For concrete columns damaged by fire, a strengthen technique using thin-walled steel tubes is proposed. To investigate the axial compression capacity of post-fire concrete columns strengthened using thin-walled steel tubes, considering tube thickness, the strengthening method, and section geometry, 12 specimens were fabricated, of which two were control columns and 10 were exposed to fire in accordance with the ISO834 temperature curve. Subsequently, eight specimens were strengthened. Axial compression tests were conducted to provide a better understanding of the strengthening technique. The experiments indicated that: (1) Different failure modes were observed for different cross section geometry—local bucking for square sections, and shear failure for circular sections. (2) The stiffness, axial strength, and ultimate deformation capacity of the enhanced columns may be rehabilitated and even better than the undamaged ones. (3) Two enhancement methods were compared. The steel tubes act as restraints, are merely subjected to tension, and provide strong restraint to the core concrete. The tube carrying load together with the concrete columns are mainly subject to compression, and likely to buckle with longitudinal strain. Finally, axial compressive equations of post-fire reinforced concrete (RC) with thin-wall steel tubes, including both square sections and circle sections, were proposed on the mechanism of concrete filled steel tubes.
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