Aggregate Testing at Level 2: Sampling, Gradation, and Quality Tests
What Level 2 Testing Covers
Level 2 aggregate testing assumes you can run routine tests and also judge whether results are valid. Core concepts: representative sampling, moisture and density relationships, sieve analysis and gradation control, and durability/quality indices.
Sampling. Every result is only as good as the sample. Know that samples must be taken from the full cross-section of a stockpile or stream, never from the surface, and that a sample reduced by quartering or a mechanical splitter must remain representative. Trap: questions that describe scooping from the top of a pile - the correct answer is almost always that the sample is not representative.
Gradation. Understand cumulative percent retained and percent passing, how to compute them from individual sieve masses, and how to check a gradation against a specification band. Watch for the difference between percent passing and percent retained, and remember that the total mass after sieving should match the original mass within a small tolerance; a large loss signals a procedural error.
Moisture and density. Distinguish total moisture from absorption, and know that bulk specific gravity changes with moisture state. For compaction control, maximum dry density and optimum moisture come from a laboratory curve, while field density is compared to it to get relative compaction.
Durability and quality. Understand what a soundness test, Los Angeles abrasion, and flat-and-elongated particle counts indicate, and that a single failing index does not automatically reject material unless the specification says so.
Worked Example
A 1000 g sample: 150 g retained on the 4.75 mm sieve, 300 g on the 2.36 mm sieve, 550 g passing. Percent passing the 4.75 mm = (1000 − 150)/1000 × 100 = 85%. Percent passing 2.36 mm = (1000 − 150 − 300)/1000 × 100 = 55%. Always subtract cumulative retained, not individual retained.
Traps to Expect
- Confusing absorption with total moisture.
- Using individual retained mass instead of cumulative retained.
- Accepting a sample that was not properly reduced.
- Treating a single out-of-band sieve as automatic rejection without checking specification tolerances.