As per IS 10262 : 2019, what is the approximate amount of entrapped air (%) to be expected in normal (non-air-entrained) concrete, when 10 mm Nominal Maximum Size of Aggregate (NMSA) is used?
Correct Answer: A. 1.5%
📚 Detailed Explanation: Entrapped Air in Normal Concrete (IS 10262:2019)
Why A (1.5%) is correct: Entrapped air is the unintentional air trapped in concrete during mixing and placing. Smaller maximum aggregate size = smaller particle voids that are harder to consolidate = more entrapped air. IS 10262:2019 Table 2 specifies these values for mix design water content calculation. With NMSA = 10 mm, the expected entrapped air is 1.5%.
IS 10262:2019 — Entrapped Air vs. NMSA
| NMSA (mm) | Entrapped Air (%) |
|---|---|
| 10 | 1.5 |
| 20 | 1.0 |
| 40 | 0.5 |
Why Smaller Aggregate = More Entrapped Air
- Smaller aggregates have more surface area per unit volume → more inter-particle contacts → more trapped air pockets.
- Larger aggregates allow better consolidation of fresh concrete, leaving fewer air voids.
- This value is used in IS 10262 mix design to calculate the absolute volume of aggregate needed.
- Note: This is entrapped air (involuntary), not entrained air (deliberately introduced by AEAs).