Flexural Behavior of R-UHTCC and Recycled Concrete Composite Beams Reinforced with Steel Bars

To promote the application of recycled concrete in construction engineering, the flexural behavior of ultra-high toughness cement-based composite (UHTCC) materials and recycled concrete composite beams was investigated in this study. Recycled aggregates were used in the production of both recycled UHTCC (R-UHTCC) and recycled concrete. A total of 10 beams were manufactured and tested under four-point bending load. The primary design parameters included concrete strength grade, R-UHTCC layer height, stirrup spacing in the pure bending section, and tensile reinforcement ratio. The effects of these parameters on the failure mode, crack width, load-midspan deflection response, ductility, load-tensile reinforcement strain response, and flexural capacity of the beams are discussed. The results indicate that limiting the use of R-UHTCC to a specific height range within the tensile zone of the beams can yield superior flexural properties compared to using R-UHTCC across the full section. The R-UHTCC and recycled concrete composite beams demonstrated good crack resistance, load-deflection response, and ductility. Compared to the R-UHTCC layer height and stirrup spacing, the influences of concrete strength and tensile reinforcement ratio on the flexural behavior of the composite beams are more significant. The maximum increase in flexural capacity and ductility index was 18.8% and 67.3%, respectively, as the concrete strength grade increased from C30 to C70. The flexural capacity increased by 64.6% as the longitudinal reinforcement ratio increased from 0.258% to 3.68%. Furthermore, a stiffness calculation method based on the effective moment of inertia was proposed and validated through experimental results. The research findings provide a theoretical and design basis for the application of R-UHTCC and recycled concrete composite beams in engineering.