Superpave and Aggregate Properties: Where Did They Come From?
Aggregate characteristics are a major factor in the performance of an asphalt mixture. In the Superpave mixture design system many aggregate criteria were included to assure the performance of the asphalt mix. These criteria included coarse aggregate angularity, uncompacted voids in fine aggregate, flat and elongated particles, clay content, and gradation parameters. The recommended limits set by SHRP on these aggregate criteria were established by years of previous research.
Numerous studies have indicated mixture stability increased with
an increase of crushed particles to replace rounded gravels and
sands; Wedding and Gaynor, Association of Asphalt Pavement Technologists
(AAPT), 1961; Field, AAPT, 1958; Campen and Smith, AAPT, 1948.
These are only a very few of the papers on the comparison of
crushed and rounded aggregates.
In 1992 Brown and Cross presented a paper at AAPT titled "A National Study of Rutting In Hot Mix Asphalt Pavements." The paper reported on an extensive study of material properties and their relationship to pavement performance. The study included 42 pavements in 14 different states. Rut depth measurements were taken, mix design information, construction records, traffic counts, and pavement samples collected for each of the pavements.
The study included a detailed laboratory testing program on samples of the asphalt mixture from the studies of rutted and good performing pavements. The data were analyzed to determine material and mixture properties and identify procedures that are necessary for the construction of rut resistant pavements. Of all the materials and mixture properties studied coarse and fine aggregate angularity correlated best to pavement rutting.
Also 1992, Kandhal, Khatri, and Motter presented a paper at AAPT titled "Evaluation of Particle Shape and Texture of Mineral Aggregates and Their Blends." In the study, 27 different manufactured and natural sands were evaluated using several different test methods. In the study all manufactured fine aggregates except one were found to exhibit uncompacted void contents of 44.5 and all the natural fine aggregates had uncompacted void contents lower than 44.5.
The FWHA Expert Task Group on Superpave Mixtures initiated an evaluation of fine aggregates to verify the Superpave recommendations. Replies from 8 states on 197 manufactured and natural sands were received. Of the 106 manufactured sands only 2 had uncompacted voids lower than 45. Of the 91 natural sands only 15 had uncompacted voids higher than 45.
The first indications from Westrack verify many of the Superpave aggregate recommendations. For the coarse graded mixes, those below the maximum density line, the coarse aggregate angularity was well below the recommended of 100/100. The actual coarse aggregate angularity of the stone was 86/72, well below the recommended values for a pavement carrying the equivalent of 50 to 75 million ESAL's. These coarse mixes have not performed well and several sections have been removed. The fine graded mixes, those above the maximum density line, had uncompacted void contents of 45. This is the value recommended by Superpave for a 50 to 75 million ESAL roadway. These mixes, though not expected to perform at such a high rate of loading, have performed better than expected. The fine mixes have out-performed the coarse mixes which did not meet the Superpave requirements.
The SHRP researchers did no research on aggregates. However,
they did build on the studies and recommendations of many researchers
who came before them and the expertise of many practitioners.
From this previous research they developed rules and recommendations
for the Superpave System. There will always be exceptions to
any rules and the same is true of Superpave. However, before
we change or completely throw out these rules to satisfy a few
exceptions we should take a close look at what we are doing and
thoroughly evaluate the situation.