Experimental Research on the Degradation of Mechanical Properties and Microstructural Evolution of Ultra-High-Performance Concrete Under Fire Conditions

Ultra-high-performance concrete (UHPC) is gaining popularity due to its superior mechanical properties, including high strength, toughness, durability, and excellent seismic performance. However, the risk of explosive spalling in UHPC structures exposed to fire poses a significant challenge, fundamentally hindering comprehensive studies on fire safety and limiting the broader engineering applications of UHPC. In this study, the mixture proportion of UHPC was developed and optimized by comparing the usage of yellow river and standard sands, different types of chemical admixtures, and different curing regimes. Furthermore, a sensitive case study was also conducted to optimize the calcined bauxite content to improve UHPC's fire resistance. For these purposes, 189 UHPC cubes, incorporating variations in particle types, superplasticizer types, and curing regimes, were tested under three thermal levels, such as 200℃,400℃, and 600℃. The microstructural evolution of heated cube specimens was then observed using a scanning electron microscope (SEM) examination. Calcined bauxite was then considered to replace the standard sand to improve the fire resistance of UHPC and enhance the residual compressive strength of cube specimens under thermal conditions. For this purpose, 0%, 20%, 40%, 60%, 80%, and 100% calcined bauxite content was designed as six case studies. Furthermore, the degradation of UHPC strength was also investigated for the corresponding six case studies by conducting the uniaxial compressive experiments. Finally, the microstructural evolution under both thermal conditions and calcined bauxite content was observed and discussed.