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|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 in the US.
Commercially, GFRC is used to make large, lightweight panels, columns and moldings profiles that are often used as façades. A GFRC systems are
considered non-structural, in that they are designed to support their own weight. The GFRC systems are considered lightweight because of the thinness
of the material, not because GFRC concrete has a significantly lower density than normal concrete
|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
While the structural properties of GFRC itself are superior to unreinforced concrete, properly designed fiberglass re bar and fiberglass structural mesh
reinforcing will significantly increase the strength of objects cast with either ordinary concrete or GFRC.
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, columns, stone range hoods, molding profiles and other
similar elements. While the weight savings due to reduced thickness is maintained, the effort of forming, mixing the lay up GFRC system is more labor
|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. 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.
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.
|Random 3D fiber reinforcing
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 2D fibers reinforcing
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 resist those tensile loads.
Random 1D fibers reinforcing
The third level of reinforcement is one-dimensional (1D) 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 slab or beams 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. Typical
proportions are equal parts by weight of sand and cement, plus water, polymer, fibers and other admixtures to also include a water repellant.
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%.
|GFRC / Casting Method
|Commercial GFRC commonly uses two different methods for casting GFRC.
Typically Spray-up is applied in two or more layers. The first layer is the face coat, much like a gel-coat in fiberglass on a boat. 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. 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. Here at Coral Stone we have
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.
|The finish look of GFRC systems
|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.
GFRC is, after all another form of concrete. So acid staining, dying and integral pigmentation are all possible. Embedment, 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.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.
Goodlettsville, TN 615-587-1259