What is GFRC?
GFRC, or Glass Fiber Reinforced Concrete, is a versatile, high-performance concrete mix that incorporates alkali-resistant glass fibers to produce a material with exceptional strength and reduced weight. Widely used in architectural applications, GFRC concrete is an ideal choice for countertops, wall panels, and custom concrete surfaces. By combining concrete with glass fibers, GFRC offers unique benefits over traditional concrete, from added durability to design flexibility.
Here are just a few of the amazing applications of GFRC:
Why Choose GFRC Concrete?
Here are some standout benefits of the preferred choice for projects requiring lightweight yet robust materials:
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Design Flexibility
GFRC concrete can be molded into a variety of shapes, textures, and finishes, allowing designers and fabricators to create unique surfaces that align with aesthetic goals without sacrificing durability.
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Lightweight Composition
Compared to traditional concrete, GFRC is up to 75% lighter. This makes GFRC concrete ideal for large, intricate designs where weight is a concern, such as countertops, sculptures, and architectural panels.
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Durability in Diverse Environments
GFRC withstands environmental stress, including moisture, temperature changes, and UV exposure, making it an ideal option for indoor and outdoor applications alike.
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Enhanced Flexural and Tensile Strength
The glass fibers embedded in GFRC create a durable matrix that provides superior flexural and tensile strength, reducing the likelihood of cracks and breakage over time.
Key Components of GFRC: What’s in the Mix?
Understanding the ingredients in GFRC is essential for creating a high-quality final product. A typical GFRC mix includes the following components:
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High-Quality Cement and Fine Sand
A well-balanced mix includes Portland cement and fine sand, often at a 1:1 ratio, to ensure a smooth texture. Fine sand (#50-#30 sieve) is ideal for consistency and surface finish. Click here for more information about the best sand for GFRC.
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Acrylic Polymer
Add polymers to enhance flexibility and reduce shrinkage. Acrylic polymers are especially beneficial as they offer UV stability, resist yellowing, and provide excellent bonding properties. Click here for more information about polymer.
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AR Glass Fibers
Use alkali-resistant glass fibers to reinforce the concrete matrix, making it stronger and more resilient to cracking and environmental wear. These fibers are crucial for maintaining the structural integrity of GFRC. Click here for more information about AR glass fibers.
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Water and Superplasticizers
As with all concrete, measure water precisely, maintaining a low w/c ratio of 0.30 to 0.35. Use superplasticizers to improve flowability without compromising strength, helping create thin sections and complex shapes with ease. The workability of the GFRC mix can be adjusted with the addition of water reducers. These agents help modify the consistency of the mix, making it suitable for various applications without sacrificing its integrity. By incorporating water reducers, you can achieve the desired workability while maintaining the essential properties of the mix. Click here for more information about superplasticizers in concrete countertop mixes.
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Other Admixtures
You may include pozzolans (such as silica fume, metakaolin or VCAS) as cement replacements. Click here for more information about the use of pozzolans in concrete countertop mix design.
Calculating the actual GFRC mix proportions can be complex, involving intricate mathematical calculations. If you would like more detail about these calculations, please see this article. For a powerful and versatile GFRC mix calculator that does all of the calculations for you, click here.
If you just want to get a mix that is already proportioned, you can purchase GFRC Kits that include the mix, the fibers, and the superplasticizer you will need.
How GFRC Concrete is Made
The production process involves unique premix or spray techniques to ensure even distribution of glass fibers, resulting in a high-quality, uniform finish.
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Spray-Up Method
Spray-Up GFRC is applied similarly to shotcrete, with the concrete mixture sprayed into the forms. The process uses a specialized spray gun with a fiber chopper 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, long fiber length and correct fiber orientation. However, the equipment can be very expensive ($20,000 or more).
Correct fiber orientation in spray-up GFRC
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Premix (“Direct Cast”) Method
In this method, glass fibers are mixed directly with the concrete. The fluid concrete mixture is then poured or sprayed into molds. This approach works well for smaller projects or pieces requiring thick layers of GFRC concrete. Premix is weaker than Spray-Up because the the fibers are shorter and placed more randomly throughout the mix.
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Hybrid Method
A hybrid method for creating GFRC involves using a hopper gun to spray a thin, fiber-free face coat (mist coat) into the mold, followed by a hand-packed or poured backer mix with fibers.
After spraying the mist coat as shown, apply the fibrous backer coat by hand.
This is an affordable way to get started, because the required hopper gun is inexpensive. However, it is critical to carefully create both the face mix and backer mix to ensure similar consistency and makeup. You must also know when to apply the backer coat so that it adheres properly to the thin mist coat but doesn’t tear it. Click here for a video about how to tell when the mist coat is ready for backer.
Most concrete countertop makers today use either a hybrid method or a premix method, which they often refer to as “direct cast”. The biggest risk with the direct casting method for GFRC is that fibers show in the finished surface of the countertop or element. This is mitigated by using specialized AR glass fibers specifically designed for concrete countertops, not for building panels.
Curing
The high polymer content of GFRC means that long term moist curing is unnecessary. Cover a freshly cast piece with plastic overnight. This could be shorter if it has gained enough strength to be uncovered and processed. Many pieces are stripped 16 to 24 hours after casting.
Processing
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 requires skill and practice.
Common Questions about GFRC
- How thick is a typical GFRC concrete countertop?-For concrete countertops, this mix is typically made at 0.75” – 1” thick. However, when casting objects like tiles, the mix is often cast at ½” thick. This thinner casting requires careful consideration in the handling and support of the piece to prevent damage.By understanding these thickness variations, you can choose the right GFRC application for your project needs, ensuring both durability and aesthetic appeal.
- How does GFRC compare to traditional precast concrete countertops?– See this article for details.
- 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. This is because GFRC tends to use about twice as much cement as ordinary concrete. The use of polymers 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. Steel reinforcing is more green than AR glass fibers, since steel is the most recycled material. Therefore steel’s use in concrete of any form boosts the concrete’s green-ness.
Interesting Facts about GFRC
- GFRC was first created in the 1940s in Russia. It wasn’t until the 1970’s that the current form came into widespread use for building façades.
- GFRC tends to run about $2.50-$3.00 per square foot for ¾” thick material. The cost increases to about $3.50-$3.75 per square foot for 1” thick material when accounting for the prices of sand, cement, admixtures, fibers and polymer.
More GFRC Training
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Free Training
View our FREE, 2.5 hour seminar “Step by Step GFRC with Mix Design” by requesting access here.
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Online Video Training
We also offer 2-hour online video training “Professional GFRC for Concrete Countertops and More”. Watching an actual GFRC countertop being constructed will help you better understand many of the topics covered in this article.
Check out the 7 minute excerpt below. See what you can learn from Jeff in only 7 minutes – and imagine what you can learn in 2 hours! Click here to learn A LOT more in “Professional GFRC for Concrete Countertops and More”.