Axial behavior of FRP confined square columns with polyurea

Objective: The aim of this study was to investigate the effect of polyurea in increasing the efficiency of GFRP confinement for the low-strength concrete. Materials and Methods: For this study, 23 square column specimens with 106 mm x 106 mm cross-sectional sizes and a height of 300 mm were produced. The sharp corners of the square columns were rounded by either 15 mm or 30 mm corner radius. Thus, stress concentrations on sharp corners are prevented. Two of the square specimens were not confined to calculate the bare concrete strength. Other specimens were confined with single and double GFRP fabrics. Ten specimens were coated with polyurea before being confined with the aim of providing more uniform hoop strain distribution on the confining jacket under axial compression. All specimens were tested under monotonic axial compressive loading. The test results were compared with the ultimate strength and strain capacity predictions of an existing design-oriented model. Six cylindrical concrete specimens with a diameter that equals to the diagonal length of square specimens (i.e. 150 mm) were produced. The cylindrical specimens were confined by either one- or two-layers of GFRP. The monotonic axial test results of these "equivalent cylindrical" specimens were utilized in the model predictions. Results: The axial monotonic stress-strain curves were obtained from the test results. The ultimate compressive strength, maximum axial and lateral strains were determined. In addition, stress- and strain- enhancement ratios were calculated. The ultimate axial strength and strain capacity of the specimens were also predicted by the design-oriented model and compared with the experimental findings. Conclusion: The highest improvement in the compressive strength was observed in the specimens having two-layers of GFRP confinement with 30 mm rounded corners (i.e. S30-2G-NP). On the other hand, the contribution of polyurea was best observed in the specimens having one-layer of confinement with 15 mm rounded corners (i.e. S15-1G). There was also an improvement in the hoop rupture strains of these specimens. Still, the homogeneous distribution of the lateral strain which is generally the case for the cylindrical specimens could not be obtained despite the polyurea coating.