Seismic Strengthening of Rectangular RC Columns Using Prestressed Cfrp Wrap

Abstract eng:
Although CFRP wrap has become a standard technique for retrofit and strengthening of different types of reinforced concrete (RC) structures, its application to rectangular RC columns with large cross-section aspect ratio, where the improvement of the confinement in the plastic hinge zone is needed, may be quite insufficient. Even though when the significant amount of CFRP layers are typically provided, the concrete is crushed along the longer side of the cross-section and the longitudinal bars are buckled in such cases. Since the CFRP wrapping has many advantages comparing to e.g. concrete and steel strengthening, an attempt has been performed to increase its efficiency in cases when the enhancement of the confinement is needed in rectangular columns with large cross-section aspect ratio. To achieve this goal the CFRP fibres were pre-stressed and the cover concrete in the plastic hinge zone of the column was removed. The new technique of pre-stressing the fibres has been proposed. The efficiency of the new strengthening technique was tested by experimental studies. Two rectangular reinforced concrete columns (non-strengthened and strengthened) with insufficient amount of lateral reinforcement and overlapping hoops were tested. The examined specimens were designed taking into account typical deficiencies that were identified in bridges with barbell cross-sections. Rectangular columns (b/h = 30/40 cm) were used to simulate the flanges of bridge columns in the scale 1:4. They were reinforced by stirrups (8/12cm), which did not provide sufficient support to prevent buckling of the longitudinal reinforcement (616). The examined column was strengthened by CFRP sheets. Upon the removal of the cover concrete, the rough concrete surface was smoothened with the layer of epoxy resin in the strengthened column. Before the installation of the jacket (3 layers of CFRP fabric) a special tensioning system was constructed. It consisted of two steel rods (h=500 mm, D =65mm) which were connected by bolts. The level of pre-stressing of CFRP fibres was about 20 % of the effective rupture strain of the CFRP. The result of the cyclic tests under constant vertical load showed that pre-stressed wrapping was very efficient. The drift capacity was increased for more than 50 %. The CFRP wrap ruptured on both sides of the strengthened column, about 5 cm from the bottom of the column at the locations of buckled bars. Afterwards the rupture of longitudinal reinforcement occurred. Only local buckling (between two successive stirrups) was identified after the removal of the CFRP wrap, whilst in case of the non-strengthened column the global buckling (over three successive stirrups) has occurred. The energy dissipation capacity of strengthened column was significantly increased (for more than two times, comparing to the nonstrengthened column).

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