AR Glass Fibers for GFRC – 3/4″
A 44-lb (20-kg) bag of 3/4″ (19mm) alkali-resistant glass fibers will make 1400 lbs of backer mix, yielding 180 sq ft of backer coat at 3/4″ thick ($0.69 per sq ft). This assumes 3% AR glass fibers by weight of total material.
Please see the full product page for important and detailed information about proportioning and mixing.
A 44-lb (20-kg) bag of 3/4″ (19mm) alkali-resistant glass fibers will make 1400 lbs of backer mix, yielding 180 sq ft of backer coat at 3/4″ thick ($0.69 per sq ft). It is also proportionate as follows:
- 1 44-lb bag of AR glass fibers
- 2 buckets of Forton polymer
- 2 bags of VCAS pozzolan
- 4 packs of Air-Minus defoamer
CCI recommends 3/4″ fibers rather than 1/2″ fibers because they make stronger concrete.
For your GFRC backer mix, add to the mist coat mix 3% AR glass fibers by weight of total material. This is a very confusing way to dose fibers, but that is how it’s done. A 3% fiber dose means for every 97 lbs of material in the mist coat mix, 3 lbs of glass fibers are added to make 100 lbs of backer coat mix. Click here for an article that explains how to calculate fiber load (and also polymer load) in GFRC concrete countertop mixes.
Do not use less than 3% fibers. Some concrete countertop teachers recommend only 2%, perhaps motivated by the desire to reduce cost of the backer mix or to make it easier to spray through ineffective spray guns. 2% is not sufficient; 3% is the minimum.
Higher fiber loads are not needed, and are not recommended due to decreased workability and the potential of compromising compaction. If extra reinforcing is required, use scrim.
On large pieces, do not place or pour one single layer of backer mix. You must apply thin (3/8″) layers and roll each layer with compaction rollers, regardless of whether you hand-apply or spray the backer. Read this article to understand why.