The FRP strips are attached to bridges using anchors. Pre-drilled holes are placed in the strips at the required spacing. The strips are then attached to the concrete under the bridge with an adhesive to hold them in place during anchor installation. At each anchor point, holes are drilled into the concrete then filled with epoxy, an anchor stud and a nut that is tightened on the strip once the epoxy has cured. “With this easy installation technique, we estimate a 12-foot-long beam can be retrofitted in an hour with a two-man crew,” says Davids.
With other FRP retrofit techniques, materials are adhesively bonded to concrete. However, field adhesion can be challenging due to environmental conditions such as prolonged exposure to moisture conditions. With mechanical fastening, holes are drilled directly into the concrete and could lead to corrosion of the internal steel reinforcement. “Mechanical fastening poses its own challenges but is a viable, more easily installed alternative in most cases,” says Davids.
The American Concrete Institute Committee 440.2R-08 “Guide for the Design and Construction for Externally Bonded FRP Systems for Strengthening Concrete Structures” gives specific guidelines for using adhesively-bonded FRP strengthening technologies. Unfortunately, there are no design guidelines for mechanically-fastened FRP systems. So Davids and his team relied on engineering principles and carefully examined test data at every step when designing this system. “We took measures to closely replicate field conditions in order to gain the most accurate data,” says Davids.
The retrofitted concrete beams underwent testing at the university’s Advanced Structures and Composites Center to determine load failure. Without the composite strips, the beams failed under 15,000 pounds of force. With the strips, that increased to under 22,000 pounds. In the field, that means heavier vehicles can cross the bridge, which is of great importance to the trucking industry.
Davids and his team use load rating guidelines from the American Association of State Highway and Transportation Office’s (AASHTO) “Manual for Bridge Evaluation” to help determine which bridges are good candidates for repair. “If a bridge is no more than 30 to 35 percent understrength and in good condition overall, it would be a perfect candidate for repair with this system,” says Davids. “There are still some fatigue resistance questions, but at a minimum we feel this system would extend the bridge life 20 additional years. And it’s possible that the life can be extended for much longer.”
Research and testing is ongoing for the FRP flexural retrofitting system. “This project has the potential to help us out greatly,” says Peabody. Davids agrees: “Considering that the Maine DOT has several hundred flat-slab concrete bridges in its inventory, this would be a worthwhile investment.”