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Chapter 9: Problem 28

When rain water percolates into the cracks of \(\mathrm{RCC}\), it further gets weakened due to the (a) formation of \(\mathrm{H}_{2} \mathrm{CO}_{3}\) (b) increase in \(\mathrm{pH}\) value (c) increase in the percentage of free lime (d) exothermic hydration process

Short Answer

Expert verified
Answer: The rainwater weakens the RCC structure when it percolates into the cracks because it forms carbonic acid (\(H_{2}CO_{3}\)), which reacts with free lime (\(Ca(OH)_{2}\)) present in the cement paste, forming calcium carbonate (\(CaCO_{3}\)). Calcium carbonate can reduce the strength of RCC structures, leading to their weakening.

Step by step solution

01

Option Evaluation - Formation of H2CO3

When rainwater percolates into the cracks of RCC, it reacts with carbon dioxide (\(CO_{2}\)) gas present in the air to form carbonic acid (\(H_{2}CO_{3}\)). This reaction can be represented as: $$ H_{2}O + CO_{2} \rightarrow H_{2}CO_{3} $$ Carbonic acid is a weak acid, which reacts with the calcium hydroxide or free lime (\(Ca(OH)_{2}\)) present in cement paste, This reaction further forms calcium carbonate (\(CaCO_{3}\)) and water. $$ H_{2}CO_{3} + Ca(OH)_{2} \rightarrow CaCO_{3} + 2H_{2}O $$ Calcium carbonate can reduce the strength of RCC structures, so this option seems plausible.
02

Option Evaluation - Increase in pH value

Regarding the increase in pH value, carbonic acid formed due to the reaction between rainwater and carbon dioxide is a weak acid. The presence of weak acids would cause a decrease in the pH value rather than increase. Therefore, this option is unlikely to be true.
03

Option Evaluation - Increase in the percentage of free lime

Rainwater reacts with carbon dioxide to form carbonic acid, which in turn reacts with free lime present in the cement paste to form calcium carbonate. This process reduces the percentage of free lime, not increasing it. Hence, this option is not true.
04

Option Evaluation - Exothermic hydration process

RCC structures undergo exothermic hydration reactions during their curing and hardening process. However, this process does not come into play when rainwater percolates into the cracks of already hardened and cured RCC structures. Therefore, this option is not correct.
05

Conclusion

Based on the above evaluations, the correct answer is: (a) formation of \(H_{2}CO_{3}\), as it weakens the RCC structure by reacting with free lime present and forming calcium carbonate.

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