Retrofitting square columns using FRP-confined crumb rubber concrete to improve confinement efficiency

Retrofitting of rectangular/square concrete columns in existing buildings is required for many reasons. Wrapping columns using fibre reinforced polymer (FRP) is one of the most effective retrofitting methods for circular columns. However, in rectangular columns, the sharpness of the columns' corners reduces the confined column performance due to the non-uniformity of the confinement pressure. In this research, three different cross-section modifications for square columns were proposed to fully utilize the FRP-confinement efficiency; namely, refined-corner (ReC) section, rounded-corner concrete-angle (RCA) section, and circularized (Cir) section. Crumb rubber concrete (CRC) with different rubber contents (0-20% as a replacement of sand by volume) and particle sizes (0.15-2.36 mm), and a similar target compressive strength (f′c) of 50 MPa, was used to form the cross-section modifiers for the RCA and Cir specimens. Confinement by FRP with wall thicknesses of 1 and 2 layers was applied to all cross-sectional modified columns. The axial and hoop stress-strain behaviours of the proposed columns were measured and compared. The results indicated that the CRC can effectively replace conventional concrete in the retrofitting of existing columns, especially when using the circularization approach.