Bond behaviour of NSM CFRP laminate strip systems in concrete using stiff and flexible adhesives

The influence of the adhesive type on bond behaviour between concrete and Carbon Fibre Reinforced Polymers (CFRP) laminate strips, in the context of Near Surface Mounted (NSM) strengthening technique, is considered as crucial for an efficient design. Thus, direct pullout tests were carried out to assess the influence of i) type of adhesive ii) CFRP cross-section and (iii) bond length on behaviour of NSM-CFRP system. Two types of stiff adhesives and one type of flexible adhesive were studied. For similar bond lengths, significantly higher maximum pullout force and bond stiffness were observed in the case of stiff adhesives, while noticeably higher slip at maximum force was achieved with the flexible adhesive. Analytical and numerical investigations were carried out in order to determine the local bond stress-slip relationships for both stiff and flexible adhesives. After demonstrating its good predictive performance, the analytical approach was used to design curves of the required anchorage lengths of NSM-CFRP system at ultimate limit state conditions. For the analytically calibrated mechanical parameters of NSM bond-slip law a two-dimensional numerical model of the direct pullout tests was worked out.