This paper presents an experimental investigation of the effectiveness of strengthening half height full size concrete columns by placing concrete jackets. Three alternative methods of concrete jacketing are investigated and results are compared with results from an original unstrengthened specimen and a monolithic specimen. The specimens were designed to represent typical ground floor columns of a concrete frame building. The unstrengthened column and the original columns of the strengthened specimens were designed to old 1950s Greek Codes. Poured concrete or shotcrete was used to construct the jackets of the strengthened specimens and, as performed in practice, various other construction procedures were carried out in order to evaluate if it is worth performing the procedures when considering the practical difficulties involved. These procedures involved welding the jacket stirrup ends together, placing steel dowels across the interface between the original column and the jacket in combination with welding the jacket stirrup ends together and connecting the longitudinal reinforcement bars of the original column to the longitudinal reinforcement bars of the jacket. In order to investigate the lower limit of the effectiveness of the technique, the case of no treatment at the interface between the original column and the jacket combined with the construction of a low strength cast in situ concrete jacket is examined. The same cross sectional dimensions and amount of steel reinforcement were used for the strengthened specimens and a control monolithic specimen. Earthquake simulation displacement controlled cyclic loading was used for the testing. The seismic performance of the tested specimens is compared in terms of strength, stiffness and hysteretic response. The effectiveness of properly constructing concrete jackets has been proved, as it was found that, under special conditions, an almost monolithic behaviour could be achieved. Even when the jacket was constructed with no treatment at the interface, a significant strength and stiffness increase was observed. It was also found that the failure mechanism and the observed crack patterns are influenced by the strengthening method. The separation of the jacket from the original column was obvious in the case when there was no treatment or other connection means performed at the contact interface between the column and the jacket. In addition, it was found that welding the jacket stirrup ends together stopped the longitudinal bars of the jacket from buckling.
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