Axial behavior of retrofitted RC columns with externally jacketed octagonal and spiral stirrups: test and FE analysis

This study presented a combined experimental, numerical, and analytical investigation into the axial load-bearing capacity of reinforced concrete (RC) columns retrofitted via external concrete jacketing reinforced with octagonal and spiral stirrups. Sixteen large-scale retrofitted RC column specimens were tested under axial compression, and this experimental program was complemented by an extensive finite element (FE) analysis of 512 numerical column models. Results revealed that the effective lateral confinement stress provided by octagonal and spiral stirrups increased with the stirrup volumetric ratio, while it decreased with an increase in the compressive strength of jacket concrete and the axial compression ratio of the existing column. Prediction formulas for the lateral strain of those stirrups at the peak load of retrofitted columns were derived, and a novel strain-based judgment method was proposed to determine the failure sequence (jacket prior to existing column or vice versa) of the two components; based on this method, the axial load-bearing limit state of jacketed RC columns was theoretically established. Furthermore, the predictive models for the axial load-bearing capacity of RC columns retrofitted with octagonal or spiral stirrup-reinforced jacketing were developed.