Seismic retrofit of low-ductile columns through concrete jacketing with inoxydable reinforcement

Abstract

Retrofit of structures often is an inevitable task especially when buildings are not designed for seismic actions or their design has followed older design codes. Many retrofit strategies have been proposed and practiced by previous researchers. Usage of fiber reinforced polymer [FRP], steel jacketing and reinforcement jacketing are among the most common retrofitting methods. For reinforcement jacketing, carbon steel has been widely employed by engineers, however, only a few applications of inoxydable reinforcements can be found in the literature. Moreover, when it comes to reinforcement jacketing, connection between the interface of original column and the jacket plays an important role and has attracted the attention of many researchers. Load transfer mechanism between original column and jacket is another field of study which has not been addressed in previous research. In this study application of inoxydable rebars for seismic retrofit of Reinforced Concrete (RC) columns was investigated. Two new connectors were used to increase the integrity between the original column and jacket. Load transfer mechanism between original column and jacket is another topic addressed in this research. This study included experimental and numerical analysis. For experimental study, 8 full scale RC columns were constructed and retrofitted with different reinforcement jacketing configurations. Numerical studies investigated the effect of different axial forces on the obtained results from experimental test. Results indicated that regardless of the employed retrofit configurations, the retrofitted columns have higher initial stiffness and ultimate strength compared to un-retrofitted columns. However, the retrofitted columns showed significantly lower ductility ratio when compared with un-retrofitted columns. All the retrofitted columns displayed a brittle failure mode in which spalling of concrete at the base of columns occurred without yield or buckling of reinforcements. Results indicated that confined jackets have higher ultimate strength and stiffness compared to un-confined jackets. However, they showed a lower ductility ratio when compared with un-confined jackets. It was observed that, when internal angle connection was used for retrofit, the highest ultimate strength, post-yield stiffness and effective stiffness were achieved. Monitoring the strain distribution between jackets and original columns revealed that confinement in jackets reduced the strain in the longitudinal reinforcement of original columns more than un-confined jackets. Strain values in the stirrups of confined jackets were significantly smaller than that of un-confined jackets. Strain ratios on the surface of concrete of confined jackets were larger than that of un-confined jackets. It is concluded that the proposed connectors have improved the ultimate strength of retrofitted columns as compared to conventionally retrofitted column, as they were unable to elevate the ultimate strengths to the level of a monolithic column.