Effect of high temperature on the performance of radiation‐protected ultra‐high performance concrete containing mixed fibers

Abstract The internal structure of ultra‐high performance concrete (UHPC) is very dense and prone to flaking in high‐temperature environments, thereby limiting its application in high‐temperature environments. In this study, polypropylene (PP) fibers were used in partially replacing steel fibers to prepare radiation‐protected UHPC‐containing hybrid fibers. The working performance, spalling behavior, mass loss, mechanical properties, γ‐ray shielding performance, and microstructure after exposure to different target temperatures (25°C, 200°C, 400°C, 600°C, and 800°C) of the radiation‐protected UHPC containing hybrid fibers were investigated. Results showed that the increase of PP fiber admixture did not have a significant negative effect on the flowability of UHPC mixes. The combination of steel fiber and PP fiber can effectively inhibit the spalling of UHPC at a high temperature. With the increase of temperature, the compressive strength and splitting tensile strength of UHPC showed a trend of first increasing and then decreasing, and the γ‐ray shielding performance gradually decreased. Compared with the normal temperature, the linear attenuation coefficient ( μ ) of UHPC at 800°C decreased by 12.6%, HVL and TVL decreased by 14.4%. Microstructural analysis showed that the porosity of UHPC increased with the increase of temperature, and the proportion of harmless pores showed a trend of increasing and then decreasing. Moreover, the high temperature led to the deterioration in the microscopic morphology of UHPC and the weakening of the bond between the steel fiber and the matrix. These findings revealed the reason for the decrease in mechanical properties of UHPC at high temperatures.