For estimating the water absorption of 4.75 mm aggregate used for concrete, the prescribed oven drying temperature (°C) is:
Correct Answer: A. 100 – 110°C
🧱 Detailed Explanation: Oven Drying Temperature for Aggregate Water Absorption
Water absorption of aggregates is determined by comparing the mass of a saturated surface-dry (SSD) aggregate sample with the same sample after complete drying. The drying temperature is a carefully specified parameter under IS 2386 (Part III): 1963 to ensure all pore moisture is driven off without damaging the aggregate’s mineral structure.
Why A (100–110°C) is the correct answer:
Water boils at 100°C at standard atmospheric pressure, so any temperature at or above this guarantees all pore water (both free and capillary) is evaporated. The upper limit of 110°C provides a small safety margin to accelerate drying, while staying well below the temperatures that could cause thermal decomposition or mineralogical changes in the aggregate. This is the same standard oven-drying range used in soil mechanics (IS 2720) and for determining moisture content in building materials.
Water boils at 100°C at standard atmospheric pressure, so any temperature at or above this guarantees all pore water (both free and capillary) is evaporated. The upper limit of 110°C provides a small safety margin to accelerate drying, while staying well below the temperatures that could cause thermal decomposition or mineralogical changes in the aggregate. This is the same standard oven-drying range used in soil mechanics (IS 2720) and for determining moisture content in building materials.
• <100°C (D: 90–100) → May not fully dry pores; results would be underestimated.
• 110–120°C (B) or 120–130°C (C) → Risk of mineralogical changes in heat-sensitive aggregates (e.g., shales, chalk); results would be unreliable.
Water Absorption Test Procedure (IS 2386 Part III)
| Step | Action |
|---|---|
| 1. Saturation | Immerse aggregate in water for 24 hours at room temperature |
| 2. SSD condition | Remove and surface-dry with a damp cloth; weigh → W1 |
| 3. Oven drying | Dry at 100–110°C for 24 hours; cool in desiccator |
| 4. Weighing | Weigh oven-dry sample → W2 |
| 5. Calculation | Water absorption = (W1 − W2) ÷ W2 × 100 (%) |
Key Concepts for Students
- 100–110°C is the standard oven-drying temperature for aggregates — the same range used in soil mechanics (IS 2720) and general material moisture testing.
- Water absorption reveals aggregate porosity — IS 383 limits absorption to ≤2% for coarse aggregate to ensure good freeze-thaw resistance and chemical durability.
- High water absorption means the aggregate will absorb mixing water, reducing the effective w/c ratio and workability. This must be corrected in the mix design (add extra water equal to absorbed water).
- The SSD condition is the mix-design reference: aggregate in SSD state adds zero net water to or draws zero net water from the mix.