Endurance Limit of Hot Mix Asphalt Mixtures to Prevent Fatigue Cracking in Flexible Pavements
National Center for Asphalt Technology -- Auburn University
Dr. E. Ray Brown
The project final report is available as NCHRP Report 646.
Performance data from well-constructed flexible pavements with a thick HMA structure, some of which have been in service for more than 40 years, show that bottom-up fatigue cracking does not occur in these pavements. This field experience suggests that an endurance limit, that is, a level of strain below which fatigue damage does not occur for any number of load repetitions, is a valid concept for HMA mixtures; its quantification could aid in the efficient design of long-life flexible pavements with a significantly reduced life cycle cost.
The idea of an endurance limit is widely recognized in other areas of material science, especially in ferrous metals. The endurance limit is usually determined from the relationship of strain to load repetitions to failure and is defined as the strain corresponding to the asymptote of the locus of points representing the fatigue life of a number of test specimens. Defining an endurance limit for HMA mixtures will result in more efficient structural design of pavements for mixtures of different characteristics. For instance, it is well known that mixtures with slightly higher binder contents have longer fatigue lives, and this would presumably translate to a higher strain level for the endurance limit of these mixtures. Other factors such as modifier type, aggregate type and gradation, binder grade, and mix volumetric properties also need to be examined in this light.
The objectives of this study were to (1) test the hypothesis that there is an endurance limit in the fatigue behavior of HMA mixtures and measure its value for a representative range of HMA mixtures and (2) recommend a procedure to incorporate the effects of the endurance limit into mechanistic pavement design methods.