Axial compression behavior of square concrete columns reinforced with longitudinal and transverse GFRP bars

• Reducing GFRP bar spacing or increasing volume ratio enhances confinement effectiveness and ductility in columns. • Spiral GFRP reinforcement improves confinement and deformation resistance more than tied reinforcement. • Closely spaced GFRP reinforcement leads to ductile failure with a second peak; wider spacing causes brittle failure. • Longitudinal GFRP bars contribute to axial strength; ignoring this leads to underestimated column capacity. • Well-designed GFRP confinement improves strength, avoids brittle failure, and enhances axial column performance. This research assesses the performance of square concrete columns that have been reinforced with transverse reinforcement and longitudinal bars made of glass fiber-reinforced polymer (GFRP) under axial compression. Seven columns were subjected to testing, which included configurations of spiral and tied transverse GFRP bars with spacings of 30 mm, 80 mm, and 200 mm. This study aimed to investigate the influence of transverse reinforcement configurations and volumetric ratios on the axial load capacity, ductility, and failure mechanisms of GFRP-reinforced columns. According to the findings, the efficacy and ductility of confinement were considerably enhanced by either decreasing the transverse bar spacing from 200 mm to 30 mm or increasing the volumetric ratio from 0.85 % to 5.65 %. Spiral transverse reinforcement was more effective than tied reinforcement in preventing deformation of longitudinal GFRP bars and providing superior confinement. The columns with a spacing of 30 mm exhibited ductile behavior and a second peak load, whereas those with a spacing of 200 mm exhibited brittle failure that was similar to that of plain concrete columns. Additionally, the transverse reinforcement spacing affected the longitudinal contribution of GFRP bars to the axial capacity, which varied from 6.1 % to 19.4 %. Neglecting the contribution of longitudinal GFRP bars to the compressive strength of the column results in a significantly lower axial load capacity than it should be. Moreover, spiral transverse bars demonstrate superior effectiveness in enhancing compressive strength and ductility compared with tied transverse bars.