## 1 Introduction  
Interfaces between concretes cast at different times are common in various repair and/or strengthening techniques, where a new, usually reinforced, concrete layer, is cast in contact with the existing structure member (e.g., the addition of a layer in the tensioned zone of a beam for flexural strengthening, the construction of an RC jacket to a column, etc.) or a new reinforced concrete element (e.g., an RC wall within an existing RC frame, etc.). The connection of the existing structure with the added elements, i.e., the behavior and the design of interfaces, is crucial for the effectiveness of the intervention technique and to reach the target of monolithic behavior of the resulting composite element. The interface overall behavior and resistance depend on the reinforcement ratio and type (reinforcing bars or industrial anchors), on the roughness of the surface of the old element (as-cast or intentionally roughened to various degrees), on the use of chemical products for the enhancement of the cohesion between the two concretes, and on the restrain of the interface, due to external compression. The design of the interfaces is decisive for the global behavior of the structures when they are subjected to cyclic actions, resulting in the degradation of the interface resistance. One of the specific characteristics of interfaces between concretes cast at different times is the frequently insufficient embedment length of the interface reinforcement due to geometrical constraints of the connected elements. This may also be the case in cracks within monolithic RC elements (natural or load-induced cracks), especially in sub-standard structures or in cracks occurring close to supports. In recognition of the significance of interfaces, numerous researchers have undertaken relevant experimental campaigns. The behavior of various types of interfaces, namely monolithic elements cracked before being subjected to shear, cold joints formed during construction, and connections between old and new concrete or between precast elements, has been experimentally studied. Available experimental results published in fifty-eight (58) papers were collected and compiled in a comprehensive database. A review of this database is presented elsewhere [1]. However, with the emphasis of this work being on the influence of external compressive stress on interfaces between concretes cast at different times and subjected to cyclic shear slip, a brief overview of the published experimental results investigating this parameter is initially provided herein. The literature is rich in tests that experimentally investigate the behavior of unreinforced interfaces between concretes cast at different times, where an external compressive stress of varying amplitude is applied [2; 3; 4; 5; 6; 7; 8; 9]. In that case, once a crack is formed along the interface, only the friction mechanism (due to the external compression) is activated. The overall behavior of the unreinforced interfaces differs significantly from that of reinforced interfaces. Indeed, in the latter, friction is due to the gradual mobilization of the interface reinforcement, while dowel action also contributes to the interface resistance. Thus, publications referring to the shear behavior of unreinforced interfaces are not reviewed herein. External compressive stress may be due to the self-weight of the structure (in horizontal interfaces), the confinement provided, e.g., by the hoops of an RC jacket to a column (along the vertical interfaces between the original column and the jacket), etc. The effect of normal compressive stress on the behavior of reinforced interfaces between concretes cast at different times was investigated in several experimental campaigns; basic information on the published tests is summarized in Table 1.  
\begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Paper**} & \multirow{2}{*}{**Number of Tests**} & \multirow{2}{*}{**Interface (11)**} & \multirow{2}{*}{**Dimension.