## 1 Introduction  
Timber-concrete composite (TCC) structures are popular for rehabilitating timber structures and building floors or bridge decks due to their increased stiffness and load-carrying capacity compared to pure timber or concrete structures. Combining timber and concrete improves the acoustic, thermal, and fire resistance performance. TCC solutions use a natural, lightweight, renewable, and durable material--wood--reducing the amount of concrete used and making it increasingly important. This approach is used in many innovative and sustainable systems [1, 2, 3, 4, 5]. The connection in TCC structural systems is crucial for efficient composite systems, with the connection performance and mechanical behavior closely related to the overall structural system behavior [2, 3, 6, 7]. The type and configuration of the TCC connection (almost fully rigid and semi-rigid) significantly impact their mechanical performance and the degree of composite action. Full composite action connections are rigid due to almost no slip and a quasi-perfect interaction between the timber and concrete; connections with full composite action are considered rigid (almost fully rigid). Semi-rigid TCC connections achieve partial interaction, resulting in non-negligible slip, a characteristic of partial composite action [2, 3, 8, 9]. The mechanical behaviors of TCC connections, including their strength, stiffness, and failure mode, are typically determined through shear tests in the short and long term [7, 10]. Numerous studies have explored the different types of connections, such as notched and glued connections, which exhibit higher stiffness, and dowel-type connections, which have lower stiffness and ductile behavior [11; 12]. Test conditions, such as the configuration of the specimen, the interface between timber and concrete, and the test procedure are crucial for evaluating the stiffness and load-carrying capacity of TCC connections. For example, [PERSON] [11] states that connections notched with glued timber blocks are sensitive to the test configuration, specimen imperfections, and measurement location, resulting in fragile behavior and deficient strains at failure. The surface of timber (due to different manufacturing methods, such as different milling machines and saws) and watering might affect the connection between timber and concrete. Tests on TCC specimens with notched connections conducted by [PERSON] and [PERSON] [13] showed that notch geometry influences the final failure mode. [PERSON] et al. [14] illustrate that the existing friction between timber and concrete in composite connections of the dowel type should not be ignored. In addition, [PERSON] and [PERSON] [2] concluded that the configuration of the specimen can influence the mechanical properties of the connection. Shear tests can be divided into push-out tests and pure shear tests. Push-out tests may be either symmetrical or asymmetrical. Symmetrical specimens are produced by connecting two concrete elements to a central timber element or by connecting two timber elements to a concrete element. Asymmetrical specimens consist of one concrete and one timber element. In single-shear tests, the load should be applied to either the concrete or timber element. In double-shear tests, the load should be applied to the center element [2; 8]. A database by [PERSON] [11] reveals that 52% of the tests are double-shear specimens, 27% are asymmetric shear specimens, 4% are pure shear specimens, and 17% are another type. [PERSON] [11] also mentioned that symmetrical specimens have a timber element in the center, accounting for 64% of the sample, while concrete-centered specimens account for 34%. Moreover, 70.7% of the connections in the sample have metal fasteners, while 23.9% have notched connections with or without metal connectors. To assess the impact of the test procedure on the performance of the connection, [PERSON] et al. [15] conducted laboratory tests. They developed shear tests on pure and asymmetric shear specimens.