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As a key load-bearing component in building structures, the effective strengthening of reinforced concrete (RC) columns is critical to enhancing their structural reliability and service life. To tackle the issue of excessive self-weight from the increasing section strengthening method and further optimize the seismic performance of encased steel strengthening, this paper presents a novel composite strengthening method for RC columns, which is characterized by using Lightweight Alkali-Activated Slag Concrete (LAASC) as the strengthening layer and an X-type encased steel structure. By conducting axial compression tests on six columns and utilizing in-depth research on small eccentric compression and hysteresis performance through numerical simulation, the specific effects of different strengthening materials and encased steel forms on the mechanical properties of the columns are systematically explored. Experimental results indicate that compared to ordinary concrete strengthening layers, LAASC can reduce the self-weight of the strengthening layer by 25%, boost the bearing capacity of the strengthened components by 37%, and enhance the vertical deformation capacity by 100%. Numerical simulation also confirms that X-type encased steel composite strengthening can effectively control bending deformation under small eccentric compression, reducing lateral deflection by 30–35% compared to un-strengthened columns. Under horizontal reciprocating loading, the cumulative energy dissipation of X-type encased steel composite-strengthened columns is 15–30% higher than that of traditional steel encased composite-strengthened columns, reflecting the diagonal bracing effect of the X-type batten plates. Full article

The structural performance of damaged open-web type of steel encased reinforced concrete (SRC) beam-columns after retrofitting was experimentally investigated. The experimental parameters were the open-web type of the encased steel and the maximum tip displacement of the columns during the initial loading. First, each column was cyclically loaded to the targeted displacement. Subsequently, the test columns were retrofitted and reloaded. The damaged portions of each column were retrofitted with the polymer cement mortar, and the epoxy resin was injected into the cracks. The experimental results indicated that the measured stiffness of the retrofitted columns was lower than the initial ones, while the displacements experienced in each column were different. The lower stiffness might be attributed to deterioration of the concrete rigidity, low rigidity of the resin and imperfect injection of the resin. Numerical analyses were also conducted to evaluate the retrofitted column behavior. The effect of the strain hysteresis of concrete at the first loading was considered for the behavior at the second loading. The analytical results predicted the experimental behaviors fairly well, which implies the validity of the analytical methods presented in this paper for evaluating the structural performance of the retrofitted SRC columns. Full article