How to choose the aggregate fraction
An aggregate fraction describes the nominal range of particle sizes. However, it does not tell you everything about the material. When you buy, you need to know the intended use and the design requirements first, and only then choose a designation from the range on offer.
What do the two numbers mean?
In a d/D designation, the numbers refer to the sieve sizes that define the grading. Do not read it as a guarantee that every single particle falls exactly between those two sizes; the specification may set a permitted proportion of other sizes.
A designation that starts with zero describes a mix that also contains fine particles. A narrower range indicates a different grading. The two groups can play different roles in layers and in drainage.
Why does grading matter?
In a properly designed mix, smaller particles can fill the spaces between larger ones. This is important when you build a layer with specific properties. It does not mean, however, that every mix will be right for every use.
Where the solution is meant to carry water away, the requirements may be different. So you need to distinguish between a structural, a drainage and a decorative function.
Matching the fraction to the job
For a sub-base, follow the requirements for the material and the layer. For a bedding layer, check the method specified for the surface. For concrete, what matters is the whole mix design and the geometry of the element, not simply buying aggregate of a similar size.
Our aggregates table confirms which fractions we offer. Do not assume a designation is available just because it appears in a guide or was used on another site.
Questions about fractions
Is a larger particle size better for a road?
Not always. What matters is the whole grading, the properties of the material and its role in the layer structure.
Can similar fractions be swapped?
A change has to be checked against the requirements. A similar range does not mean an identical material.
How should I give the fraction when ordering?
Copy the designation from the design documents or from what you agreed with your contractor, together with the intended use.
Source: GDDKiA (Polish national roads authority), sub-base of unbound mixture – document in Polish.