Layer Coefficients for NHDOT Pavement Design

At present, New Hampshire Department of Transportation (NHDOT) employs American Association of State Highway and Transportation Officials (AASHTO) Empirical Pavement Design procedure for structural design of highways (new construction, reconstruction and major rehabilitations). The AASHTO procedures uses material specific coefficients (commonly called layer coefficients) to account for the structural capacity provide by each pavement layer. The current layer coefficients used by NHDOT are a combination of the original values proposed by AASHTO in 1960s and research conducted by Janoo in 1994 (CRREL Special Report 94-30). The asphalt mixtures in use today and vehicle loadings are substantially different from the ones characterized by AASHTO during the development of the design guide in 1960s. With current use of newer asphalt binder modification technologies, allowance for recycled materials (reclaimed asphalt pavement (RAP), ground tire rubber) , and newer manufacturing and construction techniques (such as, cold in-place recycling) there is an urgent need to reevaluate the layer coefficients for materials that are currently being used in construction of NHDOT pavements. Due to lack of reliable layer coefficient values, there is high potential for over-design of pavements that translate in substantially higher spending. This study proposes to to characterize the asphalt mixtures currently used by NHDOT for determination of the actual layer coefficient values for those materials