What is Chloride extraction?
Chloride extraction, or desalination, is an advanced electrochemical process designed to remove harmful chloride ions from contaminated concrete, safeguarding the structure against corrosion.
What is Chloride extraction used for?
How does Chloride extraction work?
Chlorides destroy the protective passivation layer around steel reinforcement, leading to corrosion; the main cause of failure in reinforced concrete structures. When chloride attack is identified, chloride extraction is a vital strategy to decrease corrosion risks and boost the durability of concrete structures.
Chloride extraction in concrete structures employs an electrochemical method called ion migration to drive chlorides out of the concrete, away from the steel reinforcement and into and electrolyte solution. This process works by placing an anode in electrolyte media atop the concrete surface and applying a high current density, typically between 1 to 2 A/m². To prevent potential damage, the current density should not surpass 10 A/m² of the reinforcement surface area.
Chloride extraction effectively reduces chloride levels within 6 to 10 weeks, significantly diminishing the risk of steel corrosion. After treatment, the system, including the anode and chloride-rich electrolyte, is removed, completing the desalination process. This method stands out for its ability to rapidly and efficiently lower chloride ion concentration in concrete, offering a proactive solution to extend the lifespan of infrastructure by mitigating corrosion risks.
How do I carry out and control Chloride extraction for concrete structures?
Several standardised methodologies have been produced for chloride extraction, both by government bodies such SHRP-S-347 in the USA or private companies like Norcure in Northern Europe. These standards provide similar but varied test procedures. In the UK BS EN 14038-2:2020 is used and includes the following steps:
Evaluate the concrete structure for chloride-induced corrosion and determine the necessity for chloride extraction.
Conduct initial chloride profile testing to understand the extent of chloride contamination.
Install a temporary anode on the concrete surface. Titanium mesh is commonly used due to its effectiveness and durability.
Apply a conductive layer between the anode and concrete surface. This layer can be a sprayed conductive mortar or a pre-wetted geotextile soaked in electrolyte solution.
Use a calcium hydroxide (Ca(OH)2) solution as the electrolyte, considering the specifics of the project and potential effects on the concrete.
Connect the anode to the positive terminal and the steel reinforcement (cathode) to the negative terminal of a DC power supply.
Apply a direct current at a density recommended by the guidelines, typically between 1 to 5 A/m2 of concrete surface. The exact value depends on the specific conditions and objectives of the treatment.
The voltage should not exceed safety limits, generally around 40 to 50 V to avoid hydrogen evolution and safety hazards. Hydrogen gas evolution could lead to explosions or hydrogen-induced embrittlement in steel reinforcement, compromising the structure's integrity.
Regularly monitor the system voltage, current density, and electrolyte condition to ensure effective chloride extraction.
Adjust the current density and treatment duration based on the monitoring data to optimize chloride removal while minimizing potential negative effects on the concrete and reinforcement.
After the treatment, conduct chloride profiling again to assess the effectiveness of chloride removal.
Evaluate the structural integrity and any potential side effects, such as changes in bond strength between the steel reinforcement and concrete.
Monitor the corrosion potential and corrosion rate to assess the passivation of the reinforcement.
Remove the anode and conductive layer from the concrete surface.
Repair any damages or alterations made to the structure during the setup and execution of the extraction process.
Implement any necessary protective measures to prevent future chloride ingress such as surface coatings or surface impregnation.
The effectiveness of the treatment relies on careful planning, execution, and post-treatment evaluation to ensure long-term durability improvements to the structure. Quality assurance should include the continuous recording of voltage and current during the entire extraction process.
Concrete samples should be taken regularly from the concrete surface and analysed to establish the progress in the chloride removal. The electrolyte should be regularly checked and replenished when necessary.
What equipment and expertise are required for Chloride extraction of concrete structures?
A DC power supply capable of delivering a controlled and stable direct current at the required voltage (within the 40 to 50 V safety limits) and current density specifications.
Anode material typically, titanium mesh coated with a conductive oxide to serve as the temporary external anode, chosen for its durability and effectiveness in the chloride extraction process.
A conductive medium, such as a sprayed conductive mortar or a pre-wetted geotextile soaked in electrolyte, to ensure good electrical contact between the anode and the concrete surface
An electrolyte solution such calcium hydroxide (Ca(OH)2).
A cathode connection between the steel reinforcement (acting as the cathode) to the negative terminal of the power supply.
A digital multi-meter to measuring the voltage across the concrete structure and the current flowing through the system to ensure they remain within the specified limits.
Data loggers to record the voltage, current, and other parameters over time, allowing for the analysis of the process's progress and efficiency.
A pH meter to monitor the pH level of the electrolyte solution, ensuring it remains within a range that is effective for chloride extraction without causing damage to the concrete or reinforcement.
A chloride ion concentration meter, used to measure the concentration of chloride ions in the electrolyte solution periodically, indicating the effectiveness of the chloride extraction process.
Temperature sensors to monitor the temperature of the electrolyte solution and the concrete surface, ensuring the process does not induce thermal stress or damage.
An electrical resistance meter to assess the electrical resistance of the concrete, which can provide information about the moisture content and the effectiveness of the conductive layer between the anode and the concrete.
Hydrogen gas detectors, to ensure safety by detecting any potentially dangerous accumulation of hydrogen gas produced during the extraction process.
Automatic voltage and current controllers to adjust the voltage and current automatically based on the feedback from the monitoring devices to maintain optimal conditions for extraction.
Safety Equipment such as ventilation to prevent hydrogen gas accumulation, protective gear for operators, and equipment for detecting and mitigating flammable gases.
The installation and operation of chloride extraction equipment is an extremely specialised task requiring significant expertise that may not be available to the majority of projects. Experts in the specific field are required to ensure the success of the operation which may not be available for the extended time window required for the process.
What are the advantages of Chloride extraction for concrete structures?
By removing chloride ions, a primary cause of corrosion in reinforced concrete, chloride extraction significantly extends the service life of concrete structures.
Although initially costly, it can be more economical over the long term by reducing the need for frequent repairs and maintenance due to corrosion.
Chloride extraction is a non-destructive rehabilitation technique that does not require extensive structural alterations or removal of large sections of concrete.
Reduces the need for new construction materials and the associated environmental impact by preserving existing structures.
What are the disadvantages of Chloride extraction for concrete structures?
The setup and operation of the chloride extraction process can be expensive, especially for large structures, due to the cost of materials and equipment.
Successful implementation demands a high level of technical knowledge and expertise, which may not be readily available.
The treatment can take several weeks to months to complete, during which time parts of the structure may need to be taken out of service.
Incorrect application of the process can potentially damage the concrete or the reinforcement, especially if the voltage or current is not carefully controlled.
The effectiveness can be reduced in structures with very dense concrete or where the chloride contamination is deeply ingrained.
What are the limitations of Chloride extraction for concrete structures?
The effectiveness of chloride extraction diminishes as the depth of chloride penetration increases, making it less effective for deeply contaminated structures.
Variations in the quality and composition of the concrete can affect the efficiency of the process, with poorer quality concrete posing challenges to effective chloride removal.
Complex structural geometries can lead to uneven extraction rates and make it difficult to achieve uniform chloride removal across the structure.
While chloride extraction can significantly reduce chloride levels, it may not completely eliminate the risk of future corrosion unless combined with ongoing maintenance and protection measures.
The process involves handling electrical equipment and potentially hazardous materials, requiring strict adherence to safety protocols to protect workers and the environment.
Ancillary information
Service disruption: No
Preliminary works: Yes
Posterior works: Yes
Time consumption: Medium (one day)
Cost: High