In this graduation thesis I captured investigation studies in the field of bond stress between reinforcement bars and concrete. Investigations were carried out in accordance with standard SIST EN 10080: 2005 on normal and high strength concrete. A steel reinforcing bar with a diameter of 20 mm was inserted into a standardized mold. We created an appropriate anchor length with a rubber tube. We then poured both types of concrete into molds. Test specimens were stored for 28 days in water and, after that, we made the pull-out test. With the help of the testing device we pulled the reinforcing bars out of the concrete and recorded the data at the same time. The data was edited and shown in a graph, which shows the bond stress according to the slip between the bar and the surrounding concrete. In conclusion, the results were compared with those obtained from the bars with diameters of 12 mm. A comparison of pull-out tests carried out on the bars of 12 and 20 mm have shown that, with the use of high strength concrete, the diameter of the bars does not affect the bond between the reinforcement and the concrete.