## 1 Introduction
It is reminded that the shear slip imposed on the interface after the occurrence of a crack along the interface leads to the opening of this crack. Thus, the interface reinforcement develops tensile stresses. The tensile force of the reinforcement is equilibrated by clamping forces, resulting in compressive stresses across the interface. The conclusions of previous experimental campaigns are depicted in the provisions of currently enforced normative regulations (e.g., [18; 19]), which, however, do not address the case of interfaces subjected to cyclic loading. In the research of [PERSON] et al. [12; 13], the specimens were subjected to either monotonic or cyclic shear, with an external compressive stress applied on the interface in all specimens, equal to 2.0 MPa, throughout the test. Thus, the effect of varying this parameter on the interface behavior cannot be assessed. [PERSON] et al. [4] tested interfaces under cyclic loading under external compressive stress of relatively high magnitudes, namely 29% or 57% of the concrete compressive strength. Thus, the reinforcement crossing the interface--also of relatively high percentage yet of limited embedment length (eight times the bar diameter)--did not contribute to the interface resistance. As a result, the failure mode changed from the pullout of the short embeds under zero external stress to an extensive interface failure under high external compression, described by the authors as \"aggregate interlock failure\". The investigation of different types of interfaces, namely smooth, rough, or shear-keyed interfaces [14; 16] between concretes, or steel to concrete interfaces [15], shows an increase in the interface resistance in the presence of external compressive stress. Nonetheless, in the respective publications, no further comments were offered on the failure mode or on the overall cyclic behavior of the tested interfaces. Finally, in the tests performed by [PERSON] [17], due to the test setup, the stress perpendicular to the interface increased simultaneously with the stress parallel to the interface, and the effect of the external stress was not commented upon. In conclusion, even though the effect of external compressive stresses on interfaces was investigated by several researchers, crucial information is still missing on the effect of normal stress on the failure mode, the shear slip at which the interface resistance is reached, and the resistance degradation due to cycling. To further understand interface responses, a systematic investigation of RC interfaces subjected to cyclically imposed shear slips was undertaken at the Laboratory of RC, National technical university of Athens (NTUA), active since the early 1980s [9; 20]. The series of research programs that have been carried out comprise several test series, aiming to investigate the effect of the principal governing parameters, namely concrete strength, the percentage of reinforcement crossing the interface, the anchorage length of the reinforcing bars on both sides of the interface, the magnitude of the imposed cyclic slip, the level of normal load on the interface, etc. In the present paper, the results of part of these tests are presented and commented upon. In this portion of the test series, the effect of several parameters on the cyclic behavior of the interfaces is investigated, with the main focus being on the effect of external compressive stress acting normal to the interface. Furthermore, the model developed at NTUA [21] to calculate the interface resistance is also applied to the experimental results in order to establish its suitability in this case. In this model, both mechanisms are activated at the interface--that is, the friction and the dowel action are taken into account--while the main parameters affecting the development of the two mechanisms are considered, namely the interface roughness, the properties of concrete and steel (and the percentage of the latter), the embedment length of the reinforcement, the value of the externally applied stress, if applicable, etc.