What Is Concrete Cancer? Causes, Signs and How It Is Repaired

Concrete cancer is the common name for the corrosion of steel reinforcement inside a concrete structure, which causes the surrounding concrete to crack, spall and eventually break away. It is one of the most widespread causes of deterioration on UK reinforced concrete buildings, bridges, car parks and infrastructure.

This article explains what concrete cancer is, why it happens, how to recognise it, whether it can be repaired, and when a structure needs specialist attention.

What causes concrete cancer?

Reinforced concrete works because two very different materials complement each other. Concrete is strong in compression but weak in tension. Steel reinforcement bars embedded within the concrete carry the tensile loads. In normal conditions, the alkaline environment of the concrete surrounding the steel forms a protective passive layer that stops the steel from corroding.

Concrete cancer starts when that protective environment breaks down. There are two main causes. The first is carbonation, where carbon dioxide from the atmosphere gradually reacts with the concrete, lowering its pH over decades until the protective layer around the steel is lost. The second is chloride ingress, most commonly from de-icing salts on roads and car parks, from marine environments, or from chlorides originally present in the concrete mix.

Once the protective layer is compromised, the steel starts to rust. Rust occupies a larger volume than the original steel, up to seven times as much. That expansion generates pressure inside the concrete that eventually cracks and spalls the surrounding material away, exposing more of the reinforcement to attack. Once the process starts, it accelerates unless the cause is addressed.

What are the signs of concrete cancer?

Concrete cancer usually shows itself through a recognisable set of visible symptoms:

• Rust staining on the concrete surface, often appearing as brown or orange discolouration along cracks or below exposed reinforcement
• Fine cracking following the line of embedded reinforcement, usually parallel to the surface rather than random
• Spalling, where sections of concrete break away from the surface, sometimes exposing the reinforcement beneath
• Delamination, where the outer layer of concrete becomes hollow-sounding when tapped, indicating that the concrete has debonded from the substrate even though it has not yet fallen away
• Bulging or displacement of the concrete surface where corrosion products have built up beneath

These visible signs are often accompanied by hidden deterioration that a proper survey identifies. Areas that look sound from the outside may be extensively delaminated beneath the surface, and areas showing early rust staining may indicate much more advanced corrosion within the structure.

Where is concrete cancer most commonly found?

Certain UK building types and locations are particularly prone to concrete cancer. Multi-storey car parks are one of the most affected asset types, driven by chlorides from de-icing salt carried in by vehicles. Marine and coastal structures suffer from salt spray. Bridges and highway structures are exposed to both de-icing salt and traffic-driven wear. Buildings from the 1960s and 1970s, when construction quality control was less consistent than today, commonly show deterioration in balconies, façades and exposed structural elements.

Residential buildings with concrete balconies, walkways and external staircases are frequently affected, particularly on estates from the post-war to 1980s eras. In each case, the underlying process is the same: reinforcement corrosion driving concrete damage from the inside out.

Can concrete cancer be repaired?

Yes, in almost all cases concrete cancer can be repaired successfully, provided the repair addresses the underlying cause rather than only the visible damage. A cosmetic patch repair that ignores the corrosion mechanism will fail within a few years, often causing corrosion to accelerate in areas adjacent to the patch through what specialists call the incipient anode effect.

A proper repair follows a defined sequence. First, all delaminated and damaged concrete is removed to expose the affected reinforcement. Second, the reinforcement is cleaned, treated or replaced depending on its condition. Third, the concrete is reinstated using an approved repair mortar matched to the exposure environment and structural requirements. Fourth, where the wider structure is at risk of continuing corrosion, protection systems are installed to prevent recurrence. This may include cathodic protection, anti-carbonation coatings, or corrosion inhibitors, depending on the situation.

The extent of repair required depends on how far the deterioration has progressed. A structure identified early may need only localised patch repair. A structure where deterioration is widespread may need a phased repair programme across the affected elements. In either case, the starting point is a proper condition survey rather than a decision made from visible damage alone.

How much does concrete cancer repair cost?

Repair cost varies significantly depending on the extent of deterioration, the access required, the size and type of structure and the specification of the repair. Localised repair on a small residential balcony is a very different scale of project to full refurbishment of a multi-storey car park deck. Cost is also affected by whether the works can be delivered around continued occupation, and whether temporary safety measures need to be installed before repair works begin.

The most important cost-related decision is whether to commission a proper condition survey before committing to repair works. Repair specifications written from a survey typically avoid the cost escalations that come from unforeseen conditions surfacing on site once works are underway. A survey that costs a few thousand pounds can save tens of thousands in avoided rework and scope changes.

When to seek professional help

Concrete cancer is not something to diagnose or repair without specialist input. If your building shows any of the signs described above, particularly rust staining, cracking along reinforcement lines, or spalling of the concrete surface, the right first step is an independent concrete condition survey.

A survey identifies the extent of the damage, the underlying cause, and the prioritised repair scope, and provides the basis for either direct appointment of a repair contractor or competitive tender of the works. Where visible damage suggests immediate safety concerns, particularly on balconies, façades or overhead elements, emergency make-safe works may be needed before the longer-term repair is planned.

Closing

Concrete cancer is manageable when identified early and repaired properly. The critical decisions are made at the diagnosis stage, not the repair stage. A structure that gets a proper condition survey, an accurate specification and a repair delivered by a specialist contractor can be given decades of additional service life at a fraction of the cost of replacement.

If you would like an initial conversation about a concrete condition survey on your building, get in touch with the CRS team.