Step 4: Mixing GFRC (Pre-Blended)

GFRC (Glass Fiber Reinforced Concrete) differs from regular concrete because it doesn't contain rock aggregate. It is a mixture of fine sands, portland cement, fibers (Alkalai Resistant Glass, Polypropylene, PVA, or Natural Fibers), and some chemistry that accelerates the cure time (commonly a polymer curing agent). Making your own GFRC mix involves weighing out all of these ingredients yourself, but there are also pre-blended mixes available. Some pre-blended mixes require a liquid activator (unlike regular concrete that only requires water).

The main advantage of GFRC is that the pieces can be much thinner without losing strength. It can also be applied with a range of techniques, unlike regular wet-cast concrete. It can be sprayed through a texture hopper, packed into a form by hand, or wet-cast as usual. Because you're using less material, the formwork doesn't need to be as heavily reinforced. Another advantage of GFRC is that pieces can be demolded in 12 hours or less.

GFRC is mixed using a mortar mixer or a paddle mixer, instead of a traditional concrete mixer. Small batches under 30 lbs. can still be mixed by hand.

  • Rubber Gloves
  • Particle Mask
  • 17 gallon mixing bucket
  • Paddle Mixer
Paddle mixer:
Dual paddle, on a stand (this is a luxury). You can also use a 1/2" drill motor and a paddle, but a proper dual paddle mixer will work better, especially for something very stiff like the fibrous backing blend.

Size of bucket:
8 gallon = 50 lbs.
17 gallon = 100 lbs.
25 gallon = 150 lbs.

1. Add liquids to the bucket first. Then blend in pigment, whether it's wet or dry.
2. Add 1/2 of the total batch of the DRY components.
3. Thoroughly mix with the paddle mixer, moving the paddles around the bucket, and scraping the walls with a trowel.
4. Add 1/2 of the remaining dry mix.
5. Thoroughly mix again.
6. Add the rest of the dry material.
7. tempering the mix: the art
8. if it's too dry, add water

<p>When mixing larger jobs, how do you dry mix once the concrete mixer already mixed wet concrete?</p>
<p>when mixing large jobs, the moisture from last job will blend with newer when mixing</p>
Very detailed and easy to understand about the batch of concrete. Thank you!
Nice, thanks for this.
Really helpful. Thanks to share.
I've read your countertop book. It's not a project I'll probably ever tackle, but it was extremely interesting, as was this instructable. Thanks for sharing more of your knowledge.
wow thats some great instructable right there, thanks for the share!
thats a really helpful share a banch of thanx

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