Soundness of aggregate and its measurement

Soundness is the ability of an aggregate to resist excessive changes in volume due to changes in physical conditions such as freezing and thawing, alternate wetting and drying, or temperature changes. Unsound aggregates swell, crack and disintegrate, causing pop-outs and loss of durability of the concrete.

Laboratory measurement (IS 2386 Part 5)

Soundness is tested by accelerated attack with sodium sulphate or magnesium sulphate.

1. Take washed, oven-dried and sieved fractions of the aggregate (e.g. 10-20 mm for coarse aggregate; 300 micron to 10 mm for fine aggregate), weighing 100 g per fraction (fine) or the specified quantity (coarse).
2. Immerse the sample in a saturated solution of sodium sulphate (or magnesium sulphate) for 16-18 hours at 27 ± 1 °C. Salt crystals form in the pores.
3. Remove it, drain for 15 min, then dry it in the oven at 105-110 °C. This is one cycle.
4. Repeat for 5 cycles. The crystallisation pressure in the pores is similar to that of ice formation.
5. Wash out the salt (until the barium chloride test shows no sulphate), dry, and sieve over the original sieve.
6. Calculate the percentage loss in weight.

$$\text{Loss}\ \% = \frac{\text{original weight} - \text{final weight retained}}{\text{original weight}} \times 100$$

Limits (IS 383)

After 5 cycles, the weighted average loss should not exceed 12 per cent with sodium sulphate and 18 per cent with magnesium sulphate for coarse aggregate, and 10 per cent (sodium sulphate) and 15 per cent (magnesium sulphate) for fine aggregate.