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What is bitumen? What is asphalt?

What is bitumen? What is asphalt?

Depending on your location and/or dialect, you may use the word “bitumen” or the word “asphalt” to describe the black, viscous type of refined petroleum which is produced from crude oil in the distillation process. However, the word “bitumen” is also used to describe the naturally occurring mineral deposit, for example found in Nigeria and Canada, and the world “asphalt” is used in Britain and the US to describe the mixture of bitumen and aggregates used in road construction.

Why measure the density of bitumen/asphalt?

Knowing the density of bitumen/asphalt is essential in order to convert the volume to mass when trading. The density value is also required to categorize the asphalt/bitumen after distillation and when developing new and stronger products. 

Bitumen/asphalt measuring methods

Traditionally bitumen/asphalt is analyzed using either pycnometers or a digital density meter. There are several challenges involved.

Using a pycnometer

The standard test methods (ASTM D70 and EN 15326) require a pycnometer to be partially filled with the heated sample without splashing. In the process, the sample must not touch the inner sides of the pycnometer above the filling level – which is difficult because its neck is only about 2 cm (approx. 0.8‘’) wide. The bitumen then has to be cooled down to 20 °C. When the bitumen has cooled down to this temperature, the pycnometer has to be filled with water and thermostatted to 20 °C and the determined weights used to calculate the density of the sample. Unfortunately, this procedure is time-consuming and cumbersome, and also requires a lot of practice and skill. In addition, the achieved precision is moderate.

Cleaning after the measurement involves heating the pycnometer in an oven to soften the bitumen enough to pour out. The pycnometer then has to be soaked in toluene or mesitylene and cleaned manually to remove the remaining residues.

Using a digital density meter

The standard test method (ASTM D8188) requires a digital density meter to measure the asphalt/bitumen samples. A density meter requires less sample volume (around 3 mL to 4 mL, which is about half the amount needed by pycnometers). The density meter used ideally must have a way to keep the syringe at a temperature of around 130 °C to 140 °C so that the sample is kept in the liquid stage.

There are several advantages to using a density meter. They are quicker: Measurement with a pycnometer takes around 2 hours according to ASTM D70, whereas dedicated density meters according to ASTM D8188 often require as little as 12 minutes per measurement. Cleaning the cell of a density meter is also much easier than cleaning a pycnometer. It is sufficient to inject around 30 mL of solvent such as toluene into the measuring cell.

By following a few simple guidelines it is possible to ensure good density measurement results.

Table 1: Overview of bitumen/asphalt measuring methods with pros and cons

 

MethodProsCons
Pycnometer  
  • ASTM D70
  • EN 15326
 
 
  • Approx. 2 hours for measurement
  • Difficult and time-consuming sample preparation
  • Difficult and time-consuming cleaning
  • Requires skilled operator
  • Manual calculations to obtain result for API numbers or relative density
  • 20 mL sample required
  • 60 mL to 100 mL of cleaning solvents required
 
Digital density meter  
  • ASTM D8188
  • Approx. 10 to 15 minutes for measurement
  • Easy filling with a heated syringe
  • 3 mL to 4 mL sample required
  • 30 mL of cleaning solvents required
  • Automatic calculation of API number or relative density due to integrated conversion tables
 
 
  • No EN standard
  • More expensive equipment
 

Common units

The density of bitumen/asphalt is typically given in kg/cm3. If API numbers or the relative density are required, this needs to be calculated (manually in the case of pycnometers or automatically by the density meter, if used). 

Tables / official regulations / official approvals

The industry standard EN 15326 (equivalent to ASTM D70) requires a pycnometer to measure the density of bitumen/asphalt.

There is a new ASTM standard for using a digital density meter to measure bitumen/asphalt: ASTM D8188.