## 3 Method Proposed for Corrosion Evaluation  
As already mentioned, the method proposed herein allows for the detection of both pH and chloride ions concentration by measuring the potential difference between a 1-mm-diameter electrode used as sensor and embedded in the concrete, and an electrode used as a reference. The sensors can be inserted into the concrete, i.e., from 1 to 3 cm in depth, during the casting phase (pre-casting), or they can be placed directly into the concrete on existing structures. Regarding the measurements of the chlorides, the pertinent electrode is composed of a silver wire covered with an AgCl film. Furthermore, as for the pH measurements, an  
Figure 10: Auto PSD related to the identification of the bridge frequency from the vehicle.  
iridium wire coated with an oxide layer of iridium, obtained through a high-temperature oxidation process, is used as an electrode.  
Potential data are translated into concentration data by performing a calibration, particularized for each sensor, resulting from laboratory tests with standards as a reference. The results, in terms of correlations of the sensor calibration, are fully consistent with the [PERSON] equation. It should be noted that the TEM detection and monitoring system, as well as all non-destructive diagnostic methods (CnD), should be considered additional and complementary to the analytical laboratory procedures, in compliance with a holistic methodological approach.  
The physical principle of the sensors previously introduced is based on [PERSON]'s equation [39]:  
\[E=E^{0}+\frac{RT}{zF}ln\frac{\prod a_{i,ox}^{v_{i}}}{\prod a_{i,red}^{v_{i}}} \tag{1}\]  
where \(E\) is the electrode potential, \(E^{0}\) is the standard potential, \(R\) is the gas constant (J/mol*K), \(T\) is the absolute temperature, \(z\) is the number of exchanged electrons, \(F\) is the Faraday constant, \(a_{rad}\) is the activity of reduced species, \(a_{ox}\) is the activity of oxidized species and \(v_{i}\) is the stoichiometric coefficient. Using the Cu/CuSO4 electrode, potential values lower than 10 mV are expected if the environment of the concrete is kept alkaline. In the presence of carbonation phenomena, the potential fall could be even higher than 30 mV. During the installation and maintenance phase of the stations, tests needed to be carried out by analyzing the powders, in order to better determine the measurement uncertainty and verify the data's reliability [40].  
A peculiar aspect of detection by sensors is that the measurement is carried out on the free ions present in the capillary pores of the concrete, while the analytical ones of the laboratory are conducted on the powders extracted from the concrete in place. The latter are affected by an intrinsic variability due to the different contributions of the individual components (aggregates and binder) [41].  
Furthermore, in order to assess the degree of corrosion and to estimate the damage of the structures (estimated service life), the gradient and the speed of variation of the measured parameters could be more useful than their analytical precision. Thus, continuous monitoring is necessary. The monitoring procedure involves the use of at least three sensors for detecting the chloride concentration, and as many sensors to measure the pH. All these sensors are placed inside the concrete cover at different depths (e.g., 1 cm, 2 cm, 3 cm), in order to estimate, in addition to the local chemical parameters, the speed of penetration of both chlorides and CO2.  
Moreover, with the aim of evaluating concrete degradation by means of the sulfates, a new sensor was developed to monitor the concentration of sulfate ions in the concrete.