Self-Post-Tensioning for Concrete Beams Using Shape Memory Alloys

Abstract

This study explores the use of shape memory alloys for self-post-tensioning concrete beams. SMAs have the ability to regain their original shape after being deformed up to 6–8% strain. This shape recovery is a result of an underlying reversible solid-solid phase transformation, which can be induced by either a stress (superelastic effect) or a temperature change (shape memory effect). The shape memory effect can be exploited to prestress concrete. The heat of hydration of grout can thermally activate SMA tendons to obtain self-post-tensioned (SPT) concrete. NiTi-based SMAs are promising due to their corrosion resistance and resistance against low frequency/cycle fatigue failure. NiTiNb alloys are a class of SMAs that exhibit a wide temperature hysteresis and transformation temperatures near the service temperatures required for practical application. Here, NiTiNb shape memory alloys are studied to design an optimized SMA that can be activated using hydration heat. The material design and characterization of the SMA tendons are discussed. The temperature increase due to the heat of hydration of four commercially available grouts is investigated. The bond behavior of SMA tendons is evaluated through pullout tests. Digital Image Correlation method is used for monitoring the slippage of the SMA tendons. The feasibility of developing SPT concrete is assessed through experimental studies. The use of SMAs, which possess high fatigue and corrosion resistance, as post-tensioning tendons in concrete members will increase the service life and provide life cycle cost savings for concrete bridges. The replacement of steel tendons with SMA prestressing tendons will prevent corrosion-induced deterioration of tendons in concrete structures. The use of heat of hydration of grout to activate the shape memory effect of SMA tendons will provide self-stressing capability. This will greatly simplify the tendon installation. The need for jacking equipment or electrical source will be eliminated.