Tensile behavior and damage mechanisms of ultra-high-performance concrete with blended steel fibers under elevated temperatures

Abstract

This study investigates the residual tensile mechanical properties of Ultra-High-Performance Concrete (UHPC) that is reinforced with a blend of straight steel fibers, including both macro (S-type) and micro (M-type) fibers, after exposure to elevated temperatures ranging from 25 °C to 800 °C. The S-type fibers, having a diameter of 0.3 mm and a length of 25 mm (aspect ratio 83), and the M-type fibers, with a diameter of 0.2 mm and a length of 13 mm (aspect ratio 65), were used in varying volume fractions while maintaining a total fiber content of 2 % by volume. The experimental program evaluated the effects of these fiber blends and temperature on critical tensile performance indicators, such as initial and post-cracking strength, strain capacity, energy absorption, and fracture energy. A significant aspect of this study was the inclusion of a 24-month drying period before testing, which effectively removed residual moisture and physically bound water, thereby reducing the risk of explosive spalling commonly observed in prior research. The findings suggest that both tensile strength and energy dissipation capacity undergo a notable decline beyond 400 °C, with a marked degradation in fracture energy occurring above 600 °C. Among the various blends tested, the combination of 0.5 % S-type and 1.5 % M-type fibers exhibited the highest post-cracking strength, while the blend of 1.5 % S-type and 0.5 % M-type fibers excelled in strain capacity. Regression models were developed to correlate mechanical properties with temperature, providing valuable insights into the behavior of UHPC under extreme thermal conditions and aiding its application in fire-exposed structural contexts.