“Reinforcement is subject to corrosion in most parts of the United States, especially where road salt is heavily used and in coastal areas where there is airborne salt,” says Baur. “But all that is needed for corrosion is water and oxygen, so it's a concern even in Southern states where road salt is not used.”
Because carbon fiber grid reinforcement is non-corrosive, precasters don't need concrete cover for the sole purpose of protecting the reinforcement.
Precasters specify carbon fiber grids based on the flange width of the double-tee. “Similar to steel, you try to optimize your reinforcing material to meet the structural requirements,” says Gregg Blaszak, technical marketing manager for TechFab, a fiber manufacturer. “Wider flanges with longer cantilevers generate larger tensile forces that need to be resisted by the reinforcement. That means more carbon is needed.” Spacing between the carbon fiber strands range from 2.7 inches to 4 inches, depending on the width of the double-tee.
Metromont and High are concrete manufacturers that belong to the AltusGroup, and use C-GRID carbon fiber from Tech-Fab (also an AltusGroup member).
Using the carbon fiber, they have manufactured test double-tees with flanges as thin as 3-¼ inches, though real-life applications have used thicknesses of 4 inches or more, especially when fire-rating codes are an issue.
Theoretically, Gleich says flanges with carbon fiber reinforcement could be less than 3 inches, although he has yet to test his assumption or run models. The reservation isn't related to the ability of the tee to withstand design loads. “Getting much below 3 inches, and stripping and hauling are big issues,” he says, referring to non-structural incidents that can cause cracks. “It would be a matter of improving your processes and techniques for lifting and shipping.”
At the same time, using carbon fiber grid lets manufacturers avoid the cost of sacrificial barrier coatings on the steel. It also eliminates the need to add corrosion-inhibiting admixtures to the concrete.
Meanwhile, owners can forego applying sealers to the precast deck surface after installation and avoid re-applying these sealers every five to 10 years. Although carbon fiber costs about three times more than steel mesh, its minor incremental costs are more than offset by the long-term durability benefits.Ingenious manufacturing
Using carbon fiber has given precast manufacturers the opportunity to improve the way they introduce reinforcement into the flange. Metromont and High are commissioning an automated process for placing carbon fiber grid into the flange of the tee. The system is a major improvement compared to how the producers install steel mesh.
“When we used mesh, casting crews needed to tier it up, keep the bed clean from rust, and prevent people from walking on the bed,” says Gleich. “With the new process, there's little action after the strand is pulled. We install the tie-in plates and side connects, and then pour the concrete. The carbon fiber is automatically impressed into the wet concrete.”