Mechanical Properties of Lightweight Ultra-high Performance Concrete with Mixed Fibers after High Temperature

To investigate the effect of different fiber types and dosages on the mechanical properties of lightweight ultra-high-performance concrete(LUHPC) after high-temperature exposure, five kinds of LUHPC specimens were prepared based on 2.5% steel fiber, which were mixed with 0.2% PP, 0.4% PP, 0.2% PVA, 0.4% PVA and 2.5% steel fiber. After high temperature, the compressive strength, static modulus of elasticity, capillary water absorption test and SEM scanning were carried out to study the influence of single steel fiber, mixed with steel fiber and PP fiber, and mixed with steel fiber and PVA fiber on the mechanical properties of LUHPC after high temperature and the regulation mechanism of burst performance. The results show that there is no obvious change in the appearance of the specimen at 200℃, and the appearance damage of the specimen gradually intensifies with the increase of temperature, and micro holes appear on the surface of the specimen after the melting of PP and PVA fibers. The results show that the single doped steel fiber LUHPC bursts at 418℃, and the mixed fiber LUHPC increases the burst temperature due to the melting effect of PP and PVA. The mixed 2.5% steel+0.4% PVA fiber LUHPC can withstand 800℃ without bursting. With the increase of temperature, the mass loss rate of the specimens increased continuously. At 400℃, the mass loss rate of the specimens in each group was close to 10%, and the mass loss rate of the specimens with various fiber contents increased rapidly to about 20% before bursting. With the increase of temperature, the compressive strength of LUHPC increased slightly and then decreased significantly, and the elastic modulus decreased continuously, which conformed to the quadratic parabola change law. The compressive strength of LUHPC mixed with 2.5% steel fiber and 0.4% PVA at 800℃ is still not less than 43.5 MPa. The capillary water absorption and SEM tests showed that the microporous channels left by PP and PVA after melting improved the internal pore connectivity, which was the main reason for improving the high temperature burst resistance and strength decline of LUHPC.