Title: Mechanical Performance of Iron-Based Shape-Memory Alloy Ribbed Bars for Concrete Prestressing
Author(s): Julien Michels, Moslem Shahverdi, Christoph Czaderski, and Raafat El-Hacha
Publication: Materials Journal
Appears on pages(s): 877-886
Keywords: concrete prestressing; ductility; iron-based shape-memory alloys; recovery stress; ribbed bar; stress relaxation
A ribbed bar with a conventional shape and rib geometry as for reinforcing steel but made of a novel iron-based shape-memory alloy (Fe-SMA) is investigated regarding its mechanical performance as a method for concrete prestressing in the field of retrofitting. The alloy has the specific ability to regain its initial shape upon heating at an elevated temperature after having been permanently deformed at ambient temperature. Due to the ribbed surface, the shape recovery of a bar embedded in concrete or other cementitious materials is hindered. Hence, a tensile stress, generally known as the recovery stress, is generated in the bar itself, resulting in a prestress in the concrete. The present campaign aims at demonstrating the main mechanical characteristics of the alloy such as its behavior in tension, possible recovery stresses based on the initially applied prestrain and the subsequent maximum activation temperature, as well as stress relaxation for long-term behavior. Besides the highly ductile tensile behavior, the alloy exhibits prestresses in the range of 250 to 450 MPa (36.2 to 65.2 ksi), depending on the prestraining and activation conditions. Stress relaxation losses in the range of 10% after 1000 hours were observed.