Corrosion of reinforcing steel: why fibres prevent this problem

Corrosion of reinforcing steel is one of the leading causes of damage to concrete structures worldwide. Where traditional steel reinforcement can inevitably start to rust, synthetic and basalt fibres are fundamentally unaffected by it. In this article, we explain how corrosion occurs and why fibre reinforcement prevents this specific failure mechanism.

How does corrosion occur in concrete?

Concrete normally protects embedded steel well: the highly alkaline environment (pH around 12–13) forms a thin, protective oxide layer around the steel that inhibits further rusting. This protection disappears as soon as:

Chlorides (for example from de-icing salt or seawater) penetrate through the pores of the concrete to reach the reinforcement, or

Carbonation occurs: CO₂ from the air reacts with the concrete, causing the pH to gradually fall and the protective layer around the steel to disappear.

As soon as the steel starts to rust, it expands — rust takes up a much greater volume than the original steel. That expansion pushes the surrounding concrete outward, causing cracking. Those cracks then let through even more moisture and chlorides, which accelerates the corrosion. This self-reinforcing process can ultimately lead to crumbling concrete and reduced structural strength.

Why are synthetic and basalt fibres unaffected by this?

Synthetic fibres (polypropylene, aramid, PAN) and basalt fibres are chemically inert: they do not react with oxygen, water or chlorides the way steel does. There is therefore no possible rusting process, and hence no corrosion-driven expansion and cracking.

This does not mean that fibre reinforcement can replace traditional reinforcement in every application — in heavily structural elements, steel often remains necessary for the primary load-bearing capacity. But for applications where corrosion is the greatest risk, the absence of this failure mechanism is a significant advantage.

Where does this advantage matter most?

• Damp and chloride-exposed environments: hydraulic engineering structures, outdoor floors, car parks and roads where de-icing salt is used.

• Thin concrete covers: the thinner the layer of concrete above the reinforcement, the faster moisture and chlorides reach it. Fibre reinforcement does not carry this risk.

• Ageing infrastructure: many existing concrete structures were built decades ago with less knowledge of durable concrete cover, making them extra vulnerable to corrosion damage.

• Exposed floors: where rust staining at the surface is aesthetically undesirable.

A caveat: steel fibre is not the same as steel reinforcement

It is important to distinguish between steel fibre (loose, short fibres in the mix) and traditional steel reinforcement (bars and mesh). Steel fibre, too, can show local rust staining when exposed at the surface through wear. This is usually an aesthetic issue rather than a structural one, but it is nonetheless a reason to consider synthetic or basalt fibre instead of steel fibre for certain applications — such as wet or chemically exposed environments.

In conclusion

Corrosion is one of the leading, and most preventable, causes of concrete damage. Synthetic and basalt fibres structurally rule out this specific failure mechanism, making them particularly suitable for applications in damp, chloride-exposed or hard-to-maintain environments.