Simplified Guide to the Soundness Test for Aggregates (IS 2386 Part V - 1963)
The soundness test evaluates the durability of aggregates against weathering, particularly freeze-thaw cycles and chemical attack, which can cause disintegration. This is critical for ensuring aggregates used in concrete can withstand environmental stresses in structures like roads, bridges, and buildings.
Why Test Soundness?
Aggregates are exposed to harsh environmental conditions, such as freezing and thawing or wet-dry cycles, which can cause cracking, splitting, or crumbling. The soundness test simulates these conditions to ensure aggregates remain intact, maintaining the structural integrity and longevity of concrete.
Soundness Test Overview
What It Measures
The test measures the resistance to disintegration of aggregates when subjected to repeated cycles of immersion in a saturated sodium sulphate or magnesium sulphate solution, followed by drying. The percentage of material lost (passing a specified sieve) indicates the aggregate’s soundness.
Why It Matters
- Aggregates with low soundness loss are more durable and suitable for concrete exposed to weathering, such as in pavements or marine structures.
- High soundness loss indicates poor durability, which could lead to concrete deterioration over time.
- The test helps when service records of aggregates under actual weathering conditions are unavailable.
Key Notes
- Sodium sulphate and magnesium sulphate produce different results, so specifications must define appropriate limits for each.
- The test is more severe for closely sized aggregates than for graded aggregates, affecting limit specifications.
Apparatus
- Sieves (conforming to IS 460-1962):
- Fine Series: 150, 300, 600 microns, 1.18 mm, 2.36 mm, 4.00 mm, 4.75 mm.
- Coarse Series: 8.0 mm, 10 mm, 12.5 mm, 16 mm, 20 mm, 25 mm, 31.5 mm, 40 mm, 50 mm, 63 mm.
- Containers: Perforated (e.g., wire mesh baskets or sieves) to allow solution access and drainage without aggregate loss.
- Temperature Regulation: To maintain samples at 27 ± 1°C during immersion.
- Balances:
- Fine aggregates: ≥500 g capacity, sensitive to 0.1 g.
- Coarse aggregates: ≥5000 g capacity, sensitive to 1 g.
- Drying Oven: Maintained at 105–110°C, with an evaporation rate of at least 25 g/h for 4 hours (tested with 500 g water in 1-litre beakers).
Reagents
- Sodium Sulphate Solution:
- Prepare a saturated solution using technical-grade sodium sulphate (IS 255-1950), anhydrous (Na₂SO₄) or crystalline (Na₂SO₄·10H₂O).
- Dissolve at 25–30°C, using ≥420 g anhydrous or ≥1300 g decahydrate per litre of water to ensure excess crystals.
- Cool to 27 ± 2°C for 48 hours, stir frequently, and check specific gravity (1.151–1.174). Discard or filter discoloured solutions.
- Magnesium Sulphate Solution:
- Prepare using technical-grade magnesium sulphate (IS 257-1950), anhydrous (MgSO₄) or crystalline (MgSO₄·7H₂O, Epsom salt).
- Dissolve at 25–30°C, using ≥400 g anhydrous or ≥1400 g heptahydrate per litre of water.
- Cool to 27 ± 1°C for 48 hours, stir frequently, and check specific gravity (1.295–1.308). Discard or filter discoloured solutions.
Sample Preparation
- Fine Aggregate:
- Wash on a 300-micron sieve, dry to constant weight at 105–110°C.
- Sieve into fractions (150 micron to 4.75 mm, as per 4.1). Select ~110 g per fraction, sieve to refusal, and use 100 g samples.
- Coarse Aggregate:
- Wash and dry to constant weight at 105–110°C.
- Sieve into fractions (4.75 mm to 80 mm, as per 4.2). Use specified weights:
- 10–4.75 mm: 300 g
- 20–10 mm: 1000 g (33% 12.5–10 mm, 67% 20–12.5 mm)
- 40–20 mm: 1500 g (33% 25–20 mm, 67% 40–25 mm)
- 63–40 mm: 3000 g (50% 50–40 mm, 50% 63–50 mm)
- ≥80 mm: 3000 g per 20-mm size fraction
- Count particles for fractions >20 mm.
- All-in-Aggregate:
- Separate into fine (<4.75 mm) and coarse (>4.75 mm) fractions, testing each as above.
- Note on Small Fractions:
- If a size fraction is <5% of the sample, assume its loss equals the average of the next smaller and larger sizes (or the nearest size if one is absent).
Test Procedure
- Immersion:
- Immerse samples in sodium or magnesium sulphate solution for 16–18 hours, ensuring ≥15 mm solution coverage and a solution-to-sample volume ratio of ≥5:1.
- Maintain temperature at 27 ± 1°C. Cover containers to minimize evaporation.
- Drying:
- Drain samples for 15 ± 5 minutes, then dry at 105–110°C to constant weight (successive weights within 0.1 g for fine, 1 g for coarse aggregates).
- Cool to room temperature.
- Cycles:
- Repeat immersion and drying for the number of cycles agreed upon by the purchaser and vendor (typically 5 cycles, per common practice).
- Quantitative Examination:
- After the final cycle, wash samples until free of sulphate (test wash water with barium chloride).
- Dry to constant weight at 105–110°C.
- Sieve fine aggregates over their original sieve, coarse aggregates over specified sieves:
- 63–40 mm: 31.5 mm
- 40–20 mm: 16 mm
- 20–10 mm: 8.0 mm
- 10–4.75 mm: 4.0 mm
- Calculate loss as the percentage of material passing the sieve.
- Qualitative Examination (for >20 mm fractions):
- After each immersion, observe effects (e.g., disintegration, splitting, crumbling, cracking, flaking).
- At test end, count affected particles and classify the type of damage.
Reporting Results
Include:
- Weight of each fraction before testing.
- Percentage loss for each fraction (material passing the specified sieve).
- Weighted average loss based on the sample’s grading or the supply’s average grading (assume 0% loss for <300-micron fractions).
- For >20 mm fractions: number of particles before testing and number affected (by disintegration, splitting, etc.).
- Type of solution used (sodium or magnesium sulphate).
Example Table (Table I from the Standard)
