Evaluation of prestressing with a shape memory alloy

The paper addresses the possibility of using shape-memory effect in Nitinol (Nickel-Titanium) alloy to remedy prestress losses in aging concrete beams. Straight annealed NiTi in the low temperature martensite phase can undergo large plastic deformations, This deformation and strain energy is recoverable when the material transforms to the higher temperature (about 55 degrees C) austentite phase. This phenomena can be exploited to induce curvature, hence prestressing, by embedding NiTi wires and ropes in a structural member. Depending on the type of wire used its actuation effect can be reversed as necessary, creating the possibility of a smart prestressed concrete bridge of the future. Materials aspect studied in this project include the bond between NiTi wires and the host composite material, through mechanical testing and microscopy. An evaluation of the phase transformation is conducted using Acoustic Emission (AE) technology. AE event rate, locations and characteristics were evaluated during the actuation process.