Abstract Manager

The AASHTO 1993 is a commonly used flexible pavement design method across North America and elsewhere. A design structural number (SN) obtained from this method is converted into the thickness of different layer materials using their structural layer coefficient (a_i) values, which depend on the properties of respective materials. For example, an asphalt concrete (AC) mix with 10% air voids (AV) will exhibit a lower elastic modulus and thereby, a lower a_i value than a mix with 7% AV. This will affect the required thickness of a pavement structure on a reasonable manner. The AASHTOWare Pavement ME Design (PMED) software is the newest and most sophisticated design tool for the design and analysis of various pavements. However, highway agencies are facing challenges to implement it because of concerns related to some models and inconsistencies in predicted distresses including significant variations among software versions.         

TAC ME Design Subcommittee is now running design trials, using the latest version (v3.0) of the PMED software, to assess the predicted distresses in flexible pavements for various design inputs. In November 2023 - February 2024, several design trials were completed to assess the effect of varying AC density and voids in mineral aggregates (VMA). These design trials included the following variable inputs: climate data from nine to ten weather stations across Canada; AC mat density in the range of 90 to 97% i.e., AV contents in the range of 3% to 10%; and VMA in the compacted AC mat in the range of 13 to 17% (asphalt content in the range of 4.5 to 5.7%).  

Preliminary analysis indicated that a lower density/compaction (increased AV content) in an AC mat results in increase of predicted IRI, total and AC layer rutting, bottom-up fatigue cracking (BUFC) and thermal cracking, and an inconsistent variation of top-down fatigue cracking (TDFC). An increased VMA results in reduction of predicted IRI,  BUFC, TDFC and thermal cracking, and increase of total and AC layer rutting. In general, the predicted distresses were shown to be very sensitive to changes in AC density (air voids) and VMA. The effect of climatic condition on predicted distresses was not shown to be as expected or experienced.         

The objective of this paper is to present the above stated trial results including detailed analyses and findings. The presented information may help agencies and interested individuals in assessing the suitability of the PMED software for flexible pavement design.