## 3 Reinforcement Corrosion Monitoring Techniques
### Corrosion Assessment with Electrochemical Sensors
#### 3.1.3 Polarization Resistance (\(R_{p}\))
[171] reported that the \(R_{p}\) measurements should be performed under steady state conditions, otherwise large disparities can be found between the \(R_{p}\) values determined from DC and AC techniques. [PERSON] et al. stated that to obtain identical accuracy in \(R_{p}\) measurements, longer times are required in the frequency domain rather than in the time domain. Later in 2000, [PERSON] and [PERSON] [172] developed an algorithm (based on Fourier-transform) to measure the data of galvanostatic pulse method from the time domain into the frequency domain. The \(R_{p}\) and ohmic resistance were determined in the time domain and the impedance spectrum and disperse parameter were obtained in the frequency domain to characterize the corrosion status of reinforcement in concrete. Three years later [PERSON] et al. [173] published an approach for the analysis of galvanostatically induced transients. The proposed approach allowed the determination of values for the parameters related to corrosion of steel reinforcement such as the concrete resistance, \(R_{p}\), interfacial capacitance among others. In 2012 [PERSON] et al. [174] studied a simplified method for the application of transient analysis to in situ RCS. The authors concluded at low \(i_{corr}\) values the transient response analysis seemed to provide more accurate data than the information obtained from polarization resistance analysis possible due to the use of smaller perturbation and the removal of the concrete resistance. One of the major limitations to the use of this type of measure is that local penetration velocities, in the case of chloride-induced corrosion (pitting), are underestimated. The studies conducted for pitting corrosion, have shown that the \(R_{p}\) measurements lead to corrosion rate values 5 to 10 times lower than those obtained from direct measurements of rebar cross section [149; 175]. Some constraints are present when measuring \(R_{p}\) on real structures. For instance, the area of the working electrode (rebar network), on a real structure, is much higher than that of the counter electrode on the concrete surface. Therefore, the electrical signal is lost when the distance from the counter electrode increases [2; 74; 84; 149]. In these conditions the \(R_{p}\) values are linked to rebar surface area, which is unknown. Therefore, the measured values cannot be converted easily into the corrosion rate through the Stern-Geary equation. To overcome these limitations, [PERSON] et al. [176] have proposed a galvanostatic method for on-site \(R_{p}\) measurements. The authors introduced the concept of an external counter electrode (guard ring) for signal confinement. Another alternative is the use of electrochemical sensors in critical positions in the structure. However, in this case the working electrode area embedded in the concrete must be known. The use of this type of sensors will lead to crucial and detailed information. Furthermore, the information extracted allows modeling the service life of RCS and the acquisition of information from areas in structures with difficult access. In 1997, [PERSON] and [PERSON] [177] compared several methods to assess the corrosion of RCS. The methods studied included manual potential mapping, automatic measurements of corrosion potential with embedded reference electrodes, linear polarization resistance measurements with cut reinforcement bars, analysis of the shape of galvanostatic charging curves, and linear polarization resistance measurements of the reinforcement with a commercial instrument containing a guard ring. Commercial reference electrodes were embedded in the concrete and a stainless steel rebar was used as a counter electrode. The results showed that the \(i_{corr}\) determined with the guard ring did not agree with the results based on potential mapping or visual inspection. Later in 1999, [PERSON] [178] carried out galvanostatic measurements in bridges using sections of steel construction as reference and working electrodes.