Strengthening of Reinforced Concrete Beams with Externally Mounted Sequentially Activated Iron-Based Shape Memory Alloys.

Emanuel Strieder,Gabriele Petautschnig,Sebastian Horn,Roman Wan-Wendner,Oliver Zeman,Konrad Bergmeister,Christoph Aigner,Michael Schwenn,Marco Marcon,Pablo Castillo

doi:10.3390/ma12030345

Abstract

Iron based shape memory alloys (Fe-SMA) have recently been used as active flexural strengthening material for reinforced concrete (RC) beams. Fe-SMAs are characterized by a shape memory effect (SME) which allows the recovery of previously induced plastic deformations through heating. If these deformations are restrained a recovery stress is generated by the SME. This recovery stress can be used to prestress a SMA applied as a strengthening material. This paper investigates the performance and the load deformation behavior of RC beams strengthened with mechanical end anchored unbonded Fe-SMA strips activated by sequentially infrared heating. The performance of a single loop loaded and a double loop loaded SMA strengthened RC beam are compared to an un-strengthened beam and a reference beam strengthened with commercially available structural steel. In these tests the SMA strengthened beam had the highest cracking load and the highest ultimate load. It is shown that the serviceability behavior of a concrete beam can be improved by a second thermal activation. The sequential heating procedure causes different temperature and stress states during activation along the SMA strip that have not been researched previously. The possible effect of this different temperature and stress states on metal lattice phase transformation is modeled and discussed. Moreover the role of the martensitic transformation during the cooling process on leveling the inhomogeneity of phase state in the overheated section is pointed out.

Highlights

Shape memory alloy (SMA) strips can be used as prestressing elements to strengthen reinforced concrete (RC) members
The vertical displacement at the supports and in the center of the beam was measured with a linear variable differential transformer (LVDT) to calculate the vertical deflection of the beam considering the stiffness of the test setup
Another two LVDTs were placed at the top of the concrete beam next to the anchoring devices to monitor the relative horizontal shift of the devices due to the force in the

Summary

Introduction

Shape memory alloy (SMA) strips can be used as prestressing elements to strengthen reinforced concrete (RC) members. The shape memory effect (SME) turns a material that is deformed beyond the elastic range back into its initial shape [1,2,3]. If reshaping is prevented by mechanical fixation a stress develops within the shape memory alloy, which is called recovery stress. The application of SMAs as prestressing elements is still in an early research state [4,5]. Iron based SMAs have been used for near. Under research conditions the activation of the SMA is often done simultaneously through the whole SMA by applying electric current [5,6,10]. For a constant cross-section the electrical resistance

Results

Discussion

Conclusion