Efficiency of Mortar and Fine Aggregate Concrete in Repairing Deteriorated Axial and Eccentric Columns Subjected to Severe Environmental Conditions

Concrete structures in harsh environments face durability challenges from sulfate attack and freeze–thaw cycles, making the selection of repair materials critical. Existing studies lack systematic research on repairing columns that have experienced combined sulfate and freeze–thaw damage. This study investigates axial and eccentric concrete columns subjected to severe conditions—either 40-day sulfate dry–wet cycles or a combination of 40-day sulfate attack followed by 75 freeze–thaw cycles—which were then repaired using mortar or fine aggregate concrete. Mechanical properties were evaluated before and after repair. The results demonstrate that mortar-repaired columns exhibited 2.9%–7.7% higher ultimate load capacity than the exposed columns under both axial and eccentric loading. In contrast, columns repaired with fine aggregate concrete showed 3.6%–9.8% lower ultimate load capacity. While the lateral ductility factor improved significantly with both repair methods, the axial ductility factor and energy absorption capacity showed minimal improvement or even a slight deterioration due to uneven stress distribution. Load–deformation analysis revealed the superior performance of mortar-repaired columns, as evidenced by their enhanced axial load (Nu) and bending moment (Mu) capacity curves. Notably, the material homogeneity of mortar contributes more substantially to long-term durability than that of fine aggregate concrete, suggesting that the criteria for selecting repair materials in harsh environments need revision—particularly in regions experiencing pronounced dry–wet cycles or frequent freeze–thaw conditions.