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Material World
Material · Engineered

Asphalt concrete

Stone held together with the bottom of the oil barrel — the surface of nearly every road, and the most recycled material there is by tonnage.

Construction material

Asphalt concrete is graded stone with bitumen as the binder — typically 95 per cent aggregate and 5 per cent bitumen by mass. The stone carries the load through the interlock of the particles, and the bitumen holds it together and keeps water out.

It is a viscoelastic material, which is the fact that explains its behaviour. It is stiff under a fast load and flows under a slow one, so it spreads a wheel load and it also ruts under stationary heavy vehicles in hot weather. It gets brittle in the cold and cracks. Both of those are the binder, and bitumen grades and polymer modification exist to widen the temperature window between them.

Processing

Aggregate is dried and heated, mixed with hot bitumen in a plant, transported hot, laid by a paver and compacted by rollers while still hot. Compaction is what determines its life: an under-compacted mat has interconnected voids, water gets in, and it fails in a few winters instead of a few decades.

Warm-mix asphalt uses additives to lay at 20 to 40 °C lower, saving fuel and reducing fume exposure for the crew, and has taken a substantial share of production.

Uses

Road surfacing, overwhelmingly — most paved roads in the world have an asphalt surface — plus car parks, airport runways and taxiways, and waterproofing membranes for roofs and bridge decks.

Porous asphalt, with the fines left out, drains through the surface and reduces spray and noise, at the cost of a shorter life.

History

Natural bitumen was used as mortar and waterproofing in Mesopotamia. The modern road surface follows from macadam — John Loudon McAdam's graded broken stone of the 1820s, which is the load-carrying idea — with tar and later bitumen added as a binder to stop the dust, which is what tarmacadam was.

Refined bitumen from petroleum distillation made it cheap and available, and the motor car made it necessary.

Environmental impact

It is, by tonnage, the most recycled material there is. Reclaimed asphalt pavement is milled off, reheated and reused, and the bitumen in it is recovered along with the stone; mixes with high reclaimed content are routine and some are close to fully recycled.

That is genuinely unusual and worth stating plainly: a material whose end-of-life route is to become the same material again, at scale, without downgrading.

The less favourable side is the aggregate demand, the fuel used to heat everything, and the fact that paving a surface is itself a hydrological intervention — impermeable surfaces move rainfall into drains rather than into ground, which is a flooding question rather than a materials one and is caused by the material all the same.

Medium confidence Weak evidence

How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.

How this connects

Where a connection has been confirmed by an outside reference, that reference is named beside it.

is sourced from

  • Crude oil — material · the bitumen binder is the residue at the bottom of the distillation column

is composed of

  • Aggregate — material · 95 per cent of it by mass — the stone carries the load and the bitumen keeps the water out

is used as

  • Structural engineering — application · as pavement rather than as structure: a surface that spreads a wheel load into the ground beneath it

is used in

  • Construction — industry · the surface of nearly every paved road there is

is commonly confused with

  • Concrete — material · one is stone in bitumen and flexes, the other is stone in cement and cracks, and 'concrete' in the American sense means only the second

Sources

  • Material World
    Our own writing
  • Wikidata
    Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)

Questions this page answers

Where it comes from, and what it becomes

Follow Asphalt concrete back to what it starts as, and forward into what it becomes. Each step is a documented one — a real route material takes, not a chain of inference.

Upstream — what it comes from

  • Asphalt concrete → is composed of (95 per cent of it by mass — the stone carries the load and the bitumen keeps the water out) → Aggregate → is produced by (crushed and graded from quarried rock, and the grading is the product — a concrete mix specifies a size distribution, not a stone) → Quarrying → takes as input (the most quarried rock in the world, for building stone, aggregate and cement alike) → Limestone → is sourced from (precipitated from it, mostly by organisms building shells and skeletons out of dissolved calcium and carbonate) → Seawater → is composed of (about 96.5 per cent by mass — seawater is 3.5 per cent dissolved solids and the rest of it is this) → Water
  • Asphalt concrete → is sourced from (the bitumen binder is the residue at the bottom of the distillation column) → Crude oil
  • Asphalt concrete → is composed of (95 per cent of it by mass — the stone carries the load and the bitumen keeps the water out) → Aggregate → is produced by (crushed and graded from quarried rock, and the grading is the product — a concrete mix specifies a size distribution, not a stone) → Quarrying → takes as input (cut with diamond wire for dimension stone, and crushed elsewhere for aggregate) → Granite → is composed of (by volume, alongside feldspar and lesser mica) → Quartz
  • Asphalt concrete → is composed of (95 per cent of it by mass — the stone carries the load and the bitumen keeps the water out) → Aggregate → is produced by (crushed and graded from quarried rock, and the grading is the product — a concrete mix specifies a size distribution, not a stone) → Quarrying → takes as input (the most quarried rock in the world, for building stone, aggregate and cement alike) → Limestone → is composed of → Calcite
  • Asphalt concrete → is composed of (95 per cent of it by mass — the stone carries the load and the bitumen keeps the water out) → Aggregate → is produced by (crushed and graded from quarried rock, and the grading is the product — a concrete mix specifies a size distribution, not a stone) → Quarrying → takes as input (the most quarried rock in the world, for building stone, aggregate and cement alike) → Limestone → is composed of (in young limestone, before conversion to calcite is complete) → Aragonite
  • Asphalt concrete → is composed of (95 per cent of it by mass — the stone carries the load and the bitumen keeps the water out) → Aggregate → is produced by (crushed and graded from quarried rock, and the grading is the product — a concrete mix specifies a size distribution, not a stone) → Quarrying → takes as input (the most quarried rock in the world, for building stone, aggregate and cement alike) → Limestone → is composed of (in nodules and cavities, strontium substituting for calcium and then separating out — the mineral's commonest sedimentary occurrence) → Strontianite

These are the most distinct paths back. Asphalt concrete can be traced through others besides.

Downstream — what it becomes