Level paving field

March 12, 2007

There is no doubt that Superpave has resulted in better asphalt mixes being constructed since it was first implemented over 10 years ago. Still, it has always been recognized that mix design procedures would need to be tweaked to provide optimum performance.

There is no doubt that Superpave has resulted in better asphalt mixes being constructed since it was first implemented over 10 years ago. Still, it has always been recognized that mix design procedures would need to be tweaked to provide optimum performance.

One of the most critical items in designing high-quality hot-mix asphalt (HMA) is the liquid asphalt content. If the asphalt content is too low, durability problems may be encountered, but if the asphalt content is too high, rutting may occur. During mix design, the goal is to put as much asphalt in the mix as possible without creating a rutting or excessive bleeding problem.

The asphalt content for a given aggregate gradation is determined by laboratory compaction with the gyratory compactor. Using a higher number of gyrations produces mixtures with lower optimum asphalt content, and lower gyration levels produce mixtures with higher optimum asphalt content. It is important that the number of gyrations be carefully selected so the true optimum asphalt content for the mixture is selected.

Wanting another table

When Superpave was first adopted, a table provided guidance for the number of gyrations based on traffic level and temperature of a given climatic area. There were two basic questions about the table: Can it be simplified? And, is it right? The first question was answered in 2000 by National Cooperative Highway Research Program (NCHRP) project number 9-9. The second question has been answered by field research recently completed by the National Center for Asphalt Technology (NCAT) under NCHRP 9-9(1).

This first table showed a total of 28 potential gyration levels for seven different traffic levels and four different air temperature ranges (Table 1).

The high number of gyration levels caused confusion, especially when some projects were located in more than one traffic range or climatic area. This resulted in the need to change mix designs during the construction of a project. This created control and production problems, and most states recognized the table needed to be consolidated to minimize mix changes that would have to be made during production.

Another issue with the requirements in Table 1 was the small amount of data that was collected and analyzed to establish the gyratory requirements. Very few projects were sampled and very little data from each project was used in establishing the requirements. Since the requirement for number of gyrations is probably the most important requirement in the Superpave mix design procedure, each potential gyration level should be validated to help ensure optimum performance.

When the requirements for number of gyrations were first established, it was assumed there would be more densification in hotter climates than in cooler climates. This is a reasonable assumption, but it does not take into account that the asphalt grade used in hotter climates is higher than that used in cooler climates. The use of a higher PG grade would result in less densification in hot weather and would thus minimize the effect of climate on densification and thus gyration level.

Based on the fact that different geographical areas use different grades of asphalt, it was recommended that the effect of temperature on densification be considered negligible, since this effect would be offset by the grade of asphalt used. If the densification is not affected by air temperature, then the number of gyrations for Ndesign (the number of gyrations required for the optimum mix and the optimum asphalt content) would not be affected by air temperature.

As can be seen in Table 1, the number of gyrations required in the higher traffic levels could be as high as 172. This amount of compaction would likely result in a major breakdown of the aggregate in the mix during laboratory compaction, making the volumetric results meaningless. While these higher gyrations were required in certain situations, it is doubtful that any state actually used a gyration level of more than 125. In recent years, gyration levels in most states have been reduced to 100 or less even for the highest traffic levels.

Subtracted by 24

In the mid-1990s, NCAT began a study to look at reducing the number of gyration levels that were specified. This study was completed in 2000 with a report that recommended reducing the gyration levels from 28 possible levels down to four. Based on the NCHRP work at NCAT and an FHWA study at the Asphalt Institute, the results in Table 2 were recommended and eventually adopted by AASHTO. No work was conducted to verify the gyration levels were correct but simply to reduce the number of gyration levels in a way that would essentially provide the same mixture quality. This research effort recommended doing away with the temperature ranges in the gyration table because the PG asphalt grade selected for a particular area minimized the effect of air temperature on densification of mixtures. The traffic levels were reduced from seven to four, resulting in a total of only four recommended gyration levels. These recommendations were quickly adopted by AASHTO and are currently specified as part of the Superpave procedure. All of this work was performed under NCHRP 9-9.

It also was recognized that the Ni (Ninitial) requirements for %Gmm (percent of maximum density) needed to be modified, since many good fine-graded Superpave mixtures were being rejected because they failed the Ni requirements even though they were providing good performance. After consideration, the Ni requirements were relaxed slightly for certain traffic levels so that these quality fine-graded mixtures would meet the requirements for %Gmm.

In 2000, NCAT began a project to actually verify that the four gyration levels that had previously been adopted were correct. This study involved identifying 40 projects throughout the U.S. that could be sampled during construction and monitored over a period of four years to learn how they densified with time. This also involved using a process to calibrate gyratory compactors so that the density obtained in different compactors would be expected to be the same. The assumption for this study was that the laboratory density obtained for a given traffic level (gyration level) should be equal to the ultimate density obtained in the field.

To summarize, the initial work under NCHRP 9-9 was directed toward consolidating the table. In 2000, NCHRP 9-9 was extended to allow for a detailed evaluation of the recommended gyration levels shown in Table 2. This work was accomplished by NCAT and was completed in 2006. This work has not yet been adopted by AASHTO but it will be considered in the near future. This project was performed under NCHRP 9-9(1).

The very detailed NCHRP 9-9(1) study involved taking samples and conducting tests over a four-year period for approximately 40 projects spread throughout the U.S.

There were several questions that had to be answered in this study. For example, it was obvious that different gyratory compactors provided some difference in compaction. It had been determined by many researchers that the reason for this difference was primarily due to the differences between the external compaction angle and the internal compaction angle. Some compactors had more difference between these two angles than other compactors. This had to be accounted for in the study, even though this was learned midway through this research project. Steps were taken to consider the effect of the internal angle of gyration on density and the final results are based on an internal angle of 1.16°.

It also had to be determined how long to monitor the densificaton of HMA in place to know that ultimate density had been obtained. It was determined, based on data collection for four years, that after two years in place no significant additional densification in the HMA would be expected.

During this study, it was determined that mixes containing modified asphalts densified less than mixtures containing asphalt cements that were not modified. As a result, there are different recommendations for mixtures produced with unmodified asphalts and mixtures produced with modified asphalts. The compaction level for mixtures with modified asphalt is less than the level required for a mixture using unmodified asphalt.

It also was suggested, based on the data from this study, that the Ni and Nm (Nmaximum) requirements be deleted since neither seemed to provide any information related to the performance of the 40 projects. There have been other studies that have recommended the removal of Nm requirements but there has been less evidence that Ni should be removed. At any rate, it is clear from this and other studies that the requirements for Nm should be deleted. It is less convincing that Ni should be deleted but this requirement does appear to reject many otherwise good fine-graded mixtures. Additional work to look at the Ni requirements is recommended.

A durable four

Based on the NCHRP 9-9(1) study, the gyration levels shown in Table 3 are recommended. In general, the recommended number of gyrations is reduced from that currently specified, but these recommendations confirm what many state DOTs had already done, that is, reduce the number of required gyrations. Many DOTs had already lowered the number of gyrations required based on their experiences with Superpave.

The reduction of gyration levels also has been supported by performance studies that have indicated that the Superpave mixtures have done a good job of minimizing rutting, but durability problems have been a concern. There have been some permeability issues and compaction issues with Superpave mixtures, indicating the potential for durability problems.

Using Table 3, which requires a lower number of gyrations, will produce HMA with higher asphalt content and increased durability. This should help to reduce concerns about durability problems and should provide asphalt mixtures with improved overall performance.

About The Author: Brown is director of the National Center for Asphalt Technology, Auburn, Ala.

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