9.14. Carbonation of Concrete

The carbonation of concrete is a process where atmospheric CO₂ reacts with calcium hydroxide (Ca(OH)₂) present in the concrete.

Carbonation occurs when carbon dioxide in the air interacts with calcium hydroxide in concrete. This reaction transforms the calcium hydroxide into calcium carbonate (CaCO₃). This process leads to a decrease in the pH level of the concrete, reducing it from approximately 12.5 to below 9. This reduction in pH is crucial because it indicates the loss of alkalinity that protects the embedded steel reinforcement from corrosion.

Think of carbonation like how soda goes flat. Initially, the fizz (carbonation) is there, making the drink refreshing. When the fizz dissipates, it resembles still water that's less effective at providing that refreshing taste. Similarly, when concrete carbonates, it loses its protective alkalinity, making the steel reinforcements within more vulnerable to rusting, just as that soda loses its refreshing quality.

The most significant impact of carbonation is that it reduces alkalinity, which removes the protective shield for reinforcement against corrosion. This can lead to cracking and spalling near reinforcement.

Once the alkalinity of concrete diminishes due to carbonation, the protective barrier that prevents corrosion of the steel reinforcement is compromised. This means that the steel can start to corrode if moisture and oxygen are present. The corrosion can produce rust, which expands and can create cracks in the surrounding concrete, leading to structural issues like spalling, where pieces of concrete flake off.

Imagine a metal fence that has been painted to prevent rust. Over time, if the paint gets scratched or worn away (similar to carbonation breaking down alkalinity), the metal underneath is exposed to moisture and air and begins to rust. This rust can cause the metal to expand and break the fence apart, similar to how rusted rebar can cause cracking in concrete.

A common method to test for carbonation is the phenolphthalein indicator test. This test involves applying a phenolphthalein solution to the concrete surface, which turns pink in non-carbonated zones.

The phenolphthalein indicator test is a straightforward way to check the presence of carbonation in concrete. Phenolphthalein is a pH indicator that will change color based on the pH level of the substance it is applied to. When applied to non-carbonated concrete, it turns pink, indicating high alkalinity (above pH 9). If the concrete has carbonated, the test will not change color, indicating a lower pH and the presence of carbonation.

Think of the phenolphthalein indicator like a litmus test for acidity. Just as a litmus paper changes color depending on whether it’s in an acidic or basic solution, the phenolphthalein gives a visual confirmation of whether the concrete remains healthy and protective or has been affected by carbonation.

To control carbonation effects, adequate cover thickness, the use of dense and impermeable concrete, and surface sealers or paints are recommended.

Preventing carbonation involves implementing control measures during concrete placement and finishing. Adequate cover thickness helps shield the steel from environmental exposure. Using dense and impermeable concrete minimizes moisture ingress and limits the amount of CO₂ that can reach the steel. Additionally, applying surface sealers or paints acts as a barrier, reducing the penetration of carbon dioxide and moisture.

Imagine using a waterproof case for your phone. Just as it protects your phone from water damage, adding a protective layer to concrete ensures that harmful elements like CO₂ cannot penetrate and interact with the steel rebar, thereby preserving the structural integrity of the concrete.

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