Study on the shear multiaxial mechanical properties and damage evolution mechanisms of high-performance concrete after high temperatures

High-performance concrete (HPC) structures face the risk of exposure to fires during their service life, and the elevated temperatures induced by fire degrade the material properties. Simultaneously, shear failure under complex stress states is one of the main failure patterns of concrete. Nevertheless, there are currently limited studies on the shear multiaxial mechanical properties of HPC after high temperatures, and the response damage mechanisms remain unclear. Therefore, this article establishes different high-temperature and axial compression ratio (ACR) conditions, employing a shear multiaxial device to investigate the mechanical behavior of HPC under shear multiaxial loading. Additionally, image recognition technology is utilized to obtain the damage evolution process of HPC under various conditions. The primary findings of this study are as follows: the shear strength of HPC gradually increases with the rise in ACR, and the increase in temperature leads to a gradual increase in the influence of ACR on the shear strength of concrete. As the temperature increases, the shear strength of HPC gradually decreases. Compared to ordinary concrete, HPC exhibits lower temperature sensitivity in shear strength, with the reduction in shear strength affected by temperature ranging from 53% to 61% under different ACR. The increase in ACR delays the evolution of damage in HPC, while the rise in temperature somewhat weakens the influence of the ACR on the development of damage in HPC. Simultaneously, researchers propose a shear multiaxial failure criterion and a constitutive model for HPC after high temperatures, based on the unified strength theory of double shearing and damage theory. These findings offer a theoretical foundation for the computational evaluation of HPC structures after a fire.