## 1 Introduction  
Compared with traditional buildings, assembled structures have the advantages of short construction periods and low environmental impacts, which are conducive to the construction industry's sustainable development [1; 2; 3; 4]. Beam-column connections in assembled structures are critical cast-in-place regions where all longitudinal rebars are lapped; thus, the stress is extremely complex and prone to shear brittle failure during earthquakes, leading to the inevitable collapse of RC frame structures. To ensure assembled frame structures meet design requirements, many stirrups are usually installed at beam-column connections. However, dense reinforcements can increase construction difficulty and reduce concrete's compactness. Ultra-high-performance concrete (UHPC) has been proven to have excellent mechanical performance and durability. Fibers inside the concrete act as bridges in the matrix, inhibiting the development of cracks and dissipating energy. Stress on the crack's surface is distributed to the surrounding area, and the crack is narrow and dense. Therefore, many scholars have reduced reinforcements and improved seismic performance by using UHPC in beam-column connections' critical cast-in-place regions. [PERSON] et al. [5] applied high ductility fiber-reinforced concrete (HDC) to assembled beam-column connections' critical cast-in-place regions and tested its seismic performance under low-reversed cyclic loads. [PERSON] et al. [6] applied engineered cementitious composites (ECC) to assembled connections and studied the quasi-static cyclic behavior of special moment-resisting frame structures. Reactive powder concrete (RPC) first appeared in the 1990s. It is a type of high-performance concrete [7; 8; 9] that has high compressive strength, high tensile strength, and durability. RPC's cementitious material is industrial waste. Using industrial waste is beneficial for the construction industry's sustainable development [10]. [PERSON] et al. [11] explored RPC's basic mechanical performance using recycled fine aggregate under different curing conditions. [PERSON] et al. [12] replaced RPC's calcium carbonate with silica sand, added glass fiber, and studied its mechanical properties. [PERSON] et al. [13] prepared RPC with manufactured sand (MS) as a fine aggregate and studied the effects of different curing methods on compressive strength. [PERSON] et al. [14] prepared RPC with steel and basalt microfibers and believed that RPC could save materials and assemble structures lightweight. [PERSON] et al. [15] studied the effect of steam curing on RPC's basic mechanical performance and determined its optimal mix ratio. According to the demand, RPC's mixing ratio can be optimized by adjusting the curing conditions, the active powder type, and the fiber type. In addition, RPC can significantly improve the seismic performance of shear walls [16], beams [17; 18], and columns [19; 20]. Applying RPC to critical cast-in-place regions of assembled connections can obtain better seismic performance than monolithic concrete beam-column connections. Modified reactive powder concrete (MRPC) is a concept that combines modified reinforced composite materials and reactive powder concrete. It uses more common and environmentally friendly concrete matrix materials (mainly cement, river sand, mineral powder, fly ash, and silica fume) and uses steel fiber and polypropylene fiber as reinforcements. By utilizing the hybrid advantages of the two fibers, it has high tensile strength and high toughness. In a previous investigation, it was found that MRPC for surface strengthening of brick walls can significantly improve seismic performance [21]. MRPC and reinforcements have ultra-high bonding strength and, thus, have a potential advantage in improving the connection performance of assembled structures.