Coarse Aggregate for Concrete
Coarse aggregate typically makes up the largest single volume fraction of a concrete mix. EN 12620, the European aggregates standard, sets out how it is sized and graded, and Dmax selection has a direct effect on the water demand of the mix.
Size designation: d/D
EN 12620 designates a coarse aggregate size fraction as d/D — d is the smaller nominal sieve size and D the larger, so a 4/20 aggregate is nominally between 4 mm and 20 mm. A ‘graded’ aggregate spans a d/D range in one delivered fraction; a ‘single-size’ aggregate has d and D close together (like 10/14) and fractions are combined to build up the overall grading.
Choosing the maximum aggregate size, Dmax
Dmax is chosen against the section thickness, cover to reinforcement and reinforcement spacing — as a guide it should generally not exceed the smallest gap it must pass through. A larger Dmax reduces the water demand for a given workability (less total aggregate surface area to wet), which is one reason it is often chosen as large as the section allows; see water content.
Grading category G
EN 12620 also defines grading categories (denoted G, such as GC80/20) that set limits on the percentage passing selected sieve sizes, giving a way to specify how well-graded a coarse aggregate is. A well-graded aggregate packs more densely, leaving less void space for cement paste to fill, which helps both economy and workability.
Frequently asked questions
What does 4/20 mean on a delivery ticket?
It is the d/D size designation under EN 12620 — the aggregate is nominally graded between 4 mm (d) and 20 mm (D).
Why not always use the largest Dmax possible?
A larger Dmax must still pass comfortably through the narrowest gap in the formwork or reinforcement, and very large aggregate can also affect the finish and pumpability of the concrete, so Dmax is capped by the geometry of the section, not chosen for water-demand reasons alone.
Does aggregate shape matter as well as size?
Yes — rounded gravel and angular crushed rock behave differently in the mix even at the same Dmax; see water content for how shape affects water demand.