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1 SUMMARY Asphalt mixture fatigue response is a complex function of various parameters. As modulus increases, strain capacity decreases which decreases fatigue life. At the same time, increasing modulus will result in a decreasing phase angle which will increase the fatigue exponent, increasing fatigue life. Healing is probably a significant factor in asphalt mixture fatigue performance, with increasing phase angles correlating to better healing and improved fatigue life for non-polymer- modified binders; for polymer-modified binders, the relationship between rheology and healing is unclear. To further complicate matters, in real pavement systems softer binders and mixes will in general exhibit higher failure strains but will also tend to result in higher strains in situ, at least partly offsetting the beneficial effects of lower modulus values. The failure strain of asphalt binders tends to follow a well-defined failure envelope with respect to modulus; a power law model was used in NCHRP 9-59 to define this relationship. However, there is significant variation about this standard envelope among different binders. A fundamental question addressed in NCHRP 9-59 is what test or tests are good indicators of this inherent strain tolerance. Laboratory testing conducted during NCHRP 9-59 suggests that binder R-value is a good predictor of overall fatigue strain capacity. At a given modulus level, binders with lower R-values will exhibit higher failure strains than binders with high R-valuesâsuch as those that have been heavily oxidized. Although the SDENT test is also a reasonable indicator of inherent fatigue strain capacity, the correlation is not as good as for R-value. Considering the difficulty and cost of implementing a specification which includes the SDENT test, this procedure cannot be recommended based on research conducted as part of NCHRP 9-59. It should be noted that NCHRP 9-59 used heavily aged binders and mixture in its test program. There is a concern that the extended loose-mix aging used in NCHRP 9-59 might have resulted in an unrealistic degradation of the performance of some or all of the polymer-modified binders studied. It is therefore possible that if the materials had not been as thoroughly aged, the findings concerning polymer-modified binders might have been different. This is an important topic for further research. Although R-value is a good indicator of overall strain tolerance, modulus has an even stronger effect on failure strain and fatigue life. The Glover-Rowe Parameter (GRP) appears to relate well to binder failure strain, accounting for the effects of both modulus and R-value on failure strain. The current binder fatigue parameter, |G*| sin Î´, does not relate as well to binder failure strain, and in fact can allow binders with relatively poor strain tolerance to be used in pavement applications. The correlation between SDENT extension and binder failure strain is also not as strong as for GRP. Because the GRP has been used by numerous researchers and engineers over the past five years as an indicator of non-load associated cracking and fatigue cracking, and because validation testing suggests it correlates reasonably well to fatigue performance in the field, this parameter is being recommended as the best overall choice for a binder fatigue specification parameter to replace |G*| sin Î´. A very serious problemâprobably even more significant than that with |G*| sin Î´--is the ineffective way the current specification addresses fatigue test temperature. Ideally the binder
2 fatigue test temperature should be closely tied to average pavement temperature, but the current specification does not appear to do this well. This is in part because the test temperature is tied to both the low and high binder temperature grades, and unless an agency is careful in how it applies the specification the fatigue test temperature can be significantly higher than intended. Even when applied correctly (to base binder grades before adjustments for traffic and vehicle speed), the relationship between the current binder fatigue test temperature and average pavement temperature is not ideal. A different approach is suggested in this report, in which the fatigue test temperature is tied to the low PG grade, rather than the average of the low and high PG grades. Furthermore, several notes to the specification are suggested to make the intent of the specification clear. For example, in some areas it is common to use binders that are rated as much as two grades higher than the low PG grade given by LTPPBind. In such cases, the pavement will not only be prone to increased thermal cracking but will also be subject to significantly greater fatigue cracking unless the pavement thickness is increased. Including a note to this effect would help ensure that highway agencies are aware of this potential problem. The problem associated with binder fatigue test temperature and grade bumping does not occur AASHTO M 332 is used in place of AASHTP M 320, since M 332 does not involve change the higher binder test temperature for higher traffic levels. Replacing |G*| sin Î´ with GRP and using an improved protocol for specifying binder fatigue test temperature would solve many of the problems with the current binder fatigue specification. However, in addition to these changes an allowable range for R-value (or an equivalent parameter such as ÎTc) should be established. There are two reasons for this limitation. In thin pavements at low temperatures, binders with high R-values can result in very rapid accumulation of fatigue damage. This is likely a major reason for recently observed premature failure of pavements in Ontario and the Northern U.S. made with binders containing REOB, which tend to have high R-values. The second reason for this limitation is that in thick pavements, binders with low R-values can show relatively poor fatigue performance. By limiting both high and low R- values, both problems are addressed. Tentative allowable ranges for R-value are from 1.5 to 2.5 for binders aged with RTFOT and 20-hour PAV, and from 2.0 to 3.2 for binders aged with RTFOT followed by 40-hour PAV. The more critical limit for R-value is the upper one, as this is potentially associated with very rapid failure of thin pavements and severe distress under other high-strain applications. Additional information concerning the relationships among rheological parameters and asphalt binder performance-related properties is being collected and analyzed as part of NCHRP 9-60 and should provide additional insight into the most effective ways of incorporating R-value or related parameters into a revised specification. Data collected during NCHRP 9-60 should also allow initial estimates of the precision of R-value, ÎTc and related parameters, which is critical in establishing realistic specification limits. The current laboratory aging method for binder fatigue testing uses RTFOT conditioning followed by PAV aging for 20 hours. In NCHRP 9-59, the aging protocol used was more severeâRTFOT conditioning and 40 hours of PAV aging. For any binder fatigue test to be effective, it must include a laboratory aging method that mimics aging in the field with
3 reasonable accuracy. There has recently been much activity to address this problem, including NCHRP 9-61 which directly addresses laboratory aging of binders. Because of the likelihood of future changes in the binder aging protocol, and because of the importance of addressing problems in the binder fatigue specification as quickly as possible, initial implementation of the findings of NCHRP 9-59 should be done with the current aging protocol. Once NCHRP 9-61 is completed and its findings reviewed, the binder fatigue specification can be revisited and any changes needed to address the aging protocol made at that time. A final critical issue is what specific value the chosen specification parameter should have. The current binder fatigue parameter, |G*| sin Î´, has a maximum value of 5,000 kPa. A simple approach and one that would be easy to implement would be to use an equivalent limit on the new specification parameter. For GRP, this equivalent value is estimated to be 5,300 kPa; for practical purposes, it could probably be rounded down to 5,000 kPaâidentical to the current maximum for |G*| sin Î´. Using this equivalent value assumes that the aging protocol would initially be unchanged (RTFOT followed by 20-hour PAV). Using this equivalent GRP value and similar but more effective test temperatures would ensure that the new specification is not overly restrictive or too lax. Furthermore, the analyses presented above suggest that the primary problems with the current binder fatigue specification donât involve the specific specification value, but instead are the result of the weak relationship between |G*| sin Î´ and strain capacity, poorly defined and implemented test temperatures and an aging protocol that may not accurately reflect in situ aging. Maintaining a similarity between the current specification and the proposed binder fatigue specification should allow rapid implementation of the changes addressing two of the problems with the current binder fatigue specification. The third issueâbinder laboratory agingâis being addressed in NCHRP 9-61. Based upon the research conducted as part of NCHRP 9-59, the following conclusions concerning the relationship between asphalt mixture fatigue performance and potential binder fatigue specification parameters can be made: 1. Asphalt binder fatigue performance increases with increasing binder failure strain and increasing fatigue exponent. 2. Modulus is the most important factor affecting binder failure strain; as modulus increases, failure strain decreases dramatically. 3. The fatigue exponent for an asphalt mixture is inversely related to the binder phase angle. 4. A binderâs failure strain during fatigue loading at a given temperature and loading rate can be experimentally measured and is called the fatigue strain capacity (FSC), which is close in value to direct measurements of binder failure strain. 5. The current binder fatigue specification parameter, |G*| sin Î´, and SDENT extension are only moderately correlated to FSC. The Glover-Rowe parameter (GRP) correlates better to FSC and is a good indicator of binder failure strain under a given set of conditions. For this reason, GRP is a better choice for a binder fatigue parameter than |G*| sin Î´ or SDENT.
4 6. For thin pavementsâthose subject to relatively high strains and where pavement deflections are largely controlled by the subgradeâhigh R-values can result in very poor fatigue performance at low temperatures. For this reason, an improved binder fatigue specification should include a maximum value for R, or some equivalent control such as a limit on ÎTc. There is also evidence, though not as compelling, that very low R-values can result in poor fatigue performance for thick pavements, where the strain is controlled by the characteristics of the bound layers. 7. The current protocol for determining binder fatigue test temperature is not consistently tied to average pavement temperatures for a range of climates. An improved protocol is described in this report in which binder fatigue test temperature is tied to the low temperature PG grade, which avoids many of the problems with the way in which the binder fatigue test temperature is currently determined. 8. Several field validation sites and FHWA ALF fatigue data show reasonably good correlations between GRP and fatigue performance. Although the correlations with |G*| sin Î´ are also good in some cases, GRP appears to relate better overall to fatigue performance. 9. Using the proposed binder fatigue test temperatures, a reasonable and effective maximum value for GRP after RTFOT/20-hour PAV aging is 5,000 kPa at 10 rad/s. For RTFOT/40- hour PAV aging this limit should be raised to 8,000 kPa at 10 rad/s. 10. A reasonable and effective maximum R-value for binders aged using RTFOT/20-hour PAV aging is 2.50. For RTFOT/40-hour PAV aging this limit should be increased to 3.20. 11. It is possible that the extended loose-mix aging procedure used in NCHRP 9-59 resulted in an unrealistic loss of fatigue and fracture performance for some or all the polymer- modified binders included in the study. If so, this would affect the applicability of some of the findings of this study to polymer-modified binders and would also question the advisability of adopting extended loose-mix aging for widespread use in asphalt concrete mix design and analysis. Of particular concern in this context is whether or not the same maximum R-value should apply to both non-modified and polymer-modified binders, or if the maximum value of R should be increased or eliminated for polymer-modified binders. This is an important topic for follow up research. RECOMMENDATIONS Based upon the research conducted as part of NCHRP 9-59, the following recommendations concerning the relationship between asphalt mixture fatigue performance and potential binder fatigue specification parameters can be made: 1. The current binder fatigue test temperatures should be replaced with the values shown in Table 13 (presented later in this report and reproduced here for convenience):
5 Table 13. Proposed Binder Fatigue Test Temperatures. Low PG Grade Â°C Proposed Binder Fatigue Test Temp. Â°C -46 15 -40 17 -34 19 -28 22 -22 25 -16 27 -10 29 2. The current binder fatigue specification parameter, |G*| sin Î´, should be replaced by the Glover-Rowe parameter (GRP = |G*| (cos Î´)2 / sin Î´). As in the current specification, GRP should be determined at a frequency of 10 rad/s. The maximum allowable value for GRP after RTFOT/20-hour PAV aging should be 5,000 kPa. 3. The binder fatigue specification should include an allowable range for the Christensen- Anderson R-value of from 1.5 to 2.5, after RTFOT/20-hour PAV aging. The R-value should be calculated using the following equation: ð = ððð 2 ,â Where R = Christensen-Anderson R (rheologic index) S = BBR creep stiffness at 60 seconds, MPa m = BBR m-value at 60 seconds 4. The proposed maximum value for GRP and range for R-value given above should be considered tentative. Final values should be based on review and comment of the proposed specification by pavement engineers and researchers and collection of additional data on a wide range of binders. One important consideration is whether the precision of R is adequate for a specification involving both a minimum and maximum value. If the precision is not good enough, a specification using only a maximum value of R should be implemented. These initial proposed values are for binders aged using RTFOT/20-hour PAV. If in the future the binder aging protocol is changed, any specification limits adopted as a result of this research should be thoroughly reviewed and adjusted to accommodate changes in the binder aging protocol.
6 5. There are suitable alternatives to R-value for use in an improved binder fatigue specification. These include ÎTc and BBR stiffness at m=0.3. Some additional work would be needed to develop recommendations for specification values for either of these parameters. At least some of this research is being addressed in NCHRP 9-60. 6. Additional important data concerning the relationship between binder rheology, fracture properties and other performance-related parameters for a wide range of binders including a variety of polymer-modified binders is being generated as part of NCHRP 9- 60. the findings, conclusions and recommendations of NCHRP 9-59 should be re- evaluated after the conclusion of NCHRP 9-60.