Which of the following methods is NOT used for measuring air content in fresh concrete?
Correct Answer: A. Blaine air permeability method
📚 Detailed Explanation: Blaine Test is for Cement Fineness, NOT Concrete Air Content
Why A (Blaine air permeability method) is NOT used for air content: The Blaine apparatus forces air through a compacted bed of dry cement and measures the specific surface area (fineness) of the cement. It is used in the cement manufacturing quality control process. It has no connection whatsoever to measuring air voids in fresh concrete.
Standard Methods for Air Content in Fresh Concrete (IS 1199)
| Method | Principle | IS Reference | Suitable For |
|---|---|---|---|
| Gravimetric method | Compare actual unit weight with theoretical air-free unit weight. Air % = [(theoretical − actual) / theoretical] × 100 | IS 1199 | All concrete types; requires known specific gravities |
| Pressure method | Apply calibrated pressure to sealed vessel of concrete; Boyle's Law relates pressure change to air volume | IS 1199 | Normal density concrete; fast and most widely used |
| Volumetric method | Fill vessel with water; measure volume of air displaced | IS 1199 | Lightweight aggregate concrete (pressure method unreliable for porous aggregate) |
What the Blaine Test Actually Does
| Aspect | Detail |
|---|---|
| Standard | IS 4031 Part 2 — Fineness of hydraulic cement by Blaine air permeability |
| Sample | Dry, compacted cement powder in a cylindrical cell |
| What is measured | Specific surface area in cm²/g or m²/kg |
| Typical OPC fineness | 225–300 m²/kg (IS 269 minimum 225 m²/kg for OPC 33 grade) |
| Higher fineness means | Faster hydration, higher early strength, greater heat of hydration |
- Blaine test = cement fineness; completely unrelated to measuring air in fresh concrete.
- The three valid air content methods are: Gravimetric, Pressure, and Volumetric (IS 1199).
- Air entrainment is typically 3–7% for frost-resistant concrete; excess air reduces strength significantly.