## 5 Summary  
The actual problem of the examination of the interphase zone between the overlay and concrete substrate at different levels of observation has been presented in the article. Due to the high heterogeneity of joining materials, the multi-scale approach has been discussed. Four levels of examination (observation) of the interphase zone between the overlay and concrete substrate were distinguished based on the literature survey and the author's own experience:  
* 1 st level (macro)--at this level the interphase zone between layers is assessed primarily on the basis of destructive tests,
* 2 nd level (meso)--at this level the interphase zone is evaluated on the basis of the physical and mechanical properties of the concrete of the substrate layer and the overlay, as well as on the basis of the surface morphology of the substrate,
* 3 rd level (micro)--at this level the density, air pore structure and hardness of the concrete within the interphase zone are evaluated, as well as the changes of these properties over time,
* 4 th level (nano)--at this level the interphase zone is evaluated by investigating the effects of valence intermolecular forces occurring between the contacting surfaces, and also the migration of elements from layer to layer at the atom, intermolecular level and molecular levels. The possibility of using modern research methods and the basic descriptors evaluated using these methods were presented. Moreover, the suitability of these descriptors at a specific level of observation was indicated. Finally, the author's methodology of investigating the interphase zone between the overlay and concrete substrate with regard to the level of observation was also provided. Today, the condition of the interphase zone between the overlay and concrete substrate can be increasingly and more precisely diagnosed. This is mainly due to the wide and continually extended range of methods useful for this purpose. In recent years, the research was primarily based on macroscopic laboratory testing methods. At the meso level, testing involves the use of the NDT acoustic and thermographic methods. In turn, micro-level studies were performed using a scanning electron microscopy and mercury intrusion porosimetry. The anticipated development trends will likely be related to the applications of X-ray micro-CT and nanoindentation at nano level of observation. Especially with the proper combination of these methods, researchers will probably be able to observe the structure of pores and the density of the material at nano scale. Future work should also be related to the multi-scale optimization of the interphase zone between the overlay and concrete substrate. This can be done based on appropriate dosing of ingredients and proper method selection. The functional performance concrete concept (pointed out in [84]) can play a dominant role in the manufacturing of high quality multi-layer concrete elements. Until now, a large development in the knowledge in the field of the interphase zone between the overlay and concrete substrate has been achieved. To continue building on this knowledge, further steps need to be taken. The evaluation of the interphase zone between the overlay and concrete substrate is an ongoing problem. It is a multi-phase combination of the physicochemical, mechanical and rheological properties of joined materials at different levels of observation. This is probably one of the most challenging aspects at the moment. Thus, the specific multi-scale processing algorithms and numerical methods need to be developed for this purpose. This work was supported by the National Centre of Science, Poland under Grant number 2014/15/D/ST8/00550 [\"Evaluation of the interlayer bond of variably thick concrete layers based on nondestructive tests using artificial intelligence\"]. The author declares no conflict of interest. The founding sponsors had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, and in the decision to publish the results.