What is Concrete re-alkalisation?
Concrete re-alkalisation is a cutting-edge, electrochemical technique for reversing steel reinforcement corrosion within carbonated concrete. This sophisticated process rejuvenates the alkaline conditions necessary to restore the steel's protective passive layer, effectively stopping corrosion and extending the lifespan of concrete structures.
By applying a direct current between the steel (acting as the cathode) and an external anode immersed in a sodium carbonate solution, it facilitates the migration of hydroxyl ions deep into the concrete. This operation not only re-establishes the steel's passivity but also significantly enhances structural durability, offering a proactive solution to combat carbonation-induced corrosion.
What is Concrete re-alkalisation used for?
Reinforcement corrosion due to carbonation.
How does Concrete re-alkalisation work?
Concrete re-alkalisation is a meticulously engineered process designed to halt and reverse the corrosion of steel within carbonated concrete, effectively extending the structural life of buildings and bridges. The electro chemical process begins by establishing a direct electrical circuit between the cathodic steel reinforcement and an anode, typically surrounded by an alkaline solution such as sodium carbonate.
The current drives the alkaline solution into the concrete matrix, targeting the area surrounding the steel reinforcement. This current facilitates the movement of hydroxyl ions (OH-) towards the steel surface, where they interact with it to rebuild the protective, passive oxide layer that had been compromised by the carbonation process.
The depth and effectiveness of the re-alkalisation depends on mainly the current density applied and the duration of the treatment. A current density of 1 to 2 A/m2 is applied for a period that could range from several days to a few weeks, depending on the extent of carbonation and the volume of concrete being treated. Throughout this period, the system is carefully monitored to ensure optimal ion penetration and the uniform restoration of alkalinity across the treated area.
As the alkaline front progresses through the concrete, it systematically neutralizes the acidic conditions caused by carbonation, thereby reinstating the concrete’s original alkaline environment. This newly restored alkaline environment is essential for reactivating the steel's passivity, effectively halting ongoing corrosion and preventing future corrosion initiation. Upon completion of the treatment, the anode and any temporary setup are removed.
This method not only addresses the immediate corrosion issues but also significantly contributes to the long-term durability and integrity of the concrete structure, ensuring its continued function and safety.
Concrete re-alkalisation is particularly beneficial for structures at risk of or showing early signs of carbonation-induced corrosion, offering a proactive measure to maintain structural health without invasive reconstruction efforts. Its implementation requires specialized knowledge and equipment, making it a sophisticated solution for extending the lifespan of valuable concrete infrastructure.
How do I repair concrete structures using Concrete re-alkalisation?
A simple step by step procedure to carry out concrete re-alkalisation to BS EN 14038 and BS EN 1504 includes the following:
Evaluate the concrete structure to identify areas affected by carbonation and requiring re-alkalisation using non-destructive testing methods to assess the depth of carbonation and the extent of steel reinforcement corrosion.
Clean the concrete surface to remove loose material, contaminants, and any coatings that may hinder the re-alkalisation process.
Expose the reinforcement and clean it to achieve effective electrical conductivity.
Place an anode, typically a mesh or a conductive fabric, soaked in an alkaline solution (such as sodium carbonate) on the concrete surface. Ensure the anode system covers all areas identified for treatment.
Connect the anode system to a direct current power source, with the steel reinforcement serving as the cathode. Ensure proper electrical connections and safety measures are in place.
Apply a direct current (typically 1 to 2 A/m²) between the anode and the cathode. The current density and duration of application depend on the extent of carbonation and the size of the area being treated. Monitoring equipment may be used to ensure the process proceeds as expected.
Regularly monitor the voltage and current throughout the re-alkalisation process. Adjust the power supply as necessary to maintain effective treatment across the entire surface.
Once the treatment is complete (usually a few days to a few weeks, based on the initial assessment), disconnect the power supply. Carefully remove the anode system and clean the surface of the concrete.
Perform a final evaluation of the concrete to confirm the effectiveness of the re-alkalisation process. This may include testing for pH levels at the surface and near the reinforcement, as well as a visual inspection for any signs of remaining or potential corrosion.
Apply a protective coating or sealant to the treated surface to prevent future carbonation and corrosion. This step is critical for prolonging the effects of re-alkalisation and ensuring the durability of the concrete structure.
Document all details of the re-alkalisation process, including the areas treated, the current applied, and the duration of treatment. Plan for periodic inspections to monitor the long-term condition of the structure and the effectiveness of the re-alkalisation.
What equipment and expertise are required for repairing concrete structures using Concrete re-alkalisation?
To effectively carry out the re-alkalization of a concrete structure, a comprehensive set of equipment, expertise, and a detailed understanding of the process are essential.
Equipment Required:
An externally mounted anode mesh that will be placed on the concrete surface. The mesh should be compatible with the electrolyte used for the treatment.
A power source capable of delivering a controlled direct current to the anode system for the duration of the treatment process. According to CEN/TS 14038-1:2004, the current density should not exceed 4 A/m².
Suitable cables and connectors for establishing electrical connections between the anode system, the direct current source, and the steel reinforcement within the concrete.
An alkaline electrolyte solution, typically, a sodium or potassium carbonate to facilitate the migration of alkali ions into the concrete, thus increasing its alkalinity.
Monitoring and measurement equipment for assessing the condition of the structure before treatment, including devices for rebound hammer tests, ultrasonic pulse velocity tests, and equipment for measuring half-cell potentials, electrical resistivity, and chloride content.
What are the advantages of repairing concrete structures using Concrete re-alkalisation?
Concrete re-alkalisation effectively restores the concrete's original alkaline environment, crucial for protecting the steel reinforcement from further corrosion. By increasing the pH of the concrete, it helps in re-establishing the passive layer around the steel reinforcement.
Re-alkalisation can be a completely non-destructive test method with very little preparatory or supplementary works required.
By addressing the root cause of corrosion—carbonation-induced pH reduction—re-alkalisation offers a more permanent solution compared to surface-level repairs.
For historical or architecturally significant structures, re-alkalisation does not alter the appearance or cause damage to the exposed surfaces, allowing the preservation of the original aesthetics while extending the structure's life.
Concrete re-alkalization can be carried out while the building is still functional.
What are the disadvantages of repairing concrete structures using Concrete re-alkalisation?
Re-alkalisation effectiveness is reduced in concrete that contains certain types of reinforcement, such as prestressed steel, or where epoxy-coated or galvanized reinforcements are used.
This test method is ineffective when the corrosion is caused by chloride contamination.
Increasing the alkalinity of the concrete through re-alkalisation may trigger AAR in structures that contain reactive aggregates, leading to further structural issues.
What are the limitations of repairing concrete structures using Concrete re-alkalisation?
Re-alkalisation is most effective when applied before the onset of significant corrosion and does not repair the damage sustained. Cracks etc must be addressed before re-alkalisation can be effectively applied.
The success of the re-alkalisation process can be influenced by external factors, including the presence of atmospheric pollutants. Structures in environments with high levels of CO2 may require additional protective measures to ensure the longevity of the repair.
Ancillary information
Service disruption: No
Preliminary works: Yes
Posterior works: Yes
Time consumption: Medium
Cost: High