LIKE WATER OFF A ROADWAY

FAYETTEVILLE, Ark. - Rain on asphalt concrete can produce disastrous results, causing road surfaces to separate and deteriorate prematurely. University of Arkansas researcher Kevin Hall has developed a simple test to help highway engineers build roads that resist water damage.

Hall recently presented his findings at the National Transportation Review Board meeting in Washington, D.C.

Hot-mix asphalt concrete (HMAC) flexible pavement covers more than half of the roadways in the United States. Until 1990, virtually all HMAC roads in Arkansas were made by the Marshall method, which produced a very dense asphalt. However, in 1990 the Strategic Highway Research Project defined a new way to mix HMAC called Superpave, which is designed for the particular location where it is installed. Highway departments across the United States began aggressively installing Superpave in 1993.

"Unlike Marshall-type HMAC, which is made up of both small and large rocks, Superpave aggregate has mostly large rocks," Hall explained. "This provides more strength than old pavements, but it also means water moves through Superpave in a different way."

In HMAC permeability, the flow of fluid through a porous medium, is tested by using a falling-head permeameter (FHP). While this worked well with Marshall-type HMAC, it produced inconsistent results with Superpave.

Research began when highway engineers reported problems with Superpave "weeping," or appearing to exude moisture, according to Hall. Because this could result in long-term moisture-related pavement problems, the Arkansas Highway and Transportation Department began to study the permeability of Superpave. After two years, the wide variation in research results indicated some problem in the permeability testing.

Hall speculated that this was caused by the comparatively large spaces (voids) between the large rocks in the Superpave aggregate. The FHP analysis assumed that the void pathways were straight, vertical tube-like shapes, but there were no tests to actually analyze the void spaces.

"It is important to know exactly what is going on so the road bed can be properly prepared," Hall explained. "Water doesn’t really harm Superpave, but when the roadbed underneath gets wet, that’s bad."

Hall developed the void pathway test (VPT) to determine exactly how fluid behaved as it passed through Superpave. In the VPT a cylinder of compressed Superpave is placed in the apparatus and the sides are coated with a soap solution. Air is then blown through the sample and the location and size of the resulting soap bubbles is recorded.

After the 179 samples were studied and characterized by using VPT, the samples were sawn in half and the actual void openings were analyzed. While the results of the VPT agreed with the observed void openings, no specific pattern was observed. However, the VPT demonstrated that the void pathways were shorter and more twisted than assumed by the FHP and that they bend in a horizontal direction.

"VPT shows that water emerges from the sides of Superpave, not the bottom," Hall explained. "We also noted that in the samples tested, the fluid only penetrated halfway through the core vertically because it was carried out at the sides."

These differences have significant implications for road construction. For example, drains along roadsides can be positioned to remove water more effectively, preventing the water build-up that can damage the roadways.