15.3. Carbonation

Carbonation is the process in which carbon dioxide (CO₂) from the atmosphere penetrates into the concrete and reacts with calcium hydroxide, forming calcium carbonate. This reaction can affect the alkalinity of the concrete, particularly in structures that have a high volume of coarse aggregates.

Carbonation starts when CO₂ in the air comes into contact with the concrete surface. This carbon dioxide begins to dissolve in the moisture present in the concrete. As it penetrates deeper, it reacts with calcium hydroxide, a compound that contributes to the concrete's strength and alkalinity. This reaction leads to the formation of calcium carbonate. A well-carbonated concrete can compromise its protective properties, making it more susceptible to corrosion over time.

Imagine a sponge soaking up a drink. Just as the sponge gains water, concrete gains carbon dioxide from the air. Over time, this sweet drink (carbon dioxide) changes the sponge’s structure (the concrete), making it less effective in holding together what it’s meant to. This is similar to how carbonation can gradually weaken the concrete's defenses.

The carbonation process reduces the pH level of concrete, which can lower its alkalinity. This is significant because the high pH of uncarbonated concrete protects embedded steel reinforcement from corrosion. When carbonation occurs, it may make the environment more favorable for corrosion to take place.

Concrete is naturally alkaline, usually maintaining a pH around 12-13, which protects embedded steel from rust. When carbonation decreases this pH by forming calcium carbonate, it alters the protective barrier for the steel. If the pH drops below a certain level, the protective layer of iron oxide might break down, leading to corrosion and potentially structural failure if the steel reinforcement begins to rust.

Think of the pH level in concrete as a blanket protecting a person from the cold. If the blanket becomes thinner (due to carbonation), it cannot keep the person as warm (protect the steel from corrosion). Eventually, cold starts to creep in, leading to discomfort (corrosion), indicating that the protective layer is becoming ineffective.

SCC's dense microstructure can delay the carbonation depth, improving its durability against environmental factors. However, it's important to note that the use of lower coarse aggregate volume can sometimes result in reduced alkalinity, making the material more susceptible to carbonation.

Self-Compacting Concrete (SCC) is designed to have a very compact and dense structure, which helps limit the penetration of carbon dioxide and other environmental elements. This characteristic allows SCC to resist carbonation for longer periods, thereby enhancing its lifespan and durability under exposure to carbon dioxide. Although, if SCC is formulated with too little coarse aggregate, it might inadvertently lower the concrete's overall alkalinity, potentially leading to increased carbonation risks.

Consider a coffee thermos that keeps your drink hot. The dense insulation prevents heat from escaping, much like SCC prevents CO₂ from penetrating deeply. But if the insulation is thinner (less coarse aggregate), it may not keep heat (alkalinity) as effectively, allowing heat to dissipate faster (allowing carbonation).

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