Bitumen Grades and Uses in Road Construction

Bitumen used in the pavement in road construction is classified either according to its penetration or according to its viscosities. Viscosity grading and Superpave Performance grading (PG) systems are the most popular in America. Elsewhere in the world, a penetration grading system is applied. Selecting a grade of bitumen cement for a specific road project depends upon several factors, such as ambient temperatures, rainfall, traffic conditions, and the costs of transporting the bitumen to the site. For a view of bitumen production processes please view my previous blog “Bitumen Production Processes in Petroleum Refinery“.

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Following are the three most adopted bitumen grading systems around the world;

  1. Penetration Grade Bitumen
  2. Viscosity Grade Bitumen
  3. Performance Grade Bitumen

1. Penetration Grade Bitumen

The penetration grading system was introduced in the early 20th century. Bitumen penetration grade is classified using its penetration property which is obtained by a penetration test. The main goal for grading bitumen based on penetration is to determine its suitability for use in various construction methods and under different climatic conditions.

The penetration test indicates the hardness of bitumen and measures the stability and consistency of pure bitumen, oxidized bitumen, and emulsion bitumen. This test classifies Bitumen according to its hardness. For blown bitumen, a softening point test was also carried out in addition to the Penetration test.

This refinery bitumen consists of a wide category of penetration grade bitumen, which includes more than 5 penetration subgrades: bitumen 30/40 grade bitumen, 40/50 grade bitumen, 50/70 grade bitumen,  60/70 garde bitumen, 80/100 grade bitumen, and 100/120 grade bitumen, These subgrades differ in their level of stiffness—the lower the number, the harder the bitumen, and the higher the number the softer the bitumen. The level of stiffness of each subgrade is determined through the penetration method.

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In bitumen penetration testing, a standard needle can penetrate a bitumen binder sample when placed under a 100-gram load for 5 seconds. The bitumen penetration test is done at a temperature of 25 °C. Penetration grades are listed as a range of penetration units (1 penetration unit = 0.1 mm). The 65 penetration means that the needle will travel 6.5 mm with a 100-gram load in 5 seconds at 25 °C. Typical asphalt binders used in the paving industry are between 65 and 70 penetration and 85 and 100 penetration.

The vast majority of Bitumen used for road construction is 60/70 grade Bitumen. It’s also called paving-grade Bitumen and is suitable for road construction in a wide range of temperatures. It is usually mixed with aggregates to form hot mix asphalt and pave roads. Since it provides additional strength when mixed with the aggregates, the 60/70 bitumen is ideal for paving high-traffic roads. Additionally, this bitumen has excellent workability and cures quickly, meaning that roads can be paved faster and open to motorists sooner. Some general properties of bitumen penetration grades are shown in the table below;

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2. Viscosity Grade Bitumen

In the 1970s, the US introduced a new method of viscosity grading at 60 °C. This was to ensure a solution for construction problems and to have high-temperature performance.

The viscosity grading system not only measures penetration as in the penetration system but also measures bitumen binder viscosity at 60 °C (140°F) and  135 °C (275°F) as compared to penetration bitumen grade is tested at 25 °C. Viscosity measurement at two different temperatures indicates its suitability both at low and high temperatures. Viscosity grades are listed in viscosity units poises (cm/g/s–1) or poises divided by 100.

Different viscosity grades of Bitumen that are produced are VG 10, VG 20, VG 30, and VG 40 bitumen. Higher the number, the harder the Bitumen in the viscosity grading system.

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VG-10 is mostly used in spraying applications such as surface dressing and paving in very cold climates instead of 80/100 penetration bitumen grade. It is also used to produce Bitumen Emulsion and Modified Bitumen products.

VG-20 Bitumen

VG-20 is used for paving in cold climates and high-altitude regions.

VG-30 Bitumen

VG-30 is especially applied to construct extra heavy-duty Bitumen pavements for significant traffic loads. It can be used instead of 60/70 penetration bitumen grade.

VG-40 Bitumen

VG-40 is applied in highly stressed areas such as intersections, near toll booths, and truck parking. Because of higher viscosity, stiffer Bitumen mixes can be produced to alter resistance to road shoving and other problems related to the higher temperature and heavy traffic loads.

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In the US Bitumen has been named Asphalt or Asphalt cement. The grade with the lowest viscosity is AC-2.5 and AC-5 is used in cold regions like Canada. Thus grade AC-5 will have a viscosity of 500 poises at 60°C (140°F).

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3. Superpave Performance Grade Bitumen

The Superpave program was initiated in 1987 by the U.S. Superpave performance grade is a method of describing bitumen for road paving by taking into consideration specific factors like traffic density, climatic conditions (maximum and minimum ambient temperature, rain and snowfall, etc.), and layer location.

It facilitates the selection of the most suitable asphalt binder, aggregate, and modifier to achieve the required level of road pavement performance. Superpave performance grading is based on the idea that HMA (hot mix asphalt) bitumen binder properties should relate to the applied conditions.

The Superpave Grading system uses the same tests as used in penetration grading and viscosity grading systems. The only difference is that these tests are conducted at specific temperatures according to the climatic conditions in the area of use related to climatic conditions, and traffic conditions.

Superpave performance grading uses two numbers (PG 64-40). The first is the average 7-day maximum pavement temperature in degrees Celsius, and the second is the minimum pavement design temperature likely to be faced, in degrees Celsius. For example, a PG 64-40 bitumen grade is intended for use at a place where the average seven-day maximum pavement temperature is 64°C and the expected minimum pavement temperature is –40°C.

The pavement temperature estimations are used to determine bitumen’s suitability for use in that region. In cases where pavement temperature variation between maximum and minimum is more than 90°C, straight-run or unmodified bitumen grades are generally unsuitable, and polymer-modified type bitumens are used. For example, if the PG 78-22 sum of 78+22 is 98 then it is considered polymer-modified Bitumen, and the PG 64-10 sum is less than 90 so it is considered as Un-Modified Bitumen.

PG grades that are well-structured against thermal cracking are PG 58-34, PG 64-22, PG 64-28, and PG 76-22. PG grades that are resistant to rutting are PG 70-22, PG 70-28, PG 76-28, and PG 82-22.

Top Reference 

Mr. Sikandar Rasheed is our guest writer and co-writer of this blog. He is a BSc. Chemical Engineer and has more than 04 years of working experience as an Operations Engineer at the Asphalt Air Blowing Unit, Pak Arab Oil Refinery, Pakistan.

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