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How is GFRC made?



Introduction


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.

Sprayed


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.


Steps:


  1. 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.

  2. 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.

  3. The GRC material is sprayed and built up in thin layers until the required thickness is achieved - normally 10 - 15mm. Simple hand rollers are used to compact the material between layers.

  4. The product is left in the mould and covered with polythene to prevent moisture loss until the next day. The product is then demoulded.

  5. After demoulding the units are covered with polythene and allowed to cure for approximately 7 days. Alternatively, if a polymer curing compound is used in the mix the units can be exposed to the atmosphere immediately although it is advisable to keep them protected from direct sunlight or severe external conditions for a day or two. Reference should be made to the Polymer Supplier's instructions.


Premix


Premix mixes shorter fibers into the fluid concrete mixture which is then poured into molds or sprayed. Spray guns for premix don’t need a fiber chopper, but they can still be very costly. Premix also tends to possess less strength than spray-up since the fibers and shorter and placed more randomly throughout the mix.


Steps:

  1. The sand and cement are mixed dry and then the water/admixture and polymer (if used) are added. Generally a two speed slurry/fibre blender mixer is used. With this type of mixer, the fast speed is designed to create a smooth creamy slurry. This takes about 1 - 2 minutes. The mixer is then switched to slow speed and fibre in the form of chopped strand (length approximately 13mm) is added slowly. The fibre is blended into the mix for approximately 1 minute.

  2. Once the mix is ready, it is poured into moulds which are vibrated using a vibrating table.

  3. The product is left in the mould and covered with polythene to prevent moisture loss until the next day. The product is then demoulded.

  4. After demoulding the products are cured under polythene sheets to maintain moist conditions for approximately 7days. Alternatively a polymer curing compound can be used as described for the sprayed process.


Hybrid


One final option for creating GFRC is using a hybrid method that uses an inexpensive hopper gun to apply the face coat and a handpacked or poured backer mix. A thin face (without fibers) is sprayed into the molds and the backer mix is then packed in by hand or poured in much like ordinary concrete. This is an affordable way to get started, but it is critical to carefully create both the face mix and backer mix to ensure similar consistency and makeup. This is the method that most concrete countertop makers use.





Types of GFRC Products


Pediments and Lintels

Pediments & Lintels are designed to be used with columns in entry features and arbours. Lintels are horizontal beams that lie on top of two separate columns. They can then hold other structures up above the arch or entrance. Pediments are structures shaped like a triangular prism that lie on top of the lintel. Below are some examples of pediment and lintel combinations.

Quoins

Quoins are masonry blocks that are embedded in the corners of buildings. They provide more strength to the structure when it is made of weaker rock. It also makes the building look stronger and more stable as a whole. While quoins can be made of many materials when they are not needed structurally to keep the building up, they are normally made of stone or brick if they are being applied for physical strength and stability. In many applications, quoins are used primarily for their appearance and are finished so they are appealing to the eye.

Brackets

GFRC brackets are molds that look as if they are supporting the weight of the cornice, eave, or balcony. They are very decorative with a large amount of detail. When made from glass fiber reinforced concrete, it is not strong enough to actually hold up structures above it.

Cornice

The cornice is a decorative layer that lies above building structures or just below the ceiling indoors. It can lay between the top of the building and the roof or above windows. On the interior, it may be a design on the top of the surrounding walls of a room. Cornices can be produced in any shape with any design. They can range in styles from contemporary to temporary and range in finishes from stone to metal. The cornice also acts as a barrier for water on the roof to protect the building below from weather conditions.

Access Panels

Drywall access panels can be made from GFRC (instead of a more traditional GFRG) when placed in areas that are subject to high moisture or exterior conditions. Shop drywall access panels online ->

Custom Shapes and Sizes

The possibilities with molded products are endless. For curved surfaces or unique shapes, molded products can be a great application.See more molded products here ->

GFRC Facts


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

  2. GFRC tends to run about $12-22 per square foot for commercial applications (including shop drawings/submittals/etc.) when accounting for the prices of sand, cement, admixtures, fibers and polymer.

  3. Just like regular concrete, GFRC can accommodate a variety of artistic embellishments including acid staining, dying, integral pigmentation, decorative aggregates, veining and more. It can also be etched, polished, sandblasted and stenciled. If you can imagine it, you can do it, making GFRC a great option for creating three-dimensional concrete elements.

  4. GFRC tends to cost about 20% more than GFRG (glass-fiber reinforced gypsum).


Conclusion

The main benefits of GFRC are:Ability to Construct Lightweight Panels – Although the relative density is similar to concrete, GFRC panels can be much thinner than traditional concrete panels, making them lighter.High Compressive, Flexural and Tensile Strength – The high dose of glass fibers leads to high tensile strength while the high polymer content makes the concrete flexible and resistant to cracking. Proper reinforcing using scrim will further increase the strength of objects and is critical in projects where visible cracks are not tolerable. UP Ceilings can create custom GFRC molded products for your project. Contact us to learn more today.

GFRC LEED Information


GFRC Technical Information