Coatings, sealers, membranes and barriers

If a combination of patching and sealing is required for a carbonated structure, the patch repairs must remove all carbonated concrete around the steel. In some cases this will only require exposure of the front face but in more extreme conditions it may need concrete removal behind the steel to restore the passive alkaline environment. Much has been published about anticarbonation coatings and their effectiveness but it is not easy to find independent information about specific products. Careful comparison of product information may be required. This may include the requirement for independent testing. The principle of anticarbonation coatings are that they are porous enough to let water vapour move in and out of the concrete but 

Penetrating sealers have been recommended as a way of stopping chlorides getting into concrete. The chemistry of the process is that silanes, siloxysi-lanes and similar chemicals will penetrate the pores of the concrete and react with the water in the pores to from a hydrophobic (water repelling) layer that stops water getting in as a liquid (that may carry salt with it), but allows water vapour in and out of the concrete so that it will breath . 

The penetrating sealer is within the concrete so it is protected from physical damage and degradation by ultraviolet light, etc. The problem is that the colourless liquid must be applied so that most of it gets into the pores and there is enough water in the pores to react, but not too so much that it washes the sealer back out and pores remain uncoated. There is still some debate over the true penetration depth of sealers to good quality concrete in the field. 

In the United States many highway agencies have applied penetrating sealers to concrete bridge decks. There has been no definitive research to show whether sealers work on trafficked surface or for how long they are effective. 

The penetrating sealers approved for application to highway structures in Europe are generally solvent free and are applied in a series of coatings by thorough wetting of the surface with the sealer with a drying period between coats. 

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