ASPHALT ANSWERS

Asphalt Article December 28, 2000
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Rutting of hot-mix asphalt (HMA) pavements at intersections is a problem. Rutting takes place at intersections because the HMA exhibits lower stiffness under slow moving or standing loads compared to fast moving loads. The problem is aggravated in hot weather because the stiffness of HMA is further decreased at high temperatures. Very hot catalytic convertors of the standing automobiles also aggravate the situation. In most instances it is found that even though there is significant rutting at the intersection, there is no significant rutting in the same pavement away from the intersection.

A study of rutting at intersections conducted by the writer indicated significantly lower air voids and voids in the mineral aggregate in the HMA pavement at the intersections compared to the HMA pavement away from the intersections. This resulted from extra compaction effort imparted by the slow or standing traffic. Because of the difference in the rate of loading and compactive effort at intersections, it seems logical that there is a need to specify and use a different type of HMA at intersections.

To rectify an existing rutted intersection, the HMA pavement should be milled off to a depth of 250 mm. If the rutting is originating from the subgrade, then the subgrade needs to be improved. However, recent studies have shown that rutting usually occurs in the asphalt pavement (in the top 100 mm).

The following mixes are recommended for use at intersections carrying heavy volumes of traffic. These recommendations are based on recent experience with European mixes (such as stone-matrix asphalt) and Superpave mixes in the U.S.:

  • 50 mm stone-matrix asphalt (SMA) wearing course;
  • 50 mm stone-matrix asphalt (SMA) binder course; and
  • 150 mm dense-graded Superpave mix base course.

Stone-matrix asphalt is recommended for the top 100 mm of pavement. SMA is a very strong rut-resistant mix that will not generally densify below design air voids. The base course should be designed as a dense-graded 25 mm nominal size mix with the Superpave gyratory compactor.

However, because there is no guidance given regarding compactive effort for mixes at intersections, it is recommended to use the compactive effort for the highest traffic category (No. 7) in the Superpave. For asphalt binder it is recommended that the selected Superpave PG grade binder be two grades stiffer than that based on design high temperature of the project location.

About the author: 
Kandhal is the Associate Director at the National Center for Asphalt Technology at Auburn University. You may write him in care of the editor.
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