STREETS & HIGHWAYS/Intersection project makes asphalt a winner

Because slower speeds generally mean a higher incidence of rutting, pavement designers must recognize that intersections are different from posted-speed highways. Consequently, designing intersections is a complex process that should involve a detailed assessment of the existing roadway.

For years, highway departments were reluctant to use hot mix asphalt (HMA) on high-stress intersections, believing that the material was prone to rutting. However, a Maryland State Highway Administration (MDSHA) project proves the fallacy of that belief.

In 1994, MDSHA invited the asphalt industry to repave an intersection on U.S. Route 40. The project was unique because MDSHA told the designers that they could use any available technology and could ignore current state specifications. The only requirement of the project was that the work be accomplished within a specific budget.

The Maryland Asphalt Association formed a task force that included its members and those of the Lanham, Md.-based National Asphalt Pavement Association and the Lexington, Ky.-based Asphalt Institute. The task force first performed a forensic analysis of the roadway, including pavement coring to determine the depth of rutting in the pavement structure.

As part of the analysis, the team cut a trench across the width of the slow lane, revealing eight inches of HMA over the top of an existing portland cement concrete roadway. The analysis showed that the intersection had rutted at a rate of about one-and-a-half inches per year. Rutting was evident in the top six inches of HMA pavement.

The team then took four-inch diameter cores from various areas of the roadway to confirm that the depth was consistent. Two 10-inch diameter cores showed the various mixes that Maryland had used in its previous pavement rehabilitation projects. The 10-inch cores showed that, even in coarse mixes with large aggregate, there was very little stone-to-stone contact. (The team believed that stone-to-stone contact was necessary to withstand the normally heavy truck traffic at the intersection.)

Then designers chose to test the Superpave Mix Design System, a process that helps designers determine appropriate aggregates and asphalt binders, as well as depths needed and other important factors in using Superpave hot mixes. Using the Hamburg Wheel Tracking Device, which runs a steel wheel back and forth over a sample 20,000 times, the team tested the two larger cores for rutting performance. Both failed at fewer than 4,000 passes. The Superpave mix rutted no more than four millimeters at 20,000 passes under identical testing methods.

Because of that and the fact that they achieved greater stone-on-stone contact the Superpave mixes were chosen for the project, which involved replacement of all eight inches of the existing HMA pavement. The choice allowed the team to specify an asphalt binder that would meet both the climatic and traffic conditions of the intersection.

The removal and replacement project took 22 hours and cost $36 per square yard. The project was completed last July, giving MDSHA an opportunity to judge the merits of the HMA mix. In five-and-a-half years, the pavement had rutted less than 1/16 of an inch and was still considered to be in excellent condition. The mix worked so well that MDSHA used it to repave an adjacent intersection.