Cyclic load behavior of CFRP bars reinforced coarse aggregate UHPC hollow beams: Flexural strength degradation and ductility enhancement

Combining a hollow cross-section with coarse-aggregate ultrahigh-performance concrete (CA-UHPC) represents an innovative strategy to enhance the performance of FRP-reinforced concrete structures. This study investigates the cyclic flexural behavior of CFRP-reinforced CA-UHPC hollow rectangular beams containing double circular voids. Displacement-controlled, four-point bending cyclic tests were conducted to systematically examine how the number of load cycles—at identical displacement levels—affects failure mode, crack propagation, load–displacement response, strength degradation, and ductility evolution. Test results indicate that all specimens failed predominantly by UHPC crushing. As the number of cycles increased, beam stiffness decreased markedly, ultimate load capacity diminished, residual deformations accumulated, and crack widths widened. Specifically, after 3 and 10 cycles, ultimate capacity dropped by approximately 9% and 23%, respectively, compared with static loading, while residual displacement increased by about 20% and 95%. To quantify strength loss, a degradation coefficient was introduced, and a regression expression—based on cycle count and displacement–curvature ratio—was developed. Ductility was assessed following ACI recommendations, revealing that energy absorption and dissipation at ultimate limit state improve with increasing cycles, evidencing enhanced ductile behavior. Finally, a flexural strength prediction model—grounded in the stress-integration method and incorporating post-cracking residual tensile stresses and graded displacement cycling effects—is proposed. The model offers theoretical guidance for performance evaluation and design optimization of such high-performance composite beams under repeated loading.