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OCR for page 91
91 CHAPTER 8 Conclusions and Recommendations Conclusions Additional evaluation is needed to reconcile the difference between beam and uniaxial fatigue results, which produced The following conclusions may be drawn based on the data trends for neat versus polymer modified binders that were presented to date: the opposite of those determined with beam fatigue testing. Field observations, from the data discussed in Chapter 6 A practical definition of the endurance limit or long-life and from the NCAT Test Track, support the importance pavement would be a pavement able to withstand 500 mil- of good construction in addition to pavement thickness lion design load repetitions in a 40-year period. design and materials selection. Several techniques can be used to evaluate beam fatigue data Shift factors between laboratory and field performance, near the endurance limit. These include logarithmic extrap- based on fatigue transfer functions developed as part of this olation of the loading cycle versus stiffness curve, single- research, ranged from 4.2 to 75.8. or three-stage Weibull model using the stiffness ratio, and Pavement thicknesses for a perpetual design determined ratio of dissipated energy change (RDEC). The single-stage using the endurance limits measured as part of this study Weibull model produced fairly accurate extrapolations that were consistent with thicknesses observed in previous appear to be conservative. Extrapolations performed using studies of in-service pavements. the single-stage Weibull model resulted in the lowest vari- The MEPDG and PerRoad perpetual design methodologies ability in the mini round-robin. Therefore, the single-stage are sensitive to changes in the endurance limit. Weibull model is recommended for extrapolating low strain Considering a typical principal arterial traffic stream, the fatigue tests to confirm the existence of the endurance limit. thickness of a perpetual pavement designed using PerRoad In certain cases, the three-stage Weibull model may pro- was similar to that determined using the 1993 AASHTO vide a better fit to the experimental data. Procedures for Pavement Design Guide or MEPDG (without the endur- both methods are supplied in Appendix A. ance limit) for a 20- or 40-year design life. The thickness The data support the existence of an endurance limit for of a perpetual pavement designed with the MEPDG was each of the six mixes tested. The 95% lower prediction limit approximately 50% thicker. The predicted condition of varied from 75 to 200 ms. the pavement at the end of 20 to 40 years was signifi- All of the estimated endurance limits were above 70 ms. An cantly different, with no cracking expected in the perpet- analysis of LTPP data indicated an endurance limit of 65 ms. ual pavements versus over 20% of lane area cracking at On a log basis for normal strain fatigue tests, the repeata- 90% reliability (based on the MEPDG). Damage would bility (within-lab) standard deviation was determined to be also be expected based on a change in serviceability index 0.248 and the reproducibility (between-lab) standard devi- of 1.2 for the 1993 AASHTO design procedure. ation was determined to be 0.318. This results in within- and between-lab coefficients of variation of 5.4% and 6.8%, Recommendations respectively. Uniaxial tension testing provides a promising technique. Recommendations from this study address the following Results from the uniaxial tension test can be determined five areas: (1) investigation of the endurance limit as a mixture more quickly than beam fatigue tests, but the data are more property, (2) additional research and development to further complicated to analyze. There are difficulties controlling the development of the uniaxial tension test, (3) field testing the strain that is actually applied to the sample being tested. to investigate cracking observed in thicker LTPP sections that

OCR for page 91
92 appear to refute the existence of the endurance limit, (4) in- ments in the LTPP database that exhibit cracking, to deter- corporation of the endurance limit into pavement design, mine the cause of that cracking. and (5) cataloging endurance limit values. For pavements designed using equivalent annual or equiv- Only a single gradation and two aggregate sources blended alent seasonal temperatures, the use of a single value for the in a single mix were tested in this study. Further, only a sin- endurance limit appears to be reasonable. However, field data gle form of binder modification was evaluated. Table 2.1 presented from the NCAT Test Track indicate that pavements presented the affect of a range of factors on fatigue life (1). can withstand a cumulative distribution of strains (which Of these, binder stiffness and air void content were expected includes strain levels that exceed the mixtures' endurance limit, to have a larger affect than aggregate type and gradation. determined at a single temperature, as described in this study) Differences were observed in the predicted endurance limit and still exhibit perpetual behavior (77). Further, there is evi- based on binder stiffness. The affect of binder content and dence that the endurance limit, determined from beam fatigue in-place air voids was mixed. Few affects were observed for tests, varies as a function of temperature. Thus, future efforts the PG 67-22, but a more pronounced affect was observed for to incorporate the endurance limit into the MEPDG should the polymer modified PG 76-22. Additional evaluations should consider a distribution of acceptable strains or endurance lim- be conducted with a wider range of mixtures, binder types, its that vary as a function of temperature. and modified binders. Agencies should use Appendix A, Proposed AASHTO The samples tested in this study were short-term samples, Practice for Beam Fatigue Testing, to determine catalogs of oven aged for four hours at 275F (135C) according to endurance limit values for typical mixes, binder grades, and AASHTO R30. No long-term aging was evaluated. Nunn (10) binder sources. Only a single aggregate source and gradation indicates that the stiffness of thick pavements increases with were tested in this study. Therefore, it is difficult to assess time. This should reduce the strain at the bottom of the as- the affect of those parameters on the endurance limit. Based phalt layer. However, the endurance limit or strain "capacity" on the literature review, these seem to be secondary factors. of the mix may decrease with increased oxidative aging and The endurance limit catalog should concentrate on mixes physical hardening. The affect of long-term aging on the en- used at the bottom of the HMA layer where cracking initi- durance limit should be investigated. ates. An experimental plan should include the binder grades Samples for the uniaxial tension test can be prepared on and types of modifiers the agency typically uses in the bottom- a gyratory compactor. Uniaxial tension testing is less time- lift, laboratory compaction efforts (e.g., Ndesign) if historical consuming than beam fatigue testing. Therefore, this test experience indicates these different levels result in different method deserves additional development. Basic research optimum asphalt content, and major aggregate types. If the should be conducted to better understand the stress states in agency is considering the use of a rich-bottom layer, with or uniaxial tension samples. Better techniques should be devel- without polymer modified binder or a high-modulus base, oped to control the strain experienced by the test sample. they should also be included. Laboratory samples should be Forensic investigations should be conducted on a stratified compacted to a density representative of the level typically random sample of the thickest GPS-1, GPS-2, and SPS-1 pave- achieved in the field.