Material Laws of FRP-Strengthened RC Element in Biaxial Tension–Compression

The truss model approach is widely used to predict the behavior of fiber-reinforced polymer (FRP)-strengthened reinforced concrete (RC) elements in shear. The constitutive laws of each material components, namely concrete, steel, and FRP reinforcement, are essential in the truss model theory. This paper, which is one of a series of papers emphasizing on constitutive modeling of FRP-strengthened RC in shear, focuses on experimental and analytical investigations of constitutive laws of FRP-strengthened RC under biaxial tension–compression loading. The effect of two key parameters, specifically the softening coefficient and the Hsu-Zhu ratios, were carefully investigated by testing six full-scale panels. The results indicated that the softening coefficient and the Hsu-Zhu ratios for FRP-strengthened RC vary greatly compared to RC elements without FRP reinforcement and are heavily affected by parameters such as FRP stiffness and principal tensile strains. Furthermore, analytical expressions were proposed for the softening coefficient and the Hsu-Zhu ratios based on the experimental results.