Reviewed by Er. Inderjit Sharma (Retd.), Punjab PWD
What is soundness? It is the ability of aggregate to resist breaking down from weathering: repeated wetting and drying, heating and cooling, and freezing. Unsound stone slowly cracks and crumbles in the structure.
How is it tested? The aggregate is soaked in sodium or magnesium sulphate solution and dried in an oven, again and again (usually 5 cycles). The salt crystals that grow in the pores act like frost and try to split the stone. We then measure how much it has broken down.
This guide follows IS 2386 (Part 5).
How the Test Works
Apparatus Required
IS sieves
80 mm down to 150 micron, to split the sample into size fractions.
Wire mesh containers
Hold each fraction while it soaks; solution drains freely.
Sulphate solution
Saturated sodium or magnesium sulphate, at least 5 times the sample volume.
Oven (105–110 °C)
Dries the sample after every soaking.
Balances
500 g (0.01 g) for fine and 10 kg (1 g) for coarse fractions.
Hydrometer & thermometer
Check the solution’s specific gravity and keep it at 27 °C.
Preparing the Solution
| Item | Sodium sulphate | Magnesium sulphate |
|---|---|---|
| Salt per litre of water (at 25–30 °C) | ~420 g anhydrous or ~1300 g decahydrate | ~400 g anhydrous or ~1400 g heptahydrate |
| Keep at | 27 ± 2 °C for at least 48 h | 27 ± 1 °C for at least 48 h |
| Specific gravity before use | 1.151–1.174 | 1.295–1.308 |
Stir while adding the salt; some crystals should stay undissolved (saturated solution). Discard or filter discoloured solution.
Sample Fractions
Fine aggregate (below 4.75 mm)
Wash on the 300 micron sieve and dry. Take at least 100 g of each size: 4.75–2.36 mm, 2.36–1.18 mm, 1.18 mm–600 µm, 600–300 µm.
Coarse aggregate (above 4.75 mm)
Wash, dry and sieve. Take 300 g of 10–4.75 mm, 1000 g of 20–10 mm, 1500 g of 40–20 mm and 3000 g of 63–40 mm (and of each larger size).
Skip small fractions
A size that makes up less than 5% of the sample is not tested; it takes the loss of the nearest size.
Procedure
- Put each fraction in its wire container and immerse it in the solution for 16–18 hours, with at least 15 mm of solution above it. Keep it covered at 27 ± 1 °C.
- Take it out, drain for about 15 minutes, then dry it in the oven at 105–110 °C to constant weight. Let it cool.
- That is one cycle. Repeat for the number of cycles specified (usually 5).
- After the last cycle, wash the sample free of sulphate. Test the wash water with a few drops of barium chloride; no white cloudiness means the salt is gone.
- Dry each fraction and sieve it on the sieve given below. Weigh what is retained.
- Also look at each piece for splitting, crumbling, cracking or flaking, and note it.
Sieve for Finding the Loss (Coarse Aggregate)
| Size fraction | Sieve used after the test |
|---|---|
| 63–40 mm | 31.5 mm |
| 40–20 mm | 16 mm |
| 20–10 mm | 8 mm |
| 10–4.75 mm | 4 mm |
For fine aggregate, use the same sieve on which the fraction was originally retained.
Calculation
A = percentage of that size in the original grading of the sample. Sizes finer than 300 micron are taken as zero loss.
Try It: Soundness Calculator
Enter the original grading (%) and the weights before and after for each fraction. The values filled in are a worked example for 20 mm coarse aggregate.
Precautions
🌡️ Control temperature
Keep the solution at 27 °C during soaking; salt solubility changes with temperature and so does the test’s severity.
🧪 Check specific gravity
Check the solution with a hydrometer before each use; replace it if it’s discoloured or out of range.
🚿 Wash out all salt
Leftover sulphate adds weight and hides the loss. Wash until the barium chloride test shows no cloudiness.
📝 Note the solution used
Magnesium sulphate is harsher than sodium sulphate, so results aren’t comparable. Always report which one you used.
Practical Tips for Site Engineers
1. Typical Limits (after 5 cycles)
| Aggregate | Sodium sulphate | Magnesium sulphate |
|---|---|---|
| Fine aggregate | max 10% | max 15% |
| Coarse aggregate | max 12% | max 18% |
As per IS 383 and MoRTH. Follow your contract specification.
2. Good Site Habits
- Plan ahead: five cycles take at least 5–7 days. Send samples from a new quarry early so approval doesn’t hold up work.
- When it’s needed most: soundness matters for structures exposed to weather, frost, wetting and drying, or salty conditions: bridges, pavements, marine and canal works.
- Use it as a deciding test: MoRTH asks for a soundness test when the water absorption of aggregate is more than 2%. A porous stone that passes soundness may still be accepted.
- Look at the quarry: weathered, stained, laminated or soft-looking rock, and stone that flakes or crumbles in old stockpiles, deserves a soundness test.
- Read it with other tests: soundness, AIV, abrasion, shape and water absorption together decide acceptance.
Key Takeaways
- Soundness = resistance to weathering, tested by repeated sulphate soaking and oven drying.
- Soak 16–18 h at 27 °C, drain, oven dry; repeat for 5 cycles; wash out the salt.
- Loss per fraction (C) × original grading (A) ÷ 100 = weighted loss (D); add up all D.
- Typical limits: fine 10% (Na) / 15% (Mg); coarse 12% (Na) / 18% (Mg).
- On site: plan for the 5–7 day test; it decides acceptance when absorption is above 2%.
References: IS 2386 (Part 5): Methods of Test for Aggregates – Soundness; IS 383:2016: Coarse and Fine Aggregate for Concrete; MoRTH Specifications for Road and Bridge Works.