In short
- From the bottom, a conventional flexible highway pavement is sub-grade, granular sub-base (GSB), wet mix macadam (WMM), dense bituminous macadam (DBM) and bituminous concrete (BC).
- The sub-grade is compacted soil. Every layer above it is crushed stone: graded blends for GSB and WMM, single-size aggregate and crushed fines for the two bituminous layers.
- Stone quality tightens upwards. The aggregate impact value limit is 40 for GSB, 30% for WMM, 27% for DBM and 24% for BC in the MoRTH Fifth Revision (2013).
- Layer thickness is not a fixed list. IRC:37-2018 derives it from design traffic in msa and the effective CBR of the sub-grade, and sets only a few minimums.
Anyone buying material for a road is buying it for a particular layer, and the layer decides the grading, the test limits and the month it is needed.
A conventional flexible highway pavement has five layers. From the bottom: the sub-grade, the granular sub-base (GSB), the wet mix macadam base (WMM), the dense bituminous macadam (DBM) and the bituminous concrete wearing course (BC). The sub-grade is soil. The four layers above it, together called the crust, are all made from crushed stone.
The layers in order, from the bottom
| Layer | What it does | Material from the plant | MoRTH reference |
|---|---|---|---|
| Sub-grade | The top 500 mm of the earthwork: the foundation the pavement is designed on | None. Selected soil, compacted | Section 300, Clause 305 |
| Granular sub-base (GSB) | Drains the pavement, separates it from the soil, spreads load | Crushed stone blended to one of six gradings | Clause 401, Table 400-1 |
| Wet mix macadam (WMM) | Granular base: carries the load down to the sub-base | Crushed fractions and dust blended to one envelope | Clause 406, Table 400-13 |
| Dense bituminous macadam (DBM) | Bituminous base and binder course | Coarse aggregate and crushed fines for the hot mix plant | Clause 505, Tables 500-8 and 500-10 |
| Bituminous concrete (BC) | Wearing course: the surface traffic runs on | Smaller coarse aggregate and crushed fines, to the tightest stone limits | Clause 507, Tables 500-16 and 500-17 |
References are to the MoRTH Specifications for Road and Bridge Works, Fifth Revision (2013). A prime coat is sprayed on the finished WMM before the DBM, and a tack coat between bituminous layers. Both are bitumen and neither takes stone.
Sub-grade: the layer nobody buys
The sub-grade is the top 500 mm of the embankment or cutting, directly under the pavement. It comes from the earthwork, not the crusher, and MoRTH Table 300-2 wants it compacted to not less than 97% of maximum dry density by IS 2720 Part 8. IRC:37-2018 Clause 6.1 asks for the same on expressways, national and state highways.
It matters to a material buyer for one reason: its strength, expressed as CBR, is an input to the thickness of everything above it. A weak sub-grade means a thicker crust and more tonnes. CBR and what it does to pavement thickness explains the test.
GSB and WMM: the two granular layers
These two take most of a road's crushed stone, and they are the two a crusher plant produces as finished products. GSB is the lower one: a blend of crushed stone to one of the six gradings in Table 400-1, chosen in the contract, whose job is to drain and separate as much as to carry. GSB gradings under MoRTH Clause 401 has the full table.
WMM sits on it. One grading envelope, a limit on particle shape, water added in a mixing plant, laid by paver. It is the layer the bituminous courses bear on, and the one that shows first when a road starts to fail. The clause and its tables are in wet mix macadam under Clause 406.
DBM and BC: the bituminous layers
Above the WMM the stone is bound with bitumen in a hot mix plant, and the plant's product changes from a blend to separate sizes. DBM is the base and binder course. MoRTH gives it two gradings, with nominal aggregate sizes of 37.5 mm and 26.5 mm. BC is the wearing course, also in two gradings, with nominal sizes of 19 mm and 13.2 mm. So the hot mix plant draws on 20mm and 10mm chips and crushed fines, with a coarser fraction for the larger DBM grading.
Natural sand is not allowed in binder courses and is capped at half the fine aggregate in base courses, so most of the fines are crusher fines. Both layers limit dust passing 0.075 mm to 5 percent and water absorption to 2 percent. BC adds a polished stone value of at least 55, because it is the only layer a tyre touches.
| Layer | Aggregate impact value, maximum | Combined flakiness and elongation, maximum | Table |
|---|---|---|---|
| GSB | 40 | No limit | 400-2 |
| WMM | 30% | 35% | 400-12 |
| DBM | 27% | 35% | 500-8 |
| BC | 24% | 35% | 500-16 |
For WMM, DBM and BC the impact value is an alternative to a Los Angeles abrasion limit of 40%, 35% and 30% respectively. Source: MoRTH Fifth Revision (2013).
What IRC:37-2018 fixes and what it leaves to design
MoRTH says how each layer is made. How thick each one is comes from IRC:37-2018, the guideline for the design of flexible pavements, and the answer is a calculation, not a list. The inputs are the design traffic in million standard axles (msa), over a design period of 20 years for national and state highways, and the effective CBR of the sub-grade, which starts from four-day soaked CBR tests. For roads carrying more than 450 commercial vehicles a day, both directions together, Clause 6.4.3 says the effective sub-grade CBR should be more than 5 percent.
The guideline does set a few floors. Where the sub-base is built as separate drainage and filter layers, each is at least 100 mm, and a single filter-cum-drainage layer is at least 150 mm (Clause 7.2.2). An unbound granular base is not less than 150 mm (Clause 8.1). Those are minimums, not designs. The thicknesses on your drawings come from the designer's calculation for your traffic and your soil, and the guideline also offers cement-treated sub-bases and bases as alternatives to the granular layers.
Rigid pavement in one paragraph
A rigid pavement is a concrete road. The sub-grade and a sub-base stay, but in place of the WMM and the bituminous courses there is a layer of dry lean concrete (DLC) with a pavement quality concrete (PQC) slab on top. For the plant the order changes from blends to concrete aggregate: 40mm and 20mm, with fines to the concrete mix design.
What this means for the material order
The layers are built in sequence, so the demand arrives in sequence: GSB first, WMM behind it, DBM and BC aggregate last. A plant that knows the programme can stock each one ahead of its front. Once the cross-section is fixed, the quantity of GSB and WMM in a kilometre turns the drawing into tonnes.
We supply GSB, WMM, 40mm, 20mm, 10mm and crusher dust on our own tippers at a per-tonne delivered rate. The terms are on the aggregate supply page, and the uses of each size are on the materials page. Specifications are revised, and the clause and table named in your contract are the ones the material is tested against.
Standards and sources
- MoRTH Specifications for Road and Bridge Works, Fifth Revision (2013), Clause 305 and Table 300-2 (sub-grade)
- MoRTH Specifications for Road and Bridge Works, Fifth Revision (2013), Clauses 401 and 406, Tables 400-1, 400-2, 400-12 and 400-13
- MoRTH Specifications for Road and Bridge Works, Fifth Revision (2013), Clauses 505 and 507, Tables 500-8, 500-10, 500-16 and 500-17
- IRC:37-2018 Guidelines for the Design of Flexible Pavements, Clauses 4.3, 6.1, 6.2, 6.4.3, 7.2.2 and 8.1
- Products and delivery 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.