## 1 Introduction
In addition, as with this study, several researchers have conducted experimental studies to strengthen unreinforced masonry using overlay methods. [PERSON] et al. (2015) [PERSON] et al. (2015) strengthened unreinforced masonry by overlay polypropylene fiber reinforced mortar (PFRM) and carbon fiber mesh (CFM). [PERSON] et al. (2013) [PERSON] et al. (2013) strengthened unreinforced masonry walls with three types of polymeric mesh attached with mortar plastered on the walls. [PERSON] (2019) [PERSON] (2019) strengthened masonry walls using glass fiber mesh-reinforced mortar (G-FRM) so that the shear strength of the masonry walls increased by more than 100%, even with a 15 mm or smaller thickness of strengthening layers on the two wall faces. In most preceding studies, fiber mesh or wire mesh is usually used in the overlaying method for strengthening masonry walls. In this study, an efficient method to strengthen unreinforced masonry walls using a steel fiber-reinforced mortar (SFRM) overlay. The proposed overlaying method has the advantage of constructability during residence of occupants. In addition, plastering work does not require high workmanship and can be applied to the vicinity of openings without cutting or splicing. Above all, mortar can be integrated with masonry, considering that it has been used for the joint and finishing of masonry. However, the brittle behavior of plain mortar is similar to masonry. It is necessary to add reinforcement appropriate for plastering material to improve mechanical behavior of masonry walls after retrofit. SFRM is a cement composite incorporating steel fibers which improves brittle behavior of mortar as well as mechanical properties such as tensile strength and toughness. Generally, steel fibers are known to have better strength and stiffness than organic fibers such as Polyethylene (PE), Polypropylene (PP), Nylon, etc., but relatively long and thick dimensions make those difficult to apply steel fibers to thin mortar overlay. On the other hand straight or hook-type steel fibers commonly used in construction fields are relatively stiff and difficult to plaster in a thin layer without balls and protrusion, of which the latter may cause human injury. As a result, this study adopted Amorphous Steel Fibers (ASF), which reinforces mortar effectively in spite of its relatively short length due to better bonding characteristics owing to the thin shape and surface roughness. In this study, the effect of SFRM for strengthening masonry walls was assessed based on the compressive and diagonal tension strength for masonry prisms. Those strength parameters are used to define the lateral force-resisting capacity of masonry walls in a major failure mode such as toe-crushing or diagonal tension failure, as stipulated in ASCE 41-17 [14]. In order to build strengthened prism specimens, a recommended SFRM mixing ratio with both strengthening efficiency and overlay constructability is derived based on constructability tests for fresh mortar, compression and tension tests for hardened mortar specimens for various mixing ratios, since the characteristics of SFRM strongly depend on the type of fiber, fiber volume fraction, and mixing ratio of mortar. Then, the performance of the strengthening method is evaluated by compressive and diagonal tension tests of masonry prisms with and without SFRM overlay to which the recommended mixing ratio of the mortar. This study concentrates on two representative types of unreinforced masonry. One is made of red bricks and usually used as an exterior veneer. The other is concrete bricks, usually used for bearing walls. For easy construction, only SFRM was used, without additional glass fiber or wire mesh. Finally, the applicability of existing design equations is investigated based on the measured strengths of prism specimens.