymer), water, other admixtures and alkali-resistant (AR) glass fibers.
Some of the many benefits of GFRC include:
- Ability to Construct Lightweight Panels– Although the relative density is similar to concrete, GFRC panels can be much thinner than traditional concrete panels, making them lighter.
- High Compressive, Flexural and Tensile Strength– The high dose of glass fibers leads to high tensile strength while the high polymer content makes the concrete flexible and resistant to cracking. Proper reinforcing using scrim will further increase the strength of objects and is critical in projects where visible cracks are not tolerable.
GFRC is strong. Check out this video to see just how strong it can be:
The Fibers in GFRC- How They Work
The glass fibers used in GFRC help give this unique compound its strength. Alkali resistant fibers act as the principle tensile load carrying member while the polymer and concrete matrix binds the fibers together and helps transfer loads from one fiber to another.
Without fibers GFRC would not possess its strength and would be more prone to breakage and cracking. Understanding the complex fiber network in GFRC is a topic in and of itself. See this article for more in depth technical information about GFRC fibers.
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.
Many mix designs for GFRC are available online, but you’ll find that all share similarities in the ingredients and proportions used. Mix design isn’t a concept that can be tackled in one article, but read on for some of the basic components in a good mix. If you are simply looking for a GFRC mix calculator that does all the math for you, click here.
- 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%. Polymer dose is typically 6% solids by weight of cementitious material. Consider trying Forton VF-774, a reliable acrylic polymer choice.
- 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- Fiber content varies but is typically between 3% to 7% of the overall cementitious weight. Higher fiber content increases strength but decreases workability. Unlike most concrete mix design ingredients, fibers in GFRC are not calculated as a percentage of dry cementitious weight. Instead, they are calculated as a proportion of total weight. This makes the math to calculate fiber load in GFRC mix designs quite complicated.
- Other Admixtures– Some other elements you may choose to include in your mix include pozzolans (such as silica fume, metakaolin or VCAS) and superplasticizers.
As you can see, GFRC mix designs are quite complex and involve some confusing mathematical calculations. If you would like more detail about these calculations, please see this article. For a GFRC mix calculator that does all of the calculations for you, click here.
Commercial GFRC commonly uses two different methods for casting GFRC: spray up and premix. Let’s take a quick look at both as well as a more cost effective hybrid method.
The application process for Spray-up GFRC is very similar to shotcrete in that the fluid concrete mixture is sprayed into the forms. The process uses a specialized spray gun to apply the fluid concrete mixture and to cut and spray long glass fibers from a continuous spool at the same time. Spray-up creates very strong GFRC due to the high fiber load and long fiber length, but purchasing the equipment can be very expensive ($20,000 or more).
- 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).
Premix mixes shorter fibers into the fluid concrete mixture which is then poured into molds or sprayed. Spray guns for premix don’t need a fiber chopper, but they can still be very costly. Premix also tends to possess less strength than spray-up since the fibers and shorter and placed more randomly throughout the mix.
- 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.
One final option for creating GFRC is using a hybrid method that uses an inexpensive hopper gun to apply the face coat and a handpacked or poured backer mix. A thin face without fibers (called a mist coat or face coat) is sprayed into the molds and the backer mix is then packed in by hand or poured in much like ordinary concrete.
This is the method that most concrete countertop makers use.
This is an affordable way to get started. However, it is critical to carefully create both the face mix and backer mix to ensure similar consistency and makeup, and to know when to apply the backer coat so that it adheres properly to the thin mist coat but doesn’t tear it.
- Pros: Affordable way to get started. 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.
As a general rule, larger items, such as building cladding panels, are normally "sprayed" whereas small items are manufactured using a "premix" GRC method.
Sprayed GRC is generally stronger than premix vibration cast GRC. The reasons for this are firstly that with sprayed GRC it is possible to achieve a fibre content of 5% - 6% whereas premix GRC is limited to around 3% - 3.5%. Secondly, sprayed GRC usually has a lower water content than premix GRC.