What is GFRC concrete?
TYPE 1 PORTLAND CEMENT
30 MESH SILICA SAND
AR GLASS FIBER
PIGMENTS OR STAIN
Are all of the materials above required?
Portland cement is the binder that essentially holds everything together. In a GFRC mix there is very low water to cement ratio so it’s important to use fresh high quality cement. Make sure there is no balls clumping as this can cause issues during and after mix! The cement is available in both white and grey. Both have it’s advantages such as grey is easier to find and slightly less expensive. The benefits of white is it’s generally more consistent in color finish and when adding color pigments to the GFRC mix it is far easier to get the color you’re going for with out having to overcome the dark grey color which is inherent when using grey cement. When using white cement we go with Federal do to the availability in our area but Lehigh is a great choice as well! Portland cement can usually be found at your local mason supply store!
Pretty much every concrete application requires the use of sand however GFRC is slightly different. Our team at Redefined Concrete uses 30 Mesh silica sand which we find to be the ideal size for spray and hand lay applications. The size of the sand and aggregate is not as important if you do not plan to run the concrete through a sprayer but this is not a detail to overlook! The sand that you use must be washed, free of contaminants, and dry. Sand can hold upwards of 5% water and the ratios for the GFRC concrete mix are very important. We want to get the water to cement ratio mixed perfectly! Typically you can find any sand at masonry but silica sand is slightly more difficult to find. If you are unable to locate it locally you can always contact the silica distributor.
AR stands for alkaline resistant. This factor is very important as the more traditional e glass that would be used in fiberglass applications will not work for GFRC mixes! AR glass has a minimum requirement of 16% ZIRCONIA content to be suitable for GFRC uses. The AR glass comes in several different styles from chopped strand to skrim. We use 3/4 AR glass for our projects.
We use Vf-774 with 51% solids ratio mix that serves several purposes. Typical concrete cures through the process of hydration which is a chemical reaction between the Portland cement and water. Most concrete projects need to be wet cured for at minimum 7 days to ensure the concrete is structurally sound. The majority of the strength in concrete is created within the first 7 days. This product traps the moisture inside and eliminates the 7 day wet cure requirement. This frees up space and it sure beats having to hose down a piece of concrete for 7 days that’s laying on the shop floor! The other benefit of polymer is it helps maintain the long term flexural strain to failure property of the concrete. This was concluded by a 20 year study on the subject! Another benefit of Forton 774 is that it creates a plasticizing effect which basically helps the work-ability of the material. It’s also UV stable so there are no concerns with yellowing!
Plasticizer makes the low water to cement ratio possible. Our team uses ADVA 140 as it is excellent for spray applications and will still provide hand workability. Plasticizer enables us to have that low water to cement ratio.