Deprecated: Assigning the return value of new by reference is deprecated in /home/cci/domains/concretecountertopinstitute.com/public_html/include/common.php on line 58 Introduction to GFRC (Glass Fiber Reinforced Concrete) - Tech Talk > GFRC - Articles : Concrete Countertops, Institute, Concrete Connections, Raleigh, NC : The Concrete Countertop Institute
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Introduction to GFRC (Glass Fiber Reinforced Concrete)

Published by Jeffgirard on 2008/6/10 (97968 reads)

by Jeffrey Girard, P.E., President of The Concrete Countertop Institute

The following is an article about what GFRC is, how it works, its properties and how it is made, including mix designs, casting techniques and finishing techniques.

According to Wikipedia.com, “[g]lass fiber reinforced composite materials consist of high strength glass fiber embedded in a cementitious matrix. In this form, both fibers and matrix retain their physical and chemical identities, yet they produce a combination of properties that can not be achieved with either of the components acting alone. In general fibers are the principal load-carrying members, while the surrounding matrix keeps them in the desired locations and orientation, acting as a load transfer medium between them, and protects them from environmental damage.”

GFRC is a form of concrete that uses fine sand, cement, polymer (usually an acrylic polymer), water, other admixtures and alkali-resistant (AR) glass fibers. Many mix designs are freely available on various websites, but all share similarities in ingredient proportions.

GFRC History and Application

GFRC was originally developed in the 1940’s in Russia, but it wasn’t until the 1970’s that the current form came into widespread use.

Commercially, GFRC is used to make large, lightweight panels that are often used as façades. These panels are considered non-structural, in that they are designed to support their own weight plus seismic and wind loadings, much in the way glass window curtain walls are designed. The panels are considered lightweight because of the thinness of the material, not because GFRC concrete has a significantly lower density than normal concrete. On average it weighs about the same as ordinary concrete on a volume basis.

Façade panels are normally bonded to a structural steel frame which supports the panel and provides connection points for hanging.

Structural Properties of GFRC

GFRC derives its strength from a high dosage of AR glass fibers and a high dosage of acrylic polymer. While compressive strength of GFRC can be quite high (due to low water to cement ratios and high cement contents), it is the very high flexural and tensile strengths that make it superior to ordinary concrete. Essentially the high dose of fibers carries the tensile loads and the high polymer content makes the concrete flexible without cracking.

GFRC is analogous to the kind of chopped fiberglass used to form objects like boat hulls and other complex three-dimensional shapes. The manufacturing process is similar, but GFRC is far weaker than fiberglass.

While the structural properties of GFRC itself are superior to unreinforced concrete, properly designed steel reinforcing will significantly increase the strength of objects cast with either ordinary concrete or GFRC. This is important when dependable strength is required, such as with cantilever overhangs, and other critical members where visible cracks are not tolerable.

GFRC does not replace reinforced concrete when true load carrying capacity is required. It’s best used for complex, three dimensional shells where loads are light. Applications where GFRC makes the most sense are fireplace surrounds, wall panels, vanity tops and other similar elements. GFRC’s advantage is minimized when ordinary flat countertop-shaped pieces are being made. While the weight savings due to reduced thickness is maintained, the effort of forming, mixing and casting are similar or the same.

How the fibers work

GFRC uses alkali resistant glass fibers as the principle tensile-load carrying member. The polymer and concrete matrix serves to bind the fibers together and transfer loads from one fiber to another via shear stresses through the matrix.

Fiber reinforcement in concrete is a topic that frequently causes confusion and misunderstanding. CCI has written articles on fiber reinforcement in ordinary concrete. However, the role of structural fibers and the importance of their dosage and orientation will be discussed here.

Fiber reinforcement is a common method to increase the mechanical properties of materials. It is an important topic that is taught to many engineers interested in material science. As mentioned before, fiberglass is perhaps the most common and widely recognized form of fiber reinforcement.

In order to resist tensile loads (and thus prevent the GFRC piece from breaking or cracking), there needs to be a sufficient amount of fiber present. Additionally, the orientation of the fiber determines how effective that fiber resists the load. Finally, the fiber needs to be stiff and strong enough to provide the necessary tensile strength. Glass fibers have long been the fiber of choice due to their physical properties and their relatively low cost.

Typical GFRC mix uses a high loading of glass fibers to provide sufficient material cross-sectional area to resist the anticipated tensile loads. Often a loading of 5% fiber by weight of cementitious material is used. This means that 100 lbs of GFRC mix includes 5 lbs of glass fibers.

Finally, the orientation of the fibers is important. The more random the orientation, the more fibers are needed to resist the load. That’s because on average, only a small fraction of randomly oriented fibers are oriented in the right direction.

There are three levels of reinforcement that are used in general concrete, including GFRC.
The first is random, three-dimensional (3D) reinforcing. This occurs when fibers are mixed into the concrete and the concrete is poured into forms. The fibers are distributed evenly throughout the concrete and point in all different directions. This describes ordinary concrete with fibers. Because of the random and 3D orientation, very few of the fibers actually are able to resist tensile loads that develop in a specific direction. This level of fiber reinforcing is very inefficient, requiring very high loads of fibers. Typically only about 15% of the fibers are oriented correctly.


                            GFRC fibers

Random 3D fibers

The second level is random, two-dimensional (2D) reinforcing. This is what is in spray-up GFRC. The fibers are oriented randomly within a thin plane. As the fibers are sprayed into the forms, they lay flat, conforming to the shape of the form. Typically 30% to 50% of the fibers are optimally oriented.

                               Fibers

This orients them in the plane that the tensile loads develop in. While more efficient than 3D, 2D reinforcing is still inefficient because of the highly variable fiber orientation within a horizontal plane. Additionally, most of the fibers lie outside the zone where the tensile loads are the greatest (which is the best location to place reinforcing so as to resist those tensile loads). As mentioned in other CCI articles on reinforcing, this zone is always at the bottom surface of a countertop (or at the top in the case of a cantilever). Structural engineers are very aware of this, which is why beams have their reinforcing near the bottom.

                                                      re-enforced Beam

The third level of reinforcement is one-dimensional (1D) reinforcing. This is how structural beams are designed using steel reinforcing. It is the most efficient form of reinforcing because it uses the least amount of material to resist the tensile loads. The reinforcing is placed entirely within the tensile zone, thereby maximizing the effectiveness without wasting reinforcing in areas that don’t generate tensile loads. The middle of a countertop slab is such a zone.

GFRC mix designs

GFRC is a form of concrete that uses fine sand, cement, polymer (usually an acrylic polymer), water, other admixtures and alkali-resistant (AR) glass fibers. Many mix designs are freely available on various websites, but all share similarities in ingredient proportions.
Typical proportions are equal parts by weight of sand and cement, plus water, polymer, fibers and other admixtures.

Fiber content varies, but is generally about 5% to 7% of the cementitious weight. Some mixes go up to 10% by weight of cement. Increased fiber content adds strength but decreases workability. Cem-Fil’s Anti-Crak HP 12mm AR glass fiber is commonly used in premix applications.

Common water to cement ratios used rang from 0.3 to 0.35. However, acrylic polymer is being added, so some of the mix water comes from the acrylic polymer. This makes accurate w/c ratio calculations 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.

Acrylic is the polymer of choice over EVA or SBR polymers. 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%. Two reliable acrylic polymers are Smooth-On’s duoMatrix-C and Forton’s VF-774.

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.

Pozzolans such as silica fume, metakaolin and VCAS can be used to improve the properties of GFRC. VCAS will improve workability, while metakaolin and especially silica fume will decrease workability due to their higher water demand. Typically VCAS is used at a 20% cement replacement level.

Superplasticizers are often used to increase fluidity. However very strong superplasticizers will make spraying vertical surfaces difficult since the material will not hang on the vertical surfaces.

GFRC casting method

Commercial GFRC commonly uses two different methods for casting GFRC. One is called spray-up, the other is called premix.

Spray-up

Spray-up is similar to shotcrete in that the fluid concrete mixture (minus fibers) is sprayed into the forms. The concrete is sprayed out of a gun-like nozzle that also chops and sprays a separate stream of long fibers. The concrete and fibers mix when they hit the form surface. Glass fiber is fed off of a spool in a continuous thread into the gun, where blades cut it just before it is sprayed. Chopped fiber lengths tend to be much longer (about 1.5”) than fibers that get mixed in, since long fibers would ball up if they were mixed into the concrete before spraying.

Typically Spray-up is applied in two layers. The first layer is the face coat, much like a gel-coat in fiberglass. This face coat usually has no fibers in it and is thin, often only about 1/8” thick. The second, or backer layer has the fiber in it. The action of spraying on the fibers orients them in a thin layer, much like the layers in plywood.

Spray-up permits very high fiber loading using very long fiber length. GFRC made using the spray-up method the greatest strength. However, the equipment required to do spray-up is very expensive, often costing more than $20,000.

Premix

Premix, on the other hand, involves mixing shorter fibers in lower doses into the fluid concrete. This mixture is either poured into molds or sprayed. While the spray guns used don’t have a fiber chopper, they are nonetheless costly and require a pump to feed them (the same pump used with spray-up). Premix tends to be less strong than spray-up due to the shorter fibers and more random fiber orientation.

GFRC used for concrete countertops in large shops tends to be made using the spray-up method. However, the high equipment cost puts this out of the reach of most people.

Hybrid

An alternative hybrid method uses an inexpensive hopper gun to spray the face coat. The fiber loaded backer mix is often poured or hand packed, just like ordinary concrete. Once the thin face mix is sprayed into the forms it is allowed to stiffen up before the backer mix is applied. This prevents the backer mix from being pushed through the thin face mix.

Hopper guns are often used to spray acoustic ceilings, cementitious overlays or other knock-down surfaces. They are inexpensive and run off of larger air compressors. A very effective combination of a hopper gun and a 60 gallon air compressor can cost as little as $400-$500.

The face mix and the backer mix are applied at different times, so the makeup and consistency can be different. It is always important to ensure the gross makeup is similar, and w/c ratios and polymer contents should be the same to prevent curling. However the heavy dose of fibers in the backer mix often precludes spraying, so hand placement or conventional pouring of an SCC version is required.

 Spraying Face Coat  

                    Spraying the face coat

 Face Coat  

             Face coat ready for backer mix

Packing Backer

          Hand packing backer on upright

SCC backer

                   SCC backer in bottom

Edge close-up

                          Edge closeup

GFRC thickness

Typical countertop thickness ranges from ¾” to 1” thick. This represents the minimum thickness that a long, flat countertop can be made so that it doesn’t break when handled or transported. Smaller wall tiles can be made much thinner.

Curing and stripping

Because of the high polymer content, long term moist curing is often unnecessary. It is important to cover the freshly cast piece with plastic overnight, but once the piece has gained enough strength, it can be uncovered and processed.

Generally GFRC pieces are stripped the next day, usually 16 and 24 hours after casting. Longer curing will always yield better concrete, but the general tendency is strip soon after casting.

Processing

GFRC, depending upon the mix, the spray method and the skill of the caster may or may not require grouting to fill bug holes or surface imperfections. Often the blowback (sand and concrete that doesn’t stick to the forms) collects in the corners of the formwork, and if it’s not cleaned out before getting covered the concrete’s finished surface will be open and granular.

Sand in Corner

                   Sand buildup in corner

 Surface variations from inconsistent spraying

Out of the mold, GFRC can have the wet cast look. While not impossible, reliably achieving a perfect out-of-the-mold piece requires extensive skill, experience and a lot of luck. Often the surface is honed, which eliminates many casting variations. GFRC in this case is indistinguishable from a honed sand-mix. Since air bubbles tend to get trapped in the mix, there usually are small pinholes that need to be grouted, just like regular concrete.

Looks

GFRC is, after all another form of concrete. So acid staining, dying and integral pigmentation are all possible. Embedments, decorative aggregates, veining and all other forms of decorative treatments are possible. GFRC can be etched, polished, sandblasted and stenciled. If you can imagine it, you can do it.

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.

Cost

When accounting for the prices of sand, cement, admixtures, fibers and polymer, 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.

Visit ConcreteNetwork.com for more information about GFRC.


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