Nonlinear Features of the Bond-Slip Ascending Branch

The bond between a deformed rebar and concrete is determined in pullout tests, using cylindrical concrete specimens with centrally embedded rebars, having a predetermined number of bonded ribs. In these experiments, the external measured parameters are the applied pullout load at the rebar end and the slip. This setup does not allow examination of the evolution of local damage in the concrete near the ribs, and the experimental research yields only the average bond stress versus slip relationship, which characterizes the bond property. The bond-slip curve is composed of an ascending branch along a short slip range and a following descending branch along the major slip range. Commonly, the different studies focus on the descending branch, although the maximum bond resistance is gradually developed along the ascending branch. The present study investigates in detail the ascending branch, which is commonly described by a smooth monotonic curve. There are no experimental data in the literature on the evolution of the resistance to pullout loading and the accompanying damage along the ascending branch. This study investigates the different stages of bond resistance development up to the maximum bond stress, and examines the local damage growth and its effect on the bond-slip curve smoothness. This investigation identifies that the ascending curve is not a smooth curve at all and it is composed of several segments with localized gradient changes. A complementary experimental study using an innovative experimental setup allows monitoring the local damage and strain fields in the concrete near the ribs during this part of the experiment. These tests provide new insight and complement the present study findings that the local gradient changes along the ascending curve result from the gradual formation and extension of interior damage with an accumulating effect during loading.