Advanced grid stiffened fiber reinforced plastic tube encased concrete cylinders

In this study, 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. Two types of cylinders were prepared. One was a circular cylinder and the other was a square cylinder. For the circular cylinders, a uniaxial compression test was conducted; for the square cylinders, both uniaxial compression test and four-point bending test were conducted. A finite element analysis was implemented by considering the concrete within the cell or bay area as a Winkler foundation. The test results showed that the AGS tube confined concrete cylinders displayed a considerable positive or constructive composite action due to the mechanical interlocking. The AGS tube provided an 'active' confinement to the concrete core. The elastic range, the specific axial strength, and the specific axial strain of the circular AGS cylinders, and the specific bending strength and deflection of the square AGS beams were higher than those encased by solid FRP tubes, solid steel tubes, or steel grid tubes. The square AGS tubes were not as effective as the circular AGS tubes in confining concrete. The finite element analysis showed that increasing the thickness and stiffness of the rib would be effective in further increasing the load carrying capacity of the circular AGS tube encased concrete cylinders.