Iron-based shape memory alloys (Fe-SMA) - a new material for prestressing concrete structures

Abstract

Shape memory alloys (SMAs) have the special property of remembering their shape after being deformed, either immediately upon unloading or by heating above a critical temperature. This shape memory effect can be used to prestress concrete without the need for ducts, anchor heads, oil hydraulic jacks or duct injections. Furthermore, it is possible for SMAs to be used for reinforcing strongly curved structures because no prestress force loss due to friction occurs during prestressing. However, the only available SMAs on the market are nickel titanium (NiTi)-based and are far too expensive for the construction industry. Therefore, Empa developed a low cost, iron-based shape memory alloy (Fe-SMA) suitable for concrete reinforcement. In this paper, the shape memory effect of Fe-SMAs is briefly explained on the atomic scale. Next, investigations and developments by a multidisciplinary team at Empa during more than 10 years in the field of prestressing of concrete with SMAs are outlined. In the early 2000s, researchers at Empa investigated if SMAs could be used in civil constructions. A concrete beam was reinforced with NiTi wires and short NiTi fibers were embedded in mortar prisms. These investigations showed that it was possible to prestress concrete structures using NiTi wires. Subsequently, in collaboration with the Structural Engineering and Joining Technologies Research Laboratories at Empa, a new Fe-based SMA was developed for civil engineering applications. Next, strips made out of this alloy were produced and used for reinforcing concrete structures. In concrete bars, Fe-SMA strips were centrally embedded and the Fe-SMA strips were also used for strengthening reinforced concrete beams. Ongoing research uses ribbed Fe-SMA reinforcement bars for reinforcing large-scale beams. Finally, this paper discusses the company (reFer AG) established in 2012 to promote and introduce Fe-SMA reinforcements to the construction market. Potential applications are presented for Fe-SMA reinforcement in new concrete structures and in various strengthening applications.