Compressive behaviour and prediction model for short and slender FRP-confined GFRP bars

Glass fibre reinforced polymer (GFRP) composite bars can be used as internal reinforcement for concrete structures built in harsh environmental and service conditions. Their utilisation as longitudinal reinforcement for compression members is not widely recommended though due to the anisotropic nature of FRP, their complex failure modes, and limited research, especially for slender bars. A more recent innovation has resulted in the wrapping (confinement) of GFRP bars with FRP composites oriented in the hoop direction. These confined bars have demonstrated promising improvement in FRP bar compressive behaviour and ductility. In the current study, the experimental compressive behaviour of eighty short and slender FRP-confined GFRP bars is reported and a compressive strength model is proposed. The experimental parameters investigated are (i) wrapping layers (i.e. one, two, and three layers oriented +/- 83.3 degrees to the longitudinal axis of the bar), and (ii) slenderness ratios (i.e. 15, 30, 45, and 60). The resulting compressive load capacity and strength, stress-strain behaviour, and failure modes are then reported for each tested bar. Test results indicate that an increase in slenderness results in decreased strength. In addition, failure modes shifted from splitting to buckling of the longitudinal fibres. GFRP bars with at least two winding layers had a significantly higher compressive strength than the control GFRP bar (not including winding) and they also exhibited bi-linear stress-strain behaviour as well as ductile failure modes. A compressive strength prediction model is then developed, which is based on the modification of the JohnsonEuler buckling theory and correlates well with the experimental results. The research presented herein has the potential to contribute to the development of design guidelines for concrete compression members, such as columns, piers and piles, that are reinforced with FRP-confined GFRP bars.

Automated summary

Glass fibre reinforced polymer (GFRP) composite bars can be used as internal reinforcement for concrete structures in harsh conditions. However, their utilization as longitudinal reinforcement for compression members is not widely recommended due to the anisotropic nature of FRP and limited research. Wrapping the GFRP bars with hoop-oriented FRP composites has shown promising improvement in compressive behavior and ductility. This study investigates the compressive behavior of FRP-confined GFRP bars and proposes a compressive strength model.