NEW UNDERSTANDING COULD BOOST ASPHALT USE

The U.S. National Research Council says a significant advance in the scientific understanding of asphalt chemistry could make it easier to build pavements that last longer. Damian Kulash, executive director of NRC's Strategic Highway Research Program (SHRP), said "We now have an entirely new understanding of asphalt chemistry that has tremendous implications.

Sept. 2, 1991

3 min read

The U.S. National Research Council says a significant advance in the scientific understanding of asphalt chemistry could make it easier to build pavements that last longer.

Damian Kulash, executive director of NRC's Strategic Highway Research Program (SHRP), said "We now have an entirely new understanding of asphalt chemistry that has tremendous implications.

"This will enable highway engineers to design pavement structures that take full advantage of asphalt's chemical and physical properties. It also will enable refiners and manufacturers to modify asphalt materials so they will perform better."

SHRP is a 5 year, $150 million research program funded by the U.S. government to promote high priority activities that promise to yield savings in excess of research costs.

CHEMICAL MODEL

Under contract with SHRP, Wyoming and Pennsylvania researchers generated a chemical model that offers insight on the inner workings of asphalt. Preliminary findings, presented at the 1991 International Symposium on the Chemistry of Bitumens in Rome, are to be published later this year.

Asphalt is about 90% carbon and hydrogen, with sulfur, nitrogen, oxygen, and trace metals making up the other 10%. Those elements attach themselves to the hydrocarbon molecules in side groups called polar functional groups.

The polar functional groups control performance because they determine how asphalt molecules will "stick together" and form networks to give the asphalt strength, while allowing it to flow like a viscous liquid.

NRC said researchers discovered a specific chemical component, amphoterics, is very influential in asphalt performance. An amphoteric material is one that is both an acid and a base in the same molecule.

"Although these materials make up only 10-15% of asphalt, they appear to exert major control over the formation of molecular matrices, which in turn affects the asphalt's tendency to form pavement cracks, ruts, and potholes," NRC said.

If the network is not stiff enough, the asphalt will not be sufficiently resistant to flow and will be prone to rutting.

If the network is too stiff, it will not give and will be prone to fatigue cracking.

Moisture sensitivity, which can lead to cracks and potholes, also is a function of the network because it is the polar molecules that cause the asphalt to adhere to the aggregate.

And it is that bond that breaks down in pavements showing moisture induced damage.

Raymond Robertson, with Western Research Institute, Laramie, Wyo., said "Once we understand how asphalt works, it is much easier to make it do what we want."

His fellow researcher, David Anderson of Pennsylvania State University's Transportation Institute, said, "This is a major departure from the chemical model that has been accepted by researchers around the world for decades, but we have a great deal of evidence to support what we are saying."

SHRP is developing tests for manufacturers to use to determine the chemical and physical properties of asphalt cement and to modify it to give better performance.

Another series of tests is being developed so highway agencies can select asphalts and aggregates that will give the best performance.