Comparison Between Direct Shear and Flexural Tests on RC Elements Strengthened with SRG Composites Subjected to Cyclic Loading

Steel reinforced grout (SRG) retrofitting systems represent a suitable technique to increase the performances of reinforced concrete (RC) structures, in particular for structural elements subjected to flexure. They can be applied even to wet surfaces and they feature a better fire resistance with respect to Fiber-Reinforced Polymer (FRP) composites. Nevertheless, the failure mode of strengthened elements can be similar to that of FRPs, i.e. debonding of the externally bonded reinforcement, especially if several layers of composite reinforcement are applied to the RC element. Different studies can be found addressing the problem of bond between SRG and RC or masonry substrates, but only few of them analyzed the bond performance when multiple layers are considered. To this purpose, the present paper presents the results of an experimental campaign focused on this specific key-aspect of the bond behavior, i.e. when multiple SRG layers are considered. Single-lap shear tests were carried out on concrete prisms strengthened by a single or multiple SRG layers, addressing the differences in terms of bond behavior and failure modes. In addition, monotonic and cyclic loading were considered in order to verify the possible bond degradation when applying high amplitude cycles; results of the cyclic loading tests were compared with those from monotonic tests on single or multiple SRG layers with the same amount of reinforcement. Flexural tests were carried out on full-scale RC beams strengthened with multiple plies of SRG reinforcement, without mechanical anchorages. Beams were subjected to either monotonic or cyclic loading. Tests were carried out under displacement control in order to properly characterize the non-linear behavior, evaluating the effect of composite reinforcement in terms of failure mode and ductility. The comparison between both small (single-lap) and full-scale (beams) tests allows to investigate the applicability of the bond relationship developed at small scale to structural elements and to evaluate the possible limits of small-scale testing.