What is Asphalt?

The term Asphalt is also known as Bitumen, it is a black-colored material that has a consistency that varies from highly viscous liquid to shiny solid. The material can be obtained from the natural deposits or it can also be derived as a residue from the distillation process of the petroleum. This term is raised from the Greek word "Asphaltos". The natural Asphalt is also known as "brea" is believed to be formed in the early period by the breakdown of the organic marine deposits into petroleum. These deposits basically contain the minerals while the residual petroleum asphalt does not contain any minerals.

What is the composition of asphalt?

The composition of the asphalt is the compounds of hydrogen, oxygen, nitrogen, and carbon. Among these compounds, the major proportion is hydrogen and carbon. There are four main classes of compounds that are the component of Asphalt:

• Naphthene aromatics or naphthalene
• Asphaltenes
• Polar aromatics
• Saturated hydrocarbons

A group of different colored bottles showing components of Asphalt: First Acidaffins, Second Acidaffins, Asphaltenes, Saturated hydrocarbons, Polar compounds.
Components of Asphalt


The major resource of commercially used Asphalt is majorly extracted from petroleum. In our nature, there is a huge amount of Asphalt that is available in a concentrated form. Naturally occurring asphalt deposits are formed from the remains of ancient, microscopic algae (diatomic) and other ancient living things. The remains that are occurred are deposited at the bottom of the water bodies where the living organisms lived. Due to the effect of heat (i.e., above 50 °C) and pressure, the remains were transformed into materials such as petroleum, bitumen, or kerogen.

The Lake Bermudez of Venezuela and Pitch Lake of Trinidad & Tobago are the major resources of the natural deposits of bitumen. The Dead Sea and the La Brea Tar Pits are the sources of the natural seeps.

Types of asphalt mixes

There are a variety of applications of the asphalt mixes, these mixes must have sufficient stiffness and resistance for the deformation that helps to cope with the applied pressure. They must have adequate flexural strength to resist cracking caused by the varying pressure exerted on them. These are some of the reasons that the asphalt mixes are produced at different temperatures to meet various requirements.

When the asphalt mixes are produced at different temperatures these can be classified in three types as discussed below:

Cold mix asphalt

It is produced without heating the aggregate. The bitumen used in the mixture gets emulsified in water, which breaks either during compaction or during mixing. Cold mixes are generally used for lightly trafficked roads or pavements.

Warm mix asphalt

Warm mix asphalt is produced at a temperature around 25°C to 40°C lower than an equivalent hot mix asphalt. As less energy is involved and, fewer fumes are produced. Also, during the paving operations, the temperature of the material used is lower, thus resulting in improved working conditions for the working unit and an earlier opening of the pavement.

Hot mix asphalt or HMA

The production of the hot mix asphalt generally takes place at the temperature range of 150°C to 180°C. this surfacing is the most common type of the pavement surfacing. HMA is having many different names with which it is presented some of them are the hot mix, asphalt concrete, or blacktop. The design and the and the production methods are some of the factors by which the HMA is distinguished, it also includes the traditional graded mixes as well as stone matrix and various stone graded HMA. There are some agencies thta consider other asphalt based pavement surface like fog seals, slurry seals, and BST's that are required for the maintainence and the rehabiliation sections. Within HMA there is Reclaimed asphalt pavement (RAP) which forms in a area where the process of the recycling is considered seperately. There are three types of hot mix asphalts: 

  • Dense-graded mixes: They are generally referred to by their nominal maximum aggregate size and can further be classified as either fine-graded or coarse-graded. Fine-graded mixes have more fine and sand-sized particles than coarse-graded mixes. It is suitable for all pavement layers and traffic conditions and works well for structural, friction, leveling, and patching needs.
  • Stone Matrix Asphalt: SMA sometimes called stone mastic asphalt, is a gap-graded HMA originally developed in Europe to maximize rutting resistance and durability. The mix design goal is to create stone-on-stone contact within the mixture. Since aggregates do not deform as much as asphalt binder under load, this stone-on-stone contact greatly reduces rutting.
  • Open-graded mixes: It is designed to be water permeable. Open-graded mixes use only crushed stone (or gravel) and a small percentage of manufactured sands.

Asphalt concrete: A composite material that is most commonly used for the surfacing of the roads, parking lots, airports and the core of embankment dams is known as asphalt concrete. The mixes of asphalt are used for the construction of the pavements from the early twentieth century.

Asphalt mixture and its uses

Asphalt is being used since the age of Mohenjodaro as a water stop between brick walls of a reservoir. And its importance has been increased with every coming year with the advancement of technology. Asphalt is combined with other materials, because of its uses in a variety of applications, often without being labeled as anything other than simply “asphalt mixture”. Asphalt mixture is a mixture of aggregates, filler, and binder that is used for constructing pavements, roads, railway tracks, harbors, parking areas, airport runways, bike lanes, and also playgrounds or, other sports areas.

Aggregates like crushed rock, gravel, sand, or slags are used for making asphalt mixtures. But nowadays, certain waste products and by-products, are being used as aggregates such as construction and demolition debris, which increases the economic development of asphalt.

Diagram showing layers of Asphalt pavement. The Top Layer is Asphalt wearing surface followed by Asphalt intermediate layer, Asphalt base layer, Aggregate base and then Subsoil at the bottom.
Asphalt layers

Why asphalt is used extensively?

  • Asphalt paving is safe
    Asphalt pavement contributes to the safety of your property in several ways. Because asphalt has water managing qualities. It not only helps with skid resistance, but the dark color is also quite useful in melting ice and snow, making asphalt pavement safer for cars and pedestrians as well.
  • Asphalt is durable and long lasting
     Asphalt pavement usually lasts for about 10 - 15 years, and this estimated lifespan will increase if preventative maintenance is carried out regularly.
  • Asphalt projects can be completed quickly
    Asphalt mixture can be made quickly by its materials and also it dries out quickly.
  • Asphalt is good for noise reduction
    Asphalt absorbs road noise and therefore asphalt pavement is the “quiet” pavement option.
  • Asphalt paving is good for water drainage
    All of the asphalt mixtures are good for water drainage, but some types of asphalt mixtures perform even better. Porous asphalt allows water to move through the asphalt into a specially prepared gravel base which filters the water back into the ground.
  • Asphalt is cost efficient
    Asphalt when compared to concrete is generally much more cost-efficient. But as crude oil is one of the main components of any asphalt mix, it causes fluctuations in asphalt prices.

Note: The quality of asphalt pavement matters. Quality directly impacts how long and how well your asphalt pavement will hold up against the weather elements and repeated use over time. And that is the reason for asphalt being used widely. And to increase the effectiveness of asphalt pavement, good asphalt contractors are always keen on going with the latest advances in technology to maintain high-quality control measures.

Common Mistakes

Some people may confuse asphalt with coal tar, which has a similar black appearance, and during the early or mid-20th century, it was extensively used as the binder for road aggregates. But note that coal tar is a thermoplastic material produced by the destructive distillation of coal. It is not used now because of its higher temperature susceptibility and a large number of hazardous fumes produced by it.

Context and Applications

This topic is significant in the professional exams for both graduate and postgraduate courses, especially for:

  • Bachelor of Technology in Mechanical Engineering
  • Bachelor of Technology in Civil Engineering
  • Master of Technology in Material Sciences
  • Petroleum
  • Bitumen
  • Coal tar

Practice Problems

Q1. Coal tar is not used as a binder in making pavement because of its
a) Viscosity
b) Adhesion property
c) Temperature susceptibility
d) Magnetic properties
Correct Option: (c) 

Explanation: The temperature susceptibility is represented as the term which indicates the relation among variations in viscosity and temperature. Also, the temperature susceptibility relies on the temperature of the material. The coal tar has a high temperature, which is the reason behind not using it as a binder.  

Q2. Dense-graded mixes, stone matrix asphalt, open-graded mixes are types of which of the following?
a) Hot mix asphalt
b) Cold mix asphalt
c) Warm mix asphalt
d) None of the above
Correct Option: (a)


The hot mix asphalt is categorized into different types of mixes such as:

  • Dense-graded mixes
  • Open-graded mixes
  • Stone-matrix mixes

Q3. What is the color of asphalt?
a) Black
b) White
c) Red
d) Yellow
Correct Option: (a) 

Explanation: The standard color of asphalt is black because the combination of a binder with the bitumen results in viscous black liquid.

Q4. Which of the following is correct regarding asphalt?
a) Asphalt has high viscosity.
b) Asphalt has low viscosity.
c) Asphalt is not found naturally.
d) Asphalt is also called coal tar.
Correct Option: (a)

Explanation: Asphalt consists of high viscosity because of the presence of the aggregate bitumen in it. 

Q5. Which of the following is not present in asphalt?
a) Hydrogen
b) Sulfur
c) Oxygen
d) Phosphorous
Correct Option: (d)

Explanation: The elements like carbon, hydrogen, sulfur, and oxygen are contained in the asphalt. The element which is not present in the composition of asphalt is phosphorus.

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