## 1 Introduction  
Modular structures are designed to meet various building needs and are assembled on-site after factory production; therefore, their use is increasing due to their ease of construction and economic and environmental advantages [1, 2]. Most modular buildings are constructed using precast concrete (PC) or steel. Steel modules are generally half the weight of their concrete contemporaries, making them an attractive option because they lead to cost savings [3]. However, PC modules are heavier than steel, making it more difficult to lift their weight as the building height increases. PC modules have the added disadvantage of needing to be bonded by special connecting hardware. Recently, however, interest in using PC modules has increased due to their better seismic performance than steel modules. Precast concrete has been used widely in a number of countries with high seismicity such as New Zealand, Japan, and Chile [4]. In addition, concrete with higher compressive strength such as reactive powder concrete can be adopted in precast concrete modules to ensure structural performance [5]. The problem of PC module weight has been addressed with the development of customized construction equipment, but the structural integration problem between PC modules to ensure structural performance must still be solved. Especially, the seismic behavior of precast concrete structures is highly dependent on the characteristics of connections between the precast structural members [4]. In general, PC modules are installed with steel connectors such as shoes or splice sleeves. Several studies have been carried out over the years to evaluate the capacity of different types of connections for precast concrete members [6, 7, 8]. [PERSON] [9] carried out cyclic tests on a manufactured wall shoe, steel plate, and a test specimen connected by steel bars to determine which connection method resisted an earthquake most effectively when combined with a panel wall made of PC modules. In that experiment, the ductility and strength were excellent when the wall panels were connected with the wall shoe, but the steel bars were disadvantageous when the lateral displacement was large. [PERSON] [10] analyzed the bending behavior of PC wall modules connected by shoes, and the connections were then bonded with waterproof tape or structural adhesive. The structural performance of all the test specimens was excellent, regardless of the arrangement of the wall shoe, but when they were bonded with waterproof tape, the structural performance was degraded due to a low splicing performance between the modules. [PERSON] and [PERSON] [11] assessed a method wherein components were connected by a grouted splice sleeve in a PC member, but found that it was difficult to evaluate the nonlinear behavior of the components when connected in that way. The studies have focused on the splicing methods between vertical and horizontal members to ensure the seismic performance of modular buildings. However, if the connection between the horizontal members is not ensured, usability problems will arise and there will be difficulties in splicing performance between the vertical and horizontal members. The PC modular horizontal members have mainly been used in bridge construction due to their superior quality control and the advantages of low life-cycle cost, ease of construction, and time saving [12]. Such PC members in bridges typically use shear keys on connections to increase the shear stiffness and shear capacity between the modules. The shear keys act to inhibit the generation of shear slip in the connections, and the shear performance of the connections is greatly influenced by the shape of the shear keys. In PC construction, there are male-to-female [13; 14] or female-to-female keys [15; 16], as shown in Figure 1, and steel bars between the modules are connected using spliced sleeves or lapped splices. Finally, epoxy is used to fill in the non-shrinkage mortar between the shear keys to splice the two modules together [12; 17].