The first number a purchase desk wants on a new package is the aggregate quantity per km, and the number it usually gets is one somebody remembers from the last job. That figure was right for a different cross-section. Be a tenth out on a 60 km four-lane package and the gap is about six months of a 250 TPH plant's output.
So here is one kilometre worked out in full: the GSB quantity per km first, then WMM, DBM and BC, with every assumption written down and labelled as one. The material required for 1 km of road is only multiplication. The judgement is in which widths and densities go into it, and those have to come from your drawing, not from this page.
What the aggregate quantity per km depends on
The aggregate quantity per km of a highway is the compacted volume of each pavement layer over one kilometre, multiplied by that layer's compacted density, with the binder taken off for the bituminous layers. That makes four inputs for each layer.
- Width. The width of that layer, which is not the carriageway width and is different for every layer. It comes from the typical cross-section in the agreement or the approved drawings.
- Thickness. A pavement design output under IRC:37, set by design traffic and sub-grade CBR. It is on the pavement composition table, not in the MoRTH specification.
- Density. The maximum dry density from your own laboratory for GSB and WMM, and the compacted density in the job mix formula for DBM and BC. Rock type moves it: a dense basalt runs heavier than a sandstone.
- Binder content. For DBM and BC only. The mix is weighed with its bitumen in it, and the crusher supplies none of that.
If the layer names are new to you, the layers of a highway pavement sets out what each one is and which MoRTH clause governs it.
The section assumed in this example
A four-lane divided highway in plain terrain, new construction, flexible pavement, both carriageways alike. The carriageway and shoulder widths are those of IRC:SP:84, the manual for four-laning, in its 2019 edition. The layer widths, the crust and the densities are illustrative. Treat every row as a blank to be filled in from your own drawing and laboratory report.
| Item | Assumed here | Where yours comes from |
|---|---|---|
| Carriageway | 7.0 m, two lanes | Typical cross-section. 7.0 m a side is the IRC:SP:84 width |
| Paved shoulder | 2.5 m, same crust as the carriageway | Typical cross-section. IRC:SP:84-2019 gives 2.5 m; a later ministry guideline gives 2.0 m on four-lane work |
| Earthen shoulder | 1.5 m | Typical cross-section |
| BC and DBM width | 9.5 m | Carriageway plus paved shoulder |
| WMM width | 10.0 m | Illustrative: 0.25 m beyond each bituminous edge |
| GSB width | 11.5 m | Illustrative: under the earthen shoulder to the slope, plus 0.5 m on the median side |
| Thickness | GSB 200 mm, WMM 250 mm, DBM 100 mm, BC 40 mm | Pavement design under IRC:37 |
| Compacted density | GSB 2.25, WMM 2.30, DBM 2.40, BC 2.40 t/m³ | Laboratory MDD for granular layers, job mix formula for bituminous |
| Binder content | DBM 4.5%, BC 5.4% by mass of mix | Job mix formula. These are the MoRTH minimums for DBM Grading 2 and BC Grading 2 |
Left out on purpose: the median and its edge strip, service roads, junctions, structures and all concrete. Add them from your own bill of quantities.
GSB and WMM quantity per km
The two granular layers are worked the same way: length by width by compacted thickness, twice over for the two carriageways, then by density.
Volume = 1,000 m × 11.5 m × 0.20 m × 2 = 4,600 m³
Tonnage = 4,600 m³ × 2.25 t/m³ = 10,350 t
About 10,350 tonnes of GSB a kilometre
GSB is the widest layer and the first one laid, so it is the first tonnage a package calls for. Which of the six gradings it is makes no difference to this sum and a great deal to the plant; that is covered in GSB gradings under MoRTH Clause 401. If the drawing shows the sub-base in two layers, a drainage layer over a lower one, work them separately. They are different products from different stockpiles.
Volume = 1,000 m × 10.0 m × 0.25 m × 2 = 5,000 m³
Tonnage = 5,000 m³ × 2.30 t/m³ = 11,500 t
About 11,500 tonnes of WMM a kilometre
A 250 mm base is normally laid in two layers, which changes the rate of supply and not the total. The grading is in WMM under MoRTH Clause 406. One trap: the densities here are dry. WMM leaves the mixing plant with water in it, so a weighbridge slip for wet mix shows more tonnes than the dry aggregate in the load. Say in the order which basis the quantity and the rate are on.
DBM and BC: mix tonnes and aggregate tonnes
Bituminous layers are measured as mix, and a crusher supplies only the stone in the mix. Work out the mix tonnage, then take off the binder.
Volume = 1,000 m × 9.5 m × 0.10 m × 2 = 1,900 m³
Mix = 1,900 m³ × 2.40 t/m³ = 4,560 t
Aggregate = 4,560 t × 0.955 (4.5% binder) = 4,355 t
About 4,355 tonnes of aggregate for DBM a kilometre
Volume = 1,000 m × 9.5 m × 0.04 m × 2 = 760 m³
Mix = 760 m³ × 2.40 t/m³ = 1,824 t
Aggregate = 1,824 t × 0.946 (5.4% binder) = 1,726 t
About 1,726 tonnes of aggregate for BC a kilometre
MoRTH Table 500-10 offers DBM in two gradings: Grading 1 with a nominal aggregate size of 37.5 mm, laid 75 to 100 mm thick, and Grading 2 at 26.5 mm, laid 50 to 75 mm. So 100 mm is one layer of the first or two of the second, and only Grading 1 needs 40mm stone. BC at 40 mm is Grading 2 of Table 500-17, nominal size 13.2 mm. The split between 20mm, 10mm and dust is fixed by the job mix formula, so ask the mix designer for the bin proportions before ordering by size, and check the edition of the specification your contract cites.
Tonnes per km by product
| Layer | Compacted volume | Tonnes per km | Share | What the plant supplies |
|---|---|---|---|---|
| GSB | 4,600 m³ | 10,350 | 37% | GSB to the contract grading |
| WMM | 5,000 m³ | 11,500 | 41% | WMM, or the 40mm, 20mm, 10mm and dust it is blended from |
| DBM | 1,900 m³ | 4,355 | 16% | 20mm, 10mm and dust, with 40mm for Grading 1 |
| BC | 760 m³ | 1,726 | 6% | 10mm and dust, with 20mm for Grading 1 |
| Total | 12,260 m³ | 27,931 | 100% | Net, before wastage |
The DBM and BC rows are aggregate only, after taking off the binder.
Close to four fifths of the stone in a flexible pavement is GSB and WMM. The bituminous aggregate is the small share and the demanding one: it has to pass the tightest limits on shape and strength, and it is wanted last.
Why the lower layers are wider
Each layer has to sit on something wider than itself. A paver and a roller cannot compact to the very edge of an unsupported layer, so the layer beneath is drawn a little wider to give that edge something to bear on, and the load coming down through the pavement spreads outwards as it goes. The sub-base has a second reason. It is the drainage layer, and water moving sideways through it needs somewhere to come out, so on most cross-sections the GSB is carried under the earthen shoulder to the embankment slope.
The effect on quantity is larger than it looks. Here the BC is 9.5 m wide and the GSB 11.5 m. Take the quantity off by multiplying every thickness by the 9.5 m and the GSB is under-ordered by 800 m³ a kilometre, which is 1,800 tonnes. Over 60 km that is just over a lakh tonnes of sub-base nobody budgeted for.
Wastage, and turning tonnes into plant-months
Everything above is net, in place. An estimator adds an allowance for what is lost between the stockpile and the finished layer: spillage, edges trimmed off, the bottom of a stockpile that has picked up the ground it sat on, trial stretches, and work rejected and relaid. No clause fixes the percentage. Two to five per cent is a common planning range on granular layers, and the best guide is what your last package used.
Net quantity = 27,931 t
Allowance at 3% = 27,931 t × 0.03 = 838 t
Order quantity = 27,931 t + 838 t = 28,769 t
Call it 28,800 tonnes a kilometre for this section
A single 250 TPH three-stage plant will hold 25,000 to 30,000 MT a month on a normal two-shift pattern. That is our planning figure, and it puts this kilometre at about one plant-month.
One kilometre = 28,769 t ÷ 25,000 to 30,000 t a month = 0.96 to 1.15 plant-months
60 kilometres = 28,769 t × 60 = 1,726,140 t
Plant-months = 1,726,140 t ÷ 25,000 to 30,000 t = 58 to 69
Monthly need = 1,726,140 t ÷ 20 months = 86,307 t
Plants = 86,307 t ÷ 25,000 to 30,000 t = 2.9 to 3.5
About 17.3 lakh tonnes, and three 250 TPH plants to lay it in 20 working months
The 20 working months are an assumption: the construction period less mobilisation and the monsoon. And the average hides the shape. A road takes GSB first, WMM next and bituminous aggregate last, so the peak month sits well above the average. Sizing a plant against that peak is worked through in what TPH a package needs.
What to send for a quantity and a rate
Send the typical cross-section, the pavement composition table and the construction programme, and say which layers you want supplied. With those three a supplier can work the tonnes by product and by month, and say whether the job is a haul from an existing plant or a plant on the site. We do both. Delivered material from our own plants is on the aggregate supply page, and putting one of our plants on your package is under crusher plant operations.