Compressive behaviour of elliptical FRP tube-confined concrete columns

Elliptical FRP tube-confined concrete columns (EFCCCs) are an extension of circular FRP tube-confined concrete columns recently proposed at The Hong Kong Polytechnic University. Like their circular counterparts, EFCCCs are corrosion-resistant and can be used in situations where a column is subjected to substantially different loads in the two orthogonal lateral directions. The elliptical section of EFCCCs leads to more effective confinement of the concrete and an easier filament-winding fabrication process for the FRP tube compared with their rectangular counterparts. Existing studies on FRP-confined concrete in elliptical columns have been few. This paper first presents a systematic experimental study on the axial compressive behaviour of EFCCCs, with a focus on the stress–strain behaviour of the confined concrete. The test variables included the concrete strength, the thickness and fibre orientations of the FRP tube, and the aspect ratio of the cross-section. The test results showed that the confinement effectiveness of FRP tubes decreases with the aspect ratio and the concrete strength but increases with the FRP tube thickness. The orientations of fibres in the FRP tube were found to have a significant effect on its confinement effectiveness and the failure mode of EFCCCs. On the basis of the test results, existing stress–strain models for FRP-confined concrete in elliptical columns are evaluated. Furthermore, a new stress–strain model for the concrete in EFCCCs is proposed, which includes a more rigorous treatment of the confinement stiffness of EFCCCs and can provide more accurate predictions for the whole stress–strain curve than the existing models.