Restoration of fire-damaged high-performance concrete columns by post-fire water-CO₂ cyclic curing

This study experimentally investigates the post-fire mechanical performance and recovery potential of high-performance concrete (HPC) columns exposed to standard fire conditions followed by water-CO₂ cyclic re-curing. The columns were subjected to one hour of heating according to the ISO 834 standard fire curve and subsequently re-cured for 30 days. A comprehensive evaluation was performed, including internal temperature profiling, surface damage observation, microstructural analysis, load-displacement response, and failure mode characterization. Results show that HPC columns with a lower water-to-binder ratio (W/B) suffered more severe internal damage due to higher peak temperatures and steeper thermal gradients. Consequently, their residual load-bearing capacity was more significantly reduced than that of columns with a higher W/B. The conventional 500 °C isotherm method effectively predicted the post-fire capacity of high-W/B columns but overestimated that of low-W/B columns, for which a 400 °C threshold provided a better approximation. Water-CO₂ cyclic re-curing markedly enhanced the mechanical recovery of fire-damaged columns by promoting rehydration and carbonation reactions, which filled coarsened pores, healed microcracks, and partially sealed macrocracks, thereby restoring structural integrity. After re-curing, the load-bearing capacities of the low- and high-W/B columns recovered to 67.3 % and 100.9 % of their original values, respectively.