Fabricating, testing, and modeling of advanced grid stiffened fiber reinforced polymer tube encased concrete cylinders

In a previous study [1], a new confinement device - an advanced grid stiffened (AGS) composite tube, which was made of a lattice of interlaced fiber-reinforced polymer (FRP) ribs that was wrapped by a FRP skin - was used to encase concrete cylinders. The test results showed that the AGS tube confined concrete cylinders displayed a considerable positive or constructive composite action due to the mechanical interlocking between the grid skeleton and the concrete core. In order to better understand and design this type of novel structures, an in-depth study is needed. In this present study, a pin-guided system assisted by a collapsible mandrel was developed to filament wind the AGS tubes automatically. A 'building-block' test was conducted to reveal the step-by-step development of the composite action. After that, the effect of the rib thickness, skin thickness, and bay area on the structural behavior was evaluated experimentally. The numerical analysis was conducted using the ANSYS finite element software package by considering the nonlinear behavior of concrete and a non-associative Drucker-Prager plasticity criterion. Some new findings include (1) the grid skeleton is more effective in confining the concrete core than the skin; (2) with the same amount of FRP materials, AGS tubes have a much higher confinement efficiency than laminated FRP tubes with zero bay area; (3) thickening the ribs is more effective than widening the ribs; and (4) the finite element analysis procedure, along with the parameters taken, is reliable in modeling and designing this type of cylinders.