Shape memory alloys recover deformations induced at lower temperatures after being heated above a transformation temperature; restraint of this shape recovery generates relatively large stresses. These stresses are used to transfer corrective forces to structural systems for strengthening and repair. For this purpose, shape memory rods are pre-elongated, anchored to the structure, and subjected to electrical resistance heating to transfer corrective forces to the structure. The project used iron-based shape memory alloys of relatively low cost; the alloy composition was selected to yield relatively high and stable levels of restrained-shape recovery stresses. Laboratory tests verified the ability of pre-elongated rods anchored onto damaged structural systems to restore structural integrity through application of corrective forces. Subsequent damaging effects could also be overcome by electrical resistance reheating of rods. A reinforced concrete bridge structure with beams lacking sufficient shear strength at longitudinal bar cutoff locations was selected for field demonstration of the technology. A design methodology was developed and verified through laboratory tests simulating conditions of the selected bridge structure. Subsequently, a detailed design was developed and the approach was implemented under field conditions. Application of corrective (posttensioning) forces to structural systems using shape memory steel provides an efficient, rapid, and convenient approach for repair and strengthening of damaged or deficient structures. The relatively large recoverable strains of shape memory alloys help reduce losses of posttensioning forces. The approach suits application to diverse structures for corrective and strengthening effects.

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Constant-force springs were designed with Ni-Ti pseudoelastic alloy. Through local concentration of flexural deformations, the design produces ''pseodoelastic hinges'' under relatively small deformations. Constant moments at these ''pseudoelastic hinges'' produce a relatively constant level of spring force under increasing deformations. Local concentration of flexural deformations are achieved in this design through bracing the spring element outside the locations where deformations concentrate. Experimental data verified the desirable constant-force behavior of the springs and their desirable fatigue life, relaxation attributes, and corrosion resistance. The pseudoelastic spring design presented here is subject of approved and pending U.S. patent applications.