Mechanical performance of ultra-high-strength alkali-activated concrete at elevated temperatures

Purpose Ultra-high-performance alkali-activated concrete (UHP-AAC) has emerged as a low-carbon alternative to ordinary Portland cement-based ultra-high-performance concrete; however, its mechanical and dynamic performance after fire exposure remains insufficiently understood. Limited information exists regarding stiffness degradation, stress–strain evolution and dynamic response of steel-fiber UHP-AAC subjected to elevated temperatures. This study aims to evaluate the residual mechanical, stiffness and dynamic properties of steel-fiber-reinforced UHP-AAC exposed to temperatures between 200 and 600 °C. Design/methodology/approach Cube and beam specimens were heat-cured, oven-dried and subjected to controlled thermal exposure at a heating rate of 5 °C/min with a 2-h retention at peak temperature. Residual performance was evaluated through compressive, flexural, stress–strain and impact-based dynamic frequency testing. Findings Results indicate negligible strength loss at 200 °C, followed by progressive degradation, with compressive strength and natural frequency decreasing by approximately 49 and 45%, respectively, at 600 °C. Stiffness degradation occurred more rapidly than strength loss, accompanied by increased peak strain and reduced flexural capacity. Originality/value These findings provide insight into the thermal degradation mechanisms of UHP-AAC and demonstrate its potential as a fire-resilient, sustainable structural material, while highlighting the importance of stiffness-based post-fire assessment methods.