Damage Threshold of Near-Cross-Ply Tubes Used in Concrete-Filled FRP Tubes Loaded in Flexure

Concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs) have been studied extensively, but a lingering question remains regarding the vulnerability of the exposed tube to accidental damage or vandalism. In this paper, the effect of damage induced in the tubes on the flexural strength of CFFTs is studied. A total of 18 specimens with tubes having nearly orthogonal fiber arrangements were tested in four-point bending, including one undamaged control specimen to establish its full nominal strength ($M_{no}$). A total of 11 specimens with artificially induced damage in the form of through-thickness cuts on the tension side, and six with cuts on the compression side at midspan were also tested. Longitudinal, circumferential, and square cuts were introduced. Cut length and proximity to neutral axis were varied. It is shown that the most critical cut is the circumferential one on extreme tension. A sharp decline occurred initially in the nominal residual strength ratio ($M_n/M_{no}$), from 1.0 to 0.55 at a cut length-to-perimeter ratio of $0.02\pi D$, followed by a much less steep trend up to $M_n/M_{no}$ of 0.25 at a $0.2\pi D$ cut. As the location of a $0.03\pi D$ circumferential cut shifted from extreme tension to an 80° from the bottom, $M_n/M_{no}$ increased from 0.52 to 1.0. A longitudinal cut of $0.1\pi D$ on the tension side resulted in $M_n/M_{no}$ of 0.69 as opposed to 0.38 for the circumferential cut. Compression side longitudinal and circumferential cuts were less critical with $M_n/M_{no}$ of 0.8 to 0.9. For practical design, a case study based on ultimate static strength showed that the CFFT can tolerate a circumferential tension cut of up to $0.29\pi D$ before it has to be taken out of service.