What is meant by depassivation of reinforcement? State the two main chemical attacks that lead to depassivation
Depassivation of reinforcement refers to the loss or breakdown of the protective oxide layer that forms on the surface of reinforcing steel in reinforced concrete structures. This oxide layer, known as passivation, acts as a barrier against corrosive agents.
The two main chemical attacks that lead to depassivation are:
1. Carbonation: Carbon dioxide (CO2) present in the atmosphere or from other sources can penetrate into the concrete and react with the alkaline components, particularly the calcium hydroxide (Ca(OH)2), in the pore solution. This reaction results in the formation of calcium carbonate (CaCO3), which reduces the alkalinity (pH) of the concrete. As a consequence, the passivating layer on the reinforcing steel can be dissolved, leading to depassivation.
To determine if carbonation has occurred, you can perform a phenolphthalein test. This involves applying a few drops of a phenolphthalein solution on a freshly broken concrete surface. If the concrete turns pink, it indicates that carbonation has reached the reinforcing steel, causing depassivation.
2. Chloride attack: Chloride ions, often found in salts such as sea water or de-icing salts, can penetrate into concrete through diffusion or capillary action. When chloride ions reach the reinforcing steel, they chemically react with the iron oxide present in the oxide layer. This reaction disrupts and breaks down the protective layer, leading to depassivation.
To determine chloride attack, you can conduct a chloride ion content test on concrete samples. This involves drilling a hole in the concrete and extracting a core sample, which is then tested for chloride content. If the chloride content exceeds the threshold level, it indicates a likelihood of depassivation due to chloride attack.
In summary, depassivation of reinforcement occurs when the protective oxide layer on reinforcing steel is compromised or destroyed. Carbonation and chloride attack are the two main chemical processes that can cause depassivation. By conducting appropriate tests, such as the phenolphthalein test for carbonation and chloride ion content test for chloride attack, the presence of depassivation can be determined.