Effects of high-temperature exposure on the compressive strength and freeze–thaw durability of Reactive Powder concrete

This study investigates the synergistic degradation effects of high-temperature exposure and subsequent freeze–thaw cycles on the mechanical behaviour of steel fibre reinforced reactive powder concrete (RPC). Three fibre volume fractions (1.5%, 2.0%, and 2.5%) were used in RPC mixtures subjected to thermal treatments up to 600 °C, followed by freeze–thaw cycles ranging from 0 to 300. The residual compressive strength, surface damage, and mass loss were systematically evaluated, and the underlying deterioration mechanisms were elucidated through Scanning Electron Microscopy (SEM) and Thermogravimetry-Differential Scanning Calorimetry (TG-DSC) analysis. The results indicate that moderate heating (≤350 °C) enhances strength due to internal autoclaving and continued hydration, while temperatures above 400 °C lead to significant microstructural degradation. Microstructural analysis confirms that this degradation is driven by the dehydration of C‐S‐H gel and the α-to-β phase transformation of quartz. Freeze–thaw cycles exacerbated this damage, especially after high thermal exposure. Steel fibres effectively mitigated both thermal and freeze–thaw damage, with 2.0% content providing the optimal balance of strength and durability. Based on experimental data, an empirical model incorporating temperature, fibre content, and freeze–thaw cycles was developed to predict residual compressive strength. This model provides a robust tool for assessing RPC structural performance in cold regions after fire or thermal exposure.