Seismic retrofitting of RC walls externally strengthened by flax-FRP strips

Several studies available in the literature have demonstrated that natural fibres can be employed in externally bonded (EB) fibre-reinforced polymer (FRP) strips used to strengthen existing reinforced concrete (RC) members. The study aims to demonstrate the feasibility of retrofitting RC walls using flax-FRP (FFRP). The study consists of an experimental campaign and analytical evaluations, the latter aimed at determining the evolution of relevant damage indices during cyclic loading processes. The behaviour observed in the tests of RC walls strengthened by FFRP is compared with that of both a reference wall and that of similar specimens strengthened with more conventional composite materials (i.e., carbon-FRP or CFRP). The test results show that FFRP has the potential to be used for seismic retrofitting as a viable alternative to more common FRP materials and other traditional techniques. Indeed, RC wall specimens strengthened with FFRP demonstrated strength increases of up to 150% and ductility gains equal to about 30%. Moreover, the tests show that walls strengthened by FFRP generally dissipate more energy than those strengthened with CFRP: this is an important property for the seismic strengthening and retrofitting of existing RC structures.