Flexural performance investigation of steel-concrete composite beams strengthened with prestressed CFRP tendons in the negative bending moment region

To enhance the flexural performance of steel-concrete composite beams in the negative moment region, an external prestressing technique utilizing Carbon Fiber-Reinforced Polymer (CFRP) tendons was implemented for reinforcement. This article designed and manufactured 6 test beams for static load testing, and compared the changes in mechanical properties of the test beams before and after reinforcement, such as failure mode, deflection, CFRP tendons stress increment, strain, and cracks. A specific focus was given to exploring the impact of varying prestressing levels and the quantity of CFRP tendons on the flexural performance of composite beams in the negative moment region. Based on the plastic hinge theory, a new calculation model for the flexural bearing capacity of externally prestressed CFRP tendon-reinforced composite beams was established. The use of externally prestressed CFRP tendons reinforcement has a significant reinforcement effect, which can improve the elastic working range and ultimate bending bearing capacity of the composite beam in the negative bending moment zone, and effectively limit the development of concrete slab cracks in the negative bending moment zone. Theoretical values obtained from the proposed model in this study showed good agreement with experimental values, enabling accurate calculation of the incremental stress in externally prestressed CFRP tendons and the ultimate bearing capacity of composite beams. The study also provides a reasonable range for controlling the prestress level of CFRP tendons, which can serve as a reference for practical engineering design.