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.

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:

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