What are Asphalt Binders?

Asphalt binders are subsidiary products procured during the refining process of petroleum while manufacturing gasoline, diesel, and similar petroleum products. Asphalt binders are obtained from the residue that remains after the distillation of petroleum, this residue is further subjected to steam distillation and oxidation to remove the traces of gas oil present in the residue. Certain additives or modifiers are also added to asphalt binders to alter them to suit different applications.

Asphalt mixtures

It is a mixture of aggregates such as sand, gravel, and crushed stones bound together by asphalt binder. The asphalt mixtures are classified into different types based on the materials used for the preparation of the mixture.

Different types of asphalt mixtures

Hot-Mix Asphalt (HMA)

The asphalt binder is heated to decrease its viscosity, and the aggregates are dried to remove moisture before mixing it with the binder. The aggregates are mixed with a binder for asphalt cement at 95 °C, virgin asphalt at 150 °C, and polymer modified asphalt at 166 °C.

Warm-Mix Asphalt (WMA)

WMA is prepared by adding additives like zeolites and wax to asphalt before mixing it with aggregates. This reduces the temperature of mixing resulting in lower carbon dioxide emissions.

Cold-Mix Asphalt

An emulsifying agent is added to the asphalt in water before the aggregate is mixed with the mixture. The addition of emulsion makes the asphalt less viscous and makes it more workable.

Cutback Asphalt Concrete

This form of asphalt is similar to cold-mix asphalt except that the binder is dissolved in a petroleum by-product before it is mixed with the aggregates. Once the asphalt is laid, the additive evaporates and the mixture will have properties similar to cold-mix concrete.  

Modified Asphalt Mixtures

A modifier is a natural or artificial organic or inorganic material added to asphalt mixtures to modify its properties according to the requirement. Rubber and thermoplastic elastomer-modified asphalt such as Styrene-Butadiene-Styrene (SBS)-modified asphalt, Plastic- and synthetic resin class-modified asphalt such as epoxy resin-modified asphalt and Resonance-type polymer-modified asphalt are the three different types of modified asphalt mixtures.

Asphalt concrete used as a pavement topmost layer
CC BY-SA 4.0 | Image Credits: https://commons.wikimedia.org | Pirehelokan

Tests on Asphalt Binders

The behavior of Asphalt binders highly depends on the surrounding temperature. At low temperatures, the binder tends to be stiffer, vulnerable to cracking while high temperatures make the binder viscous. There are different tests used to study the rheological properties of asphalt mixtures to use them as Superpave (Superior Performing Asphalt Pavements)

Direct Tension Test (DDT)

It is a test used to measure the critical cracking temperature of the asphalt binders. Asphalt tends to become stiffer at lower temperatures (-20 degrees), to measure the failure stress and strain this test is carried out. If the binder has a DDT value of 1% or more, the asphalt binders are said to be elastic at low temperatures.

Bending Beam Rheometer Test (BBR)

This test gives a measure of the flowability of the binder at low temperatures. A constant static load is applied to a beam made up of asphalt binders and the strain undergone is noted, indicating the stiffness of the binder at low temperature.

Dynamic Shear Rheometer Test (DSR)

This test is done to study rheological properties like the viscous and elastic properties of the binder at high temperatures and intermediate temperatures. The test temperature depends on the pavement temperature conditions on the location where the Superpave is to be laid. The code followed for this test is American Association of State Highway and Transportation Officials (AASHTO) T 315, 2020. A thin sample of Asphalt binder is placed in between two circular test plates. The bottom plate is fixed, while the top plate is moved up and down to simulate the shearing force, a 1-second creep load is applied to the asphalt binder sample after which the sample is allowed to remain undisturbed for 9 seconds. The resulting phase angle between the input and output signals gives the measure of the viscous and elastic properties. At a phase angle of 90 degrees, the binder is viscous, at a phase angle of 45 degrees the binder is viscoelastic and at a phase angle of 0 degrees, the binder is elastic.

Multiple-Stress Creep and Recovery Test (MSCR)

This test uses code AASHTO TP70 and AASHTO MP19. This test is followed by the DSR test. Initially, lower stress is applied for 10 cycles, then the stress value is increased for the remaining 10 cycles. In the MSCR test, higher levels of stress and strain are applied to the binder, to simulate actual asphalt pavements. In the performance grade system, the high-temperature parameter is measured by applying an oscillating load to the binder at very low strain, the performance grade high temperatures parameter does not accurately represent the ability of polymer modified binders to provide rutting resistance.

Rolling Thin Film Oven Test (RTFOT)

A thin film of binder which is usually exposed to heat and air pressure while mixing and placing is tested for the effect of heat and air pressure on it. This thin film simulates the coating of the aggregate on the pavement. During the test, a thin film of binder is poured into a cylinder which is rotated at a certain mixing temperature with air pressure, so that actual exposure on asphalt pavements can be simulated and the change in thin-film is noted.

Context and Applications

Asphalt binders and mixtures play a vital role in the following areas of engineering:

  • Bachelors of Technology in Transportation Engineering
  • Bachelors of Technology in Civil Engineering
  • Masters of Technology in Transportation Engineering
  • Masters of Technology in Civil Engineering

Practice Problems

1. What is the full form of SBS-modified asphalt binders?

  1. Styrene-Butadiene-Styrene
  2. Strong Bituminous Superpave
  3. Superpave Bitumen Sheet
  4. Styrene Bitumen Superpave

Answer: Option a

Explanation: It is a type of rubber modified asphalt binders and its full form is Styrene-Butadiene-Styrene.

2. What is the function of modifiers in Asphalt?

  1. To increase fatigue life
  2. To increase the tensile strength of the pavement
  3. To provide rutting resistance
  4. All of the above

Answer: Option d

Explanation: The main purpose of adding modifiers is to improve rheological properties like rutting resistance, ductility, anti-fatigue, anti-aging, and resistance to low temperature cracking and prevent permanent deformation of pavement in long-term.

3. What is the role of NCHRP in Transportation planning?

  1. To conduct research factors affecting highway construction
  2. To conduct research on bridges
  3. To design and construct tunnels
  4. To research various aspects of road construction

Answer: Option a

Explanation: NCHRP is a national research program formed in the U.S. in 1962 to conduct research in acute problem areas that affect highway planning, design, construction, operation, and maintenance nationwide.

4. What is VECD?

  1. It is a type of damage that occurs in asphalt pavements.
  2. It is a research organization for highway research.
  3. It is a theory to predict fatigue cracking of pavement.
  4. It is a modifier used in asphalt.

Answer: Option c

Explanation: The viscoelastic continuum damage (VECD) theory is a method for the characterization of fatigue cracking susceptibility of asphalt mixtures.

5. Which of the following decreases the penetration value and increases the softening point of asphalt?

  1. Nano-silica
  2. Nano-clay
  3. Butadiene
  4. Asphaltenes

Answer: Option a

Explanation: Nano-silica tends to decrease the penetration value and increase the softening point of asphalt while nano-clay was found to increase the penetration and decrease the softening point temperature.

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