Abstract Manager

Highway agencies across Canada and elsewhere have developed their own specifications for asphalt concrete (AC) mixes and their placement to meet certain characteristics. The specified AC mix types can also vary within a jurisdiction depending on the past practices, new developments, local weather, available materials, traffic loads and construction project scope and/or schedule. However, there is a lack of systematic understanding of the impact of selecting an AC mix type, the specified mix properties and construction requirements on flexible pavement performance under prevailing traffic loads and climatic exposures.

The AASHTOWare Pavement ME Design (PMED) software is a comprehensive tool for the design and analysis of pavement structures incorporating various design input parameters including different properties of AC mixes. The TAC ME Design Subcommittee has been evaluating the Pavement ME Design (PMED) software since 2007. In September 2024, design trials were completed to investigate the impact of different AC mix types on the predicted distresses in new flexible pavements. They included four types of AC mixes for the surface and three types of AC mixes for the bottom layers. Each AC mix had its distinct design input values based on Manitoba (Bit. B, SP12.5, SP19.0) and Ontario (SP12.5, SMA12.5, SP19.0 and SP25.0) specifications. The design trial matrix consisted of 12 combinations of AC surface and bottom layers, which were run for 11 climate stations across Canada. These resulted in a total of 132 design runs for this study. All other inputs remained unchanged.

The preliminary analyses of results from this study indicated that, in general, the predicted IRI, rutting, bottom-up fatigue cracking and top-down fatigue cracking are sensitive to AC mix density, asphalt binder contents and voids in the mineral aggregate (VMA). Changes in asphalt binder content in AC bottom layer had no or negligible impact on the predicted distresses. An increased asphalt binder content and VMA lead to an increase of IRI (only in Montreal), AC layer rutting, total rutting and bottom-up fatigue cracking. A lower AC mix density results in reduction of bottom-up and top-down fatigue cracking only for certain AC mixes included in this study.

The objective of this paper is to present the above specified trial results including detailed analyses and findings. The presented information may help agencies and interested individuals in assessing the suitability of the PMED software and the impact of selecting an AC mix type or specifying an AC mix characteristic on pavement performance.