Which of the below property of aggregates is not desirable?
Correct Answer: A. Smooth texture
📚 Detailed Explanation: Why Smooth Texture is Undesirable in Concrete Aggregates
The surface texture of an aggregate affects how well the cement paste can mechanically bond to it. A rough, granular surface provides countless micro-anchorage points for the hardened cement paste, forming a strong mechanical interlock. A smooth or glassy surface offers almost no such anchorage.
Why A (Smooth texture) is the undesirable property: Smooth-textured aggregates (like polished river gravel or glassy rocks) reduce the bond area and mechanical interlocking with cement paste. This weak interface — called the Interfacial Transition Zone (ITZ) — becomes the weakest link in the concrete, limiting overall compressive and tensile strength. The other options are all desirable: well-graded aggregates minimise voids; angular aggregates interlock well; using smaller maximum aggregate size suits thin sections.
Surface Texture Classification and Bond Quality
| Surface Texture | Examples | Bond with Cement |
|---|---|---|
| Glassy / Smooth | Chert, polished gravel | Very weak |
| Smooth | River rounded aggregate | Weak |
| Granular | Sandstone | Moderate |
| Rough | Crushed basalt, granite | Good |
| Honeycomb / Porous | Pumice, lightweight aggregate | Good (large surface area) |
Key Concepts for Students
- Rough surface = stronger bond with cement paste, leading to higher tensile and flexural strength in concrete.
- The Interfacial Transition Zone (ITZ) between aggregate and paste is typically the weakest zone in concrete; rough-textured aggregates improve it.
- Well-graded and angular aggregates are desirable — they fill voids efficiently and interlock well, increasing concrete strength and reducing cement content.