Mechanism of tensile load sharing in hybrid reinforced concrete flexural members

Abstract

This paper investigates the flexural behavior of Hybrid Reinforced Concrete (HRC), a composite material combining conventional rebars and short, randomly distributed fibers. The synergy between these components enhances serviceability stiffness, deflection control, ultimate strength, and load-sharing efficiency. Flexural experiments were conducted on nine HRC beams with varying proportions of flexural, skin, and fiber reinforcement. A closed-form analytical model was used to predict the moment-curvature and load-deflection responses, showing good agreement with experimental data for sectional strain, neutral axis depth, and deflection throughout the loading history. Results demonstrate that fiber significantly improves structural performance by delaying crack instability and increasing post-cracking stiffness. Replacing 50-80% of longitudinal reinforcement with a 1.25% volume fraction of steel fibers increased the serviceability moment capacity by 43-65%. The fiber bridging mechanism was found to carry a substantial portion of the tensile load (74-92%) in the cracked concrete matrix within the serviceability range, leading to higher stiffness retention. However, reducing the longitudinal reinforcement by as much as 80% and replacing it with up to 1.25% fibers decreased the ultimate load by 10-50%.

Key findings

• Fiber bridging carried 74 to 92% of the tensile force in the cracked concrete over the whole serviceability range, and the tension efficiency of the concrete rose by as much as 83%.
• Replacing 50 to 80% of the longitudinal reinforcement with 1.25% steel fibers by volume raised the serviceability moment capacity by 43 to 65% and lowered the ultimate load by 10 to 50%.
• In the RC beams V1 and V2 the simulation shows an unstable response immediately after the first crack. In the HRC beams V4 and V6 no sudden drop is observed, because the residual tensile strength of the fiber concrete shares the load after first crack.
• The moment-curvature analysis follows the tensile strain at the extreme fiber through the whole loading history, which gives the neutral axis location and the load sharing between reinforcing bars and fibers at the serviceability limit state. The provisions of ACI 544.4R-18 and the fib Model Code 2020 for residual tensile strength focus on the ultimate limit state.
• After first crack the simulated curvature is lower than the experimental one. The experimental curvature is not measured directly but estimated from the average tension strain of the reinforcing bars and the concrete compression strain at the top surface, and localized cracking at the strain gauge affects it.