In short
- 10mm goes into the wearing course and finer concrete, 20mm into DBM and structural concrete, 40mm into lean concrete, drainage layers and mass concrete.
- Nominal size names the sieve that nearly all of the stone passes. A 20mm aggregate is a fraction: mostly passing 20 mm and mostly retained on the next deck down.
- Bituminous mixes are blends. MoRTH's DBM gradings have nominal sizes of 37.5 mm and 26.5 mm, and its BC gradings 19 mm and 13.2 mm.
- Layer thickness limits stone size. IRC:37-2018 prefers a compacted granular layer to be at least 2.5 times the nominal maximum aggregate size.
- An order needs the size, the grading or code it must meet, tonnes per month, the delivery point and the billing unit.
Ask a crusher for aggregate and the first question back is what size. The short answer is the one most buyers want: 10mm for the top of a road and for thin concrete, 20mm for dense bituminous macadam and structural concrete, 40mm for lean concrete, drainage and mass work.
That answer is right as far as it goes. It leaves out that the three sizes are fractions of a grading and not complete materials, that most layers in a road are blends of all of them, and that the thickness of the layer sets the largest stone you can use.
What nominal size means
A 20mm aggregate is not a pile of 20 mm stones. It is the material that passed one screen deck and was held on the next one down, so nearly all of it is smaller than 20 mm and nearly all of it is larger than the 10mm cut below. The name is the upper sieve. Some oversize and undersize is allowed, and the governing grading table says how much.
The sizes are also yard names. Specifications are written on test sieves that do not match them: MoRTH's wet mix macadam table, for one, is set out on 45, 22.4 and 11.2 mm sieves. So the size is where an order starts. The grading table is what the material gets tested against.
Which size goes where
| Size | Typical uses | Where in a road or structure |
|---|---|---|
| 40mm | Dry lean concrete, drainage layers, soling, mass concrete | Lean concrete and drainage |
| 20mm | DBM, structural concrete, precast, pavement quality concrete | Bituminous macadam and structural concrete |
| 10mm | Bituminous concrete, surface dressing, slab and beam concrete, paver blocks | Wearing course and finer concrete |
| Crusher dust | Fine aggregate, paver bedding, block and brick making, filling and levelling | Fine aggregate and filler in the mixes above |
| GSB and WMM | Highway sub-base and base course, haul roads, hardstanding | Graded mixes of every size, to MoRTH Clauses 401 and 406 |
Uses as listed on our materials page. The mix design and the contract specification decide the proportions.
Sizes in the bituminous layers
No bituminous mix is made from one stockpile. The hot mix plant draws 20mm, 10mm, dust and sometimes 40mm from separate cold bins and blends them into a grading envelope. The nominal size of that blend is fixed by the specification and tied to the layer thickness.
| Mix | Nominal aggregate size | Layer thickness |
|---|---|---|
| DBM Grading 1 | 37.5 mm | 75-100 mm |
| DBM Grading 2 | 26.5 mm | 50-75 mm |
| BC Grading 1 | 19 mm | 50 mm |
| BC Grading 2 | 13.2 mm | 30-40 mm |
MoRTH Specifications for Road and Bridge Works, Fifth Revision (2013), Tables 500-10 (DBM) and 500-17 (BC). Standards get revised and the contract governs.
Read that table against the stockpiles. DBM Grading 1 needs stone up to 37.5 mm, so a 40mm fraction has to be on site. BC Grading 2 is built mostly from 10mm and dust. Tell the crusher which grading the mix design uses before the decks are set, because the top deck for a 26.5 mm mix is not the top deck for a 19 mm one.
Layer thickness limits the size
A layer cannot be compacted properly if its largest stones are close to its own thickness. IRC:37-2018 Clause 7.2.2 puts a figure on it: a compacted granular layer should preferably be at least 2.5 times the nominal maximum aggregate size, and not less than 100 mm. On that rule a mix with a 40 mm maximum wants 100 mm of compacted thickness. The thinner the layer, the smaller the stone.
In concrete the limit comes from the member. The stone has to pass between the bars and into the cover, which is why 20mm is the default for structural work, 10mm is used in thin or congested sections, and 40mm is kept for lean and mass concrete. The governing rule sits in the structural design code and the approved mix design.
Single-size and graded aggregate
Single-size aggregate is one fraction between two adjacent decks: the 40mm, 20mm and 10mm on a plant's stockpile list. Graded aggregate is a controlled blend from a top size downwards, sold as '20mm down' or all-in. Batching plants and hot mix plants want single sizes, because they can proportion each one and correct a drifting grading with a bin setting.
GSB and WMM are graded by definition. WMM under MoRTH Table 400-13 runs from 100% passing 53 mm down to no more than 5% passing 75 micron, and it is mixed in a plant with controlled water, not ordered as three sizes and blended on the road by a grader. The layers of a highway pavement shows where each mix sits.
Shape matters as much as size
Two loads of 20mm with the same sieve result can behave differently in the mix. Flat and elongated stone passes the same mesh as cubical stone, then breaks under the roller and takes more bitumen or cement to coat. MoRTH limits combined flakiness and elongation to 35% for WMM, DBM and BC (Tables 400-12, 500-8 and 500-16), and IS 383:2016 Clause 5.3 allows 40% for concrete. The flakiness and elongation index explains the test and what in the plant controls it.
How one plant makes every size
All of these come off the same plant. Ours run a primary jaw, a secondary cone and a tertiary stage feeding screen decks, and the decks decide the products: change the mesh and the same crushed rock is split at different sizes. If the specified grading changes mid-project we change the decks, which is a shift of work inside the crushing, not a renegotiation.
What no plant can do is make one size without the others. Crushing down to 10mm makes dust, and making 20mm leaves 10mm behind. Supply runs best when the buyer takes the sizes in something like the proportion the rock yields them, and the rate reflects it when he does not.
How to order
- The product: 10mm, 20mm, 40mm, dust, GSB or WMM.
- The grading or code it has to meet: the MoRTH table and grading number, or IS 383 with single-size or graded stated. 'As per standard' is not a specification.
- Tonnes per month, and for how many months. Below roughly one tipper load a day a local yard will serve you better than a plant.
- The delivery point: nearest town, chainage or road head. Inside about 60 km of one of our plants, a delivered rate is usually competitive.
- The unit. We bill by the weighbridge tonne. If your BOQ is in cubic metres, converting tonnes, cubic metres and brass shows the arithmetic.
With those five in one message, aggregate supply can be discussed usefully on the first call. The materials page lists what comes off our plants.
Standards and sources
- MoRTH Specifications for Road and Bridge Works, Fifth Revision (2013): Tables 500-10 and 500-17 (DBM and BC gradings), Table 400-13 (WMM grading), Tables 400-12, 500-8 and 500-16 (combined flakiness and elongation)
- IRC:37-2018, Guidelines for the Design of Flexible Pavements, Clause 7.2.2
- IS 383:2016, Coarse and Fine Aggregate for Concrete (Third Revision), Clause 5.3
- Product uses, plant configuration and supply terms: SM Infra's own operating record
Your contract and its technical schedules override anything written here. Check the clause before you build to it.