Bond properties between steel and concrete form essential input for crack width calculations. For hardened concrete these properties are well known. There is, however, hardly any information about the bond properties of young concrete. As a consequence of this the reliability of crack width predictions in hardening concrete is not very high. In order to improve our knowledge about the bond between steel and hardening concrete, pull-out tests have been performed on normal strength and high strength concrete according to RILEM Recommendations. Reinforcement bars ( 16 mm) were cast in cubes, with rib length 200 mm. The specimens were cured at different curing temperatures (20°C, 30°C, 40°C isothermal and semi-adiabatic). This was realised by using temperature-controlled moulds. In order to find a correlation between the development of cube compressive strength and pull-out strength the test specimens were cured under the same conditions and tested at the same age (after 8h, 24h, 30h, 48h and 28 days). From the experimental results the development of bond stress is formulated in an alternative bond-slip relation. This formulation could be implemented as a user-defined model in DIANA for a more accurate prediction of the cracking behaviour of reinforced concrete at early ages.