Shear strengthening of reinforced concrete beams using shape memory alloys

Abstract

[eng] Shear failures in RC members are associated with brittle collapses, for this reason, shear strengthening of existing structures is often required. This paper presents an innovative system for retrofitting shear critical reinforced concrete (RC) beams, taking advantage of the shape memory effect of shape memory alloys (SMAs). The new system uses the SMAs as active external reinforcement thanks to the recovery stresses that SMAs are able to generate during their activation under restrained strain. An experimental campaign has been carried out on small-scale RC beams with rectangular cross-section (80-mm wide and 150-mm deep) strengthened in shear by means of 3 mm diameter Ni-Ti-Nb wires externally installed around the beams, forming a pseudo-spiral, in order to actively confine, or transversally prestress, the RC members. The experimental results show an increase of the shear strength around 90 % and 115 % of the retrofitted beams depending on the strengthening configuration, and also a more ductile shear failure. The effect of the geometrical imperfections occurred during the installation of the external reinforcement has been studied, and the possible prestressing losses due to these imperfections have also been evaluated. It has been concluded that the Ni-Ti-Nb wires show a promising performance despite possible imperfections in small-scale members. Finally, a previously developed shear model has been used to predict the shear strength of the tested beams. The recovery stresses considered for the pseudo-spirals were derived from separate material tests simulating similar imperfections. The predictions given by the shear model are satisfactory, with an average value of the ratio Vtest/Vrd equal to 1.14 and a coefficient of variation of 8.4 %.