Reviewed by Er. Inderjit Sharma (Retd.), Punjab PWD
What is specific gravity? It tells you how heavy the aggregate is compared with the same volume of water. A stone with specific gravity 2.65 is 2.65 times heavier than an equal volume of water.
Water absorption tells you how much water the stone soaks into its pores, as a percentage of its dry weight.
Why it matters: both values are needed for concrete and bituminous mix design, for correcting mixing water, and as a quick check of stone quality. This guide follows IS 2386 (Part 3) for aggregate larger than 10 mm.
The Four Moisture States of Aggregate
Apparatus Required
Balance (weighs in water)
Accurate to 0.5 g, with a hook to hang the basket below it in water.
Wire basket
6.3 mm mesh or finer; holds the stones under water.
Water tank
Watertight; keeps at least 5 cm of water over the basket.
Absorbent cloths
Two dry cloths to wipe the stones surface-dry.
Oven
Dries the sample at 105–110 °C for 24 hours.
Airtight container
Keeps the dried sample from picking up moisture while it cools.
Weighing in Water
Procedure
- Take a representative sample of at least 2 kg. Wash it to remove dust and fine particles.
- Put it in the wire basket and lower it into distilled water at 22–32 °C, with at least 5 cm of water above the basket.
- Lift the basket about 25 mm and drop it, 25 times, to remove trapped air.
- Leave it to soak for 24 ± ½ hours.
- Jolt it 25 times again and weigh the basket + aggregate in water (A1).
- Take out the aggregate. Put the empty basket back in the water, jolt it 25 times and weigh it (A2). A = A1 − A2 is the weight of the saturated aggregate in water.
- Spread the aggregate on a dry cloth and wipe it gently with a second cloth until no water film is visible but the stones still look damp. Spread it one stone deep and leave it for at least 10 minutes, away from sunlight and heat.
- Weigh the surface-dry aggregate in air (B).
- Dry it in the oven at 105–110 °C for 24 ± ½ hours, cool it in an airtight container and weigh it (C).
- Do at least two tests and report the mean.
Calculation
A
Weight of saturated aggregate in water (A1 − A2)
B
Weight of saturated surface-dry aggregate in air
C
Weight of oven-dry aggregate in air
Try It: Specific Gravity Calculator
Enter your weights in grams. The values filled in are a worked example.
Bonus: correct the mixing water
Precautions
🫧 Remove air
Jolt the basket 25 times before each weighing in water, so no air bubbles cling to the stones.
🧽 True SSD
Wipe until the shine of the water film disappears. Don’t over-dry: if stones turn light in colour, they have lost pore water.
⚖️ Weigh quickly
Weigh B straight after surface drying, before the stones dry further in air.
🌡️ Water temperature
Keep the water at 22–32 °C and make sure the basket never touches the tank sides or bottom.
Practical Tips for Site Engineers
1. Typical Values
| Stone | Specific gravity | Water absorption |
|---|---|---|
| Granite | 2.6–2.8 | 0.3–1% |
| Basalt / trap | 2.8–3.0 | 0.5–1% |
| Quartzite | 2.6–2.7 | 0.5–1% |
| Limestone | 2.5–2.7 | 0.5–2% |
Typical ranges only; actual values depend on the quarry. For road works, MoRTH generally limits water absorption to 2% (if higher, a soundness test decides acceptance). Follow your contract specification.
2. Good Site Habits
- Correct the mixing water every day: the mix design assumes SSD aggregate. Dry aggregate steals water from the mix (stiff concrete, low slump); wet aggregate adds water (weak concrete). Use the calculator above to adjust.
- Test each new quarry or source: specific gravity and absorption change from source to source. Recheck when the source changes.
- Read high absorption as a warning: above about 2% usually means porous or weathered stone that may be weak or not durable. Do soundness and impact tests too.
- Use the right value in calculations: SSD for concrete mix design and yield; bulk and apparent for bituminous mixes.
- Wet stockpiles in hot weather: sprinkling coarse aggregate keeps it close to SSD and avoids sudden slump loss.
- Quick quality hint: a stone that is unusually light for its size, or that darkens a lot when splashed with water, is likely porous. Test it before approval.
Key Takeaways
- Weigh in water (A), surface dry in air (B) and oven dry (C).
- Bulk SG = C ÷ (B − A); SSD SG = B ÷ (B − A); apparent SG = C ÷ (C − A).
- Water absorption = (B − C) ÷ C × 100.
- Soak 24 h in water at 22–32 °C; jolt 25 times; oven dry 24 h at 105–110 °C; do two tests.
- On site: correct mixing water for aggregate moisture; absorption above 2% is a warning sign.
References: IS 2386 (Part 3): Methods of Test for Aggregates – Specific Gravity, Density, Voids, Absorption and Bulking; IS 10262: Concrete Mix Proportioning; MoRTH Specifications for Road and Bridge Works.