SEISMIC performance of RC square columns strengthened by bidirectional basalt-fiber-reinforced polymer (BFRP) composite laminates with fan-shaped anchors

Abstract

For seismic retrofitting of existing reinforced concrete (RC) structures, multiple strengthening demands may be required simultaneously including shear, flexural, and axial strength, as well as ductility. However, satisfying such multiple demands using conventional materials and strengthening systems may be challenging. As compared to commonly used unidirectional fibers reinforced polymers (FRP) composites laminates, bidirectional FRP laminates offer an effective methodology for providing strength and ductility in different directions. In this study, seven reinforced concrete (RC) columns specimens confined by bi-directional basalt-fiber-reinforced polymer (BFRP) composite jackets at plastic hinge regions with and without fan-shaped composite anchors are evaluated. All specimens are subjected to low-frequency full-reversal cyclic lateral loadings up to failure. Several design parameters are assessed to identify its impact on the behavior of confined columns that include: (i) composite laminate types, (ii) number of plies, (iii) strengthening scheme, and (iv) presence of anchorages. Service and ultimate seismic performance of each specimen including failure modes, rupture strain, and ductility are described and investigated. Experimental results indicated that ultimate failure mode changes from a shear failure, in the case of unidirectional composite jacket, to a combined flexure/shear failure mode when bidirectional [0°/90°] composite jackets are utilized. However, results indicated that the circumferential (hoop) fibers of a bidirectional composite jacket have lower ultimate hoop strain as compared to those obtained for columns with unidirectional composite jackets. Results also showed that the use of composite anchors effectively improved working strain of longitudinal fibers [0°], especially for columns with bidirectional composite jackets. In this case, tensile strain increased by 1.5 times as compared to specimens without composite anchors. Furthermore, the presence of anchors improved flexural capacity of confined columns. In addition, an analytical model to predict flexural capacity of RC columns confined with bidirectional laminates with anchors is proposed, which was verified using experimental results. Results of this study indicated that the use of bidirectional FRP composites laminates jackets with fan-shaped anchor enhanced both strength and ductility of RC columns.