Bond behavior of reinforced high-strength concrete under high-cycle fatigue pull-out loading

In this paper experimental investigations of the bond behavior between high-strength concrete and steel reinforcement under static and cyclic loading are presented. The tests were mainly conducted with beam-end specimens and supplementary with classical pull-out samples. The experimental program included 67 quasi-static and 56 cyclic tests on two high-strength concretes and one normal strength concrete, whereby a very short bond length of twice the bar diameter was used. The results of the static tests showed a linear relationship between the bond strength and the concrete compressive strength for a pull-out failure. In the static beam-end tests, splitting cracks were generally observed resulting in a lower bond strength than that from the pull-out tests. Based on the static bond strengths, samples were exposed to cyclic loading with up to 20 million load cycles. In these tests, both the influence of the upper stress level and the loading frequency was investigated. In general, the number of load cycles until fatigue failure varied greatly despite almost the same stress range. The causes of the excessive scattering are discussed in particular in relation to existing S-N curves. With regard to the increase in slip under cyclic load, both the specimens with and without failure showed a clear dependence on the concrete compressive strength. Based on these results, a modified approach for the displacement factor is presented. Finally, the results are critically reviewed and recommendations for further investigations are given.