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

This work presents the results of an experimental and numerical investigation on the flexural behaviour of reinforced concrete (RC) slabs strengthened by carbon fibre reinforced polymers (CFRP) laminates applied according to the near surface mounted (NSM) technique, using stiff and flexible adhesives. Two study variables were analysed: i) the adhesive type and ii) the existence or not of pre-cracking on the slabs. The results show a clear dependence of slab's flexural performance on the adhesive type: the use of flexible adhesive yields to 80% of the maximum load achieved with stiff adhesives. The existence of pre-damage did not affect the structural behaviour of the slabs. CFRP rupture was observed with the use of stiff adhesives while CFRP debonding has occurred with the flexible one. Finally, a higher ductility was observed when using flexible adhesive. A numerical model was worked out and calibrated to simulate the flexural behaviour of the tested slabs. The numerical simulations showed a very good agreement with the experiments. Besides the very good predictive performance in terms of load-displacement behaviour, the numerical model also correctly reproduced the failure modes obtained in the experiments and the differences in the bonding mechanisms of slabs strengthened with the stiff and flexible adhesives. It was demonstrated that the proposed numerical model can be used in engineering practice for the analysis and design of NSM CFPR strengthening systems for existing RC structures.