Making Roads That Don't Rut

FAYETTEVILLE, Ark. -- Two researchers at the University of Arkansas are paving the way for smoother roads by developing a test to determine the long-term performance of asphalt pavements.

Dr. Kevin Hall, associate professor of civil engineering, and graduate student Stacy Williams, a two-time recipient of the U.S. Department of Transportation’s Dwight D. Eisenhower Transportation Fellowship, are trying to reduce the cost of highway degradation by developing a way to make roads that do not rut. This would substantially improve driver safety and decrease car repair expenses while producing roads that last longer.

Williams and Hall will present their findings on January 11 and 12 at the Transportation Research Board meeting in Washington, D.C.

Hall and Williams study flexible pavements - more commonly known as asphalt - which represent more than half of all road surfaces in the United States. Currently there is not a good correlation between laboratory and field data. Their findings indicate a need to develop a readily-available, simple test that can be used to screen flexible pavements before they are constructed to ensure they will perform as expected.

Americans travel an average of 17,000 miles per year in their cars. And most people know those can be bone-jarring, teeth-rattling miles. In 1997, the Environmental Working Group estimated that Americans spend $4.8 billion dollars annually repairing cars damaged by roads.

Most states experience premature flexible pavement failures due to both rutting and stripping. "Stripping is usually caused by water," Hall explains. "It is the physical separation of the asphalt binder from the aggregate in a mix."

Although national standards for asphalt exist, asphalt paving mixes do not always perform according to expectations because asphalt is not a "one size fits all" product. Asphalt is a mixture of aggregates (rock and sand), polymer and asphalt cement binders, and air. However, many different polymers and asphalt cements can be used. In addition, rock and sand differ widely in shape composition. For example, Williams explains that "natural sand causes rutting because it is round and acts like ball bearings. So you have to use manufactured sand."

The correct formula for an asphalt mixture depends on the environment and traffic levels where it will be used. Many factors, such as temperature, wetness, and slope impact on asphalt’s performance. Hall and Williams’ research focuses on helping builders make a better asphalt for paving roads by developing a test to run on each mixture to determine if that batch will perform appropriately in a specific situation.

"In the past, the components of the mixture were judged on experience," Williams explained. "They knew certain types worked pretty well. But different locations are going to act in different ways."

Creation of hot-mix asphalt concrete mixes that resist rutting, the most common failure of pavements, was one of the major goals of the national Strategic Highway Research Program (SHRP). Asphalt has been used for paving roads since the 1940s. And until 1990, most asphalt was designed the same way, no matter what the application. In 1990 SHRP defined a new way to design asphalt mixes called 'Superpave,’ which models the fundamental characteristics of the materials rather than relying on the contractor’s experience. State and local highway departments, who bear the burden of building and maintaining more than 3.9 million miles of paved roads, began implementing Superpave aggressively in 1993.

However, as Superpave became widespread, some questions began to emerge about its durability. Superpave mixtures, which are more flexible, let water through more readily than the traditional dense asphalt mixes. When applied over old asphalt, the Superpave mixture can allow water to seep through to the old surface and puddle, potentially making the roads weaker and allowing them to rut more easily. It can also cause pavements to "weep," oozing water back onto a surface after the rest of the road is dry, causing slick spots and increasing the potential for accidents.

The key to making Superpave perform up to its expectations, according to Hall, is to test the actual mixture that will be used in a specific application. By subjecting the mixture to actual field conditions, he hopes to provide builders with a way to know if, or when, their roads will rut before they are built. Their equipment can test the mixture at a variety of temperatures and wetness conditions.

"If you can identify problems ahead of time - potential rutting or weakness - you can better determine what types of pavements should go on the road," explained Williams. "We’re talking billions and billions in tax dollars, and we want to be sure what we put out there is going to last. It’s a huge expense to build a road, so we want to make sure we build it right. An extra week and a few dollars up front to test it before it is built has got to be worth it."

The Transportation Research Board (TRB) is a unit of the National Research Council, the principal operating agency of the National Academy of Sciences and the National Academy of Engineering. The Board’s mission is to promote innovation and progress in transportation by stimulating and conducting research, facilitating the dissemination of information, and encouraging the implementation of research results.