## 2 Corrosion of RCS
### Chloride Induced Corrosion  
The corrosion of RCS induced by Cl\({}^{-}\) is the most frequent cause and has major costs and implications on the structural safety. Cl\({}^{-}\) may be introduced during the manufacturing phase of the concrete by contamination of the raw materials or may come from external sources; e.g., seawater, de-icing salts used in the cold season in some countries or hydrochloric acid from the decomposition of polyvinyl chloride in the event of fire in a building. Cl\({}^{-}\) can be found in concrete in two forms: free or bound. Among the bound chlorides, the strongly and weakly bound are distinguished. Strongly bound chlorides are chemically combined mainly in the form of calcium chloraluminates and calcium chloroferrites [9]. The weakly bound chlorides are adsorbed on the pore walls [9]. The hydrated compounds of the cement may be combined with the Cl\({}^{-}\) to a certain extension, regardless of whether their origin is external or whether they are introduced with the constituents. Of course, if the Cl\({}^{-}\) are introduced with the constituents the reaction will be favored. The free chlorides are dissolved in the interstitial solution and are the main players in the corrosion of RCS. These can destroy the passive film of the reinforcing steel by reducing the pH of the pore water. The solubility of Ca(OH)\({}_{2}\) is reduced by increasing the moisture content, due to the hygroscopic properties of the salts present in the concrete (CaCl\({}_{2}\), NaCl), and increasing the electrical conductivity of the concrete [2]. The bound chlorides can be converted to the free form, for example, by the carbonation of the concrete and thus constituting a potential reserve of Cl\({}^{-}\). Recently, [PERSON] et al. [10] reported an experimental study where the interaction between concrete carbonation and chloride attack was discussed. The authors reported that the carbonation of cement paste resulted in a release of bound chlorides, which was related to the decomposition of [PERSON]'s salt and with the decomposition of calcium-silicate-hydrate gel.  
Chloride induced corrosion of reinforcement in concrete has been extensively studied in the last five decades under different conditions, mostly in relation to the chloride threshold value (CTV) or critical chloride content [11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24]. CTV may be expressed either as the total chloride content as a percentage by weight of cement/concrete, or as the molar ratio between [Cl\({}^{-}\)] and [OH\({}^{-}\)]. The [Cl\({}^{-}\)]/[OH\({}^{-}\)] ratio reflects the ratio of aggressive ions to inhibitive ions that lead to corrosion initiation in a solution environment and depend on the pH, temperature, moisture and surface finishing of the rebar [25].  
The chloride-induced corrosion of RCS may be divided in two phases; corrosion initiation phase and propagation phase. The initiation phase is characterized by the penetration of the Cl\({}^{-}\) until the steel surface and depends on its rate, on the CTV and on the thickness of the concrete cover. The propagation phase initiates from the moment in which the conditions for the steel depassivation are reached until spalling and cracking of the concrete occurs [7].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \(d_{carb}/d_{cant}\) & Concrete Condition & **Rebar Condition** & **Risk of Corrosion** \\ \hline \(<\)0.5 & No cracking & Passive & Negligible \\ \hline \(>\)0.5 & No cracking & Passive & Low \\ \hline \(\approx\)1.0 & Small cracking & Low to moderate corrosion & High \\ \hline \(>\)1.0 & \
\begin{tabular}{c} Cracks, minor \\ detachment/spalling \\ \end{tabular} & Moderate to high corrosion & Very High \\ \hline \(>>\)1.0 &
\begin{tabular}{c} Cracks, high \\ detachment/spalling \\ \end{tabular} & High corrosion with substantial & Very High/Severe \\ \hline \hline \end{tabular}
\end{table}
Table 1: Risk of corrosion due to carbonation of concrete according to [PERSON] [1].