University of California Pavement Research Center

In California, fatigue cracking is considered to be the most important type of distress affecting the performance of asphalt concrete pavements on major state highways. This report describes the results of a laboratory study of the fatigue response of a typical California asphalt concrete mix to define the effects of degree of compaction (as measured by air-void content), asphalt content, and aging on this performance parameter. The test results are then used in analytical simulations to estimate the effects of asphalt and air-void contents (with and without long-term-aging) on pavement performance. These simulations demonstrate that accurate construction control of air void content is more important than accurate control of asphalt content relative to the design target values. For example, a mixture targeted at 5-percent asphalt and 5-percent air voids will suffer a 30-percent reduction in fatigue life if the air-void content exceeds its target by 1-percent but only a 12- percent reduction if the asphalt content is shy of its target by 1 percent. Complicating this matter, however, is the likelihood that smaller-thanspecified asphalt contents will result in increased air-void contents. A mix design and analysis system for fatigue is presented; this system was initially developed as a part of the Strategic Highway Research Program (SHRP) Project A-003A. Its most attractive features include the ability to consider not only laboratory fatigue test results but also the anticipated environment and the ability to make risk assessments about design choices. Refinements to the original SHRP developed methodology are described, including improved procedures for computing pavement temperature profiles as well as calibrations which reflect uniquely California conditions. Analyses of "rich-bottom" pavements (pavements with larger asphalt content in the bottom lift) suggest added potential for improved pavement performance. Finally, a series of recommendations are presented for enhancing the fatigue performance of California pavements which include changes to current construction specifications and/or construction quality assurance procedures.