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Introduction to GFRC (Glass Fiber Reinforced Concrete) and its Benefits
August 8, 2022
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.
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 stands for Glass Fiber Reinforced Concrete. It is a composite comprised of Portland cement, fine aggregate, water, acrylic co-polymer, alkali resistant glass fiber chopped strand reinforcement and additives. In many international markets, it is referred to as GRC – Glassfibre Reinforced Concrete. The glass fibers strands for concrete, much as steel reinforcing does in conventional concrete. The glass fiber reinforcement results in a product with much higher flexural and tensile strengths than normal concrete, allowing its use in thin-wall casting applications. GFRC is a lightweight, durable material that can be cast into nearly unlimited shapes, colors and textures. 
Durability was poor with the original type of glass fibers since the alkalinity of cement reacts with its silica. In the 1970s alkali-resistant glass fibers were commercialized. Alkali resistance is achieved by adding zirconia to the glass. The higher the zirconia content the better the resistance to alkali attack. AR glass fiber roving should have a Zirconia content of more than 16% to be in compliance with internationally recognized specifications (EN, ASTM, PCI, GRCA, etc).
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.
Casting GFRC

There are two basic processes used to fabricate GFRC – The Spray-Up process and Premix process.

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 roving 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 chopped strand 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.

GFRC 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.
GFRC 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 is very difficult and requires great skill.
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. This sets them equal to each other. The use of fiberglass for 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. 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.
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