Mode II fracture energy and interface law for FRP - Concrete bonding with different concrete surface preparations

Results of an experimental campaign on delamination of CFRP sheets and plates bonded to concrete are presented. In particular, the effect of different surface preparations (grinding or, alternatively, sand blasting) is investigated. Six specimens have been tested by using a particular experimental set-up where the CFRP reinforcement is bonded to concrete and its back side is fixed to an external restraining system. The adopted set-up allows for a stable delamination process, with progressive transition between two limit states (perfect bonding and fully delaminated plate). Both strain gages along the FRP plate and LVDT transducers have been used. Experimental results showed that the technique for surface preparation may have a significant influence on the value of mode II fracture energy. Starting from experimental data, non linear interface laws have been calibrated. It is shown that, when concrete surface has been prepared by sand blasting, high values of fracture energy are attained with a smaller value of peak shear stress and significantly higher slips. Morever, FRP - concrete interface law exhibits a less brittle behavior of the softening branch.