Take a drive on any Texas highway and you'll see no shortage of orange traffic signs warning of road construction. In fact, to the state's irritated drivers, it seems as if half the highways in Texas are constantly under repair.
But new software developed by a team led by Texas A&M Engineering researchers takes into account weather and traffic conditions to help builders design better, longer-lasting roads.
The Asphalt Research Consortium, or ARC, was founded more than 10 years ago to address a fundamental issue—asphalt, says Dallas Little, professor in the Zachry Department of Civil Engineering and a distinguished member of the American Society of Civil Engineers. Specifically, the ARC focuses on the bond between asphalt and its various aggregates, as well as asphalt's susceptibility to damage from aging and moisture.
"Say where you want a road to be — Houston or Fargo, N.D. — what kind of traffic the area has, what kind of asphalt type, and so on. PANDA can account for all these factors in the performance predictions.
A major result of the research consortium's work is PANDA (Pavement Analysis Using Nonlinear Damage Approach), a state-of-the-art, three-dimensional computational code that can predict when and where pavement damage will occur. And this prediction can help road planners and builders build better pavement.
The congressionally mandated consortium was funded by a transportation bill (now under extension) through the Federal Highway Administration with about $30 million over six years to evaluate asphalt infrastructure performance.
"We answer to the U.S. Department of Transportation, state departments of transportation, industry, academics and area leaders—all representing the driving public," says Little, who is a Distinguished Member of the American Society of Civil Engineers. "The end goal is this model that will be able to enhance pavements to extend the lives of our roads and their sustainability."
Complex material, complex research team
Pavement is complicated, Little says. It's made of a very fine mineral filler and aggregate (both sand and gravel-sized rock), bound together by the viscous, sticky black liquid called asphalt that comes from crude oil. The complexity of the composite is due to the physical and chemical reactions between components. Asphalt is dynamic, and air voids and moisture in the mixture can harden and stiffen the material over time.
To address this complexity, the interdisciplinary consortium brings together experts in materials science and engineering, chemistry, and transportation. Little, with his expertise in materials engineering, leads Texas A&M's research team. His insight into asphalt material feeds into the PANDA model. Eyad Masad, a professor at Texas A&M University at Qatar, brings computation expertise to the team. Charles Glover, professor in the Artie McFerrin Department of Chemical Engineering, is an expert in asphalt chemistry and is part of the Texas A&M team, along with Robert Lytton, a micromechanics expert in the civil engineering department. And Rashid Abu Al-Rub, a former assistant professor at Texas A&M, lent his invaluable expertise in structural mechanics.
And that's just at Texas A&M. A major ARC partner is the Western Research Institute at the University of Wyoming. Little says WRI has some of the best asphalt chemists in the world because of the institute's previous life as a U.S. Department of Energy research facility with particular expertise in shale oil.
Researchers at the University of Nevada–Reno are studying tire–pavement interaction to work that knowledge into PANDA. Other collaborators include Advanced Asphalt Technologies in Sterling, Va., the University of Wisconsin-Madison, the University of Texas at Austin, the University of Nebraska–Lincoln and the University of Illinois at Urbana-Champaign.
"ARC is big and diversified ," Little says. "We have many researchers who are focused on solving one of this country's major infrastructure problems."
Current models make simplifying assumptions and gloss over important aspects. Better life-cycle information can make for a more thorough model to help save money and extend road life.
"We want to be able to predict performance—any type of damage, whether from fatigue cracking, rutting or moisture damage, or a combination of all," Little says. "But most existing models are based on empirical methods and experimentation."
The PANDA project is different because it integrates fundamental material properties of asphalt and the mineral aggregate components into a mechanistic model. And to do that, researchers must truly understand asphalt: What properties cause what kind of damage? Can that knowledge be put into a model or computational tool to help planners and builders design pavements? And can existing materials be used to build these roads, or will new materials need to be designed?
Road builders have to build different road types in different areas on the basis of information specific to those areas, such as climate and traffic conditions, Little says.
"These conditions affect how pavement will perform over a certain life cycle," Little says. "So we have to know if the pavement will sustain those conditions, whether and how it will deteriorate, what precautions should be taken, and what kind of maintenance plan will need to be developed. The basis for all this is a good model founded on fundamental material properties, and it's all part of PANDA."
Road builders have to build different road types in different areas on the basis of information specific to those areas, such as climate and traffic conditions.
Little says one notable advancement of PANDA is a rather sophisticated approach: applying the dynamics of an actual wheel as it moves over pavement. Researchers at the University of Illinois, ARC's most recent partner, offer invaluable expertise in this area. Now PANDA can actually model a realistic moving wheel, not just a static load.
"Say where you want a road to be—Houston or Fargo, N.D.—what kind of traffic the area has, what kind of asphalt type, and so on," Little says. "PANDA can account for all these factors in the performance predictions."
The software model is also important for predicting performance of pavement on a microstructural level.
"We can also use PANDA to design the asphalt itself," Little says. "We can predict how a certain mixture will behave if you change constituents—the type of asphalt binder, the rock source, the air void content, whether the voids are saturated with water, or if modifiers including nanomaterials or polymers are used. PANDA can help predict how much the overall pavement properties will change when the microstructure changes, so it's a material design tool as well."
For an example of the nanoscale investigations being considered for input into PANDA, graduate students are using an atomic-force microscope to determine what happens when polymers are added to asphalt. Knowing the nano-, meso- and microstructural changes that occur can help pavement engineers assess specific mechanisms and therefore develop better ways to modify material properties at all length scales.
"This is a much smaller scale than has ever been considered before in this field," Little says. "So incorporating this information into our model is a big step. We are not quite there yet, though, but that is our next step."
The model also accounts for asphalt's complex material behavior.
Little says that asphalt is almost like a living body: Damaged asphalt can partially "heal" and fix itself when left alone and under certain conditions. This healing property—what drives it and how it can be used to extend the life of pavement—is another aspect that the research team has studied and incorporated into the model.
"PANDA is a virtual experiment that will give the highway community the ability to easily perform 'what if' scenarios and decide how best to build a road that will last."
The team is completing software development, validating the software in laboratory and field tests, and working to make the program more user-friendly for wider use.
"PANDA is a virtual experiment," Little says. "PANDA is capable of evaluating how pavement will respond under loading as the pavement ages and is exposed to air and moisture. This tool will give the highway community the ability to easily perform 'what if' scenarios and decide how best to build a road that will last."