KERCHER 
 ENGINEERING, INC.

  “If a job is worth doing,  it is worth doing right!”

 
Superpave – Part 2

The Hot-Mix Asphalt Material for the 21st Century

Superpave system provides a new approach to specifying component materials, mixture design and analysis, and pavement performance prediction in order to consistently produce superior performing asphalt mixtures.   In the past, a one-size-fits-all approach was used for the majority of projects.  Today, the Superpave system provides highway agencies with the tools to optimize the mix design in order to meet the specific needs of each individual project.  This new system can be used for the mix design of new pavements or overlays used on rehabilitation projects, as well as, for low volume or high volume roadways.

The Superpave system was developed to address the distresses commonly found in asphalt (flexible) pavements that were discussed in Part 1 of this series of articles – rutting, low temperature cracking, fatigue cracking and raveling.  The following table provides a summary of how each distress is addressed by the Superpave system. 

* Improved materials are not a cure-all for fatigue cracking.  All pavements still require a proper pavement design that takes into account vehicle traffic loads (especially heavy trucks), soil strength, drainage, design life, etc.

The Superpave system has four components:

  1. Performance Graded (PG) Binders. Superpave provides a procedure for selecting the best asphalt binder (the glue that holds the mixture together) for the project’s specific requirements including the range of temperatures and level of traffic.
  2. High Quality Aggregates.  Recommended aggregate characteristics to enhance the long-term performance of HMA were developed.
  3. Laboratory Compaction. Superpave requires new performance-based testing procedures using the Superpave Gyratory Compactor to determine the optimum mix design.
  4. Performance Testing and Prediction Models.  Results from the testing procedures are used to predict long-term performance of the proposed mixture.

  PG BINDERS

Maybe the most significant improvement of the Superpave system is the introduction of performance graded asphalt cements, referred to as PG Binders.  Superpave provides a new system for selecting, testing and specifying asphalt binders. The asphalt cement binder is evaluated in the laboratory for the entire range of temperatures that the pavement is expected to be subjected to over the course of its design live, including extreme high and low temperatures.  It must possess sufficient physical properties to withstand the seven-day mean high temperature during the summer in order to resist rutting.  In addition, the binder must possess sufficient physical properties to withstand the one-day low temperature during the winter so as to resist low temperature cracking.

 
 

Physical Properties necessary for Performance





The PG Binder is simply specified by the pavement temperature extremes in which the binder should perform satisfactorily such as PG 70 –22 (pronounced “70 minus 22”), The “minus 22” is the minimum one-day temperature that the binder must withstand.

 
 





The “70” stands for the seven day average temperature (Celsius scale) that the binder must withstand.  The higher the grade, the stiffer the binder, the more rut resistant the mix should be.

The “minus 22” is the minimum one-day temperature that the binder must withstand. The lower the grade, the softer the binder, the more resistant to thermal cracking
 
 





Don’t worry about having to run around with a thermometer or call the local weather bureau.  The Delaware Department of Transportation has determined what PG Binder grades should be used for which situations.  This information will be provided in Part 3 of this series in the Travel Log. 

One question that may come up is why not specify one binder for all situations.  The answer is quite simple – Money.   The greater the difference between the high and low temperature, the greater the cost of the binder.




LABORATORY COMPACTION

Another innovation of Superpave is the gyratory compactor used to compact the sample mixtures in the laboratory.  This new compactor does a better job of simulating the compaction that actually occurs in the field compared to other methods previously used.  Additionally, it provides a continuous analysis of various mixtures by capturing data during the compaction process.  Data from the analysis provides the information necessary to design HMA that possess a strong aggregate skeleton to resist rutting and an optimum binder content to combat raveling and rutting.

 
 

  Compactor 





PERFORMANCE TESTING AND PREDICTION MODELS

One of the major developments that resulted from the Superpave research project was performance-based tests and performance prediction models.  Results from the analysis of laboratory testing data allows engineers to estimate the performance life of a particular mixture or the time it will take to reach a certain level of rutting, fatigue cracking or low temperature cracking. 

 

Superpave Pavement Performance Prediction





SUMMARY

As a result of these improvements, Superpave produces HMA that contains a high percentage of larger aggregate that is durable and angular, while minimizing the amount of fine aggregate in order to provide stone-on-stone contact.  This high degree of larger, high quality aggregate with stone-on-stone contact produces a strong, structural framework that is necessary to resist rutting. 

At the same time, Superpave’s PG binders are able to perform over the full range of expected temperatures, thus being able to resist low temperature cracking during the winter and remaining sufficiently stiff during extend warm periods in the summer.  These benefits combined with a mixture that contains the optimum binder content makes the Superpave system the HMA technology for the 21st Century.