Abstract
This study investigates the mechanical properties and microstructure of basalt fiber (BF) and nanoalumina (NA)-modified ultra-high-performance concrete with recycled aggregates (UHPC-RA) under high-temperature conditions. The effects of different replacement rates of recycled aggregates (RAs), BF content, and NA content on the compressive strength, splitting tensile strength, and elastic modulus were evaluated at ambient temperatures and after exposure to 200 °C, 400 °C, 600 °C, and 800 °C. The results show that mechanical properties decrease with temperature rise, but specimens containing BF exhibited improved crack resistance and better high-temperature integrity. The incorporation of NA enhanced the thermal stability and heat resistance of the concrete. Digital image correlation (DIC) was used to monitor real-time surface deformation, and scanning electron microscopy (SEM) analysis revealed improved microstructure with reduced porosity and cracks. This study demonstrates that the combination of BF and NA significantly enhances the high-temperature performance of UHPC-RA, which holds promising potential for applications in environments subjected to elevated temperatures.