Which characteristic of fine aggregates indicates their ability to retain water and affects the workability of concrete or mortar mix?
Correct Answer: D. Water absorption
📚 Detailed Explanation: Water Absorption of Fine Aggregates
When fine aggregates are in a dry state, they absorb mixing water into their pores. This absorbed water is no longer available to lubricate the mix, effectively reducing the free water content and lowering workability.
Why D (Water absorption) is correct: Water absorption directly measures how much water an aggregate retains. High absorption = more water locked in pores = less free water = reduced workability. Option A (particle size distribution) affects grading and surface area but not water retention per particle. Option B (specific gravity) is used in mix design volume calculations, not water retention. Option C (fineness modulus) is a single-number measure of average particle size, not a water retention indicator.
Aggregate Characteristics and Their Role
| Property | What It Measures | Effect on Workability |
|---|---|---|
| Water Absorption | Water retained in pores | High absorption = lower workability |
| Particle size distribution | Grading of particles | Well-graded = better workability |
| Specific gravity | Density ratio | Mix design, not direct workability |
| Fineness modulus | Average particle size index | Coarser FM = slightly better workability |
- Aggregates should be in Saturated Surface Dry (SSD) condition for accurate mix design — absorbed water is accounted for.
- High water-absorption aggregates require extra water in the mix design to compensate and achieve target workability.