Get crumbling, rust-stained concrete checked and repaired safely

Rust stains, cracks or pieces falling off a balcony, beam, column or car park? Compare quotes from structural engineers who survey the concrete, test it and specify the repair.

Concrete is strong when it is squeezed, and it relies on steel bars inside it to resist pulling and bending. If the concrete over the steel is thin or cracked, has slowly reacted with carbon dioxide in the air (carbonation), or has picked up salt from de-icing or sea air, moisture and air reach the steel and it rusts. Rust takes up more space than the steel it came from, so it pushes the concrete away: first cracks and rust stains, then pieces falling off, known as spalling. Left alone, the steel can weaken the beam, slab or balcony. A structural engineer surveys and tests the concrete, judges how serious it is, and specifies a repair that deals with the rusting steel, restores the concrete over it and protects it afterwards.

How a professional can help

Surveying and testing the concrete: They inspect the structure and take readings or samples. These show how thick the concrete over the steel is, how far carbonation has reached, how much salt is present and where the bars sit, so the repair rests on measurements.
Judging how safe it is today: They check how much steel has been lost and whether the balcony, beam or slab can still carry its loads. They advise on closing it off, propping it or limiting its use while repairs are arranged.
Specifying the repair: They set out how to cut out damaged concrete, treat or replace the rusting bars and apply repair mortar to restore the cover. The materials suit the exposure and the original concrete.
Protecting the repair: They specify coatings, sealers or waterproofing to slow further carbonation and keep water out, and detail drainage and falls so water doesn’t sit against the concrete.
Options for larger structures: On bigger or more exposed structures, they advise whether specialist treatments are worth considering alongside patch repairs. Some use a small electric current to stop corrosion. They also set out a long-term maintenance plan.
Reports for managers and freeholders: They produce survey reports with photographs, defect plans and priorities, which block managers, freeholders and leaseholders can use to plan and fund the work in stages.

Is this right for you?

You can see rust stains, cracks or loose concrete on a balcony, the underside of a slab, or a column.
You manage a block of flats or a car park and need a condition survey and a prioritised repair plan.
A surveyor’s or lender’s report on a purchase mentions rusting steel bars in concrete or “concrete cancer”.
A concrete lintel over a window or door is cracking, rusting or dropping.

How the project usually runs

Visual survey: The engineer looks at all accessible concrete, noting cracks, rust staining, hollow-sounding areas found by tapping, exposed bars and leaks, and marks the defects on plans or photographs.
Testing: Where needed, small areas are opened up and samples taken to measure how thick the cover is, how far carbonation has gone and how much salt is present. The results show how widespread the problem is and what is causing it.
Repair priorities: The engineer recommends repairs in order of priority, from urgent propping or making safe through to patch repairs and protective coatings, phased where the building stays in use.
Written scope and quotes: A written scope and drawings are issued so specialist concrete repair contractors can price the same work, and the engineer can help you compare their quotes.
Repair and inspection: The engineer inspects the exposed steel and prepared concrete before it is covered, and signs off the finished work for the owner’s records.

What affects the cost

How much of the structure is affected, and how deep the corrosion and carbonation go.
How many balconies, columns, beams or slab undersides need repair, and how hard they are to reach.
Propping or temporary support needed while the damaged concrete is broken out.
Whether the building must stay occupied or in use, which affects phasing and protection.
Testing needed to establish the cause, such as cover surveys, carbonation tests and salt sampling.
Whether protective coatings, waterproofing or drainage improvements are included to protect the repair.

Safety, responsibility and approvals

If a balcony or slab is cracking or shedding concrete, keep people off it until an engineer has inspected it. Many can be repaired once the damage has been assessed.
In a block of flats, the lease or title deeds usually set who arranges and pays for concrete repairs, so involve the freeholder or managing agent early.
Repair work often doesn’t need planning permission, but listed concrete buildings need consent for changes to their fabric, and Building Regulations can apply where structure is replaced or altered.
Planning, Building Regulations and heritage rules differ across England, Scotland, Wales and Northern Ireland. Structural engineers cover the whole UK.

Questions people ask

What is concrete cancer?

It is a popular name for the steel bars inside concrete rusting. Moisture and air reach the steel, the bars rust and swell, and the concrete cracks and breaks away. It isn’t a sign that the concrete itself is rotten, but the steel can lose strength, so it should be assessed rather than just patched over.

Is a rusting balcony safe to use?

Not necessarily. Rust stains, cracks or falling concrete mean the steel inside is corroding, and you can’t judge by eye how much strength has been lost. Ask an engineer to inspect it as soon as you can. They can say whether it needs temporary support or should stay out of use until it is repaired.

Why does the steel inside concrete rust?

Concrete normally protects steel, but over time it can carbonate as carbon dioxide from the air reaches the bars, and salts from de-icing, sea air or contaminated materials can attack the steel. Thin cover and cracks let water in. Repair needs to deal with the cause as well as the damage that shows.

Can concrete damage be fixed with simple patch repairs?

Sometimes, for small areas, if the surrounding concrete is sound. But if corrosion is widespread, patches can fail as new damage appears next to them. A condition survey shows whether patching is enough or a wider repair or protective system makes more sense in the long run.

Who is responsible for repairing a concrete balcony in a block of flats?

It depends on your lease or title deeds and who owns the structure. In England and Wales, the freeholder or management company is often responsible for the structure and recovers costs from leaseholders through service charges, but leases vary. A solicitor or managing agent can explain your position, and an engineer’s survey is usually the starting point.

Available across the UK

Professionals cover the whole of the UK: England, Scotland, Wales and Northern Ireland. Planning and Building Regulations differ between the four nations, and your professional will work to the rules that apply where your property is.