# Electrochemical Diagnostics of Sprayed Fiber-Reinforced Concrete Corrosion
## 2 Corrosion Henomena  
In reinforced concrete structures, corrosion of the reinforcement is the key factor directly affecting their durability [21; 22]. Rebars typically corrode because the physical and chemical processes occurring in the environment of the structure reduce the protection of the reinforcement provided by the concrete cover. This is usually caused by gradual carbonation of concrete, the impact of chlorides, or other factors which, by compromising the concrete cover, indirectly cause the corrosion of the reinforcement [22; 23; 24; 25]. Corrosion caused by concrete carbonation in the concrete cover usually results from the impact of the carbon dioxide in the air, as well as changes of the ambient temperature and humidity [26; 27]. In reinforced concrete structures, the concrete (if sufficiently waterproofed and made with appropriate and properly mixed ingredients) provides basic and also the most efficient protection of the reinforcement [monograph]. The protective role of concrete is related to its alkalinity (pH \(\approx\) 12.5-13.5) which results in passivation on the interface between the concrete and the steel (i.e., formation of a micro-layer with very  
Figure 1: Examples of steel fibers used for concrete reinforcement [own study]. Figure 2: Size and distribution of cracks in reinforced concrete elements subjected to tension: (**a**) plain concrete; (**b**) fiber–reinforced concrete (FRC) [own study on the basis of Reference ([13], p. 266, figure 9.10)]. low ionic conductivity), which practically prevents corrosion of the rebars. Unfortunately, prolonged negative impacts of the physical and chemical factors of the external environment, as well as potential mechanical damage, gradually deteriorate the protective capabilities of the concrete cover. Carbonation gradually reduces the pH value of concrete, with the neutralized areas of concrete spreading inwards and eventually reaching the passive film (Figure 3). If the pH value of concrete drops below \(\sim\)11.8, the passive film is destroyed, directly triggering electrochemical corrosion of the reinforcement. Carbonation usually affects larger, surface layers of concrete. The level of carbonation of concrete can be determined directly on the structure using simple diagnostic methods (e.g., phenolphthalein test, thymolphthalein test, or Rainbow test) or by more complex laboratory analysis (X-ray diffraction (XRD), differential thermal analysis (DTA), scanning microscopy or IR spectroscopy). The second frequent cause of corrosion of the primary reinforcement bars in reinforced concrete structures is so-called chloride corrosion [23; 28], which takes a markedly different form than carbonation. Chloride corrosion is a pitting spot corrosion which can burst the concrete cover from the inside without any visible changes on the surface of the concrete element (Figure 4). This type of corrosion affects structures located at seasides which either have direct contact with seawater, or are exposed to the so-called sea fog, and also all types of civil engineering structures (mostly roads) that in winter are de-iced with agents containing sodium chloride (NaCl) [24; 29]. The spread of chloride corrosion is often mediated by frost, as the water freezing in the concrete pores generates stress, which leads to cracking of the concrete, thus opening the way for chloride ions which diffuse into the concrete cover and damage the passive film which protects the reinforcement. Electrochemical reactions result in corrosion cells forming on the surface of the reinforcement, and the corrosion process advances rapidly. Structures on which chloride corrosion is identified usually require renovation. Reinforcement corrosion can also be indirectly caused by all types of concrete damage arising from other types of corrosion (deterioration due to sulfate attack, corrosion caused by leaching of  
Figure 4: Reinforcement corrosion caused by the penetration of chloride ions [own study]. Figure 3: Reinforcement corrosion caused by carbonation of the concrete cover [own study].