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22 The concept of a perpetual pavement requires the knowl- edge of the HMA endurance limit. The main purpose of this study was to validate the endurance limit for HMA using laboratory beam and uniaxial fatigue tests with rest periods between loading cycles and develop the mathematical algo- rithm to insert this endurance limit into a predictive fatigue damage methodology. A comprehensive study was performed to estimate the endurance limit of a wide range of conven- tional HMA due to healing that occurs during the rest periods. Extensive laboratory displacement-controlled beam and uni- axial tension-compression fatigue tests were performed. Hot mix asphalt was used with three binder grades, two binder contents, two levels of air voids, three test temperatures, four values of rest periods between loading cycles, and three levels of applied strain. A fractional factorial statistical design was used in order to minimize the number of tests and still obtain the necessary results. The study used the principle that the endurance limit occurs due to healing of the asphalt mix that happens during the rest period between loading cycles or stress pulses due to moving traffic. Two models were developed from the beam and uniaxial fatigue tests that can predict the SR or the PSR at various test conditions which can be related to the healing gained during the rest period. The strain that allows for com- plete healing was obtained to estimate the endurance limit below which a very large number of load repetitions (tech- nically an infinite number) can be applied to the pavement without accumulation of fatigue damage. After developing the models, a concept of integrating the endurance limit in the traditional strain-Nf fatigue relationships was discussed as a step toward incorporating the endurance limit in the AASHTOWare Pavement ME Design. The following are the key findings of this study. 1. HMA exhibits an endurance limit that varies with mix- ture properties, temperature, and pavement design condi- tions. There is no single value of the endurance limit for all conditions. The endurance limit varies depending on binder grade, binder content, air voids, temperature, and the rest period between load applications. 2. Mixtures using softer binders exhibit higher endurance limits than mixtures using stiffer binders. 3. High binder contents and low air voids produced high endurance limit values compared to low binder contents and high air voids, which showed low endurance limits. 4. Endurance limit values were higher at high temperatures, which correspond to soft mixtures compared to low tem- peratures that correspond to stiff mixtures. 5. HMA stiffness (modulus) was found to be an excellent surrogate property that takes into account all of the pri- mary mix variables: binder grade, binder content, air voids, and temperature. This concept, however, needs to be used carefully since air voids and binder content can counteract each other and create the same stiffness but may have different endurance limits. However, it should also be recalled that the classical AC fatigue model used in the AASHTO ME Design also additionally adjusts fatigue life through the mix air void and effective bitu- men contents. 6. For a loading period of 0.1s the rest period that ensures complete healing ranges from 5 to 10s for the beam fatigue results. The uniaxial fatigue results showed a threshold value of 3s. 7. The models developed in this project can accurately estimate the endurance limit without testing beyond 20,000 cycles if the rest period is adequately long (3 to 10s). The number of loading cycles has little effect on the endurance limit for tests with long rest periods since damage will be healed by the end of each loading cycle. 8. The endurance limit values from the beam fatigue test yielded differing strain magnitudes compared to those of the uniaxial fatigue test. The predicted endurance limit values based on the beam fatigue model with three bind- ers ranged from 22 microstrain to 223 microstrain. The C H A P T E R 5 Summary and Findings
23 endurance limit values based on the uniaxial fatigue model with one binder ranged from 1 microstrain to 82 microstrain. It would be desirable to determine if expanding the uniaxial endurance limit study to the same three binders would result in a more favorable compari- son of the endurance limit values of the two tests. 9. The beam fatigue model (Equation 3) is proposed for future endurance limit studies. The beam fatigue test is more established with a larger database available in the literature than the uniaxial test. The model obtained from the uniaxial test study (Equation 9) was based on limited data. Moreover, the uniaxial test is more time consuming than the beam fatigue test and requires an enormous attention to detail. 10. The endurance limit model obtained in this study from the flexural beam fatigue approach can be incorporated in Pavement ME Design. This would support the future design of perpetual pavements that could accommo- date a large number of truck loads without accumulated fatigue damage. Such designs would require close con- trol of layer thicknesses and material properties so that the strain does not exceed the endurance limit for the expected load spacing (rest period). Suggested Future Research This research developed an integrated model to predict heal- ing and endurance limits for conventional HMA mixtures. To gain a better understanding of the endurance limit for the full range of asphalt mixtures, suggested future research includes: ⢠Field verification of the model for a wide variety of pave- ment cross sections and different combinations of HMA stiffness, rest period, and number of load applications. Ver- ification can best be achieved by using existing databases, such as the LTPP, or using accelerated loading facilities to control critical design properties. ⢠Coding of the model for its incorporation in the AASHTO- Ware Pavement ME Design software. ⢠Investigation of the healing-based endurance limit method for other types of mixes such as recycled asphalt pavement, recycled asphalt shingles, warm mix asphalt, asphalt rub- ber, and mixtures prepared with polymer modified or highly chemically modified asphalt binders. ⢠Investigation of the contribution of the different compo- nents of the mix such as binder, mastic, and aggregate to the endurance limit.
