Retrofitting of Plastered Low-Strength Square Concrete Columns: Axial Performance of CFRP Jacketing without Plaster Removal

Abstract Seismic retrofitting of existing reinforced concrete (RC) columns with fiber-reinforced polymer (FRP) jackets typically requires removing plaster to ensure direct bonding. Although widely practiced, plaster removal increases labor, cost, and environmental impact and can damage the weak concrete surfaces common in substandard buildings. Previous experimental studies have mostly examined FRP confinement applied directly to concrete; however, the axial behavior of columns strengthened without plaster removal remains largely unexplored. In particular, the combined effects of plaster thickness, plaster strength, and FRP jacket configuration have not been systematically evaluated for square columns with retained plaster. This study addresses this gap by experimentally investigating the monotonic axial behavior of plastered concrete columns retrofitted with carbon FRP (CFRP) jackets. Twenty unreinforced square columns (275 × 275 × 550 mm), cast with low-strength concrete (approximately 10 MPa), were tested under axial compression. The parameters varied were plaster thickness (12.5, 25.0, and 37.5 mm), plaster compressive strength (approximately 4.5 and 9.0 MPa), and CFRP jacket configuration (one or two layers). Test results show that, within the engineering-relevant axial strain range (≤1.5%), CFRP jacketing over plastered surfaces effectively maintains the axial load-carrying capacity regardless of plaster properties, demonstrating adequate confinement. These findings confirm the feasibility of CFRP retrofitting without plaster removal and provide a practical framework for strengthening RC members constructed with low-strength concrete. A new constitutive model was developed to represent the axial stress–strain response of plastered CFRP-jacketed concrete, explicitly incorporating plaster thickness, plaster strength, and CFRP jacket configuration. The proposed model captures the experimental behavior with high accuracy and offers a reliable tool for the design and assessment of plaster-interlayered CFRP-strengthened columns.