A hybrid bonding system for improving the structural performance of FRP flexurally strengthened concrete beams

In this paper results of an experimental test are presented in which eighteen notched concrete beams strengthened with carbon fiber (CF) sheets in flexure are subjected to four-point bending. The objective of the test is to seek improved bonding configurations for FRP to concrete interfaces, through which optimal flexural strengthening performance can be achieved. Two types of sheet geometries (conventional CF sheet and new CF strand sheet) and three types of adhesive bonding systems (stiff adhesive bonding, ductile adhesive bonding, and the hybrid adhesive bonding are applied in the test. Through combining the sheet geometry and adhesive bonding) system in different ways, five bonding configurations are formed as follows for comparisons: (1) conventional CF sheet + stiff adhesive bonding; (2) conventional CF sheet + ductile adhesive bonding; (3) CF strand sheet + stiff adhesive bonding; (4) CF strand sheet + ductile adhesive bonding; and (5) conventional CF sheet + hybrid adhesive bonding. Efficiencies of these bonding configurations are evaluated in terms of the serviceability limit state (SLS) and ultimate limit state (ULS) performance of strengthened beams. It is found that the new type of CF strand sheet is superior to the conventional CF sheet. In addition, the hybrid use of two types of adhesives has advantages over a single use of stiff or ductile adhesive bonding system when there is a necessity to optimize simultaneously the SLS and ULS performance.