Concrete spalling is the breakdown of concrete caused by the steel reinforcement inside it corroding. Water and chlorides reach the steel through cracks or failed waterproofing. Rust occupies up to seven times the volume of the steel it replaces, so it pushes the surrounding concrete outward until it cracks, bulges and falls away. In Sydney apartment buildings it appears first on balcony edges, soffits and exposed slab nosings, because those surfaces take the most water and have the least cover over the steel.
How to recognise it early
Rust staining on a soffit, a hairline crack running parallel to a balcony edge, or a hollow sound when the concrete is tapped all point to spalling that has started under the surface. Concrete rarely fails without warning. By the time a piece falls, the corrosion has usually been running for years.
Why patching alone fails
Filling the hole without treating the steel and the water path guarantees a repeat. Repairing spalling properly means breaking back to sound concrete behind the corroded bar, cleaning or replacing the steel, applying a corrosion inhibitor, reinstating with a repair mortar matched to the parent concrete, and then fixing whatever let the water in. That last step is the one most often skipped.
The waterproofing question
Balcony membranes have a service life. When a membrane fails, water tracks to the slab edge and the steel corrodes from the outside in. A spalling repair on a balcony that keeps its failed membrane is money spent twice. Any scope worth approving addresses the membrane and the drainage at the same time as the concrete.
Who arranges the work
On a strata scheme, common property spalling is the owners corporation's responsibility under section 106 of the Strata Schemes Management Act 2015. See who pays for common property repairs.
Who can carry it out
If the building is class 2, the work must be done by a registered building practitioner who can make compliance declarations. See what class 2 registration means. Vertex holds registration BUP0002293 and carries out spalling repair, waterproofing and facade restoration across Sydney. Our strata and remedial page sets out the full scope.