Rebar corrosion and concrete spalling, explained

The short version

Reinforced concrete works because steel bar inside it carries tension the concrete can't. The steel only lasts because new concrete is strongly alkaline, which keeps a protective film on the metal. Restoration problems begin when that protection breaks down and the bar starts to rust. Rust takes up far more room than the steel it came from, and that expansion splits the concrete and pushes the surface off. The brown stain, the drummy patch, the chunk on the sidewalk — all of it is the rebar expanding.

How the protection breaks down

Two fronts attack the steel on the BC coast, and they often work together.

Carbonation is slow chemistry. Carbon dioxide from the air reacts with the concrete and gradually lowers its pH, advancing inward from the surface as a front. When that front reaches the depth of the bar, the alkaline protection is gone and corrosion can start anywhere along the steel. Older concrete with thin cover loses this race sooner.

Chloride is the faster killer here. Salt — carried in the air off the water in West Vancouver and the downtown waterfront, or tracked into parkades on winter tires — travels through the porous concrete to the steel. It doesn't need to lower the pH; above a threshold concentration at the bar it breaks the passive film in spots and drives intense, localized pitting corrosion. This is why buildings near the ocean and salted parkade slabs deteriorate ahead of sheltered inland structures.

Why the concrete cracks off

Here's the part that surprises people. The load on the building usually isn't what cracks the concrete. The rust does. As steel corrodes it converts to iron oxides that occupy several times the original volume. Inside the tight confinement of the concrete, that expansion generates pressure the cover can't resist. First you get a crack tracking the line of the bar, then delamination — the cover separates but hangs on — and finally a spall, where the piece drops and leaves rusted steel exposed to the weather. Now water and chloride reach the bar directly and the whole cycle speeds up.

The field signs, in the order you usually see them

• Rust-coloured staining bleeding down a concrete face or balcony soffit — the first visible signal, often years before a chunk falls.
• Map cracking or fine cracks running parallel to where rebar sits below the surface.
• A hollow or drummy sound when the concrete is tapped — the cover has delaminated from the steel but has not dropped yet.
• Bulging or a visible bump in the concrete surface as the expanding rust pushes outward.
• Loose fragments, exposed rusted bar, or a fresh spall scar where a piece has already let go.
• Efflorescence — white salt deposits — at crack mouths, a sign water is moving through the concrete.

On residential towers this shows up first at balcony slab edges and undersides. On parkades it's the suspended slab soffit and the column bases near where cars track salt and water. If you're a strata council member doing a walk-around, rust staining and drummy concrete are the two things worth noting and reporting — you don't need instruments to catch them early.

What actually causes it on a given building

• Carbonation: atmospheric CO₂ lowers the concrete pH from the surface inward until it reaches the bar and strips its passive film.
• Chloride ingress: sea-salt aerosol off the Strait of Georgia and de-icing salt in parkades migrate to the steel and break down protection locally, even at high pH.
• Low concrete cover: bars cast too close to the surface — common in older construction — give the corrosion front a shorter trip.
• Cracking from any cause: shrinkage, overload, or thermal movement opens a fast path for water and chloride straight to the steel.
• Standing water and failed membranes: balcony and parkade decks that no longer drain keep the concrete saturated, accelerating everything.

How it gets confirmed before anyone repairs

A surface that looks bad and a surface that is structurally compromised are not the same thing, and you can't tell them apart by eye. That's why a restoration starts with a condition assessment, not a quote. An engineer sounds the concrete to map the delaminated areas, runs half-cell potential readings to find where the steel is actively corroding, measures carbonation depth on a fresh core, and pulls chloride samples at the level of the bar. The results define how much concrete has to come out and how the repair is designed under CSA S448.1, Repair of reinforced concrete in buildings and parking structures. The rainscreen requirements that came out of BC's leaky-condo era — with an estimated $4 billion in damage across more than 900 buildings, per BC's Homeowner Protection Office — were meant to keep water out in the first place; on older buildings that predate them, corrosion is often well underway by the time anyone looks.