Sieve Analysis That Fails a Gradation Band on One Sieve
A 20 kg sample of 19.0 mm nominal-maximum crushed limestone is reduced by quartering and sieved. Results (percent passing): 25.0 mm 100; 19.0 mm 96; 12.5 mm 58; 9.5 mm 41; 4.75 mm 12; 2.36 mm 6; 1.18 mm 4; 300 µm 2; 75 µm 1.1. The specification band for the 12.5 mm sieve is 40–70% passing, and for the 4.75 mm sieve 0–10% passing. The 12.5 mm result is inside the band; the 4.75 mm result (12%) exceeds the 10% limit. The producer argues the sample was 'borderline' and asks for a retest. The technician notes the sample was reduced from a stockpile that had been rained on the previous day and that the 75 µm result is 1.1%, above the 1.0% limit for the application.
1. Evaluate whether a retest is technically justified, and identify what a retest can and cannot resolve.
Read the worked answer
Two independent failures exist: 4.75 mm at 12% vs. a 10% maximum, and 75 µm at 1.1% vs. a 1.0% maximum. Neither is a rounding-level miss except the fines, which is marginal.
A retest is justified only if there is a documented reason to doubt sample representativeness - and here there is one: the stockpile was rained on, which can concentrate fines at the surface and alter the fine end of the gradation. However, a retest cannot 'fix' a genuine nonconformity; it can only establish whether the first sample was representative. The correct procedure is to resample from a different location and depth using the established sampling plan, reduce by a method that preserves fines (avoiding loss of minus-75 µm material during quartering), and test the new sample. Averaging the two results is not appropriate for acceptance; each sample is evaluated against the limit.
The producer's 'borderline' framing is not a technical argument. The 4.75 mm exceedance is 20% over the limit and is unlikely to be explained by sampling alone.
2. The 75 µm result is 1.1%. Describe the handling errors that most commonly inflate or deflate this value and how they should be controlled.
Read the worked answer
The minus-75 µm fraction is the most handling-sensitive part of a gradation. Common errors:
- Loss of fines during quartering or splitting (deflates the value) - control by using a splitter or careful coning-and-quartering and by not blowing dust away.
- Oven-drying at excessive temperature or for too long, which can cause agglomeration of clay-sized particles that then fail to pass the 75 µm sieve (deflates the value) - control by drying at the specified temperature and disaggregating gently.
- Incomplete washing during the wash gradation, leaving fines on the coarse fractions (deflates the passing value) - control by washing to visually clear water.
- Contamination from a dirty sieve or from the previous sample (either direction) - control by cleaning and inspecting sieves.
Given the marginal 1.1% result, the technician should verify by wash gradation on a properly reduced subsample and confirm the sieve is clean and undamaged before concluding nonconformity.