A prestressed mechanically fastened fiber-reinforced polymer (MF-FRP) retrofit system was previously developed to provide a rapid, cost-effective means of extending the service life of deteriorated prestressed concrete bridges. Designed for short-term use and to be installed without specialized equipment, the system enables certain degraded bridges to remain open, often without load posting, until full permanent replacement projects can be planned, scheduled, and completed.
The static performance of the MF-FRP system was previously verified through laboratory tests of full-scale prestressed channel beams removed from structures approximately 54 years old. In these prior NCDOT research efforts, new MF-FRP components were installed in the laboratory on old beams. The prior research was followed by installations of the MF-FRP system on three in-service prestressed concrete C-channel bridges in North Carolina, each of which was about 54 years old at the time of retrofit and remained in service for approximately another two years while replacement projects were planned, scheduled, and implemented.
Specimen SCU1 at failure
Demolition of two of these bridges during replacement presented the opportunity to salvage five retrofitted beams and seven companion control beams as part of the current research project. The current effort included full-scale static and fatigue tests of the salvaged girders to evaluate the condition and performance of the aged beams, some of which included field-applied retrofits subjected to about two years of environmental exposure and traffic loading.
Results show that when the MF-FRP system is installed on both stems of a channel member, the capacity of that member is increased beyond the original design levels. Installations of MF-FRP on a single stem of a channel member are not recommended due to poor experimental performance. Fatigue tests designed to simulate up to an additional three years of field service demonstrated that strengthened members performed well in fatigue, with significant useful life remaining in the repaired members at the time they were removed from the field. All observed fatigue failures occurred in the concrete rather than in the mechanical components of the MF-FRP system, enabling inspection guidance to be proposed and indicating good durability of the MF-FRP system.
Based on these findings, the recommended service life of the MF-FRP retrofit can be extended from two years to five years, provided retrofitted members are regularly inspected.
