## 2 Materials and Methods
### Inspection Procedure  
Visual inspection was carried out in accordance with [27]. Overall, several types of damage were found, including longitudinal cracks in the center of the pavement slabs, transverse cracks, limited corner breakouts, and sealing damage at joints and cracks. At one of the toll plazas, the phenomenon of map cracking (or \"crazy\" cracking), similar to the pattern of alligator cracking in asphalt pavements, was observed (e.g., Figure 3a), followed by site-specific potholes (e.g., Figure 3b) of considerable size. In addition, the longitudinal and transverse cracks appeared to be connected in most cases and ran along adjacent slabs (e.g., Figure 3c). Finally, the presence of faulting was observed in some fully damaged slabs (e.g., Figure 3d), with the individual parts of a cracked slab behaving completely independently of each other under load. Figure 3: Examples of distresses at the concrete pavements: (**a**) map cracking, (**b**) potholes, (**c**) interconnected cracks, and (**d**) faulting at damaged slabs. Figure 2: Pavement cross-section. Based on the visual inspection findings, the assessment with the FWD could be a further aid in differentiating the condition of the examined pavements. It is worth mentioning that the FWD is a testing system that uses a load pulse applied stationary to the pavement surface to simulate a moving truck by dropping a known mass from a given height onto a steel plate embedded in the pavement surface [15]. The result of this process is a typical deflection profile, the so-called deflection basin, which is shown in Figure 4. The deflections of the pavement are usually recorded with a series of sensors/geophones at predefined distances from the center of the load plate. During the test, random samples of slabs were loaded in the direction of travel at each toll station (Figure 5a) so that the entire test area was covered. The focus was naturally set on the damaged slabs, but intact slabs were also tested for comparison purposes. Based on the deflection basin recorded at each location, two deflectometric indicators were evaluated, namely:  
* \(\mathrm{D}_{0}\) (\(\upmu\)m): the maximum central deflection, recorded in the loading plate directly beneath the loading point. This indicator reflects the overall pavement condition. Figure 4: Typical FWD-based deflection basin. Figure 5: (**a**) FWD system, (**b**) test setup at an edge, (**c**) test setup at a corner, and (**d**) test setup at a crack. * D\({}_{1800}\) (\(\upmu\)m): the deflection recorded from the outer sensor/geophone located at a distance of 1800 mm from the loading center. This indicator is known to reflect the substructure condition. It should be noted that there are no guidelines or rules for the magnitude of the FWD load for concrete pavements, but the general trend is to use higher loads than for asphalt pavements to activate the proper pavement responses and provide meaningful data [16]. In this study, a load of 95 kN was applied and distributed on a 150 mm-diameter loading plate, in accordance with the authors' previous relevant experience [15]. The FWD load was then applied near the slab edges (Figure 5b) and corners (Figure 5c) to assess the deflection differences within a single slab and evaluate the durability of the pavement. The slabs subjected to durability testing were again randomly selected to cover a large portion of the study area. It is noted that durability is likely to be more critical for airfield pavements, as the movement of aircraft does not always coincide with the slab's axis; therefore, the behavior of the slab at the edges and corners is equally important.