What are Bituminous Materials?

The word 'bituminous' in Bituminous materials come from the word bitumen, which is popularly known as 'asphalt' in the United States. Asphalt is a heavier substance that is obtained by the fractional distillation of crude oil, after removing the lighter components such as gasoline and diesel. Asphalts are known to have good adhesive and waterproofing properties. Depending upon the grade needed, asphalt may be refined several times in the fractional distillation column.

Apart from distillation, asphalt also occurs naturally. Traces of asphalt can be found at the beds of historic rivers, where organisms decay under the influence of extreme heat and pressure to form fossils.

In ancient times, asphalt has been used as a medicine to treat several diseases. In the present era, asphalt finds its use to line the bottom of ships and acts as an important binder, typically known as asphalt binder or bituminous binders in building materials in several construction projects. Complex hydrocarbons form the basic structure of asphalt with calcium, iron, Sulphur, and oxygen as their primary contents.

Types of asphalt

Based on the property, uses, and specifications, there are various kinds of asphalt. Some of the few kinds of asphalts are outlined below.

Penetration grade asphalt

These grades of asphalt are refineries by the products obtained after fractional distillation of crude oil. Refineries produce such grades of asphalts based on different magnitudes of viscosities. The name 'penetration grade' comes from the penetration test which is carried out to distinguish asphalt. The penetration value for a typical road asphalt ranges from 15 to 450, whereas it is 25 to 200 for commonly used asphalts. The BS EN 1426 and BS EN 1427 provides the penetration and softening points of a particular asphalt.

Oxidized asphalt

The refinery-produced asphalt is further reduced to bituminous materials with higher molecular weights. These types of bituminous materials are produced by treating soft refinery-produced asphalts with processed air in controlled temperature and pressurized environments. Asphaltenes and Maltese are two primary bituminous materials produced by this process. These bituminous materials find their wide applications in road paving, coating roofs, and pipes.

Cutback asphalt

Cutback asphalt or cutback bitumen is a grade of bitumen whose viscosity is temporarily reduced by the addition of volatile oil. Upon evaporation of the volatile oil, the bitumen regains back its viscosity. The amount of viscosity can be controlled by the amount of dilution of volatile oil and the property of volatility of oil mixed.

This grade of asphalts is primarily used for laying roads. During surface dressing of roads, the viscosity of such asphalt is reduced to initiate ease of surfacing and laying.

Natural asphalt

Natural asphalts or Brea, are the type of bitumen formed during ancient civilizations due to the burial of organic materials subjected to high degrees of pressure and heat. The constituent materials of natural asphalt are the primary hydrocarbon chains with minor proportions of oxygen, nitrogen, and Sulphur.

The primary use of natural asphalts during ancient times was for road construction, sealing water rocks and it was used as a primary waterproofing material in households.

Types of Bituminous mixtures

To obtain various properties of bitumen, different materials are mixed to serve different purposes. The main materials which constitute asphalt pavement or asphalt concrete for construction are coarse and finer aggregates, filler, and binder. Asphalt pavement is a hard surface formed by an aggregate mixture of crushed stones, sand, asphalt binders, and asphalt itself, these constituents are known as asphalt mixtures.

A typical asphalt pavement primarily consists of four courses namely surface course, binder course, base course, and subbase course.

An asphalt mixture is often termed a bituminous mixture, based on the content of the materials that form an asphalt pavement, bituminous mixtures are classified into the following types:

Open graded bituminous mixture

This grade of bituminous material has coarse aggregates and the binder as the mixture content. These grades possess the property of porosity and have excellent frictional properties.

Gap graded bituminous mixture

These kinds of mixtures are characterized by the presence of fine aggregates, fillers, and binders. This kind of grade offers good fatigue properties and high tensile strength.

Well graded bituminous mixture

This category of the mixture implements all the asphalt mixtures for pavement construction. They are typically denser compared to other mixtures and have good compressive and tensile strength.

Bitumen stability

The pavement constructed by bitumen for roads and highways is subjected to continuous loads due to the movements of vehicles. Under such conditions the pavement undergoes deformations. The ability of a pavement to resist deformations is given by its stability.

The stability of bitumen is controlled by three parameters, friction between the particles of bitumen mixture, cohesive property of bitumen, and bond between the particles or mixtures. The stability majorly depends on the strength or quality of the binder used.

For instance, half-warm and hot-mix asphalts are extensively used in the United States, as it offers good stability properties, flexibility, water-repelling properties, and weather resistance properties. This kind of asphalt has quick hardening properties and requires less manpower for surface dressing, which is an added advantage in this present pandemic of coronavirus disease 2019 (COVID-19).

Context and Applications

The topic finds its major existence in the undergraduate and post-graduate degree courses of

  • Bachelors of Technology in Civil Engineering
  • Masters of Technology in Civil Engineering
  • Bachelors in Science in Chemistry

Practice Problems

1. Which of the following forms the constituent part of asphalt concrete?

  1. Base course
  2. Binder
  3. Filler
  4. Fine aggregates

Answer: Option a

Explanation: The base course forms one of the primary parts of asphalt concrete followed by the sub-base course, surface course, and binder course.

2. When soft bitumen is mixed with processed air, which of the following bituminous materials are formed?

  1. Asphaltenes only
  2. Maltenes only
  3. Both asphaltenes and maltenes
  4. None of these

Answer: Option c

Explanation: When soft refinery-produced bitumen is treated with processed air, within the controlled temperature and pressure, asphaltenes and maltenes are produced.

3. Which of the following parameters determines the stability of bitumen?

  1. Friction between the particles
  2. Cohesive property of bitumen
  3. Bond between the particles
  4. All of these

Answer: Option d

Explanation: Friction between the particles, cohesive property of the bitumen, and bond between the particles determines the stability of a bitumen.

4. Which of the following asphalts has good water-repelling properties?

  1. Natural asphalt
  2. Oxidized asphalt
  3. Hot-mix and warm-mix asphalt
  4. Cutback asphalt

Answer: Option c

Explanation: Hot-mix and warm-mix asphalts have good water-repelling properties which makes them suitable for road construction projects.

5. Which of the following is a method used to temporarily reduce the viscosity of asphalt?

  1. Adding volatile oil
  2. Adding additives
  3. Adding bituminous mixtures
  4. All of these

Answer: Option a

Explanation: Adding certain grades of volatile oil in proper dilution reduces the viscosity of asphalts which makes it well suitable for road constructions.

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