A concrete using an air entrained cement:
Note: This is a flawed question as multiple options are correct. The official commission did not consider any single option correct in its revised answer key.
🔬 Understanding Air-Entrained Concrete
Air-entrained concrete is created by adding a special admixture (an air-entraining agent) that creates billions of stable, microscopic air bubbles throughout the concrete mix. These bubbles are not the same as entrapped air, which are larger, irregular voids caused by incomplete compaction. The intentionally entrained air bubbles fundamentally change the properties of both fresh and hardened concrete.
📝 Detailed Analysis of Properties
This question is considered ambiguous because several of the statements are true effects of using air-entrained cement. Let's analyze each one.
(a) has strength less than 10% to 15%
This is a true statement. The air bubbles are essentially voids, and they reduce the cross-sectional area of solid material, which lowers the compressive strength. For typical amounts of entrained air (4-8%), a strength reduction of 10-20% is expected.
(b) has more resistance to weathering
This is a true statement. This is the primary reason for using air-entrained concrete. The microscopic bubbles provide empty space for water to expand into when it freezes. This relieves internal pressure and dramatically improves the concrete's resistance to freeze-thaw cycles and de-icing salts, which is a major form of weathering.
(c) is more plastic and workable
This is a true statement. The tiny air bubbles act like microscopic ball bearings, lubricating the mix. This makes the fresh concrete more cohesive, plastic, and easier to place and finish, significantly improving its workability.
(d) is free from segregation and bleeding
This is an exaggerated but directionally correct statement. The improved cohesion from the air bubbles makes the mix less prone to segregation (separation of heavy aggregates) and bleeding (rising of excess water to the surface). While it may not be completely "free from" these issues in a poorly designed mix, it drastically reduces them.
📊 Summary of Air Entrainment Effects
| Property | Effect of Air Entrainment |
|---|---|
| Durability (Freeze-Thaw) | Greatly Increased (Primary Benefit) |
| Workability & Plasticity | Increased |
| Segregation & Bleeding | Reduced |
| Strength | Reduced |
| Permeability | Reduced |
💡 Study Tips
- The Main Trade-Off: The key concept of air entrainment is that you sacrifice some strength to gain a large improvement in durability (especially freeze-thaw resistance) and workability.
- Primary Purpose = Durability: If a question asks for the *main reason* or *primary purpose* of air entrainment, the answer is always to improve resistance to weathering and freeze-thaw cycles.
- Recognize Flawed Questions: Be aware that sometimes questions are poorly phrased. If multiple options seem correct, as in this case, it's important to understand why each one is a valid property of air-entrained concrete.