GFRC requires specialized equipment to mix and cast. This page explains which equipment to use and why.
The mixing procedure for GFRC is very important. GFRC must be high shear mixed. GFRC is very sensitve to the mix consistency – viscosity, flowability, workability. The basic GFRC mix design has been proportioned to make it very easy to crete a flowable mix design and still use a low water-cement ratio for strong concrete.
In ordinary concrete mix design, which is aggregate rich, the traditional way to create a flowable mix is to use quite a bit of a very powerful superplasticizer. The mix is intended to be poured into a mold that fills by gravity. In contrast, GFRC is sprayed onto the surface of a mold that may be vertical or even upside down. The mix has to be flowable so it can be sprayed and achieve a very high quality surface finish with no pinholes, but it also has to stick to the mold. Trying to make a flowable GFRC mix by using superplasticizer in the same amounts and methods for conventional concrete will result in a mix that won’t stick or is too runny.
In addition, because GFRC uses a cement-rich mix, the mix is sensitive to good cement particle dispersion for high early strength and quick set times. What high shear mixing does is it mechanically disperses the clumped cement particles rather than relying on large aggregate and large amounts of superplasticizer to do the same thing. With conventional concrete, the large aggregate (gravel) creates shearing action as it tumbles in the mixer. GFRC needs mechanical mixing at high energy to achieve that cement dispersion. You cannot use the conventional barrel/tumble style precast mixers for GFRC. They are too slow and don’t put enough energy into the mix.
High shear mixing takes only a minute. Here’s how to do it:
See the product page for the Collomix Xo6 for more information. For less than $600, the Collomix is the least expensive way for concrete countertop pros to mix GFRC properly. Commercial high shear mixers cost $5000 and up.
Dual-headed mixers are also available, but we do not recommend them. The single-headed Collomix (X06) provides a higher head speed (580 rpm at high speed) than the dual-headed mixer (430 rpm), plus the X06’s head has a greater diameter (160mm vs 140mm). Neither of these sounds very significant, but shearing energy is related to the square of the diameter, so the larger head spinning faster gives a lot more energy than a smaller head. Since the dual-headed mixer’s blades intermesh, you cannot change the blades to other shapes, as recommended above to prevent sloshing.
A simple drywall hopper gun can be used to spray a thin mist coat, and then you hand-apply layers of fibrous backer coat. We recommend the Kraft EZY Deck Pro hopper gun for spraying mist coat. It costs less than $100. Purchase the Kraft sprayer here.
We do not recommend the Mortar Sprayer “3 Jet Stucco” backer coat sprayer. It is designed to spray non-fibrous mortar/stucco onto a wall, where precision isn’t important. When spraying GFRC backer through it, it is very hard to control the amount of material sprayed, which leads to variable strength in the final product. It spits out tons of material explosively, and you have to move FAST to make sure it doesn’t dump out too much (for a thin backer layer).
Backer must be applied in thin coats (1/4″ or 6mm), and each coat must be compaction rolled to orient the fibers correctly and ensure bonding with the previous layer. This article explains why.
We have seen a disturbing trend in the concrete countertop industry of using a single thick coat of backer, or creating a flowable mix (“SCC”) and pouring in the backer. Perhaps this was a misguided attempt to save time, because hand-applying backer coats and rolling them takes the majority of time required to create GFRC. There are backer sprayers that can speed up production time when you are ready to upgrade to larger equipment. It could have been an attempt to save money by not having to buy compaction rollers. However, this approach is simply the wrong way to do GFRC and results in far lower tensile and flexural strength, which is the whole point of GFRC. Click here for an article that explains the danger of this approach.
The Concrete Countertop Institute sells compaction rollers in 2 sets: the rollers you absolutely need when you are first starting out, and the additional rollers that are helpful when you’re doing more and larger projects.
GFRC Equipment Store:
You may purchase GFRC equipment in the CCI store. We have selected only the best and most necessary GFRC equipment and materials: everything you need and nothing that you don’t, with clear instructions for how and why to use it.
More about GFRC:
See this page for information about how to make GFRC, plus training options.