concrete . Glass fiber concretes Glass Fiber Reinforced Concrete (GFRC) is a type of fiber reinforced concrete. are mainly used in exterior building façade panels and as architectural precast concrete. Contents [hide hide]]
1 Composition o
1.1 Laminates
2 History 3 Properties o
3.1 Sandwich panels
4 External links
[edit edit]]Composition Glass fiber reinforced concrete(GFRC) consists of high strength glass fiber embedded in a cementitious matrix matrix.. In this form, both fibers and matrix retain their physical and chemical identities, while offering a synergism synergism:: a combination of properties that can not be achieved with either of the components com ponents acting alone. In general, fibers are the principal load-carrying members, while the surrounding matrix keeps them in the desired locations l ocations and orientation, acting as a load transfer medium between them, and protects them from environmental environmentald damage. In fact, the fibers provide reinforcement for the m atrix and other useful functions in fiber-reinforced composite materials. Glass fibers can be incorporated into a matrix either in continuous lengths or in discontinuous (chopped) lengths. [edit edit]]Laminates A widely used application for fiber-reinforced concrete is structural laminate laminate,, obtained by adhering and consolidating thin layers of fibers and m atrix into the desired thickness. The fiber orientation in each layer as well as the stacking sequence of various layers can be controlled to generate a wide range of physical and mechanical properties for the composite laminate. However, GFRC cast without steel framing is commonly used for purely decorative applications suc h as window trims, decorative collum ns, exterior friezes, or limestone like wall panels. [edit edit]]History The potential for using a glass fiber reinforced concrete system was recognized by Russians in the 1940s. The early work on glass fiber reinforced concrete went through major modifications over the next few decades [edit edit]]Properties The design of GFRC panels proceeds from a knowledge of its basic properties under tensile tensile,, compressive, bending and shear forces, coupled with estimates of behavior under secondary loading effects such as creep, thermal and moisture movement. m ovement.
There are a number differences between structural metal m etal and fiber-reinforced composites. For example, metals in general exhibit yielding and plastic deformation whereas most fiber-reinforced composites are elastic in their tensile stress-strain characteristics. However, the dissimilar nature of these m aterials provides mechanisms for high-energy absorption on a microscopic scale comparable to the yielding process. Depending on the type and severity of external loads, a composite laminate m ay exhibit gradual deterioration in properties but usually would not fail in catastrophic manner. Mechanisms of damage development and growth in m etal and composite structure are also quite different. Other im portant characteristics of many fiber-reinforced composites are their non -corroding behavior, high damping capacity and low coefficients of thermal expansion. Glass fiber reinforced concrete architectural panels have general appearance of pre-cast concrete panels, but are different in several significant ways. For example, GFRC panels will, on the average, weigh substantially less than pre-cast concrete panels due to their reduced thickness. The low weight of GFRC panels decrease superimposed loads on the b uilding’s structural components. The building frame becomes more economical. [edit edit]]Sandwich
panels
A sandwich panel is a composite of three or more materials bonded together to form a structural panel. It takes advantage of the shear strength of a low density core material and the high compressive and tensile strengths of the GFRC facing to obtain high strength to weight ratios. The theory of sandwich panels and functions of the individual components may be described by making an analogy to an I-beam -beam.. Core in a sandwich panel is comparable to the web of an I-beam, which supports the flanges and allows them to act as a unit. The web of the I-beam and the core of the sandwich panels carry the beam shear stresses. The core in a sandwich panel differs from the web of an I-beam in that it maintains m aintains a continuous support for the facings facings,, allowing the facings to be worked up to or above their yield strength without crimping crim ping or buckling. Obviously, the bonds between the core and facings must be capable of transmitting shear loads between these two components thus making the entire structure an integral unit. The load carrying capacity of a sandwich panel can be increased dramatically by introducing light steel framing.. The light steel stud framing will be similar to conventional steel stud framing for walls, except, framing that the frame is encased in a concrete product. Here, sides of the steel frame are covered with two or more layers of GFRC, depending on the type and magnitude of external loads. The strong and rigid GFRC provides full lateral support on both sides of the studs, preventing studs from twisting and buckling b uckling laterally. The resulting panel is light li ght weight in comparison with traditionally reinforced concrete, yet is strong and durable and can be easily handled.
Introduction to GFRC (Glass (G lass Fiber Reinforced Concrete) Published by Jeffgirard on 2008/6/10 (58182 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 mi x 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.
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.
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.
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 mi x 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. CemFil’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 wil l 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 i nto 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 sprayup 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 the face coat
Face coat ready for backer mix
Hand packing backer on upright
SCC backer in bottom
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 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 c oncrete 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.
Glass Fiber Reinforced Concrete How to use GFRC for better decorative panels and countertops by Bill Palmer, ConcreteNetwork.com Columnist
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 m aterial 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 O wens-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 m any 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 internall y, 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 crack s-and that's a durable material. GFRC will outlast precast concrete, cast stone, even s ome 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 po zzolan), 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, i ncluding 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 applica tions 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 real ly 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 moistroom 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 m ix. 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 Minerals 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 fum e 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)
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 (selfconsolidating) 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. Man y differentarchitectural 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 m any 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 m ovement. 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 r ock, 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 m ethod, 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 articleSurrounding Fireplaces With Beautiful Decorative Concrete Work .
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 cur ing 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.
Looking for a material that is lighter in weight and durable? Glass fiber reinforced concrete (GFRC) delivers unparalleled design flexibility along with a limitless variety of colors, textures and shapes. Glass fiber reinforced concrete (GFRC) is gaining popularity with concrete contractors and countertop fabricators, who are being presented architectural plans with custom designs. Traditional fabrication materials limit how quickly they can complete their stage of the project. In today’s market, extended labor hours eat up what profit margins are available, making it difficult to keep their business profitable. With Glass fiber reinforced concrete (GFRC), designers are given the opportunity to maximize the architects imagination by utilizing the light weight, high early strength, sprayable attributes that are inherent to the Glass fiber reinforced concrete (GFRC) process.
Why is Glass fiber reinforced concrete (GFRC) selected over other building materials?
Glass fiber reinforced concrete (GFRC) offers unparalleled versatility in applications such as: interior and exterior wall panels, shower walls, concrete countertops, facades, lightweight architectural trim, balusters, fireplaces and many other precast opportunities. If you have experienced using traditional Glass fiber reinforced concrete (GFRC) formulas before, you realize that there is a learning curve in the precise measuring and spraying of the Face Mix.
What is the startup cost for Glass fiber reinforced concrete (GFRC)?
Typical Glass fiber reinforced concrete (GFRC) formulas have a high start up cost due to buying minimum quantities on all its components, most contractors’ average $1,500-$3,000 in startup Glass fiber reinforced concrete (GFRC) material costs. Through relentless research and testing, the Xtreme Series technology by SureCrete, developed a hybrid Glass fiber reinforced concrete (GFRC) formula that has addressed the learning curve of GFRC and reduced the mixing inconsistencies by creating it as a two component mix with a start up cost around $250.
What are the design restrictions for Glass fiber reinforced concrete (GFRC)?
The Xtreme Series Glass fiber reinforced concrete (GFRC) mix design is a blank white canvas wit h the ability to take on any color scheme, texture or shape you can imagine! The SureCrete Design team has 7 reproducible texture finishes that our taught in the introductory PreCast Concrete 4-Day training course. These textures can be combined, used individually and altered with many different coloring agents for an unlimited color and texture combination. For the first time concrete is a manufacturable fabrication material with consistent, r eproducible looks and finishes. Here are just three of the Xtreme Series finishes:
What makes it a hybrid Glass fiber reinforced concrete (GFRC) system?
Typical Glass fiber reinforced concrete (GFRC) formulas are made up of cement, sand, polymers, plasticizers, acrylics, and high amounts of glass AR fiber. The Xtreme Series technology uses cement, sand along with 3 types of fiber (PVA, Glass AR, and Nano). There are NO polymers and acrylics in its formula. Why is this a unique attribute? GFRC fabricators now have a larger temperature swing during the mixing and spraying stage of 45-85 degrees Fahrenheit, compared to typical 70-75 degrees with traditional GFRC formulas. The second component to this mix is the Precast Modifier, this product does lose its potency even if it freezes during delivery of material, reducing waste and loss of time for the fabricator. The three fibers used are unique because they are preblended into the 50 lb bag mix. When you take advantage of this technology you quickly realize how it accelerates the learning curve of how to mix GFRC. The Glass AR, PVA and nano fibers are already blended into the 50 lb bag, reducing errors during both mixing and application. The Xtreme Series material exceeds the performance of typical GFRC formulas with its impressive 10,500 PSI compressive strength and 1,450 PSI flexural strength. With all these features combined, GFRC fabricators achieve demold times between 8-12 hours after spraying.
Glass fiber reinforced concrete (GFRC) was first introduced to the building industry in the early 1970’s in the United Kingdom. Today, it is one of the most popular and innovative building materials used throughout the United States, Europe, Middle East and Asia.
What is GFRC?
It is a composite of Portland cement, fine aggregate, water, acrylic co-polymer, glass fiber reinforcement and additives. The glass fibers reinforce the 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. There are two basic processes used to fabricate GFRC – the Spray-Up process and the Premix process. The Premix proc ess is further broken down into various production techniques such as spray premix, cast premix, pultrusion and hand lay-up.
How is it used? GFRC is primarily used as an exterior façade or cladding material for both new construction and for recladding or restoration of existing building facades. For these applications, the Spray-Up process is generally used and the GFRC “skin” is panelized on a steel stud frame and weighs 20-25 lbs per square foot. Due to its extreme flexibility in
design and function, it is also used extensively in applications not requiring a steel stud frame and is generally produced using one of the Premix proc esses. These applications include architectural ornamentation (column covers, cornices, window and door surrounds, etc), terra cotta restoration and re placement, fireplace surrounds, concrete countertops, faux rocks and planters. Without the frame, GFRC will weigh 7 -10 lbs per square foot.
