Concrete carbonation is one of the main degradation mechanisms affecting reinforced concrete structures exposed to the atmosphere. It is a slow but progressive chemical-physical process that gradually reduces the protective capacity of the concrete cover and triggers corrosion of the steel reinforcement.
The phenomenon is particularly relevant in the maintenance of infrastructure such as bridges, viaducts, multi-storey car parks and industrial structures, where concrete durability is a fundamental performance requirement — and where timely intervention can make the difference between controlled repair and advanced structural deterioration.
What is concrete carbonation?
Hardened concrete naturally maintains a strongly alkaline environment, with a pH between 12 and 13, which provides passive protection to the steel reinforcement. Over time, carbon dioxide (CO₂) present in the air penetrates the porous matrix of the material and reacts with calcium hydroxides, progressively lowering the pH of the system — a process known as carbonation.
When the carbonation front reaches the depth of the reinforcement, the passivation condition is lost and the steel becomes vulnerable to corrosion, particularly in the presence of moisture and oxygen. From that point on, the degradation process accelerates significantly.
Causes that accelerate the process
The rate of carbonation depends on several factors, related both to material quality and to environmental exposure conditions:
- Porosity and compaction of the concrete: porous or poorly compacted concrete offers reduced resistance to CO₂ diffusion.
- Water/cement ratio: high values increase matrix porosity and accelerate penetration of the carbonation front.
- Presence of cracks or microcracks: these provide preferential pathways for CO₂ and other aggressive agents.
- Exposure to wet-dry cycles: these promote the penetration of aggressive agents and accelerate degradation processes.
- Urban or industrial environmental conditions: higher concentrations of CO₂ in anthropised environments significantly accelerate the process.
Exposed and inadequately protected structures become progressively more vulnerable over time, regardless of the quality of the original concrete.
Effects on reinforced concrete structures
The main effect of carbonation is corrosion of the steel reinforcement. This process triggers a series of consequences that directly affect the durability and safety of the structure:
- formation of oxidation products with volume increase, generating internal pressure within the concrete
- progressive spalling of the concrete cover
- surface cracking and scaling
- loss of bond between steel and concrete
- reduction of structural performance over the long term
This is a typical degradation phenomenon in infrastructure exposed to atmospheric agents, which if not managed promptly can compromise the safety and functionality of the structure.
Diagnosis and assessment of carbonation
Assessing the extent of carbonation is a fundamental step in reinforced concrete maintenance. Before any repair intervention, it is essential to accurately determine the level of degradation and the extent of the carbonation front.
The main investigation techniques include:
- Phenolphthalein indicator tests on surfaces or extracted cores: the reagent changes colour as a function of pH, allowing the carbonation front to be precisely visualised.
- Measurement of carbonation front depth on fresh sections.
- Cover depth survey using a covermeter, to assess the residual safety margin.
- Assessment of the condition of the reinforcement, to verify the presence and extent of ongoing corrosion.
These analyses allow the level of degradation to be accurately defined and repair interventions to be planned with the most appropriate system for the specific situation.
Anti-carbonation protection systems
Prevention is based on the use of surface protection systems capable of reducing CO₂ diffusion into the concrete, without impeding water vapour permeability — a fundamental requirement to avoid condensation and delamination problems.
Among the most widely adopted solutions are elastomeric coatings and specific protective systems for cementitious surfaces, designed to achieve an optimal balance between CO₂ barrier performance and vapour permeability. These systems comply with the performance requirements set out in EN 1504-2, the European standard defining requirements for products used in the protection of concrete structures.
The anti-carbonation treatment cycle: operational phases
A typical protection and repair cycle consists of several sequential operational phases, each of which must be completed and verified before proceeding to the next:
- Surface preparation and cleaning: removal of dirt, residues and loose material to ensure adhesion of the protective system.
- Removal of degraded sections and volumetric reinstatement: elimination of carbonated or detached concrete and reconstruction of the section with certified structural mortars.
- Treatment of corroded reinforcement: application of corrosion inhibitors or passivating agents to exposed reinforcement where required.
- Application of consolidating primer: to increase the surface cohesion of the substrate and improve adhesion of subsequent layers.
- Application of the anti-carbonation protective coating: application of the elastomeric system or specific protective cycle, with in-process thickness control.
- Surface finish with protective and aesthetic function.
The aim of the system is to create a continuous barrier that limits the penetration of aggressive agents and extends the service life of the structure, restoring its original performance.
Fields of application
Anti-carbonation protection systems are used in all reinforced concrete structures exposed to significant environmental conditions. The most common applications include:
- Bridges and viaducts: structures with high atmospheric exposure, subject to wet-dry cycles and elevated corrosivity conditions.
- Tunnels and road infrastructure: environments with high concentrations of CO₂ and chemical agents from vehicular traffic.
- Multi-storey car parks: structures particularly vulnerable due to the combination of atmospheric exposure, water infiltration and chemical agents (de-icing salts, fuels).
- Industrial buildings: structures exposed to aggressive environments with high concentrations of CO₂ and chemical substances.
- Exposed concrete facades: where material durability is both a structural and an aesthetic requirement.
In these contexts, material durability is an essential design parameter, and the choice of the most appropriate protection system must be assessed on a case-by-case basis by a specialist applicator.
An integrated approach to durability
Managing carbonation-related degradation is not limited to surface protection alone, but forms part of a broader programme of planned reinforced concrete maintenance. Intervening only at the surface without addressing the causes and effects of ongoing degradation means postponing the problem, not solving it.
The most effective interventions combine:
- Preliminary condition assessment, to accurately define the level of degradation and plan the most appropriate treatment cycle.
- Repair of deteriorated sections, using structural mortars conforming to EN 1504.
- Reinforcement protection, with certified inhibitors and passivating agents.
- Application of EN 1504-2 certified anti-carbonation systems, with documented thickness and performance control.
This approach allows the original performance of the structure to be restored and degradation processes to be significantly slowed, reducing maintenance costs over the structure’s service life.
Why work with a specialist applicator
Correctly managing concrete carbonation requires technical expertise that goes well beyond the simple application of a product. Preliminary diagnosis, selection of the most appropriate protection system, substrate preparation and in-process thickness control are all factors that directly influence the final performance and durability of the intervention.
FES Global Group manages the entire process: from preliminary technical assessment and repair of deteriorated sections, through certified application of anti-carbonation systems to final documentation in accordance with EN 1504. With over twenty years of experience on bridges, viaducts, multi-storey car parks and industrial structures across Europe and internationally, FES has the expertise and certifications to address any level of degradation — from the most straightforward cases to the most structurally complex situations.