The quality of concrete can be improved by using
Correct Answer: C. graded aggregates
📚 Detailed Explanation: Graded Aggregates and Concrete Quality
The grading of aggregate refers to the distribution of particle sizes in a sample. A well-graded aggregate follows a grading curve that ensures the right proportions of large, medium, and small particles, so that smaller particles fill the gaps between larger ones progressively, minimising void content.
Why C (graded aggregates) is correct: Graded aggregates improve concrete quality because: (1) They minimise void content in the aggregate skeleton; (2) Less cement paste is needed to fill voids, reducing shrinkage and heat; (3) The denser packing improves compressive strength; (4) Lower permeability improves durability. Single-size aggregate (Option A) has the highest void content and requires the most paste. Fine aggregate only (B) or coarse aggregate only (D) cannot produce structural concrete on their own — they are individual fractions that must be combined in the correct proportions.
Effect of Aggregate Grading on Concrete Quality
| Aggregate Type | Void Content | Paste Required | Concrete Quality |
|---|---|---|---|
| Single-size (gap-graded) | Highest (~40–50%) | Most | Weakest, most permeable |
| FA only or CA only | High | High | Not structural |
| Well-graded (combined) | Lowest (~28–35%) | Least | Best strength and durability |
Key Concepts for Students
- IS 383 specifies grading envelopes for both fine and coarse aggregate; these envelopes define what counts as “well-graded.”
- A well-graded combined aggregate for concrete typically follows the Fuller's parabola: p = 100 × (d/D)^0.5, where d = particle size, D = maximum size.
- Graded aggregates also improve workability because the surplus paste (beyond void-filling) lubricates particle surfaces.