Feasibility of iron-based shape memory alloy strips for prestressed strengthening of concrete structures

Abstract

Near-surface mounted reinforcement (NSMR) is a strengthening method for concrete structures, such as buildings or bridges. NSMR involves strips or bars that are glued into grooves in the cover of the concrete. In this paper, a feasibility study is presented that uses iron-based shape memory alloy (Fe-SMA) strips instead of fiber-reinforced polymer (FRP) strips for NSMR. SMAs can more easily be prestressed than FRP. Because prestressing of SMAs does not require any mechanical jacks and anchor heads, the additional openings on the concrete surface beside the grooves that are needed to clamp the NSMR are significantly smaller.The recovery stresses (i.e., the prestresses) were investigated in a tensile testing machine combined with a climate chamber. The temperature of the strips was increased up to 160°C to provoke the phase transformation in the SMA. The bond behavior of the Fe-SMA strips glued into a groove with cement-based mortar was studied in lap-shear experiments using a 3D image-correlation measurement system. The result was compared with the bond behavior of CFRP strips glued with epoxy. Finally, two concrete bars with lengths of 70cm were each reinforced with an Fe-SMA strip. After the concrete was cured, the Fe-SMA strips were activated (i.e., prestressed) by resistive heating, and the prestressing effect on the concrete bar was measured on the concrete surface using a mechanical strain gauge.The study demonstrated the general feasibility of Fe-SMA strips in prestressed NSMR. The recovery stresses were in the range of 250–300MPa. A sufficient bond behavior was observed. Concrete bars could be successfully prestressed with a centrally embedded Fe-SMA strip.