A Tool For Evaluating HMA Volumetrics and Field Compactibility
By Wayne Jones, P.E.
Controlling the volumetrics in hot mix asphalt (HMA) is not a new concept and, in fact, has been around for over a century. In 1903, Bitulithic Macadam, an early HMA design based on volumetrics, was patented by Frederick J. Warren, founder of Warren Brothers Company in Boston, Massachusetts. Back then, Mr. Warren designed an experiment to determine the optimum size and gradation of aggregate particles needed to fill a container of known volume. Starting with a cubic foot of solid rock and a cubic foot container, he broke the rock into subsequently smaller and smaller pieces and then tried to put the pieces back into the container.
Experimenting with different sized particles and a more continuous gradation of particle sizes, Warren eventually found he could get up to 90 percent of the original volume back in the bucket.
Warren apparently did all this experimenting on controlling the voids in the mixture, not based on any great insight that it would lead to longer life pavements, but merely to control the amount of voids in the mix. Like all good competitive “low-bid” contractors, he wanted to control how much of the expensive, imported liquid asphalt was needed to fill the remaining 10 percent air voids in the aggregate.
With liquid asphalt selling for more than $350 per ton in parts of the United States, controlling the voids in HMA is even more critical today for contractors to remain competitive. The state-of-the-practice Superpave (SP) mix design process is also based on volumetrics. Under the SP process, the individual aggregates must pass a series of toughness tests before they can be considered for use in the HMA. Next, the approved aggregates are combined in a series of one-point tests to develop an aggregate structure that holds up under laboratory compaction requirements based on anticipated future traffic levels. Once an acceptable aggregate structure has been determined, then an asphalt content is determined that produces 4 percent air voids at the design compactive effort.
Where we are today?
Put a group of experienced mix designers together in a room, and the stories of especially difficult mix designs start to flow. Each designer in turn will tell a tale of an ever increasing number of one-point mix designs that he or she had to perform before coming up with an acceptable aggregate structure. Usually any number between 5 and 25 will be topped by the next commentator in the conversation. So it appears, after a century of progress, the process of selecting an acceptable aggregate gradation that meets all of the volumetric requirements is still based on the individual mix designer’s experience and a trial-and-error approach.
All of this experimenting takes time and can be very costly, not only in terms of laboratory costs, but the project can be delayed while waiting on an approved mix design. In today’s competitive world, such long delays are unacceptable. This is where the Bailey Method can be most valuable.
In the late 1970s, Robert D. Bailey, now retired from the Illinois Department of Transportation, began to develop a series of analytical procedures to evaluate mixtures being proposed by contractors in the district where he was the Chief Materials Engineer. As he began to refine the procedure, he used this new analytical tool to predict mixture volumetrics—often to the amazement, and sometimes the frustration, of the contractors’ quality control personnel—and to offer suggestions on how to make the necessary gradation changes to meet the volumetric requirements.
As the use of this mixture analysis technique began to spread across Illinois and Indiana, it became known as the “Bailey Method.” The procedure eventually came to the attention of at least two of the Transportation Research Board’s committees on HMA mixtures, which jointly published a technical circular in October 2002, entitled, Bailey Method for Gradation Selection in Hot-Mix Asphalt Mixture Design.
So what is this analytical tool?
The Bailey Method of gradation evaluation focuses on the aggregate properties that affect the way aggregates fit together (or pack) in a confined space or volume (See Figure 1). To analyze the packing factors, the method defines four key principles that break down the overall combined aggregate blend into four distinct fractions (See Figure 2). Each fraction is then analyzed for its contribution to the overall mix volumetrics.
By comparing the size of particles that fit into the voids between the largest aggregate pieces to the size of the largest aggregate pieces found in a fraction, ratios can be developed that are an indication of how well all the particles in the fraction fit together. Once a mix designer has been taught the principles of the Bailey Method and how to apply them, he can then begin to predict how changes in the factors that affect packing will change volumetrics and compactability of a particular mixture.
The Four Main Principles of the Bailey Method:
Provides an entirely new definition of what is coarse and fine, and how to determine the volume of each:
What coarse particles create voids and which ones fill them?
Which fraction (coarse or fine) is in control of the overall structure?
Concentrates on the coarse fraction of the overall aggregate blend and how the particle sizes are distributed:
How does the coarse fraction pack together and what is the volume of voids in the coarse aggregate?
How does the fine fraction pack?
To what extent is the coarse fraction compactable or susceptible to segregation?
Concentrates on the coarse part of the fine fraction and how it relates to the packing of the overall fine fraction.
Looks at the fine part of the fine fraction and how it relates to the packing of this portion of the combined blend.
All four principles of the Bailey Method are interactive and must be monitored for changes. If a gradation changes, then all four principals must be reviewed, because if one changes, the other three will change also.
Will it work on all type of mixes?
Documented use of the Bailey Method continues to flow in from contractors all across the nation and around the world. The principles of the method have been used in such far away places as Canada, France, the Middle East, the Caribbean and China. If it works in these places across the world, chances are the principles of the Bailey Method can be applied to mixes in your area.
Is the Bailey Method only a tool for the mix design lab?
No, remember the method analyzes the overall aggregate gradation in the mixture. Consequently, the principles also work on production mix to predict how changes in aggregate gradations going into the plant will affect the overall mixture volumetrics coming out of the plant. This will allow the Quality Control technician to make the necessary gradation changes to keep the mixture within specification.
Can anyone learn the techniques? Yes, and to get the technology in the hands of those who need to analyze mixtures, the Asphalt Institute has developed a one-day Bailey Method Workshop entitled, The Practical Guide to Achieving HMA Volumetrics. The class is taught by the Field Engineering staff of the Institute, and is intended to provide the participants with the ability to predict the packing properties of their own individual aggregates.
The information from this class will allow the participants to immediately begin to evaluate their existing mixtures and laboratory blends using the principles they learn in the workshop. By attending a workshop and then going home and immediately beginning to use the analytical techniques, the Bailey Method will quickly become a valuable asset to any quality control program.
For those who wish to learn about the Bailey Method in more detail, the Institute offers an intensive 2 1/2-day course at their headquarters in Lexington, Kentucky. The principle instructor for this course is William Pine of Heritage Research Group of Indianapolis, Indiana, who worked for Mr. Bailey during the nine years he was with the Illinois DOT.
To learn more about either of these courses or to register for future offerings, please visit the Institute’s website, www.asphaltinstitute.org.
|Wayne Jones is the Asphalt Institute’s Field Engineer for the Columbus, Ohio, area.