Based on coarse aggregate’s shape, what shape is mostly recommended for good bonding and high strength in concrete?
Correct Answer: B. Angular aggregates
🧱 Detailed Explanation: Why Angular Aggregates Give the Best Bond in Concrete
The shape of coarse aggregate particles significantly influences both the fresh properties (workability) and the hardened properties (strength, bond) of concrete. IS 383:1970 classifies aggregate shape into rounded, irregular, angular, flaky, and elongated categories, each with distinct implications for mix design.
Why B (Angular aggregates) is the correct answer:
Angular aggregates, produced by crushing hard rock in jaw or cone crushers, have sharp edges, fractured faces, and a rough surface texture. These characteristics provide two critical advantages:
1. Mechanical interlocking: The irregular, angular geometry prevents particles from sliding past each other, creating a “locked” aggregate skeleton that resists shear and tensile stress.
2. Superior bond with cement paste: Rough, freshly fractured surfaces have high surface energy and micro-roughness, enabling the cement paste to grip the aggregate at the interfacial transition zone (ITZ) with far greater adhesion than smooth surfaces.
Angular aggregates, produced by crushing hard rock in jaw or cone crushers, have sharp edges, fractured faces, and a rough surface texture. These characteristics provide two critical advantages:
1. Mechanical interlocking: The irregular, angular geometry prevents particles from sliding past each other, creating a “locked” aggregate skeleton that resists shear and tensile stress.
2. Superior bond with cement paste: Rough, freshly fractured surfaces have high surface energy and micro-roughness, enabling the cement paste to grip the aggregate at the interfacial transition zone (ITZ) with far greater adhesion than smooth surfaces.
Rounded aggregates (C) have the best workability but weakest bond. Flaky (D) and elongated (A) aggregates increase water demand and weaken the mix.
Aggregate Shape — Workability vs. Bond Strength
| Shape | Source | Workability | Bond Strength | Recommended For |
|---|---|---|---|---|
| Rounded | River beds, seashores | Best | Lowest | Mass concrete, plain concrete |
| Irregular | Pit sand, glacial | Moderate | Moderate | General concrete |
| Angular | Crushed rock quarries | Lower | Best ✅ | High-strength, structural RCC |
| Flaky / Elongated | Sedimentary rocks | Very low | Low | Avoid for structural use |
Key Concepts for Students
- Angular aggregate gives the best bond because rough, fractured surfaces grip the cement paste; smooth rounded surfaces create a weaker interfacial bond.
- The trade-off: angular aggregate increases water demand (more water needed for the same slump). Mix design must account for this by adjusting the w/c ratio or using a plasticiser.
- High-strength concrete (M40 and above) and prestressed concrete mandatorily use crushed (angular) aggregate — rounded aggregate cannot develop the required interfacial bond.
- Flaky and elongated aggregates should be limited because they create planes of weakness and increase surface area, requiring more cement paste, reducing economy.