## 3 Test Results
### Test Series C20 and C21  
Test series C20 and C21 were manufactured with notched connections. For all specimens of this series, the timber length in front of the loaded edge of the notch was 300 mm, which is 15 times the notch depth of 20 mm. The tests were stopped when the displacement between the timber and concrete reached 20 mm at the notch (EN 26891 [23] defines a minimum of 15 mm). Figure 15 shows the load-displacement curves with ductile compression of the timber in front of the loaded edge of the notch up to 20 mm. Figure 16 shows the same results up to 1 mm displacement. An example of a test specimen before and after testing is shown in Figure 17. The results of test series C21 are shown in a similar way in Figures 18 and 19. The strong decrease with a subsequent increase in stiffness within test C21.1 in Figure 18 may be due to a measurement error and can be neglected. All tests within both test series showed a final timber compression failure mode up to 20 mm displacement at the notch (see Figures 17 and 20). Figure 14: Load–slip curve. \begin{table}
\begin{tabular}{c c c} \hline
**Specimen** & \(K_{\text{ser}}\) **[kN/mm]** & \(F_{\text{max}}\) **[kN]** \\ \hline C1.21 & 49.4 & 74.8 \\ \hline C1.22 & 31.4 & 45.6 \\ \hline Mean & 40.4 & 60.2 \\ \hline \end{tabular} Note: Applied level of torque: 75 Nm.  
\end{table} 
Table 5: Stiffness values (\(K_{\text{ser}}\)) and maximum load (\(F_{\text{max}}\)). Figure 16: Load–displacement curve for test series C20 (up to 1.0 mm) [20]. Figure 17: Specim setup for test series C20: (**a**) before testing; (**b**) after ductile timber compression failure [20]. Figure 15: Load–displacement curve for test series C20 [20]. Figure 21 shows the ductile timber compression failure in front of the loaded edge of the notch after 20 mm displacement. This type of final failure mode is typical for the  
Figure 19: Load–displacement curve for test series C21 (up to 0.5 mm) [20]. Figure 20: Specim setup for test series C21: (**a**) before testing; (**b**) after ductile timber compression failure [20]. geometry of test series C20 and C21. Similar tests conducted in [13] or in [20] showed that a ductile final failure mode occurs for a long length of timber in front of the loaded edge of the notch (15 times the notch depth). During the testing of series C20 and C21, some loud noises were heard, especially at the beginning of the tests. However, no concrete cracking was observed during or after the tests. Upon evaluating the load-displacement curves, it was concluded that the heard noises were likely caused by the adhesive friction break-off resulting from direct contact between the timber and concrete. Typically, the timber in the notch area is watered 30 and 10 min before concreting. However, it is not clear whether this was done during the manufacturing of the specimens of test series C20 and C21, which may explain the observed friction break-off. For the symmetric test series C21, a clear trend was observed when comparing all levels of friction break-off within the five tests carried out for both observed levels of friction break-off. Both levels were significantly lower than 0.4 times the estimated maximum load, \(F_{\text{est}}\). As a consequence, no meaningful stiffness values could be derived, which usually is done in this load-level range according to [23]. All asymmetric tests series C20 showed a similar behavior. Adhesion friction break-off occurred shortly after 0.4 times \(F_{\text{est}}\) was reached. A nearly rigid bond can be assumed if there is adhesive friction between the timber and concrete. Therefore, no reasonable stiffness values, \(K_{\text{ser}}\), could be derived for test series C20 and C21. However, all tests within test series C20 and C21 resulted in a final failure mode as a ductile timber compression failure in front of the loaded edge of the notch. Table 6 shows the maximum load-carrying capacity, \(F_{\text{max}}\), per notch obtained in the tests of both series.