Bond behavior of iron-based shape memory alloy reinforcing bars embedded in concrete

Iron-based shape memory alloys (Fe-SMA) have shown a strong potential for strengthening applications in concrete structures. This paper presents an experimental study on the bond strength and bond-slip behavior of ribbed Fe-SMA bars embedded in concrete. A total of nineteen pullout tests were conducted to study the bond behavior of 16-mm Fe-SMA bars embedded in concrete specimens with different levels of passive confinement. Prior to specimen fabrication, the bars were pre-strained (at 4% or 8%) and then activated by resistive heating after concrete casting. The experimental study considered different activation temperatures (160 °C, 300 °C, and no activation) to evaluate their effects on bond performance. Five specimens with conventional steel bars were also tested to establish a direct comparison with conventional reinforcement. Unconfined pullout specimens failed by splitting of concrete in most of the cases, while confined specimens presented pullout failures and higher bond strengths. In general, the Fe-SMA bars presented similar bond performance as conventional steel bars. However, the bond strength of unconfined specimens was reduced with the activation temperature. For confined specimens, the activation temperature had a minor effect on the bond strength. Local bond stress-slip relations derived from tests have also been compared with two analytical bond-slip laws available in the literature. Finally, the transfer length of Fe-SMA bars in prestressing applications has been estimated based on the experimental data and a simple analytical model of the bond stress distribution along the embedment length of a bar.