Why coat concrete?

• Concrete is far from a durable and long-lasting material
• Concrete has higher levels of moisture and alkalinity than other building materials
• Unprotected concrete will deteriorate when exposed to very aggressive environments - especially in industrial areas, areas exposed to severe elements such on the coast and areas with high atmospheric pollution, high humidity or frequent weather changes from wet to dry

What are the special problems of concrete?

• Concrete is a porous material
• Unprotected concrete can absorb water, sodium chloride (salt), carbon dioxide (CO₂) and acid precipitation, which in time will affect its integrity.

Chloride ions

Salt can be present from de-icing of roads, the sea on coastal structures or air-borne salt spray. When there is ingress of salt the chloride ions penetrate deep into the concrete. They eventually accumulate around the steel reinforcement bars, destroying the protective film at the steel/concrete interface. With oxygen and moisture present corrosion of the steel begins.

Carbonation

When CO₂ gas dissolves in any water in the pores, it reacts with the cement to form carbonates which reduce the concrete pH. Carbonation can be a problem in reinforced concrete as a pH of around 7 is below the passivity threshold of steel.

When the steel starts to corrode, its volume increases 4-6 times - resulting in enormous pressure on its surroundings, causing cracks and ruptures in the concrete. Water will then penetrate, the concrete is further weakened and steel corrodes.

Combining protection and decoration

Coatings for concrete differ in their main aims from that of decorative paints. The primary objective of a coating must be to maintain the integrity of the concrete structure and protect any reinforcing steel against corrosion.

Sometimes a protective coating must not only repel water but also be breathable to allow water vapour to escape from the concrete out to the air.

Presentation is also of key importance. The cosmetic effect must in some cases combine long lasting colour retention with additional protection. Jotun Concrete protection systems achieve the twin aims of protection and decoration.

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