What is concrete cancer?

Concrete cancer is the common name for corrosion of the steel reinforcement inside concrete. As the steel rusts, it swells and pushes the surrounding concrete apart. The result is cracking, rust stains, and eventually pieces of concrete breaking away. The concrete itself isn’t the problem. The rusting steel is.

Why the steel starts to rust

Fresh concrete is highly alkaline, and that alkalinity protects the steel inside it. A thin passive layer forms on the surface of the bars and stops them rusting, even though concrete is never completely dry. Concrete cancer starts when that protection is lost. There are two main ways this happens.

Carbonation

Carbon dioxide from the air slowly works its way into concrete and reacts with it, lowering its alkalinity. This front moves inward over years. Once it reaches the depth of the steel, the bars lose their protection and can start to corrode when moisture and oxygen are present. Dense, well-compacted concrete carbonates very slowly. Poor quality or porous concrete can carbonate through its cover in a fraction of the time.

Chlorides

Chlorides break down the protective layer on steel even when the concrete is still alkaline. They come from salt spray in coastal areas, salty groundwater, pool water, and in older buildings sometimes from additives used in the original mix. Buildings close to the bay or the ocean are more exposed, which is why coastal apartment blocks often show concrete cancer earlier than buildings inland.

Why it gets worse over time

Rust takes up several times the volume of the steel it forms from. That growth puts the concrete around the bar under tension it was never designed to carry, so it cracks. The cracks let in more water and air, which speeds up the corrosion, which causes more cracking. Left alone, the concrete covering the steel eventually separates and falls away, a stage called spalling. By then the bars may have lost a meaningful amount of their thickness.

What makes a building more likely to get it

• Not enough cover. The concrete between the steel and the surface is the steel’s protection. Where bars sit too close to the surface, carbonation and chlorides reach them sooner.
• Water getting in. Failed waterproofing on balconies, roofs, and planter boxes keeps the concrete damp, or lets it wet and dry over and over. Both let the steel corrode faster. Steel in fully saturated concrete corrodes more slowly, because oxygen can’t easily reach it.
• Cracks left open. Every open crack is a shortcut to the steel.
• Age. Many buildings from the mid twentieth century were built with less cover and lower strength concrete than current standards call for.
• Exposure. Salt air, tidal zones, car parks, and pool surrounds are all harder on concrete than a dry internal slab.

Where concrete cancer usually appears

The most common places are balcony slab edges and soffits, car park ceilings and columns, concrete facades and window hoods, retaining walls, pool surrounds, and bridges. Anywhere that is regularly wet, exposed to salt, or built with thin cover is worth checking.

Is concrete cancer dangerous?

It can be. In the early stages the damage is mostly cosmetic, but it does not stop on its own. As it progresses, loose concrete can fall from balconies, facades, and ceilings, which is a safety risk for anyone below. In advanced cases the loss of steel can reduce the strength of the element, and a structural engineer should assess it. The earlier it is repaired, the smaller and cheaper the repair usually is.

Can concrete cancer be fixed?

Yes. A proper repair removes the damaged concrete, treats or replaces the corroded steel, and rebuilds the section with a repair mortar. Patching over the top without dealing with the steel rarely lasts. Even a well-done patch can speed up corrosion in the steel just outside it, known as the halo or incipient anode effect, which is why contractors can add galvanic anodes or corrosion inhibitors around a repair.