Cathodic Protection of Reinforced Concrete Structures
Reinforced concrete is known for its durability and strength, in part due to the passive oxide film that forms around the reinforcing steel. This protective film is created by the alkaline Portland cement used in the construction process, which acts as a barrier against corrosion. However, this oxide film can be weakened or destroyed when aggressive ions such as chlorides come into contact with the concrete surrounding the reinforcing steel, leading to corrosion of the steel.
Corrosion is an electrochemical process that involves the formation of anodic and cathodic areas on the steel. When there is a continuous connection between the anodic and cathodic areas through steel bars and immersion in the same electrolyte, a complete circuit is formed for the corrosion cell. This results in corrosion occurring at the anodic regions, which continue to deteriorate until failure occurs at these areas. The presence of continuous steel bars in the concrete can facilitate the corrosion process by providing a pathway for the completion of the corrosion cell circuit, leading to further degradation of the reinforcing steel.
Cathodic protection is a corrosion control method for steel in contaminated concrete, where the embedded reinforcing steel is made cathodic. This results in the accumulation of hydroxyl ions around the steel, making it passive and resistant to corrosion for a longer time. The cathodic protection system involves electrically connecting the reinforcing steel to another metal that becomes the anode, either with or without the use of an external power supply. There are two types of cathodic protection systems: sacrificial passive systems and impressed current systems.
Sacrificial passive systems use a less noble metal, such as zinc, as the anode. This metal sacrificially corrodes to protect the reinforcing steel. Zinc sheet anodes coated with a conductive hydrogel adhesive can be applied to the surface of the concrete, or zinc or zinc alloys can be sprayed onto the concrete using arc spray or flame spray equipment. Embedded galvanic anodes can also be used, which are installed within the repaired concrete and connected to the reinforcing steel.
Impressed current cathodic protection systems, on the other hand, use an external power supply to force a small amount of electric current through the reinforcing steel to counteract the corrosion process. Anodes made of materials like platinum, which corrode at a very slow rate, are used in these systems. There are various types of impressed current systems, including surface-mounted anodes without overlays, conductive mastic anodes for vertical surfaces, plate-type anodes glued to the concrete surface, surface-mounted anodes with overlays for horizontal surfaces, mesh-type noble metal anodes fixed to concrete with pins, conductive polymer concrete strips containing a noble metal anode, embedded anodes system, saw slot anodes created by sawing slots in the concrete surface and filling them with a conductive polymer concrete, and anodes placed at the level of reinforcement during new construction but without direct contact with the reinforcing steel.
It’s important to note that cathodic protection is not a replacement for corroded steel and impressed current systems are not recommended for prestressed concrete structures due to the potential for hydrogen-induced embrittlement. Additionally, proper installation of the system requires confirming the electrical continuity of the reinforcing steel and ionic conductivity of the concrete. Sacrificial passive systems can be used for post-tensioned structures after detailed corrosion analysis.