## 2 FRC Under Chloride Ingress  
Chloride-induced corrosion of rebar is a complex electrochemical process that results in a reduction in the rebar cross-section, consequently increasing internal stress in the rebar and adversely affecting its mechanical behaviour. For this process to occur, sufficient oxygen and chloride concentrations [10] need to be reached at the level of rebar in concrete, and a closed electrical circuit must be formed.  
Due to the abundance of oxygen in both the atmosphere and the ocean, usually, the build-up of chlorides dictates when the corrosion process is initiated [20]. Once the chloride concentration reaches the critical threshold (\(C_{\text{crit}}\), expressed as a percentage of binder mass) [21; 22], the protective layer on the rebar surface--formed in the highly alkaline environment of fresh concrete--is damaged, and chloride-induced corrosion is initiated.  
In the ensuing electrochemical reaction, the rebar itself acts as a mixed electrode [23], where anodes (localised areas where chloride ions break down the protective passive layer) and cathodes (intact passive regions) coexist. The steel bar serves as the metallic pathway, connecting these anodic and cathodic sites electrically. Meanwhile, the concrete pore solution functions as the electrolyte, enabling the movement of ions to complete the electrical circuit. As a consequence, two simultaneous electrochemical reactions occur: an anodic reaction, where the iron in the steel rebar is oxidised (resulting in rebar degradation), and a cathodic reaction, where the oxygen is being reduced, which allows for the corrosion process to be sustained.  
The movement of chlorides through the concrete matrix (and subsequent build-up until \(C_{\text{crit}}\)) may be caused by the pressure gradient (permeation), concentration gradient (diffusion), capillary forces (absorption) or any combination of these factors, which exist between the interior and the exterior of the concrete element [10]. Therefore, the design codes (i.e., ACI 318 [24] or Eurocode 2 [25]) focus on preventing chlorides from reaching \(C_{\text{crit}}\) as a means of achieving adequate durability of the structures. This may be achieved by strictly limiting the presence of free chlorides in component materials or by slowing down their ingress. Namely, porosity is directly proportional to permeability and may be reduced by lowering the water-cement (\(w/c\)) or water-binder (\(w/b\)) ratios, whereas the diffusion effects can be mitigated by increasing the concrete cover depth (\(c\)) to rebar.  
Cracks are also recognised as another significant factor in chloride ingress and subsequent rebar corrosion. There is ongoing debate regarding the precise impact and significance of certain crack characteristics, such as the crack widths (\(w\)), the number and spacing of cracks (crack frequency) or crack orientation relative to the rebar. Locally, cracks act as preferential pathways for chloride ingress, connect internal pores [26] and may diminish the protective role of concrete cover. Since cracks are unavoidable in RC (even under service loads), their impact is specifically addressed by limiting the permissible crack widths (\(w_{\text{max}}\)) in design codes [24; 25].