Gradation and Particle Size Analysis
Sieve analysis fundamentals
Gradation is determined by sieve analysis for material retained on the No. 200 (75 µm) sieve and by washing to determine material finer than No. 200. Know the standard sieve series and the difference between individual and cumulative percent retained, and how to compute percent passing. A candidate must understand that the sum of percent retained on all sieves plus percent passing the finest sieve should total 100% - a quick arithmetic check that catches transcription errors.
Traps in gradation questions
Common traps: (1) confusing percent retained with percent passing; (2) forgetting to include the wash loss in the total mass; (3) using an overloaded sieve (too much material on one sieve causes blinding and inaccurate separation); (4) reporting gradation on a wet-mass basis without correcting for moisture. Also watch for questions about nominal maximum size versus maximum size - nominal maximum is the largest sieve retaining more than 10%, while maximum size is the smallest sieve through which the entire sample passes.
Worked example
A 1000 g dry sample: 100 g retained on 9.5 mm, 300 g on 4.75 mm, 500 g on 2.36 mm, 100 g passing 2.36 mm. Percent passing 9.5 mm = 900/1000 = 90%; passing 4.75 mm = 600/1000 = 60%; passing 2.36 mm = 100/1000 = 10%. The nominal maximum size is 4.75 mm because 40% is retained above it (more than 10%).
Rule of thumb
Keep individual sieve loads moderate - a common guideline is to avoid overloading sieves so particles cannot blind the openings; if a sieve is overloaded, split the sample and re-sieve.