Experimental evaluation and microscopic analysis of the sustainable ultra‐high‐performance concrete after exposure to high temperatures
Liang Luo, Mingming Jia, Hongwei Wang, Xuanhao Cheng
Abstract
Abstract The incorporation of recycled aggregates (RA) into ultra‐high‐performance concrete (UHPC) for sustainable construction offers a compelling solution for recycling waste construction materials while meeting high‐performance requirements. As a newly developed construction material, UHPC lacks performance data related to temperature. To address this knowledge gap, this study investigates the effect of high‐temperature exposure on the performance of UHPC incorporating recycled aggregates (RA) and steel fibers (SF). Five exposure temperatures (25, 200, 400, 600, and 800°C) were selected to simulate thermal conditions associated with structural fire scenarios. Specimens were prepared with five different mix proportions, varying the RA replacement rates (0%, 25%, 50%, 75%, and 100%) and SF content (0%, 1%, 1.5%, and 2%). After thermal exposure, a series of tests were conducted to evaluate physical properties (density, porosity, and color changes) and mechanical properties (compressive strength and stress–strain behavior). Additionally, X‐ray diffraction was used to analyze microstructural changes, detecting phase transformations and thermal decomposition products. The results indicate that the temperature has a significant impact on both physical and mechanical properties. As the temperature rises, apparent density decreases, porosity increases, and color changes provide visual cues for estimating fire temperatures. Compressive strength initially rises, peaking at 400°C with a 10%–30% enhancement, but declines sharply beyond this threshold. Higher SF content and lower RA replacement rates mitigate mechanical degradation, with optimal performance observed at 300°C and 2% SF content. X‐ray diffraction analysis reveals that the microstructure deteriorates progressively with increasing temperature, as phase transformations and microcracks reduce material density. A predictive model for the stress–strain relationship at high temperatures is proposed, considering the combined effects of temperature, RA content, and SF inclusions. This study provides critical insights into the thermal performance of UHPC with RA, offering a scientific foundation for practical engineering applications in fire‐resistant structures.