Monotonic and cyclic experimental testing of concrete confined with shape memory alloy spirals

Numerous studies have shown that concrete confinement can greatly improve the seismic behavior of reinforced concrete (RC) structures and compared to passive confinement, active confinement, which involves prestressing of the concrete member in the transverse direction prior to the application of loading, is more effective in increasing the strength and flexural ductility of concrete elements. Some recent studies proposed a promising seismic retrofitting scheme of using a new class of material known as Shape Memory Alloys (SMAs) in the form of transverse reinforcement (spirals) to provide active confinement to concrete structures. The application of active confinement using these new spirals is simple and only requires heating the spiral with no need for mechanical prestressing. However, existing research on SMA confinement is mainly focused on monotonic loading and only a small amount of specimens were tested. Therefore, it is important to explore the behavior of SMA confined concrete in more depth under cyclic loading. In this study, first, a series of concrete cylinders confined using SMA spirals with different pitch spacing are subjected to uniaxial cyclic compression tests. The results help in understanding the effect of SMA spirals pitch spacing in the behavior of SMA confined concrete. Second, the effect of applying SMA confinement to concrete under cyclic loading is then investigated experimentally through comparing it with that of concrete confined using conventional steel spirals with similar level of confining pressure and the same spiral spacing. Experimental results show that SMA confined concrete is much more effective in improving concrete strength and ductility compared to conventional steel confined concrete. Last, the differences in stress-strain behavior between the cyclically and monotonically loaded SMA confined specimens are explored experimentally and found that the monotonic stress-strain behavior represents an envelope for the corresponding specimens tested under cyclic loading.