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Repair Techniques

The repair techniques presented have been categorised into four groups, which are as follows: Patch Repair
By far the most common repair technique is the application of concrete patches to damaged or deteriorated concrete. Furthermore, when other remediation techniques are being applied in order to limit the extent of on-going corrosion mechanisms or to prevent their re-occurrence. Patch Repairs are also used to reinstate the spalled or delaminated areas of concrete.

Electrochemical Process

Conventional patch repair is, and will always remain the primary method of repair of reinforced concrete structures suffering from corrosion damage to the reinforcement. Even when electrochemical techniques are used, patch repairs are invariably carried out first. However, the patch repair requirements are less onerous when combined with electrochemical treatment. Electrochemical techniques provide a useful set of methods for preventing or limiting further damage to structures affected by reinforcement corrosion.

Although the underlying principles are essentially the same, it is convenient to classify the electrochemical processes into three generic categories:

Cathodic protection (CP):

In cathodic protection, the corroding anodic areas of steel are made cathodic by the supply of electrons from an anode applied either to the concrete surface or embedded.  There are two ways of applying cathodic protection to structures: Galvanic and Impressed Current CP

Electrochemical chloride extraction (ECE) – is also known as desalination or chloride extraction (CE). The fundamental principle involved in ECE is similar to that of CP. The only major differences are the period and level of current application. CP is essentially a permanent installation involving an application of current in the region of 5-20 mA/m2 of steel whilst ECE is a temporary treatment where a much higher current density in the range of 0.5-2.0 A/m2 of steel is applied over a period of weeks. The chloride ions migrate to the concrete surface where they are removed.

Electrochemical Realkalisation is used for carbonated reinforced concrete structures and entails the re-establishment of alkalinity around the reinforcement and in the cover zone. Alkali ions are electrically driven toward the steel which, with the production of hydroxyl ions at the steel, repassivate the steel and reduce corrosion activity to a negligible level. The electrolyte is highly alkaline and drawn into the carbonated cover concrete by electro-osmosis where it acts as a buffer zone. This process is applied as a single treatment taking a few days for each zone treated.

Corrosion Inhibitors

Corrosion inhibitors are one of a variety of techniques that can be employed in an effort to suppress and control the rate of steel corrosion in concrete structures particularly in the case of hidden or latent damage, although their long term effectiveness in reinforced concrete is still open to debate and the subject of detailed research. Due to the large number of commercially available concrete corrosion inhibitors, which vary widely in their respective formulations and inhibitive properties, categorisation is difficult. However, it is possible to divide concrete corrosion inhibitors into two generic categories.

Concrete admixture inhibitors - used as a preventative measure.

Surface applied and drilled-in inhibitors - used as a curative or preventative measure.

These two generic categories can be further subdivided into anodic, cathodic and ambiodic (mixed) inhibitors depending upon the formulation of the inhibitor.

Surface Treatments

While concrete is often considered a durable material it may in certain circumstances require protection. Following deterioration and repairs, or in the absence of sufficient cover to protect reinforcement, there is often a requirement to provide protection against penetration by water, carbon dioxide, sulphur dioxide and salts.

Protection against long-term deterioration is also a possible requirement. Additionally, there is an increasing need for decorative or colouring treatments to improve the quality of the surroundings where these contain large areas of concrete, or even for colourless waterproofing treatments to prevent accumulation of dirt and biological growths.

Three generic types of surface treatment are available for the decoration and protection of concrete surfaces, designed to control chemical ingress as well as moisture movement. They are described as follow

Film-formers – these are coating systems based on either organic resins such as styrene butadiene and acrylic copolymers or inorganic resins such as potassium silicate, which form a protective/decorative film on the surface of the concrete. Coatings may be endowed with special properties, such as the ability to bridge moving cracks whilst maintaining film integrity.

Film-forming coatings for concrete are principally decorative protective elastomeric products. They are formulated to form a barrier against the ingress of carbon dioxide, and other deleterious substances, yet allow the free passage of water vapour. They should exhibit a proven resistance to weathering, and maintain their elastomeric and barrier properties in service, which will often be a maintenance free life in excess of 10 years. Test certificates should be sought to demonstrate these properties.