Iron-Based Shape Memory Alloys in Civil Engineering – from Early Developments to Market Implementation

Efficient structural maintenance is one of the crucial areas in structural engineering for the decades to come. While demolition and complete rebuilding is more and more avoided, retrofitting in case of structural deficiencies and/or change of use is constantly increasing. Traditionally, non-prestressed applied (and occasionally prestressed) composite or steel reinforcements are applied in case a structure requires an upgrade in order to meet criteria at both service load (and fire) and ultimate load state. This paper presents a powerful alternative to currently available technologies: iron-based shape memory alloys (Fe-SMA). Shape memory alloys (SMAs) are able to regain their initial shape after having been deformed (prestraining). This recovery in strain can either occur immediately after unloading, or upon an activation at elevated temperatures. By prohibiting strain recovery, a tensile stress (recovery stress) develops in the alloy, which can be introduced as a compression (prestress) force to the parent structure via an end-anchorage. While Nickel-Titanium SMAs are well-established in other industries, iron-based SMA offers an economically more viable solution specifically for the construction industry. After early developments in Japan in the second half of the 20th century, a well-adapted alloy composition was patented by Empa about twenty years ago. This paper presents early R&D activities at the level of material behavior towards first prototype developments for applications in structural engineering, specifically for the repair of existing constructions. It is shown how these first inventions paved the way for production at industrial scale of commercially available products. The manuscript discusses short- and long-term mechanical properties, advantages for on-site applications, structural behavior at service, fire, and ultimate load state, and how life expectancy of existing structures can be significantly enhanced by applying the said methods. Eventually, economic and ecological aspects of the presented retrofitting techniques are also discussed.