1. Introduction
Reinforced concrete is one of the most widely used construction materials in modern buildings and infrastructure. Its combination of compressive strength, durability and compatibility with steel reinforcement makes it suitable for foundations, columns, beams, slabs, balconies, bridges, car parks and other structural elements. However, reinforced concrete is not immune to deterioration. One of the most important mechanisms that can reduce the durability of reinforced concrete is carbonation.
Carbonation is a chemical process in which carbon dioxide (CO₂) from the atmosphere penetrates into concrete and reacts with alkaline compounds within the cement paste. As carbonation progresses through the concrete, it reduces the material's alkalinity. When the carbonation front reaches the embedded steel reinforcement, the protective alkaline environment around the reinforcement can be lost. This may allow corrosion of the steel to begin when sufficient moisture and oxygen are present.
Anti-carbonation coatings are protective surface treatments designed to reduce the penetration of carbon dioxide and other potentially harmful substances into concrete. They form a protective barrier on the concrete surface while allowing the substrate to maintain an appropriate degree of vapour permeability, depending on the coating system.
The use of anti-carbonation coatings is an important part of concrete repair and preventative maintenance. By slowing carbonation and limiting the entry of aggressive agents, these coatings can help preserve the protective environment surrounding reinforcement, reduce the risk of corrosion-related damage and extend the service life of concrete structures.
This document explains what anti-carbonation coatings are, why they are necessary, how they work and how they contribute to protecting the structural integrity and durability of buildings.
2. What Is Concrete Carbonation?
Concrete normally has a highly alkaline internal environment, largely due to compounds produced during cement hydration. This high alkalinity creates a passive layer around embedded steel reinforcement. The passive layer helps protect the steel from corrosion.
Carbon dioxide in the atmosphere can gradually enter concrete through its pores and capillary network. It then reacts with alkaline components in the cement paste. One important reaction involves carbon dioxide reacting with calcium hydroxide to form calcium carbonate.
A simplified representation is:
Ca(OH)₂ + CO₂ → CaCO₃ + H₂O
As this reaction occurs, the alkalinity of the concrete decreases. The carbonation process generally progresses inward from the exposed concrete surface, creating what is known as a carbonation front.
If the carbonation front reaches the level of the reinforcement, the steel may lose its passive protection. Where moisture and oxygen conditions are suitable, corrosion can subsequently develop.
Therefore, carbonation itself does not necessarily mean that concrete immediately loses its structural capacity. Rather, it creates conditions that can initiate reinforcement corrosion, which can then lead to significant deterioration.
3. What Are Anti-Carbonation Coatings?
Anti-carbonation coatings are specially formulated protective coatings applied to concrete surfaces to reduce the penetration of carbon dioxide and other environmental contaminants.
They are generally applied to exposed concrete elements such as:
- External walls
- Concrete façades
- Columns and beams
- Balconies
- Car parks
- Bridge structures
- Concrete soffits
- Retaining structures
- Industrial buildings
The coating acts as a protective layer between the concrete substrate and the external environment.
A suitable anti-carbonation coating normally has a high resistance to carbon dioxide diffusion. At the same time, the system may be designed to allow water vapour to escape from the concrete. This balance is important because preventing CO₂ ingress while allowing appropriate vapour movement can help reduce the risk of moisture becoming trapped within the concrete or coating system.
Anti-carbonation coatings can be based on different binder technologies, including acrylic, elastomeric and other polymer-modified systems. The appropriate product depends on exposure conditions, substrate condition, required durability, appearance and manufacturer specifications.
4. Why Are Anti-Carbonation Coatings Necessary?
4.1 Protection Against Reinforcement Corrosion
The principal purpose of an anti-carbonation coating is to reduce the risk of reinforcement corrosion caused by carbonation.
Steel reinforcement embedded in sound, highly alkaline concrete is normally protected by passivation. Carbonation lowers the alkalinity of the concrete. Once the carbonation front reaches the reinforcement, this protective condition can be compromised.
Corrosion causes the steel to expand. The corrosion products occupy a greater volume than the original steel, generating internal pressure within the surrounding concrete. This can result in:
- Cracking
- Delamination
- Spalling
- Loss of reinforcement cross-section
- Exposure of additional steel
- Progressive deterioration of the concrete
An anti-carbonation coating helps slow the carbonation process and therefore helps preserve the alkaline protection around the reinforcement for longer.
4.2 Extending Service Life
Buildings and concrete structures are designed to remain functional for many years. However, environmental exposure can progressively reduce their durability.
Applying an appropriate protective coating can reduce the rate at which damaging agents enter the concrete. This can delay deterioration and extend the period before major repair or replacement is required.
This is particularly valuable for existing buildings where the concrete remains structurally sound but requires additional protection to maintain its long-term performance.
4.3 Reducing Maintenance Requirements
Concrete deterioration can result in expensive maintenance operations. Once reinforcement corrosion has caused significant cracking or spalling, repairs may involve breaking out damaged concrete, cleaning or replacing reinforcement, applying corrosion protection, reinstating the concrete and applying a protective finish.
Preventative protection can reduce the likelihood or severity of these problems.
Anti-carbonation coatings therefore form part of a planned preventative maintenance strategy, rather than being considered solely as an aesthetic finish.
5. How Anti-Carbonation Coatings Work
The effectiveness of an anti-carbonation coating is based primarily on its ability to restrict the movement of carbon dioxide through the coating.
The concrete surface contains pores and capillaries through which gases can move. Without suitable protection, atmospheric CO₂ can gradually penetrate the concrete.
A correctly specified coating creates a relatively resistant surface layer. This increases the effective diffusion resistance encountered by CO₂, slowing the rate at which carbonation progresses into the concrete.
The coating can also provide additional environmental protection against factors such as:
- Rainwater
- Atmospheric pollutants
- Wind-driven moisture
- Certain salts and contaminants
- Ultraviolet radiation, depending on the formulation
The coating therefore acts as part of the building's external protective envelope.
However, an anti-carbonation coating is not a substitute for repairing defective concrete. If the substrate contains active cracks, delamination, exposed reinforcement or significant corrosion damage, those defects normally need to be addressed before the coating system is applied.
6. Importance of Surface Preparation
The performance of an anti-carbonation coating depends heavily on the condition and preparation of the concrete substrate.
Before application, the surface should generally be:
- Sound
- Clean
- Free from loose or friable material
- Free from excessive dust
- Free from oil, grease and other contaminants
- Appropriately repaired
- Sufficiently dry or within the moisture limits specified by the coating manufacturer
Existing cracks may require assessment and appropriate treatment. Areas of delaminated or loose concrete should normally be removed and repaired using a compatible repair system.
Where reinforcement is exposed and corrosion has occurred, the reinforcement should be assessed and treated as part of the concrete repair process.
Poor preparation can result in inadequate adhesion, blistering, premature coating failure and reduced carbonation resistance.
7. Application of Anti-Carbonation Coatings
Application methods vary according to the product, but anti-carbonation systems may be applied using brushes, rollers or spray equipment.
A typical system can consist of:
- Substrate preparation
- Concrete repairs where required
- Primer or preparatory coat
- First protective coating layer
- Second or subsequent coating layer
- Inspection and quality control
The number of coats and required dry-film thickness should be determined according to the product specification and environmental exposure.
Application conditions are also important. Temperature, humidity, substrate moisture, rainfall and dew point can affect coating performance. Coatings should therefore be applied within the manufacturer's specified environmental limits.
Quality control should include checking surface preparation, coverage, coating thickness, curing conditions and visual uniformity.
8. How Anti-Carbonation Coatings Help the Structure
8.1 Maintaining Reinforcement Protection
The most significant structural benefit is the preservation of the concrete's ability to protect embedded reinforcement.
By slowing CO₂ penetration, an anti-carbonation coating can delay the loss of alkalinity at reinforcement level. This can help maintain the passive condition of the steel for a longer period.
8.2 Reducing Corrosion Risk
Once reinforcement corrosion develops, it can initiate a cycle of cracking and concrete deterioration. Limiting carbonation can reduce one of the factors responsible for initiating this corrosion process.
This does not mean that an anti-carbonation coating eliminates all corrosion risks. Reinforcement can also corrode because of chloride contamination, cracking, moisture exposure and other mechanisms. Nevertheless, controlling carbonation can be an important part of the overall durability strategy.
8.3 Preventing Concrete Spalling
Corrosion-induced expansion can generate tensile stresses within the concrete cover. Continued corrosion can cause cracking and eventually spalling.
By reducing the likelihood of carbonation-induced reinforcement corrosion, protective coatings can help reduce the risk of this deterioration mechanism.
8.4 Preserving Structural Capacity
Reinforcement contributes significantly to the tensile and flexural capacity of reinforced concrete. Severe corrosion can reduce the cross-sectional area of reinforcement and weaken the bond between the steel and concrete.
By helping to delay corrosion, an anti-carbonation coating can contribute indirectly to preserving the structural performance of reinforced concrete elements.
It is important to recognise that coatings do not increase the original structural capacity of a concrete member. Their principal role is durability protection and deterioration control.
9. Economic and Environmental Benefits
The use of anti-carbonation coatings can provide economic benefits by reducing the frequency and severity of concrete repairs.
Major concrete repair projects can require:
- Specialist labour
- Scaffolding or access equipment
- Concrete removal
- Reinforcement treatment
- Repair mortars
- Waste disposal
- Reinstatement
- Recoating
- Potential disruption to building occupants
Preventative protection can help delay these interventions.
There can also be environmental benefits. Extending the service life of an existing structure reduces the need for premature demolition and reconstruction. This can reduce demand for new concrete, reinforcement, transportation and associated construction activities.
For large buildings and infrastructure assets, extending service life can therefore contribute to more sustainable asset management.
10. Limitations and Design Considerations
Anti-carbonation coatings should not be regarded as a universal solution for every form of concrete deterioration.
Before specifying a coating, the condition of the concrete should be assessed. Relevant factors can include carbonation depth, chloride contamination, cracking, reinforcement condition, moisture exposure and previous repairs.
If reinforcement corrosion is already advanced, simply applying a coating over the surface may not resolve the underlying problem. Structural or concrete repairs may be required first.
The coating must also be compatible with the substrate and environmental exposure. External concrete exposed to severe weather, freeze-thaw conditions, pollution or high levels of moisture may require a specialised protective system.
Correct specification, substrate preparation and application are therefore essential.
11. Conclusion
Anti-carbonation coatings are an important protective technology for reinforced concrete buildings and structures. Their primary function is to reduce the penetration of atmospheric carbon dioxide into concrete and slow the progression of carbonation.
Carbonation becomes a structural durability concern when it reaches embedded reinforcement and reduces the alkaline conditions that normally protect the steel from corrosion. Corrosion can subsequently cause reinforcement expansion, cracking, delamination and spalling, potentially reducing the long-term performance of reinforced concrete elements.
By providing a high-resistance protective barrier against carbon dioxide, an appropriately designed anti-carbonation coating can delay the carbonation process and help preserve the protective environment surrounding reinforcement. This can reduce the risk of carbonation-induced corrosion and contribute to a longer service life.
The benefits extend beyond corrosion prevention. A well-maintained coating system can help protect concrete from environmental exposure, reduce maintenance requirements, minimise the frequency of major repairs and contribute to the sustainable management of existing buildings.
However, the coating should always be considered as part of a wider concrete durability strategy. Correct assessment of the existing structure, suitable repair of defects, appropriate surface preparation, compatible materials and controlled application are essential to achieve effective long-term performance.
In conclusion, anti-carbonation coatings help protect the durability rather than directly increasing the structural strength of a building. Their value lies in slowing deterioration and protecting the reinforced concrete from one of the important mechanisms that can lead to reinforcement corrosion. When correctly specified and maintained, they can help preserve concrete structures, extend service life and reduce the long-term cost and disruption associated with major repair works.