FRP-Confinement of Hollow-Core Concrete Cylinders and Prisms

The use of hollow-core reinforced concrete (RC) sections for bridge piers has become a popular engineering practice to obtain a reduction of the self-weight (especially in seismic zones) and a better structural efficiency in terms of the strength/mass and stiffness/mass ratios. In contrast to this popularity in practice, scientific studies on the mechanical behavior of such structural elements are limited. The use of Fiber Reinforced Polymer (FRP) materials for external confinement of hollow core columns and piers is an almost unknown field at the moment. The research work presented herein aims at evaluating the influence of various experimental parameters on the effectiveness of FRP jackets applied to hollow concrete columns. Hollow-core concrete prisms and cylinders were tested under uniaxial compression to study the stress-strain relationship before and after FRP jacketing. A range of experimental parameters were investigated: different concrete strength, type of fibers, number of wrap layers, column shape and dimensions, and for square and rectangular sections, the corner radius and the cross-sectional aspect ratio. Axial strain was measured by LVDTs, while strains in the fibers were recorded by electrical strain gauges. Circular columns wrapped with FRP showed a significant increase in terms of both strength and ultimate displacements. Results obtained by laboratory tests were close to those recorded for FRP-confined concrete, which means that the increase in ultimate load was found to be comparable to that found in full circular sections. Rectangular columns showed a lower increase in ultimate capacity, compared to circular sections, even if the results related to ultimate axial displacement encourage adopting this technique for seismic retrofit to fulfill higher ductility requirements in both prismatic and cylindrical columns.