MANEY - Corrosion education manual (EFC 6) 3rd Edition
|Publication Date:||1 January 2001|
In general, reinforced concrete has proved to be successful in terms of both structural performance and durability because the concrete provides chemical and physical corrosion protection of the rebars. The alkaline pore solution passivates the steel and the concrete cover prevents or at least retards the ingress of corrosion-promoting substances. However, there are instances of premature failure of reinforced concrete components due to corrosion of the reinforcement. The two factors provoking corrosion are the ingress of chloride ions from deicing salts or sea water or the reaction of the alkaline pore solution with carbon dioxide from the atmosphere, a process known as carbonation. As a result of the corrosion reaction the cross section of the rebars is reduced and rust is formed. This process can cause cracking or spalling of the concrete and dangerous loss of structural stability.
From the point of view of the corrosion protection of the rebars two different situations have to be distinguished:
• on new structures, the most effective measure for durability can be achieved in the design stage by using adequate concrete cover and high concrete quality. This will prevent aggressive substances as e.g. chloride ions from deicing salts or sea water, from reaching the rebars within the design life. Additional protective measures can be applied such as admixtures to concrete to decrease its permeability, the use of more corrosion resistant materials for the reinforcement (e.g. stainless, galvanised, or epoxy coated steels), electrochemical protection systems (e.g. preventive cathodic protection) or others.
• on existing structures the deterioration process may have reached different stages according to age, exposure condition, concrete cover and quality: for a corrosion risk situation or at the onset of corrosion, preventive measures may be applied, whereas in severely corroding structures repairs have to be conducted.
Inhibitors, which are chemical substances that prevent or retard corrosion by action at the steel/ concrete interface, have been proposed (and used) both as preventive measures for new structures and as repair measures for existing reinforced concrete structures. The method of application differs: in new structures inhibitors are admixed in sufficiently high concentrations to the fresh concrete, on existing structures, where the onset of corrosion has to be prevented, inhibitors are applied at the concrete surface; for repair work inhibitors can be present in paints for the reinforcement or in repair mortars.
Edited by: B. Elsener