A large number of concrete bridges need to be repaired due to durability problems. This research focuses on evaluating the bond quality of the repair material, which has the highest influence on the durability of the repair. Onsite experimental research is performed on the vertical surfaces of the column of the Klara Jug bridge, which is located in the southern part of the Zagreb. Since the experimental research is carried out in parallel with the ongoing repair works on other bridge elements, the experimental conditions correspond to the real construction site conditions. Simulation of the actual site rehabilitation process was carried out on the test fields, which included removal of the damaged concrete by hydrodemolition at a depth of 1.5–2 cm, washing of the concrete by roto nozzle and wet spraying of the repair mortar. The results of the pull-off tensile bond strength show that there is an optimum period for spraying of repair mortar after washing the concrete surface. Optimal time for application of mortar is 90 min after completion of washing of the vertical concrete surface by the roto nozzle. In other weather conditions, the application time depends on the expected evaporation of water from the surface. The tensile strength of the bond between concrete and the repair mortar is 62% lower if the concrete is not washed by the roto nozzle prior to the application of the repair mortar. This investigation can serve as a practical guide for the repair of vertical surface of concrete elements of bridges, overpasses and viaducts.