Corrosion
Why rust inside concrete stays hidden until it is expensive
By the CORRSENS team
Concrete protects the steel inside it, until it stops. Here is how rust starts, why you cannot see it, and why early inspection costs far less than repair.
Steel inside concrete is protected twice. The concrete covers it, and the concrete is alkaline, which keeps a thin protective film on the steel surface. As long as that film holds, the steel does not rust.
How the protection fails
Two things break the film. Chlorides, from sea salt in the air or from de-icing salts, work their way through the concrete to the steel. Carbonation, where carbon dioxide slowly reduces the concrete’s alkalinity, does the same over decades. Once the film is gone and there is moisture and oxygen, the steel starts to rust.
Why you do not see it
Rust takes up much more space than the steel it came from. It pushes outward from inside, so the first sign is often a rust-coloured stain or a fine crack along the line of a bar. By the time concrete falls away, which engineers call spalling, the steel has already lost some of its thickness.
The damage is done long before it shows.
Why early inspection matters
Repairing a small patch of rusting steel is a small job. Repairing a girder or a pier where the steel has lost section is a large one, and sometimes means closing the structure. Finding the problem early moves the work from the second kind to the first.
This is the reason CORRSENS exists. India loses about 4.3% of its GDP to corrosion every year (Nomura Research Institute, Built to Last, 2026), and a great deal of that cost starts as damage nobody could see.
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