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.
Coarse aggregate in UK specifications
UK coarse aggregates are declared against BS EN 12620 using the d/D size notation (for example 4/20 or 10/20), and PD 6682-1 sets out the UK guidance on the categories normally specified for concrete. BS 8500-2 then adds the requirements that are specific to UK concrete, including those for recycled and recycled concrete aggregates. For most general structural work the choice is between a 20 mm and a 10 mm maximum size, with 40 mm used mainly in mass concrete and some foundations.
- 20 mm (4/20) is the everyday UK default for reinforced concrete in slabs, beams, walls and foundations where bar spacing and cover allow it.
- 10 mm (4/10 or 2/10) suits thin sections, congested reinforcement, pumped toppings and some precast work, but needs more paste and therefore more cement for the same strength and workability.
- 40 mm reduces paste demand in large pours; check that the pump, cover and reinforcement spacing can accept it.
Recycled and secondary aggregates in BS 8500
The UK has made wider use of recycled aggregate (RA) and recycled concrete aggregate (RCA) than many countries, and BS 8500-2 sets requirements and limits on where they can be used in structural concrete. RCA, crushed from graded concrete of known origin, is usually treated more favourably than mixed recycled aggregate. Before using either, confirm with the specification and the producer that the proposed replacement level is permitted for the exposure class and strength class, and use the tested particle density and water absorption in the mix calculation — recycled materials often absorb considerably more water than natural gravel or crushed rock.
Alkali–silica reaction: a UK-specific check
Alkali–silica reaction (ASR) is managed in the UK through guidance that considers the reactivity of the aggregate combination, the alkali content of the concrete and the exposure to moisture. BRE Digest 330 and Concrete Society Technical Report 30 are the commonly cited references. In practice, the producer normally assesses the aggregate classification (low, normal or high reactivity) and limits the reactive alkali content of the mix accordingly; SCMs such as fly ash and GGBS are often part of the strategy. If you are specifying a mix for a wet or buried environment, ask how ASR has been addressed rather than assuming the aggregate supplier has done it.
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.