# Long-Term Effects in Structures: Background and Recent Developments
## 3 Long-Term Effects in Concrete
### Corrosion  
The corrosion of embedded steel stands as a primary cause behind the premature deterioration of reinforced concrete structures globally [64].  
The causes of corrosion in reinforced concrete (RC) structures can be attributed to several factors, including the following:  
* Carbonation: Carbon dioxide (CO\({}_{2}\)) from the atmosphere reacts with calcium hydroxide in concrete, reducing its pH and neutralizing its alkalinity. As the pH level decreases to a critical point, the passive layer becomes unstable, leading to potential steel corrosion. Throughout the propagation stage, the corrosion rate is contingent upon various factors, including the presence of water and oxygen. Carbonation-induced corrosion results in general corrosion of steel when it comes into contact with carbonated concrete [65].
* Chloride Attack: Chloride ions, often present in seawater, or marine environments, can penetrate concrete and reach the embedded steel reinforcement. Chloride attack can initiate and accelerate corrosion by breaking down the passive film on the steel surface.
* Moisture and Oxygen: Corrosion of steel reinforcement in concrete requires the presence of moisture and oxygen. Water ingress through cracks or pores in the concrete provides the necessary electrolyte for the electrochemical corrosion process, while oxygen facilitates the oxidation of iron in the steel.
* Cracks and Poor Concrete Cover: Cracks in concrete structures can provide pathways for aggressive substances such as chlorides and carbon dioxide to reach the embedded steel reinforcement, accelerating corrosion. Inadequate concrete cover over the reinforcement also increases the risk of corrosion by reducing the protective barrier between the steel and external corrosive agents [66].
* Microorganisms (bacteria): Microbiologically influenced corrosion occurs when microorganisms, such as colonize the concrete surface and create conditions conducive to corrosion. These microorganisms produce metabolic by products that can degrade the protective passive film on embedded steel reinforcement and promote localized corrosion.  
With respect to the abovementioned factors, carbonation and chloride attack are the most common cause of corrosion [67]. Various analytical methods for predicting basic corrosion mechanisms in reinforced concrete are proposed. Estimating the depth of carbonation in concrete can be accomplished using classical approach from CEB [68] and modified [69] diffusion models. The composition of concrete plays a significant role in the diffusion of CO\({}_{2}\), while humidity strongly influences the rate of carbonation. Surface materials with high compactness and impermeability can noticeably delay the carbonation process of concrete. Similarly, chloride concentration within the concrete can be determined using diffusion models [69]. Various models have been proposed to incorporate the time-dependent diffusion coefficient for concretes [70; 71; 72; 73]. It is important to account for variations in the diffusion coefficient due to concrete cracking and changes in the water-to-cement ratio. Detailed discussions on models are provided in [74].