What is Asphalt?

Asphalt is everywhere in urban areas. Though we use it pretty much everyday to get where we need to go, we probably give very little thought to the black stony mixture under our tires and feet.

Asphalt is a thick dark brown or black material that is derived from petroleum, sulfur, crushed stone and gravel, which is also known as aggregate. Once other petroleum-based products have been refined and processed, tar remains at the bottom of the barrels. This byproduct contains some sulfur but consists of 80% carbon, giving it its rich black color. The tar is scraped from the bottom of those barrels and combined with sand or gravel to create what we know as asphalt.

There are several types of asphalt due to different processes of mixing:

Hot mix asphalt concrete (HMAC) has a two-part production process that is done before the asphalt and aggregate are mixed together.

  • Heating of the asphalt binder
  • Removing moisture from the aggregate

The mixing temperatures vary based on the makeup of the asphalt.

  • Virgin asphalt is mixed at 300⁰F
  • Polymer modified asphalt is mixed at 330⁰F
  • Asphalt cement is mixed at 200⁰F

Warm mix asphalt (WMA) is created by adding asphalt emulsions, wax, zeolites or even water to the asphalt binder before mixing. The benefits of this process are:

  • Mixing and laying temperature are significantly lowered which results in lower fossil fuel consumption.
  • The lower the fuel consumption, the less aerosol, vapors and carbon dioxide are released, the less fumes there will be. This improves air quality for construction workers and passersby.
  • Lower laying temperature means the asphalt will be ready for use sooner which is an asset to construction sites that have rigid time constraints.

Cold mix asphalt is produced by using soap and water to emulsify the asphalt before combining it with the sand or gravel. This process thins the asphalt and makes it less sticky and easier to work and compress. Once enough water has evaporated, the emulsion is broken and the cold mix asphalt takes on the properties of HMAC. The most common use for cold mix asphalt is patching material, but it is strong enough to be used on roads that are not frequently traveled.

Cut-back asphalt is produced when the asphalt binder is dissolved in kerosene or another refined form of petroleum before mixing it with the aggregate. In a dissolved state, the asphalt has a lower viscosity level making it easier to work and compress. The refined petroleum that was used for dissolution evaporates after the asphalt is laid. More and more often, this type of asphalt is being replaced by asphalt emulsion due to pollution concerns.

Mastic asphalt (sheet asphalt) is produced when hard grade bitumen is heated in a green cooker until it becomes a thick, sticky liquid. Next, the bitumen is mixed with the aggregate to be cooked and matured for 6 to 8 hours, and then it is ready to be transported to the job site for laying. Sometimes other additives such as anti-stripping agents and polymers are mixed in to improve the quality of mastic asphalt.

Natural asphalt is derived from bituminous rock, which can only be harvested in parts of the world where sedimentary rocks have been infused with upwelling bitumen.

Asphalt in its various forms has many uses, but its most common use is as a surface to drive on. Asphalt, also known as blacktop, can support the weight cars, trucks and semis making it an ideal material for roads and parking lots. The surface is easy on tires while still providing traction for slowing and braking. The same material that is used to pave roads is also used to repair them. Asphalt fills potholes and closes cracks. Asphalt is also a popular surface for recreations areas such as basketball and tennis courts. One lesser known use for asphalt is as a roofing material. Flat commercial roofs can be sealed by pouring liquid asphalt on top. Also, some roofing shingles are made from asphalt, which has usually been mixed with sand aggregate.

Asphalt provides us with a smooth surface on which to drive our vehicles. It seals roofs to protect the building underneath from water damage. It patches cracks and holes when a surface has fallen into a state of disrepair. Considering the fact that asphalt is almost everywhere and has so many important uses, it’s amazing how much it goes unnoticed.