Seismic performance of shear-critical RC columns strengthened by multiple composites considering shifted failure zone

Strengthening reinforced concrete (RC) columns utilizing advanced composites, including fiber-reinforced polymer (FRP), polyethylene terephthalate (PET) FRP, and engineered cementitious composite (ECC) has attracted extensive efforts for decades. Remarkable achievements have been made in understanding the performance of the retrofitted RC columns that failed in the strengthened area. However, localized strengthening may force failure to shift outside the retrofitted zone, and these cases are rarely reported in literature. This paper focuses on the seismic performance of shear-critical RC columns that failed outside the strengthened zone. The main test parameters considered are the axial compression force (ACF) ratio and the strengthening materials used (ECC, carbon FRP (CFRP)-ECC, or PET-ECC). The results indicate that the failure mode and failure zone depend on the strengthening conditions. Weak strengthening (ECC only) will not result in a shift of the failure zone but strong retrofitting combinations do (CFRP-ECC and PET-ECC). Although the shear failure shifted to the unstrengthened zone, strengthening can still significantly improve ductility, energy-dissipation capacity and stiffness. A larger ACF can change the failure modes from shear to flexural or from shear-flexural to shear-compression. Shear strength models with and without considering the ductility-based reduction effects are evaluated.