What Is Aggregate and Why Does Gradation Matter?
The particle-size mix of an aggregate shapes how it performs, not just its size
What aggregate is
Aggregate is the general term for granular material — sand, gravel, crushed stone — used as a component in concrete, asphalt, and as a standalone base or fill material. It's typically split into fine aggregate (sand-sized) and coarse aggregate (gravel/crushed-stone-sized).
Why gradation is a separate property from size
Two aggregate samples can have the same maximum particle size but very different gradations — one might be concentrated mostly in one size range (gap-graded), while another has a smooth spread across many sizes (well-graded). A sieve analysis is the standard test used to determine this distribution.
Why a well-graded mix generally performs better
In a well-graded aggregate, smaller particles fill the voids between larger ones, producing a denser overall structure once compacted — this generally improves strength and reduces the volume of binder (cement or asphalt) needed to fill remaining voids. A gap-graded or uniformly-sized aggregate leaves more void space, which can require more binder or produce a weaker, more permeable result, depending on the application.
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Sources
ASTM C136
Standard Test Method for Sieve Analysis of Fine and Coarse Aggregates.