Concrete mixer charging sequence and mixing time: Which procedure best reflects correct practice to avoid cement loss and ensure uniform mixing?
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AWet the drum with about 10% of the batch water, charge aggregates and cement, then add the remaining water and mix for at least 1 minute; ensure no cement is lost from the drum
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BCharge all dry materials first, add the entire water at once, and discharge immediately to avoid segregation
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CAdd cement alone with full water, then add aggregates and mix for 20 seconds
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DAvoid wetting the drum to prevent sticking and reduce cement loss
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EAdd 10% of water after all ingredients and withhold the remaining water until discharge
Answer
Correct Answer: Wet the drum with about 10% of the batch water, charge aggregates and cement, then add the remaining water and mix for at least 1 minute; ensure no cement is lost from the drum
Explanation
Introduction / Context:Proper charging and mixing sequence in drum mixers prevents cement paste balling, reduces material loss, and achieves a uniform mix consistency. Shortcuts create strength variability and surface defects.
Given Data / Assumptions:
- Conventional tilting or non-tilting drum mixer on site.
- Batch water measured accurately.
- Target is homogeneous, well-coated aggregates with no paste loss.
Concept / Approach:Good practice is to dampen the drum with a small portion of water (about 10%) to reduce dusting and sticking, charge aggregates and cement, then introduce the remaining water during mixing to achieve the specified water–cement ratio uniformly. Minimum mixing time of about 1 minute (often 1–2 minutes after all materials are in) is required, and loss of cement from the drum is unacceptable.
Step-by-Step Solution:Pre-wet drum with ~10% water to minimize paste loss.Charge coarse/fine aggregates and cement; start rotation.Add remaining water steadily; continue mixing ≥ 60 seconds after all materials are in.Check for no cement spilling at the mouth; adjust drum angle/speed as needed.
Verification / Alternative check:Manufacturer instructions and site QA checklists align with this sequence to ensure consistent slump and reduced variability across batches.
Why Other Options Are Wrong:(b) and (c) lead to balling, dusting, and poor coating; (d) increases sticking and loss; (e) starves the mix of water during mixing, producing non-uniformity.
Common Pitfalls:Under-mixing; dumping full water at once; re-tempering with extra water at discharge.
Final Answer:Wet the drum with about 10% of the batch water, charge aggregates and cement, then add the remaining water and mix for at least 1 minute; ensure no cement is lost from the drum.