Insights into the fracture behavior of multi-scale fiber reinforced ultra-high performance cementitious composites after exposure to high temperatures

Using multi-scale hybrid fibers can remarkably enhance the mechanical properties of ultra-high performance cement-based composites. However, the evolution of fracture properties of multi-scale hybrid fiber reinforced ultra-high performance cementitious composites (MFRUHCC) with temperature remains to be revealed, which is of great significance to the safety of engineering structures in a fire. This study delves into the fracture behavior of MFRUHCC reinforced with hybrid steel fibers, polyethylene fibers, and multi-length carbon fibers through three-point bending tests of pre-notched beams after exposure to temperatures of 20 degrees C, 200 degrees C, 400 degrees C, 600 degrees C, and 800 degrees C. The crack propagation, load-crack mouth opening displacement curves, strain distribution, and microstructural change are analyzed. Results illustrate that the hybrid utilization of carbon fibers and polyethylene fibers can improve the fracture energy and fracture toughness, but adding excessive fibrs can induce adverse effects. Although the steel fibers exhibit a slight influence on the initial fracture toughness, they can remarkably enhance the fracture energy and unstable fracture toughness. The highest unstable fracture toughness of MFRUHCC with 0.4 % short carbon fibers, 0.8 % medium carbon fibers, 0.4 % long carbon fibers, and 1.0 % polyethylene fibers and steel fibers is up to 48.99 MPa & sdot;m1/2. It is found that carbon fibers can restrict the propagation of microcracks and strengthen the interface between steel fibers and the matrix, while the steel and polyethylene fibers can bridge the macrocracks, producing a strong synergistic toughening effect. However, this effect can be weakened above 400 degrees C due to the thermal degradation of steel fibers. A micromechanical model is developed to estimate the fracture toughness by considering the contribution of various fibers and temperature. These findings provide essential insights into the high-temperature behavior of MSHFRC, contributing to the advancement of more resilient and fire-resistant composite materials for engineering applications.