The Constructor

Cathodic Protection of Reinforced Concrete Structures

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Cathodic Protection of Reinforced Concrete Structures

In reinforced concrete, a passive oxide film is created around the reinforcing steel by the alkaline Portland cement, which protects the steel from corrosion. This oxide film is weakened or destroyed when aggressive ions, like chlorides, attack the concrete around the reinforcing steel causing corrosion of the reinforcing steel. Corrosion is an electrochemical process having anodic and cathodic areas formed on the steel. When the circuit of the corrosion cell is completed by the presence of continuous steel bars connecting anodic and cathodic areas and by immersion in the same electrolyte, corrosion occurs at the anodic regions. The corrosion continues until failure occurs at the anodic area. Cathodic protection is a method to control the corrosion of steel in contaminated concrete that works by making the embedded reinforcing steel cathodic. When the steel becomes cathodic, hydroxyl ions are accumulated around it making it passive for longer time. The reinforcing steel is electrically connected to another metal that becomes the anode with or without the application of an external power supply. The cathodic protection systems that work in the absence of an external power supply and in the presence of a less noble metal (like zinc) to act as anode are referred to as sacrificial passive systems. In case of an impressed current cathodic protection, an external power supply is used to force a small amount of electric current through the reinforcing steel to counteract the flow of current caused by the corrosion process. A metal, like platinum, serves as anode which corrodes at a very slow rate. Cathodic protection is used to protect almost any type of reinforced concrete structure, including horizontal slabs, walls, towers, beams, columns and foundations. However, this method has the following limitations: Sacrificial passive cathodic protection systems may be provided using one of the following methods: a) Zinc sheet anodes, precoated with a conductive hydrogel adhesive, are applied to the surface of the concrete. The formed anode is called zinc hydrogel anode and its appearance is improved by coating with various paints. b) Zinc or zinc alloys are sprayed to the concrete using arc spray or flame spray equipment. c) Embedded galvanic anodes are embedded within the repair concrete connected with the reinforcing steel. The anodes are installed along the perimeter of concrete patch repairs to protect adjacent areas from corrosion due to the anodic-ring effect. The impressed current cathodic protection systems contain an anode, a DC power source, and connecting cables. Various types of these systems based on the anode installation and its application are as follows: a) Surface-mounted anodes without overlays are mounted on the surface of the concrete and do not require a cementitious overlay. However, the wear resistance of the system is reduced. b) Conductive mastic anodes consist of anodes embedded on the surface of concrete having a conductive coating. They are used on vertical surfaces, ceilings, and columns. c) Plate-type anodes consist of manufactured anode plates glued to the concrete surface. d) Surface-mounted anodes with overlays are generally used on horizontal surfaces and require a cementitious overlay of 13 mm minimum thickness. e) A mesh of a noble metal anode is fixed to concrete with multiple pins, called mesh-type noble metal anodes, which are covered with a cementitious material. f) Conductive polymer concrete strips consist of a series of conductive polymer concrete strips containing a noble metal anode that is fixed to the concrete surface and covered with a cementitious overlay. g) Embedded anodes system is embedded in the surface of the concrete or is placed at the level of the reinforcement in new construction. h) Saw slot anodes are made by creating a series of small depth and width saw slots in the concrete surface which are filled with a noble metal anode and a conductive polymer concrete. i) Anodes can be placed at the level of the reinforcement during new construction. This anode must not have direct contact with the reinforcing steel.
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