Fracture Properties of High-Performance High-Ductility Alkali-Activated Concrete Under Thermo-Mechanical Coupling: Effect of Fiber Hybrid Ratio

This study investigated the fracture behavior of high-ductility alkali-activated composites (HDAACs) under thermo-mechanical coupling. Fracture tests were conducted on hybrid polypropylene/polyethylene (PP/PE) fiber-HDAAC with varying PP fiber replacement ratios (0%, 25%, and 50%) and coupled temperatures (0 °C, 30 °C, 70 °C, 100 °C, and 150 °C). The fracture mechanisms were analyzed through failure modes, mode I fracture energy (<i>G</i>_F), and the J-integral method. The results showed that below 100 °C, specimens exhibited ductile failure with a main crack along the notch accompanied by stable matrix cracking, with <i>G</i>_F peaking at 16.47 kJ/m². At 150 °C, fiber melting led to a reduction in <i>G</i>_F to 2.01 kJ/m². Initial cracking energy (<i>J</i>_IC ≈ 0.1 kJ/m²) remained stable, while unstable fracture energy (<i>J</i>_IF) peaked at 70 °C and dropped sharply at 150 °C. The energy consumed by matrix cracking showed (<i>J</i>_m) a similar trend to that consumed by fiber pull-out and fracture (<i>J</i>_b), with <i>J</i>_m/<i>J</i>_C = 0.4-0.5. Higher PP replacement reduced both <i>J</i>_m and <i>J</i>_b. The fracture behavior differences under thermo-mechanical coupling versus post-heating were mainly due to fiber exposure timing. This study highlights the critical influence of thermo-mechanical coupling on HDAAC fracture mechanisms, offering guidance for designing HDAACs for high-temperature applications.