Concrete mixer charging sequence and mixing time: Which procedure best reflects correct practice to avoid cement loss and ensure uniform mixing?

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.