AbstractThis paper investigates the effects of axial preload on concrete columns that have been strengthened by placing concrete jackets. Most of the research in the world in the area of strengthening concrete columns has been performed without taking into account preloading effects. It is normal experimental practice to apply an axial load after the construction of a jacket. In reality, preloading is the true situation when strengthening existing buildings due to service loading effects. The present paper presents experimental data so that the effects of preloading can be compared with a number of other similarly constructed and tested specimens. In addition, the results are compared to an original column and a monolithic column. A theoretical approach has been developed so that different concrete strengths, jacket strengths and applied axial loads could be converted to equivalent mechanical characteristics and axial loads and only the influence of the applied axial preload could be examined. The results showed that preloading improves the strength and the deformation capacity but decreases the initial stiffness.