The current paper studies the force transfer mechanisms in retrofitted columns containing dowels crossing the interface of old and new concrete. It includes 10 specimens of square section (150x150x500mm) of 24,37 MPa nominal concrete strength with 4 longitudinal steel bars of 8mm diameter (500 MPa nominal strength) and different transverse reinforcement ratios (ω c = 0,075, ω c =0,15 ). All columns were subjected to initial axial compression up to maximum load. After repair with thixotropic high strength concrete, all columns were retrofitted with RC jacket of 80mm width of 31,8 MPa concrete strength, including 4 longitudinal bars of 8mm diameter (500 MPa nominal strength) and different confinement ratios(ω cj = 0,035/0,071/0,142 ). The retrofitted columns contain dowel bars of either 10 or 14mm diameter that cross the interface. Two loading patterns were selected to test the strengthened columns, the behavior of which is investigated through P-δ, energy absorbed diagrams and the levels of ductility achieved. The results indicate that the initial damages affect the total behavior of the column and the capacity of the interface to shear mechanisms and to slip: a) the maximum bearing load of old column is decreased affecting at the same time the loading capacity of the jacketed element, b) suitable repair of initially damaged specimens increases the capacity of the jacketed column to transfer load through the interface.