Aggregate Quality Control Tests: What Does Each Test Show?
A guide to common aggregate tests, including grading, sand equivalent, Los Angeles abrasion, soundness and absorption, and how to interpret their results.
Quality and technical information
How to Read a Grading Curve: A Practical Guide for Aggregates
A grading curve shows how particle sizes are distributed. Learn continuous, gap and uniform grading and the coefficients of uniformity and curvature.
The grading curve is the most widely used chart in aggregate assessment. It shows what share of a material passes each sieve, and that largely explains how a mix behaves in concrete, asphalt and road layers.
How the curve is made
A sample passes through a standard sieve series (ASTM C136) and the mass retained on each sieve is recorded. The horizontal axis shows sieve size, usually on a logarithmic scale, and the vertical axis shows percent passing.
Three common curve shapes
| Type | Appearance | Material behaviour |
|---|---|---|
| Continuous | Gentle, even slope | High density, few voids |
| Uniform | Very steep slope | Many voids, good permeability |
| Gap | A nearly flat section | One size range is missing |
This is why base material is continuously graded so it can compact, while drainage material is closer to uniform so that water can pass.
Coefficients of uniformity and curvature
Two numbers sum up the curve. If D10, D30 and D60 are the sizes that 10, 30 and 60 percent of the material passes:
- Coefficient of uniformity: Cu equals D60 divided by D10
- Coefficient of curvature: Cc equals D30 squared, divided by the product of D10 and D60
In the Unified Soil Classification System, gravel with Cu of at least 4 and sand with Cu of at least 6 count as well graded if Cc lies between 1 and 3.
Reading a curve against standard limits
Reports usually plot two limit curves from a standard and one sample curve. Falling between the limits is necessary but not sufficient. A curve that runs from near the upper limit to near the lower limit may be within range and still make an unbalanced mix.
A control note
One curve shows the state of one sample. Comparing curves from successive shipments shows whether a material is uniform. For sand, fineness modulus makes this comparison simpler. Other tests are reviewed in the guide to aggregate tests.
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