Short Answer:

Carbonation in concrete is a chemical reaction where carbon dioxide from the air reacts with calcium hydroxide in the concrete to form calcium carbonate. This process reduces the pH of concrete and moves slowly from the surface inward over time.

As carbonation progresses, it lowers the protective alkalinity around the steel reinforcement. This loss of alkalinity allows moisture and oxygen to reach the steel bars, causing corrosion. Rust expands inside the concrete, leading to cracks, spalling, and weakening of the RCC structure if not controlled.

Detailed Explanation

Carbonation in Concrete

Carbonation is a natural process that takes place when carbon dioxide (CO₂) from the air enters the concrete and reacts with calcium hydroxide [Ca(OH)₂], a compound formed during cement hydration. The reaction produces calcium carbonate (CaCO₃) and water, which leads to a reduction in the concrete’s pH level. Fresh concrete has a high pH (around 12.5 to 13.5), which protects steel reinforcement from corrosion by forming a passive film on its surface.

As carbonation progresses deeper into the concrete, the pH falls to about 8.5–9.0. At this lower pH, the passive protective layer on the steel reinforcement breaks down, allowing moisture and oxygen to initiate corrosion of the steel bars. The expansion of rust leads to internal pressure, cracking, and eventually spalling (breaking off) of the concrete cover, compromising the durability and safety of the structure.

Carbonation in Concrete

1. How Carbonation Occurs

• Carbon dioxide from the atmosphere diffuses into the porous concrete.
• It reacts with calcium hydroxide to form calcium carbonate:
  CO₂ + Ca(OH)₂ → CaCO₃ + H₂O
• This process is slow and depends on factors such as concrete quality, exposure, and moisture content.
• It begins at the surface and progresses inward with time.

2. Signs of Carbonation and Damage

• Formation of fine cracks or surface scaling.
• Discoloration and weakening of the concrete surface.
• Rust stains appearing on the surface due to steel corrosion.
• Spalling or breaking of concrete cover as reinforcement expands.

3. Factors Influencing Carbonation

• Porosity of Concrete: High water-cement ratio leads to more pores, increasing carbonation rate.
• Concrete Cover Thickness: Thicker covers delay carbonation from reaching steel.
• Humidity: Optimum humidity (around 50–70%) speeds up the carbonation process.
• Quality of Curing: Poor curing increases concrete permeability, allowing more CO₂ to enter.
• Environmental Exposure: Urban areas with higher CO₂ levels face more carbonation risk.

Effect on Reinforcement

1. Loss of Alkalinity

• The high pH in concrete maintains a passive protective film on steel.
• When carbonation lowers the pH below 9, this film dissolves, exposing steel to corrosion.

2. Corrosion of Steel Bars

• Moisture and oxygen react with exposed steel to form rust (iron oxide).
• Rust expands in volume up to 2–3 times, creating internal pressure.

3. Cracking and Spalling

• Expansion due to rust formation cracks the concrete.
• Pieces of concrete may fall off, exposing more steel and accelerating damage.

4. Reduction in Structural Strength

• Corroded steel loses its diameter and tensile capacity.
• The bond between steel and concrete weakens, reducing structural performance.

Conclusion

Carbonation in concrete is a chemical process where carbon dioxide reacts with concrete compounds, reducing pH and leading to reinforcement corrosion. This results in rusting, cracking, and weakening of RCC structures. To prevent this, high-quality concrete, adequate cover, proper curing, and protective coatings are essential. Regular inspection and timely repair can help in maintaining long-term durability of concrete structures.