How What and Why Blog: Specific Gravity of Asphalt

Specific Gravity of Asphalt: What Equipment Do I Need?

Asphalt specific gravity, often measured using the Rice Test method, is a key Hot Mix Asphalt (HMA) characteristic used to calculate the percentage of air voids in compacted HMA. This calculation, performed in compliance with AASHTO T 209 and ASTM D2041 is used in Superpave mix design for the determination of theoretical maximum density and in-place air voids. It is also used to calculate the amount of asphalt absorbed in a HMA mixture and to provide target values for compaction operations. A Rice Shaker device is key in this test to provide controlled agitation during the deairing process while assuring precise repeatability.

What is the Purpose of Determining the Specific Gravity of Asphalt?

Along with air void determination, measurements obtained from the test establish the target density for compaction operations, as well as the amount of bitumen absorbed due to porosity of the individual aggregate particles. If the percent air voids are too low, rutting and shoving of the pavement can occur in use. If the percentage of air voids is too high, the pavement can be subject to moisture damage, exhibit decreased strength and have a shorter fatigue life when compared to correct air void percentages.

How Does it Work?

The focus of the Rice Test is to compare the mass of the sample to the volume of the sample, excluding the air voids. The mass is determined by measuring the dry mass of the loose HMA sample either at the beginning of the test or after it has been dried at the end of the test. The volume is calculated by dividing the mass of the water displaced by the sample, by the unit weight of water.

To obtain mass and volume, it’s necessary to remove free and entrapped air from the asphalt mixture sample. A vacuum pump is required to remove the free air, while a Rice Shaker provides the agitation of the pycnometer necessary to remove the entrapped air from the mixture.

An asphalt sample is prepared for testing by heating to a constant 135°C, and separating into loose particles by hand. The sample is placed into a calibrated pycnometer and weighed, then water at 25°C is added to cover the sample. A cover is placed on the pycnometer, and a vacuum is applied while it is being agitated on the Rice Shaker to remove entrapped air. A final weight to determine volume is taken by suspending the pycnometer with sample in water or by completely filling the pycnometer with water.

*This procedure is outlined in detail in the ASTM D2041/AASHTO T 209 test methods.

What are the Benefits of a Rice Shaker?

  • Consistent, repeatable agitation for specific gravity determinations enhances accuracy and repeatability
  • Variable vibration control allows for setting optimum agitation
  • Automatic operation minimizes errors and frees technician from hand agitation
  • Also useful in specific gravity determinations for both fine aggregates and soils

What do we Recommend?

Gilson’s Rice Shaker is specially designed for consistent, automated shaking of Asphalt Rice Test vacuum pycnometers, freeing the lab technician for other tasks. The built-in 0-99min., 59 sec. digital timer has ± 0.25 second accuracy, vibration speed control and a three position switch for manual or timed operation. Speed controls regulate agitation to avoid stripping of asphalt, and fitted top and bottom plates provide an exact fit for SG-16A and SG-18A Pycnometers. Optional Adaptors allow use with larger SG-15 Pycnometer, Heavy-Wall Filter Flasks, Volumetric and LeChatelier Flasks, and SG-2 Specific Gravity & Absorption of Fine Aggregate Jar and Top.