Round-ended concrete-filled FRP tubes subjected to monotonic axial compression: Experimental study and design-oriented model

Round-ended cross sections, characterized by a rectangular middle segment with two semi-circular ends, are widely adopted in bridge piers due to their superior strong-axis bending resistance and aesthetic appeal. This study addresses a research gap in understanding the axial compressive behavior of round-ended concrete-filled FRP tubes (RE-CFFTs). While extensive research has been conducted on conventional CFFT sections (i.e., circular, square, rectangular, and elliptical sections), RE-CFFTs remain largely unexplored. Round-ended CFFTs can be considered a geometric intermediate between rectangular and elliptical sections, offering improved FRP confinement compared to rectangular CFFTs and superior constructability compared to elliptical CFFTs. A systematic experimental program involving 28 specimens was conducted to investigate the influence of cross-sectional aspect ratio ρ (i.e., the ratio of section length-to-semicircle diameter, ranging from 1.0 to 3.0 with an interval of 0.5) and FRP tube thickness on their axial compressive performance. It was found that all the RE-CFFT specimens failed by FRP hoop rupture located in or near the semicircular region of the cross-section, with the concrete core showing major diagonal cracks. The maximum hoop strain of RE-CFFTs was generally found at the semicircle midpoints or the connection portions of the semicircles and flat segments of the cross section. The aspect ratio ρ significantly affected the compressive stress-strain response and the hoop rupture strain of round-ended CFFTs. Within the ranges of the examined parameters, specimens with aspect ratio ρ ≤ 1.5 exhibited typical bilinear ascending curves, while those with ρ ≥ 2.0 displayed a descending second branch (i.e., softening behavior), potentially followed by rehardening. Both the peak stress and ultimate axial strain of confined concrete decreased progressively with increasing ρ. When the aspect ratio ρ ≥ 2.5, the strength enhancement ratio approached 1.0, suggesting empirically that a maximum aspect ratio ρ of 2.0 could serve as the threshold for strength enhancement in RE-CFFTs. Finally, a new design-oriented model was proposed to predict the axial stress-strain responses of RE-CFFTs, incorporating parameters considering the effective confinement area and the sectional aspect ratio. Validation against experimental data demonstrated the accurate prediction of the proposed model for key parameters including peak stress, ultimate strain, and overall curve profiles.