PERTHConcrete spalling: when the steel inside rusts
Concrete spalling (concrete cancer) in coastal Perth: how chloride and carbonation rust the reinforcement, why balconies are worst, AS 3600 exposure
Concrete looks permanent. It is not. When the steel reinforcement inside it starts to rust, the concrete cracks apart from the inside, and in coastal Perth that process has a head start. It is called concrete spalling, or concrete cancer, and it is a material deterioration defect under AS 4349.1 that turns major once the reinforcement is exposed.
What is actually happening
Reinforced concrete is a partnership: concrete takes the compression, embedded steel takes the tension. The steel only survives because the concrete around it is highly alkaline, which keeps it passive and rust-free. That protection depends on the steel being buried under enough sound concrete, a dimension the standards call cover.
Two things break the protection. Carbonation, where carbon dioxide slowly reacts into the concrete and lowers its alkalinity, and chloride attack, where salt penetrates to the steel. Once either reaches the reinforcement, the steel begins to rust. Rust occupies far more volume than the steel it replaces, so it pushes outward with enormous force and cracks the cover off from within. The concrete falls away, or spalls, exposing rusting steel behind it.
Why Perth's coast makes it worse
Chloride is the accelerant, and Perth's coast supplies it in the air. AS 3600, the concrete structures standard, sets durability by exposure classification, and coastal structures sit in the more severe classes precisely because salt-laden air drives chloride into concrete. A balcony a few streets from the beach lives in a harsher environment than the same balcony inland, and it ages accordingly.
This is the same coastal band that softens roof battens through delignification and corrodes tie-down straps. Salt does not attack one thing at a time.
Where it shows up
Suspended concrete elements are the ones to watch: balconies, suspended slabs, concrete cantilevers, and the edges of elevated floors. Older concrete infrastructure, retaining walls, and anything cast with thin cover or poor compaction is vulnerable. The early signs are rust staining bleeding through the surface, fine cracking that follows the line of the reinforcement below, and drummy or hollow-sounding patches where the cover has debonded but not yet fallen.
The visible spall is the late stage. By the time a chunk has dropped off a balcony soffit, the steel behind it has been rusting for a long time, and there is usually more of the same nearby that has not let go yet.
When it is a major structural defect
Under AS 4349.1 clause 1.4.10, spalling is a major defect once it threatens safety, utility or leads to further deterioration. Reinforcement that has lost section to rust has lost strength, and on a suspended balcony that is a safety question, not a cosmetic one. Spalling is also progressive by nature: once the cover is breached the process feeds itself, so it does not stabilise on its own the way some cracking does.
What an inspection can tell you
We report the visible evidence: rust staining, crack patterns following reinforcement, spalled areas, and drummy cover found by sounding. What a visual inspection under AS 4349.1 cannot do is measure how far carbonation or chloride has penetrated, or how much steel section is left, because that needs testing. Where the evidence points to active spalling on a structural element, the honest report says the extent needs an engineer's investigation, and explains why. How it gets repaired, and by whom, is covered in our guide on rectifying concrete spalling.
On a strata or apartment inspection this matters twice over, because a spalling balcony or common-property slab can arrive as a special levy addressed to every owner.
The chemistry, in plain terms
Concrete spalling, sometimes called concrete cancer, is worth understanding at the level of what is physically happening, because the mechanism explains both why it is progressive and why the coast makes it worse. Reinforced concrete is concrete with steel bars inside it for tensile strength, and the concrete is supposed to protect that steel from moisture. When moisture and, crucially, salt or other aggressive agents reach the steel, it begins to rust. Rust occupies a much greater volume than the steel it forms from, so as the reinforcement corrodes it expands, and that expansion pushes outward against the surrounding concrete with real force. Eventually the concrete cover cracks and breaks away, exposing more steel to moisture, which accelerates the corrosion, which breaks away more concrete. It is a self-reinforcing cycle, which is why spalling gets worse over time rather than stabilising, and why catching it early matters.
The visible result is concrete that is cracking, flaking, or breaking away, often with rust staining, and sometimes with the corroded reinforcing bar itself exposed. But the visible damage is the late stage of a process that has been going on inside the concrete for some time, which is part of why spalling can be more advanced than it first appears, and why an area showing early signs deserves a closer look at what is happening around it.
Why Perth's coast accelerates it
Salt is the great accelerant of concrete spalling, and Perth's long coastline means a great deal of the city's building stock sits in a salt-laden environment. Salt-bearing air carries chloride to concrete surfaces, and chloride is particularly effective at breaking down the protection that concrete normally gives to the steel inside it and at driving corrosion once it reaches the reinforcement. A concrete element that might last a very long time inland can spall far sooner in a coastal setting, simply because the salt is constantly at work. Balconies, suspended slabs, concrete elements exposed to the weather, and any reinforced concrete near the coast are the places where the risk is highest.
This coastal dimension is why concrete spalling is a particular concern in Perth's coastal apartment and multi-storey stock, much of which is reinforced concrete standing directly in the salt environment. In those buildings, spalling of the structural concrete is one of the failures that can become a major and expensive repair, and in a strata context an expensive shared one. The combination of reinforced concrete, coastal exposure, and age is the recipe, and a great deal of Perth's coastal building stock has all three.
When it is major, and what an inspection can see
Not all spalling is a major structural defect. Minor surface spalling of a non-structural element is a maintenance and durability issue. Spalling that is affecting a structural element, a beam, a suspended slab, a column, or that is extensive and progressive, is a major defect, because it is reducing the capacity of the concrete to do its structural job and it will get worse. The judgement about which side of the line a given instance falls on turns on where the spalling is, how advanced it is, and whether the affected element is carrying load, and it is exactly the kind of assessment a buyer needs made rather than left to them to guess.
A visual inspection can identify the signs of spalling, the cracking, flaking, rust staining, and exposed reinforcement, and can assess how extensive and how advanced the visible damage is, and read the setting, coastal exposure, age, the type of element affected, to weight the risk. What it cannot do from a visual inspection alone is determine the full extent of the corrosion inside the concrete or the precise remaining capacity of an affected structural element, because that can require specialist investigation and testing. So where spalling affects structural elements or appears extensive, the report identifies it as a major defect and recommends the appropriate structural or specialist assessment, rather than either dismissing it or overstating what a visual look can conclude. The point is to catch it, weight it honestly, and direct the buyer to the closer look when the situation genuinely calls for one.
Common questions
What is concrete cancer?
Concrete spalling, or concrete cancer, is where moisture and salt reach the steel reinforcement inside concrete. The steel rusts and expands, cracking the concrete off from within. AS 3600 governs concrete durability and cover.
Why is it worse near the Perth coast?
Chloride from salt-laden air is the accelerant. AS 3600 places coastal structures in more severe exposure classifications because salt drives chloride into the concrete to the reinforcement.
Is concrete spalling a major structural defect?
Under AS 4349.1 clause 1.4.10 it is major once it threatens safety, utility or further deterioration. Reinforcement that has lost section to rust has lost strength, which on a suspended balcony is a safety matter.
This comes up on every apartment & strata inspection and pre-purchase building inspection we carry out.
Read next: material deterioration: the defect Perth's coast accelerates and fixing concrete spalling: trades, methods and AS 3600.
Sources
Read rather than recalled. Summarised here; the documents themselves are Crown copyright and are linked rather than reproduced.