Axial Compressive Behavior of Slender Circular Columns Made of Green Concrete and Double Layers of Steel and GFRP Reinforcement

Green concrete made with ground granulated blast-furnace slag (GGBS) has presented itself as a sustainable alternative to traditional concrete. Similarly, the noncorroding fiber-reinforced polymer (FRP) reinforcing bars have demonstrated reliable structural performance at a competitive price. The combination of these materials substantially improves the service lifespan for reinforced concrete (RC) structures. Earlier research investigations and existing standards do not account for FRP reinforcement in the design for compressive forces. This study revisits this design approach while experimentally investigating the axial compressive behavior of slender circular GGBS-based green concrete columns reinforced with double layers of steel and glass fiber reinforced polymer (GFRP) bars and spirals. The experimental program consists of nine large-scale specimens loaded concentrically. All specimens had a diameter of 260 mm and a length of 2,500 mm, yielding a slenderness ratio of 38.5. Two control specimens were reinforced with a single steel layer having 1.2% and 2.5% longitudinal reinforcement ratios. The remaining seven specimens had an outer layer of GFRP reinforcement and an inner layer of steel reinforcement. The investigated parameters were: (1) longitudinal reinforcement ratio; (2) spiral reinforcement ratio; and (3) spiral reinforcement configuration (pitch and diameter). The experimental findings revealed that all tested columns failed in a material-type failure. However, the slenderness effects reduced the strength of the hybrid-reinforced columns compared with the control counterparts. The steel-reinforced control specimens demonstrated load-carrying capacities that agreed well with nominal capacity predictions under existing code. Conversely, the nominal capacity predictions in current codes and standards underestimated the strength of the hybrid specimens. A proposed equation from the literature that considers the compression contribution of GFRP bars was also evaluated, and a proposed correction factor was introduced to count for second-order effects. Due to the limited number of specimens in this study, future research is warranted to refine this proposed correction factor further.