Introduction

Soundness is the property of aggregates to resist excessive volume changes and disintegration when subjected to weathering conditions.

This test determines the resistance to disintegration of aggregates caused by saturated solutions of sodium sulphate or magnesium sulphate. It furnishes information helpful in judging soundness when service records are not available.

Mechanism

The test simulates weathering action through salt crystallization in aggregate pores:

Process Steps

1. Salt Penetration: Saturated sulfate solution penetrates aggregate pores during immersion
2. Crystallization: During drying (105–110 °C), salt crystals form within pores
3. Expansion: Upon re-immersion, salt crystals rehydrate and expand, creating internal pressure
4. Disintegration: Repeated cycles cause cracking, splitting, and breakdown of weak aggregates

Simulated Conditions

This process simulates:

• Freezing and thawing cycles
• Wet-dry cycles
• Marine environment exposure
• Chemical attack from sulfate-bearing soils

Formulas

Individual Fraction Loss

$$ \text{Percentage Loss} = \frac{W_1 - W_2}{W_1} \times 100 $$

Where:

• $W_1$ = Initial weight of fraction before test (g)
• $W_2$ = Final weight retained on specified sieve after test (g)

Weighted Average Loss

$$ \text{Weighted Average Loss} = \frac{\sum (P_i \times L_i)}{100} $$

Where:

• $P_i$ = Percentage of each size fraction in original sample
• $L_i$ = Percentage loss of that size fraction

Interpretation

Lower percentage loss values indicate better soundness and greater resistance to weathering conditions. Specifications typically limit the weighted average loss to acceptable thresholds based on aggregate type and intended use. For most concrete applications, the maximum recommended weighted average loss is about 5% for fine aggregates and about 12% for coarse aggregates.