9.15. Corrosion of Reinforcement in Concrete

Causes:

• Ingress of chlorides (from sea water, de-icing salts)
• Carbonation-induced pH drop
• Moisture and oxygen presence

Corrosion of reinforcement in concrete is primarily caused by three factors: the ingress of chlorides, which can come from sources like seawater or de-icing salts; a drop in pH due to carbonation, which diminishes the alkaline environment that protects steel; and the presence of moisture and oxygen, which are essential for the corrosion process to occur.

Think of the steel reinforcement as a piece of iron left in a damp garage. Over time, if the air is humid and there are contaminants like salt in the environment, the iron will rust. Similarly, when reinforcing steel in concrete is exposed to moisture, oxygen, and harmful elements like chlorides, it begins to corrode.

Mechanism:

• Electrochemical cell formation between anodic and cathodic regions
• Formation of rust which has 2–6 times the volume of steel → causes cracking and spalling

The corrosion process occurs through the formation of electrochemical cells in the presence of moisture and oxygen. This means that different areas on the steel surface can become either anodic (where oxidation occurs and metal is lost) or cathodic (where reduction occurs). As the steel oxidizes, it converts to rust, which has a much larger volume than the original steel. This expansion leads to internal pressure build-up, causing the surrounding concrete to crack and spall.

Imagine blowing up a balloon. Initially, it's tight but as you keep adding air, it expands. If the balloon's surface is weak, it might burst. In a similar way, the rust takes up more space than the steel, leading to cracks and bubbling in concrete until it fails.

Types:

• Uniform corrosion
• Pitting corrosion

Corrosion can manifest in different ways, with the two main types being uniform and pitting corrosion. Uniform corrosion spreads evenly across the surface, leading to a gradual loss of material. On the other hand, pitting corrosion is localized, resulting in small, deep pits that can severely weaken the steel even if the overall loss of material is less significant. This type of corrosion is particularly dangerous because it can lead to sudden failure.

Consider a metal surface under a blanket of uniform rust. It's like a smooth sack that has evenly lost its strength, making it weaker overall. In contrast, pitting corrosion is like a sponge developing deep holes in specific areas, potentially collapsing under pressure despite retaining shape elsewhere.

Protection Strategies:

• Use of corrosion inhibitors
• Epoxy-coated or galvanized bars
• Cathodic protection systems
• Corrosion-resistant steel (CRS)
• Adequate concrete cover and low permeability

To prevent corrosion, various strategies can be employed. Corrosion inhibitors can be added to the concrete mix to slow down the process. Using epoxy-coated or galvanized steel bars provides a protective layer, while cathodic protection systems can redirect corrosion currents away from steel. Furthermore, utilizing corrosion-resistant steel (CRS) enhances durability, and ensuring adequate concrete cover combined with low permeability reduces the chances for water and contaminants to reach the steel.

Think of it as dressing for protection against the rain. Just as you might wear a raincoat or use an umbrella to keep dry, structures can be protected against corrosion by using inhibitors, special coatings, and proper materials, preventing the adverse effects of water and contaminants.

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