What is GFRC concrete?

For thousands of years we’ve used a variety of mixes to create concrete/cement, GFRC is a more recent innovation that was introduced in the late 1960s. GFRC simply stands for Glass-Fiber-Reinforced-Concrete. It is a composite comprised of Portland cement, sand, water, acrylic co-polymer, ar glass fiber reinforcement and additives. The addition of alkaline resistant glass fibers gives the concrete an increased tensile and flexural strength. In regular concrete applications increased tensile strength is achieved with the addition of re-bar. There are many benefits to using GFRC over standard concrete but there are also several drawbacks. Glass fiber reinforced concrete is more expensive to use than regular concrete due to the additives and labor that it takes to create. Another drawback is unlike regular concrete which is abundant in nearly any hardware store you may have difficulty finding some of the additives you need to make gfrc. It is an excellent product for demanding environments where weight and strength cannot be compromised. Below we will go over some of the items used to create gfrc along with mixing ratios.
 
Interesting fact: The Romans made their concrete by mixing lime, seawater, and volcanic rock to form mortar.
 
There are a number of ways to mix gfrc concrete but the ingredients that are implemented are relatively universal. All add mixtures serve a purpose ranging from bonding to long term durability. Below is a list of commonly used materials to create the GFRC
Type 1 Portland Cement

TYPE 1 PORTLAND CEMENT

30# Silica Sand

30 MESH SILICA SAND

AR GLASS FIBER

Iron Oxide Pigment

PIGMENTS OR STAIN

Water

H2O

Acrylic Polymer

ACRYLIC CO-POLYMER

Plasticizer

PLASTISIZER

Are all of the materials above required?

While all of the above items are not required for a GFRC mix they are highly recommended. GFRC has a very low water to cement ratio and unlike conventional concrete, gfrc acquires a majority of its strength within just a few days. The additives are what makes this possible. Standard concrete doesn’t achieve 50% hardness until approximately 7 days and you also need to make sure the concrete remains wet during cure. If you would like more information about each additive and what it does to the mix check out some of the drop downs below. 
 

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.

This one is pretty simple… just make sure there are no iron salt or other contaminants like rust in the water. It can actually add a weird color to the concrete!

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.

There are two ways to color concrete. There is intrinsic (pigments mixed in the concrete) or extrinsic (topcoat stain). We use intrinsic iron oxide pigments for our gfrc concrete. 

GFRC concrete mix ratios

Below we list the mix ratios that our team uses for our products. Please note the mix tables below that gives the spray up ratios is NOT for applications where you mix the glass fibers in the slurry. The chart is for professional applications that use glass chop guns which automatically mix the slurry and fibers as you’re spraying. If you are making a product such as a table and you will be using a hopper you’ll want to spray on what we’ll refer to as the face coat. Mix the face coat using the ratios in the chart below for the premix but DO NOT add glass fibers for this coat. Once this is mixed, use the hopper to spray on a layer over the entire face of the mold. After you’ve achieved the desired thickness you can then mix up the backer layer. The backer layer will use the ratios from the premix chart below including the glass fibers. The backer layer will be applied by hand and we recommend rolling over the layer with a roller or a bubble buster to remove as much of the air bubbles as possible.

Primary use cases for GFRC concrete

Although GFRC is superior to traditional concrete it is not as often used. As we mentioned above the materials required to make gfrc are not as commonly found as ready mix concrete. The challenge of finding the needed ingredients and there is also the labor factor that has to be counted for as it’s more time-intensive to make as well. The most common areas you will see gfrc concrete used is exterior façade or cladding material for both brand new constructions and for recladding or restoration of existing projects. In these applications, gfrc weighs between 8-11 lbs per sq ft. Another common area you will see it used is in the interior of homes on countertops and tables. The fibers within the concrete make it so that the concrete does not have to be as thick and in return makes each piece much lighter. In smaller applications, there really is a huge advantage to using GFRC over traditional concrete.