Concrete, as most people know, is strong under compression but weak under tension, and is commonly strengthened by casting it around, e.g. a grid of steel reinforcing bar (“re-bar”). Glass Fiber Reinforced Concrete (GFRC) is pretty much exactly what it sounds like: concrete reinforced with glass fibers. As in most composite materials, the fiber elements in GFRC can be carefully oriented, or randomly distributed, in the solid matrix. The nice thing about the latter method is that you can just mix the reinforcing fibers into the bulk concrete and don’t have to pre-position them in the mold. GFRC concrete panels can be much thinner and lighter than metal-reinforced slabs, and the glass fibers are not subject to corrosion. It is typically cast in a thin section of around 1/2" to 3/4" thick.
GFRC is a strong molded product with great moisture resistance. The most sought-after value-add of GFRC is its strength. When embedded into a concrete matrix, glass fibers act as load-carrying members; concrete itself is inherently strong, so the synergistic combination of the two naturally creates a sturdy exterior and weather resistant material. This characteristic is why glass fiber reinforced concrete is so weather-resistant and long-lasting. While also being prepared to paint once set up.
Compared to traditional concrete, GFRC is exceptionally lightweight. Depending on its makeup, glass fiber reinforced concrete can weigh as much as 75% less than traditional concrete. This makes it a versatile material for use in commercial applications and also reduces end-cost. Lighter materials are easier to ship, install, and maintain.
When most people think of concrete they don’t think of highly-detailed aesthetic applications. But in fact, GFRC is actually a perfect material for creating small-scale decorative finishes. As a surrogate for plaster, stonework, and millwork, glass fiber reinforced concrete is today used in everything from fireplace surrounds to statues to artificial rockwork.
How It's Made
The glass fibers used in GFRC help give this unique compound its strength. Alkali resistant fibers act as the principle tensile load carrying member while the polymer and concrete matrix binds the fibers together and helps transfer loads from one fiber to another. Without fibers GFRC would not possess its strength and would be more prone to breakage and cracking.
Commercial GFRC commonly uses two different methods for casting GFRC: spray up and premix. Let’s take a quick look at both as well as a more cost effective hybrid method.
Glassfibre Reinforced Concrete (GRC) is generally manufactured by either the "sprayed" process or the "premix" process. Premix GRC can either be vibration compacted, or manufactured using a self compacting GRC mix. The method chosen is normally dictated by factors such as strength requirements, size of mold, architects specification etc.
As a general rule, larger items, such as building cladding panels, are normally "sprayed" whereas small items are manufactured using a "premix" GRC method.
The application process for sprayed GFRC is very similar to shortcrete in that the fluid concrete mixture is sprayed into the forms. The process uses a specialized spray gun to apply the fluid concrete mixture and to cut and spray long glass fibers from a continuous spool at the same time. Sprayed GFRC creates very strong product due to the high fiber load and long fiber length, but purchasing the equipment can be very expensive ($20,000 or more).
Sprayed GRC is generally stronger than premix vibration cast GRC. The reasons for this are firstly that with sprayed GRC it is possible to achieve a fibre content of 5% - 6% whereas premix GRC is limited to around 3% - 3.5%. Secondly, sprayed GRC usually has a lower water content than premix GRC.
The water and admixture (and polymer if used) are placed in a "high shear mixer" and the sand/cement are slowly added until a smooth creamy slurry is achieved. The consistency of the slurry can be checked using a simple slump test kit. Mixing time is about 1 - 2 minutes.
When ready the mix is transferred to a "pump/spray unit". The pump conveys the slurry at a regulated rate of flow to the spray gun. At the spray gun fibre, in the form of a roving, is chopped to a length of approximately 32mm and added to the slurry. The two materials are projected onto the mold surface using an air supply from a compressor.