## 2 Experimental Program  
### Mechanical and Geometric Properties  
The experimental study comprised three test series (C1, C20, and C21) that evaluated different test conditions. These conditions included the geometry of the test specimen, the contact interface between the timber and concrete, and the load protocol (static and/or cyclic). Test series C1 aimed to investigate the stiffness of the connection under various contact conditions, torque levels, and numbers of load cycles (cyclic tests). Static tests were conducted to determine the load capacity after all cyclic tests. The study aimed to investigate the behavior of the TCC connection under reduced contact force between the timber and concrete by altering the interface type. In addition, the study aimed to evaluate the impact of the interface type on the friction levels. Test series C20 and C21 focused on evaluating the effect of the geometry of the specimens on the load capacity and stiffness of the connection in static tests. Test series C1 was conducted on symmetrical push-out test specimens using a dowel-type connection system, specifically the Wurth Assy plus self-tapping VG screw (Wurth, Kunzelsau, Germany), along with the FT connector, in accordance with the European Technical Approval ETA-13/0029 [18]. The test specimens were made of glued laminated timber and concrete with strength classes of GL 24 h and C 25/30 (see Figure 1 and Table 1). The precast concrete element manufacturing process includes the installation of the FT connector (see Figure 2), to ensure the precise application of one screw per shear plane, at a 30\({}^{\circ}\) angle to the timber fibers and with a length of 230 mm, fully embedded in the timber element. Figure 1 shows the position of the measurement device located in the horizontal area of the screw between the timber and concrete. Linear variable differential transformers (LVDTs) with a capacity of 20 mm were used. The load was applied using a 200 kN actuator, and the load capacity was recorded using a 200 kN load cell. Test series C20 and C21 were produced as asymmetric (inclined under 80\({}^{\circ}\)) and symmetric push-out test specimens, respectively. As TCC connections, notches were milled into the timber elements. As material, glued laminated timber with a strength class of GL 24h and concrete C 30/37 was used. In each shear plane, a Spax 8 \(\times\) 160 screw with a washer head and a partial thread was inserted at a 90\({}^{\circ}\) angle to the timber fibers in the center of the notch to prevent uplift. This follows the guidelines of standard ETA-12/0114 [19], and the screws were applied before concreting (see Figure 3 and Table 1). Test series C1 was developed using symmetrical specimens with timber in the center of the concrete elements. Each specimen consisted of two precast concrete elements measuring 500 \(\times\) 500 \(\times\) 70 mm and one timber element with a cross-section of 100 \(\times\) 200 mm and a height of 500 mm. The elements were joined by applying a screw per shear plane with a diameter of 10 mm and a length of 300 mm. The screws were inserted through the FT  
Figure 3: (**a**) Asymmetrical push-out specimen of series C20; (**b**) symmetric push-out specimen of series C21 (dimensions in millimeters) [20]. Figure 2: FT connector and screw before tightening on push-out test series C1. connector screws after the concrete had cured. The specimens were assembled with clamps to ensure the correct positioning of all elements (see Figure 2). Test series C20 was produced as asymmetric specimens, with a concrete element measuring 80 \(\times\) 200 \(\times\) 624 mm and a timber element measuring 100 \(\times\) 200 \(\times\) 674 mm. The notch dimensions were 160 \(\times\) 200 \(\times\) 20 mm, where a screw with a diameter of 8 mm and a length of 160 mm was applied (see Figure 3a). Test series C21 consisted of symmetrical specimens with concrete in the center of the timber elements.