## 1 Introduction  
The durability of layered concrete elements, both newly executed elements and also surface corroded elements that were repaired by the application of repair concrete, is strongly dependent on an appropriate adhesion between layers [1; 2; 3; 4]. This appropriate adhesion is defined in standard as the desired minimum value of interlayer adhesion [5]. Figure 1 shows the semantic and etymological context of the examination of the interphase zone based on [6]. Figure 1: The semantic and etymological context of the evaluation of the bond between concrete layers (based on [6]). Therefore, this examination should be understood as an assessment of the phenomenon of the adhesion between two layers of a certain thickness made of a composite material that is composed of both aggregate and a liquid cement matrix that is hardening over time (Figure 1). Between the two materials, a thin layer or border in the form of an \"interface\" is created. However, according to [7], unlike the \"interface\" that is created after the application of the overlay onto an existing concrete substrate, it should be referred to as an \"interphase\" due to the maturation of overlay concrete. In construction practice, the evaluation of the interphase zone between the overlay and a substrate is conducted macroscopically and is based on the evaluation of the pull-off adhesion value (\(f_{\text{b}}\)) using the destructive pull-off method [5]. The higher the \(f_{\text{b}}\) value, the better the bond is. Based on these tests, the \"adhesion map\" of the assessed surface of an element can be obtained, which can be used e.g., for the rough location of defective regions. Based on macroscopic examinations, it is also possible to assess whether the delamination occurred at the contact between layers (adhesion damage), or in the material of the overlay or substrate (cohesive damage), however, this does not explain why it happened. Recently, more and more different types of actions are being undertaken in order to increase the adhesion between layers to reach a value above the desired minimum value defined in the standard. These actions may include the following:  
* appropriate treatment of the surface of the substrate, resulting in an increase in the effective relative surface area, orientation of the surface texture, removal of the cement laitance, surface exposure of the aggregate, etc.,
* increase the maximum aggregate grain size in concrete substrate,
* the applications of different adhesive substances,
* strengthening of the concrete substrate using impregnating and bonding agents,
* modification of the concrete composition, especially the overlay, with additives and nano-additives that \"seal\" its structure,
* modification of the concrete composition, especially the overlay, with admixtures that affect the viscosity of the concrete mix. The examination of the interphase zone between layers at lower levels of observation could be helpful in these actions. This would require the use of modern testing methods other than the pull-off method. By knowing the distribution and structure of air pores in concrete in the interphase zone, it is possible to deliberately select an additive that will reduce porosity. In turn, knowledge of the chemical composition of the concrete in the interphase zone can help in the selection of a suitable bonding agent that is appropriate for increasing the adhesion in the interphase zone. Nowadays, \"enhancement\" of the adhesion is mainly performed experimentally and it involves samples of different microstructures being subjected to tests until there is one that has the desired parameters. Taking the above into consideration, the purpose of this paper is to indicate the possibilities of using both available modern testing methods and descriptors that are assessed using these methods in order to evaluate the interphase zone between the overlay and concrete substrate at various levels of observation.