Experimental and Numerical Study on Reinforced Concrete Columns Strengthened with Lightweight Alkali-Activated Slag Concrete and X-Type Encased Steel

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.