Depth of Carbonation

Carbonation depth is an important factor to consider when evaluating the durability of concrete structures. It refers to the extent of carbon dioxide penetration into the concrete, which affects the pH level of the cement paste that surrounds the steel reinforcement. When the pH drops below 11, the protective layer on the steel begins to deteriorate, leaving it vulnerable to corrosion.

To assess carbonation depth, the phenolphthalein method is commonly used. This involves applying a solution of phenolphthalein indicator to freshly exposed concrete surfaces, such as those from broken structures or split cores. The indicator appears pink on alkaline surfaces with pH values greater than 9, but becomes colorless on surfaces with lower pH values. The change in color indicates the location of the depassivation front, where the pH has dropped enough to remove the protective coating from the steel.

It's important to note that corrosion can occur in the zone ahead of the depassivation front, where the pH has not yet dropped enough to trigger a color change in the indicator. This means that even if the indicator appears pink, there may still be some risk of corrosion in the surrounding area.

The phenolphthalein method is covered by methods such as EN 14630, which outlines the test methods for determining carbonation depth in hardened concrete. By using this method, engineers and inspectors can accurately evaluate the risk of corrosion in concrete structures and take appropriate measures to ensure their durability.

Overall, understanding carbonation depth and using appropriate testing methods is crucial for maintaining the integrity and longevity of concrete structures. By staying vigilant and proactive in monitoring concrete conditions, we can ensure the safety and reliability of our built environment.