Glass-fiber reinforced concrete (GFRC) isn't new to the concrete industry but its use in concrete countertops is a recent phenomenon. GFRC provides increased tensile and flexural strength, reducing the possibility of cracking, even at the narrow inside corner openings for a sink.
As a decorative precast contractor, Brandon Gore, owner of Gore Design, Tempe, Ariz., switched to GFRC six years ago for constructing countertops, specialty floor tiles, and architectural pieces, because of its benefits:
- GFRC castings weigh less because they are made thinner.
- Only a mold surface for one side of a casting is needed.
- Steel reinforcement isn't necessary, which eliminates the time-consuming process of constructing reinforcement cages.
- Mechanical vibration is not required, even for complex shapes, and there are virtually no bug holes.
- Transporting work costs less with little risk of cracking.
Most importantly, switching to GFRC offered Gore the opportunity to expand his business to an international market.
Types of glass fiber
Fiberglass is sensitive to the high alkalinity of concrete so the manufacturer coats the glass fiber to prevent them from coming in contact with cement paste. Typically referred to as alkaline resistant (AR) fibers, these should be used for GFRC work. Do not use type A or E fibers.
Glass fibers are sold as individual fibers, as fibers bundled in multifilament strands, or as mats that are placed in a mix and covered with wet concrete. Individual “chopped” fibers can be purchased in lengths up to 1 1/2 inches long, however, Gore prefers 1/2-inch lengths for his projects. When contractors buy bales of multifilament strand, they can chop them into desired lengths with a glass fiber chopper gun mounted at the end of a shotcrete spray nozzle. As the concrete mix is pumped through the nozzle, it is accelerated by compressed air and mixed with the GFRC fibers as it travels toward a mold.
The amount of fibers in a mix is expressed as a ratio of the weight of the fiber compared to 100 pounds of cementitious product. Gore says the usual dosage range is between 2.5% to 4%. To put this in perspective, adding 1 1/2 pounds of fibers (the typical pre-packaged weight) to 1 cubic yard of typical concrete is about 0.2%.
Gore uses one basic mix for his countertops and then adds components for different steps. The base mix consists of 23 1/2 pounds of portland cement (a 1/4 bag), 20 pounds of fine sand, 1 quart of acrylic polymer modifier, finely ground reclaimed fiberglass filler (he prefers this to adding pozzolans), and water. This mix is used for the initial slurry coat application. Then he adds one pound of 1/2-inch-long chopped AR glass fibers to the mix for vertical areas or a polycarboxylate superplasticizer to make a flowable mix ideal for horizontal casting.
Polymer is a necessary ingredient to the GFRC process because it forms a film around the fibers, improving flexural strength and reducing brittleness in the finished product. To reduce initial setting time, accelerating admixtures also can be added, but be sure they are compatible with the superplasticizer you are using.
Casting a countertop
The first step is to place a slurry coat without glass fibers directly onto the mold surfaces. This prevents the glass fibers from being visible on the surface of the finished product. The thickness of this application is usually 1/8 to 1/4 inch thick and can be sprayed on with a hopper gun or other spraying equipment. Gore waits for the water sheen of the mix to disappear before placing the GFRC application. You also can lightly push your finger into the mix; if nothing sticks to your finger it is ready.
When working with vertical surfaces in a mold, cast these sections after the slurry coat has set. Gore mixes the GFRC stiff enough to apply vertically with trowels. He casts horizontal work last by adding a superplasticizer to the GFRC mix to make it very flowable, which reduces air voids and increases the bond to the slurry coat. Finally, Gore advises demolding the day following placement, or as soon as temperatures allow.
GFRC is becoming more popular with decorative precast contractors who make countertops and architectural precast concrete work. Gore conducts training seminars for those who want to learn more. You can visit his Web site, www.gfrcworkshop.com, for more information.