Triaxial strength and failure criterion of plain high-strength and high-performance concrete before and after high temperatures

Triaxial tests were performed on 100 mm × 100 mm × 100 mm cubic specimens of plain high-strength and high-performace concrete (HSHPC) 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, using three layers of plastic membrane with glycerine were placed between the compressive loading plate and the specimens; the tensile loading planes of concrete samples were processed by an attrition machine, and then the samples were glued-up with the loading plate with structural glue. The failure mode characteristic of the specimens and the direction of the crack were observed and described. The three principally static strengths in the corresponding stress state were measured. The influence of the temperatures and stress ratios on the triaxial strengths of HSHPC after exposure to high temperatures was also analyzed. The experimental results showed that the uniaxial compressive strength of plain HSHPC after exposure to high temperatures does not decrease completely with the increase in temperature, the ratios of the triaxial to its uniaxial compressive strength are dependent on the brittleness–stiffness of HSHPC after different temperatures and the stress ratios. On this basis, a new failure criterion with the temperature parameters is proposed for plain HSHPC under multiaxial stress states. It provides the experimental and theoretical foundations for strength analysis of HSHPC structures subject to complex loads after subjected to a high temperature environment.