## 1 Introduction  
Precast concrete (PC) technology has been widely used in construction of parking lots, discount stores, logistics warehouses, etc. to achieve rapid completion [1, 2, 3]. In recent years, PC construction has been adopting composite PC technology to achieve a similar structural performance for connections to monolithic construction. Such composite construction can provide an economic solution by combining precast and cast-in-place (CIP) concretes with the efficiency and integrity of monolithic concrete structure. However, previous studies [4, 5, 6, 7] have found that the interface shear transfer capacity was essential to achieve monolithic action, because horizontal shear behavior between PC beam and CIP slab can affect the development of flexural strength, diagonal shear strength, and deformation capacities for a composite beam. Therefore, composite PC beams should be designed to have a horizontal shear strength greater than the horizontal shear force that may cause interface shear failure under bending deformation. Otherwise, the composite action of beams decreases, causing a significant reduction of stiffness and flexural capacity after interface shear failure. Several factors have been found to influence the horizontal shear behavior of a composite PC beam, including the interface condition, the ratio of shear connectors across the interface, the yield strength of the shear connector, the compressive strength of the concrete, the density of the concrete, and the presence of an externally applied force. The roughness and surface conditions of interface are closely related to the horizontal interface strength without shear connectors. Horizontal shear strength increases with surface roughness up to a 6 mm amplitude from the test results [4; 8]. Furthermore, the use of shear keys extending transversely across the interface is not more effective in resisting the horizontal shear than a fully roughened surface [4]. The concrete compressive strength of the weaker between substrate and overlay restricts the horizontal shear transfer capacity of the interface. Test results revealed that an increase in weaker concrete strength improved the horizontal shear capacity [4; 6; 7; 8]. [PERSON] and [PERSON] [5] investigated the effect of concrete strength and clamping stress based on the test results of 16 composite concrete beams with roughened surface. The clamping stress caused by the shear connector across the interface is effective after a slip of 0.5 mm. Meanwhile, horizontal shear strength has been mostly studied using push-off tests with normal-weight concrete (NWC) [9; 10; 11; 12; 13; 14; 15; 16; 17; 18] and push-off tests with lightweight concrete (LWC) [19; 20; 21; 22; 23]. The difference in concrete density is clearly distinguished in the failure mode. NWC was destroyed along the surface between aggregate and binder, while LWC had a fracture surface penetrating the aggregate. Therefore, the interface surface using NWC forms a surface by protrusion of the aggregate, so it has a shear transfer mechanism by aggregate interlocking. On the other hand, the horizontal interface between the PC beam and the CIP slab is located either on the compression side or on the tensile side of the cross-section depending on the direction of moment. In the case of positive moment, it is classified as C-type interface due to the compressive stress on the CIP slab. In the case of negative moment, it is classified as T-type interface due to the tensile stress on the CIP slab. In previous studies [24; 25; 26], the horizontal shear performance of the composite beams was evaluated through three phases of studies. In the phase 1 study, three beam-column joint specimens were tested to investigate the flexural behavior of continuous PC composite beams. However, their strengths were limited by the sliding failure of interface despite the use of sufficient shear reinforcements for the composite action.