All About Asphalt

Paving Design

*Information on this page obtained from

Design Considerations

All your asphalt paving projects have some things in common:

  1. Water is the enemy.  The three most important considerations for designing the pavement structure are drainage, drainage and (you guessed it) drainage.  Water saturation can erode the base structure and weaken the bond between the aggregates and asphalt binder.  Most potholes form during the spring when the ground and road base are saturated and weakened with snow melt and springtime rains.  Those April showers will bring potholes if the water can’t get away quickly.
  2. Over time, asphalt exposed to sunlight, heat and air oxidizes and becomes more brittle and loses its elasticity.  A little preventive maintenance (see Maintenance) at regular intervals can greatly prolong the life of your asphalt paving project.
  3. If you bend a coat hanger back and forth and back and forth, it will bend several times until the metal fatigues and finally breaks.  The same is true for asphalt pavements.  Fatigue cracks have a tell-tale pattern that looks like alligator skin.  Having a thick enough, strong enough base under your asphalt paving project, prevents the pavement from flexing so far each time.  This lets you extend the time before your pavement fatigues and cracks.  Having too thin pavements over a too soft base (not enough structure) is an all too common problem on places like parking lots and driveways.
  4. Garbage in, garbage out.  If your asphalt paving project doesn’t use quality materials, it may look good right after paving but not so much later on.  (See Materials section for tips on selecting quality materials.)

Hint:  Whatever the application, your asphalt paving project should be designed with good drainage, enough structure to meet the project’s needs and the right mix design.

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.

Perpetual Pavements

If you want your asphalt paving project to last forever, perpetual pavements are for you.

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

Structural Overlays

Rural County Overlay

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.


Road Rehabilitation

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

Contact us now for a free estimate and quick reliable service. A qualified estimator will meet with you personally to evaluate your asphalt paving needs and promptly provide a free written estimate. Alaniz Construction also excels at developing pavement maintenance plans for your long term needs.

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