Aggregate Sampling and Sample Reduction
Why sampling dominates error
Almost every aggregate test result is only as good as the sample that reached the balance. A candidate must know the difference between a field sample (material taken from the stockpile, belt, truck, or roadway) and a test sample (the reduced portion actually tested), and must recognize that the test sample must be representative of the field sample. Sampling from the surface of a stockpile is a classic trap: segregation puts coarse particles at the toe and fines in the core, so samples are normally taken from the upper portion of a stockpile or from a belt/truck where the full gradation is present.
Reduction methods
Know the two accepted reduction techniques: quartering (mixing, flattening into a cone, quartering, and retaining opposite quarters) and the mechanical splitter (riffle box) with an even number of chutes. Traps include: using a splitter whose chute width is too small for the top size, failing to mix the sample before reduction, and reducing a wet sample without accounting for moisture. A wet sample cannot be reduced reliably by splitting because fines adhere to coarse particles.
Worked rule of thumb
A useful rule: the minimum field sample mass should be large enough that after reduction the test sample still exceeds the minimum mass for the largest particle size expected. If the top size is 25 mm (1 in.), a minimum test sample on the order of 10 kg is a reasonable working figure for gradation, and larger for tests requiring multiple splits. When in doubt, take more material and split down - never "top up" a small sample.
Traps in questions
Questions often ask which sample is most representative (answer: a properly collected belt or splitter sample, not a grab sample from the pile face). Also watch for confusion between sample mass minimums and test portion sizes for individual tests.