Confinement behavior of plain concrete columns jacketed with ultra high‐performance concrete under axial compression: Experimental and analytical study

Existing buildings and bridge components often require strengthening due to aging, deterioration, and increased loading. Ultra‐high‐performance concrete (UHPC) has emerged as a promising strengthening material due to its high strength, tensile ductility, and durability. This study presents combined experimental and analytical investigations on circular normal‐strength concrete (NSC) columns fully jacketed with UHPC and axially loaded on the composite section. The parameters considered include UHPC jacket thicknesses of 15, 30, and 45 mm, and two interface conditions, namely untreated (NS) and water‐pressure‐jet‐treated (WPJ) surfaces. The results demonstrate that UHPC confinement significantly enhances axial load and deformation capacities, with performance improving as the UHPC jacket thickness increases. WPJ surface preparation enhanced bond integrity and strain compatibility between the NSC core and the UHPC jacket, resulting in improved composite interaction, more efficient load transfer, and greater confinement effectiveness compared to specimens without surface treatment. An analytical model is developed by incorporating thick‐walled cylinder theory to predict load–displacement and load–hoop strain responses. The predicted axial capacities closely matched the experimental results, with a mean strength ratio of 0.97 with a COV of 3%. It also yielded a mean stiffness ratio of 1.04 with a COV of 8%, demonstrating the reliability of the proposed modeling approach.