Steel in reinforced concrete

Steel is one of the most widely used materials in civil construction, mainly as a reinforcement in reinforced concrete. The combination of concrete and steel provides a composite material that is highly resistant to both compressive stress (managed by the concrete) and tensile stress (managed by the steel). However, steel is subject to oxidation, a phenomenon that can seriously compromise the structural integrity of reinforced concrete works.

Steel oxidation is an electrochemical reaction that occurs when the metal is exposed to moisture and oxygen. In the context of reinforced concrete, steel is embedded in a cement matrix, which initially protects it from external agents thanks to the high alkaline pH (pH > 12.5). Under these conditions, a passive oxide layer (Fe₂O₃) forms on the surface of the steel, which acts as a barrier against further corrosive attacks.

However, over time, various factors can break this balance:

1. Carbonation of concrete: This is a process in which carbon dioxide (CO₂) present in the air penetrates the reinforced concrete and reduces the pH of the cement matrix. When the pH drops below a critical value (around 9-10), the passive layer on the steel destabilizes, allowing corrosion to advance.
2. Chlorides: Another important factor that induces corrosion is the presence of chlorides, often derived from marine environments or the use of de-icing salts. Chlorides can penetrate through the pores of the concrete and reach the steel, where they break the passivating film even at high pH, ​​quickly triggering the oxidation process.
3. Porosity of concrete: A concrete with high porosity allows greater penetration of aggressive agents such as water, oxygen and chlorides, favoring the corrosion of the steel.
4. Cracking: Cracks in the concrete, due to structural loads or shrinkage, can expose the steel directly to the external environment, accelerating the corrosion process.

Steel corrosion leads to the formation of corrosion products (rust) that occupy a larger volume than the original metal. This increase in volume generates internal pressures that can cause cracking and delamination in the concrete, compromising the adhesion between the steel and the concrete. In the most severe cases, the loss of section of the steel due to corrosion reduces the load-bearing capacity of the structural element, putting the stability of the structure at risk.

Corrosion Mitigation Techniques

There are several strategies to prevent or reduce the oxidation of steel in reinforced concrete, which can be divided into preventive measures and active protection techniques:

1. Increased concrete coverage: A greater thickness of concrete around the reinforcement offers greater protection against the penetration of external agents. In addition, a concrete with low permeability reduces the access of oxygen and moisture to the steel.
2. Use of pozzolanic cements or additives: Materials such as pozzolans or fly ash improve the density of the concrete and its resistance to the penetration of aggressive agents. Some additives such as corrosion inhibitors can be used to interrupt the oxidation process directly in contact with the steel.
3. Surface treatments: Applying protective coatings on the concrete surfaces, such as hydrophobic paints or sealants, helps prevent the ingress of moisture and aggressive agents. The use of waterproofing membranes, especially in marine or aggressive environments, is an effective technique to limit the exposure of concrete to corrosive agents.
4. Stainless or galvanized steel: In particularly aggressive situations, such as those exposed to chlorides, the use of stainless steel can be a solution. Stainless steel has a very high resistance to corrosion due to its high chromium content, which forms a protective oxide. Alternatively, galvanized steel, coated with a layer of zinc, can offer protection due to the anodic nature of zinc compared to iron, which corrodes preferentially.
5. Sacrificial cathodes and cathodic protection: This technique consists of using a less noble metal, such as zinc or magnesium, which oxidizes instead of steel. Impressed current cathodic protection is another technique used in large structures, where an electric current is applied to prevent corrosion of the steel.
6. Use of high-performance concrete:

   High-performance concrete (HPC) with a reduced water-cement ratio and the use of microsilica significantly improve durability and resistance to penetration by aggressive substances.

Corrosion inhibitors are chemicals added to concrete that slow or prevent the corrosion of steel within the concrete. These inhibitors work by creating a protective barrier on the surface of the steel or by altering the chemical environment around it, thus preventing the onset of corrosion or slowing its progress. Among the most commonly used for reinforced concrete are:

1. Calcium nitrite: Calcium nitrite (Ca(NO₂)₂) is one of the most common and widely used inhibitors. It acts as an anodic inhibitor, preventing the formation of oxides on the surface of the steel. Calcium nitrite is particularly effective against chloride-induced corrosion, neutralizing the effect of chlorides that would otherwise promote corrosion. Additionally, it does not reduce the mechanical performance of concrete, making it a popular choice in aggressive environments, such as marine environments or those exposed to de-icing salts.
2. Organic inhibitors: These are chemical compounds based on amines or polar compounds that form a protective film on the surface of the steel. Among these, alkylamines and amine-derived compounds are widely used. They work by reducing the rate at which steel reacts with corrosive agents, mainly by forming a thin hydrophobic layer that prevents the access of moisture and oxygen.
3. Phosphates: These are anodic inhibitors that, when dissolved in the porous solution of concrete, form a protective layer of iron phosphate on the surface of the steel, thus preventing the initiation of corrosion. Although less common than nitrites, they are used in certain applications where environmental conditions require it.
4. Amine-based inhibitors: Amines are often used as corrosion inhibitors because they can adsorb onto the surface of steel and form a chemical barrier that slows or prevents corrosive attack. A common example is ethanolamine, which helps form a protective film on the surface of the steel, reducing the access of water and oxygen, key factors in corrosion.
5. Migrating Corrosion Inhibitors (MCI): These are a class of additives that have the ability to diffuse into the concrete matrix and reach deep into the rebar. They are often used in already hardened concrete, as they penetrate through the pores of the concrete. These inhibitors can be either anodic or cathodic and often use organic substances such as amines. The ability to migrate to the steel makes these inhibitors particularly useful in repair and maintenance projects of existing structures.
6. Benzoates: Benzoates, such as sodium benzoate, can be used as cathodic inhibitors, preventing corrosion in areas where the steel receives electrons during the corrosion reaction. These inhibitors are effective under uniform corrosion conditions, but are less common than calcium nitrites and phosphates.
7. Combination Inhibitors: Some corrosion inhibitors use a combination of multiple chemicals to provide synergistic protection. For example, the combination of calcium nitrites and organic inhibitors can improve the overall effectiveness of the corrosion protection system. These combined systems offer performance advantages over a wide range of environmental conditions and stresses.

The use of corrosion inhibitors in reinforced concrete can significantly increase the service life of structures, especially in chloride-exposed environments such as marine structures, bridges or roads treated with de-icing salts. These inhibitors are often added during the concrete mixing phase, but can also be applied by impregnation or diffusion into existing structures.

Corrosion Monitoring

Monitoring the corrosion of steel in reinforced concrete is essential to ensure the safety and durability of structures, as it allows early detection of signs of degradation. Specific techniques and devices allow the progression of corrosion to be assessed and timely interventions to be taken, reducing repair costs and extending the useful life of buildings.

Corrosion Monitoring Techniques

1. Corrosion potential: This technique measures the electrochemical potential of steel in concrete, detecting potential differences that may indicate the onset of corrosion. A reference electrode is used, such as the copper/copper sulphate electrode or the silver/silver chloride electrode. Measurements with negative potentials compared to these electrodes may indicate a high risk of corrosion.
2. Linear polarization resistance (LPR): With this technique, a controlled current is applied to the steel and the change in electrochemical potential is measured. LPR allows the corrosion rate of steel to be determined, providing quantitative information on degradation. It is considered a precise technique, but requires a good electrical connection to the reinforcement.
3. Electrochemical Impedance Spectroscopy (EIS): EIS is an advanced technique that uses an alternating current to measure the resistance and capacitance of the system. It provides detailed data on the electrochemical properties of steel and concrete and can detect early corrosion. It is widely used in laboratories, but can also be applied in the field with suitable equipment.
4. Galvanostatic Pulsed Technique (GPT): This technique uses pulses of galvanostatic current to determine the polarization of steel. The data collected allows the polarization resistance to be calculated and therefore the corrosion rate. It is a non-destructive technique, suitable for monitoring critical areas of the structure.
5. Chloride and Carbonation Analysis:

   In addition to electrochemical techniques, chemical analysis of chlorides and carbonation in concrete is essential for corrosion monitoring. Concrete samples are taken and analyzed in the laboratory to measure the chloride concentration and the depth of carbonation, providing indirect but useful data to assess the risk of corrosion.

Corrosion Monitoring Tools

1. Half-Cell Potential Meter: Used to measure electrochemical potential, this device consists of a reference electrode and a voltmeter to read the potential difference between the steel and the electrode. This instrument is portable and easy to use in the field for a quick diagnosis of the state of corrosion.
2. Polarization Resistance Corrosion Probes: These are devices installed in structures to measure polarization resistance. LPR probes provide direct data on the corrosion rate and can be connected to data acquisition systems for long-term monitoring. They are often used in particularly aggressive environments, such as marine or industrial structures.
3. Electrochemical Impedance Spectroscopy (EIS) Analyzers: These are advanced equipment that provide detailed data on electrochemical conditions. Often used in laboratories for in-depth analysis, they are also available in portable versions for field monitoring, especially in research and development projects.
4. Galvanostatic Pulsed Probes (GPT): These are specialized probes that apply controlled pulses of current to the structure. These probes are used to monitor polarization changes and calculate corrosion rates with high accuracy.
5. Fiber Optic Systems: Innovative monitoring systems use corrosion-sensitive fiber optics to monitor pH, chlorides, and moisture within concrete. These systems can detect critical environmental changes and are used for continuous monitoring in high-value structures, providing real-time data.
6. Wireless Corrosion Monitoring Systems:

   Wireless technologies allow data to be collected from sensors installed in the structure without the need for cables. These systems can transmit data to a central monitoring station, facilitating continuous monitoring of corrosion conditions even in hard-to-reach areas.