Effectiveness of CST on the Behavior of GFRP Bar-Reinforced Square Concrete Columns under Different Loading Conditions

The behavior of fiber-reinforced polymer (FRP) bar-reinforced concrete columns with various cross-sections transversely reinforced with FRP bar ties has been studied extensively in the literature. However, FRP bar ties in the FRP bar-reinforced concrete columns are characterized by the lower tensile strength in the bent sections than in the straight sections and the propensity of slippage failure at the overlapping sections. The lower tensile strength at the bent sections (about 30-60% of the tensile strength of the straight sections) of FRP bar ties is due to the kinking of innermost fibers at the bent sections, which significantly affects the performance of FRP bar-reinforced concrete columns. The slippage failure of the FRP bar tie at the overlapping sections is due to the opening of the FRP bar tie legs before the column attains maximum axial load. This research study aims to investigate experimentally and analytically the effectiveness of a new type of carbon FRP (CFRP) strip ties (CST) for the glass FRP (GFRP) bar-reinforced normal strength concrete columns under different loading conditions. In this study, a total of 42 steel and GFRP bar-reinforced concrete specimens with a square cross-section of 200 mm×200 mm were experimentally tested up to failure. Thirty-six specimens had a height of 800 mm and were tested as columns under concentric, uniaxial eccentric, and biaxial eccentric loads. The remaining six specimens had a length of 1500 mm and were tested as beams under four-point bending. Several key parameters have been studied: type of longitudinal reinforcement (steel and GFRP bars); transverse reinforcement ratio; square (S), square overlapping square (SS), and square overlapping circular (SC) configurations of the steel ties and CST; spacing of the CST (50, 75, 100, and 125 mm); and loading conditions. The results of the tested specimens are presented and discussed in terms of axial load carrying capacity, failure modes, strains in the GFRP bar and CST, ductility, and confinement efficiency of the specimens. In addition to the experimental study, analytical studies were conducted for the peak axial load-bending (𝑃 − 𝑀) interaction diagrams (corresponding to the first and second peak loads) of the steel and GFRP bar-reinforced concrete specimens for the different configurations of the steel ties and CST.