For more than 10 years we can find many reports in industrialised countries all over the world, that prove the effectiveness of the protection of rebars (reinforcement bars) in concrete. If a rebar is protected with cold galvanisation (a cathodic protection) against corrosion initially, then its lifetime can be prolonged for a period exceeding 100 years.
Corrosion and the mechanism of concrete cracking Traditionally the highly alkaline conditions inside concrete provide a passivating environment for the reinforcement. A thin layer of oxides forms on the steel surface. This oxide layer is stable in the alkali rich solution and protects the steel against further corrosion. The steel is unlikely to rust as long as the passivating conditions remain.
Well compacted concrete and an adequate cover of the rebars also provide a physical diffusion barrier against corrosion by reducing the penetration of atmospheric carbon dioxide (CO2), oxygen and moisture (3 elements that are necessary to initiate and sustain corrosion reactions).
This passivation can be destroyed due to a reduction in alkalinity by ingress of atmospheric carbon dioxide (CO2) (carbonation) or sulphur dioxide (SO2) (industrial climates) or by the ingress of the aggressive chloride ions (e.g. from marine environments) or de-icing salts (e.g. on roads) that can locally breach the passivation.
The rust created on the rebars can increase up to 2 times the volume of the steel and due to the high internal stresses, eventual cracking of the concrete or even complete destruction will follow.
History of the protection of reinforcing steel in concrete
For more than 30 years people in industrialized countries all over the world have tried to protect concrete from cracking by: –
- Making a better compacted concrete.
- Adding corrosion inhibitors to the concrete.
- Active and passive protection of the reinforcements
The first logical protection that was tried on the reinforcing steel in concrete was the existing range of paints. Coating rebars with an ordinary paint is of course no solution for the corrosion of the rebars: some years ago, very few paints sustained the high pH of fresh concrete ( about Ph 13). Moreover, a coating is porous. Penetration of moisture and oxygen cannot be prohibited.
Protecting rebars with a coating is even experienced with fusion-bonded epoxy, a very high priced coating, technically difficult in application. But it brings no solution to the corrosion problem of rebars.
Galvanising rebars with the hot-dip process does not bring the solution either: cracks, caused by the bending or rebending of the rebars, will easily destroy the galvanising layer. Rebars that have been galvanised by the hot-dip process do not provide enough adhesion for the concrete. Moreover, the rebars need to be galvanised in a workshop, which means that they have to be restricted in size. This will also cost time due to distance and transport, which can be very expensive in some parts of the world.
The latest developments in the attempts to protect reinforcing steel in concrete against corrosion prove to be ineffective as well. In North America reinforcing concrete can be done with rebars made out of polypropylene, a very expensive material, but still no solution: polypropylene does not provide an adequate adhesion for the concrete. In Germany a big metal company will offer you stainless reinforcing steel (at a very high price of course), but this is still no solution: it does not provide an adequate adhesion for the concrete and rebending a stainless steel rebar is practically impossible without failure.
Protecting the reinforcing steel with Cold Galvanisation
In comparison with all other methods of protection that have been tried out on reinforcing steel, a thin layer of cold galva (about 40 µm) introduces the best protection on the market at a very low cost effective price.
Cold galvanisation is a single-pack zinc coating easy to apply by brush, roller, spraying or dipping under any atmospheric condition. It offers a better cathodic protection than hot-dip galvanisation. This was proven in Europe, in the USA and in Asia, both in laboratory and in field-testing. A cold galva layer is a homogeneously distributed zinc layer, containing about 96% cathodic, active zinc, pure to 99.995 %.
The surface preparation on the reinforcing steel can be done either by grit blasting to SA 2.5 (commercial blasting WJ-1-NACE-5SSPC-SP12) with roughness degree Ra 12.5 for new steel with still the milscale on, or on rusted rebars by taking off the rust steel using a mechanical steel brush or by power tool cleaning by means of ultra high pressure water jetting (100 to 200 bars).
A cold galva layer is flexible and compressible and will not crack nor is damaged by bending and rebending the rebars. This is surely an important aspect in view of the fact that rebars are roughly manipulated with severe risk of getting damaged. Moreover, it offers a good adhesion with concrete.
The drying time of the cold galva before the contact with the concrete may be very short. As soon as the cold galva is touch dry, the concrete can be cast. From the moment that the fresh concrete will encapsulate the reinforcing steel protected by a cold galva layer, some oxidation of the cold galva layer will take place (due to the pH of the fresh concrete) with the consequence of the formation of some zinc salts on the surface of the coating. They will seal off the cold galva layer completely, making it an even better protective layer.
Cold galvanisation will protect the rebars, and consequently the concrete, for a period of time exceeding 100 years.
There are examples of contractors having installed a brush and spray unit on site to apply the cold galvanisation with a minimum investment and a high performance.