Large-Scale Experimental Study on the Retrofitting Effect of Concrete Filling in Hollow Reinforced Concrete Columns

When hollow reinforced concrete (RC) columns are retrofitted with concrete filling, the hollow interior cannot always be accessed to treat the internal surface. Without this treatment, an optimal bond between the newly introduced concrete and the existing column cannot be established; as a result, the two parts do not behave as a single unit. This study conducted cyclic loading tests on three types of large RC column specimens: a hollow RC column, a hollow RC column with a solid section at the column base, and a hollow RC column retrofitted with concrete filling (lacking inner surface treatment). The aim of this study was to investigate the effectiveness of a concrete-filling retrofitting method for hollow RC columns when the filled and existing portions behave separately, assuming a situation in which the bond between the filled and existing portions cannot be strengthened. The results indicated that concrete-filling retrofitting can suppress the inward buckling of the internal longitudinal bars, thereby enhancing the flexural deformation capacity of hollow RC columns to some extent. However, this did not prevent the significant axial settlement and reduction in the lateral load capacity that occurred when the inner longitudinal bars buckled. This is because the filled portion cannot resist axial forces when the filled and existing portions behave separately. Additionally, it was suggested that retrofitting with concrete filling does not substantially improve the shear resistance of a hollow RC column when the filled section behaves separately from the existing section.