If you're looking for an alternative to traditional precast concrete countertops, here it is: glass fiber reinforced concrete (GFRC). GFRC is a composite building material that uses glass fibers instead of steel to reinforce the concrete. The result is a lighter weight product with a higher flexural strength than traditional steel reinforced concrete.
The key to GFRC is alkali-resistant (AR) glass fibers and a Polyplex curing compound, says Jim Ralston, owner of Urban Concrete Design in Arizona. "These fibers are capable of resisting the alkali attack that occurs when the fibers are mixed with normal hydrating portland cement," Ralston says. "That's the backbone of the whole thing. This random matrix of fibers transferred throughout the entire countertop is the strength of the GFRC."
Even though GFRC has been around the concrete industry for years, there are still many contractors who aren't aware of or knowledgeable about its use, especially in concrete countertops. Ralston, who has 25 years of master mould making and precast experience and is also a supplier of GFRC materials, says his company has been creating GFRC countertops and other GFRC pieces for over five years. In fact, every concrete countertop Urban Concrete Design creates uses GFRC, Ralston says.
There are many reasons Ralston prefers to use GFRC over traditional wet casting when it comes to concrete countertops. One of the biggest reasons is the weight factor. One example: Ralston created a GFRC reception desktop for a Bosch showroom in Scottsdale, Ariz. The desk was 12 feet long, 3 feet 7 inches wide and 2 feet 7 inches high. "Typically, in precast, that is going to weigh between 4,000 and 6,000 pounds. With GFRC it was 1,400 pounds," Ralston points out.
Ralston says typically the largest GFRC countertops he does are 5 foot by 10 foot pieces. But he has done countertops as long as 20 feet. And when it comes to cantilevered edges, Ralston says he will usually do them up to 24 inches. "Because of the flexibility, the GFRC actually works against itself at some point. Once the edge gets beyond 24 inches you get into an area where you need an external frame for support because the GFRC will begin to flex under its own weight," Ralston says. An admixture must be added to the GFRC to allow it to hang vertically for these drop down edges, Ralston adds.
When it comes to how large contractors can make GFRC countertops, Ralston says it's based on two factors, neither of which directly relate to the GFRC itself. "It all depends on the individual fabricators and what size casting decks they have. Another factor is how they are able to handle larger pieces," he says. If a contractor doesn't have the room to make or the ability to transport large pieces it's probably best to stick to multiple, smaller sections.
For the Bosch showroom - which also included two 9-foot-long countertops, an egg-shaped water feature and concrete benches - Ralston created a 14 foot by 4 foot deck for creating all the molds. "Off of that deck I could do the entire project. It was made out of medium density overlay plywood and coated with polyester resin so I could make pieces seamless," he says.
Another benefit of GFRC is the material has a short setup, curing and stripping time, Ralston says. "I can strip, finish, seal and install in less than 72 hours. With regular wet cast sometimes it takes 7 to 10 days to cure, and then they have to finish and install," he says.
Transitioning to GFRC countertops is not a huge leap. Most contractors doing wet cast concrete countertops will have all the tools needed for a GFRC countertop, Ralston says. The difference is in the materials, specifically the admixtures, curing compound and AR glass. "The curing compound and AR fibers are a challenge because if you're remote from a larger metropolitan area they aren't that accessible yet," he explains.