Efficiency of hybrid reinforced concrete beams with low reinforcement ratios

Abstract

This research explores the serviceability-level characterization of hybrid reinforced concrete (HRC) beams with low reinforcement ratios in the range of 0.052%-0.262% under flexural test. Hybrid reinforced structural members use continuous reinforcement with randomly distributed chopped fibers in the matrix. The study focuses on the immediate post-cracking response of flexural beams to analyze parameters such as stiffness, deflection, strain transfer mechanisms, and ultimate strength response. The influence of steel fibers on flexure performance, ductility, and tensile strain, as well as the sharing of the flexural load are also studied. HRC beams are shown to have a higher first crack strength (15-24%) and stiffness in the serviceability domain compared to conventional RC beams regardless of the reinforcement ratio. The addition of up to 1.25% fibers contributes to the sharing of the flexural load such that reducing longitudinal reinforcement between 50-80% was associated with decreasing the ultimate load between 10-50% while still increasing the post-crack stiffness. Crack localization was found in both RC and HRC sections within a range of reinforcement ratios. The addition of fiber while maintaining the same reinforcement ratio confirmed the reduction in ductility. However, by adopting a lower limit of 3.0 for the deflection ductility ratio, HRC beams with up to 0.75% of steel fibers were found to be ductile, even despite presented softening behavior after the peak load.

Key findings

• The first-crack strength of the HRC beams was 15 to 24% higher than that of the RC beams.
• At a reinforcement ratio of 0.052%, raising the steel fiber volume fraction from 0.75% to 1.25% increased the service-level stiffness and delayed yielding of the longitudinal reinforcement.
• At a fiber dosage of 0.75%, raising the reinforcement ratio from 0.052% to 0.131% increased the ultimate load by 32% and the ultimate deflection by 56%.
• Reducing the longitudinal reinforcement by 80% lowered the ultimate load by 10 to 50%, depending on the fiber content.
• Skin reinforcement increased the ultimate flexural strength by 28%.
• With a lower limit of 3.0 on the deflection ductility ratio, only the RC beams and the HRC beams with up to 0.75% steel fibers were ductile. By the ductility requirements of the fib Model Code 2020, all HRC beams with reinforcement ratios of 0.052 to 0.131% and up to 1.25% steel fibers were ductile.
• Over fiber contents of 0.5 to 1.25% and reinforcement ratios of 0.052 to 0.262%, the fibers raised the post-cracking stiffness and improved crack control, while the reinforcement ratio governed the ultimate flexural strength and the deformation capacity.