Considerable age of a very high number of bridges conjointly with a steadily increasing amount of traffic and changes in design philosophy (e.g. earthquake engineering) have made maintenance needed and retrofitting become more and more important over the years. Retrofitting can become necessary both for flexure or shear enhancements. Existing steel solutions for shear strengthening are very laborious and complex, and the durability of the steel construction is questionable. As an alternative to steel solutions, carbon fiber reinforced polymer (CFRP) sheets or strips are used for shear strengthening of reinforced concrete beams. But, prestressing of CFRP sheets or strips is hardly applicable. However, a prestressing of a shear strengthening has the advantages that the width of existing shear cracks can be reduced and the stresses in the internal steel stirrups are reduced.
Therefore, in this study, a new iron-based shape memory alloy ('memory-steel') in the form of U-shaped (stirrups) ribbed bars with a nominal diameter of 12 mm were used in combination with sprayed mortar for shear strengthening of reinforced concrete (RC) structures. The memory-steel bars were activated with electric resistive heating. The activation resulted in a prestress of about 300 N/mm² in the memory-steel reinforcement and consequently in vertical compressive stresses in the web of the RC beams. Large-scale experiments on T-beams with a height of 0.75 m and a total length of 5.2 m were performed to show the practicability and efficiency of the memory-steel shear strengthening. Promising results have shown that the new strengthening system works well in practice. The shear capacity could be increased significantly. Furthermore, at the serviceability limit state, the prestressed memory-steel stirrups reduced the overall beam deflections, the stresses in the internal steel stirrups, the number of cracks, and the crack widths.