Bitumen & Asphalt Extraction

Asphalt & Bitumen Extraction

The three major test methods, solvent extraction, nuclear asphalt content gauge and ignition furnace are discussed here.  Each method offers a way to determine asphalt content and aggregate gradation from an HMA sample. The asphalt content and gradation test can be used for HMA quality control, acceptance testing or forensic analysis.  

What’s the difference between Bitumen and Asphalt?

Bitumen is actually the liquid binder that holds asphalt together. The term bitumen is often mistakenly used to describe asphalt. A bitumen-sealed road has a layer of bitumen sprayed and then covered with an aggregate This is then repeated to give a two-coat seal.

Asphalt is produced in a plant that heats, dries and mixes aggregate, bitumen and sand into a composite mix. It is then applied through a paving machine on site as a solid material at a nominated or required thickness, relative to the end use. Asphalt results in a smoother and more durable surface than a bitumen-sealed road.

Solvent Extraction

Solvent extraction, the oldest of the three test methods, uses a chemical solvent (trichloroethylene or toluene) to remove the asphalt binder from the aggregate.  Typically, a loose HMA sample is weighed and then a solvent is added to disintegrate the sample.  The asphalt binder/solvent and aggregate are then separated using a centrifuge (see Figures 7 and 8) and the aggregate is weighed.  The initial and final weights are compared and the difference is assumed to be the asphalt binder weight.  Using this weight and the weight of the original sample a percent asphalt binder by weight can be calculated.

Today, the solvent extraction method is only sparingly used due to the hazardous nature of the specified solvents.

The standard solvent extraction test is:

ASTM D2172 / D2172M - 11 
Standard Test Methods for Quantitative Extraction of Bitumen from Bituminous Paving Mixtures

Nuclear Asphalt Content Gauge

A nuclear asphalt content gauge (see Figure 9) measures asphalt content by estimating the actual number of hydrogen atoms contained within a sample. The gauge detector counts only thermal (low energy) or “slow” neutrons thereby making the detector count proportional to the number of hydrogen atoms in the sample. Since asphalt is a hydrocarbon, the more hydrogen atoms, the more asphalt. A calibration factor is used to relate thermal neutron count to actual asphalt content.

The nuclear asphalt content gauge offers a relatively quick (4 to 16 minutes depending upon desired accuracy) method for measuring asphalt content.  Since the gauge actually measures hydrogen nuclei and then correlates their number with asphalt content, anything affecting the number of hydrogen nuclei in the sample can be a potential source of error.  Because water contains a significant amount of hydrogen (H2O), anything that adds moisture to the sample (e.g., moisture in the aggregate pores) is a potential error source.

Ignition Furnace

The ignition furnace test developed to replace the solvent extraction method, determines asphalt binder content by burning off the asphalt binder of a loose HMA sample.  Basically, an HMA sample is weighed and then placed in a 538°C (1072°F) furnace (see Figure 10) and ignited.  Once all the asphalt binder has burned off the remaining aggregate is weighed. The initial and final weights are compared and the difference is assumed to be the asphalt binder weight. Using this weight and the weight of the original sample, a percent asphalt binder by weight can be calculated.

A correction factor must be used with the ignition furnace because a certain amount of aggregate fines may be burned off during the ignition process.