PSI - Issue 33

Francesco Freddi et al. / Procedia Structural Integrity 33 (2021) 371–384 Author name / Structural Integrity Procedia 00 (2019) 000–000

375

5

As the calcium carbonate forms, due to its lower solubility compared to the calcium hydroxide, it deposits in the concrete pores leading to the clogging phenomenon which reduces the material porosity. Moreover, the mechanical properties of the concrete also are affected. In fact, an increment of the material stiffness and compressive strength is typically observed. To account for these changes, the material modulation functions d i (φ) have been introduced d � � � � � � � � �� ������� � � ��������� � � � (9) Another critical aspect tied to the carbonation process is the reduction of the pH value within the concrete. In fact, the hydration process of the cement paste promotes an alkaline environment with high pH values (≈13) in which the reinforcement bars develop a passive layer protecting them from corrosion phenomena. However, carbonation of concrete reduces the pH to more neutral values (< 9) that lead to the de-passivation of the steel rebars, causing the initiation of the corrosion process. The pH within the concrete is evaluated via the following equation �H � �� � ������ � �� � �Ca�OH� � �� (10) 2.2. Corrosion Once the carbonation front reaches the surface of the bars, the pH of the concrete pore solution reduces to more neutral value which leads to the dissolution of the protective passive layer. Due to the nature of the homogeneous advancement of the carbonation front, the de-passivation of the steel bars is uniform, creating a microcell corrosion condition where anodic and cathodic areas are placed in adjacent locations. Corrosion consists in an electrochemical process in which reinforcement bars are affected by the anodic dissolution of the iron into the pore water accordingly to the following the semi-reaction Fe → Fe 2+ + 2e - (11) The free electrons produced in the anodic half-cell reaction are consumed in the cathodic reduction of the oxygen present in the proximity of the steel rebar � � O 2 + � � H 2 O + e - → OH - (12) The combination of the two semi-reactions lead to the formation of “red rust” deposits in proximity of the cathodic areas following the complete reaction Fe 2+ + 2OH - → Fe(OH) 2 (13)

Fig. 2 Microcell corrosion system