## 3 Reinforcement Corrosion Monitoring Techniques
### Potentiodynamic Method  
The application of a small amplitude potentiodynamic scan, is one of the preferred methods used to obtain a reliable measurement of \(R_{p}\) in laboratory studies when the scan rate is slow [180; 181]. Potentiostats with automatic compensation of ohmic drop are used. Amplitudes during measurement may range from 10 mV to 30 mV. Potentiostatic Method  
This method estimates \(R_{p}\) values from the application of small anodic potential pulses starting from the OCP, usually +10 mV. The value of the current that is generated is recorded for a certain time period. A schematic representation of the transient obtained in laboratory experiments after the application of a potential pulse of +10 mV is shown in Figure 2. \begin{table}
\begin{tabular}{c c} \hline \hline
**Construction Steel Conditions** & \(i_{corr}\)/\(\mu\)A cm\({}^{-2}\) \\ \hline Passive & \textless{}0.1 \\ Low to moderate corrosion & 0.1 to 0.5 \\ Moderate to high corrosion & 0.5 to 1.0 \\ Very high & \textgreater{}1.0 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Construction steel conditions as a function of the values of \(i_{corr}\) in RCS measured in laboratory and in situ [46; 136; 178; 179]. Figure 2: Schematic representation of the transient obtained after the application of a potentiostatic pulse of +10 mV [182]. \(R_{p}\) sensors have been applied to new and existing structures [80; 182]. In the case of existing structures, the sensors have to be fixed after concreting [80; 182]. The electrolytic contact between the original concrete and the sensors is much more difficult to establish in a non-intrusive way and a few systems have been proposed [35; 78; 80; 152; 183; 182]. Sensors are generally fixed inside holes. The electrolytic contact between the original concrete and the sensor is established by pressing the sensor against the concrete or by using sealing materials. [PERSON] et al. in 2008 [80; 182], proposed three different embedded sensors for corrosion rate measurement, which were designed to be applied to existing RCS. The sensors were used to assess the effect of two surface treatments and a corrosion inhibitor on the control of steel reinforcement corrosion. The measurements were performed using a three-electrode system (Figures 3 and 4). Stainless steel electrode of 12 mm diameter was used as counter electrode; a steel electrode of construction (8 mm in diameter) was used as working electrode, with an average length of 10 and 9 mm, respectively, spaced 8 mm apart. As reference electrode Ti/TiO\({}_{2}\) was used (Figure 3) [80; 82]. The determination of \(R_{p}\) using this method is obtained from the following expression:  
\[R_{P}=\Delta E\cdot A\cdot\left(\frac{1}{I_{3}-I_{1}}-\frac{1}{I_{2}-I_{1}} \right)R_{p}=\Delta E\cdot S\bigg{(}\frac{1}{I_{3}-I_{1}}-\frac{1}{I_{2}-I_{1} }\bigg{)} \tag{5}\]  
where: \(R_{p}\)--Polarization resistance (\(\Omega\) cm\({}^{2}\)); \(\Delta E\)--Potentiostatic pulse applied (V); \(S\)--area of the working electrode (cm\({}^{2}\)); \(I_{1}\)--Current intensity measured at time t, immediately before the application of the potential pulse (A); \(I_{2}\)--Current intensity value measured at time t at which the potential pulse (A) is applied; \(I_{3}\)--Current intensity measured at the baseline reached at the end of the experiment (A). Figure 5 shows the sensors attached in two different ways. In type M the sensors were covered with a porous mortar and in type T the sensors were covered with original concrete. Figure 4: (**a**) Fixing the \(R_{p}\) sensors Figure and (**b**) Filling with mortar [182]. Figure 3: Schematic representation of the \(R_{p}\) sensor (WE—working electrode, CE—counter electrode and REF—reference electrode). Reprinted with permission from _Sensors_ MDPI [82]. A third sensor using a non-intrusive method was proposed. In this method the working electrode is constituted by a section of rebar surrounded by the original concrete (Figures 6 and 7).