Concrete mixing time for uniform colour and consistency when using ordinary Portland cement (OPC) Select a suitable total mixing time in a standard batch mixer to obtain uniform paste coating and colour.
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A0.5 to 1.0 minute
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B1.0 to 1.5 minutes
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C1.5 to 2.0 minutes
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D2.0 to 3.0 minutes
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Emore than 3.0 minutes is mandatory for all batches
Answer
Correct Answer: 1.5 to 2.0 minutes
Explanation
Introduction:Mixing time influences homogeneity, workability, and strength. Insufficient mixing causes colour streaks and honeycombing; excessive mixing can increase temperature and cause segregation. The question seeks a practical range for OPC concrete in typical drum/batch mixers.
Given Data / Assumptions:
- Ordinary Portland cement.
- Conventional on-site tilting drum or pan mixer.
- Normal slump concrete, standard aggregate grading.
Concept / Approach:
Standards and good practice suggest a minimum time to coat all particles with paste and achieve uniform colour. For common on-site mixers, the total mixing time after all materials are in the drum is typically around 1.5 to 2 minutes.
Step-by-Step Solution:
1) Load sequence ensures part mixing during charging, but uniformity requires post-charging mixing.2) Empirical studies show diminishing returns beyond ~2 minutes in standard mixers.3) Therefore, select 1.5 to 2.0 minutes for uniform colour and consistency.Verification / Alternative check:
Perform a field uniformity check: visual colour uniformity and absence of cement balls; slump tests between successive batches should be similar.
Why Other Options Are Wrong:
0.5–1.0 minute is too short; 1.0–1.5 minute may still be borderline; 2.0–3.0 minutes can be acceptable but often unnecessary and may risk temperature rise; a blanket rule of >3 minutes is not mandatory.
Common Pitfalls:
Starting the stopwatch before all materials are in; not accounting for mixer performance; ignoring admixture manufacturer guidance.
Final Answer:
1.5 to 2.0 minutes