Exposure to high temperatures causes a significant deterioration in the mechanical properties of ultra-high-performance concrete. Considering the characteristics of multiscale defects, it is expected to improve the residual tensile performance of ultra-high-performance concrete by incorporating multiscale fibers (including polyethylene fibers, steel fibers, calcium sulfate whiskers, carbon fibers, and carbon nanotubes). An investigation is conducted to examine the impact of multiscale fibers, CE, and temperature ranging from 25 to 800 °C on several aspects of multiscale fibers ultra-high-performance concrete (MSFUHPC), including first cracking strength and strain, ultimate strength and strain, failure mode, as well as the tensile stress-strain relationship. The results show that above 400 °C, multiscale fibers can enhance the first cracking strength, and the effects of calcium sulfate whiskers and CE on the first cracking strain depend on the temperature and their content. It is found that Steel fibers and CE can enhance the strain-hardening ability, and multiscale fibers and CE can increase the residual tensile strength and peak strain at high temperatures. At room temperature, multi-scale fibers and CE increase the maximum tensile strength and ultimate strain of MSFUHPC by 1.28 and 45.92 times, respectively. Moreover, multiscale fibers can refine the pore structure and reduce the porosity at 800 °C. While CE can decrease the increasing rate of porosity with temperature. Furthermore, the uniaxial tensile stress-strain relationship of MSFUHPC after being subjected to high temperatures is suggested based on experimental results, taking into account the influence of multiscale fibers, CE, and temperature.
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