Congested reinforcement may lead to difficulties with compacting concrete and reduce the connection efficiency. To overcome this problem, using large-diameter longitudinal rebar to replace medium-diameter longitudinal rebar to reduce the number of longitudinal rebars may be a useful mean. However, the seismic behavior of precast concrete (PC) columns with different-diameter longitudinal rebars was still unclear. In order to evaluate the influence of large-diameter longitudinal rebar replacement on the seismic behavior of PC columns, a series of large-scale reinforced concrete (RC) columns adopting similar concrete strength, longitudinal rebar ratio, and transverse rebar ratio was fabricated and tested. Six of the columns were prefabricated with grouted sleeve connections and the remaining two were cast in place (CIP) for reference. The longitudinal rebar diameter varied from 18 mm to 32 mm. A low-cycle reversed horizontal load was applied to study their seismic performance, including failure modes, load-bearing capacity, hysteresis behavior, stiffness degeneration, and energy-dissipation capacity. The test results showed that the PC column with large-diameter longitudinal rebar replacement performed similarly to CIP columns in general. The column with large-diameter longitudinal rebar suffered significant bond-slip between longitudinal rebar and concrete, especially for columns with a high axial compressive ratio of 0.6. It may be of detriment to the seismic behavior of the columns to some extent. Additionally, with the increase in the diameter of longitudinal rebar, the ductility and energy-dissipation capacity of PC columns were reduced slightly. In the grouted sleeve region, a local rigid zone was formed, making its overall lateral stiffness higher than that of corresponding CIP columns. It is recommended to extend the strengthening zone, with closer transverse reinforcement, to two times the column depth of the PC columns with grouted sleeve connections, as the plastic hinges may be shifted upward.
