Static cyclic in-plane shear response of damaged masonry walls retrofitted with NSM FRP strips – An experimental evaluation

An experimental study was conducted to assess the effect on strength and ductility of retrofitting unreinforced masonry (URM) shear panels with near surface mounted (NSM) carbon fibre reinforced polymer (CFRP) strips. A total of sixteen wall panels, 1200 mm × 1200 mm, were subjected to vertical pre-compression combined with increasing reversing cycles of in-plane lateral displacement. All wall panels were previously tested (prior to retrofitting) under compression and cyclic shear using three different pre-compression levels resulting in various levels of damage. The damaged walls were repaired, retrofitted with NSM FRP strips and retested under pre-compression stress levels of 2.8 MPa, 2 MPa and 1.4 MPa. The retrofitted walls displayed higher displacement capacities compared with URM walls highlighting the effectiveness of retrofitting URM walls under earthquake loading. The ultimate loads were not enhanced due to retrofitting under higher pre-compression levels. However the presence of the reinforcement did restore the ultimate loads to those observed for the original undamaged URM state. This meant that overall, the reinforcement was effective in increasing the energy dissipation capacity of the walls compared to URM. The improvements in the behaviour of the URM walls due to retrofitting were generally similar, irrespective of the amount of damage the URM walls experienced prior to retrofitting. The paper discusses the effect on strength, displacement capacity, energy dissipation and ductility achieved by FRP retrofitting of the damaged (lightly and highly) URM panels compared to the undamaged URM panels under different pre-compression levels. The broader aim of the research is to identify techniques for improving the seismic performance of existing URM walls under in-plane shear loading.