The Basics of Glass Fiber Reinforced Concrete (GFRC)

Oct 16th 2023

The Basics of Glass Fiber Reinforced Concrete (GFRC)

The Basics of Glass Fiber Reinforced Concrete (GFRC)

Glass Fiber Reinforced Concrete (GFRC), also known as Glassfibre Reinforced Concrete (GRC), is a versatile engineered material composed of cement, aggregate, polymer, and fibers. This amalgamation creates a remarkably lightweight yet incredibly robust substance. GFRC is renowned for its exceptional flexural strength, making it an ideal choice for a myriad of interior and exterior applications such as wall cladding, fire pits, countertops, furniture, and more.

Understanding GFRC Mix Composition

One of the distinctive features of GFRC is its unique mix composition when compared to traditional concrete. The key components in a GFRC mix include:

1. High Cement Proportion: GFRC mixes typically consist of an equal ratio of cement to sand, which is relatively high compared to conventional concrete mixes.

2. Glass Fiber Reinforcement: Instead of utilizing steel for reinforcement, GFRC employs glass fibers as the primary reinforcement material. These fibers significantly contribute to the material's remarkable strength and durability.

3. Polymer Resin: An acrylic polymer resin is added to the mix to enhance the material's strength, act as a curing aid, and provide numerous other benefits.

4. Additional Components: Many GFRC mixes may also include pozzolans, defoamers, and shrinkage reducers to further enhance their properties.

Basic GFRC Mix Design

Here's a breakdown of the basic components in a typical GFRC mix design:

It's important to note that this information provides only a fundamental understanding of the GFRC mix design process. There are various factors and variables that can influence the final outcome, and therefore, independent practice and testing are crucial.

The GFRC Process Overview

The GFRC fabrication process involves several key steps to ensure the material's proper composition and successful casting. Let's delve into the process:

A. Prepare Supplies & Equipment

1. Ensure all necessary equipment and tools are set up and ready for use.

2. Prepare the casting forms by cleaning and applying a release agent. Remove any caulk or residue, cover screw heads with tape, and complete other necessary preparations.

3. Cut and prepare any foam, scrim, or inlays that will be included in the project.

B. Calculate Recipe & Materials Needed

1. Determine the batches for the face and backer coats based on your project's requirements. Use a mix calculator for accurate measurements.

2. Decide on the total number of batches to be mixed, considering factors like job size, ambient temperature, the type of mixing equipment, and the working time. Avoid mixing more material than can be used within 20 minutes.

C. Mixing

1. Begin with the Mist Coat (Face Coat) by mixing one batch using a handheld mixer to create a thin, milkshake-like consistency. Add plasticizer as needed to maintain the desired texture.

2. For the Backer Coat, mix one batch with fibers for the initial backer coat. Dispense only half of the mix into a bucket, ensuring it can be used before it begins to set. Adjust the consistency based on your preference, with thinner mixes being easier to work with and achieving better consolidation.

3. Follow these steps for mixing:

  • Add mix water and polymer to the mixing bucket.
  • Dry blend sand, pigment, cement, and pozzolan (PVA fibers for the face mix) in a separate bucket.
  • Add approximately 80% of the dry blended material to the water and mix on low speed until well blended. For backer coats, add AR Glass fibers after thoroughly mixing and blend them in on low speed.
  • Repeat for subsequent and final backer coats.
  • Mix on high speed for 1-2 minutes to shear the mix until it reaches a loose, smooth consistency, similar to a milkshake.
  • Add the remaining dry blended material and mix on low until well blended. Adjust the consistency with plasticizer if necessary.

D. Casting

1. Start with the Mist Coat:

  • Fill the hopper gun about halfway.
  • Set the air pressure to around 90 PSI and regulate it at the gun to achieve the desired results (usually around 45 PSI at the gun).
  • Test spray in an area away from the forms to ensure the gun is operating correctly.
  • Begin spraying from a corner, working outward, concentrating on edges first and then open areas. Clear any sand rebound that accumulates and ensure the material looks wet when applied.
  • Do not allow the mist coat to dry; it should remain soft to promote proper bonding.

2. Backer Coat Options:

- You can apply the backer coat with a backer coat gun, maintaining a higher air pressure (100+ PSI) and spraying an even, thin layer over the surface. Use chip brushes and compaction rollers to ensure proper consolidation.

- Alternatively, you can apply the backer coat by hand, taking care not to push it through the mist coat. Use chip brushes and compaction rollers to achieve the desired thickness and proper consolidation.

3. Make sure to address any corners, edges, and open areas thoroughly. It's crucial to avoid any gaps or air pockets between layers, and reinforcing fibers should be correctly oriented to maximize strength.

E. Curing

1. Gently clean excess cement from the forms.

2. Mist the concrete surface with water and cover it with plastic to ensure wet curing for at least 24 hours (or 36 hours in cooler temperatures). The plastic should fully enclose the mold and be secured to retain moisture.

3. Enhance curing by using moving blankets, foam sheets, or other means to maintain heat and minimize gaps between the mold and plastic. It's vital to keep the surface hydrated for the first 24-36 hours.

F. Demolding

1. You can demold GFRC as soon as 24-36 hours after casting, provided the concrete is at room temperature. Be cautious when handling the material, as it is still relatively soft. Faster curing can be achieved with accelerators and heat.

2. Once removed from the mold, place the casting on stickers or shims to allow air movement on all sides of the concrete for even curing.

3. Avoid placing any objects on top of fresh concrete, as this could cause "ghosting."

G. Slurry Coat (Optional)

1. If necessary, create a slurry coat using the following recipe:

  • Dry Ingredients: Cement (320 grams), Alto-Pozz (80 grams), Microspheres (up to 60 grams).
  • Wet Ingredients: Mix 1 part KongKrete Polymer with 3-4 parts water.

2. Mix only the amount of slurry that you can use within 15 minutes. If it hardens, you can add Optimum plasticizer to extend working time.

3. Dampen the surface lightly and spread the slurry into voids, ensuring it is packed thoroughly. Use hand tools, a grout float, sponge, or any suitable equipment to achieve this.

In conclusion, understanding the basics of Glass Fiber Reinforced Concrete (GFRC) is essential for those looking to work with this innovative construction material. By following the process of preparation, mixing, casting, curing, demolding, and optionally applying a slurry coat, you can create a wide range of durable and visually appealing projects, from countertops and wall cladding to furniture and fire pits. Remember that practice, experimentation, and meticulous attention to detail are key to mastering the art of working with GFRC.