Why concrete spalls
Reinforced concrete relies on steel bars to take tension, and concrete around them to protect them. When moisture and air reach the steel, it rusts, and rust takes up more space than steel. That expansion pushes the concrete cover off in flakes and chunks, exposing more steel and speeding up the process. Causes include thin concrete cover, carbonation of the concrete over time, and chlorides from salt.
Where it appears
- Concrete lintels over windows and doors, see lintel replacement
- Balconies and walkways, see concrete balcony repairs
- External steps and stairs, see concrete staircases
- Garage and warehouse floors, see warehouse floor repairs
- Concrete frames and canopies on commercial and post-war buildings
Coastal locations such as Tynemouth and Whitley Bay experience salt in the air, which increases the risk of reinforcement corrosion.
Proper repair method
A lasting repair follows a set sequence. The loose and cracked concrete is broken out until sound concrete is reached, and the reinforcement is exposed fully around its circumference. Rust is removed from the steel by grit-blasting or wire brushing, and the bars are treated with a protective coating. Where the bar has lost significant section, extra reinforcement is added. The area is then rebuilt in layers with a suitable repair mortar compatible with the existing concrete. Finally, a protective coating helps resist moisture and carbon dioxide.
Why patching alone fails
Covering rusty steel with fresh mortar without cleaning and treating it leaves the corrosion process running. The patch will crack and fall off, often with new cracks forming close by. Poor repairs can also create new corrosion zones around the edges.
Structural concerns
If spalling is extensive, or in a load-bearing member such as a beam, balcony or lintel, a structural engineer should assess it. We stop and advise if we find significant section loss. Repair works might fall under Building Regulations; see GOV.UK and the Planning Portal.
Frequently asked questions
Why is the concrete flaking off with rust stains?
The reinforcing steel inside is rusting and expanding, which cracks and pushes off the concrete cover. The steel must be cleaned and protected as well as the concrete repaired.
Is spalling concrete dangerous?
It can be if it is in a structural element or overhead, since pieces may fall and the steel loses strength. Have it checked sooner rather than later.
