How Concrete Filling Fundamentally Changes Stress-Strain Curve of Angle-Ply FRP Tubes in Tension

Angle-ply (+/- 55 degrees) fiber-reinforced polymer (FRP) tubes are widely available and have been used in concrete-filled FRP tube (CFFT) members. Two observations have been reported regarding the behavior of these tubes in tension: a remarkably nonlinear stress-strain response and a significant increase in their tensile strength and stiffness when filled with concrete. To better understand these phenomena, a robust finite-element model is developed using LS DYNA software and validated against a diverse experimental database. It showed that the nonlinear behavior of the tube is mainly due to matrix cracking perpendicular to the fibers and to a lesser extent due to in-plane shear along diagonal bands. Concrete filling restrains the large radial and circumferential contraction of the hollow tube under longitudinal tension, thereby generating significant hoop tensile stresses and consequently a state of biaxial tensile stress. A failure envelope under such stress combination was developed and far exceeded uniaxial strength in either direction. A parametric study was performed on 68 new models with various properties. The longitudinal tensile strength (sigma(max)) of CFFT tubes with fiber angles (theta) relative to longitudinal axis of 35 degrees, 45 degrees, 55 degrees, 65 degrees, and 75 degrees increased 2.9, 4.1, 3.3, 2.8, and 1.4 times, respectively, that of hollow counterparts. Design-oriented equations were developed to represent the enhanced longitudinal bilinear stress-strain curve when the tube is filled with concrete. It can be used for flexural strength calculations of CFFTs, which would otherwise be grossly underestimated if calculated using hollow tube properties reported by the manufacturer or established from longitudinal coupon tests or from classical lamination theory.

Automated summary

Angle-ply fiber-reinforced polymer (FRP) tubes exhibit nonlinear behavior and increased tensile strength and stiffness when filled with concrete. This is mainly due to matrix cracking and in-plane shear, with concrete filling restraining contraction and generating hoop tensile stresses.