Abstract
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.
Highlights
Built infrastructure, such as buildings, bridges, highways, dams, tunnels, water supply systems, and many others guarantee the proper functioning of our daily living standards
Similar as in usual reinforced concrete, the concrete protects the reinforcement for corrosion and larger covers are recommended for real applications
Rupture of the internal steel stirrups was hearable and some were visible via the large cracks (Fig. 17)
Summary
Built infrastructure, such as buildings, bridges, highways, dams, tunnels, water supply systems, and many others guarantee the proper functioning of our daily living standards. Prestressing a reinforced concrete structure is an even more efficient technique in terms of durability and serviceability, as cracks and deformations can be reduced or at least prevented from further growing This technique is a common construction method all over the world: a large number of bridges are fully or partially designed this way, especially to overcome large spans and to guarantee the mentioned satisfactory behavior under service loads. The application of shape memory alloy stirrups can provide the possibility to implement prestressed shear strengthening. Shape memory alloys have the characteristic that they, after having been pre-deformed, move partly back to their initial shape when subjected to high temperatures This characteristic leads to a completely new philosophy in terms of a prestressing technique for concrete constructions based on shape memory effects [13–. [31] used 25 mm wide and 0.5 mm thick iron-based shape memory alloys (Fe-SMA) strips to shear strengthen 2.3 m long and 0.35 m high T-beams. A further aim of the investigation was to show the practicability of the new strengthening technique for on-site applications
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