Effects of Local Stirrup Deficiency on Load-Bearing Performance and Crack Evolution in GFRP-Strengthened RC Beams

To address the problem of local stirrup deficiency in existing reinforced concrete (RC) beams, three-point bending tests were conducted on glass fiber-reinforced polymer (GFRP)-strengthened beams. A total of 16 specimens in 8 groups were designed to comparatively investigate the bearing capacity, load–deflection curve characteristics, ductility and crack evolution of specimens with two stirrup configurations, i.e., full-length uniformly distributed stirrups and a 300 mm mid-span stirrup-deficient zone, under 0 to 3 GFRP layers. The results indicate that mid-span stirrup deficiency significantly impairs the strengthening efficiency of GFRP. Compared with stirrup-deficient beams, beams with full-length stirrups exhibit 6.1–15.1% higher ultimate bearing capacity; they better enable multi-layer GFRP to exert its strengthening efficiency, with more remarkable improvement in sectional stiffness and a gentler post-peak descending branch. The deterioration effect of stirrup absence on ductility is evident, and beams with full-length stirrups show a 6.6–18.1% higher ductility coefficient. This study reveals the synergistic mechanism between stirrups and GFRP. For beams with full-length stirrups, GFRP delivers superior crack confinement performance on major cracks and results in more uniform crack distribution, while stirrup-deficient beams exhibit larger crack widths and non-uniform crack distribution.