## 1 Introduction  
Unreinforced masonry is a representative structural component that is vulnerable to earthquakes. Significant structural damage and collapse of masonry buildings accompanied by significant property loss and casualties were observed in many past earthquake events [1, 2]. One reason for such severe damage is that many masonry buildings are composed of unreinforced masonry walls, as these buildings were designed before the establishment of seismic design codes or are exempted from seismic design requirements owing to their relatively small sizes. Masonry supports gravity loads effectively by its compressive strength but cannot sustain combined stresses owing to its low tensile strength and brittle behavior. Diverse modeling techniques have been developed for unreinforced masonry structures using refined finite elements or macro-elements [3]. In addition, innovative survey techniques using 3D laser scanning or digital photogrammetry have been developed and combined with seismic performance assessment based on the finite element modeling [4, 5]. Considering the vulnerability of unreinforced masonry buildings is necessary; however, a retrofit technique that enables a low-cost and short duration construction is required to promote the voluntary retrofit of such buildings, which are typically privately owned. Among the many retrofit techniques for masonry buildings, strengthening masonry walls by attaching or plastering reinforcement materials such as fiber-reinforced polymer (FRP)or mortar reinforced with a fiber mesh is advantageous in that implementation can be performed outside the building, thereby enabling retrofitting while the buildings are occupied. [PERSON] et al. [6] tested masonry walls strengthened with glass fiber grids and prestressed steel bars subjected to cyclic loading. [PERSON] et al. [7] tested masonry wallettes reinforced with twisted steel bars inserted in grooves formed on mortar joints or brick surfaces and evaluated the improvement in ductility. [PERSON] et al. [8] verified the strength and ductility of unreinforced masonry strengthened with lime grout injection. [PERSON] et al. [9] and [PERSON] et al. [10] attached steel strips to existing masonry walls in a diagonal or vertical direction and anchored their ends to adjacent existing structural components. [PERSON] et al. conducted both numerical and experimental studies on the bond-slip behavior of FRP materials for strengthening masonry [11]. [PERSON] et al. [12] conducted cyclic loading tests on masonry walls strengthened with FRP sheets. [PERSON] et al. [13] sprayed engineered cementitious composites reinforced with polyvinyl alcohol fibers on masonry walls and observed an increase in the strength and ductility under cyclic loading. In addition, [PERSON] et al., [PERSON] et al., [PERSON] et al. and [PERSON] [14; 15; 16; 17] investigated diverse strengthening techniques for masonry walls by plastering high-strength mortar or mortar reinforced with diverse fibers or meshes such as carbon fiber mesh, polymeric net, and glass fiber reinforced polymer mesh. [PERSON], and [PERSON] investigated confinement effect obtained with fabric reinforced cementitious mortar jackets for masonry columns experimentally and proposed methods to predict enhanced axial strengths [18; 19]. This study investigates the performance and key mechanical properties of masonry strengthened with an amorphous steel fiber-reinforced mortar (ASFRM) overlay. Among the various fibers used in concrete or mortar mixtures, amorphous steel (AS) fibers are adopted because of their superior bonding characteristics and relatively short and thin shapes, which are suitable for plastering small thicknesses. Although existing studies have typically utilized mortar overlays reinforced with grid-type meshes, only AS fibers are used as the main reinforcement material in this study. The mesh can be excluded because steel fibers exhibit high strength and stiffness compared with organic fibers such as polyethylene (PE) and polypropylene.