How to Read Aggregate Gradation Curves
A gradation curve is the aggregate's fingerprint: one line that says whether the product sells, the plant is healthy, and the customer's concrete will behave. Learning to read the shape — not just the pass/fail — is what separates QC technicians from QC professionals.
01Orient the axes
X-axis: sieve size, plotted logarithmically, coarse on the right. Y-axis: cumulative percent passing, 0 at bottom, 100 on top. Every point answers one question — what fraction of the sample is smaller than this opening.
02Draw the spec band first
Overlay the specification limits (ASTM C33 size number, DOT table, customer spec) as an envelope. The product must live inside the band at every control sieve. A curve hugging one edge of the band is technically passing and practically warning.
03Read the shape
A smooth S-curve = well-graded, sizes distributed for tight packing. A steep near-vertical curve = uniform/single-size (open-graded — what a #57 or #89 chip should be). A flat shelf in the middle = gap-graded: sizes missing, segregation and finishing problems ahead.
04Pull the three working numbers
D50 (the size at 50% passing) tells you central coarseness at a glance. The 200-wash (% passing #200) is the cleanliness number — dust kills bond and blinds screens. For sands, compute fineness modulus: sum the cumulative retained on the standard sieves ÷ 100; concrete sand wants roughly 2.3–3.1.
05Diagnose the plant from the drift
Curves moving coarse over weeks: screens blinding or wear opening the crusher setting. Fines creeping up: worn liners, wet screening trouble, or a crusher choking. One wild sample is sampling error; three trending samples is maintenance's problem. Plot the history, not the day.
06Check the blend math when fixing
Blending streams combines linearly at each sieve: blend %passing = Σ(stream fraction × stream %passing). A product failing #8 by four points can often be rescued with 15–20% of a finer stream — compute it before touching the screen deck.
This is bread-and-butter ShotRock QC work — gradation forensics, spec-compliance root cause, and blend fixes with the math shown.
Need this done right on a real project?
ShotRock provides independent due diligence, reserve estimation, mine engineering, and economics — senior-level only, worldwide.
Figures are approximate, compiled from public sources as of mid-2026, and rounded for readability. Rankings shift with markets, mergers, and new discoveries. Nothing here is investment, legal, or engineering advice for a specific site — for that, hire an engineer. We know one.