Ductility and lateral load performance of substandard RC columns retrofitted with CFRP over plastered Surface

Abstract Seismic strengthening of existing reinforced concrete (RC) columns often involves applying carbon fiber-reinforced polymer (CFRP) jackets, which typically requires the removal of existing plaster layers to ensure direct bonding. While this practice is widely adopted, it increases construction costs, prolongs retrofit duration, generates considerable plaster waste, and may damage already low-strength concrete substrates. Previous experimental research has mainly focused on CFRP jackets applied directly to bare concrete, demonstrating their effectiveness in improving load-carrying capacity and ductility. However, the combined effects of plaster thickness, plaster strength, and CFRP jacket configuration on the seismic performance of square-section columns with retained plaster layers remain insufficiently explored. This study experimentally investigates the seismic performance of eight full-scale substandard RC columns (300 × 300 × 1350 mm) constructed with low-strength concrete (~10 MPa) and widely spaced transverse reinforcement, tested under reversed cyclic lateral loading while maintaining constant high axial load ratio of 60%. The experimental variables include three plaster thicknesses (12.5, 25, and 37.5 mm), two plaster compressive strengths (~3.5 and ~8.5 MPa), and single- or double-layer CFRP jackets. Test results demonstrate that retaining the plaster layer does not prevent significant ductility enhancement; in several cases, plastered CFRP-jacketed specimens achieved drift capacities comparable to or exceeding those of unplastered counterparts. These findings provide new experimental insights into the feasibility and effectiveness of CFRP jacketing over plastered concrete surfaces, highlighting a practical retrofitting approach that minimizes construction time, and environmental impact while maintaining structural performance.