# Cyclic Behavior of Long Concrete Interfaces Crossed by Steel Screws
## 6 Comparison between Long and Short Interfaces
### Failure Mode and Overall Behavior  
The data of Table 3 show that the cracking and the damage of long interfaces was similar to their short counterparts. In short rough interfaces, the typical failure mode was that of the formation of a crack in the old part, parallel to the interface, close to the end of the embedment length of the reinforcement, denoting the creation of a concrete cone. In long rough interfaces, the failure was due to the crack parallel to the interface (concrete cone) and, in one case, also to the formation of concrete struts in the old part. These failures in the long rough interfaces were recorded for an applied shear slip significantly larger than the one corresponding to the attainment of the maximum interface resistance (\(\mathrm{s_{u}}\)). Regarding the short and long smooth interfaces, a difference was observed in their failure mode: the first failed due to a diagonal crack developed in the new part, whereas the latter failed due to the crack along the interface, which caused the fracture of the dowels, for a rather large value of the applied shear slip. In general, in both short and long interfaces reinforced with concrete screws, the falling branch of the shear force vs. shear slip curves was not steep and, thus, the behavior of the interfaces was not characterized by pronounced brittleness. Figure 8 shows the degradation of the maximum resistance due to cycling. Although the long and short interface specimens were not directly comparable, as they differed in the roughness of the interface (Table 2), it seems that, as a rule, the short interfaces exhibited more significant degradation due to cycling than the long ones (comparing, for example, specimens R-24/G/65/0.1 and L-R-23/G/65/0.1). The same observation was valid for smooth interfaces, as the comparison between specimens S-25/G/65/0.1, S-32/G/65/0.1, and L-S-27/G/65/0.1 showed, even though the difference was not so pronounced. In Figure 13, the normalized envelopes are plotted for the first cycle of short and long interfaces to better investigate the asymmetry of behavior in the two loading directions. The data plotted in Figure 13 reveal that the envelopes for long interfaces were quite similar in the two loading directions. This characteristic was not observed for the short interfaces, where the asymmetry of resistance between the two loading directions was rather significant. Figure 13: Hysteresis loop envelopes for all (long and short) interfaces: (**a**) rough and (**b**) smooth. On the other hand, the data presented in Table 3 show that, although the maximum resistance, \(\tau_{\text{u}}\), of long interfaces did not differ significantly from that of the short ones, it was mobilized at smaller shear slip values for the rough interfaces, whereas the long interfaces seemed to exhibit longer falling branches (Figure 14) after the attainment of the maximum resistance until failure occurred and the test was terminated. The difference was much more pronounced in the case of smooth interfaces. This characteristic, paired with the symmetry of hysteresis loops in the two loading directions, showed that the behavior of long interfaces presented more favorable properties than their short counterparts. Furthermore, the model developed by the authors for the calculation of the maximum resistance of interfaces [2], although based on experimental data obtained basically through short interfaces, could quite accurately predict the maximum resistance of long interfaces as well (Table 2). It should be noted that, when calculating the maximum shear resistance of an interface, the relevant equation for concrete breakout [22] was also accounted for. The tensile stress developed in the bars was taken as the smaller of two values, namely, the tensile stress due to pullout, provided by the manufacturer of the screws, and that corresponding to concrete cone failure, according to [22].