The rocks which have very high strength are
Correct Answer: D. granite
📚 Detailed Explanation: Strength of Rock Types
The strength of the aggregate directly affects the potential strength of concrete, especially in high-strength concrete where the aggregate may govern the failure mode. Different rock types formed through different geological processes have widely varying strength values.
Why D (granite) is correct: Granite is a coarse-grained igneous rock composed of quartz, feldspar, and mica, formed by slow cooling of magma deep in the earth's crust. This slow crystallisation produces an interlocked crystalline structure with very high strength (150–280 MPa compressive strength) and excellent abrasion resistance. Comparisons: Limestone (A) is a sedimentary rock with variable strength (30–150 MPa) — soft limestones are unsuitable for concrete; Sandstone (B) is a sedimentary rock with even lower strength (20–100 MPa) due to loose cementation of sand grains; Basalt (C) is also a strong igneous rock (100–300 MPa) but is typically harder and less available than granite in India. Granite is the universally cited example of a very high-strength rock for aggregate.
Compressive Strength of Common Rocks
| Rock Type | Formation | Typical UCS (MPa) | Suitability as Aggregate |
|---|---|---|---|
| Sandstone | Sedimentary | 20–100 | Variable; weak varieties rejected |
| Limestone | Sedimentary | 30–150 | Good if hard; soft varieties rejected |
| Basalt | Igneous (extrusive) | 100–300 | Excellent; dense, strong |
| Granite | Igneous (intrusive) | 150–280 | Excellent; high strength aggregate |
Key Concepts for Students
- Granite = highest strength among common building rocks — the standard exam answer for this type of question.
- Rock strength must exceed the design concrete strength; for M60+ high-strength concrete, granite or basalt aggregates are specified.
- Aggregate should not be the weak link: IS 456 and IS 10262 require that aggregate Crushing Value (ACV) and Impact Value (AIV) meet minimum standards for the grade of concrete.