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.

Perpetual Pavements use multiple layers of asphalt surfaces.  The pavement design begins with a strong yet flexible asphalt base layer that resists the repeated bending of tensile strains caused by traffic, and thus stops cracks from forming in the bottom of the pavement.  A strong intermediate asphalt layer (binder course) completes the permanent structural portion, and the final layer (wearing course) is a rut-resistant asphalt surface that lasts many years before needing restoration.  When the wearing course starts to show distress or when it loses its skid resistance, the top layer is milled off, recycled and replaced with a new wearing course.  Perpetual pavements can be maintained easily and cost-effectively without removing the road structure for reconstruction, saving time and money.  A perpetual pavement provides a sustainable, durable, safe, smooth, long-lasting roadway without expensive, time-consuming, traffic-disrupting reconstruction or major repair.

Additional Resources

• Perpetual Pavement Synthesis
• Asphalt Pavement That Lasts

Over time repeated traffic loading can weaken (fatigue) the pavement structure, and growing traffic counts require higher structural strength.  When more strength is needed, it’s time for a structural overlay, that is, one or more layers of new asphalt surfacing.  The existing road should be in good shape, and any distresses should be fixed before the overlay is done.  A good tack coat (a thin layer of asphalt applied to the old surface) is essential in bonding the old and new layers.  Testing has demonstrated that firmly tacked layers improve overall pavement strength and provide better performance than untacked layers.

When the ride becomes really rough, when there are severely deteriorated cracks, moderate alligator  cracking (fatigue), potholes or patches everywhere, or when there are ruts, it’s time to rehabilitate the pavement.

The most common practice is to use a milling machine to mill off the top, distressed layers. Then, any base or drainage problems are corrected.  Finally, a layer of new asphalt pavement is applied to fill the area that was milled off.  This process is referred to as “mill and fill.”  The millings or reclaimed asphalt pavement (RAP) may be taken back to a asphalt plant to be re-used in new asphalt.  They can also be recycled in place by adding new liquid asphalt and correcting any deficiencies in the mix (CIR).

If the distresses are through the whole pavement structure, or if the base has been damaged, then it’s time for full depth reclamation (FDR).  If new pavement layers are put over a poor base, they will fail prematurely.  All old pavement should be removed, the drainage and base repaired, and new asphalt pavement applied.

Additional Resources

• Flexible Pavement Rehabilitation Manual