Coupling effects of axial static pressure ratio and high temperature on dynamic mechanical properties and crushing fractal characteristics of concrete under static-dynamic coupled loads

Abstract

Concrete in buildings is susceptible to the coupling effects of high temperature and static-dynamic coupled loads in case of fire. To explore the coupling effects of axial static pressure and high temperature on dynamic mechanical properties and crushing modes of concrete under static-dynamic coupled loads, static-dynamic coupled loading tests were conducted on plain concrete (PC) with axial static pressure ratios (0, 0.2, 0.4, 0.6, 0.8) and temperatures (20 °C, 200 °C, 400 °C, 600 °C, 800 °C) as variable conditions. The stress-strain relationship, strength characteristics, dynamic increase factor (DIF), deformation characteristics, energy absorption characteristics and crushing fractal characteristics of concrete were analyzed. It is shown in the results that when axial static pressure ratio (ASPR) does not exceed 0.4, axial static pressure can close the microcracks and fine pores in concrete, thus significantly improving the dynamic strength characteristics and DIF, and decreasing the deformation characteristics and crushing degree of concrete. The effect of axial static pressure is most obvious at ASPR of 0.4. When ASPR exceeds 0.6, axial static pressure causes microcracks and pores to expand again, thus reducing the dynamic strength characteristics, and increasing the deformation characteristics and crushing degree of concrete. Axial static pressure can significantly change the percentage of energy absorbed before and after the peak stress of concrete. High temperature can cause serious damage deterioration to concrete when it exceeds 400 °C, thus obviously decreasing the dynamic strength characteristics and energy absorption characteristics, and improving the deformation characteristics and impact crushing degree of concrete. The dynamic compressive strength and energy absorption characteristics are the best at 200 °C. The DIF value is the largest at ASPR of 0.4 and temperature of 200 °C. 400 °C is the turning point of the transition from the dominant influence of ASPR to that of high temperature. Fractal dimension is used to depict the impact crushing degree of concrete, and the larger it is, the higher the crushing degree of concrete is.