Galvanic Protection Of Reinforced Concrete Structures


The deterioration of reinforced concrete is principally due to the corrosion of the reinforcements, which occurs when the concrete has lost its ability to protect them. External pollutants found in air or water, enter through the micropores in the concrete. In time, the chemical properties of the concrete in the vicinity of the reinforcement are altered and its pH drops. Once the ambient moisture penetrates through to the steel, corrosion begins.

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Cathodic Protection is a technique based on electrochemical principles to prevent corrosion on metal structures or protect metal structures erected in aggressive environments from corrosion. This effect may be obtained by inducing a continuous current between an electrode, known as the anode, and the metal, which requires protection, known as the cathode. This circuit lowers the electrical potential of the metallic element thus reducing the speed at which it corrodes. The cathodic process may be initiated in two different conditions:

1. If corrosion on the metal element has already started, there is a condition of cathodic protection: condition which acts to reduce the corrosion activity until it stops.
2. If there is no corrosion on the element, it may be defined as preventive cathodic protection in that it impedes corrosion from the start.

Cathodic protection may be achieved in two ways:
1. Impressed Current System
2. Galvanic Anode System
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The impressed current system relies on an external transformer to develop the current required. The positive pole is connected to an anodic disperser, which is usually an insoluble anode (for example: high silicon iron, graphite, activated titanium, etc.) while the negative pole is connected to the structure requiring protection. The main advantage of the galvanic anode system is that it does not require any external energy source. When two different types of metals are connected and embedded in a suitable electrolyte the metal with the most negative electric potential will oxidise and protect the metal with the least negative potential. Aluminium and zinc are generally used to protect steel if the electrolyte is sea-water or concrete, magnesium is used for elements embedded in the ground and fresh water, while iron is used for copper alloys and stainless steel.

Galvanic Cathodic Protection
The phenomenon described above may be used to protect reinforced concrete structures. Electrochemical techniques are employed to annul or prevent corrosion or to limit it within certain acceptable limits. To reach this target the reinforcement must be polarised using cathodes. Cathodic protection/prevention may be achieved by connecting the steel reinforcement to sacrificial zinc anodes, which, thanks to their more negative electrical potential, protect the steel reinforcement rods and keep them free from corrosion. The aim of protection in the concrete is to bring the reinforcement to a passive state or reduce activity on its surface. In structures polluted by chlorides, the current also provokes an increase in the pH level and keeps the chlorides away from the steel, and both occurrences promote the formation of a film of passivity. In carbonated structures, on the other hand, the current only promotes an increase in the pH level, which may be increased from 9 (carbonisation condition) to 12-13, values, which take the steel reinforcement from an active state to a passive state. Cathodic prevention is based on the fact that corrosion of the steel reinforcement is not initiated as long as its electrical potential is kept lower than the electrical potential of corrosion. By connecting the two metals (carbon steel-zinc), the reinforcement is kept passive and initiation of the phenomenon is impeded, even if there are high levels of chlorides present. Another aspect which must be considered is the density of current required for cathodic protection/prevention. In old, deteriorated structures this value is between 5 and 20 mA/m2, while for prevention on new structures the value is between 0.2 and 2 mA/m2. In the first case, since the reinforcement is highly active, the initial demand for current will be very high (in the order of 15/20 mA/m2), which will then gradually reduce to a lower level (in the order of 4-5 mA/m2) as soon as a passive state has been reached (generally 6 to 12 months after installation). This means that an important advantage of galvanic cathodic protection is that it is self-regulating according to the real demand for current, which the reinforcement requires as time goes by.

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Advantages of Galvanic Systems
- No external wiring or energy sources are required
- Easy to install
- Generate enough current even in dry environments (in compliance with EN 12696 European Standards)
- Difference in potential impedes corrosion with no risk to the structure
- Self-regulating
- Guarantee long-lasting protection and each installation may be calculated and sized according to technical and economical requirements
- Selective, targeted repairs may be carried out
- No maintenance is required for the entire service life of the anodes
Products from the Mapesheild Range are particularly recommended for the active protection of:
- Floor slabs in raised car-parks
- Beams and columns
- Pre-fabricated reinforced concrete structures
- Front edges of balconies
- Concrete floors
- Piles, abutments, friezes, kerbs and floor slabs on bridges and viaducts
- General reinforced structures, in particular those exposed to aggressive environments such as those close to the sea
- Beams, pillars, piles and steel tanks
MAPEI has developed a range of products especially for galvanic cathodic protection/ prevention:
Mapesheild I:
four different types of internal galvanic anodes in various sizes and masses of zinc to guarantee protection for most types of reinforced concrete structures. The very shape of MAPESHIELD I gives it a large anodic surface, a characteristic, which, together with the special conductive paste, makes the product extremely efficient and long-lasting. MAPESHIELD I anodes ensure that the steel reinforcement is depolarised in compliance with the prescriptions in the EN 12696 European standard “Cathodic protection of steel in concrete”.
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Mapeshield I is composed of a zinc core with a large surface area covered by special conductive paste which keeps the system active over the years. Mapeshield I is available in 2 different lengths and 4 different masses so the system may be used in most structures. The surface which the anode can protect depends on its size (the bigger the anode, the larger the area it protects) while the mass, which is proportional to the amount of metal it contains, effects its duration.
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Mapeshield I is particularly recommended for protecting reinforcement rods against corrosion in structures requiring repair work and also offers a number of advantages if applied on new reinforced structures for protection against corrosion, especially if they come into contact with aggressive agents.

Application examples:
- Piles and abutments on bridges and viaducts
- Floor slabs
- Pre-fabricated reinforced concrete structures
- Front edges of balconies
- Concrete floors (such as car-parks)
- Foundations
- Basins
- Pre-fabricated structures (buffer panels, beams, etc.)
Mapesheild E 25:
adhesive zinc plates applied directly on the surface of new and repaired concrete. Apart from its simple application procedure, this type of galvanic anode allows the steel reinforcement to be protected without having to remove the concrete unless necessary. This leads to a considerable saving in time and costs. MAPESHIELD E 25 ensures that the steel reinforcement is depolarised in compliance with the prescriptions in the EN 12696 European standard “Cathodic protection of steel in concrete”.

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Mapeshield E 25 is particularly recommended for protecting reinforcement rods against corrosion in structures which do not require repair work and to reduce or block oxidation in structures requiring repair.
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Application examples:
- Piles and abutments on bridges and viaducts
- Floor slabs
- Car parks
- Pre-fabricated reinforced concrete structures
- Beams and columns
- Front edges of balconies
Mapesheild S:
a product developed for galvanic cathodic protection of steel structures exposed to atmospheric conditions. This type of anode is also comprised of a thin auto-adhesive zinc plate applied directly on the surface requiring protection. MAPESHIELD S conforms to the prescriptions in the UNI 10781 Standard “Requirements of self-adhesive zinc tape, application methods and checking the coating”.
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Zinc plate with adhesive backing for galvanic cathodic protection against the corrosion of steel structures exposed to atmospheric conditions. Mapeshield S is particularly recommended for protecting flat, round and complex shaped metal structures which have a surface to which zinc plates may be directly applied.
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Application Examples:
- Beams
- Columns
- Piles
- Pipe-work
- General metal structures
Mapeshield S is available in 50-metre-long rolls at a width of 100, 200 or 300 mm. To guarantee protection, the zinc plate must be applied on the entire surface of the structure, which requires protection. Once applied, no other operations need to be carried out because contact with the structure is guaranteed by the adhesive.
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Conclusion Mapeshield range of corrosion protection system is the ideal products for corrosion protection. Other products used for corrosion protection system are:
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Some projects like Genua Construction of new Chanel, Residential structure of Italy, Industrial structure, UAE, Restoration of Cooling towers, Spain, Underpass terminal 1, Rome Airport, Parchi Viaduct, Milano are few references where Mapeshield products have been used.
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Spokesperson:
Mr. Santhosh Prakash
Vice President (Product Management)
Mapei Construction Products India Pvt. Ltd.,
E: s.prakash@mapei.co.in, infoindia@mapei.co.in
W: www.mapei.co.in