Fracture behavior of lightweight multiscale fiber-reinforced ultra-high-performance concrete after exposure to high temperatures

Ultra-high-performance concrete (UHPC) is prone to uncontrolled crack propagation and brittle failure, especially under fire events. Steel, polyethylene, and carbon fibers can halt thermal cracking from micro to macro levels, effectively enhancing the crack resistance and toughness of UHPC. Additionally, cenospheres are lightweight hollow structure materials, which can reduce the density and thermal conductivity of UHPC. Thus, in the present article, the multiscale fibers and cenospheres are incorporated to create a lightweight multiscale fibers UHPC (MSFUHPC) with enhanced workability and durability at elevated temperatures. Experimental research involving fourteen mix proportions across five temperatures is conducted to study the fracture behavior of this novel material. The results indicate that cenospheres help mitigate the decrease in compressive strength between 400°C and 600°C, with their reduction effect on strength diminishing from 14% at 20°C to 0% at 600°C. The excellent dispersion of the cenosphere enhances the cracking resistance of MSFUHPC between 400°C and 600°C, achieving twice the initial fracture toughness and 1.5 times the unstable fracture toughness compared to the groups without cenosphere. Furthermore, the hollow structure of the cenosphere improves energy dissipation and delays the fracture process of MSFUHPC from 20°C to 400°C. Within this range, the improved effect of the cenosphere on fracture energy rises from 36% to 87%. Among the lightweight MSFUHPC designs, 0.6% 1 mm carbon fiber (CF) + 0.6% 3 mm CF + 50% cenosphere design shows the best performance between 400°C and 600°C. This not only verifies the effectiveness of this novel material but also provides guidance for mixture design.