Asphalt Facts

asphalt facts

Asphalt is used as a surface paving material for roads, driveways, parking lots, sidewalks, bike paths and more.  It is a good material for surface paving because placing a smooth layer of asphalt is possible.  It is also relatively inexpensive, yet durable.  Here are some scientific asphalt facts and some facts about how asphalt paving is built and used.

Asphalt is a natural occurring material also known a bitumen.  It is a tar-like petroleum substance that is a lot like hard dried pancake syrup.  For many years ancient people found the asphalt and used it for waterproofing their boats and things they wanted to store water inside. They also used asphalt to help keep water out of their buildings.

Separately, people like the Romans and other ancient civilizations built roads by laying down large stones.  This allowed people, horses and goods to pass, even when the ground was very soft.  The large stones, however, were very heavy and required intensive labor to build so this type of road was not practical for long distances.  In the late 1700’s a man named John Loudon McAdam realized that smaller stones held together well if they had a certain percent of large stone, smaller stone and tiny fine stones.  He developed a surface paving known as water-bound macadam.  In this case, water was used to bind together all of the stones.  This was a huge advancement in pavement technology and allowed construction of many more roads because they were easier to build and cost much less.

Asphalt Facts Paving

Rollers are used to make the asphalt pavement smooth when it is hot

In 1824, in Paris France the first recorded road was built that replaced water-binding with asphalt binding.  It was understood that water was a problem with the water-bound macadam.  The stone could not hold together.  Since asphalt had been used form many years as a waterproofing material, it was a natural fit in keeping the water out of the road paving.

Since this time engineers, scientists and researchers have learned more about the science of how the asphalt binder performs.  Many different advancements in materials are been made to allow people to produce the millions of miles of asphalt roads used throughout the world.

Asphalt Facts

  • Asphalt is a mixture of aggregate (crushed rock) and liquid asphalt binder.
  • Asphalt is placed hot because the binder flows like liquid when it is hot enough.  When the binder is liquid it coats the aggregate and allows spreading, rolling and compaction of asphalt pavement, making it smooth and durable.
  • Asphalt pavement is considered a flexible pavement.  Asphalt is flexible not rigid so it actually moves under a heavy load.  Asphalt pavements that are designed correctly, rebound to their original shape after the heavy load has passed.
  • Asphalt is a viscoelastic material.  It behaves as both an elastic material and a viscous material.  Loading and rebounding to its original shape is dependent on time.  The longer that asphalt is loaded, the less likely that it will return to its original shape.  The worst case for asphalt is parking a heavy truck for a long time on a very hot pavement.  The combination of high temperature and loading for a long time can result in pavement that does not return to its original shape.  This is called pavement rutting.  Rutting is common in truck parking areas and near traffic lights were there is a lot of truck traffic.
  • All liquid asphalt is not the same.  Binders are rated on a PG scale.  The PG scale is the lab-tested temperature range where the binder performs well.  For example, a PG 64-28 binder performs well within a high temperature of 64 degrees Celsius and a low temperature of -28 degrees Celsius.  Typically, the wider PG range binders have better performance, but cost more.  A PG 76-28 binder  performs better than a PG 64-22 binder.
  • Natural PG asphalt binders typically have limited performance over a wide temperature range. To increase the range, asphalt plants can add polymers such as SBS (Styrene-Butadiene-Styrene) and SBR (Styrene-Butadiene-Rubber). The polymers improve strength and flexibility performance of the asphalt, but also add cost to the asphalt.
  • A majority of asphalt for paving is made in an asphalt plant.  An asphalt plant is a place that mixes the asphalt ingredients so it is ready for placement.
  • Asphalt is delivered from the plant to the job in asphalt trucks.  When the asphalt trucks get to the job site, they lift their truck beds and dump the asphalt out the back.  Asphalt truck drivers are very skilled at slowly lifting their truck bed so enough material dumps out the back, but not too much.
  • When it is cold, asphalt truck drivers cover their truck bed with insulation to keep it warm enough to place when they get to the job site.