Near-Surface-Mounted Fiber-Reinforced Polymer Strips for Strengthening of Reinforced Concrete Girders

Abstract

The AL 97 bridge over I-65 near Letohatchee, Alabama, has been found to have deficient negative-moment capacity under current loading standards. A near-surface-mounted fiber-reinforced polymer (NSM FRP) strengthening system has been proposed (Alexy 2009) to repair the deficiency. A survey of relevant literature regarding the intermediate-crack (IC) debonding of NSM FRP revealed tests generally with reinforcement ratios and concrete compressive strengths higher than those present in the Letohatchee bridge; similarly, few previous tests involved specimens cracked prior to strengthening. To generate experimental data for direct correlation with the bridge, a laboratory test series was developed to study the performance of the NSM system. Eight specimens were fabricated with Grade 40 steel reinforcement and 3000 psi concrete, cracked, repaired with NSM FRP strips, subjected to service-load cycles, and tested by monotonic loading until failure. Simultaneously, a nonlinear, layer-by-layer flexural analytical model—to account for strain hardening in the steel, tension stiffening of the concrete, and the cracked nature of the specimens at repair—was used to produce theoretical performance data for comparison to the experimental results alongside the capacity predictions resulting from the limiting design values recommended by ACI 440 (2008), Standards Australia (2008), and Seracino et al. (2007). Analysis of the data obtained from both the service-load cycles and the monotonic loading showed no evidence of bond degradation; on the contrary, examination of the state of the specimens after failure and all experimental results indicates a strong bond through ultimate capacity. The capacities predicted by the three evaluated design methods were nearly equivalent at all levels of reinforcement for the parameters of the test and proved conservative. The viability of the design recommended by Alexy (2009) was confirmed.