Durability and Soundness Testing
What durability tests measure
Durability tests assess an aggregate's resistance to weathering and breakdown. The soundness test cycles the aggregate through repeated immersion in a saturated salt solution (sodium or magnesium sulfate) followed by oven drying, and measures the resulting mass loss by sieve. The Los Angeles abrasion test measures resistance to fragmentation and wear by tumbling aggregate with steel balls in a rotating drum and reporting percent loss. Freeze-thaw and slake durability tests address related mechanisms.
Traps in durability questions
Common traps: (1) confusing soundness (chemical/weathering resistance) with abrasion (mechanical wear resistance); (2) forgetting that soundness loss is reported by sieve size fraction, not just a single number; (3) using the wrong salt or the wrong number of cycles; (4) ignoring that softer aggregates may break down during handling, inflating loss. Also watch for questions about deleterious materials - clay lumps, friable particles, and organic impurities - which are separate tests from soundness.
Worked example
If a 500 g sample of a given size fraction loses 25 g after the specified soundness cycles, the loss is 25/500 × 100 = 5.0%. Whether that passes depends on the specification limit for that fraction and salt type - sodium sulfate and magnesium sulfate limits differ, so always check which salt the specification references.
Rule of thumb
Soundness limits are typically expressed as a maximum percent loss per sieve fraction, and magnesium sulfate tends to produce less severe loss than sodium sulfate. Do not compare a magnesium-sulfate result against a sodium-sulfate limit.