Experimental testing and constitutive modeling of concrete confined with shape memory alloys

Concrete confinement has been proven to be effective in increasing concrete strength and ductility, which are essential characteristics for structures prone to extreme loading events, such as earthquakes. Andrawes and Shin (2008) first proposed the idea of using shape memory alloy (SMA) spirals to apply active confinement to concrete prior to loading without the need for mechanical prestressing and some research has been done on this topic in the past few years. Results so far show the promise of this new technique for seismic applications. However, more in depth investigation of this new confinement technique is in need to better understand concrete behavior under confinement provided by SMAs. This research aims at addressing five main issues related to this new confinement technique. These issues are: 1) the relatively high cost of the commonly used Ni-based SMAs, which limits the widespread application of this new technique; 2) lack of knowledge related to the application of this new confinement technique to non-circular concrete sections; 3) lack of knowledge related to applying SMA confinement to newly constructed concrete columns as transverse reinforcement; 4) limited knowledge about the stress-strain behavior of SMA confined concrete; 5) lack of analytical models that are able to predict three dimensional (3-D) behavior of concrete confined with SMA spirals. This research focused on addressing the previously stated knowledge gaps. First, to promote and facilitate the application of this novel confinement technique, the present research studied the characteristics of an unconventional, cost-effective type of SMAs, namely Fe-based SMAs, to explore their feasibility of being used in applying active confinement to concrete elements. Second, this research proposed an innovative method to implement SMA confinement in non-circular concrete sections and validated its efficacy by comparing it to conventional methods. Tests were carried out on SMA confined concrete prisms and their stress-strain behaviors were compared with those of concrete prisms confined with fiber reinforced polymer (FRP) jackets. Third, an experimental study on implementing SMA spirals as internal transverse reinforcement in newly constructed square concrete columns was carried out. In this experimental program, a short square concrete column was designed, constructed with internal longitudinal steel rebars and transverse SMA spirals. Lastly, a comprehensive experimental program was conducted on SMA confined concrete cylinders with different concrete strengths, spiral pitches, confining pressure values, loading types and loading protocols to explore their effects on the stress-strain behavior of SMA confined concrete. A plasticity model was derived and validated based on these concrete cylinders test results within the framework of Drucker-Prager plasticity model. Unlike existing models in the literature, this model is able to take into account the unique behavior of SMA confined concrete, which involves a combination of both active and passive confinement, and is capable of simulating the 3-D stresses of SMA confined concrete