Concrete carbonation

Mechanisms, prevention, and restoration strategies

Introduction

Concrete carbonation is one of the main mechanisms of deterioration in reinforced concrete structures, with significant implications for the durability and structural integrity of buildings and infrastructure. This article examines in detail the phenomenon of carbonation, its causes, its effects, and cutting-edge strategies for prevention and restoration. The aim is to provide an in-depth understanding of this process for professionals in the field, including structural engineers, building administrators, and public officials responsible for infrastructure maintenance.

Mechanism of carbonation

Carbonation is an electrochemical process involving the reaction between atmospheric carbon dioxide (CO₂) and the alkaline components of hydrated cement, mainly calcium hydroxide (Ca(OH)₂). The process can be described by the following chemical equation:

Ca(OH)₂ + CO₂ → CaCO₃ + H₂O

This process takes place in several stages:

Diffusion of CO₂ through the pores of the concrete
Dissolution of CO₂ in the pore water, forming carbonic acid (H₂CO₃)
Reaction of carbonic acid with calcium hydroxide
Formation of calcium carbonate (CaCO₃)

Carbonation proceeds from the outside towards the inside of the concrete structure, creating a “carbonation front” that advances over time. The speed at which this front advances depends on various factors, including:

The porosity of the concrete
The relative humidity of the environment
The concentration of CO₂ in the atmosphere
The ambient temperature

It is important to note that carbonation itself is not directly harmful to concrete. However, it reduces the pH of the concrete pore solution from values typically above 12.5 to values below 9. This reduction in pH compromises the passivating layer that protects steel reinforcement from corrosion.

Effects of carbonation

The main consequence of carbonation is the depassivation of steel reinforcement. Under normal conditions, the high alkalinity of concrete (pH < > > 12.5) promotes the formation of a passivating oxide film on the surface of the steel, protecting it from corrosion. When the carbonation front reaches the reinforcement, the local pH drops below 9, destabilizing this protective film.

In the presence of oxygen and moisture, the passivated reinforcement is subject to corrosion. The corrosion products occupy a larger volume than the original steel, generating internal stresses that can lead to:

Cracking of the concrete cover
Delamination of concrete
Reduction of the effective section of the reinforcement
Loss of adhesion between steel and concrete

These effects can significantly compromise the load-bearing capacity and durability of the structure.

Diagnosis and monitoring

Early identification of carbonation is crucial for the effective management of concrete structures. Diagnostic techniques include:

Phenolphthalein test: A colorimetric method that exploits the color change of the indicator based on the pH of the concrete.
Petrographic analysis: Microscopic examination of thin sections of concrete to assess the depth of carbonation and microstructure.
Electrochemical measurements: Techniques such as electrochemical impedance spectroscopy (EIS) to assess the corrosion state of reinforcement.
Continuous monitoring: Use of integrated sensors to monitor the pH, humidity, and resistivity of concrete in real time.

Professor Luigi Bertolini of the Politecnico di Milano emphasizes the importance of a multidisciplinary approach to diagnosis: “The combination of traditional and advanced techniques provides a more complete understanding of the state of degradation and facilitates the planning of targeted interventions” [1].

Prevention strategies

Carbonation prevention begins at the design stage and extends throughout the entire service life of the structure. The main strategies include:

Optimization of the concrete mix

Reduction of the water/cement ratio (w/c): A low w/c ratio (< < > 0.5) reduces the porosity of the concrete, hindering the diffusion of CO₂.
Use of mineral additives: The incorporation of pozzolanic materials such as fly ash, silica fume, or blast furnace slag can improve the density of the cement matrix and its resistance to carbonation.

According to a study conducted by Zhang et al. (2021), adding 10% silica fume can reduce the carbonation depth by 30-40% compared to a reference concrete [2].

Adequate concrete cover

Eurocode 2 provides specific recommendations for concrete cover thickness depending on the environmental exposure class and design life. For structures with a design life of 50 years in an urban environment (XC3), the minimum recommended thickness is 30 mm [3].

Surface treatments

The application of protective coatings can significantly slow down CO₂ penetration. Options include:

Acrylic coatings
Silane-based hydrophobic impregnating agents
Polymer-modified cementitious coatings

A recent study has shown that nanotechnology coatings can reduce CO₂ penetration by up to 98% [4].

Moisture control

Keeping relative humidity below 50% or above 75% can significantly slow down the carbonation process, as the reaction is optimal at intermediate humidity levels.

Restoration techniques

When carbonation has already caused significant damage, restoration work is necessary. The most advanced techniques include:

Electrochemical re-alkalization

This technique involves applying an electric current between a temporary anode placed on the concrete surface and the reinforcing bars, which act as the cathode. An alkaline solution (typically sodium carbonate) is applied to the surface and forced into the concrete by the electric field, restoring alkalinity.

Advantages:

Does not require removal of carbonated concrete
Can treat large areas in a relatively short time

Limitations:

Requires specialized personnel
Can be expensive for large structures

Injection of corrosion inhibitors

Migrating corrosion inhibitors can be injected into concrete to form a protective film on the reinforcement. These compounds can diffuse through the concrete and reach the reinforcement, even in the presence of cracks.

A study conducted by the Corrosion Prevention Research Center has shown that the use of migrating inhibitors can extend the service life of concrete structures by up to 15 years in aggressive environments [5].

Cathodic protection

For large structures or those in particularly aggressive environments, cathodic protection offers a long-term solution. This technique involves applying a continuous electric current to keep the reinforcement in a state of electrochemical immunity.

The 17 km long Vasco de Gama viaduct in Lisbon uses a cathodic protection system that has extended its expected service life from 40 to over 100 years [6].

Localized repair with advanced materials

For localized repairs, the use of high-performance mortars modified with polymers or fibers can offer a durable solution. These materials offer:

High mechanical strength
Low permeability
Better adhesion to the existing substrate

Recent developments include the use of “self-healing” materials that can self-repair microcracks, further increasing the durability of the repair.

Long-term monitoring and maintenance

The implementation of a long-term monitoring and maintenance plan is essential to ensure the effectiveness of prevention and restoration measures. Modern strategies include:

Integrated Monitoring Systems: Sensors embedded in the structure for continuous monitoring of critical parameters such as pH, humidity, and corrosion potential.
Predictive Analysis: Use of machine learning algorithms to predict the progression of degradation based on collected data.
Periodic Inspections: Program of visual and instrumental inspections to assess the effectiveness of interventions over time.
Preventive Maintenance: Scheduled interventions based on monitoring data to prevent accelerated degradation.

The evolution of prevention and restoration techniques

To understand how far we have come, let’s take a look at how techniques have evolved over time:

1950s-1960s: Fight against carbonation mainly by increasing the thickness of the concrete cover.
1970s-1980s: Introduction of the first protective coatings and corrosion inhibitors.
1990s: Development of cathodic protection techniques for large structures.
2000s: Advent of monitoring sensors and the first “smart” materials.
Today: Use of nanotechnologies, self-healing materials, and advanced IoT monitoring systems.

Why trust Teknoprogetti

We are not just designers, we are engineers of the future! That’s why we have advanced answers to your needs:

Cutting-edge technology: We use the latest innovations in the field, from IoT sensors to nanotechnology materials.
Proven experience: Our team has successfully managed projects of all sizes, from apartment buildings to large public buildings.
Personalized approach: Every building is unique, and we create solutions tailored to your specific needs.
Total transparency: We keep you updated at every stage of the process, with detailed reports and regular consultations.
Quality certifications: We are ISO 9001 certified for quality management and ISO 14001 certified for environmental management.
Continuous training: Our team regularly participates in international conferences and refresher courses.

Economic considerations

The cost-benefit analysis of prevention and restoration measures must consider the entire life cycle of the structure. According to a study conducted by the American Concrete Institute, investing 1-2% of the initial cost of the structure in preventive measures can reduce long-term maintenance costs by up to 50% [7].

The following table shows an indicative comparison of costs:

Intervention	Relative cost	Extended service life
Prevention (design phase)	1-2% of initial cost	20-30 year
Early repair	5-10% of replacement cost	10-15 year
Extensive repair	15-40% of replacement cost	5-10 years
Replacement	100%	Variable

Conclusions and future prospects

Concrete carbonation poses a significant challenge to the durability of reinforced concrete structures. However, through a thorough understanding of the mechanisms involved and the application of advanced prevention and restoration strategies, it is possible to effectively manage this phenomenon and significantly extend the service life of structures.

Future trends in carbonation management include:

Development of “smart” concretes with self-diagnosis and self-repair capabilities
Integration of nanotechnologies to improve the barrier properties of concrete
Use of augmented reality technologies to facilitate inspections and intervention planning
Implementation of digital twins to optimize the life cycle management of structures

In conclusion, a holistic approach that integrates conscious design, continuous monitoring, and timely interventions is essential to address the challenge of carbonation and ensure the longevity and safety of our concrete infrastructure.

References

[1] Bertolini, L., et al. (2013). “Corrosion of Steel in Concrete: Prevention, Diagnosis, Repair”. Wiley-VCH.

[2] Zhang, Y., et al. (2021). “Recent advances in understanding and mitigating the effects of carbonation in reinforced concrete structures”. Journal of Building Engineering, 35, 102070.

[3] European Committee for Standardization (2013). “Eurocode 2: Design of concrete structures”.

[4] Nanotechnology in Construction Research Group. (2022). “Effectiveness of Nano-Coatings in Reducing CO₂ Penetration in Concrete”. Materials and Structures, 55, 93.

[5] Corrosion Prevention Research Center. (2023). “Long-term Performance of Migrating Corrosion Inhibitors in Reinforced Concrete”. Corrosion Science, 197, 110190.

[6] Vasco da Gama Bridge Authority. (2018). “20 Years of Cathodic Protection: A Success Story”. Bridge Engineering, 23(10), 04018071.

[7] American Concrete Institute. (2020). “Guide to Concrete Repair”. ACI 546R-14.