Effect of thermal cycling on the mechanics and microstructure of ultra-high performance concrete

This study investigates the mechanical and microstructural responses of Ultra-High Performance Concrete (UHPC) subjected to a substantial number of thermal cycles upto 300 times from ambient temperature to 60°C. The experimental findings reveal a distinctive pattern of behavior in the static compressive strength, characterized by an initial increase, followed by a subsequent decline, and a subsequent modest resurgence as the thermal cycling progressed. Notably, the peak compressive and flexural strengths were attained after 120 thermal cycles, whereas the maximum dynamic compressive strength was observed after 60 thermal cycles. It is worth highlighting that the dynamic increase factor (DIF) exhibited an inverse trajectory in comparison to the static compressive strength. After 120 thermal cycles, the stress-strain curves under impact loads exhibited pronounced strain softening characteristics, with an oscillation state duration of 250μs, which surpassed that of the ascending and descending phases. The observed variances in macroscopic mechanical properties can be attributed to several pivotal factors, including the quantity of hydration product, the mean chain length (MCL) of the calcium-silicate-hydrate (C-S-H) gel, the distribution of pore volume, and the presence of micro-cracks. This comprehensive examination serves as a valuable theoretical and empirical foundation for advancing the utilization of UHPC in applications subjected to prolonged and intricate thermal conditions.