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Glass Fiber Reinforced Concrete

How to use GFRC for better decorative panels and countertops

GFRC can be used to create durable and exquisitely detailed ornamental concrete. NEG America

When someone says fiberglass, we think of insulation or boats or Corvettes, but maybe we should think of concrete. Technically, fiberglass is simply very fine glass fibers. The material used to make boats or other products, although called fiberglass, is really glass fiber reinforced plastic—glass fibers in a polymer matrix. If, instead of the polymer, we use portland cement and sand, the resulting material is glass fiber reinforced concrete--GFRC or sometimes GRC (the Brits call it glassfibre reinforced concrete).

The problem with using glass fibers as reinforcement for concrete is that glass breaks down in an alkaline environment--and there's almost nothing more alkaline than concrete. You may have heard of concrete being damaged by alkali-silica reactivity (ASR) when there is reactive silica in the aggregate. Glass is primarily silica. The original GFRC in the 1940s rapidly lost strength as the glass was destroyed by the alkaline environment. In the 1970s alkali-resistant (AR) glass fibers were perfected by Owens-Corning and by Nippon Electric Glass (NEG) leading to a rapid increase in applications.

GFRC has been used for the past 30 years to produce many concrete products, especially thin architectural cladding panels, but also for ornamental concrete such as domes, statues, planters, and fountains. Recently, decorative concrete artisans have discovered the benefits of GFRC for decorative panels (such as fireplace surrounds), concrete countertops, and artificial rock work.

Countertops with integral sinks remain crack free when made with GFRC. Concast Studios in Oceano, CA

Artificial rocks made with GFRC look real at a fraction of the weight. Innovative Rock & Water

Benefits of GFRC

There are lots of good reasons to use GFRC for thin sections of concrete:

• Lighter weight: With GFRC, concrete can be cast in thinner sections and is therefore as much as 75% lighter than similar pieces cast with traditional concrete. According to Jeff Girard's blog post titled, The Benefits of Using a GFRC Mix for Countertops, a concrete countertop can be 1-inch thick with GFRC rather than 2 inches thick when using conventional steel reinforcement . An artificial rock made with GFRC will weigh a small fraction of what a real rock of similar proportions would weigh, allowing for lighter foundations and reduced shipping cost.

Large artificial rocks made with GFRC are lighter allowing rock
features where real rock would be impossible. NEG America

• High strength: GFRC can have flexural strength as high as 4000 psi and it has a very high strength-to-weight ratio.
• Reinforcement: Since GFRC is reinforced internally, there is no need for other kinds of reinforcement, which can be difficult to place into complex shapes.
• Consolidation: For sprayed GFRC, no vibration is needed. For poured, GFRC, vibration or rollers are easy to use to achieve consolidation.
• Equipment: Expensive equipment is not needed for poured or vibrated GFRC with a face coat; for sprayed GFRC, equipment generally costs about $10,000.
• Toughness: GFRC doesn't crack easily—it can be cut without chipping.
• Surface finish: Because it is sprayed on, the surface has no bugholes or voids.

Smooth surfaces are easily achieved using the two-coat GFRC process. Concast Studios in Oceano, CA

• Adaptability: Sprayed or poured into a mold, GFRC can adapt to nearly any complex shape, from rocks to fine ornamental details.
• Durability: According to ACI 544.1R-96, State of the Art Report on Fiber Reinforced Concrete, "The strength of fully-aged GFRC composites will decrease to about 40 percent of the initial strength prior to aging." Michael Driver, division manager with Nippon Electric Glass, a major manufacturer of AR glass fibers, disagrees. "There's never a durability issue. Water can't get in—there are no cracks—and that's a durable material. GFRC will outlast precast concrete, cast stone, even some natural stone." Durability has been increased through the use of low alkaline cements and pozzolans.
• Sustainable: Because it uses less cement than equivalent concrete and also often uses significant quantities of recycled materials (as a pozzolan), GFRC qualifies as sustainable.

GFRC ornamental concrete is lightweight
because it is hollow. NEGAmerica

• Cost: GFRC as a material, however, is much more expensive than conventional concrete on a pound-for-pound basis. But since the cross sections can be so much thinner, that cost is overcome in most decorative elements. "When you keep the thickness to about ¾ inch, the material cost is typically less than $2.00/square foot," said Driver. "Because of the high modulus of elasticity of the glass, it replaces all of the steel, but once you get into 4-inch slabs, the GFRC becomes cost prohibitive."
• "GFRC hasn't caught on like it could because of the mix design," said Driver. "When you have so much cement, your chemistry changes and you have a lot of variables to control. Many regular concrete guys have problems and end up disgruntled. It takes some time to become proficient with GFRC. There's a lot to know, a lot of variables. Training is key."

Glass Fibers for GFRC

With the original E-glass fibers, durability was a big issue since the glass broke down and lost strength. The glass fibers used in GFRC since the 1970s are alkali-resistant glass and the durability issue has mostly gone away. A few things you should know about AR glass fibers:

Fibers are disbursed throughout the concrete mix. meldUSA in Raleigh, NC

Chopped fibers are only ½ inch long. Rich Fibers & Systems

• There are several manufacturers of AR glass fiber, including NEG America, Nycon, Rich Fibers & Systems, and Owens Corning. Owens Corning recently bought Saint-Gobain's Vetrotex glass fiber business (Cem-FIL) and has transferred sales and marketing of its concrete-reinforcing fibers (including AR glass fibers) to Continental Marketing's Rich Fibers & Systems.
• The alkali resistance of AR glass fibers is a result of adding zirconia (zirconium oxide) to the glass—the best fibers have zirconia contents of 19% or higher.
• The fibers used for countertops, fireplace surrounds, and other decorative applications are high integrity (meaning the strands don't break down into individual filaments) and are usually ½-inch (13-mm) long or a combination of 13, 19, and 25 mm.
• "A strand will contain anywhere from 50 to 200 filaments," said Michael Driver, Division Manager with Nippon Electric Glass America. "That's what gives you the pseudo-ductility. Since you haven't locked in every filament you get fiber pull-out and that's what provides the ductility. The tensile strength of the glass fiber is higher than that of steel and the modulus of elasticity is 3 times that of concrete so that when you put stress into the concrete system the glass absorbs the energy and won't allow it to crack. Polypropylene fiber is great for reducing plastic shrinkage cracking but it can't stop tensile stresses in the hardened concrete--it can't stop cracking. With glass you don't really get any elongation—if it does fail it's more of a brittle failure. The yield strength and the ultimate strength are basically the same. Some people see that as bad but it means you simply design to stay within the yield strength."

A fiber roving in a spool must be chopped by a chop gun
as the GFRC is mixed and applied. Rich Fibers & Systems

• AR-glass fibers are also available as roving, which is a spool of a continuous length of multiple strands of glass fiber twisted together (typically 28 strands in a roving with 200 filaments per strand). In regular spray-up GFRC, this roving is fed into a chopper gun which cuts the fibers to a specific length as they are mixed with the concrete as it is sprayed.
• Glass fiber is also available as a scrim, which is a fiber fabric. This is placed into areas that might have a tendency to crack. "Our scrim isn't woven," said Driver, "the fibers are laid on top of each other and glued together using an organic substance."
• Another fiberglass reinforcement for concrete and mortar is lath—SpiderLath makes AR glass lath that can be used as a base coat for stone veneer or stucco or as reinforcement in concrete countertops.

Glass lath can't corrode and has higher strength than steel. Spiderlath

• Chomarat North America (which makes C-Grid, a carbon fiber reinforcement grid that is used in countertops) also makes a fiberglass grid reinforcement they call MeC-GRID, which is bonded with an epoxy resin.
• Glass fibers, according to Nycon, "pose no health hazards, since fibers with diameters greater than 3 microns cannot be inhaled." AR glass fibers are typically 13 or 14 microns in diameter.
• Nycon's Bob Cruso recommends a blend of fibers, using glass fibers in combination with polyvinyl alcohol or nylon fibers to control plastic shrinkage cracking.

Mixes and Materials for GFRC

Traditional spray-up GFRC is a low water-cement ratio mix. Most decorative GFRC products, other than artificial rocks, are made with a two-layer process with a very thin (1/8 to 3/16 inch) face coat and a thicker backing layer.

• Sand and cement are typically used at a ratio of about 1 to 1, although some mix designs call for slightly higher cementitious materials content (see "GRFC Mix Design," Concrete Décor, June/July 2008).
• With its high cement content and low water-cement ratio (0.33 to 0.38), GFRC can dry out quickly and not gain full strength. Traditionally, GFRC panels were cured in a moist-room for 7 days. Today, more commonly, this is overcome by using an acrylic polymer additive which serves as a curing compound to prevent the mix water from evaporating. The acrylic is typically in liquid form. NEG America's Mike Driver recommends using 5% acrylic solids by weight of cement, which he says will result in the same strength you would get from a 7-day wet cure.

GFRC can be sprayed directly onto the mold with
the proper equipment. NEG America

• The acrylic also gives you concrete that gains strength rapidly. GFRC panels and countertops are ready for use within 3 days. Mike Wellman, Concast Studios in Oceana, Calif., uses 30% liquid acrylic emulsion and 70% water in his mixes.
• The fibers are added to the mix at about 2% to 3% for premixed GFRC or 4% to 6% by weight for spray-up mixes.
• Many GFRC experts will also use silica fume, metakaolin, or other pozzolans in their mix. This reduces the permeability of the concrete, making it more water-resistant and also reduces the alkalinity of the concrete, which means it doesn't affect the glass—both of these factors mean increased concrete durability. Vitro Materials makes a pozzolan material they call VCAS 160 (for vitrified calcium alumino silicate—more on VCAS. VCAS 160 is made primarily from waste E-glass, making it a "green" material, since it replaces cement with an industrial byproduct. Vitro Materials has shown that VCAS 160 (formerly called VCAS Micron HS) has 10% lower water demand than silica fume or metakaolin, can be used at cement replacement levels up to 30%, and is white in color. Research sponsored by NEG America demonstrates that the ideal replacement rate is 25% of the total cementitious materials—at that level strength gain is not delayed and all ASR is controlled.

Typical GFRC Mix (Premixed)

Chopped AR glass fibers—2 to 3% by weight for premixed; 4% to 6% for spray-up Acrylic polymer emulsion—5% acrylic solids by weight of cement Type I or II cement Sand:cement equals approximately 1:1 Pozzolan (VCAS) at 10 to 25% cement replacement Admixtures: superplasticizer (high-range water reducer, such as a polycarboxylate) for face coat and pourable (self-consolidating) back coat Color—dry or liquid in face coat

• With a two-coat system, the face-coat mix contains no fibers that would be visible when polished but does contain the integral color, so you only have to pay to color a small amount of concrete. Often a superplasticizer is added to this mix.
• The backing layer contains the glass fibers but no color. This layer is what provides the strength.
• The backing layer may also contain a high-range water reducer (superplasticizer), if it is to be poured into place. For sections that need to hold a vertical shape, such as sinks or drop edges in countertops, no plasticizer is used in order to keep the mix stiff.
• Keeping the water-cement ratio and polymer content about the same in the face-coat mix and the backing layer is important so that the shrinkage characteristics of both layers are similar and you don't get curling.

This unique fireplace surround was created using GFRC.
Absolute ConcreteWorks

Manufacturing GFRC Pieces

There are three methods for making concrete elements using GFRC: traditional hand spray-up, vibration casting, and sprayed premix.

• The traditional, and perhaps still the best, way to manufacture precast GFRC elements is by hand spraying the GFRC into a mold. This is how most precast GFRC architectural cladding panels are made and also most ornamental precast GFRC. With the direct spray-up method, you need a concentric chopper gun, which is fed by a spool of GFRC roving pulled into the chopper gun and blended at the nozzle. This mix has a higher fiber content (4 to 6%) than can be achieved with premix and is the recommended method for larger panels. It does, however, require experienced workers, expensive equipment, and rigorous quality control.

Rock panels are created using spray-up GFRC. Eldorado Wall Co.

• Vibration casting uses premixed GFRC poured into a mold and vibrated to achieve consolidation. This is a much simpler method, but requires water-tight molds and doesn't work well with rock molds.
• Sprayed premixed GFRC, with chopped fibers in the mix, requires a peristaltic pump and a special spray head. This method requires less expertise than the hand spray-up method and results in higher strengths than with vibration casting.

Larger architectural elements are created by directly spraying the premixed GFRC into a mold. NEGAmerica

Most decorative GFRC pieces, especially countertops, or fireplace surrounds are made using a two-layer approach. The facing layer is the thin decorative layer and the backup layer is thicker and contains the glass fibers.

• The face coat is normally sprayed into the mold using a drywall hopper gun (see Jeff Girard's posting on spraying GFRC). This layer is about 1/8 to 3/16 inch thick.
• "One square foot of countertop requires only about 2 pounds of concrete mixture for the face coat," said Mike Wellman, Concast Studios, Oceana, Calif. "It's pretty thin so with my mixer I'm able to do a 200 square foot job--about the biggest kitchen there is. This allows me to do the whole thing with one batch to insure color consistency."

Countertops are best made using a two-coat process. Concast Studios in Oceano, CA

• "We let the face coat set to where it's moist but won't move—about ½ hour to 1 hour," said Wellman.
• The GFRC backer coat is then placed. Most decorative contractors either pour this layer or trowel it on by hand. The thickness of this layer is in the range of ¾ to 1 inch, depending on the size of the panel and the loads it will be carrying.
• The GFRC layer is typically placed in two layers of about 3/8 inch and compacted using rollers or a vibrating table.
• Mixers for GFRC need to provide a lot of shear at both low and high mixing speed—high for the low water-cement ratio concrete mix then low to prevent breakage when the glass fiber is added. Power-Sprays is a British company, represented in the U.S. by NEG America, that specializes in GFRC equipment. They make an excellent upright mixer. You can also use a handheld mixer, such as those from Collomix or even a mixer blade on an electric drill. "The limitation for most guys is having a mixer that can mix enough volume and has the power to mix fiberglass in well," said Wellman.

A handheld electric mixer works well for GFRC. Collomix

• With the polymer addition, GFRC sets fairly quickly. Depending on conditions, panels can be stripped and polished within 24 hours, although Wellman waits 3 days for the concrete to gain nearly its full strength

Making GFRC Panels Decorative

GFRC panels can be given nearly any decorative treatment as normal concrete. The application dictates what works best:

• Architectural panels are often cast using various form liners. The surface finish can be sand blasted, acid etched, or polished. Various tints of gray, white, and buff can be achieved using colored cements or pigments.
• Many GFRC ornamental pieces are shot or cast using white cement and light color tints. Stone or clay brick pieces can be embedded in panels, although consideration should be given to the differential shrinkage characteristics of the different materials. Many different architectural features are best produced using GFRC.

Ornamental architectural accents can be created with GFRC.
J&M Lifestyles in Randolph, NJ

• Countertops are typically made using a face coat and a solid integral color is often the method of choice. "We use integral color in the face coat," said Mike Wellman, Concast Studios, Oceana, Calif. which makes countertops and fireplace surrounds. "Sometimes we will do an acid stain but the majority of our clients stick with the straight integral color." Wellman typically polishes the countertop to a high gloss finish, but offers many varieties. Read more about Concast Studios' work.

GFRC countertops can be finished using virtually any decorative concrete techniques.
Absolute ConcreteWorks in Seattle, WA

• Countertops can be produced without the facing coat, although if polished the fibers will be visible. "Some of our customers like the fibers to show," said NEG America's Mike Wellman. "If it's acid etched or acid washed, they don't mind the fibers and they actually blend in with the color."
• With face coats, broadcasting aggregate or embedding decorative elements is a good choice. "Since I'm spraying in the initial face coat I'm able to broadcast aggregate in which lets me get flowing movement," said Wellman. "I can sprinkle in glass or seashells and when polished and exposed it gives the illusion of movement. With wet cast it's trickier to get that movement and make it look good."
• Rock features typically use GFRC panels that are sprayed against molds made using real rock features. Steve Holmes, vice president of Eldorado Wall Company, a Boulder, Colo. maker of rock climbing walls, says that the first coat they spray has no glass fiber. "The chop gun has mud-only and mud-and-glass triggers. The first thin coat has no fibers then we bring the thickness up to ¾-inch nominal with the GFRC mix."

Although the structure for this climbing wall has all of the appearance of real rock,
modular handholds are attached for climbing. Eldorado Wall Co.

• To create rocks, the GFRC panels are mounted on a structural steel framework. "The panels can be oriented in different directions," said Eldorado Wall's president John McGowan, "then we plaster the seams and sculpt them to blend the panels into a rock feature." To create the patches, said Holmes "we place lath and rebar into the seams then start with a scratch coat then apply the sculpt coat. This is done with a field mix based on a shotcrete recipe." Coloring the rocks is done with a variety of techniques Eldorado has developed over the years.
• Jim Jenkins of JPJ Technologies teaches artificial rock making. His method, however, does NOT use GFRC but rather a composite fiber-reinforced polymer concrete material that he invented and has perfected. "Our panels are ¼ to ½-inch thick," said Jenkins, "where a GFRC panel will be 1-1/2-inch thick. Our material can be cut easily with a circular saw and yet is stronger than GFRC. The seams between panels are patched with the same material used to make the panels so they behave, look, and stain the same." A sister company, Synthetic Rock Solutions, sells premanufactured rock panels that can be used to assemble rock features.

Artificial rocks require artistry in color application to obtain a realistic appearance.
Synthetic Rock Solutions in Amity, OR

• Coloring rock and water features entails a lot of artistry. Multiple colors and techniques are blended to produce realistic color, as described in "Geo-Illusions" in the December 2007/January 2008 issue of Concrete Décor.
• Ornamental GFRC fireplace surrounds have become very popular, due to their light weight and durability. Check out what Sierra Concrete Designs does with this application in the article Surrounding Fireplaces With Beautiful Decorative Concrete Work.

Fireplace surrounds are an ideal application for GFRC.
Sierra Concrete Designs

GFRC Resources

There's a ton of information available on GFRC and also some training and free technical advice:

• Nippon Electric Glass America will provide training and technical support. NEG America's Michael Driver recommends that manufacturers interested in producing GFRC products need to participate in hands-on training prior to any attempts at making this type of material. "Most GFRC mixes contain more cement than aggregate and typically include acrylic latex polymers for curing," he said. "Variables such as cement chemistry, aggregate gradation and shape, mix temperature, water chemistry, water reducing admixture type, AR glass fiber aspect ratio, fiber orientation, fiber content, and curing conditions are just a few that need to be considered. Without knowledge of these variables, the novice GFRC manufacturer can become discouraged after just a few failed attempts."
• The Precast/Prestressed Concrete Institute has a wealth of information on GFRC manufacturing in PCI MNL-128 Recommended Practice for Glass Fiber Reinforced Concrete and PCI MNL-130 Quality Control Manual for GFRC.
• ACI's state of the art report on fiber reinforced concrete, ACI 544.1R-96, is slightly old now (1996) but has a long chapter on GFRC.
• Stromberg Architectural Products, which manufactures architectural accents, has an extensive handbook on GFRC that is available for free download.
• The Glassfiber Reinforced Concrete Association, the GRCA, is a British-based but international association dedicated to advancing knowledge on GFRC. They have an excellent brochure on GFRC applications, called "GRC in Action," that is available for free download.