Flexural Behaviour of Reinforced Concrete Elements Retrofitted Using External Unbonded Superelastic Shape Memory Alloy Bars

Steel reinforced concrete (RC) framed structures are seismically designed for safety, where the earthquake energy is dissipated through yielding of the steel bars. During strong earthquakes, high inelastic deformations are allowed to take place. This allowance results in significant residual deformations. Thus, following an earthquake, the structure might be deemed irreparable, requiring its demolishing and replacement. If the frame can be designed to regain its original shape following an earthquake, then the structure will be repairable. This kind of design can be achieved by using smart materials such as Shape Memory Alloys (SMAs). SMAs have unique properties, which make them distinctive when compared to other metals and alloys. These properties give them the ability to undergo large deformations and return to their undeformed shape upon unloading (superelasticity). Under cyclic loading, the flag shape stress-strain relationship of SMA provides the ability to dissipate large amounts of energy. In addition, they have good corrosion and fatigue resistance. These unique properties of SMAs have motivated researchers to utilize them as reinforcing bars in RC structures. The first part of this study aims at providing in-depth understanding of the flexural behaviour of SMA RC beams. A sectional analysis method, that predicts the flexural behaviour of SMA RC beams during both loading and unloading stages, is adopted and validated using available experimental data. A parametric study is then carried out to investigate the effect of different geometrical properties. Recommendations for the optimum amount and length of SMA bars are drawn based on results of this study. Retrofitting RC structures can be needed to minimize the seismic residual deformations occurring in the structure following an earthquake event. It can also be needed to upgrade their capacities and/or address the deterioration happening overtime. Innovative and cost effective retrofitting techniques are continuously being developed. In the second part of this study, a new technique for retrofitting RC beams in flexure is introduced. The technique is based on using external unbonded superleastic SMA bars. The technique is first assessed using the Finite Element (FE) method. A simplified sectional analysis approach is then presented, validated, and used to conduct a parametric study. Results of the parametric study are used to develop equations to predict changes in the beam behaviour covering its residual displacement, stiffness, and the dissipated energy because of the suggested retrofitting technique. Beam-column joints (BCJs) or RC framed structures are designed to satisfy the strong column-weak beam concept, where severe inelastic deformations are allowed to occur in the beam. Minimizing these inelastic deformations can be needed to make the structure repairable. In addition, one of the problems for existing RC structures designed per older standards (pre 1970s) is the inadequate anchorage for the beam reinforcement in the joint area. In the third part of this study, the applicability of using external unbonded SMA bars to retrofit RC BCJs is investigated. A FE model is first developed and validated. A simplified model is then proposed and validated using the developed FE model. The simplified model is used to conduct a parametric study to investigate the behaviour of SMA retrofitted RC BCJs. Results of the parametric study are used to develop equations to decide the optimum length and amount of the SMA bars. Superelastic SMA bars can be used to minimize the inelastic deformations happening in RC frame structures after seismic events. Minimizing these inelastic deformations make the structure repairable. In the fourth part of this study, the seismic performance of RC frames retrofitted using external superleastic SMA bars is investigated and compared to the behaviour of a regular steel RC frame structure. Nonlinear time history analysis is performed for a six storey RC frame structure located in high seismic region. After performing the analysis, two retrofitted frames are assumed. Analysis is performed again for the two frames at the load intensities causing failure of the steel RC frame. The performance of the retrofitted frames is compared to the steel RC frame. The retrofitted frames showed lower level of damage at failure and they tolerated higher earthquake intensities. The suggested retrofitting technique reduced the maximum drifts by 10% to 15%, and the residual drifts by 50% to 70%