Steel reinforcing bars are key components in reinforced concrete (RC) structures. As concrete alone has very low tensile strength the addition of rebar remarkably increases the strength of RC members, especially when subjected to bending moment and shear force. Since both concrete and reinforcing must work together and stress must be transferred effectively between these two materials, no treatment of the rebar surface (e.g. anticorrosion treatment, painting) should be conducted before concreting, as would be the case for structural steel members directly exposed to environmental forces. For this reason, in RC structures the role of anticorrosion protection is played by concrete cover (the distance between the steel rebar and the external concrete face). However, when concrete is exposed to extremely aggressive conditions (e.g. marine environment, coastal environment or industrial zones) various factors, such as carbonation or alkali-aggregate reaction, can cause rapid concrete deterioration and therefore the initiation of steel reinforcement corrosion occurs at stages much earlier than in other less-extreme environments.
In this short article the most common methods for mitigating the risk of rebar corrosion are presented and discussed.
The type of interventions for the protection of rebar from corrosion fall into three categories:
- External surface solutions,
- Repair of concrete; and,
- Electrochemical methods.
External (Concrete) Surface Solutions
These methods focus on the application of adequate measures to prevent water and chloride ingress into the concrete structure. They include application of coating or membrane layers that can “isolate” the concrete structure. Moreover, migrating corrosion inhibitors (MCI) can be applied on the concrete surface which then penetrates into the concrete to create a film where the face in contact with the rebar is hydrophobic and the end towards the concrete is hydrophilic.
Another measure that falls into this category and is very common in the protection of bridge structures is hydrophobic impregnation. This solution involves applying water repellent agents, the most common of which are the silanes, on the concrete surface, thus rendering the concrete of the surface hydrophobic and preventing water from reaching the reinforcement.
Other solutions to protect concrete include use of pore liners, which render the external surface water repellent, or pore blockers, which react with the concrete in the proximity of pores to form an insoluble material.
The application of repair methods on a concrete member depends largely on the degree of damage to the concrete and the reinforcing steel. Damage to concrete can be quantified as the percentage of member depth affected by carbonation, or alkali-aggregates reaction.
For reinforcing steel, measurements showing significant loss of rebar cross section indicate high extent of corrosion.
If the damage on concrete is limited to the cover, patch repairs are commonly applied. Such method consists of removing the physically deteriorated concrete of the existing cover, cleaning the reinforcement and adding mortar.
If the damage to the concrete member is more prominent, concrete replacement is usually applied. This is performed through hydro-demolition of the existing concrete until the depth where contamination end is reached, replacement of the rebar that shows significant cross section loss due to corrosion, and finally concreting.
It should be noted the collaboration between parent concrete and the new concrete or mortar is a crucial factor for the efficiency of both aforementioned methods. For this reason shrinkage-compensated mortars / concretes are often used. Fiber-reinforced concrete is also gaining significant popularity for such applications as it offers more effective waterproofing compared to conventional cast-in-place concrete.
Various electrochemical methods have been proposed to protect steel from corrosion. Some of these include chloride extraction, reverse osmosis and re-alkalisation. However, cathodic protection is nowadays considered to be the most efficient method to prevent corrosion and for this reason is extensively used in retrofitting to RC structures.
While the methods presented in the previous paragraphs aim at eliminating the cause of corrosion initiation, cathodic protection protects the steel rebar against corrosion in a more active manner. The method’s principle consists in connecting the steel rebar to be protected to a sacrificial material with unpolarised potential. Then a power supply connects the sacrificial material (anode) to the steel (cathode) providing the necessary amount of electric current. This ‘circuit’ results in the corrosion of the sacrificial material instead of the reinforcing steel. As, in reinforced concrete structures, steel reinforcement components are usually in contact with each other, this method results in the protection of a significantly larger zone of reinforcement than the ones connected to the circuit. Various types of cathodic protection methods have been proposed and applied successfully. Some great examples of this are the Immersed Current Cathodic Protection, the Galvanic Cathodic Protection or the Hybrid Cathodic Protection.