The steps and what materials you will need for your decorative GRP concrete project – how to work with fiberglass concrete

A vanity made with GFRC.

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Glass fiber reinforced concrete (GRP) has long been used in building facades and was originally developed for its superior performance in seismic zones. The decorative concrete industry embraced it not only for its strength, but also for its versatility for creating texture, color and shape that would otherwise be impossible with traditional precast.

The basic principles of GFRC are simple. Typical mix designs use a cement-rich mix that combines some cement with some graded, dry sand. With the addition of water, hardening polymer, and fiberglass for reinforcement, GFRC is typically cast to a thickness of 0.75 to 1 inch. Mixtures can be modified for spray applications or plasticized with a water reducer to create self-setting concrete.

Materials, simplified

Buddy Rhodes concrete productsAs one of the pioneers in the decorative concrete industry, Buddy Rhodes Concrete Products has developed user-friendly materials for the concrete professional. Here’s what you need for a typical GFRC project:

• A GFRC mix of cement, sand, dry curing polymer and admixtures. For example, the Buddy Rhodes GFRC Blended Mix already contains the right ingredients and high-performance additives for making thin, lightweight concrete or for any GFRC project.
• 3/4 ”long, chopped Alkaline Resistant (AR) glass fibers are added separately to provide critical flex and tensile strength.
• AR Glass Fiber Scrim is a fabric that can also be added for additional, targeted reinforcement.
• Super plasticizer / water reducer can be added as a wetting agent to reduce water content and improve final strength.

First formwork

Buddy Rhodes concrete productsThe formwork for GRP should not be porous and have watertight seams. Melamine-faced particle board or laminates are a common choice for large, flat surfaces. Flexible urethane or silicone rubber molds are also a good option, ideal for complex parts or production molds that can be poured into over and over again.

Casting: The spray technique with a hand-made backer mix

The GFRC spray technique is carried out in two steps: a surface layer (top / mist coating) without fibers, followed by a structural layer (carrier mix) containing AR glass fibers. No fibers are used in the topcoat mixture, as the finished surface should not have any fibers. Start by adding 1 gallon of water and 2 ounces of water reducer to a 17.5 gallon bucket. Slowly add a 50 pound. Bag of GFRC Blended Mix and mix thoroughly with a hand held mortar mixer with a high viscosity paddle. Let the mixture sit for 10 minutes until it is “wrongly set” and then mix again. If necessary, add a water reducer to get the right consistency.

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Pour the mixture into a drywall hopper gun for spraying. Your spray should come out of the gun like a wet splash. This first crucial layer creates the surface detail of the casting and should be about 1/8 of an inch thick. Use a bristle brush to brush the top coat in the form to force out any air pockets, especially in the corners and edges of the form, to consolidate the mixture.

Once the top coat is sprayed, start mixing the top coat. Timing is important – you don’t want the topcoat to be completely dry or the backercoat won’t stick properly. Ideally, the top layer mixture should begin to stiffen so that the top layer is not pushed through and there is still a good connection between the face and top layer. For the backer mix, mix more material just as you did for the face mix. Add 1.5 lbs towards the end of the mixing process. of AR fiber per 50 lbs. made of mixed concrete. Slowly blend in the AR fibers as you decrease the mixer speed. Mixing the fibers too vigorously will compromise their structural integrity. Apply this behind your face coat. Total thickness depends on the size of the project, but 1 inch is typical. You can make a liquid mixture that flows over the face coating, or you can make a thicker mixture that is applied by hand. A thicker mix is ​​especially useful for vertical walls.

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It is recommended that you trowel the back of the part, especially if it needs to rest on or butt against a flat surface. No perfection is required on the back of the part as the back is not visible. However, for installation it is important to have a flat, smooth surface. After all of the concrete is in place, cover with plastic wrap and allow to cure for at least 16 hours at room temperature (73 ° F / 23 ° C).

After the part is demolded, fill in any insect holes with a matching paint paste, buff them lightly with 200 grit diamond pads, and seal them before installing. The same GFRC mix mix can be used for more traditional surfaces such as polished and hard-trowelled in-situ concrete. It’s also a great starter mix design for wet casting mixes. Add up to 25 lbs. 3/8 ”aggregate or similar to increase yield and reduce shrinkage.

A vanity made with GFRC.Buddy Rhodes concrete products

Buddy Rhodes Concrete Products offers a diverse range of materials that go beyond the GFRC Blended Mix. Create ultra-thin high-performance concrete with our ECC Blended Mix. Try the classic Buddy Rhodes Signature Pressed finish with Craftsman Mix. Or create lightweight boulders, sculptures, and themed environments with Buddy Rhodes Vertical Mix. They also offer over 400 concrete pigments, topical glazes, sealers, and other accessory products for any artisanal concrete project. How-to videos, technical support, and more can be found on the website: www.buddyrhodes.com.