Post-fire compressive mechanical behaviors of concrete incorporating coarse and fine recycled aggregates

Understanding the damage mechanisms and residual mechanical properties of concrete after high-temperature exposure is essential for evaluating and repairing fire-damaged structures. However, research on recycled aggregate concrete (RAC), particularly those incorporating fine recycled aggregates (RAs) or a combination of coarse and fine RAs, remains limited. This study addresses this gap by performing both macro- and micro-level analyses of RAC. At the macro level, 225 cylindrical specimens were tested after exposure to temperatures of 20, 200, 400, 600 and 800 °C. The study examined how factors such as the water-to-binder ratio, RA substitution rates and the compressive strength of the parent concrete affected key compression performance metrics, including spalling, failure modes, compressive strength, elastic modulus, Poisson's ratio, peak strain and stress-strain curves. At the micro level, mercury intrusion porosimetry was used to provide further insights into the pore structure of the mortar, complementing the macro-level findings. Key findings include: (i) Due to their porous structure, RAs reduce spalling frequency between 600 and 800 °C without significantly affecting crack propagation or failure morphology. (ii) At room temperature, coarse and fine RAs significantly lower the compressive strength and elastic modulus of hardened concrete while increasing lateral expansion, particularly when the RA quality is poor. (iii) After high-temperature exposure, RAC demonstrates higher relative strength and elastic modulus than natural aggregate concrete (NAC) with the same water-to-binder ratio, suggesting that the porous microstructure of RAs helps preserve mechanical properties, with the residual strength and elastic modulus of both RAC and NAC converging between 600 and 800 °C. (iv) Poor-quality or high-content RAs lead to a sharper decline in nominal stress during the descending phase of the dimensionless stress-strain curve, especially when fine RAs are used. (v) The study proposes uniaxial compressive constitutive models and conversion relationships for residual mechanical properties, tailored to various exposure temperatures and RA contents.