What Happens Beneath the Plaster , and Why Paint Can Make a Difference
In a 30-year-old apartment building, the visible damage , faded walls, plaster cracks, moisture stains , is often just the tip of the iceberg. Behind these traces lies a slow, steady chemical process occurring centimeters deep: the carbonation of concrete.
It has no smell, no sound, no visible sign , but if it reaches the reinforcement, it can jeopardize the structural integrity of the entire building. External paint, in this context, is not just an aesthetic upgrade , it can act as a barrier to delay the process.
When concrete is freshly poured and removed from formwork, it has a very high pH , around 12.5–13. This alkaline environment creates a "passive" oxide film around the steel reinforcement, protecting it from rust. Like an invisible anti-corrosion shell.
Over the years, however, atmospheric CO₂ gradually penetrates the pores of the concrete. There it reacts chemically with calcium hydroxide (Ca(OH)₂) to produce calcium carbonate (CaCO₃). This reaction gradually lowers the pH. The "carbonation front" advances inward , millimeter by millimeter, year after year.
When the pH in the reinforcement zone drops below ~9, the protective film collapses. The steel begins to rust , and this rust does not stop on its own.
Rust (iron oxides) is not just a stain. It has a particularly dangerous property: its volume is up to 6 times larger than the original steel. This expansion creates enormous internal pressures within the concrete.
Initially, micro-cracks appear on the surface, easily ignored. Gradually, the concrete cover "breaks" , and in some cases pieces fall off, exposing the corroded reinforcement. This is not a "paint problem" or a "plaster problem" , it is a structural problem that requires engineering assessment.
The visible signs , brown stains on the surface, swollen plaster around beams, exposed rebar at balcony corners , mean the damage is already underway. They cannot be "fixed" with a coat of paint.
Paint does not stop carbonation , but it can significantly slow it down, provided it is applied before the damage reaches the reinforcement. It works as a dual barrier:
The resin structure creates a film that partially blocks the diffusion of carbon dioxide into the concrete. This slowdown can add years of life to the reinforcement , especially if the carbonation front is still far from the steel. Effectiveness depends on the resin type and, above all, the film thickness.
Moisture is the catalyst for corrosion. Without moisture, even at low pH, steel does not actively corrode. A water-repellent system contributes indirectly to protection by keeping the concrete mass drier.
The rate of carbonation is not constant. It depends on interacting factors , some controllable, some not.
Concrete quality is the most important factor: dense concrete with a low water/cement ratio resists much longer. Unfortunately, in constructions of the '70s–'90s the quality was not always high.
Cover thickness (the distance from the surface to the first reinforcement) plays a decisive role. Current regulations require at least 25–30 mm, but in older buildings we often find 10–15 mm , inadequate in urban environments.
Ambient humidity has an inverse effect: the maximum carbonation rate occurs at 50–70% relative humidity , typical of the Greek climate. And in urban centres, the increased CO₂ concentration from traffic further accelerates the process.
Carbonation progresses proportionally to the square root of time (√t). This means that external protection in a building's early years has disproportionately high value , while delaying repairs costs exponentially more.
Not all paints work equally as a CO₂ barrier. Elastomeric systems , due to their greater film thickness (200–400 μm) , create a thicker barrier and bridge micro-cracks. This means fewer openings through which CO₂ can penetrate.
Silicone-based paints offer a different advantage: excellent balance between CO₂ barrier and breathability. This is important because a system that completely blocks diffusion may trap moisture , which, as we saw, accelerates corrosion.
Conversely, simple thin-film acrylics (80–120 μm) offer less resistance to CO₂ diffusion. In buildings where carbonation is already approaching the reinforcement, this difference can translate into years of service life.
If carbonation has already reached the reinforcement and corrosion is underway, paint alone solves nothing. The signs are clear: exposed rebar, disintegrated concrete, crumbled cover.
In this case, the proper approach includes: removal of damaged concrete, cleaning the reinforcement of rust, application of anti-corrosion protection, restoration with specialized repair mortars, and finally application of a protective coating system.
The order of work is critical , painting over a damaged substrate does not hide the problem, it merely delays its reappearance slightly.
Carbonation is not theory , it is a reality that affects every building with a reinforced concrete structure. The right façade paint, if applied in time and with technical criteria, can add years of life to the reinforcement.
The choice of system is not just aesthetic. In a 30-year-old building, it can be part of the strategy for protecting the structural frame. And that is not solved by a nice colour shade , it is solved by the right chemistry, the right thickness, and the right timing.
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