The article presents the results of multi-sensor evaluation of the concrete within the interphase between overlay and existing substrate with regard to pull-off adhesion. It has been shown that both the effective surface area of the existing concrete substrate and the contribution of the exposed aggregate on this substrate, as a result of concrete substrate surface treatment, have a significant impact on pull-off adhesion. The highest adhesion was obtained when the surface of the existing concrete substrate was shot-blasted. This method of surface treatment provides both a high coarseness of the surface of the existing concrete substrate and considerable exposure of the aggregate on this surface. In order to clarify why this method of surface treatment of existing concrete substrate is advantageous with regard to the possibility of obtaining high adhesion, scanning electron microscopy (SEM) was used. SEM microstructural analysis was performed on concrete cubic specimens taken from the interphase zone between the overlay and existing concrete substrate. The results of these studies, including the contact type between the overlay made of cement mortar and the existing concrete substrate, are presented in the study.