A major cause of damage in reinforced concrete (RC) structures such as bridges, building, and wharves is aging and structural deterioration under severe environments. Such deterioration as concrete cracks, corrosion of steel, and deformation of structural members can significantly degrade the structural performance and safety. Therefore, effective and easy-to-use methods are required to repair and strengthen these concrete structures. In addition, it sometimes needs to increase the load capacity of structures for carrying additional loads due to the change of the purpose of structures and the new extension of existing structures. In this study, the stress-strain curves of the specimens with a low ratio of confinement (represented by the area ratio) show a descending trend after reaching the maximum stress, while the stress.strain curves of the specimens with a high ratio of confinement show an ascending trend until failure. Constitutive models of a confined concrete are developed based on the compressive test results. Good agreement is observed between the experimental (actual) and analytical (predicted) stress-strain curves, demonstrating the validity of the proposed constitutive model and the parameter values. Such a model is useful for design and analysis of concrete structures strengthened by the proposed narrow strips of CFRP laminates.