Evaluation of the residual strength of concrete incorporating high quantity of functionalized recycled aggregates and free PCM exposed to elevated temperatures

Abstract

Current studies on PCM-modified concrete mainly focus on thermal regulation, while limited attention has been paid to the residual mechanical behavior of recycled aggregate concrete incorporating free PCM under high temperatures. This study investigates the combined effects of two-phase change material (PCM) incorporation methods, direct liquid addition and form-stable absorption, on the residual mechanical behavior of concrete subjected to elevated temperatures. Recycled aggregates (RA) were pre-treated via paraffin impregnation to produce functionalized recycled aggregates (RAP), and free liquid paraffin was incorporated at different dosages. Five concrete mixtures were designed to evaluate the individual and combined effects of the two PCM incorporation methods. Workability and residual strength were assessed through slump tests and heating cycles at 20 ◦C, 200 ◦C, 400 ◦C, and 600 ◦C. Microstructural analysis was conducted using SEM and TGA techniques. The results indicate that high RAP and free PCM contents reduce compressive strength at ambient temperature due to weakened interfacial transition zones (ITZs). However, moderate PCM addition induces a transient thermal buffering effect around 200 ◦C, which alleviates stress concentration and delays crack propagation. TGA results confirmed paraffin volatilization at approximately 165 ◦C, followed by the progressive decomposition of Ca(OH)₂ and CaCO₃ at higher temperatures. Overall, the study reveals a transition from physical pore-related weakening at low temperatures to thermochemical degradation mechanisms at high temperatures. These findings provide practical guidance on the optimal dosage of PCM and RA replacement ratio for balancing thermal regulation and structural integrity in fire-safe concrete design.