## 3 Test Results  
### Test Series C1  
For all tests, the stiffness values \(K_{\text{ser}}\) were evaluated. The stiffness of the connection in a situation of direct contact between the elements was compared to the stiffness determined according to ETA-13/0029 [19], where \(K_{\text{ser, ETA}}=9.45\) kN/mm, as shown in all the diagrams. Test series C1.1 and C1.2 were manufactured with direct contact (DC) between the timber and concrete. For test series C1.1 (see Figure 9), a torque of 60 Nm and two load cycles were applied. Subsequently, the specimen was unloaded, and a torque of 80 Nm was applied, and three more load cycles were applied (see Table 4). The preliminary tests showed that the stiffness tends to increase with the number of load cycles and the torque applied to the screws (see Figure 9). It is important to note that these results are only indicative due to the limited number of load cycles. Information on the load speed is given in Table 3, which has been calculated by reaching \(F_{\text{est}}\) after 5 min. However, a different load speed may affect the resulting stiffness values. As the speed was constant for all tests, no information can be given for the influence of a varied speed. Figure 10 shows the results of tests C1.3 to C1.8 under different conditions. As observed in the preliminary tests, the stiffness of the connection is influenced by the type of the contact surface. In this group of tests, specimens with DC (C1.3 and C1.6) showed a continuous increase in stiffness as the torque levels increased. The observed behavior may be attributed to the increase in friction effects as the torque increases, resulting in higher pressure between the timber and concrete. Test C1.4 showed a different behavior under the same test conditions (intermediate layer P) as C1.7. In terms of stiffness, this test showed an almost constant behavior even when the torque level of the screws increased from 30 Nm to 60 Nm and from 60 Nm to 80 Nm. Tests C1.5 and C1.8 with a PV intermediate layer showed similar stiffness values for all torque levels. However, a slight increase in the stiffness was observed after each load cycle. This increase was not as significant as in the tests with DC and P. Based on the results, it can be concluded that the PV intermediate layer eliminated any friction effect, resulting in stiffness values similar to the reference value (\(K_{\text{ser, ETA}}\)). Figure 9: Resulting stiffness values \(K_{\text{ser}}\) of push-out tests C1.1 and C1.2. Tests C1.9 to C1.12 were conducted with ten load cycles per torque and aimed to investigate the stiffness of the connection after each load cycle. The study was performed with torque levels of 30 Nm and 80 Nm, and intermediate layers DC and PV were used, as they represent the two extreme friction situations. The results are given in Figure 11 except for those from test C1.10, which were excluded due to an error in the displacement measurement system. The results show a significant difference in the stiffness between tests C1.11 and C1.12, which had a PV intermediate layer, and the CD situation (C1.9). The stiffnesses of C1.11 and C1.12 increased steadily between each test cycle, but with a much smaller amplitude compared to test C1.9, which had direct contact between timber and concrete. Tests C1.13 to C1.16 were conducted using the same load protocol, torque levels, and interlayer type as tests C1.9 to C1.12. Upon analysis of Figure 12, it was observed that tests C1.13, C1.15, and C1.16 show similar results compared to tests C1.9, C1.11, and C1.12. Furthermore, the results showed a comparable tendency to tests C1.13 and C1.14, both with direct contact between the timber and concrete but with different torque levels. The use of plastic foil and Vaseline as an interlayer resulted in similar stiffness values even at higher torque levels. However, the absence of an interlayer led to increased stiffness values at higher torque levels.