Concrete structures are crucial in our infrastructure, but their durability can be compromised by corrosion, especially of steel reinforcement. Cathodic protection (CP) systems are used to address this. This article explores why monitoring concrete corrosion, especially with CP systems, is vital for infrastructure longevity and safety.

Understanding Corrosion in Concrete

Corrosion of steel reinforcement in concrete occurs when chloride ions penetrate the concrete and come into contact with the embedded steel, initiating electrochemical reactions that lead to the formation of rust. This process not only compromises the structural integrity of the concrete but also results in aesthetic deterioration and potential safety hazards.

The Role of Cathodic Protection Systems

Cathodic protection systems are widely recognized as an effective means of mitigating corrosion in concrete structures. These systems work by either imposing a direct current to polarize the steel reinforcement to a more negative potential (impressed current CP) or by providing sacrificial anodes that corrode preferentially to protect the steel (galvanic CP). By altering the electrochemical environment at the steel surface, CP systems effectively arrest the corrosion process and extend the service life of concrete structures.

Common Types of Cathodic protection

The reinforcing steel is electrically connected to another metal that becomes the anode with or without the application of an external power supply. The cathodic protection systems that work in the absence of an external power supply and in the presence of a less noble metal (like zinc) to act as anode are referred to as sacrificial passive systems. In case of an impressed current cathodic protection, an external power supply is used to force a small amount of electric current through the reinforcing steel to counteract the flow of current caused by the corrosion process. A metal, like platinum, serves as anode which corrodes at a very slow rate. Cathodic protection is used to protect almost any type of reinforced concrete structure, including horizontal slabs, walls, towers, beams, columns and foundations. However, this method has the following limitations:

• Cathodic protection does not replace corroded steel.
• Impressed current cathodic protection systems are not recommended for general usage on prestressed concrete structures because hydrogen produced can make the high-strength steels brittle in nature.
• Passive sacrificial systems can be used for post- tensioned structures after detailed corrosion analysis.
• Electrical continuity of the reinforcing steel and ionic conductivity of concrete must be confirmed during system installation.

Importance of Monitoring Corrosion

• Early Detection
  Chloride ions are one of the primary causes of corrosion in reinforced concrete structures. By monitoring chloride ingress, any increase in chloride concentration within the concrete can be detected early on. This early detection can help prevent or mitigate corrosion damage to the reinforcement steel. even in the presence of CP systems. Despite the protective measures afforded by CP, localised areas of corrosion can still occur due to factors such as coating damage or insufficient current distribution. Timely detection enables prompt intervention to address these issues before they escalate into larger, more costly problems.
• Performance Evaluation 
  Continuous monitoring provides valuable data on the performance of CP systems, ensuring that they are functioning as intended. By regularly assessing parameters such as current output, potential measurements, and corrosion rates, stakeholders can identify any deviations from expected performance and take corrective actions as necessary to optimise CP system efficiency.
• Maintenance Planning
  Monitoring corrosion facilitates informed decision-making regarding maintenance and repair strategies. By understanding the extent and distribution of corrosion within the concrete structure, stakeholders can prioritise maintenance activities, allocate resources effectively, and minimise downtime, thereby maximising the cost-effectiveness of infrastructure management.
• Compliance and Assurance
  Regular corrosion monitoring helps ensure compliance with regulatory standards and industry best practices. By demonstrating a proactive approach to corrosion management through comprehensive monitoring programs, stakeholders can instill confidence in the integrity and safety of concrete structures, fostering trust among regulatory authorities, investors, and the public.
• Optimisation of Cathodic Protection System
  Cathodic protection systems work by providing a negative electrical charge to the reinforcing steel, thus preventing it from corroding. Monitoring chloride ingress helps in optimising the operation of cathodic protection systems. By understanding the rate of chloride ingress, adjustments can be made to the cathodic protection system to ensure that it provides adequate protection against corrosion.
• Assessment of Structural Health
  Monitoring chloride ingress provides valuable information about the overall health of the concrete structure. Excessive chloride ingress indicates a higher risk of corrosion, which can compromise the structural integrity of the building over time. Regular monitoring allows for timely maintenance and repair activities to be carried out, ensuring the longevity and safety of the structure.
• Cost Savings
  Early detection of chloride ingress and corrosion allows for proactive maintenance and repair strategies to be implemented, which can ultimately lead to cost savings. By addressing corrosion issues early on, the need for more extensive and expensive repairs or even structural replacement can be minimised.

Methods of Corrosion Ingress Monitoring

The most effective method of measuring ingress of corrosion by chlorides is via a multi-anode corrosion ladder probe, that allows for measurements of chlorides at set depths between the rebar and the cover face of the concrete and provides valuable insights into the condition of the structure and the effectiveness of CP systems, enabling informed decision-making and proactive maintenance planning, prior to chlorides coming into contact with the rebar, and also at the point of the rebar within the concrete.

Multi-Anode Corrosion Ladder Probe