Pre- and post-yield bond, tension-stiffening, and cracking in ultra-high performance fiber reinforced concrete

In reinforced concrete elements, tension-stiffening strongly influences deflections and crack-widths at the serviceability limit, and plastic hinge rotation at the ultimate limit. Modeling has shown the impact of tension-stiffening at the ultimate limit to be particularly important when ultimate failure is governed by reinforcement rupture, which has been is common in ultra-high performance fiber reinforced concrete (UHPFRC) elements. It is further known that the magnitude of tension-stiffening and crack opening is proportional to both the reinforcement ratio of the tension chord and the diameter of the reinforcing bar. Despite this understanding, very little testing has been undertaken to quantify the bond between larger diameter reinforcement and UHPFRC and also on the resulting impact to tension-stiffening and concrete cracking. To address these issues, in this paper a series of bond and tension-stiffening tests are undertaken on ultra-high performance concretes both with and without steel fibers. The result of the experiments is used to develop a local bond stress slip relationship applicable to both pre- and post-yield. When implemented in a partial-interaction model, it is shown that the new bond model can be used to predict pre- and post-yield tension-stiffening and crack-widths.