How Carbonation Affects Concrete Structures and How to Detect It
Concrete is the backbone of modern construction. From bridges and tunnels to commercial buildings and homes, its strength and durability are unparalleled. However, despite its robustness, concrete isn’t immune to deterioration. One silent but serious threat is carbonation—a naturally occurring chemical process that can compromise the structural integrity of reinforced concrete over time.
Let’s explore how carbonation works, why it matters, and how to detect it early to prevent costly damage.
What Is Carbonation in Concrete?
Carbonation is a chemical reaction that occurs when carbon dioxide (CO₂) from the air penetrates the concrete and reacts with calcium hydroxide (Ca(OH)₂) in the cement paste. This reaction produces calcium carbonate (CaCO₃), which reduces the pH of the concrete.
Under normal conditions, concrete has a high pH of around 12.5–13.5, which protects embedded steel reinforcement from corrosion. However, as carbonation lowers the pH, the protective oxide layer on the steel deteriorates. This makes the steel susceptible to corrosion—a key trigger for structural degradation.
Why Carbonation Is a Problem
While carbonation itself doesn’t directly weaken concrete, the resulting corrosion of embedded steel leads to:
Expansion of corroded steel, which creates internal pressure
Cracking and spalling (flaking) of concrete
Reduced load-bearing capacity
Accelerated structural failure if left untreated
Over time, these issues increase maintenance costs and pose serious safety risks—particularly in aging infrastructure and poorly maintained buildings.
Where and When Does It Occur?
Carbonation progresses from the concrete surface inward. It’s influenced by several environmental and material factors:
High CO₂ exposure, especially in urban or industrial areas
Porous or poorly compacted concrete
Low cement content or inadequate curing
Thin concrete cover over reinforcement
Dry environments, which favor CO₂ diffusion
Older buildings constructed before modern building standards are especially vulnerable. For this reason, regular inspections are essential—especially in critical infrastructure and heritage structures.
How to Detect Carbonation in Concrete
Early detection is the key to effective intervention. The most widely used method to detect carbonation is the Phenolphthalein Test:
A small core sample or surface section is exposed.
Phenolphthalein solution is sprayed onto the surface.
Areas with alkaline (non-carbonated) concrete turn pink or purple, while carbonated areas remain colorless.
This visual method is simple and cost-effective, but for deeper analysis, carbonation depth measurement using drilled samples and laboratory testing is recommended.
Laboratories like Test Concrete Laboratory offer comprehensive carbonation testing, using accurate instrumentation to assess pH levels and carbonation fronts. This data informs repair strategies and helps extend the service life of the structure.
Mitigating and Repairing Carbonation Damage
Once detected, the extent of carbonation damage guides the next steps. Common mitigation strategies include:
- Surface Coatings: Anti-carbonation coatings can act as a barrier to CO₂ and moisture.
- Cathodic Protection Systems: These reduce or prevent corrosion in reinforced steel.
- Concrete Replacement or Repair: Severely damaged sections can be removed and recast with protective additives.
- Carbonation-resistant Concrete Mix: Use of pozzolanic materials and proper curing improve durability in new builds.
Combining preventive design with ongoing monitoring creates a robust defense against carbonation-related deterioration.
Safeguard Your Structures Today
Carbonation may be invisible at first, but its effects can be devastating if ignored. Fortunately, with accurate detection and expert analysis, you can identify risks early and protect your assets effectively.
At Test Concrete Laboratory, we offer UKAS-accredited carbonation testing services designed to pinpoint vulnerabilities and guide cost-effective repairs. Whether you’re maintaining residential, commercial, or civil infrastructure, our expertise ensures your structure remains strong, safe, and compliant.
Book a consultation today at TestConcrete.co.uk and take proactive steps to extend the life of your concrete structures.
