The feasibility of new strengthening methods using a novel iron-based shape memory alloy (Fe-SMA), also known as 'memory steel', has been shown in several scientific investigations. These techniques have high potential for being cost effective ways to strengthen and prestress existing concrete structures. For more than two years, these new strengthening methods have also been available on the construction market. Several site applications have been carried out so far, including externally applied memory-steel strips and near-surface mounted ribbed memory-steel bars. This study describes selected site applications.
The first site applications present the installation and activation of end-anchored memory-steel strips ('re-plate'). The strips were mechanically fixed to the concrete substrate using a direct-fastening system with nails. Activation of the shape-memory effect and hence prestressing was accomplished either by resistive or infrared heating. On one object, successful reduction of existing crack width was demonstrated and monitored with Omega-deformation gauges. The temperature of the strips was monitored with thermocouples or a built-in sensor.
In a second application category, memory-steel bars ('re-bar') were used to strengthen a cantilever slab of a residential building in the negative bending moment area. After grooves were cut into the concrete surface, the bars were inserted and bonded at both ends with a cementitious mortar. The shape-memory effect was activated by infrared heating of the bar over its unbonded length. After the bar-temperature had reached ambient level again, the remaining groove was also filled with mortar. In this site-application, heating of the memory-steel was monitored with temperature sensors.
Building owners, contractors, as well as structural engineers responded very positively to the novel strengthening techniques. The presented examples demonstrate the successful application of new effective strengthening methods for retrofitting of existing concrete structures using memory-steel products.