EXPERIMENTAL INVESTIGATION ON SEISMIC PERFORMANCE OF UNREINFORCED MASONRY WALLS STRENGTHENED WITH LIGHTWEIGHT ENGINEERED CEMENTITIOUS COMPOSITES

Abstract

Historically, Masonry buildings are known to be one of the most seismically vulnerable structures due to their low capacity to resist tensile stresses and lateral loads developed during seismic activity. Over two thousand non-earthquake-resistant masonry buildings were widely built in the Almaty region – Kazakhstan's most seismically dangerous area between the 1950s to 1980s. The Almaty region is known to have a long seismic cycle, indicating that stress repeatedly builds up over a long period and has a high potential for a rapid release of a hazardous earthquake in the future. Thus, the development of effective techniques such as retrofitting masonry structures with the application of engineered cementitious composites to improve the seismic performance of such buildings is essential. Engineered cementitious composites (ECC) have been studied more for structural retrofitting applications, such as seismic retrofitting on unreinforced masonry (URM) walls because they exhibit a ductility strain hardening response under tension in contrast to conventional concrete. Literature shows that using normal-weight designed cementitious composite can greatly increase masonry walls’ strength against cyclic in-plane horizontal loads. However, it has limited improvement in the deformation capacity and ductility of the wall, which is an important criterion for seismic resistance. This restriction might be due to the incompatibility between the wall and the composite in terms of strength and stiffness. It is suggested that lightweight engineered cementitious composites (LWECC), which have lower strength and stiffness and are more compatible with brick walls, can address the restriction. To investigate the seismic performance of the URM walls retrofitted with LWECC through an experimental study, three URM walls with an aspect ratio of 0.55 were tested under cyclic in-plane loadings. The walls were chosen as 1/3 of an interior prototype load-bearing wall segment of a typical old non-earthquake-resistant four-story residential URM clay brick building found in Almaty region, Kazakhstan built between the year, 1957 to 1967. One wall was a URM wall without any form of retrofit as a reference specimen and the other two walls were retrofitted by troweling 15 mm thick layers of LWECC on both faces of the wall. The composites were chosen to have two different compressive strengths of 27.10 MPa and 11.13 MPa respectively, exhibiting tensile strength and tensile strain capacities of 3.75 MPa (1.48%) and 2.63 MPa (1.80%) 3 respectively. In-plane wall testing was conducted by applying a constant gravity load of 248 kN to induce an average compressive stress (σ0) of 0.51 MPa, maintained to represent the axial stress on the prototype wall with 3-story wall load above then increasing cyclic lateral loads applied to the top left end of the wall until failure or 15% strength reduction was observed. The test walls were instrumented with five linear variable differential transformers (LVDTs) and four string potentiometers to measure the horizontal top displacements, all deformations, uplifts, and crack propagations experienced by each wall specimen. Results from the in-plane wall testing showed that retrofitting the URM walls with the lightweight composites, although exhibiting a limited increase in the lateral load-resistant capacity of the walls by average values of 7.21% for 27.10 MPa LWECC retrofitted wall and 10.14% for 11.13 MPa LWECC retrofitted wall, there were significant improvements in the ultimate displacement and wall ductility capacities. Retrofitting of the wall with 24.10 MPa LWECC caused an average increase of 58.44% and 151.91% in the ultimate displacement and wall ductility capacities respectively and retrofitting of the wall with 11.13 MPa LWECC caused an average increase of 122.78% and 151.18% in the ultimate displacement and wall ductility capacities respectively. The findings show that this retrofitting method can significantly increase the ductility and displacement capacity of unreinforced masonry (URM) walls, resulting in a change from brittle failure to a more ductile mode of failure while maintaining wall integrity under significant lateral deformations. Furthermore, it was deduced that the usage of a lighter composite material produces improved ductility, hence boosting the efficiency of the strengthening strategy. In particular, the lightweight composite material's improved ductility significantly raises brick walls' ductility and displacement capacity.