Experimental study on shear behavior of square RC columns wrapped with CFRP strips under lateral static and pendulum impact loadings

To study both the static and dynamic shear resistance of reinforced concrete (RC) columns with a small shear-to-span ratio retrofitted by carbon fiber reinforced polymer (CFRP) strips, two groups of eleven square RC column specimens with a shear-to-span ratio of about 1.4 were designed, fabricated and tested. The first group having four specimens was tested under lateral static loading, and the other group with seven specimens was subjected to lateral pendulum impact loading. The effects of stirrup ratio, CFRP reinforcement ratio, and impact velocity on failure mode, lateral shear resistance and deformation capacity were investigated. The results demonstrated that with similar static theoretical shear capacities, the CFRP-strengthened column (without stirrups) exhibited higher static deformation capacity (+88 %) than the column with internal steel stirrups (without CFRP). When subjected to lateral impact loading, the columns developed low strains on both the stirrups and CFRP strips before the peak impact force, both of which contributed little to the punching shear resistance of the columns. An existing analytical model of calculating the punching shear resistance of RC members under lateral hard-impact loading was modified by considering the variation of inertial force with the impact force, which showed good prediction on the occurrence of punching shear failure or not. An energy-based index with a threshold value of 0.1 can be used to assess the diagonal shear failure of RC members wrapped with or without CFRP strips under lateral hard-impact loading during the global response stage.