Influence of adhesive type on the flexural behaviour of RC slabs strengthened with NSM-CFRP systems

Abstract

In the context of the strengthening of reinforced concrete (RC) structures, near surface mounted (NSM) technique can be used by applying carbon fibre reinforced polymer (CFRP) laminates on the concrete cover of the structural elements to be strengthened. An adhesive is used to fix the CFRP to concrete, which is responsible by the stress transfer between the concrete and the CFRP, assuming a key role for the success of the strengthening system. The influence of the adhesive type on the overall performance of these strengthened structures presents some research gaps in knowledge. In fact, only a reduced number of studies were found in the literature evaluating the influence of the mechanical characteristics of the adhesives on the performance of the strengthened elements. This paper presents the results of an investigation on the flexural behaviour of RC slabs strengthened with NSM CFRP systems using stiff (Adhesives 1 and 2) and flexible (Adhesive 3) adhesives. For this purpose, an experimental program was carried out, being considered two study variables: (i) the adhesive type and (ii) the existence or not of pre-cracking on the structural element. Flexural slab tests were used to characterize the differences on the slab’s structural behaviour depending on the parameters tested. Regarding to the results obtained, it is clear the dependence of the response force versus mid-span vertical deflection and mid-span CFRP strains on the adhesive type as well as on the presence or absence of pre-cracking. However, it was verified that the maximum load attained is less dependent on the mention parameters. Thus, with the flexible adhesive, the maximum load attained is about 80% of the maximum load achieved in the tested slabs where stiff adhesives were used. The maximum load achieved is similar with presence or absence of pre-cracking. The failure of the CFRP laminate was observed with stiff adhesives while with the flexible one, CFRP debonding was observed. Finally, a higher ductility was observed by using flexible adhesive.