Order 20mm aggregate from three crushers and you can receive three different materials, all correctly described. The size on the challan is a nominal size: a label for a band between two sieves, with a tolerance at each end. Whether the tolerance matters depends on what the stone is going into.
This page takes the three sizes a highway job orders most, 10mm, 20mm and 40mm aggregate, and sets out what each may legally contain and which layer or mix uses it. Concrete sizes are from IS 383:2016 and road references from the MoRTH Specifications for Road and Bridge Works, Fifth Revision. Check the edition your contract cites.
What a nominal size means
Nominal size is the sieve that nearly all of the material passes, not the size of every piece. IS 383:2016, the Indian standard for coarse and fine aggregate for concrete, sets the limits for single-sized aggregate in Table 7. Figures are per cent passing by weight.
| IS sieve | 40mm | 20mm | 10mm |
|---|---|---|---|
| 63 mm | 100 | – | – |
| 40 mm | 85–100 | 100 | – |
| 20 mm | 0–20 | 85–100 | – |
| 12.5 mm | – | – | 100 |
| 10 mm | 0–5 | 0–20 | 85–100 |
| 4.75 mm | – | 0–5 | 0–20 |
| 2.36 mm | – | – | 0–5 |
The same table also gives 63, 16 and 12.5 mm single sizes, and graded aggregate of 40, 20, 16 and 12.5 mm nominal size. A dash means no limit on that sieve.
Read the 20mm column. Up to 15 per cent of the load may be coarser than 20 mm, provided all of it passes 40 mm. Up to 20 per cent may be finer than 10 mm, and up to 5 per cent may pass 4.75 mm, which is not coarse aggregate at all.
Coarser than 20 mm = up to 15% = 3.0 t
Finer than 10 mm = up to 20% = 4.0 t
of which under 4.75 = up to 5% = 1.0 t
Between 10 and 20 mm = at least 65% = 13.0 t
Over a third of an in-specification load can sit outside the 10 to 20 mm band
A load at those limits passes. So does a load of clean 12 to 20 mm stone with none of that in it. They will not behave the same in a mixer, which is why a mix design is tied to a source and why changing supplier in the middle of a pour is a bigger step than it looks.
Graded aggregate is a different product under the same name. Graded 20mm in Table 7 needs 90 to 100 per cent passing 20 mm and 25 to 55 per cent passing 10 mm: in effect 20mm and 10mm already blended. Say which one you want.
40mm aggregate uses: mass concrete, WMM and drainage
40mm is the size for concrete with room in it. MoRTH Table 1700-7 allows a 40 mm maximum nominal size in well steining, well caps, pile caps and solid piers and abutments. Plain concrete under foundations takes it for the same reason: fewer, larger stones have less surface for the cement paste to coat.
In the pavement, stone of this size is the coarse fraction of wet mix macadam, whose grading runs up to 53 mm, and of DBM Grading 1. Outside it, 40mm is drainage and filter stone behind walls and soling under floors and hardstanding.
20mm aggregate uses: structural concrete, PQC and DBM
20mm is the default. Most reinforced concrete is designed around it. MoRTH Table 1700-7 fixes 20 mm for RCC girders, slabs, kerbs and piles and for all prestressed work, and IS 456 holds the nominal maximum size to a quarter of the thinnest dimension of the member, and lower again where the bars are close together. For ordinary sections that lands on 20 mm.
On the road, 20mm is the coarse fraction of dry lean concrete and, with a slightly coarser cut alongside it, of pavement quality concrete. In bituminous work it is the stone that makes DBM Grading 2 and the top end of bituminous concrete Grading 1.
10mm aggregate uses: wearing course and tight sections
10mm does three jobs. It is the intermediate fraction in almost every graded mix, filling the gaps between 20mm stones in concrete, DLC, DBM and WMM. It is the main stone in bituminous concrete Grading 2. And as clean single-sized chips it is surface dressing: MoRTH Clause 509 lists 19, 13, 10 and 6 mm chips, single-sized and cubical, with the size chosen by traffic and by how hard the existing surface is.
In concrete, 10mm takes over where 20mm will not fit: thin toppings, precast items, paver blocks, and sections where the bars are too close for larger stone to pass. Sizes finer than 10mm are screened to order. Our standard single sizes are 40mm, 20mm and 10mm, and anything outside that is a question for the plant.
How the trade sizes map to MoRTH bituminous mixes
Bituminous specifications do not use 40, 20 and 10. MoRTH names each mix by its nominal aggregate size on the 37.5, 26.5, 19 and 13.2 mm sieve series, defined as the largest specified sieve on which any of the aggregate is retained.
| Mix | Nominal size | Layer thickness | Fractions usually blended |
|---|---|---|---|
| DBM Grading 1 | 37.5 mm | 75–100 mm | 40mm, 20mm, 10mm, dust |
| DBM Grading 2 | 26.5 mm | 50–75 mm | 20mm, 10mm, dust |
| BC Grading 1 | 19 mm | 50 mm | 20mm, 10mm, dust |
| BC Grading 2 | 13.2 mm | 30–40 mm | 10mm, dust |
Fifth Revision. The last column is trade practice, not specification: the job mix formula fixes the fractions and their proportions, and a hot mix plant will often ask for an intermediate cut as well.
So an order for a hot mix plant should quote the mix and the table, and the crusher should cut its decks to the bituminous sieves. Stone screened to concrete sizes can be blended into a DBM envelope, but the 26.5 and 13.2 mm control sieves fall between the concrete sizes, which leaves the blend less room to move.
Why shape matters as much as size
A sieve measures one dimension. A slab of stone 30 mm long and 8 mm thick goes through a 20 mm opening on its edge and is counted as 20mm aggregate. In a mix it is nothing of the kind. It bridges, traps voids, snaps under a roller and needs more binder or paste to coat.
So shape is specified on its own. MoRTH limits the combined flakiness and elongation index to 35 per cent for WMM, DBM, BC and pavement concrete, and IS 383:2016 sets 40 per cent for concrete aggregate generally. Shape comes from the rock and from how the last crushing stage is run. A cone kept choke-fed at the right setting, or a VSI, makes cubes; a starved cone on worn liners makes flakes. The test is explained in flakiness and elongation index and the machines in jaw, cone and VSI crusher stages.
Which size goes into which layer
| Use | 40mm | 20mm | 10mm |
|---|---|---|---|
| Wet mix macadam | Coarse fraction | In the blend | In the blend |
| Dry lean concrete | Not used | Coarse fraction | In the blend |
| Pavement quality concrete | Not used | Coarse fraction | In the blend |
| RCC and prestressed concrete | Not used | Main stone | In the blend |
| Mass and foundation concrete | Main stone | In the blend | In the blend |
| DBM Grading 1 | Coarse fraction | In the blend | In the blend |
| DBM Grading 2 | Not used | Coarse fraction | In the blend |
| BC Grading 1 | Not used | Coarse fraction | In the blend |
| BC Grading 2 | Not used | Not used | Main stone |
| Surface dressing | Not used | Rarely, as 19 mm chips | Single-sized chips |
| Drainage and soling | Main stone | Sometimes | Not used |
A guide to ordering, not a mix design. Pavement quality concrete also takes a cut between 20 and 31.5 mm where the plant makes one.
Ordering by size
Give the supplier the use, not only the size. 20mm for structural concrete, 20mm for DBM Grading 2 and 20mm for DLC are three different cuts off the same plant. Add the standard the stone will be tested against, the monthly quantity and the delivery point, and remember the order will be weighed, not measured: the conversions are in aggregate weight in cubic metres, CFT and brass. Our plants produce 40mm, 20mm and 10mm with the decks set to the grading the contract cites. The sizes are listed on materials, and rates and lead distances start on the aggregate supply page.