Repair and Strengthening of Concrete Structures Through Application of Corrective Posttensioning Forces with Shape Memory Alloys

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.