## 3 Reinforcement Corrosion Monitoring Techniques
### Potentiodynamic Method
As reference electrode Ti/TiO\({}_{2}\) sensor was used and as counter electrode (CE) a stainless steel rod was used (Figure 6). The authors concluded that the \(R_{p}\) embedded sensors are promising tools to assess the \(i_{corr}\) of steel in existing concrete contaminated with chlorides. The methodology based on the use of sections of rebar, surrounded by the original concrete, as working electrode, for \(i_{corr}\) measurements, should be preceded of meticulous assessment since, due to the heterogeneity of concrete, different corrosion states may be present [80; 182]. Figure 5: (**a**) Picture of the methodologies used to fix the sensors. (**b**) Schematic of the specimens instrumented with the sensors under study. The methodologies used to fix the sensors are presented as well as the reinforcement and the working electrode (WE) of the sensors [182]. Figure 6: Picture of two R\({}_{\text{P}}\) sensors using a non-intrusive method [182]. The galvanostatic method is the application of a current pulse, from 10 \(\mu\)A to 20 \(\mu\)A, with duration between 8-10 s. It is a fast and non-destructive technique that provides information about the steel condition as well as information about concrete resistivity. When a pre-selected constant current (\(I_{app}\)) is applied to the system, the potential increases and the polarization of the steel occurs. Assuming that the system is described by a _Randles_ circuit, the potential of the rebar \(V(t)\) at a given time (\(t\)) can be expressed by:  
\[V_{t}(t)=I_{app}\bigg{[}R_{p}\bigg{(}1-\ell^{\frac{-t}{R_{p}+dt}}\bigg{)}+R_{c} \bigg{]} \tag{6}\]  
where: \(R_{p}\) is the charge transfer resistance at the surface of the steel rebar which is equivalent to the polarization resistance [183], \(C_{dl}\) is the double layer capacitance at the surface of the steel rebar and \(R_{c}\) is the ohmic resistance of the concrete between the surface electrode and the steel rebar. By adjusting the function to the experimental results, following the procedure described by [PERSON] et al. [184] it is possible to obtain the values of \(R_{p}\) and \(R_{c}\). After an equilibration time the transient response reaches a steady-state potential (\(V_{max}\)), where:  
\[\log_{e}\left(V_{max}-V_{t}\right)=\log_{e}\left(I\,R_{p}\right)-\left(t/(R_{p }C_{dl})\right) \tag{7}\]  
Plotting \(\log_{e}\left(V_{max}-V_{t}\right)\) vs. \(t\) gives a linear graph with a slope equal to \(1/R_{p}C_{dl}\) and intercepts at \(\log_{e}(IR_{p})\). The \(i_{corr}\) can be obtained from Stern-Geary equation (Equation (4)). In the last two decades this method has been widely studied [146; 172; 185; 186; 187; 188; 189] and is the basis of the development of commercial equipment with external probes for corrosion rate measurement [190; 191]. Electrochemical Impedance Spectroscopy (EIS)  
EIS measurements have been used as a non-destructive technique since the 80's in the study of steel corrosion in concrete [81; 192; 193]. This technique provides information about several parameters, such as bulk concrete properties, the presence of films on the surface and interfacial corrosion and mass-transfer phenomena. The most used frequencies are between 100 kHz and 1 MHz, which obtain information on the resistivity of the concrete [194], transfer of charge, and diffusion phenomena [81; 160; 161; 162; 193; 195; 196; 197; 198; 199; 200]. Frequencies between 20 kHz to about 100 MHz have been used in the study of the dielectric behavior of concrete [197; 201]. EIS is a powerful technique for the characterization of a diversity of electrochemical systems and for determination of the contribution of electrode or electrolytic processes in these systems [202; 203; 204; 205]. It can be used to study an electrochemical system assuming that an electrical equivalent circuit (EEC) can represent the behavior of the steel embedded within the concrete [161; 164; 166; 195; 197; 198; 206; 207; 208; 209; 210]. However, due to the heterogeneity of concrete, the analysis of the EIS results is challenging.