Home / Concrete spalling repairs
Concrete cancer doesn't stop on its own.
Spalling concrete is a symptom. The disease is corroding reinforcement inside the slab, and it keeps expanding until something removes the cause. We diagnose it properly, repair it to engineering standard, and protect it so the same section is not back in five years.
01, The mechanism
The concrete is not the problem. The steel is.
"Concrete cancer" is the common name for a straightforward process: the reinforcing steel inside a concrete element begins to corrode, and corroding steel expands.
Reinforcement sits inside concrete precisely because concrete is highly alkaline, and that alkalinity forms a passive layer around the steel that stops it rusting. Break that chemistry and the protection is gone. As the steel corrodes it can expand to several times its original volume, and because it is cast into a rigid material with nowhere to go, that expansion pushes outward. First you get hairline cracking. Then rust staining that bleeds through the surface. Then delamination, where a section of concrete separates from the substrate but has not yet fallen. Finally spalling, where the face lets go entirely.
The important point for a building owner is that this is a progressive process, not a static defect. Once the passive layer is compromised and moisture and oxygen have a path to the steel, corrosion continues. Rendering over it, patching it with a cosmetic filler or repainting the elevation does not stop it. It hides the evidence while the reinforcement underneath keeps losing section, which is why so many buildings end up paying for the same area twice.
It also becomes a safety matter well before it becomes a structural one. A spalling soffit, balcony edge or façade element above a footpath, entry or car park is a falling-object risk, and that is usually the point at which an owners corporation, facilities manager or insurer starts asking for a documented rectification plan rather than another patch.
Patching spalled concrete without treating the reinforcement is not a repair. It is a delay, and it is usually an expensive one.
Pro-Build Plus, practice principle02, Why it happens
Four causes, and Melbourne supplies most of them.
Identifying which mechanism is driving the deterioration matters, because it changes the repair specification and it changes what has to be done to stop it recurring.
Carbonation
Carbon dioxide from the air penetrates the concrete over decades and gradually neutralises its alkalinity. When that carbonation front reaches the depth of the reinforcement, the passive layer is lost and corrosion begins. This is the most common driver in older inland buildings and it is why deterioration often appears across an entire elevation of similar age at once.
Chloride attack
Airborne salt is aggressive to embedded steel and it does not need carbonation to do damage. Bayside and coastal exposure across Port Phillip means a large share of Melbourne's building stock is dealing with chloride-driven corrosion, which tends to be more localised, more severe and faster moving than carbonation.
Insufficient cover
Cover is the depth of concrete between the surface and the steel, and it is the primary defence. Where original construction left reinforcement too shallow, or where a slab edge or balcony nib was poured tight, deterioration shows up early and in predictable patterns regardless of how well the building has been maintained.
Water ingress
Failed membranes, blocked drainage, perished sealant joints and defective flashings keep concrete persistently wet, and sustained moisture accelerates every other mechanism. Concrete deterioration and façade envelope failure are very often the same problem presenting in two different places.
03, Diagnosis
Scope it properly, or price it twice.
Visible damage is almost never the full extent. Delaminated concrete can sound hollow long before it cracks, so an accurate scope depends on investigation rather than what can be seen from ground level.
Access and map the whole element
Close-up inspection of the affected elevations, soffits, balconies and structural elements, with delamination survey work to find the areas that are already compromised but have not yet spalled. This is what turns "there is a patch of damage near level four" into a defined quantity.
Establish the mechanism
Cover measurement, carbonation depth testing and chloride sampling where required, so the repair is specified against the actual cause rather than an assumption. This is the step most commonly skipped, and skipping it is why repairs fail early.
Specification and structural input
Where the deterioration affects structural capacity, or where the extent is significant, a structural engineer defines the repair specification and any propping or staging requirements before work begins.
A defined, documented package
You receive a quantified scope, a methodology, an access strategy and a programme built around the building remaining occupied. That is the document an owners corporation can put to a vote and an insurer can rely on.
04, The repair
Break out to sound concrete. Treat the steel. Reinstate. Protect.
A durable concrete repair follows a sequence, and every step in it exists because skipping it causes failure. Damaged and delaminated concrete is broken out back to sound material, and critically the excavation continues behind the reinforcement rather than stopping at the face of it. Corrosion on the back of a bar that has been sealed over will keep working, and the repair will fail from the inside.
Exposed reinforcement is then cleaned back to bright steel and treated. Where section loss is significant, supplementary or replacement reinforcement is installed to the engineer's specification. The prepared area is reinstated with a proprietary repair mortar selected to be compatible with the parent concrete, because a repair material that is substantially stronger or stiffer than what surrounds it concentrates stress at the bond line and creates the next failure.
Finally the element is protected. Depending on the mechanism identified during investigation this may include an anti-carbonation or protective coating system to slow re-carbonation and resist chloride penetration, along with rectification of the water path, drainage or membrane defect that was keeping the concrete wet. Where deterioration is widespread and advanced, longer-term options such as corrosion inhibition are assessed on their merits rather than applied by default.
Everything is documented as it goes: locations, quantities, what was found, what was treated and what was installed. That record is what gives an owner, a committee and an insurer confidence that the work was done to specification and not simply covered over.
05, Where we work
135 projects. Every one of them occupied.
Concrete rectification rarely happens in an empty building. It happens above a functioning entry, off a resident's balcony, or across a car park that has to keep operating. That constraint is our normal working condition, not an exception.
Typical elements we rectify include balcony slabs, edges and nibs, soffits and undercrofts, columns and blade walls, car park slabs and headers, planter boxes and podium structures, and precast or in-situ façade panels. Deterioration frequently appears in more than one of these at once, because they usually share an exposure condition or a water path.
Because we deliver the full remediation cycle, concrete rectification can be programmed alongside cladding removal, façade replacement or compliance audit works rather than run as a separate project with its own access hire, its own disruption and its own separate bill. On most buildings that is the difference between one access programme and two.
06, Common questions
Asked by owners, committees and facilities managers.
Next step
Get the extent measured before you price the repair.
Speak with a remediation specialist about what your building is showing. If it turns out to be isolated and early, we will tell you that too.