Concrete Cancer

Steel rusts, and rust takes up more room than steel.

Concrete cancer isn't the concrete failing — it's the reinforcing bar inside it corroding. As the steel rusts it expands to several times its original volume, and the concrete around it has nowhere to go. First a hairline crack, then a rust stain, then the concrete face lifts off and drops.

It doesn't stop on its own. Once the cover is cracked, more water and air reach the steel, the corrosion accelerates, and the affected area spreads along the bar in both directions.

What you're looking at

• Rust staining — brown weeping down a soffit, slab edge or balcony underside

• Cracking that follows a straight line — tracking the reinforcement below rather than wandering like a shrinkage crack

• Drummy concrete — sounds hollow under a tap, meaning the face has already debonded and is held on by nothing

• Spalled sections — concrete already off, bar exposed and actively rusting

Why it's happening 

Two causes, and Sydney has both.

Carbonation. CO₂ from the air works into the concrete over decades and neutralises the alkalinity that was protecting the steel. Once that protective layer is gone, any moisture reaching the bar starts corrosion. This is why it shows up on buildings from the 60s through the 80s almost on schedule.

Chlorides. Salt air on coastal and harbourside buildings drives chloride into the concrete, which attacks the steel directly. Eastern suburbs, Northern Beaches and harbour-front stock get this badly, and it's worse where the concrete cover over the bar was thin to begin with.

Balconies, slab edges, soffits, awnings and parapets go first — they're exposed on more faces and usually have the least cover.