Asphalt pavement is the defacto standard surfacing material for road construction because of its low cost, durability, and flexibility. It is a simple material, yet highly engineered for diverse climates and applications.
Roadbuilding is an ancient activity, with the first evidence of constructed roads dating back to 4000 BC. The Romans developed highly advanced roadbuilding technology, with some Roman-built roads still in serviceable condition. The Romans also used asphalt as a caulk and sealant on baths, water fixtures, and aquaducts.
Roadbuilding relies on multiple layer construction. The base layer must be strong and stable enough to bear the loading of traffic down into the foundation soil or rock, while the upper layer or wearing course must resist the abration and wearing effects of traffic and of the climate.
According to the National Asphalt Paving Association, asphalt was used as a binder material for road construction as far back as 625BC in Babylon. Asphalt was reintroduced into road construction during the Industrial Revolution in England, as road building technology advanced to facilitate goods movement.
In 1900, “bitulithic” pavement, a mixture of bitumen (bitu) and stone (lithos) was first patented in the US by Frederick J. Warren. This was the start of modern asphalt pavement.
The asphalt pavement we use today is still a mixture of coarse aggregates (rock and stones), fine aggregates (sand and stone dust), and a binder composed of bitumen and modifying oils and polymers.
Hot-mix asphalt is produced in stationary or portable plants according to a “mix design”. The mix design specifies the appropriate proportions of aggegates of various sizes as well as the weight and properties of the bitumen binder.
In the plant, measured amounts of coarse and fine aggregates are heated and blended with the bitumen at a controlled temperature to produce hot-mix asphalt material. The hot-mix emerges from the plant as a tarry viscous material that is generally stored at temperatire inside an insulated silo before being transported to the construction site in a dump truck.
Asphalt pavement is produced by spreading a smooth, even mat of the hot mix asphalt on to the road base using machines called pavers, then compacting the mat with rollers. The compacting process compresses the loose mat into a tight interlocked matrix of stones and dramatically increases the binder contact between stones, which gives the asphalt pavement its strength.
Although the “recipe” for asphalt might appear straightforward, research and advancement into the materials and processes continues to this date.
Some examples of recent technological advancements in asphalt pavement:
- Warm-mix asphalt, which is produced as lower temperatures and improves paving workability and reduces energy and carbon emissions.
- Polymer modified bitumens that have wider operating climates
- Anti-strip additives that improve wear life by reducing oil loss from the surface
- New synthetic binders made from plant stock that can replace natural bitumen
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