## 3 Reinforcement Corrosion Monitoring Techniques  
There are several methods for corrosion monitoring of RCS [1; 45; 46; 47; 48; 49; 50; 51; 52; 53; 54], which, generally, are categorized as non-destructive methods (NDM) or destructive methods (DM). These methods allow the implementation of different methodologies with different degrees of complexity and cost, from which information on the progression and kinetics of corrosion phenomena can be obtained [55; 56; 57; 58; 59; 60]. These methods can be used in new, existing or repaired structures. The situation becomes more complex in repaired structures since this type of structures involves the use of different repair products and interfaces. In the last few decades, the development of models to predict the service life of RCS has led to the need to quantify the \(i_{corr}\) under different conditions of natural exposure. The measurement can be carried out in a destructive way, by means of the direct measurement of the reduction of the section of the rebar, or non-destructively through the use of instrumental methods. The destructive methods give us indication of the average speed of corrosion, the NDM allow to quantify the amount of metal that is corroded at a certain instant (e.g., instantaneous corrosion rate). The recognition of the degradation of a structure is generally initiated by visual observation together with the collection of elements related to its history (type of structure, elements described in the project, year of execution, materials used, previous interventions, etc.). Visual inspection allows the assessment of corrosion damage on the surface of RCS. The appearance of the corroded area may provide valuable insight about the cause and extension of corrosion. Nevertheless, this method is extremely dependent on the inspector's experience. Its effectiveness for detecting surface discontinuities due to steel corrosion and hidden corrosion is limited [57; 61]. On the basis of this preliminary information, a sampling plan must be drawn up at representative sites of the structure such as characterizing sites of high risk of contamination or structural elements of major relevance. Concrete characterization tests are carried out for the collected samples. These results are compared with the project requirements and the adequacy of the type of concrete used, considering the environment that the RCS is exposed to. The carbonation depth of the RCS is determined with pH indicators (phenolphthalein) at different depths [5; 8; 62; 63; 64]. Considering the results obtained, in some areas, the concrete can be removed and direct observation of the reinforcement is performed. However, the DM are limited to sampling so they may not give an idea of the whole structure, which is extremely important in a RCS with large heterogeneities, both in terms of material and of exposure environment. Furthermore, in cases where it is necessary to assess the evolution of corrosion over time, such as following a repair intervention, it may not even be advisable to use DM. Within this context, NDM have been developed, in order to provide quick information about the entire structure. Ideally,these methods should be able to detect the corrosion state of the steel inside the concrete, the main causes of corrosion and the evolution of the corrosion phenomena over time. In the 1970s, [PERSON] and [PERSON] started the development of non-destructive electrochemical methods, for the assessment of corrosion in RCS, with the measurement of corrosion potential [65; 66]. According to [PERSON] et al. [5] the measurement of galvanic current [67], corrosion rate, resistivity, reduction limit current of oxygen and chloride content was followed in the early 1990s. The application of the methods mentioned above requires the use of external sensors, placed on the surface of the concrete, or sensors embedded in the concrete. The use of external sensors allows measurements to be taken on the entire exterior surface of the concrete.