Experimental study of concrete columns confined with shape memory alloys

This experimental study focuses on investigating the cyclic behavior of reinforced concrete (RC) columns retrofitted with an innovative active confinement technique. In order to enhance the ductility and strength of RC columns, passive confinement techniques such as using steel jackets and fiber reinforced polymer (FRP) sheets have been widely accepted and used. However, previous studies have shown that the improvement in concrete strength and ductility resulting from active confinement is far more superior to passive confinement. Most of the attempts for applying active confinement using conventional materials have yielded little success due to difficulties associated with applying the confinement pressure on site. This paper proposes a new technique where the active confining pressure is applied using the shape memory effect associated with the heating of Shape Memory Alloys (SMAs); a technique that will cause large external confinement pressure to be applied on the column without excessive work or labor. In this study, four reduced-scale RC columns are prepared and tested under quasi-static cyclic loading. Two of the tested columns are retrofitted with SMA spirals with and without additional glass FRP (GFRP) wraps, the third column is confined passively with GFRP wraps only, and the fourth column is tested in its un-retrofitted condition (as-built). The load-deflection results show that the actively confined columns exhibit superior performance to passively confined and unconfined columns. This superior behavior is primarily attributed to the increase in concrete strength and ultimate strain associated with active confinement.