Abstract Manager

A large portion of Canada’s highway network is low volume roads with thin pavement structures and surface layers. Currently, no generally accepted design method available for these roads. As such, the pavement design methods or the selected thickness of surface and base layers vary widely among the jurisdictions. The AASHTO 1993 pavement design guide recommends a minimum of 25.4 mm (1 in.), 50.8 mm (2 in.) and 63.5 mm (2.5 in) asphalt concrete (AC) surface layer for a design ESALs of ≤50,000; 50,001-150,000; and 150,001 to 500,000, respectively. This paper focuses on the outcomes of the thin asphalt pavement design analysis using the AASHTOWare Pavement ME Design (PMED) software (v2.6)

The TAC ME Design Subcommittee has been evaluating the PMED software since 2007-2008. In February - March 2022, design trials were completed to assess the suitability of the PMED software for thin (<100 mm AC surfaced) flexible pavement design. These design trials included: i) climate data from 14 weather stations across Canada; ii) two untreated native subgrade (high plastic clay and silty sand) soils with resilient moduli values of 35 and 60 MPa, respectively; iii) 100 mm granular base with 100 mm subbase and 150 mm granular base with no subbase; and iv) varying AC thickness from 50 mm to 150 mm (in 5 to 30 mm increments).  

The trial results have shown that the predicted IRI and total rutting generally decrease with an increase in AC layer thickness. However, uniform trends of the predicted IRI and rutting were observed for an AC layer thickness of ≥105 mm and ≥80-100 mm (depending on the subgrade stiffness), respectively. An increase or inconsistent variation in the predicted bottom-up fatigue cracking and AC layer rutting were observed for an increase in AC layer thickness within the thin AC thickness range. Uniform trends for the predicted bottom-up fatigue cracking and AC layer rutting were observed when the thickness of AC layers were ≥100mm and ≥100-105 mm, respectively. There was no or negligible effect of AC layer thickness on the top-down fatigue cracking. Overall, these trial results indicated that the minimum thickness of AC layer should be ~100 mm (4 inches) for using the PMED software in pavement design and analysis.

The objective of this paper is to present the above stated trial results, analysis and findings. The presented information may help agencies and interested individuals in assessing the suitability of the PMED software when designing thin asphalt pavements for low volume roads.