Behavior of large-scale columns strengthened with basalt fiber-reinforced polymers sheets or bars and hybrid FRPs

Basalt fiber-reinforced polymers (BFRP) have recently become a widely used material for increasing the ultimate load capacity of reinforced concrete (RC) columns and improving their behavior throughout their entire life. So, an experimental program consisting of 19 large-scale RC columns of low strength has been prepared to examine the behavior of strengthened RC columns at all stages of loading. In this program, the columns were divided into two groups. The main parameters include the strengthening materials (BFRP wrap, BFRP bars, and carbon fiber reinforced polymers (CFRP) wrap), the strengthening techniques (fully or partially wrapping, the number and direction of layers), the cross-sectional shapes (square, rectangular, and circular), and the loading types (eccentric and concentric). The confinement of BFRP wraps significantly improved the service load and the ultimate failure load. The hybrid systems achieved an adequate level of confinement of the strengthened columns, increased the maximum load by 43.61 %, and absorbed energy by 477.2 % compared to the control column. BFRP bars used as a near-surface mounted material improved the behavior of the columns by up to 60 % and 411 % in maximum load and absorbed energy, respectively. The ACI and ECP codes predicted good estimates for the stress of columns strengthened with a single layer but underestimated for columns strengthened with a single layer and BFRP bars. Based upon the existing test data, BFRP-RC is recommended. Furthermore, this research can support using BFRPs as strengthening materials.