Gyration Level for Superpave Gyratory Compactor (SGC) for Airfield Mixtures

 

Current Federal Aviation Administration (FAA) specifications for compaction of airfield mixtures allow the engineer to select the method of asphalt mixture compaction for mix design and quality assurance testing, as either the Marshall hammer (ASTM D6926) or the Superpave gyratory compactor (SGC) (ASTM D6925). For airfield pavements designed to support aircraft up to 60,000 pounds, the required laboratory compactive effort is either 50 blows with the Marshall hammer or 50 gyrations with the SGC. For airfield pavements designed for aircraft above 60,000 pounds, the required laboratory compactive effort is either 75 blows with the Marshall hammer or 75 gyrations with the SGC.

 

Figure 1. Illustrations of Marshall hammer and SGC compaction.

The Marshall hammer compaction was used for many decades prior to the development of the SGC and has been considered to result in mixtures with satisfactory performance. However, since the introduction of the SGC as an option in FAA specifications, the SGC has gradually become the method of choice for most FAA projects, particularly in the eastern U.S. Some concern remains among airfield asphalt pavement engineers that specimen densities from Marshall and SGC compaction are not equivalent, and the differences may result in airfield asphalt mixtures that may perform differently in service.

Research backed by FAA and AAPTP

To address this issue, a study was sponsored by the FAA through the Airport Asphalt Pavement Technology Program (AAPTP) to determine the number of SGC gyrations (NEQ) equivalent to
50- and 75-blow Marshall hammer compaction.
The study was conducted by NCAT and the University of Nevada, Reno (UNR).

Although the FAA previously sponsored research projects to assess the appropriate level of design gyrations when using the SGC, the literature review revealed that historical correlations between Marshall and SGC compaction have been established in single laboratories, likely with single operators, and with a limited number of mix designs.

For a more comprehensive evaluation, this research study gathered 51 airfield mixtures (P-401 and P-403) from across the United States, representing various climatic zones, aggregate types, and binder grades. The mixtures were collected from airfield paving projects constructed in 2023 and 2024. Figure 2 shows the locations of the airport projects where the mixtures were collected. 

Figure 2. Locations of projects where mixture samples were obtained for this study.

Determining NEQ through comparative testing

Forty-four of the mixtures sampled were 75
gyration/blow designs (28 Superpave and 16 Marshall), and seven mixtures were 50
gyration/blow designs (five Superpave and two Marshall). The mixtures gathered were split between the NCAT and UNR laboratories by location, with mixtures in the western part of the country tested at UNR and most of the mixtures in the eastern part of the country tested at NCAT. Both laboratories followed consistent protocols for specimen preparation, compaction, and analysis. Prior to testing, a limited number of mixtures were tested at both labs to verify reproducible results. 

The approach to determine the NEQ consisted of finding the number of gyrations with the SGC that yielded the same average compacted specimen bulk specific gravity (G_mb) as determined from the 50-blow or 75-blow Marshall hammer, since G_mb is the only mixture property affected by compactive effort. To “normalize” the compaction data on a common scale, G_mb results for each mixture were divided by its maximum specific gravity (G_mm) and expressed as %G_mm. The steps used to determine N_EQ are outlined to the right and illustrated in Figure 3.

1. Determine the mixture G_mm.

2. Compact specimens with 50- or 75-blow Marshall compaction and determine the G_mb of the specimens.

3. Determine the relative density (%G_mm) of the Marshall specimens.

4. Compact the first set of specimens in the SGC at 50 or 75 gyrations. 

5. If the average SGC %G_mm was greater than the average Marshall %G_mm, then compact the second set of SGC specimens at a lower number of gyrations. If the average SGC %G_mm was less than the average Marshall %G_mm, then compact the second set of SGC specimens at a higher number of gyrations.

6. Determine the linear regression equation between the number of gyrations and %G_mm at the two SGC compaction levels.

7. Use the linear regression equation to determine N_EQ at the number of gyrations corresponding to the average Marshall %G_mm.

Figure 3. Graphical illustration of determining N_EQ  for an asphalt mixture.

Using the N_EQ results of the 75-blow/75-gyration and 50-blow/50-gyration mixtures, cumulative distribution functions were obtained as presented in Figure 4 (below). These distributions excluded N_EQ results that were outside of the tested SGC compactive efforts. Figure 4a shows the cumulative distribution function of N_EQ-75 from the combined NCAT-UNR database with 41 datapoints. The mean N_EQ-75 of this dataset was 77 gyrations with a standard deviation of 21 gyrations. Figure 4b shows the cumulative distribution function of N_EQ-50 with six data points. The mean N_EQ was 58 gyrations with a standard deviation of 18.

Figure 4. Cumulative Distribution Functions of
N_EQ-75  (a) and N_EQ-50 (b) for the Combined NCAT
and UNR Data.

These results confirm the recommendations of previous studies that have led to the current FAA P-401 and P-403 specifications that allow either 50-blow or 50-gyrations mix designs for airports servicing aircraft with a maximum of 60,000 pounds, and either 75-blow or 75-gyrations for all other FAA airport facilities.

The overall findings of this rigorous research study, incorporating an analysis of over 50 airfield mixtures from across the country, with a wide array of aggregate, asphalt binder, and mixture properties, validated the existing P-401/403 options for compactive efforts for asphalt mix designs.
Table 1 (next page) shows the recommended Marshall and SGC compactive efforts for airport pavements servicing aircraft above and below 60,000 lbs.

Table 1. Recommended compactive efforts for P-401 and P-403 mix designs.

This research included multiple laboratories and a larger number of mixtures than in past research efforts. It led to similar correlations between Marshall blows and SGC gyrations as past similar research, but integrated multi-laboratory variability, which is important to consider since, in practice, within a given state or region, there will be multiple qualified laboratories developing P-401 or P-403 mixtures as opposed to a single laboratory.

Only a limited number of 50-blow Marshall mix designs were obtained from across the country, as they are used only for airfields servicing relatively low-weight aircraft.

Click here to view the full published report, titled "Validation of Gyration Level for Superpave Gyratory Compactor (SGC) for Mix Design and Control of Airport Asphalt Mixtures."

   

Contact Carolina Rodezno for more information about this research.