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91 Conclusions The following conclusions may be drawn based on the data presented to date: ⢠A practical definition of the endurance limit or long-life pavement would be a pavement able to withstand 500 mil- lion design load repetitions in a 40-year period. ⢠Several techniques can be used to evaluate beam fatigue data near the endurance limit. These include logarithmic extrap- olation of the loading cycle versus stiffness curve, single- or three-stage Weibull model using the stiffness ratio, and ratio of dissipated energy change (RDEC). The single-stage Weibull model produced fairly accurate extrapolations that appear to be conservative. Extrapolations performed using the single-stage Weibull model resulted in the lowest vari- ability in the mini round-robin. Therefore, the single-stage Weibull model is recommended for extrapolating low strain fatigue tests to confirm the existence of the endurance limit. In certain cases, the three-stage Weibull model may pro- vide a better fit to the experimental data. Procedures for both methods are supplied in Appendix A. ⢠The data support the existence of an endurance limit for each of the six mixes tested. The 95% lower prediction limit varied from 75 to 200 ms. ⢠All of the estimated endurance limits were above 70 ms. An analysis of LTPP data indicated an endurance limit of 65 ms. ⢠On a log basis for normal strain fatigue tests, the repeata- bility (within-lab) standard deviation was determined to be 0.248 and the reproducibility (between-lab) standard devi- ation was determined to be 0.318. This results in within- and between-lab coefficients of variation of 5.4% and 6.8%, respectively. ⢠Uniaxial tension testing provides a promising technique. Results from the uniaxial tension test can be determined more quickly than beam fatigue tests, but the data are more complicated to analyze. There are difficulties controlling the strain that is actually applied to the sample being tested. Additional evaluation is needed to reconcile the difference between beam and uniaxial fatigue results, which produced trends for neat versus polymer modified binders that were the opposite of those determined with beam fatigue testing. ⢠Field observations, from the data discussed in Chapter 6 and from the NCAT Test Track, support the importance of good construction in addition to pavement thickness design and materials selection. ⢠Shift factors between laboratory and field performance, based on fatigue transfer functions developed as part of this research, ranged from 4.2 to 75.8. ⢠Pavement thicknesses for a perpetual design determined using the endurance limits measured as part of this study were consistent with thicknesses observed in previous studies of in-service pavements. ⢠The MEPDG and PerRoad perpetual design methodologies are sensitive to changes in the endurance limit. ⢠Considering a typical principal arterial traffic stream, the thickness of a perpetual pavement designed using PerRoad was similar to that determined using the 1993 AASHTO Pavement Design Guide or MEPDG (without the endur- ance limit) for a 20- or 40-year design life. The thickness of a perpetual pavement designed with the MEPDG was approximately 50% thicker. The predicted condition of the pavement at the end of 20 to 40 years was signifi- cantly different, with no cracking expected in the perpet- ual pavements versus over 20% of lane area cracking at 90% reliability (based on the MEPDG). Damage would also be expected based on a change in serviceability index of 1.2 for the 1993 AASHTO design procedure. Recommendations Recommendations from this study address the following five areas: (1) investigation of the endurance limit as a mixture property, (2) additional research and development to further the development of the uniaxial tension test, (3) field testing to investigate cracking observed in thicker LTPP sections that C H A P T E R 8 Conclusions and Recommendations
appear to refute the existence of the endurance limit, (4) in- corporation of the endurance limit into pavement design, and (5) cataloging endurance limit values. Only a single gradation and two aggregate sources blended in a single mix were tested in this study. Further, only a sin- gle form of binder modification was evaluated. Table 2.1 presented the affect of a range of factors on fatigue life (1). Of these, binder stiffness and air void content were expected to have a larger affect than aggregate type and gradation. Differences were observed in the predicted endurance limit based on binder stiffness. The affect of binder content and in-place air voids was mixed. Few affects were observed for the PG 67-22, but a more pronounced affect was observed for the polymer modified PG 76-22. Additional evaluations should be conducted with a wider range of mixtures, binder types, and modified binders. The samples tested in this study were short-term samples, oven aged for four hours at 275°F (135°C) according to AASHTO R30. No long-term aging was evaluated. Nunn (10) indicates that the stiffness of thick pavements increases with time. This should reduce the strain at the bottom of the as- phalt layer. However, the endurance limit or strain âcapacityâ of the mix may decrease with increased oxidative aging and physical hardening. The affect of long-term aging on the en- durance limit should be investigated. Samples for the uniaxial tension test can be prepared on a gyratory compactor. Uniaxial tension testing is less time- consuming than beam fatigue testing. Therefore, this test method deserves additional development. Basic research should be conducted to better understand the stress states in uniaxial tension samples. Better techniques should be devel- oped to control the strain experienced by the test sample. Forensic investigations should be conducted on a stratified random sample of the thickest GPS-1, GPS-2, and SPS-1 pave- ments in the LTPP database that exhibit cracking, to deter- mine the cause of that cracking. For pavements designed using equivalent annual or equiv- alent seasonal temperatures, the use of a single value for the endurance limit appears to be reasonable. However, field data presented from the NCAT Test Track indicate that pavements can withstand a cumulative distribution of strains (which includes strain levels that exceed the mixturesâ endurance limit, determined at a single temperature, as described in this study) and still exhibit perpetual behavior (77). Further, there is evi- dence that the endurance limit, determined from beam fatigue tests, varies as a function of temperature. Thus, future efforts to incorporate the endurance limit into the MEPDG should consider a distribution of acceptable strains or endurance lim- its that vary as a function of temperature. Agencies should use Appendix A, Proposed AASHTO Practice for Beam Fatigue Testing, to determine catalogs of endurance limit values for typical mixes, binder grades, and binder sources. Only a single aggregate source and gradation were tested in this study. Therefore, it is difficult to assess the affect of those parameters on the endurance limit. Based on the literature review, these seem to be secondary factors. The endurance limit catalog should concentrate on mixes used at the bottom of the HMA layer where cracking initi- ates. An experimental plan should include the binder grades and types of modifiers the agency typically uses in the bottom- lift, laboratory compaction efforts (e.g., Ndesign) if historical experience indicates these different levels result in different optimum asphalt content, and major aggregate types. If the agency is considering the use of a rich-bottom layer, with or without polymer modified binder or a high-modulus base, they should also be included. Laboratory samples should be compacted to a density representative of the level typically achieved in the field. 92
