Select the option that is true: (A) If the aggregate to cement ratio of a concrete mix is increased the concrete mix becomes lean. (R) The quantity of paste available for lubrication decreases, if the aggregate to cement ratio is increased.
📚 Detailed Explanation: Aggregate-Cement Ratio and Lean Mix
This assertion-reason question tests understanding of how the aggregate-cement ratio affects both the classification (rich vs lean) and the workability of fresh concrete.
Assertion A is TRUE: A “lean” mix has a high aggregate-cement ratio (more aggregate, less cement). A “rich” mix has a low aggregate-cement ratio (more cement, less aggregate). Increasing the aggregate-cement ratio makes the mix leaner.
Reason R is TRUE and CORRECTLY EXPLAINS A: When the aggregate-cement ratio increases, there is proportionally less cement paste per unit volume. Cement paste (cement + water) is what lubricates aggregate particles, coats their surfaces, and provides workability. Less paste = less lubrication = lower workability = the characteristics of a lean mix.
Rich vs. Lean Mix Comparison
| Property | Rich Mix (low A/C) | Lean Mix (high A/C) |
|---|---|---|
| Cement content | High | Low |
| Paste volume | High | Low |
| Workability | Better | Lower |
| Strength (same w/c) | Higher potential | Lower potential |
| Cost | Higher | Lower |
Key Concepts for Students
- More aggregate relative to cement = lean mix; more cement relative to aggregate = rich mix.
- At a constant w/c ratio, increasing aggregate-cement ratio reduces both workability (less paste) and achievable strength.
- R correctly explains A because the physical mechanism — reduced paste for lubrication — is exactly why a high A/C mix is lean.