Pavement Design

Currently, to design a pavement, engineers assign each material a structural coefficient based on the strength of the material.  For example, a compacted aggregate base has a low structural coefficient, while an asphalt pavement layer has a much higher structural coefficient.  The structural coefficient is multiplied by the layer thickness to determine a structural number (SN).  These structural numbers are then added together to calculate the total structure of the pavement.  Projected traffic counts with percent trucks, climate and other factors are used to determine the optimal total structure.  The pavement engineer uses this information to design the type and numbers of layers in the pavement.  Some agencies use a similar method of calculation but slightly different number (California Bearing Ratio, CBR), however the concept is the same.

This diagram shows a typical pavement design.  The pavement structure is comprised of many layers.  The bottom most layer is the existing soil or sub-grade.  The next layer is an aggregate base layer.  This layer is sometimes stabilized with asphalt, cement or fly ash.  This is followed by one or more layers of asphalt pavement.  The bottom layer of the asphalt pavement is often called the base lift (course).  The asphalt paved surface course (also called the wearing course) is the top layer, and is responsible for the smooth ride and skid resistance.  The surface course may be made of multiple layers, depending upon the structural needs.  The uppermost asphalt layer is usually made with smaller size aggregate than the asphalt base course.  This allows for a tight and closed surface texture that keeps water and other weather elements out.

The NEW Mechanistic Empirical Design Procedure (MEPDG)

The National Cooperative Highway Research Program (NCHRP) has developed a new pavement design and analysis tool, The Mechanistic-Empirical Design Guide for New and Rehabilitated Pavement Structures.  The guide employs mechanistic-empirical approaches:  Empirical, based on the results of experiments or experience in real life performance and mechanistic, based on laboratory determination of pavement responses such as stress, strains and deflections due to loading (mathematical models).

These approaches provide a more realistic characterization of in-service pavements and provide uniform guidelines for designing the in-common features of asphalt, rigid, and composite pavements. By using these approaches, engineers can create more reliable pavement designs.

The new guide incorporates procedures for performing traffic analyses, includes options for calibrating to local conditions, and incorporates measures for design reliability. Engineers can use the guide to predict pavement performance in terms of pavement distress over time.  Such as fatigue, rutting, and thermal cracking in asphalt pavements.

This new design approach (MEPDG) is being implemented by each state as Departments of Transportation gather material performance and strength data to input into the design models.  Please contact your state Asphalt Paving Association for updated infomation regarding this implementation.