Exploring 4 Common Methods for Grading Bitumen You Need to Know

Do you know why it is important to grade Bitumen? you may think to know the types or find the best bitumen. The answer is, to find suitable bitumen for specific applications. Finding suitable bitumen is essential to build strong and durable infrastructure. Based on the stiffness of the bitumen at a predetermined temperature, it is classified.  The hardness of the bitumen varies with the temperature. As the temperature changes hardness of the bitumen also changes. Hence, classifying the bitumen depending on the stiffness is essential to building structures that withstand thermal cracking, rutting and deformations.

 

There are four common ways used for grading bitumen. In this blog, you're going to explore the four different methods used for grading bitumen and their advantages and limitations.

1.       Chewing Grading System
2.       Penetration Grading System
3.       Viscosity Grading System
4.       Superpave Performance Grading System

 v  Chewing Grading System

Chewing tests were carried out in the 19^th Century when there was no development of penetration tests. As the name refers, the test is done by chewing the bitumen by experienced quality inspectors in the US. The bitumen sample can be either rejected or accepted depending on the test results. A significant limitation of this method is that the temperature used to test the bitumen is very less compared to today's bitumen testing temperature which directly impacts the durability of structures.

v   Penetration Grading System

 

The Penetration grading system is an empirical test which has been usage more than 100 years. Penetration grading was introduced by the American Society for Testing Materials (ASTM) in 1903 for grading bitumen. This method involves inserting a needle through 100 g bitumen at a specific temperature of 25 degrees Celsius for 5 seconds. The Penetration value is calculated by the penetration length of the needle through bitumen and estimated in the penetration unit.  1 penetration unit is equal to 0.1mm. Penetration value indicates the hardness of the bitumen whether it is softer or stiffer. Based on the penetration value, bitumen can be classified into subgrades such as penetration grade 10-20,20 – 30, 40- 50, 60 - 70, 120-150, and 200 to 300. The penetration-grading system is widely used to select suitable bitumen for road construction worldwide.

 

However, there are some limitations of the penetration grading system as it is not based on fundamentals and is established on empirical. Additionally, the penetration grading system cannot be applied to polymer-modified bitumen as it uses empirical value for the test.

v  Viscosity Grading System

 

To derive a solution for construction issues and to have high performance in higher temperatures, the US introduced the Viscosity Grading System in 1970. The viscosity grading system test is conducted at 60 Celsius and 135 Celsius which are the maximum temperature of the road surface in summer and mixing temperature respectively. Unlike the penetration grading system, the viscosity grading system is standard for grading the bitumen as it is based on fundamental value. In the US, bitumen is called Asphalt cement or Asphalt, so in the US, viscosity grade is known as AC grade and classification is made on AC grades such as AC 30, 60, 40 etc. and in India graded as VG 30, 40, 50 etc.

 

The lower the viscosity value, the softer the bitumen. The higher the viscosity value, the stiffer the bitumen. In cold areas like northern countries, lower viscosity grade bitumen has been used and in superheated areas like southern countries higher viscosity grade bitumen has been used. Depending on the climatic conditions, bitumen can be chosen at the appropriate viscosity grade.

One of the significant advantages of viscosity grading bitumen is reduces the potential of tender mixes due to the minimum specified kinematic viscosity value at 135 Celsius. Unlike penetration grades, the viscosity grading system offers the same rutting performance for different viscosity grades.

v  Superpave Performance Grading System

 

The Superpave Performance Grading System is developed in the research of Strategic Highway Research Planning (SHRP) for the performance-based grading method from 1987 to 1992. This method is developed to analyse or evaluate the specific performance of bitumen to solve many constructional and engineering problems. Unlike the viscosity grading system, which is rational, The Superpave performance grading system is used to evaluate the performance of the bitumen material when it is used.

 

The PG grading system is based on the climatic conditions and where the bitumen is used. The Superpave PG system tests and specifications are expected to prevent three main damages such as thermal cracking, rutting and fatigue cracking. The ultimate goal of the Superpave grading system is to fully characterize the asphalt binders that have been used in the Hot Mix Asphalt pavements. Unlike the Superpave grading system, the penetration and viscosity grading system fails to fully illustrate bitumen used in HMA pavement.

 

To conclude, choosing the right bitumen for a specific application is very important for the durability and longevity of structures. Moreover, the four methods of grading bitumen help identify suitable asphalt binders for your project requirements. To know more details on how to select the asphalt binder for specific applications under specific climatic conditions, contact Petra Oil. We, at Petra Oil, deliver exceptional quality bitumen suitable for various climatic conditions and applications and will help you to choose the right bitumen solution for your project needs.