An activation guideline for the resistive heating process of iron-based shape memory alloy embedded in concrete

In new and existing reinforced concrete (RC) structures, extensive research has proven that iron-based shape memory alloys (Fe-SMA) can be used as prestressing materials. As for the Fe-SMA embedded in concrete, the resistive heating method is usually adopted to activate its shape memory effect to develop prestress in the concrete. The prestressing process of the Fe-SMA involves multiple physical fields such as electric currents, temperatures, and mechanics. An accurate and standardized implementation of the activation process is essential for the Fe-SMA to effectively generate the expected prestress in the structure. However, data drift can occur when thermocouples measure the temperature of the Fe-SMA through which current flows and non-contact heat sensors can not measure the temperature of the Fe-SMA embedded in the concrete. It is also difficult to theoretically estimate the activation time and temperature of the Fe-SMA in concrete. The pre-test or empirical values are usually required to estimate the activation current and time for reaching the target temperature, compromising the construction efficiency. Additionally, the selection of activation parameters varies wildly for Fe-SMA with different specifications and sizes. There are currently no standardized guidelines for the electrical resistive activation of the Fe-SMA. Therefore, a multi-physical finite element model was developed to study the self-prestressing process of the Fe-SMA embedded in concrete and predict the temperature distribution and prestress of the Fe-SMA and concrete. The effects of the current density, the Fe-SMA shape and size, and the concrete cover depth were analyzed. In the end, an activation guideline for the electrical resistive heating of the Fe-SMA was offered to facilitate the construction and activation process in field engineering.