Externally bonded FRP and mechanically fastened by nailing: A new technique to postpone debonding of FRP sheets in unreinforced concrete slabs

The present study evaluates various techniques for flexural strengthening of slabs by bonding carbon fiber-reinforced polymer (CFRP) sheets and then develops a new bonding method called externally bonded reinforcement and mechanically fastened by nailing (EBR-MFN). A total of 18 unreinforced unidirectional concrete slabs with the dimensions of 500 x 200 x 40 mm in nine groups were tested under four-point flexural loading. These groups consisted of slabs without FRP (control slabs), FRP slabs strengthened with near-surface mounted (NSM), conventional externally bonded reinforcement (EBR), bore-epoxy anchorage (boring), externally bonded reinforcement on grooves (EBROG), externally bonded reinforcement in grooves (EBRIG), EBR-MFN-2x1N (with two nails), EBR-MFN-2x2N (with four nails), and EBR-MFN-4x2N (with eight nails) methods. The performance of the proposed EBR-MFN approach was compared to the other techniques using the load-displacement curve and failure mechanism. The findings showed that the EBR-MFN approach significantly improved the slab performance by postponing the debonding between the concrete surface and CFRP sheets. Also, the EBR-MFN method with two, four, and eight nails compared to the EBR samples increased the load-carrying capacities by 19.62, 38.13, and 48.62%, respectively. Although the EBR-MFN technique with eight nails had an easy and quick installation process with less environmental pollution, its load-carrying capacity was only about 14% fewer than the EBROG method. In specimens reinforced by the EBR-MFN method, the failure mode was changed from debonding to Separation of a thin layer of concrete + CFRP slight rupture.