Glass Fiber Reinforced Concrete, commonly called GFRC, allows concrete to be cast in relatively thin sections while using alkali-resistant glass fibers as reinforcement.
There are many ways to formulate GFRC. The mix designs below are a practical starting point that we have used successfully at Fishstone for architectural concrete, concrete countertops, furniture, sinks, panels and other decorative concrete applications.
Both of the batches shown below produce approximately 1/3 cubic foot of material.
Understanding a GFRC Mix
A GFRC mix is more than cement, sand and glass fiber. The performance and workability of the concrete are influenced by several components working together:
- Portland cement provides the primary cementitious binder.
- Fine silica sand provides aggregate and helps build the body of the mix.
- Pozzolan can supplement the cementitious system and contribute to particle packing and overall mix performance.
- Acrylic polymer is commonly used in traditional GFRC formulations.
- Superplasticizer improves flow while limiting the amount of additional water required.
- Defoamer helps control entrapped air.
- PVA fiber can be used in the face coat for additional micro-reinforcement.
- AR glass fiber provides the primary reinforcement in the structural backer coat.
GFRC Face Coat Mix Design
We sometimes refer to the face coat as the beauty coat because it is the material placed directly against the mold and ultimately becomes the visible surface of the finished concrete. The goal is a workable mix capable of reproducing mold detail while minimizing visible fibers and excessive entrapped air.
Fishstone GFRC Face Coat Recipe
| Ingredient | Amount |
|---|---|
| Portland Cement – Type I | 18.5 lb |
| Bottle-Pozz | 4.5 lb |
| Silica Sand – approximately #40–50 sieve | 23 lb |
| Water | 6–6.5 lb |
| KongKrete Liquid Acrylic Polymer | 2.3 lb |
| PVA Fiber – 8 mm, optional | 85 g |
| Optimum 380 Superplasticizer | 23–42 mL |
| C-64 Defoaming & Densifying Admixture | 5–10 mL |
Approximate Yield: 1/3 cubic foot.
Why No AR Glass in the Face Coat?
- The face coat is intended primarily to create the visible surface of the casting. Large amounts of AR glass fiber can become visible at the surface and can make it more difficult to achieve fine mold detail.
- The structural reinforcement is normally concentrated in the backer coat.
- PVA fiber may optionally be incorporated into the face coat as fine secondary reinforcement.
GFRC Backer Coat Mix Design
The backer coat provides most of the structural reinforcement in a traditional GFRC casting.
The basic cement, sand, pozzolan, water and polymer proportions remain similar to the face coat. The major difference is the addition of a substantially higher amount of alkali-resistant glass fiber.
Fishstone GFRC Backer Coat Recipe
| Ingredient | Amount |
|---|---|
| Portland Cement – Type I | 18.5 lb |
| Bottle-Pozz | 4.5 lb |
| Silica Sand – approximately #40–50 sieve | 23 lb |
| Water | 6–6.5 lb |
| KongKrete Liquid Acrylic Polymer | 2.3 lb |
| AR Glass Fiber | 2.6 lb |
| Optimum 380 Superplasticizer | 23–42 mL |
| C-64 Defoaming & Densifying Admixture | 5–10 mL |
Approximate Yield: 1/3 cubic foot.
PVA fiber is generally not necessary in the backer coat, although its presence will not normally create a problem.
The AR glass fiber should be incorporated after the base concrete has been thoroughly mixed.
Adjusting GFRC Workability
GFRC needs enough flow to consolidate properly, but simply adding more water is generally not the best way to create that flow. The amount of Optimum 380 superplasticizer and water can be adjusted within the working range of the formulation to produce the consistency required for the casting method.
Different applications may require different consistencies. A sprayed or mist face coat needs to move differently than a hand-applied backer coat. A self-consolidating GFRC mix requires still another level of flow.
Temperature, cement chemistry, aggregate moisture and other raw-material variations can also affect the way a GFRC mix behaves. For this reason, the recipes above should be considered a proven starting formulation rather than an inflexible formula.
Why Use Acrylic Polymer in GFRC?
Acrylic polymer has traditionally played an important role in GFRC mixes.
In a properly designed GFRC system, polymer can contribute to the handling characteristics and performance of thin architectural castings and allows traditional GFRC systems to be produced without the extended wet-curing requirements associated with some non-polymer formulations.
Fishstone's KongKrete Liquid Acrylic Polymer is designed for use in GFRC and other high-performance cementitious applications.
Why AR Glass Fiber Is Important
Not all glass fiber is appropriate for concrete.
GFRC uses alkali-resistant, or AR, glass fiber because Portland-cement concrete creates a highly alkaline environment. The AR glass fiber becomes the primary reinforcement within the backer coat and allows GFRC components to be produced much thinner than conventional steel-reinforced concrete sections. Fiber quantity, fiber length, dispersion and orientation all influence the behavior of the finished casting.
Superplasticizer and Water Control
- One of the most important principles in high-performance concrete is separating flow from water content.
- Adding water makes concrete easier to mix, but additional water also changes the cementitious system.
- A high-range water reducer such as Optimum 380 allows the GFRC mix to become considerably more workable without relying entirely on additional mixing water.
- Always make small adjustments and evaluate the mix before adding additional admixture.
Controlling Air and Pinholes
High-performance cementitious mixes can trap significant amounts of air during mixing.
Entrapped air can contribute to:
- Surface pinholes
- Bugholes
- Reduced surface density
- Incomplete mold reproduction
Fishstone's C-64 Defoaming & Densifying Admixture is included in this GFRC formulation to help control unwanted air within the mix.
Mixing technique, mold preparation, vibration and placement technique also influence the finished surface.
GFRC Face Coat vs. Backer Coat
| Characteristic | Face Coat | Backer Coat |
|---|---|---|
| Primary purpose | Finished visible surface | Structural reinforcement |
| AR glass fiber | Normally omitted | Yes |
| PVA fiber | Optional | Generally unnecessary |
| Surface detail | Very important | Secondary |
| Reinforcement level | Low | High |
| Placement | Directly against mold | Applied behind face coat |
The two layers are designed to work together rather than as unrelated concrete mixes.
GFRC Mixing Sequence
A consistent mixing sequence is important when reproducing a GFRC formulation.
A practical general sequence is:
- Measure all ingredients accurately before starting.
- Begin blending the cementitious materials and aggregate.
- Incorporate the liquid components and admixtures while developing the desired consistency.
- Mix until the base concrete is uniform and free of unmixed powder.
- Make final flow adjustments gradually.
- For the backer coat, add AR glass fiber only after the base mix has developed.
- Mix the fiber only long enough to achieve good dispersion.
Common GFRC Mix Problems
The mix is too stiff
Avoid immediately correcting a stiff mix by adding large amounts of water. First evaluate the superplasticizer dosage, temperature and mixing sequence.
The mix is too fluid
Too much water or excessive superplasticizer can make placement difficult and may affect fiber suspension. Make adjustments gradually.
The mix contains excessive air
Check mixing technique and defoamer dosage. High-speed mixing can introduce considerable air into some GFRC formulations.
AR glass fiber is clumping
The base concrete should be fully mixed before introducing the AR glass fiber. Add the fiber gradually enough to achieve even dispersion.
The mix is setting faster than expected
Ambient temperature, material temperature and cement chemistry can significantly affect working time. Keep materials as consistent as possible and account for temperature when batching.
GFRC Materials Used in This Recipe
Fishstone supplies the specialized ingredients needed to reproduce and modify this GFRC formulation, including:
- Bottle-Pozz pozzolan
- KongKrete Liquid Acrylic Polymer
- AR Glass Fiber
- PVA Fiber
- Optimum 380 Superplasticizer
- C-64 Defoaming & Densifying Admixture
Portland cement, silica sand and water can typically be sourced locally.
GFRC Mix Design FAQ
What does GFRC stand for?
GFRC stands for Glass Fiber Reinforced Concrete.
What type of glass fiber should be used in GFRC?
GFRC should use alkali-resistant glass fiber designed for cementitious environments.
Can regular fiberglass be used in GFRC?
GFRC formulations are designed around alkali-resistant glass fibers. Standard fiberglass should not simply be substituted for properly specified AR glass reinforcement.
How much AR glass fiber is in this Fishstone GFRC recipe?
The backer formulation shown above uses 2.6 pounds of AR glass fiber per approximately 1/3 cubic-foot batch.
Does the GFRC face coat contain AR glass fiber?
The Fishstone face-coat formulation shown above does not use AR glass fiber. The primary AR glass reinforcement is incorporated into the backer coat.
Why is superplasticizer used in GFRC?
Superplasticizer provides increased flow and workability without obtaining all of that additional flow by adding more water.
Why is a defoamer used?
GFRC and other high-performance mixes can entrain or trap air during mixing. A defoaming admixture helps reduce unwanted air and improve surface quality.
Can this GFRC recipe be modified?
Yes. GFRC formulations commonly require adjustment for materials, casting methods, temperature and desired consistency. This recipe is intended as a proven starting point.
Start With a Proven GFRC Mix and Adjust From There
There is no single GFRC recipe appropriate for every fabricator and every application. The formulation above provides a practical starting point built around cement, fine aggregate, pozzolan, acrylic polymer, high-range water reducer, air control and alkali-resistant glass reinforcement. Once you understand the function of each ingredient, the mix can be adjusted intelligently for the casting method and performance you are trying to achieve.
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