Effects of Asphalt Film Thickness on Field Performance


The overall consensus among asphalt materials engineers agrees sufficient aggregate coating promotes durability of asphalt mixtures. However, many disagree on which mix parameter best controls durability in design and production. Some suggest durability is ensured by specifying a minimum volume of voids in mineral aggregate (VMA), while some argue permeability provides better control. Yet, others claim a more direct but controversial approach of specifying the average thickness of coating is more rational. While specifications controlling durability should ideally be established on the merit of field performance, instead they are largely based on conclusions from laboratory investigations and accelerated testing. To date, no studies have conclusively quantified the effect of these parameters on long term durability in the field. Though this study did not explore permeability, analysis of 458 pavement sections over a 9 year period overwhelmingly confirm a direct relationship between durability and AFT while no such conclusion can be made for VMA. Sections with an AFT less than 8.0 microns had a 75% to 95% probability of severe surface raveling within 9 years. As AFT approached 9.0 microns, the probability decreased to less than 20%. Results also indicate AFT significantly affected the rate and severity of flushing, while VMA had a significant impact on rate only. AFT was also found to significantly affect resistance to severe rutting, while the effect of VMA was limited. Both fatigue resistance and fatigue severity was significantly influenced by AFT only. It is recommended that AFT be specified between 9 and 12 microns to ensure durability and prevent flushing. Agencies not using the Iowa AFT equation should increase the recommended range.

Meeting Name

Transportation Research Board 90th Annual Meeting (2011: Jan. 23-27, Washington, DC)


Civil, Architectural and Environmental Engineering

Keywords and Phrases

Aggregates; Asphalt mixtures; Durability; Mix design; Pavement design; Pavement performance

Document Type

Article - Conference proceedings

Document Version


File Type





© 2011 Transportation Research Board, All rights reserved.

Publication Date

27 Jan 2011