Experimental investigations of GFRP-reinforced columns with composite spiral stirrups under concentric compression

In this study, a new way to transversely reinforce square concrete columns using glass fiber reinforced polymer (GFRP) composite spiral stirrups composed of several outer rectangular GFRP stirrups and an inner spiral GFRP stirrup was proposed. The double restraints of the composite spiral stirrups overcame the shortcoming of the relatively lower elastic modulus of GFRP compared with steel and provided effective lateral restraint for the core concrete. The axial compressive performance of GFRP-reinforced square concrete columns with composite spiral stirrups was experimentally investigated. The test variables, including the stirrup spacing, stirrup diameter, longitudinal bar type and stirrup configuration, were discussed. The results indicate the following: (1) By the application of GFRP composite spiral stirrups, both a higher core concrete strength and ultimate load could be achieved compared to GFRP rectangular stirrups and GFRP composite rectangular stirrups. (2) Except for specimens 12S-150G-G and 8S-100G-G, the load versus axial concrete strain curves of GFRP-reinforced columns with composite spiral stirrups did not drop after reaching the peak but went through a stable stage, showing excellent ductility. (3) With a decrease in stirrup spacing from 100 mm to 50 mm, secondary peaks appeared on the load versus axial concrete strain curves. The decrease in the stirrup spacing effectively enhanced the core concrete strength and ductility of the columns. (4) Considering the contribution of the GFRP bars and the restraint effect of stirrups, a new method to calculate the ultimate bearing capacity of GFRP-reinforced columns with composite spiral stirrups was proposed and verified with experimental results. • A novel GFRP stirrup configuration for restraining the core concrete was proposed. • Axial compression tests were conducted considering the stirrup spacing, diameter, configuration and longitudinal bar type. • Results revealed using GFRP composite spiral stirrups improved the ultimate load and ductility of the columns. • A new method to calculate the ultimate bearing capacity was proposed and verified with experimental results.