Modification of epoxy binder with multi walled carbon nanotubes in hybrid fiber systems used for retrofitting of concrete structures: evaluation of strength characteristics

The rapid development in infrastructural facilities necessitates an efficient approach for the repair and retrofitting of concrete structures and, confinement method using fiber reinforced polymer is a promising one. The commonly used carbon and glass fibers for confinement poses environmental and performance issues. The present study addresses these two major aspects by considering natural fibers along with modification of epoxy binder to impart ductile behavior ie., to investigate the effectiveness of multiwalled carbon nanotubes (MWCNT) incorporated synthetic and natural fiber reinforced polymer (FRP) systems as the external confinement. MWCNT is incorporated in 0.5–1.5wt.% in epoxy nano and epoxy multiscale and there is significant enhancement in tensile and fracture properties of the composites up to 1wt.%, beyond which it declined due to agglomeration. Various strength tests were performed with sisal, basalt, carbon and hybrid sisal-basalt FRP systems with different FRP layer thickness on plain concrete cylinders. From the test results it is outlined that external confinement with MWCNT incorporated FRP improved the axial load-carrying capacity, energy absorption and ductility of concrete with respect to that of control specimens. Compared with unconfined specimens, those strengthened with MWCNT modified hybrid FRP wraps containing sisal and basalt fibers recorded increments of 114% and 87% in their load-carrying capacity and energy absorption, due to the intrinsic rigidity of hybrid fibers and epoxy modification. Furthermore, the outcomes indicate that MWCNT incorporated hybrid sisal-basalt FRP confined specimens exhibited superior properties and the low strength of natural FRP confinement compared to artificial FRP can be improved by epoxy modification. The outer jacketing resisted abrupt and catastrophic failure to a great extent.