Application of NiTiNb shape memory alloys in the end region of prestressed concrete bridge girders

Due to the growing demand for long-span bridges, pre-tensioned bulb-tee (BT) girders with numerous prestressing strands are utilized in bridge construction. To efficiently place a large number of prestressing strands, the girders are designed with wide flanges, a narrow web, and a deep height. This recent bridge design trend raised concerns about splitting cracks at the end region of the girders as the prestress is released with accompanying high bursting and/or spalling forces. In this study, the applicability of shape memory alloy (SMA) in the girder end region is evaluated numerically using finite element method (FEM). First, an experimental study is conducted to investigate the recovery stress (prestressing force) of NiTiNb bars inside the concrete through electrical heating. In the numerical analysis phase, the recommended model in the PCI Bridge Design Manual (BT-72) is analyzed using FEM and compared to numerical models in which one or two end steel stirrups are substituted with SMA stirrups capable of applying prestress in the vertical direction by heating. The end region behavior is estimated qualitatively by evaluating the maximum principal strain at web regions and quantitatively observing the crack propagation. SMA stirrup shows a significant impact in reducing the degree and propagation of end region cracks.