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.
Quality and technical information
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.
To someone unfamiliar with it, an aggregate test report is a collection of numbers. But each number answers a specific question: what size is the material, how clean is it, how long will it last, and how does it behave with water and cement?
Grading
Sieve analysis (ASTM C136) is the basis of most assessments. Its result is a curve showing the percent passing each sieve, which is compared with standard limits. Fineness modulus comes from the same results. For details, see Fineness modulus of sand.
Cleanliness
Two main tests belong in this group:
- Material finer than the 75 µm sieve (ASTM C117): measures fines by washing the sample.
- Sand equivalent (ASTM D2419): shows the ratio of sand to clay-like material by a quick method suited to site control.
The organic impurities test (ASTM C40) also shows whether sand contains matter that may disturb cement setting.
Durability and strength
| Test | Main question | Example limit |
|---|---|---|
| Los Angeles abrasion (ASTM C131) | How well does the rock resist impact and abrasion? | In ASTM C33, a maximum of 50% for many classes |
| Sulfate soundness (ASTM C88) | How stable is the rock against weathering? | In ASTM C33, a maximum of 12% with sodium sulfate or 18% with magnesium sulfate for coarse aggregate |
These limits are examples from one standard. The limit a project needs depends on its class, exposure and technical specification.
Density and absorption
Relative density and absorption (ASTM C127 for coarse and ASTM C128 for fine aggregate) feed directly into mix design. Without these figures you cannot convert material mass to volume or correct mix water for aggregate moisture.
Shape and fractured faces
The percentage of flat and elongated particles (ASTM D4791) and of fractured particles (ASTM D5821) matter especially for asphalt and road layers, because the stability of those layers depends on particle interlock.
Sampling properly
A test result is only as valid as its sample. A sample taken only from the surface of a stockpile does not represent the material, because coarse particles roll to the bottom. Samples must be taken from different points and reduced by a standard method.
Interpreting results
No single test decides the quality of a material. Review results together and against the project specification. The full list of methods is in the quality and standards section.
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