How Is Bitumen Graded?
It’s important for bitumen to be graded before it can be used in a construction project. This grade is primarily based on the stiffness of the bitumen at predetermined temperatures. Normally, these are 50, 60, and 130 degrees C, compared to the average temperature of bitumen (25 degrees C).
This method of testing has been devised in order to determine the bitumen’s ability to resist thermal cracking — i.e. physical damage brought about by heat and near-constant friction, causing cracks in the surface of the solid bitumen. Bitumen that can resist thermal cracking will have a higher grade. The ability of the bitumen to resist thermal cracking depends on a number of various factors, including the origin of the bitumen and the method that it was refined.
Different Methods For Grading Bitumen
Antiquated now, the chewing test was the preferred method of grading bitumen in the 19th century. As its name implies, the Chewing Test required experienced inspectors to chew the bitumen and then either reject or accept it. Because of the tolerances of the human body, the temperatures of the test were much lower than later tests and thus much less accurate.
First performed in 1903, the Penetration Test involves inserting a loading needle into a bitumen sample maintained at a steady 25-degree celsius in a water bath for a period of no longer than 5 seconds. This will produce a penetration angle that is measured as:
1 penetration unit = 0.1 mm.
The higher the penetration value, the softer the bitumen becomes. The American Society for Testing and Material Standard D 946 gives 5 penetration grades for the bitumen binders. They are:
- Hardest Bitumen Grade 40 –50
- 60 –70
- Softest Bitumen Grade 200-300
However, while this Penetration Test was an improvement on existing testing methods, it nevertheless had several inherent flaws. These were:
- This method of penetration grading is empirical and not fundamental
- This method cannot be used for polymer modified bitumen
Developed in the 1970s, Viscosity Testing was a further step forward after Penetration Testing. Viscosity Testing was designed to improve the general bitumen product.
This is because up until this point construction in the United States had used a bitumen with a penetration grade of 60 – 70; however, this grade showed a lower viscosity at 135 degrees centigrade — which suggested that it was more prone to thermal cracking and was causing mixing problems during the construction process. This was especially true for the southern states — which tended to exceed the 60 degrees that penetration testing was performed at.
Advantages of Viscosity Testing include:
- The viscosity grading system retains minimum performance in terms of fatigue cracking.
- The potential on tender mixes can be minimized with the minimum specified values of kinematic viscosity at a temperature of 135 degrees Celsius.
- The maximum allowable temperature susceptibility can be established by specifying the minimum value of penetration at 25 degrees and the kinematic viscosity at 135 degrees.
- The suppliers can provide the users with accurate asphalt mixing and temperature values for construction. This is possible because of the measurement of viscosity at two temperatures.
Superpave Performance Grade
The newest way to grade the performance of bitumen is by evaluating its performance while in use. It was developed by the Strategic Highway Research Planning between 1987 and 1992.
Features of the Superpave Performance Grading system include:
- It includes tests for bitumen binders.
- The field performance by the engineering principles will influence the physical properties of the bitumen.
- In order to understand the developing performance of the bitumen, pains are made to replicate its performance for a period of 5 to 10 years.
- The tests and specification of Superpave system intend to avoid the following three main damages in bitumen:
- Fatigue cracking
- Thermal cracking.
These failures happen at high, intermediate and low temperatures respectively.
- The pavement is taken for testing for the entire range of temperature between 50 and 135 degrees celsius.
- During Winter a bending beam rheometer and direct tension tester are used to measure the bitumen rheological properties at the project site.
Contact Bituchem Today
At Bituchem, we only use the highest grade bitumen and asphalt for our service. This is to ensure that surfaces laid with our products enjoy a maximum lifespan. For more information about Bituchem, our products, or any enquiries whatsoever, please contact us today. Our experienced and passionate staff will be able to help you with any enquiries.