The Alkali-Aggregate Reaction (AAR) in concrete is a deleterious chemical process. Which of the following conditions is NOT a prerequisite for this reaction to occur?
- AHigh alkali content in the cement.
- BPresence of reactive silica or carbonates in the aggregate.
- CA constant moisture level where relative humidity exceeds 80%.
- DHigh concentrations of C₃A in the cement paste.Correct
Solution
Concept
Alkali-Aggregate Reaction (AAR)
Formula
N/A
Step-by-step solution
1. Identify the prerequisites for Alkali-Aggregate Reaction (AAR): high alkali content, reactive aggregates, and relative humidity exceeding 80%. 2. Recognize that C₃A (tricalcium aluminate) relates to early strength and sulfate resistance, not AAR. 3. Conclude that high concentration of C₃A is NOT a prerequisite for AAR, making Option (d) the correct answer.
Explanation
High concentrations of C₃A in cement are responsible for early strength development and sulfate resistance, but C₃A is not involved in the chemical process of Alkali-Aggregate Reaction (AAR). AAR specifically requires alkalis from the cement, reactive silica or carbonates from the aggregate, and high moisture levels.
Shortcut
To easily remember the prerequisites for Alkali-Aggregate Reaction (AAR), use the mnemonic "ARM": - Alkali: High alkali content in cement - Reactive Aggregate: Reactive silica or carbonate in aggregate - Moisture: High relative humidity (exceeding 80%)Note: C₃A relates to early strength and sulfate resistance, not AAR.
Common mistake
A common mistake is misreading the word "NOT" in the question and choosing options A, B, or C because they are recognized as actual requirements for the Alkali-Aggregate Reaction. Another frequent error is confusing C₃A with alkalis or assuming C₃A plays a role in AAR, when it is actually associated with early strength, heat of hydration, and sulfate attack.
Correct answer: D) High concentrations of C₃A in the cement paste.