Multiaxial tensile–compressive strengths and failure criterion of plain high-performance concrete before and after high temperatures

Multiaxial tensile–compressive tests were performed on 100 mm × 100 mm × 100 mm cubic specimens of plain high-performance concrete (HPC) at all kinds of stress ratios after exposure to normal and high temperatures of 20, 200, 300, 400, 500, and 600 °C, using a large static–dynamic true triaxial machine. Friction-reducing pads were three layers of plastic membrane with glycerine in-between for the compressive loading plane; the tensile loading planes of concrete samples were processed by attrition machine, and then the samples were glued-up with the loading plate with structural glue. The failure mode characteristic of specimens and the direction of the crack were observed and described, respectively. The three principally static strengths in the corresponding stress state were measured. The influence of the temperatures, stress ratios, and stress states on the triaxial strengths of HPC after exposure to high temperatures were also analyzed respectively. The experimental results showed that the uniaxial compressive strength of plain HPC after exposure to high temperatures does not decrease completely with the increase in temperature, the ratios of the triaxial to its uniaxial compressive strength depend on brittleness–stiffness of HPC after different high temperatures besides the stress states and stress ratios. On this basis, the formula of a new failure criterion with the temperature parameters under multiaxial tensile–compressive stress states for plain HPC is proposed. This study is helpful to reveal the multiaxial mechanical properties of HPC structure enduring high temperatures, and provides the experimental and theory foundations (testing data and correlated formula) for fire-resistant structural design, and for structural safety assessment and maintenance after fire.