Permissible dust content in aggregates: identify the correct upper limits of dust for different grading/workability conditions used in concrete production
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A5% of total aggregates for a low-workability mix with coarse grading
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B10% of total aggregates for a low-workability mix with fine grading
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C20% of total aggregates for a high-workability mix with fine grading
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Dall the above
Answer
Correct Answer: all the above
Explanation
Introduction / Context:Fine dust in aggregates alters water demand and cohesion. Codes permit different dust limits based on grading and target workability so that mixes remain cohesive without excessive water.
Given Data / Assumptions:
- Three scenarios—coarse/low workability, fine/low workability, fine/high workability.
- Limits are expressed as percentage of total aggregates.
Concept / Approach:Coarser grading tolerates less dust (around 5%). Finer grading in low workability permits ~10%. High workability mixes with fine grading can handle up to ~20% fines as cohesion is needed to resist segregation.
Step-by-Step Solution:Check each limit for each case as per standard practice.All three statements align with permissible ranges.Hence, choose all the above.
Verification / Alternative check:Guidance found in IS 383 and concrete QC manuals supports these upper bounds under specified conditions.
Why Other Options Are Wrong:
- Any single choice alone omits the other valid cases specified by codes.
Common Pitfalls:
- Failing to adjust water content or admixtures when fines content is high.
Final Answer:all the above