NCAT Reports on Maximum Pavement Thickness and Recalibration of Asphalt Layer Coefficient

Therefore, many agencies are continuing to follow the 1993 AASHTO Pavement Design Guide.  This approach used asphalt layer coefficients based on the original AASHTO Road Test from the late 1950s.  Since then, the asphalt materials used today are not the same.  Today’s asphalt has better materials control, more advanced designs and performance graded binders.  However, except for a few DOTs, they still use the default asphalt layer coefficient of 0.44.  This report outlines procedures to recalibrate the asphalt layer coefficients for today’s asphalt.  Like the state of Alabama that adopted a coefficient of 0.54, the overall pavement structure was reduced by almost 20 percent.  The reduction saved money and did not compromise pavement performance.

The second major report completed by NCAT focused on designing long-life flexible pavements.  Using work conducted at the NCAT Test Track, researchers have proposed a new approach to evaluating stresses and strains in a pavement.  Instead of a single value that is typically conservative and leads to over-designed pavement, a distribution of stresses and strains experienced by the pavement can be used to design an optimal long-life structure at a lower cost.  This report also proposes an approach to set maximum pavement thicknesses that will result in a long-life pavement.  Looking at subgrade soil strength, base strength and pavement loading, a maximum asphalt thickness is calculated.  When conventional design methods (MEPDG or 1993 AASHTO) result in thicknesses greater than the maximum, the pavement owners can save money and not compromise structural performance by using the maximum thickness. 

Check the NCAT website for these and other technical reports.