Hybrid confinement mechanism of large-small rupture strain FRP on concrete cylinder

This study aims to investigate the hybrid mechanism of concrete cylinders confined with different types of fiber reinforced polymer (FRP). The adoptive FRP sheets include three types of low rupture strain FRPs (carbon fiber polymer (CFRP), glass fiber polymer (GFRP) and basalt fiber polymer (BFRP)) and two types of high rupture strain FRPs (polyethylene terephthalate fiber polymer (PET-FRP) and polyethylene naphthalate fiber polymer (PEN-FRP)). More hybrid gradients are designed to exposit the hybrid mechanism of large-small rupture strain FRPs. A total of 39 FRP-confined concrete cylinders are conducted under axial compression. The tested parameters are FRP types and the number of FRP layers. The failure modes, stress-strain responses, lateral confined pressures under different axial stresses, the apparent Poisson's ratio variation and energy-dissipating capacities are analyzed to reveal the mechanical mechanism of the specimens. The results show that the hybridization of the FRP sheets with rather different elongation capacities brings a pseudo ductility behaviour of concrete cylinders. That definitely improves the strain capacity, but the strength improvement relies on the adequate axial stiffness discrepancy between higher elongation FRP and lower elongation FRP sheets. By the hybridization, the utilization of lower elongation FRP sheets is markedly improved. The concrete cylinders confined by hybrid FRP sheets with similar elongation capacities behave like to the single FRP confined ones. Moreover, the predicted formulas are proposed and capture well the ultimate state of hybrid FRP confined concrete cylinders.