Flexural behavior of ultra-high-performance fiber reinforced concrete beams strengthened with titanium alloy and GFRP bars

This study aims to experimentally evaluate the effectiveness of the near-surface mounting (NSM) approach for restoring or upgrading the flexural behavior of ultra-high performance fiber reinforced concrete (UHPFRC) beams. The experimental matrix was developed to investigate the structural response of beams exhibiting simulated deficiencies in longitudinal reinforcement, thereby requiring flexural strengthening. Two NSM rebars were used: glass fiber reinforced polymer (GFRP) and titanium alloy bars (TiABs). Five RC beams were constructed and underwent four-point flexure testing. The tension reinforcement rebars in the RC beams employed a hybrid arrangement of steel, titanium, and GFRP bars. The first beam (B1-CS) used only steel rebars, serving as a control specimen. The second beam (B2-CT) comprised only TiABs. The third beam (B3–2T) contained two NSM TiABs and one steel rebar, and the fourth beam (B4–1T) consisted of one NSM TiAB and two steel rebars. The fifth beam (B5–1G) contains two steel rebars and one NSM GFRP rebar. The results indicate that using TiABs as the primary reinforcement, rather than all-steel rebars (as in B2-CT), increased the ultimate load capacity by 59.9% compared with B1-CS. In addition, using TiABs as NSM rebars increased the ultimate load capacity up to 45.0%, and the use of NSM GFRP bars exhibited a relatively small difference of 6.9%, respectively, compared with B1-CS. The implementation of NSM TiABs increased the ductility capacity of the beam compared to steel rebars, with an increase of up to 64.7% compared to B1-CS. Based on the experimental results, the predictive efficacy of the analytical formulation suggested by the American Concrete Institute (ACI) was evaluated. The ACI formulation showed good agreement with the experimental results.