## 3 Results and Discussion  
### Push-out Tests and Failure Modes  
Figures 6 and 7 illustrate all push-out specimens after failure. The specimens were divided into two sets according to observed failure modes. Mode I (specimens #1-#3) was an adhesive-type failure occurring at the interface between the steel plate and the adhesive layer. No damage was observed in the concrete cube. The entire adhesive layer remained on the concrete. Mode II (specimens #4-#10) included a cohesive-type failure or mixed cohesive-adhesive failure. In mode II, three sub-groups were distinguished: mode IIa (specimens #4-#6), mode IIb (specimen #7) and mode IIc (specimens #9-#10). The damage in modes IIa and IIb included two phases. In the first phase, the damage was mainly cohesive with a failure in the bulk of the concrete cube. The second phase was a separation of the steel plate from the concrete. In mode IIa, a single diagonal crack originated from the interface between steel and concrete, and then the cohesive (specimen #6) or mixed cohesive-adhesive failure (specimens #4, #5) occurred in the bulk of the adhesive layer. In mode IIb, a crack appeared in the middle of the concrete cube and then the adhesive failure occurred. The failure in mode IIc was cohesive (specimen #8) and mixed adhesive-cohesive (specimen #9) with a separation in the bulk of the adhesive layer. Figure 8 shows the force-displacement curves for all tested specimens. In mode I, the shape of curves for each specimen is similar. The sudden drop of the force at the moment of the adhesive failure can be observed. The maximum force was 47.4 kN (specimen #1), 49.2 kN (specimen #2) and 47.6 kN (specimen #3). The curves for the specimen failures categorized as modes IIa and IIb exhibited two drops of the force. The first drop is associated with the appearance of the crack in the concrete,  
Figure 6: Failure modes of tested steel-concrete specimens (last stage in the testing machine): mode I including specimens #1 to #3—adhesive failure and mode II including specimens #4 to #10—cohesive-type failure or mixed cohesive-adhesive failure. Figure 7: Failure modes of tested steel-concrete specimens (photographs of the damaged steel-concrete interface after removing from the testing machine): mode I including specimens #1 to #3—adhesive failure and mode II including specimens #4 to #10—cohesive-type failure or mixed cohesive-adhesive failure. while the second is associated with the separation of the steel plate. In the case of specimens with diagonal cracks, the maximum force was 54.6 kN (specimen #4), 59.2 kN (specimen #5), 58.5 kN (specimen #6). For the specimen with the crack in the middle of the concrete cube (specimen #7), the maximum force was 72.3 kN. The last two curves corresponding to mode IIc are characterized by the sudden drop of the force at the moment of the cohesive or mixed adhesive-cohesive failure. The maximum force was 82.5 kN (specimen #8) and 86.1 kN (specimen #9). Results of the interface slip between the steel and concrete in both vertical and horizontal directions are presented in Figure 9. For each specimen, the slip values are given for this interface at which the failure occurred (i.e., the right interface for specimens #1, #2, #6 and the left interface for specimens #3, #4, #5, #7, #8, #9). The moment of initiation of the crack in the concrete cube was indicated by the dashed lines (specimens #4-#7). In the case of the adhesive or mixed adhesive-cohesive failure, the measured slip varied around zero almost to the end of the degradation process. For specimens #2, #3 and #9, the monotonic increase of the slip was observed at the end of the test. The maximum values of the slip for the specimens representing mode I and mode IIc was about 0.05 mm. In the case of specimens with the cohesive failure in the bulk of the concrete cube, a sudden increase of the slip was observed in specimens #4, #5 and #7. A different behavior can be seen for specimen #6 for which the monotonic increase occurred after the cracking of concrete cube.