The Basics of Glass Fiber Reinforced Concrete (GFRC)

GFRC BASICS 

Glass Fiber Reinforced Concrete (GFRC), Also known as Glassfibre Reinforced Concrete (GRC),is an engineered material that contains cement, aggregate, polymer & Fibers. These ingredients are combined within a cementious matrix producing an extremely lightweight yet durable material.

GFRC’s extremely high flexural values are ideal for interior & exterior application such as wall cladding, fire pits, countertops, furniture, etc.

Some of the main differences with GFRC mixes compared to standard concrete include:

• Utilizes a high proportion of cement (usually 1part cement to 1 part sand)
• Uses Glass Fibers for primary reinforcement instead of steel
• Adds an all acrylic polymer resin to increase strength, act as curing aid amongst many other benefits within the mix.
• Many mixes use Pozzolans, defoamers, shrinkage reducers for added benefits.

Basic GFRC Mix Design

This information is intended to provide a basic understanding of the process and mix design of GFRC. There are many variables that can affect the outcome. Independent practice and testing should be done.

GFRC Process Overview

Table of Contents:

A) PREPARE SUPPLIES & EQUIPMENT

B) CALCULATE RECIPE & MATERIAL NEEDED

C) MIXING

D) CASTING

1) FACE COAT

2) BACKER COAT

E) CURING

F) DEMOLDING

G) PROCESING

• A.PREPARE SUPPLIES & EQUIPMENT
  • 1.Ensure all equipment and tools are staged and ready to be used.
  • 2.Forms are clean and prepped for casting. Remove any caulk and / or residue, apply release agent, cover any screw heads with tape, etc.
  • 3.Cut / prep any foam, scrim, inlays, etc. that will be cast into the project. *See Recommended Tooling & Materials list

• B.CALCULATE RECIPE & MATERIAL NEEDED
  • 1.Batch (Weigh out) face and backer batches (Refer to Mix Calculator).
  • 2.Total number of batches should be determined based upon job size, temperature, mixing equipment and comfort. Do not mix up more than can be used in 20 minutes.

• C.MIXING
  • 1.Mist Coat (Face Coat): Mix up one batch using hand held mixer in bucket to be sprayed in as mist coat. This should be enough to spray two (first and second) separate mist coats into molds. Mix should have the consistency of a thin milkshake. Add plasticizer to maintain consistency before re-filling hopper gun.
  • 2.Backer Coat: Mix up one batch with fibers for initial backer coat. Only dispense ½ mix into bucket (amount you can use before mix begins to set). Mix to desired consistency (thinner is easier to work with and achieves easier consolidation, thicker will hold on verticals better). Second half should be monitored and can be brought back to desired consistency using plasticizer (Optimum 380) if more fluidity is desired.
  • 3.Order of Mixing
  • i.Add mix water and polymer to mixing bucket
    • ii.Dry blend Sand, Pigment, Cement, Pozzolan (PVA fibers for face mix) in separate bucket
      • iii.Add approximately 80% of dry blended material to water and mix on low speed until well blended.
        • i.For backer coats only: AFTER mix is thoroughly mixed add AR Glass fibers and mix on low speed until blended.
        • ii. If you are spraying backer with backer gun you may find it is easier to mix up a smaller batch of backer (using hand mixer) for initial backer coat. This will be based on your personal preference.
        • iii.Repeat step 2 for subsequent and final backer
      • v.Once mix looks even mix on high speed for 1-2 minutes to shear the mix. Once complete the concrete should be a loose, smooth consistency like a milkshake.
      • iv.Add balance of dry blended material and mix on low until well blended. Add Plasticizer if needed to achieve desired consistency. Scrape sides of bucket

• D.CASTING
  • 1.Mist Coat
    • i.Fill hopper gun about halfway.
    • ii.Air pressure to the gun should be around 90PSI and then regulated at the gun to achieve desired results (usually around 45 PSI at the gun). If material spatters the pressure is too low and if the material has excessive sand rebound or looks dry and sandy the pressure is too high and /or held too far away from the surface.
    • iii.Test spray an area away from the forms (cardboard, scrap material, etc.) to ensure gun is operating properly and any unwanted debris is flushed out.
    • iv.Holding the gun 6”-12” (15cm-25cm) from surface start in a corner and work your way out concentrating on the edges first then open areas using a random spray pattern.
    • v.As you go clear any sand rebound that accumulates with compressed air or vacuum especially in corners or along edges.
    • vi.Continue until the entire form is coated with at least 1/16”(2mm) coverage (should never exceed 1/8” (3mm)).
    • vii.Brush/poke material into corners and edges using chip brushes slightly damp and spray another light coat to cover any area that the brush has uncovered.
    • viii.Material should look wet when applied. If it looks dry or “sandy” try lowering the pressure to the gun and/or hold the tip of the gun closer to the surface or wet the mix out further using plasticizer.
    • ix.IMPORTANT: mist coat should not be allowed to dry. It should be soft enough to disrupt the surface when brushing with chip brushes. If it does look dry or needs more moisture spray lightly with a diluted mix (1part polymer: 1Part water) and lightly brush surface before applying more mist or backer. This will help the bond.
    • x.Be sure to CLEAN gun by completely taking it apart. When reassembling use Vaseline or equivalent to lube all moving and threaded parts

• 2.Backer Option#1- Applying Backer with Backer Coat Gun
  • i.Air pressure to the gun should be 100+ psi to the gun
    • ii.Only fill hopper 1/2 full and spray an even thin layer over surface.
    • iii.Using chip brushes poke up into the corners and use compaction rollers with just enough pressure to ensure there are no air pockets between coats
    • iv.You can modify the backer gun so it can reach into tighter areas using ½” pipe fittings
    • v.Be sure to CLEAN gun by completely taking it apart. When reassembling use Vaseline or equivalent to lube all moving and threaded parts

• 3.Backer Option #2- Applying Backer by Hand Placing
  • i.If a mist coat has been sprayed give it a little time to stiffen up prior to applying backer so the backer does not push through. This may take longer in colder months than warmer. Do not let the mist coat completely dry out as this will inhibit the bond between the two layers. Mist coat should no longer look “glossy”.
    • ii.Scoop a small amount of backer mix into forms and spread gently and evenly over mist coat ensuring full coverage. It is important to make the first backer as thin as possible. This will make consolidation (of the mist and backer) at this critical point more efficient and easier.
    • iii.Using chip brushes poke up into the corners and use compaction rollers on flat areas to roll/compress with just enough pressure to ensure there are no air pockets between coats.
    • iv.Spread the backer by hand or trowel in thin layers (no more than 3/8” (9mm thick)).
    • v.Roll all corners, edges and open areas using compaction rollers after each layer has been applied. The rollers help flatten the material, bond the layers, and properly orient the reinforcing fibers to provide maximum strength.
    • vi.Place any scrim or foam gently in the forms without pushing the backer layers below or above to prevent ghosting.
    • vii.The Scrim MUST be completely covered/encapsulated without air pockets or it will create a weak point.
    • viii.Second & further backer Coat
      • a.Same as initial backer
      • b.Use depth gauge to ensure desired thickness
      • c.Shape form back to desired finish

• E.Curing
  • 1.Gently clean off excess cement from forms. Mist concrete surface with water and completely cover with plastic to wet cure for at least 24 hours (36 hours in cooler temperatures). Plastic should completely cover the mold and be held down to retain all moisture.
  • 2.Moving blankets, foam sheets or some means to hold in heat will benefit curing and minimize gaps between the mold and plastic
  • 3.It is important to hold in moisture. The underside of the plastic should have moisture on it and the concrete should look hydrated (not dry) for the first 24-36 hours to insure proper curing. The plastic should be lifted and the concrete should be misted regularly.

• F.Demolding
  • 1.Demolding GFRC can occur as soon as 24-36 hours after casting. Concrete should be at room temperature. Keep in mind, the concrete is still soft and should be handled carefully (Faster curing can be obtained with accelerators and/or heat). Concrete left in the mold for longer than 24 hours should be misted regularly to maintain even hydration.
  • 2.Once removed from mold place casting on stickers or shims placed evenly to support concrete. This is to allow air movement on all sides of concrete to cure evenly.
  • 3.Do not place items on top of fresh concrete as “ghosting” may occur.
  • 4.Beware uncured concrete will be soft when grinding or polishing. Be careful to not gouge or remove too much material.

• G.Slurry Coat
  • 1.Dry ingredients- (you can mix up as much dry slurry and store in buckets until ready to use)
  • 2.Wet ingredients- (mix up as much as you want and store in sealed container until ready to use) - 1part Kongkrete polymer to 3-4 parts water.
  • 3.Only mix up enough slurry that you can use in 15 minutes. If slurry hardens you can add Optimum plasticizer to extend working time.
  • 4.Lightly dampen surface then mix slurry into a peanut butter consistency using polymer and water solution then spread into voids. Make sure the slurry is packed into the entire void using hand, grout float, sponge or whatever works.

Basic Slurry Recipe