Quasi static in-plane behavior of full scale unreinforced masonry wall with and without retrofitting

Abstract

Unreinforced masonry (URM) is the oldest and most widely used construction material in the world. URM is not able to carry tensile forces due to its low tensile strength. The main structural elements that resist earthquakes in these buildings are the old URM walls which were designed to resist mainly gravity loads. Their vulnerability is caused by the failure of unreinforced masonry walls due to the in-plane and/or out of plane seismic load. This study presents an experimental investigation on the behaviour of in-plane loaded URM wall retrofitted with ferrocement technology. A quasi-static cyclic test on full-scale wall has been performed on unreinforced masonry (URM) wall. Unreinforced masonry wall was built consisting of a clay masonry panel and a reinforced concrete base slab. Wall assemblies had aspect ratio of 0.75. Walls were investigated for full ferrocement coverage with extra base slab-wall panel joint lamination. Wire mesh steel having opening sizes 12 X 12 mm was considered for ferrocement encasement. One wall was kept unretrofitted only to be used as a control model. Behaviour of the strengthened walls under a combination of a vertical load and lateral reversed cyclic loading was compared to the control models to observe improvement of lateral load resistance capacity. The retrofitted walls exhibited highly inelastic cyclic behaviour due to hysteretic energy dissipation. Equivalent viscous damping was calculated to be within the range of 8 to 22 %. Both specimens underwent a moderate degree of strength and stiffness degradation due to the damage accumulated from repeated cyclic deformation and increasing displacement. Stiffness of the specimens decreased gradually under cyclic loading. But, Stiffness of the retrofitted specimens did not degrade at the same rate as that of the unretrofitted samples. The initial stiffness amplified up to 52% in case of retrofitting by ferrocement. Fully ferrocement encased walls showed the highest increase in terms of stiffness. The progressive damage accumulated by the walls during the course of the cyclic tests caused significant strength and stiffness degradation in the force-deformation behaviour. Before formation of first crack, the specimen retrofitted with ferrocement overlay withstood about two times more lateral load than the corresponding unretrofitted specimen. The post-cracking strength was greatly enhanced by the presence of ferrocement, which was almost 1.6 times for the failure load. The strengthening also improved the total energy dissipation by a factor ranging from 49 % to 68% for the walls. The energy dissipation was almost 1.4 times higher than that of the control wall. Ductility also increased by retrofitting the walls with ferrocement. Hysteresis loops showed higher ductility for retrofitted specimen than that of the control specimen. Numbers of cracks were significantly fewer for the specimens retrofitted with ferrocement overlay. Unretrofitted specimen exhibited rocking failure pattern, whereas retrofitted specimen showed rocking and to some extent flexural mode of failure. Finally, retrofitting URM walls by ferrocement overlay may be quite useful as it can be applied easily by local construction workers, at an affordable cost. The ductile properties of ferrocement layers make the URM resistant to the in-plane load without having collapsed.