July 10th, 2006
By R. Michael Anderson, PE and Dwight Walker, PE
After more than 10 years of practical use of Superpave, most engineers and contractors agree: Overall, Superpave is successful. Performance grading (PG) binder system improves the performance identification of liquid asphalt. Superpave's method for selecting material composition is based on local traffic and temperature conditions, enabling asphalt binders and aggregate mixtures to achieve the desired performance. Superpave introduced the mixture volume method to many institutions and suppliers. Other advances include mixture aging / conditioning procedures and the ability to estimate pavement compaction.
Even with these successes, experience tells us that there is still some work to be done. Work is under way to improve the characterization of asphalt binders, mixture design procedures and performance testing.
PG combined materials to improve this PG system is one of the most widely praised parts of Superpave program. However, most binder experts believe that it is necessary to develop a more suitable test method for testing modified asphalt.
The new multiple shear creep recovery test (MSCR) is promising, rather than the PG-Plus test that many institutions have adopted. The MSCR test can be performed using the same sample and dynamic shear rheometer (DSR) as the widely used AASHTO M320 parameter test.
Another potential improvement in evaluating modified asphalt is the development of a method to accurately simulate the aging of hot mix asphalt (HMA) binders during mixing, compaction, and use. At present, the rotating film oven test (RTFOT, AASHTO T240) is used to simulate the aging of the binder. But experience shows that RTFOT may not be suitable for modified asphalt.
During the RFTOT test, some modified asphalt binders will not flow freely in the rotating bottle. This prevents the asphalt from being exposed to hot air in the form of a continuously moving film. NCHRP 9-36 is such a research project, the purpose is to develop a set of laboratory procedures applicable to both pure asphalt and modified asphalt. Now that the research project has been completed, a report can be submitted shortly.
Mixture design improvement
Further refinement of the Superpave mix design program is in progress. One of the common concerns is the number N of rotations during the compaction of the laboratory.
Superpave's mix design program requires the use of a rotary compactor. This device can compact the samples in the laboratory to approximately the same density as the actual asphalt mixture in use. The compaction level used in this procedure is completely independent of the project traffic level.
The research purpose of the NCHRP 9-9 (1) project is to provide an authoritative on-site verification of the current Ndesign value. This research has been completed and a research report is expected to be released next year. Ndesign level is expected to decrease.
The NCHRP 9-33 project is in progress and its purpose is to develop an improved mix design procedure and manual. The new procedure is applicable to dense gradation, open gradation and intermittent gradation mixtures.
This procedure may include volume design methods, simple performance test equipment (SPT), and test methods for testing moisture sensitivity. This report will be completed in about 2 years.
Fine-grained mixture vs coarse-grained mixture When Superpave plan was just introduced, vigorously promote the use of coarse aggregate mixture. Most coarse graded mixtures perform well. But when the Nevada's WesTrack test facility started operation, the question arises whether these mixtures are sensitive to small changes in gradation or asphalt content.
Some coarse graded compounds have produced rutting or permanent fatigue damage. Others have some water seepage, especially when the density is slightly lower than the technical requirements. Therefore, many institutions that previously used coarse graded mixtures are now also allowing the use of fine graded mixtures on large-capacity highways.
Another material issue is the degree of stone-to-stone contact in the mix. In many mixtures used, coarse particles actually float in the mixture and are not stressed. SMA's ability to resist rutting has shown the value of achieving close contact between stones.
Regarding the assessment of the susceptibility of mixtures to small changes in gradation, the method being used by some asphalt technical experts is the Bailey method. This method was issued by the Transportation Research Commission in 2002 in E-C044, which is based on the understanding of the characteristics of aggregate pouring. The orientation of the aggregate particles in the mixture will affect the pore structure and the density of the compacted asphalt mixture. Experience with the Bailey method shows that some coarse graded asphalt mixtures can also be sensitive to small changes in the grade (that is, less than 2% of the coarse screen size), so that the density of the mixture will also change by 2%. Generally, finely graded mixtures do not exhibit this level of sensitivity.
The demand for Bailey's law knowledge has been increasing in recent years. To this end, AI and Bill Pine hold a course titled "Achieving Volumetrics and HMA Compactability" in Lexington every winter.
Superpave has established strict requirements for ingredient aggregates. These requirements include: angle, cleanliness and shape to ensure good performance. However, aggregates that meet these requirements are not always available locally. Specification makers must balance the cost of imported materials with the desired performance. Local materials may be suitable for some applications, but in high flow or high stress locations, it makes sense to use selected aggregates or modified asphalt.
The ultimate goal of asphalt mixture design is to find the right type and ratio of materials (aggregate and asphalt) in order to achieve suitable performance, such as rutting and cracking under actual operating conditions (traffic, speed and climate). Note: "Appropriate performance under actual operating conditions" means that cost must be one of the issues to be considered. For low-flow road surfaces, it is unnecessary to design a strong aggregate skeleton with modified asphalt. In these cases, local materials are absolutely acceptable.
Until the performance test is widely used, most users will continue to rely on the mixture volume performance as a replacement for performance-related characteristics. Years of historical experience show that there is a correlation between the pore characteristics of asphalt mixture (that is, density, pores and pores in mineral aggregates) and their performance. However, this relationship is not absolute. Mixtures with volume characteristics that meet the requirements may have poor performance, which is possible (WesTrack is an example). Prior to routine testing based on the performance characteristics of asphalt mixtures, the potential risk of damage due to the use of performance-related alternative characteristics, no matter how low, must be accepted.
Although Superpave is not a perfect system, it does have many advantages over earlier systems. With further development and improvement, it should continue to provide asphalt mixtures that meet and exceed performance expectations.
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