Plug into concrete protection
The Corrosion Preventation Association explains cathodic protectionCorrosion of embedded steel, through the action of de-icing salts, airborne sea salts, or salts cast into the concrete at the construction stage, is arguably the most serious cause of deterioration of reinforced concrete structures.
Water companies too, are beginning to realise that conventional 'cut out the chlorides' and repair procedures are never-ending and, therefore, becoming very expensive. As a result, many are now concentrating more on electro-chemical techniques as a practical, long-term solution, for repairing and monitoring reinforced concrete.
Cathodic protection is employed on new and existing structures either suffering from, or liable to be affected by, corrosion of embedded steel. Its use eliminates the danger of cracking and spalling, which would occur as a result of embedded steel corrosion, thus enabling the structure to realise its full design life.
Impressed current cathodic protection systems involve the permanent installation of a low-voltage, controlled electrical system which causes the normally damaging anodic reactions to occur at an installed durable anode, thus preventing the steel from corroding. The anode can be applied on the surface, within an overlay, inside joints, or drilled into small holes in the structure. It is the only treatment to provide a constantly monitorable and controllable protection.
Cathodic protection is a technically-sound solution since it deals directly with the cause of the problem and performs across the entire area being treated. This is not possible with conventional concrete repair without removing all of the contaminated or carbonated concrete. Such extensive repair is prohibitive both in terms of practicality and cost, making cathodic protection the most economical solution. When installing the technique to existing structures, all that is required is some cosmetic patching to repair cracks and areas which have broken away. The system has been successfully installed on car parks, bridge decks and support structures, tunnels, commercial and residential buildings.
In the water industry, reservoirs, water towers and other containment vessels have received the treatment. Barr Beacon Reservoir, located near Perry Barr, Walsall, is a typical example. It is one of many underground reservoirs within the asset management of South Staffordshire Water. On this project the technique was used to monitor and protect the roof support columns of the underground structure.
C-Probe was contracted to design, supply, install Ebonex conductive ceramic anodes, commission and manage a pilot scheme with the objective of providing a greater understanding of the behaviour of the reinforced concrete columns supporting the roof of the reservoir.
To this end, the system was designed for both the east and west tanks. A cathodic protection system and a system to detect early corrosion were installed to a total of eight columns. In both tanks, two columns had a discrete anode cathodic protection system installed and two columns were fitted with embedded corrosion potential and corrosion rate monitoring probes. On the cathodic protection columns it was decided to investigate the effectiveness of the system to both 100% and 50% of the column height, since the reservoir is filled to at least 60% capacity all of the time and, as such, the most susceptible area was likely to be in non-water saturated areas immediately below and above the water line. The installation, incorporated into an integrated neural network system (managed via the C-Probe internet interactive management system - AiMS) has now been operating successfully for more than twelve months. Valuable information is being acquired to predict the on-going maintenance requirements of the structure.
Cathodic protection of reinforced concrete was first practised on buried reinforced concrete pipes in the 1950s using conventional buried pipeline cathodic protection technology.
The earliest application of cathodic protection of atmospherically exposed reinforced concrete was to a bridge deck in northern California in 1972. By 1984, in the UK, there was increasing concern regarding the costs of repair and maintenance of structures and buildings arising from the corrosion of reinforcement. At the time UK market for cathodic protection in the remediation of reinforced concrete was estimated to total around 15,000m² per annum, but by 1999, members of the Corrosion Prevention Association (CPA) had treated an estimated 200,000m² in the UK and more than 2Mm² across the globe.
Cathodic protection is increasingly becoming the choice for informed engineers, marine, harbour and water companies and others who are responsible for concrete structures. Most structures requiring repair due to chloride related corrosion will show cost savings in excess of a factor of two when comparing long term repairs incorporating cathodic protection to the old 'cut out the chlorides' repair procedures. In some cases a cathodic protection solution has cost less than 20% of that for the more traditional approach and less breaking-out means reduced temporary works.
Related electro-chemical techniques of chloride extraction and realkalisation are available for the remediation of reinforced concrete structures. These generally have a limited application than cathodic protection, but can provide valuable remediation solutions for particular structures.
For more information, the CPA has published a Cathodic Protection Status Report
and information sheets relating to various aspects of cathodic protection and
the other electrochemical techniques