Corrosion of the reinforcing steel in concrete is a worldwide problem. However, if corrosion effects are considered in the design phase and the right decisions made prior to construction, concrete construction longevity can be extended. Article supplied by the Australasian Corrosion Association.
The corrosion of steel in concrete is accelerated in harsh environments, especially where high salt levels or extreme temperatures can accelerate the rate of decay. Usually, the most exposed elements deteriorate first but because the active corrosion may take five to 15 years to initiate cracks in the concrete, much of the actual corroded reinforcement is not visible.
As the degradation of the steel and weakening of the concrete occurs from the inside and may not be seen for many years, it is often referred to as ‘concrete cancer’. According to Ian Godson, managing director of Infracorr Consulting PL, it might take up to 15 years before any cracking is visible.
“It is a hidden problem which means that, when you find it, it is often well advanced, very much like the tip of the iceberg,” he says.
The two commonest causes of concrete corrosion are carbonation and chloride or ‘salt attack’.
The alkaline (high pH) conditions in concrete form a passive film on the surface of the steel reinforcing rods, thus preventing or minimising corrosion. Reduction of the pH caused by “carbonation” or ingress of chloride (salt) causes the passive film to degrade, allowing the reinforcement to corrode in the presence of oxygen and moisture. A voltage differential of approximately 0.5 V is set up between the corroding (anodic) sites and the passive (cathodic) sites resulting in a corrosion cell where electrons move through the steel from anode to cathode. The rate of the reaction is largely controlled by the resistance or resistivity of the concrete. Acid forms at the anodic (corroding) site which reduces the pH and promotes the corrosion of the steel.
In broad terms, when carbonation, chlorides and other aggressive agents penetrate concrete, they initiate corrosion that results in cracking, spalling and weakening of concrete infrastructure. As reinforcing rods rust, the volume of the rust products can increase up to six times that of the original steel, thus increasing pressure on the surrounding material which slowly cracks the concrete. Over the course of many years, the cracks eventually appear on the surface and concrete starts to flake off or spall.
Carbonation is the result of CO2 dissolving in the concrete pore fluid and this reacts with calcium from calcium hydroxide and calcium silicate hydrate to form calcite (CaCO¬3).
Within a relatively short space of time, the surface of fresh concrete will have reacted with CO2 from the air.
Gradually, the process penetrates deeper into the concrete and after a year or so it may typically have reached a depth of one millimetre for dense concrete of low permeability, or up to five millimetres for more porous and permeable concrete depending on the water/cement ratio.
Chlorides, usually from seaside splash or windblown locations, migrate into the porous concrete over time, causing corrosion when the concentration of chlorides reaches critical levels at the reinforcement. In addition, older structures may have utilised calcium chloride as concrete ‘set accelerators’ at the time of construction, again resulting in serious corrosion issues.
Concrete corrosion repair and prevention
The traditional method of concrete repair is to remove the cracked and spalling concrete to a depth of 20-30mm behind the reinforcing bars to fully expose the rusted material and remove the contaminated concrete from the steel. All the corroded material is then removed and the steel treated or replaced, after which specialist repair concrete mortars are applied and the surface made good. A modern development is for the repair mortars to be polymer modified to improve adhesion and resist further ingress of contaminants. Coatings are commonly used in combination with patch repairs to reduce further entry of carbonation or chlorides.
These “patch repairs” that remove the contaminated concrete from the deteriorating sections often do not address this hidden corrosion and result in accelerated deterioration to the surrounding areas, commonly failing again within three to five years. One of the limitations of patch repairs is that you have to remove large quantities of sound concrete to solve the problem, causing significant noise and disruption to the building occupants.
The main alternative to patch repair is cathodic protection
One type, Impressed Current Cathodic Protection (ICCP), is a technique whereby a small, permanent current is passed through the concrete to the reinforcement in order to virtually stop the corrosion of the steel.
The main benefit of ICCP is that the extent of removal and repair of concrete is vastly reduced, with only the spalled and delaminated concrete required to be repaired. Once installed, the ongoing corrosion can be controlled for the long term, eliminating future spalling and deterioration even in severely chloride or carbonation contaminated concrete.
The selection of anode systems is the most vital design consideration for a durable and efficient ICCP system. Incorrect selection and placement of the anode system can result in poor performance and vastly reduced life of the installation.
While the technique is relatively simple in theory – inserting anodes into the concrete at set spacing attached to the positive terminal of a DC power supply and connecting the negative terminal to the reinforcing steel (ICCP systems commonly operate at two to five volts) – the drawback is that you need lots of cables and permanent power supplies which results in this technology being mainly restricted to civil structures such as wharves and bridges with very rare applications to buildings.
A relatively recent development has been Hybrid CP, which utilises zinc anodes installed in drilled holes with the anodes powered for an initial period of around 10 days. The high initial CP current totally passivates the steel reinforcement, migrating chloride away from the bars and restoring an alkaline (high pH) environment in the concrete.
Following the initial impressed current phase, the temporary power supply and cables are removed, with the anodes then connected to the reinforcement via locally placed junction boxes to provide ongoing galvanic protection. This relatively low galvanic current maintains the ongoing passive condition at the reinforcement and prevents further concrete damage. Hybrid CP systems are usually designed to give a 30 year or longer design life.
Hybrid CP offers all the advantages of ICCP, including corrosion control and reduced concrete removal, without the high cost and maintenance of power supplies, cables and control systems. Areas and structures that were previously difficult and uneconomical to treat with ICCP can now be protected using Hybrid CP technology. This includes small scale and remote structures including those situated in non-powered sites such as bridges, marine dolphins and culverts. In the case of building repairs, Hybrid CP offers significant advantages over ICCP by eliminating the need for unsightly and costly cabling and power supplies.
- The Australasian Corrosion Association (ACA) works with industry and academia to research all aspects of corrosion and throughout the year conducts training courses and hosts seminars across New Zealand. Further information: www.corrosion.com.au.