Experimental investigation of axial strength of damaged concrete-filled FRP tubes

Abstract

Axial compressive strength of circular concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) benefits greatly from a significant concrete confinement by the tube. Accidentally or deliberately induced damage to the tube would undermine the CFFT strength. In this study 33 CFFT short columns of 144–168 mm (5.67–6.61 in.) outer diameter (D) and 420–460 mm (16.53–18.11 in) height (H) (i.e. H/D = 2.74–2.92) were tested under concentric loading to failure. Tube damage was artificially induced in the form of through-thickness linear cuts at mid-height. The cut length (x) and orientation (θ) with respect to the longitudinal axis were varied. Two types of GFRP tubes were studied, namely a nominal [±55o] angle-ply tube with diameter-to-thickness ratio (D/t) = 35 and a [86°/−12°] near cross-ply with D/t = 48, to investigate two commonly available but distinctly different tubes. The effect of cuts in the two tubes studied was very similar. A nonlinear descending trend can be seen in axial strength ratio (Pn/Pno), relative to the intact CFFT strength (Pno), with x/H ratio. Initially, a rapid decent from 1.0 to 0.6 occurs in (Pn/Pno) (i.e. reduction in confinement effectiveness fcc’/fc’ from 2.0 to 1.2), as x/H ratio increases from zero to 20 % (i.e.x/D = zero to 60 %) followed by a lower rate decline to 0.52 (fcc’/fc’=1.0) at x/H = 60 % (x/D = 175 %). Vertical and diagonal cuts significantly affected strength, whereas horizontal cuts had insignificant effect on strength. Strain measurements revealed stress concentrations at the tip of the cut, where crack propagations occurred in the tube. A design-oriented simple model is presented to capture the strength reduction of the damaged CFFT column.