How asphalt is made

Asphalt is the thick black leftover from the bottom of the distillation tower, mixed hot with gravel.

stations: 5
things upstream: 2
raw sources: 2
step from the ground: 1

The short version

Asphalt starts as bitumen, the heaviest residue left at the bottom of the refinery's distillation towers. Heavier crude oils are boiled further under vacuum so lighter oils leave and a thick binder remains. At an asphalt plant, the binder is heated and coated onto hot gravel and sand, then laid while still hot.

Distill off the light oils

Crude oil from heavier grades is boiled in a distillation tower, and everything lighter than the bottom residue rises away. What drains from the base is thick and black.

Goes in
Crude oil
Boil it again under vacuum

The residue is boiled again at low pressure, so more oil leaves without the heat cracking the molecules. The remaining refined bitumen is similar in thickness to cold molasses.
Adjust the binder

Refiners blend or treat the bitumen to hit a target hardness. Some grades are modified with polymers or with air to make them stiffer or more elastic.
Dry and heat the aggregate

At the asphalt plant, gravel and sand are dried in a rotating drum. They must be hot and dry for the binder to stick.

Goes in
Gravel and crushed stone
Coat the stone in binder

Hot bitumen is mixed with the stone in a pugmill until every piece is coated. The mix is kept hot in a silo and loaded into trucks.

End of the lineAsphalt Next it goes into asphalt road, asphalt shingle.

Worth repeating

About 70% of annual bitumen production goes to road construction.
The largest natural deposit of bitumen in the world is the Pitch Lake in southwest Trinidad.
In American English the manufactured binder is usually called asphalt, while much of the rest of the world calls it bitumen.