The short answer
A flexible highway pavement is built in five layers from the bottom up: compacted subgrade, granular sub-base (GSB), wet mix macadam (WMM) base, dense bituminous macadam (DBM) and a bituminous concrete (BC) wearing course. Each layer is stiffer and costlier than the one below it, and each calls for a different grading of crushed stone.
A road is a way of spreading a truck's wheel load until the ground underneath can carry it. Each layer takes the pressure it receives from above and hands a lower pressure to the layer below. The best material goes at the top, where the stress is highest, and the cheapest at the bottom, where there is most of it.
01The layers, bottom to top
| Layer | Usual compacted thickness | Material | Its job | MoRTH section |
|---|---|---|---|---|
| Subgrade | Top 500 mm of the formation | Selected soil, compacted | Carries everything above it | 300 |
| GSB | 150 to 300 mm | Graded crushed stone or gravel | Spreads load, drains water out | 401 |
| WMM | 150 to 250 mm | Graded crushed stone mixed at controlled moisture | The main granular structure | 406 |
| DBM | 50 to 100 mm and more | Coarse graded stone bound in bitumen | Structural bituminous base | 505 |
| BC | 30 to 50 mm | Finer graded stone bound in bitumen | The surface: grip, sealing, ride | 507 |
Thicknesses are set for each project under IRC:37 from the design traffic and the strength of the subgrade. A prime coat goes over the WMM and a tack coat between bituminous layers.
02Subgrade
The subgrade is not bought, it is made from the best soil available along the alignment, and it is the only layer whose strength the designer has to accept instead of specify. Its strength is expressed as a CBR value, and every layer above is sized from that figure. A weak subgrade means a thicker and more expensive pavement for the whole length it runs under.
It is also the layer that hides its faults longest. Short compaction looks identical to full compaction on the day. How it is specified and tested is in embankment and subgrade compaction.
03Granular sub-base
GSB does two things. It thickens the pavement cheaply, and it gives water a way out sideways before it softens the subgrade. The second job is why its grading matters so much. A sub-base with too many fines holds water like a sponge. One with too few will not lock together under a roller. The six gradings and where each is used are in GSB grading under Clause 401.
04Wet mix macadam
WMM is the layer that carries the traffic. It is crushed stone of several sizes proportioned to a dense grading, brought to the right moisture in a mixing plant, laid by a paver and rolled until the stones interlock. Its strength comes from stone bearing on stone, so particle shape and grading count for more than in the sub-base. The details are in WMM under Clause 406, and the two layers are compared in GSB vs WMM.
05The bituminous layers
Above the granular base the stone is bound with bitumen. DBM is the thick, coarse layer that takes bending. BC is the thin, dense one that vehicles touch. Both are made in a hot mix plant from single-size fractions, typically 20 mm, 10 mm, 6 mm and stone dust, blended to a job mix formula.
This is where aggregate quality is tested hardest. The stone must be strong, cubical, clean and able to hold bitumen in the presence of water. A quarry whose rock is fine for GSB can fail every one of those. The limits are set out in tests on aggregate for road work.
06Which product goes where
| Layer | Product ordered | Notes |
|---|---|---|
| GSB | GSB to the specified grading | One blended product, top size 53 or 75 mm |
| WMM | WMM, or 40 mm, 20 mm, 10 mm and dust for blending | Often mixed at the paving contractor's own plant |
| DBM and BC | 20 mm, 10 mm, 6 mm and dust | Proportions fixed by the hot mix design |
| Concrete structures | 20 mm, 10 mm and sand | To the concrete mix design |
| Dry lean concrete under a rigid pavement | 40 mm and smaller, blended | Where the road is concrete instead of bituminous |
All of these come off one three-stage plant by changing screen decks, which is why a single order can cover four layers. The full list of what we produce is on the materials page.
07A rigid pavement, briefly
A concrete road replaces the bituminous layers and the WMM with a slab. The usual build is subgrade, GSB, about 150 mm of dry lean concrete, and a pavement quality concrete slab on top. It uses less graded base material and a great deal more 20 mm, 10 mm and sand.