The North Carolina Department of Transportation (NCDOT) uses the 1993 AASHTO Guide for Design of Pavement Structures to determine the minimum pavement stiffness that will ensure pavement longevity. For design, the contribution from any given layer is calculated by the product of that layer's thickness and a structural layer coefficient that captures the overall quality and structural benefit of the material. This research has investigated the value of these layer coefficients for North Carolina pavement design. The relevant literature on development of the structural layer coefficients during the AASHO test road project, NCDOT design practices, and the development and identification of structural layer coefficients in other states and agencies was first reviewed and summarized. Then, databases containing pavement performance and material properties were compiled using existing data sources. Next, the structural layer coefficients for paving materials in the pavement performance database were calibrated by using the measured performance data and Excel Solver. These calibrated coefficients were then compared against projected performance using more modern materials.
It was found that the optimal layer coefficients for asphalt concrete base mixtures was 0.2 and that the ABC layer coefficient was 0.14 regardless of traffic levels. For the surface and intermediate mixtures on non-interstate roadways with 20-year cumulative ESALs less than 3 million, the optimal layer coefficient was found to be 0.24. For interstates and roadways with 20-year cumulative ESALs greater than or equal to 3 million, the optimal layer coefficient for surface and intermediate mixtures was found to be 0.43. With respect to current materials, the performance simulations showed inconsistent results between rutting and fatigue performance and thus no systematic changes were identified. The research team has made the following main recommendations regarding layer coefficients in North Carolina.
- The calibrated layer coefficients for non-interstate pavements designed using a 20-year cumulative ESAL less than 3 million, should be changed from 0.44 to 0.24 for asphalt concrete surface and intermediate mixtures and from 0.3 to 0.2 for asphalt concrete base mixtures. It is recommended to retain the current value of 0.14 for the ABC layer.
- The calibrated layer coefficients for interstate pavements at all traffic levels and all other roadways designed using a 20-year design ESAL equal to or greater than 3 million, should be kept at 0.44 for asphalt concrete surface and intermediate mixtures and changed from 0.3 to 0.2 for asphalt concrete base mixtures. It is recommended to retain the current value of 0.14 for the ABC layer.
Other recommendations are made as part of this research project as well as a guideline for monitoring and cataloging the long-term performance of North Carolina roadways are also provided.
