Different types of concrete CP system by impressed current
- 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.
- 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.
- Plate-type anodes consist of manufactured anode plates glued to the concrete surface.
- Surface-mounted anodes with overlays are generally used on horizontal surfaces and require a cementitious overlay of 13 mm minimum thickness.
- 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.
- 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.
- Embedded anodes system is embedded in the surface of the concrete or is placed at the level of the reinforcement in new construction.
- 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.
- Anodes can be placed at the level of the reinforcement during new construction. This anode must not have direct contact with the reinforcing steel.
Different type of sacrificial passive cathodic protection services.
- 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.
- Zinc or zinc alloys are sprayed to the concrete using arc spray or flame spray equipment.
- 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.
History of RCC
The use of steel in constructing the bridges, industrial infrastructures, buildings starts nearly the late 19th century onward. Steel cast in concrete develops a passive iron oxide film that makes it highly resistant to corrosion .Corrosion of steel in concrete is normally prevented by the chemical properties of hydrated Portland cement, but the protective mechanism can fail in high-chloride environments. This is actually a myth, in reality the protective passivation film on steel does not always destroyed by high concentration of chloride ions. The experiment shows the protective mechanism is unaffected even at the high chloride ion concentration in seawater (~20,000 ppm) except where air bubbles adhere to the steel surface due to which anode created and corrosion start. All concrete contain some chlorides, but steel corrodes only in small percentage of reinforced concrete. Many times this oxide film is also weakened or destroyed when aggressive ions, 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.
Concrete Cathodic Protection System 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/MMO Coated Titanium, serves as anode which corrodes at a very slow rate.