## 3 Reinforcement Corrosion Monitoring Techniques
### Corrosion Assessment with Electrochemical Sensors
#### 3.1.1 Open circuit Potential (OCP)  
In RCS the concrete acts as an electrolyte and the steel develops a potential dependent on the characteristics of the surrounding concrete. Open circuit potential (OCP) is basically the measurement of the potential (in V or mV) in relation to a reference electrode [114; 115; 116; 113]. A schematic representation for OCP measurements is shown in Figure 1. Examples of reference electrodes are saturated calomel (SCE), copper/copper sulfate (CSE), silver/silver chloride or even activated titanium [80]. This method is often controversial in terms of interpretation since the OCP measurements depend on the conditions of the RCS (e.g., moisture levels and Cl\({}^{-}\) amount) that may affect the readings leading to inaccurate results [61; 116]. The information extracted from OCP indicates either a low or high probability of corrosion occurrence but it does not provide any information on the corrosion rate [84].  
According to ASTM C876-15 [117], the probability of steel corrosion, considering the OCP values, is shown in Table 2[50]. If the steel potential in the concrete is lower than \(-276\) mV vs. SCE there is a 90% chance of corrosion. The construction of corrosion potential maps according to ASTM C876-15 is the most applied electrochemical technique in the corrosion risk assessment of RCS. However, it is generally accepted that OCP should always be complemented with other measurements.  
Several embeddable solid reference sensors in concrete for OCP monitoring have been developed [118; 119; 120; 71; 121; 122; 72; 123; 73; 74; 75; 76; 77; 78; 79; 80; 81; 82; 83; 84; 85; 86; 87; 88; 89; 90; 91; 92; 93; 94; 95; 96; 97; 98; 99; 100; 101; 102; 103; 104; 105; 106; 107; 108; 109; 110; 111; 112; 113; 114; 115; 116; 117; 118; 119; 120; 121; 122; 123; 124]. Saturated calomel electrodes (SCE) and saturated copper sulfate electrodes, due to their compatibility and ease of use, are used for laboratory studies in alkaline environments--simulated concrete pore solutions (SCPS). Liquid based SCE are not suitable for embeddable sensors and their use is therefore restricted to laboratory environments [79]. Ag/AgCl reference electrodes are generally used in RCS to measure the rebar corrosion potential [79] or to determine the chloride content in SCPS [73; 120; 121; 122; 123; 124; 125; 126]. Some reported that Ag/AgCl reference electrodes embedded in mortar specimens are stable only for short periods [127] while others found good long-term stability [128]. Solid type manganese dioxide (Mn/MnO\({}_{2}\)) reference electrodes appear to be more stable and reliable in RCS. [PERSON] et al. [79] showed that the half-cell potential of embedded Mn/MnO\({}_{2}\) sensor in concrete was consistent with a good stability for an exposure period of 24 months.  
OCP measurement is a very simple and useful technique and the construction of potential mapping, according to ASTM C876-15, allows identification of anodic and cathodic sites in RCS. Nevertheless, it should be kept in mind that OCP values provide only probabilistic information on the occurrence of corrosion.