Concrete spalling in Melbourne: what causes it, and how to spot it early.

The short version: spalling is what happens when the steel reinforcement inside concrete starts to rust. Rust takes up more room than steel, so as it grows it pushes the concrete off from the inside — the staining, cracking and falling chunks you can see are the late stage of a process that's been going for a while.

What's actually happening

Reinforced concrete relies on the alkaline concrete around the steel to keep it from corroding. Two things break that protection down. Carbonation — carbon dioxide from the air slowly reacting into the concrete — lowers the alkalinity over years until the steel is no longer protected. And chlorides — salt — punch through faster, which is why buildings near Port Phillip Bay and the coast tend to show spalling sooner. Add moisture and oxygen through cracks or perished coatings and the steel rusts. Because rust occupies several times the volume of the original steel, it expands and forces off the concrete "cover" over the bar. That's spalling.

The early signs worth acting on

• Rust staining — brown/orange bleeding through the surface, often before anything has cracked. This is the cheapest stage to catch it.
• Cracking along the steel — fine cracks that run in straight lines following the reinforcement below.
• Drummy concrete — tap it and it sounds hollow; the cover has already debonded even if it hasn't fallen yet.
• Flakes and "blows" — small pieces letting go on balcony edges, soffits, slab edges and the underside of structures.

Where we see it most on Melbourne buildings: balcony edges and soffits, exposed slab edges, plant rooms and rooftops, and car park structures. Anywhere the concrete is exposed, older, or has been getting wet.

Spalling vs delamination — what’s the difference?

These get used interchangeably and they aren’t the same thing. Delamination is the concrete separating internally along a plane — usually at the level of the reinforcement — while the surface is still in place. Tap it and it sounds hollow or “drummy”, but nothing has fallen. Spalling is the next stage: the delaminated cover actually breaks away and leaves an open patch with the bar exposed.

The practical point is that delamination is the stage you want to find things at. A drummy area is already failed concrete — it just hasn’t let go yet. A survey that only records visible spalls will underestimate the real extent of the problem, often by a wide margin, which is why the repair area on site is regularly larger than the one on the photos.

How it’s properly assessed

A visual look tells you where it has already failed. Working out how far it actually extends takes a bit more:

• Drummy survey — tapping or chain-dragging the surface to map delaminated areas that haven’t spalled yet. Low-tech, and still the most useful single test.
• Cover meter — locates the reinforcement and measures how much concrete cover sits over it. Shallow cover is usually why one elevation is failing and another isn’t.
• Carbonation depth — a phenolphthalein indicator on a fresh break shows how far the carbonation front has moved in, and whether it has reached the steel.
• Chloride sampling — drilled dust samples tested for chloride content, relevant on bayside and coastal buildings where salt is the driver.
• Half-cell potential survey — maps where the steel is actively corroding, including under concrete that still looks and sounds sound.

On a high-rise façade this is exactly the kind of survey rope access suits, because the crew can work the elevation methodically rather than sampling only what a lift can reach. It usually forms part of a wider façade inspection report with a photographic defect register.

Why catching it early matters

Spalling is progressive — it never stops on its own, and the longer it runs the more steel section is lost and the bigger the repair. There’s also a safety angle: spalled concrete falling from height is a genuine hazard to people below, which is why councils and owners corporations take loose concrete on a façade seriously. Acting at the staining stage is a small repair; acting after a balcony edge has let go is a much larger one.

How it’s repaired — by rope access

For localised and edge spalling we repair it on rope, without a scaffold program. The sequence is the same one used at ground level, delivered at height:

1. Mark and cut the perimeter. The repair area is set out to include the drummy concrete found in the survey, not just the visible spall, and saw-cut to a clean edge so the reinstatement isn’t feathered to nothing.
2. Break out to sound concrete. Including behind the bar — the steel has to be exposed all the way round, or the corrosion simply continues on the hidden face.
3. Clean and assess the steel. Abrasive cleaning back to bright metal, then a judgement on section loss. Badly reduced bars are supplemented or replaced rather than coated over.
4. Protect. A corrosion-inhibiting primer, and on larger or chloride-driven repairs, sacrificial galvanic anodes set into the patch.
5. Reinstate and coat. Polymer-modified repair mortar built up in layers and cured, then an anti-carbonation coating over the wider elevation to slow the next round.

That’s our concrete spalling repair service, and it pairs naturally with broader façade repairs when the cause was water getting in.

The patch-repair trap worth knowing about

There’s a counter-intuitive problem with spot repairs that catches a lot of buildings out. When you put fresh, highly alkaline mortar into a patch, the repaired section becomes electrochemically different from the older carbonated or chloride-affected concrete immediately around it. That difference can drive new corrosion in a ring just outside the patch you’ve only recently paid for — the “incipient anode” or halo effect.

It’s the reason a building can have a repair done and see fresh cracking appear around the edges of it a year or two later, and it’s why sacrificial anodes and a whole-elevation coating strategy are worth discussing rather than treating each spall as an isolated patch. A repair scope built one spall at a time, with no view of the elevation as a system, tends to come back.

What drives the cost of a repair

We quote scope by scope, but the variables that move the number are consistent:

• Extent — and specifically the delaminated area, not the visible one. This is the single biggest driver and the one most often underestimated before a drummy survey.
• Depth and steel condition — a shallow cover repair is quick; a bar with real section loss needs supplementing and takes far longer.
• Access — rope access removes the scaffold or swing-stage line item on scattered high-level repairs, which is usually where the saving sits. Compare the access methods here.
• Cause — chloride-driven spalling on a bayside building needs a different, more involved strategy than straightforward carbonation inland.
• Coating and protection — whether you treat the spalls only, or protect the whole elevation so the cycle slows down.

Preventing the next round

Because water is so often the trigger, spalling rarely travels alone — it’s usually worth checking the water-ingress paths and perished joint sealants at the same time. Keeping water out of the façade is the cheapest corrosion protection there is. Beyond that: get an anti-carbonation coating onto exposed elevations when they’re next repainted, and put the building on a maintenance cycle so the next signs get caught at the staining stage rather than the falling-concrete stage.