Pavement Layers
A pavement is a layered load-spreading system. This guide covers each layer, what it is made of, how thick it should be, and what it looks like when it fails.
Recommended thickness
| Application | Surface course | Binder / base course | Unbound base |
|---|---|---|---|
| Village access road | TST surface dressing | — | 150 mm ABC |
| Residential street | 40 mm AC wearing | 50 mm ABC | 200 mm aggregate base |
| Urban road | 50 mm AC wearing | 75 mm ABC | 250 mm aggregate base |
| Provincial highway | 50 mm AC wearing | 100 mm ABC | 250 mm base + 200 mm sub-base |
| National highway | 50 mm AC wearing | 125–175 mm ABC | 300 mm base + 250 mm sub-base |
| Motorway | 50 mm SMA or AC | 175–225 mm ABC | 300 mm base + 300 mm sub-base |
| Industrial yard | 50 mm AC | 100 mm ABC | 300 mm base |
| Parking area | 40 mm AC | 50 mm ABC | 200 mm base |
Mix types
| Designation | Description | Density (kg/m³) | Typical use | Standard |
|---|---|---|---|---|
| Asphaltic Concrete Wearing Course | Class A / Class B | 2,340 | 40–50 mm wearing course | NHA GS Item 305 |
| Asphaltic Base Course | ABC | 2,360 | 50–100 mm base | NHA GS Item 306 |
| Asphalt Treated Base | ATB | 2,350 | Heavy-duty base | NHA GS |
| Bituminous Macadam | Open-graded base | 2,180 | Profile correction | NHA GS |
| Triple Surface Treatment | TST / chip seal | — | Surface dressing | NHA GS Item 303 |
| SMA | Stone Matrix Asphalt | 2,400 | Heavy traffic corridors | AASHTO M 325 |
| Recycled asphalt (RAP) | Milled material | 2,000 | Base, shoulders, access tracks | AASHTO M 323 |
| Aggregate Base Course | Granular base | 2,200 | 150–300 mm | NHA GS Item 304 |
| Sub-base | Granular sub-base | 2,150 | 150–300 mm | NHA GS |
| Bitumen 60/70 | Penetration grade binder | 1,030 | Binder and tack coat | AASHTO M 20 |
| Bitumen 80/100 | Penetration grade, cold regions | 1,025 | Northern areas | AASHTO M 20 |
| Emulsion, tack coat | Cationic emulsion | 1,010 | Between lifts | NHA GS Item 302 |
Waste and compaction allowance
Loose material behind the paver occupies roughly 20% more volume than the finished compacted surface. Add edge trimming, material left in the vehicle body and normal measurement tolerance, and you need an allowance on top of the calculated tonnage.
| Situation | Allowance | Reason |
|---|---|---|
| Simple rectangle, machine-laid, experienced crew | 5% | Minimal trimming and predictable compaction |
| Standard driveway or car park | 10% | Normal edge detail and some hand work at the perimeter |
| Curved edges or several separate zones | 15% | More offcuts and more hand placement |
| Hand-laid work and small patches | 15% | Hand compaction is less consistent |
| Recycled planings | 15–20% | Higher compaction factor and less predictable spread |
| Unbound granular base | 20–25% | Significant consolidation under the roller |
Round the final figure up to the nearest half tonne. Being short means a cold joint in the middle of the surface and a second delivery charge, which costs far more than the extra material would have.
The base underneath
The unbound layer under the surfacing does more structural work than the surfacing itself, and it is where most premature failures actually originate. A thinner bound layer over a deep, well-compacted, free-draining base will outlast a thicker bound layer over a shallow one.
| Subgrade | Approximate CBR | Base under domestic access | Base under commercial |
|---|---|---|---|
| Well-graded gravel | 20–40 | 100 mm | 150 mm |
| Sand, well drained | 10–20 | 100–150 mm | 150–200 mm |
| Silty sand | 6–10 | 150 mm | 200 mm |
| Loam | 5–8 | 150–200 mm | 200–250 mm |
| Silty clay | 3–5 | 200–250 mm | 250–300 mm |
| Heavy clay, poorly drained | 2–3 | 250–300 mm plus geotextile | 300–450 mm plus stabilisation |
| Peat or organic soil | under 2 | Excavate and replace | Excavate and replace |
Two additions repay their cost on marginal ground: a separation geotextile between the subgrade and the granular layer, which stops stone punching down into soft clay and can save 50–100 mm of aggregate; and proof-rolling with a loaded vehicle before any stone is placed, which finds the soft spots while they are still cheap to deal with.
Remember that the granular base is usually the larger tonnage of the two. A 100 m² area with 200 mm of base at 2,100 kg/m³ needs 42 tonnes of stone against roughly 12 tonnes of surfacing at 50 mm. Budget and order for both.
Standards and who administers them
Surfacing work in Pakistan is specified under NHA General Specifications and AASHTO material standards, administered by the National Highway Authority (NHA) and provincial highway departments. In practice that governs three things you will see on any quotation or drawing:
- The mix designation. Each layer has a defined material with a stated aggregate size, binder grade and target void content. This is what determines the density used in your tonnage calculation.
- The layer thicknesses. Minimum and maximum lift thicknesses are tied to the nominal aggregate size, because a lift thinner than about twice the largest stone cannot be compacted and one thicker than about three times it cannot be compacted through.
- Compaction acceptance. Density is normally specified as a percentage of the laboratory maximum rather than an absolute figure, because the maximum varies with the mix design.
For private domestic work none of this is legally binding, but the same physics applies. Asking a contractor which specification they are working to, and getting the compacted thickness written into the quotation in millimetres, is the single most useful thing you can do to make competing quotations comparable.
Diagnosing surface problems
| What you see | Where it originates | What to do |
|---|---|---|
| Loose aggregate on the surface (ravelling) | Wearing course — oxidised binder | Surface dressing or a thin overlay |
| Longitudinal depressions in the wheel path (rutting) | Wearing or binder course | Plane off and replace with a stiffer mix |
| Interconnected polygonal cracking | Structural — base or subgrade | Reconstruction; an overlay will reflect through within two years |
| Straight cracks along the paving line | Construction joint | Seal, then overlay |
| Regular cracks across the carriageway | Binder grade against climate | Seal; specify a softer low-temperature binder next time |
| Localised low spots and ponding | Subgrade settlement | Excavate, rebuild the formation and repave that area |
| Crumbling edges | Unsupported edge | Backfill against the edge or install an edge restraint |
| Potholes | Water plus existing cracking | Full-depth patch, then fix the drainage that caused it |
Maintenance schedule
| Interval | Action | Why it matters |
|---|---|---|
| Within the first 48 hours | Keep all vehicles off | The mat is still cooling and takes a permanent impression easily. |
| First 30 days | Avoid turning the wheel while stationary; support trailer jacks on a board | The binder has not fully hardened and point loads leave marks. |
| Within 6–12 months | First surface dressing or seal, if specified | Slows oxidation of the binder. Applying it sooner traps volatiles and softens the surface. |
| Annually | Sweep, clear drainage, inspect edges and joints | Standing water at an edge is the most common cause of premature failure. |
| Every 1–2 years | Seal any cracks that have appeared | A sealed crack costs a fraction of the pothole it becomes. |
| Every 3–5 years | Reseal and, on car parks, re-line | Restores surface texture and appearance. |
| Every 10–15 years | Consider a surface course overlay | Resets the wearing layer while the structure below survives. |
| 20–25 years | Full reconstruction | Only when the structure itself has failed, not merely the surface. |
Deferred maintenance compounds quickly. A crack sealed early costs very little; the same crack left for two winters admits water, softens the base and becomes a full-depth patch. The economics of surfacing maintenance are almost entirely about intervening before water reaches the layers below.
Calculator
Frequently asked questions
How many tonnes do I need?
Area in m² × compacted thickness in m × 2.34. A 100 m² area at 50 mm needs about 11.7 tonnes.
How many m² does a tonne cover?
About 8.5 m² at 50 mm compacted.
What allowance should I add?
10% for standard work, 5% for simple machine-laid rectangles, 15% for curved or hand-laid areas.
Does the calculator handle several areas?
Yes — add a zone per shape, each with its own thickness and mix type. Multi-layer structures and irregular sites are entered directly.
Which standards apply?
NHA General Specifications and AASHTO material standards, administered by the National Highway Authority (NHA) and provincial highway departments.