Traffic Densification of Asphalt Concrete Pavements

Abstract

A 5-year study of newly constructed pavements showed that a reduction in in situ air voids occurred both within and between wheelpaths for highways with an average daily traffic (ADT) load of less than 10,000 vehicles. Regardless of the level of voids immediately after construction, mixtures in the upper 65 mm (2.5 in.) within the wheelpath indicated a reduction in voids by 3 to 5 percent (e.g., from 10 to 6 percent voids), and by between 2 to 4 percent between the wheelpaths. Because only limited densification occurred below this depth for lower–traffic-volume facilities, reducing the mix design level of air voids from 4 percent to 2 percent for the lower lifts was suggested so that lower initial voids could be obtained during construction. An evaluation of older pavements indicated that moisture damage to the lower pavement layers was typical; thus, a change in mix design procedures might also help improve durability by increasing the film thickness. Pavements with high traffic volumes (>50,000 ADT) consistently indicated an increase in voids over time in the upper lift [40 mm (1.5 in.)], little change in the middle 65 mm (2.5 in.), and a decrease in the bottom 65 mm (2.5 in.). The hypothesis suggested to explain these findings was that a loss of material in the upper lifts was occurring, most probably due to moisture damage as the upper, more permeable wear course, commonly used in Minnesota, allowed water trapping at the wear and binder course (i.e., less permeable) interface. A further investigation of in situ void changes on an interstate indicated that for a pavement constructed with the same fine gradation in all lifts, traffic compacted the mixtures in a manner similar to that in low-volume roads. When the initial in situ voids increased from around 7 percent to nearly 10 percent, the influence of traffic on the densification was substantially increased.