## 3 Reinforcement Corrosion Monitoring Techniques
### Corrosion Assessment with Electrochemical Sensors  
Several publications using electrochemical embedded sensors to monitor corrosion of RCS [73; 78; 79; 80; 46] including sensors with wireless communication of data [91; 92; 93; 94; 95] have been found. In 2009, [PERSON] and [PERSON] [88], published a paper where several examples of recorded data for corrosion potential, electrical resistance and corrosion rates were presented. The three case studies discussed showed that corrosion monitoring is achievable using different sensors and methods. The authors concluded that due to the variation of \(i_{corr}\) found in RCS in natural exposure, a methodology to determine representative values of the \(i_{corr}\) in order to assess with accuracy the service life of a RCS must be established [88]. Later, in 2013, [PERSON] et al. published a paper entitled \"Condition survey with embedded sensors regarding reinforcement corrosion\" [96]. The authors discussed the different types of sensor systems available based on different methods, including electrochemical. The main conclusion from this study was that the experience with monitoring systems has increasedsignificantly [96]. In the same year [PERSON] et al. [97] published a manuscript proposing a fast weak polarization method to test the corrosion state of reinforcements based on electrochemical polarization dynamics. Additionally, the authors developed a new corrosion sensor to monitor the corrosion state of concrete cover based on the proposed method. The authors concluded that the corrosion sensor effectively monitored the chloride penetration into concrete with insignificant influence of the relative humidity in the concrete [97]. Corrosion of new and existing RCS can be evaluated by different methods. The most important are listed below:  
* Open circuit potential (OCP);
* Surface potential;
* Concrete resistivity (\(\rho\));
* Polarization resistance (\(R_{p}\));
* Noise analysis;
* Galvanic current (\(i_{gal}\)). Most of the sensors found in the literature were developed to be installed in structures during the construction phase. The installation of a corrosion monitoring system using embedded sensors should not be seen as a generic procedure, to be applied to all structures. Instead, it is of particular interest to install these systems in structures located in particularly aggressive environments; in structural elements that are difficult to access; in structures where damage has already been detected; or structures that have been repaired and is of interest to follow the performance of the repair [3]. The selection of parameters to be assessed (e.g., temperature, \(i_{corr}\), pH, \(\rho\), Cl\({}^{-}\)) as well as the type of instrumentation to be used may be different depending on whether the structure is under construction, already built, or in the process of being repaired. In a corrosion monitoring system, the parameters associated with the electrochemical behavior of steel or the evolution of corrosive conditions in the concrete should be followed. Temperature is an important parameter since all the chemical and electrochemical processes are thermally activated. Therefore, the corrosion of the reinforcement also depends on the temperature [84; 98; 99; 100; 101; 102; 103; 104]. The knowledge gained in the last few decades has led to the development of sensors that are able to monitor different parameters simultaneously. For example, [PERSON] and [PERSON] developed a sensor that monitors the concrete \(\rho\), pH, chloride content, and microcell corrosion current [105]. The authors showed that the sensor, when in contact with paste, exhibited good sensitivity and reliability over the year of monitoring. Ideally, embedded sensors must be stable in an alkaline environment, insensitive to chemical and thermal variations of concrete, weather tolerant and have the ability to allow the passage of small currents with a minimum of polarization and hysteresis effects.