"Fatigue cracking is one of the most influential distresses that govern the service life of asphalt concrete pavements. Fatigue cracks are due to repeated traffic loading and/or temperature cycling over extended periods that induce combinations of tensile and shear stresses in asphalt concrete layers. These stresses initiate microcracks and cause them to propagate, densify, and coalesce to form macrocracks. For many years, significant research efforts have focused on developing reliable fatigue prediction models. These models usually relate the initial response (such as tensile strain or dissipated energy) of asphalt mixture to the fatigue life. As a result, they cannot accurately account for complex damage evolution under realistic loading conditions (e.g., multi-level loading, changing rest periods, varying loading rates, etc.) that occur throughout the service life of pavement systems. These models are simple to use because the only response of the mixture that needs to be measured is at the initial stage of fatigue testing. However, with the advanced automatic control/data acquisition systems available today, this advantage of simplicity is not substantial.
Development of a fundamentally sound fatigue model serves two important purposes. For pavement engineers, this model can provide accurate information on fatigue performance of asphalt concrete under realistic loading conditions, leading to better assessment of fatigue life of a new pavement or the remaining life of an existing pavement. For materials engineers, the fatigue model founded on basic principles in mechanics provides relationships between material properties (chemical or mechanical) and model parameters, which can be used for selection or design of more fatigue-resistant binders or mixtures.
The principal objectives of the research were:
- to investigate the causes for early fatigue failure of WesTrack pavements using the viscoelastic continuum damage model,
- to evaluate the effects of mix variables (e.g., asphalt content, air voids content, aggregate gradation) and testing conditions (e.g., temperature and moisture) on fatigue performance of asphalt concrete using the viscoelastic continuum damage fatigue model,
- to verify or calibrate the fatigue performance prediction by the viscoelastic continuum damage fatigue model using actual performance data from the experimental pavement sections, and
- to evaluate the viscoelastic continuum damage fatigue models with different levels of simplification under varying mix and testing conditions to develop a simple test method for fatigue cracking.
This report presents the findings from direct tension and indirect tension tests. From the direct tension testing, a methodology was developed by which the material response under any uniaxial tensile testing condition (type of loading and temperature) can be predicted from the material response obtained from a single testing condition. The methodology makes use of a uniaxial constitutive model for asphalt concrete that is based upon the elastic-viscoelastic correspondence principle and work potential theory, a continuum damage theory based on thermodynamics of irreversible process. Uniaxial tensile testing is performed under controlled crosshead mode for both cyclic and constant rate to failure tests. Various strain amplitudes, frequencies, and rates are applied at several test temperatures. A single characteristic curve can be found that describes the reduction in material integrity as damage grows in the specimen regardless of the applied loading conditions (cyclic versus monotonic, amplitude/rate, frequency). The characteristic curve at any temperature can be found by utilizing the time-temperature superposition principle and the concept of reduced time. Eight WesTrack mixtures are tested and the methodology is used to successfully predict the fatigue damage at different testing conditions from a single condition. A test and analysis procedure for the fatigue characterization of asphalt mixtures based on this methodology is proposed and potential applications are discussed.
In this report, also presented are the viscoelastic characterization of asphalt concrete in indirect tensile testing and the development of a simple performance test for fatigue cracking. The analytical solutions to calculate creep compliance and center strain from displacements measured on the specimen surface were developed based upon the theory of viscoelasticity. These developments were verified by 3-D finite element viscoelastic analysis and tests. A simple performance test was developed based on these solutions and work potential theory. To evaluate its validity, the indirect tensile tests were performed on WesTrack asphalt mixtures varying aggregate gradations, asphalt contents, and air void contents. Fracture energy obtained from indirect tensile strength testing and creep testing was highly correlated with field performance of these mixtures at WesTrack. A combination of indirect tensile creep and strength testing was proposed as a simple performance test for fatigue cracking.
The research findings in this report may not be sufficiently complete to be implemented routinely by state highway agencies yet. However, they clearly demonstrate great potential as a cost-effective performance test/analysis procedure for the fatigue cracking evaluation of asphalt concrete. The findings from this research project have been reported at the 2002 TRB meeting and the 2002 AAPT meeting. At the AAPT meeting, the principal investigator of this project participated in the panel discussion and also was invited to make a presentation on the findings from this project at the Symposium on Physical Tests for Fatigue Cracking Evaluation of Asphalt Mixtures. The responses from these national meetings were very positive in that through this research project a solid theoretical/experimental foundation has now been established in fatigue cracking evaluation of asphalt mixtures. In order to maximize the advantage afforded by these findings, further research is necessary in which the developed procedures are tested with a wider range of mixture types and performance.
