The future of binders and asphalt mixtures is a bright one indeed. Increasingly, state transportation departments and local agencies will adopt Superpave―and will improve it as their chosen method of mix design. As state testing staffs be
The future of binders and asphalt mixtures is a bright one indeed. Increasingly, state transportation departments and local agencies will adopt Superpave―and will improve it as their chosen method of mix design. As state testing staffs become smaller relative to their workloads, more states will move to statistically-based quality assurance programs.
The use of stone matrix asphalt or SMA will increase. And if the cost of fuel continues to rise and air quality regulations get tougher, we will see more warm mix asphalt. Full scale tests on warm mix are expected this year. Someday, researchers hope to be able to automate quality control testing so that samples are never touched by human hands!
A range of tests will be developed for both binders and mixtures that will enable technicians to predict more accurately the mechanistic behavior of asphalt pavements under given loadings and temperature conditions. “We’re planning to convert from a sawed-off shotgun to a deer rifle with a scope,” says Gary Fitts, Senior Field Engineer, Asphalt Institute, San Antonio, Texas. “We want to reduce the scatter in our predictions. We can improve our ability to predict performance if we use properties that have applied physical characteristics.”
The future will bring greater emphasis on using reclaimed asphalt pavement in mixtures, Fitts says. The industry will improve the effectiveness of using recycled material.
The next five years should see more emphasis on mixture research than binder research, says Rick Holmgreen, Asphalt Technical Support Manager, ConocoPhillips Co., Houston, Texas. A number of research programs across the country focus on modeling an asphalt pavement’s behavior―such as reflective cracking or permanent deformation―based upon certain properties measured in the laboratory. Holmgreen ticks off a list of universities doing such research: Arizona State, Texas A&M and the University of Illinois, to name a few.
Lowered Design Compaction
Most experts agree that Superpave’s design level of laboratory compaction, as measured by N-design, will be adjusted downward over the next few years. At the National Center for Asphalt Technology(NCAT), research will wind up this summer on NCHRP 9-9(1). Ray Brown, Director of NCAT says, “The bottom line is the recommendation that N-design will be reduced by a certain amount.”
“I do not think we have a good sound basis for N-design numbers as they exist in Superpave,” says Gerry Huber, Associate Director of Research, Heritage Research Group, Indianapolis, Indiana.“The numbers are reasonable but I think they are too high.”
High levels of compaction in the gyratory can lead to problems when designing mixes with soft aggregates, Huber says. The aggregate skeleton gets crushed in the gyratory mold, yet rollers on the pavement do not compact the mix to that degree. For example, you can design a mixture in the gyratory to have 4 percent air voids, but the aggregate is crushed. The same mix can have 6 to 6.5 percent air voids under field compaction―which then makes it very difficult to get 93 percent of maximum theoretical density in the field, says Huber.
High values for N-design limit the ability of states to use local aggregates that may be more marginal in quality, according to Mark Blow, Senior Field Engineer, Asphalt Institute, Sioux Falls, South Dakota. He says many states have already reduced N-design, because high costs of transportation and aggregates force them to use local aggregates. “High values for N-design, utilized on historically accept-able local materials, reduces the VMA, leaves less room for asphalt, and results in drier mixes,” says Blow.
Blow cites Iowa as an example. State mix designers adapted local, existing mixtures to the gyratory system of design, so that local aggregates could be use.