Smart Prestressing with Shape-Memory Alloy

Shape-memory effect in nickel-titanium (nitinol) alloy was successfully utilized as a way of inducing additional prestressing in concrete. Strands made with NiTi (nickel/titanium) shape-memory alloy (SMA) wires were elongated beyond their plastic limit and subsequently were embedded in model concrete (mortar) beams. Upon electrical heating, a martensite-to-austenite phase transformation takes place and the material undergoes large shrinkage strains. This strain energy can be used to generate a significant prestressing force in concrete. Potentially, this provides a means of creating a “smart bridge,” where the amount of prestress can be increased or decreased as needed. Such a structure could actively accommodate additional loading, or remedy prestress losses over time. Properties of the SMA material and the micromechanics of its bond with concrete were investigated. Strands made with SMA wires were electrically actuated to induce deflection and failure in concrete (mortar) beams. Optical microscopy and the acoustic emission technique were used to understand the actuation mechanisms.