Prestressing effect of shape memory alloy reinforcements under serviceability tensile loads

Abstract

Repairing and strengthening of existing aged steel-reinforced concrete structures is a major challenge. Today, much of the repair work completed is insufficient and brittle. A promising new solution for repair and strengthening tasks is the use of iron-based shape memory alloy (Fe-SMA). The pre-strained Fe-SMA components enable the pre-stressing of existing building components due to the heat-triggered contraction of the steel. Thus, deflections can be reduced or even recovered. In addition, the cracking process can be adapted, and an improvement in the load, under which the first crack appears, is possible. In this paper, the effects of pre-stress generated by activated Fe-SMA rebars, which were centrally embedded inside of a concrete specimen, are shown. The objective of the study is to quantify the improvement in the loads of the first crack and show the influences of the pre-stressing on the load-bearing behavior and the cracking process. For this purpose, axial tensile tests were performed on concrete bars with height, width, and length of 50 mm, 70 mm, and 900 mm, respectively. These were compared to usual construction steel rebars, pre-strained but nonactivated Fe-SMA rebars, and activated Fe-SMA steel rebars. The evaluation of crack patterns and openings was done using digital image correlation (DIC). The pre-stressing of the concrete causes an increase in the first crack loads of more than 150%, which indicates a clear improvement in the state of serviceability limit.