Relationships Between the Fatigue Properties of Asphalt Binders and the Fatigue Performance of Asphalt Mixtures (2022)

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78 Conclusions and Suggested Research Conclusions On the basis of research conducted as part of NCHRP 09-59, the following 12 conclusions concerning the relationship between asphalt mixture fatigue performance and potential binder fatigue specification parameters can be made: 1. The fatigue life of an asphalt concrete mixture depends on many factors. Those of primary interest in a binder fatigue specification are applied binder strain, binder failure strain, and the fatigue exponent. Fatigue life increases with decreasing binder applied strain relative to failure strain and increasing fatigue exponent. 2. Binder failure strain is primarily a function of binder modulus, with failure strain decreas- ing dramatically with increasing modulus. At any given modulus value, the failure strain of all asphalt binders will fall into a relatively narrow range, creating a well-defined failure envelope. Failure strain, however, does show some variability about this standard failure envelope among binders. 3. The fatigue exponent for an asphalt mixture is inversely related to the binder phase angle. 4. For polymer-modified binders, the phase angle value used for calculating the fatigue expo- nent is not the measured value, but the value calculated using the Christensen-Anderson rheological model and the R-value calculated at a modulus value above 10 MPa. This is necessary because for polymer-modified binders the measured phase angle does not reflect the phase angle of the continuous binder phase of the binder but is instead heavily influenced by the polymer network. 5. A binder’s failure strain under fatigue loading and a given set of conditions can be calculated as FSC, which appears to be reasonably close to direct measurements of binder failure strain such as the DT test. The FSC of a binder is an important factor in determining fatigue performance and is a good basis for a specification test. 6. The current binder fatigue specification parameter, |G*| sin δ, and SDENT extension are only moderately correlated to FSC. The GRP correlates better with FSC and indicates binder failure strain well under a given set of conditions. For this reason, GRP is a better choice for a binder fatigue parameter than |G*| sin δ. 7. For thin pavements, high R-values can result in poor fatigue performance at low tempera- tures. For thick pavements, low R-values can result in poor fatigue performance. For optimum pavement fatigue performance, both low and high R-values should be avoided, although the relative performance of binders with high R-values appears to be worse than that for binders with low R-values. 8. The current protocol for determining binder fatigue test temperature is not consistently tied to average pavement temperatures for a range of climates. The binder fatigue test temperature in general is too high for grades PG 70-XX and higher, and too low for grades PG 58-XX and lower. Grade adjustments for traffic volume and speed can elevate binder C H A P T E R 4

Conclusions and Suggested Research 79   fatigue test temperatures unless addressed properly by local agencies. Selecting binders with low-temperature PG grades that do not meet the requirements for the local climate can also elevate the binder fatigue test temperature. These issues create a significant problem in the current method for determining binder fatigue test temperature. An improved protocol is described in this report in which binder fatigue test temperature is tied to the low- temperature PG grade. In combination with several appropriate notes clearly describing the intent of the specification, this avoids many problems with the way in which the binder fatigue test temperature is currently determined. 9. Several field validation sites and FHWA ALF fatigue data show reasonably good correla- tions between GRP and fatigue performance. Although the correlations with |G*| sin δ are also good in some cases, GRP appears to relate better overall to fatigue performance. 10. 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. 11. A reasonable and effective range for R-value for binders aged using RTFOT/20-hour PAV aging is from 1.50 to 2.50. For RTFOT/40-hour PAV aging, this range should be shifted to a range of 2.00 to 3.20. 12. It is possible that the extended loose-mix aging procedure used in NCHRP 09-59 resulted in an unrealistic loss of fatigue and fracture performance for some or all of the polymer- modified binders included in the study. If so, this would affect the applicability of some study findings to polymer-modified binders and would also call into 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 the same maximum R-value should apply to both non-modified and polymer-modified binders, or whether the maxi- mum value of R should be increased or eliminated for polymer-modified binders. This is an important topic for follow-up research. Some additional useful information on this topic is being generated as part of NCHRP 09-60. Guidance On the basis of research conducted as part of NCHRP 09-59, the following six proposals can be made concerning the relationship between asphalt mixture fatigue performance and potential binder fatigue specification parameters: 1. The current binder fatigue test temperatures should be replaced with the values shown in Table 13 (presented earlier in this report and reproduced here for convenience). 2. The current binder fatigue specification parameter, |G*| sin δ, should be replaced by the Glover-Rowe parameter (GRP = |G*| cos2 δ/sin δ). 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 Equation 16 (presented earlier in this report and reproduced here for convenience): 2 3,000 1 ( ) ( ) ( ) = − R log log S log m where R = Christensen-Anderson R-value (rheologic index), S = BBR creep stiffness at 60 seconds (MPa), and m = BBR m-value at 60 seconds. Low PG Grade °C Proposed Binder Fatigue Test Temp. °C −46 15 −40 17 −34 19 −28 22 −22 25 −16 27 −10 29 Table 13. Proposed binder fatigue test temperatures.

80 Relationships Between the Fatigue Properties of Asphalt Binders and the Fatigue Performance of Asphalt Mixtures 4. This proposed maximum value for GRP and range for R-value should be considered tenta- tive. Final values should be based on review and comment on the proposed specification by pavement engineers and researchers and on collection of additional data for a wide range of binders. One important consideration is whether the precision of R is adequate for a specifi- cation 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. A second issue is whether the same maximum R-value should apply to both non-modified and polymer- modified binders or whether the maximum value of R should be increased for polymer- modified binders. 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. 5. There are suitable alternatives to R-value for use in an improved binder fatigue specification. These include ΔTc and BBR stiffness at m = 0.30. Some additional work would be needed. 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, are being generated as part of NCHRP 09-60. The findings, conclusions, and proposals of NCHRP 09-59 should be reevaluated after the conclusion of NCHRP 09-60. Implementation Plan The following sections outline activities the research team believes will be helpful in imple- menting the findings of this research. Suggested implementation activities are discussed as early implementation activities, late implementation activities, and barriers to implementation. Early Implementation Activities—Up to a Year After Project Completion Early implementation activities start with final review of this report by the panel and publi- cation of the NCHRP 09-59 final report. This will be followed by presentations by the research team at various technical meetings, including FHWA ETG binder and mixture meetings, various TRB committee meetings, regional asphalt user-producer group meetings, and state highway agency meetings. An important aspect of these presentations should be requesting that interested parties evaluate a variety of binders using the proposed revised specification and share their results with the pavement engineering community. Early implementation activities should also include submission of papers to the Association of Asphalt Paving Technologists or TRB. A person or group active in AASHTO who is supportive of the proposals of this research should initiate and continue to shepherd their adoption. An important early implementation activity will be completion of NCHRP 09-60, which includes testing and analysis closely related to NCHRP 09-59. Many NCHRP 09-59 binders will be further tested as part of this project. The findings, conclusions, and proposals of NCHRP 09-59 should be reevaluated after completion of NCHRP 09-60. Several issues were not addressed as part of NCHRP 09-59 but could be addressed through one or more follow-up research projects. If findings of this research are initially implemented using the current aging protocol, the 16 NCHRP 09-59 binders should be retested using RTFOT/20-hour PAV aging. Retesting should include as a minimum determination of the GRP values and R-value. It would probably be useful to perform BBR tests on these binders after RTFOT/20-hour PAV aging, to verify the proposed range in R is suitable when values are calculated from BBR data

Conclusions and Suggested Research 81   rather than DSR data. Some of this testing is planned as part of NCHRP 09-60; this additional work would only include BBR tests on those binders not scheduled for testing in NCHRP 09-60. Consideration should also be given to determining the SDENT extension after RTFOT/20-hour PAV aging. An important topic for follow-up research to NCHRP 09-59 is the effect of different binder- and mixture-aging procedures on fatigue and fracture performance. As mentioned in several sections of this report, the inherent strain tolerance of the polymer-modified binders included in NCHRP 09-59 did not appear to be significantly better than that of most non-modified binders. That is, at a given GRP value, the failure strain of polymer-modified and non-modified binders was similar. The fatigue model used in this project would still predict that the polymer-modified binders would mostly perform substantially better than the non-modified binders in actual pavements, but only because they were generally softer and so had better strain tolerance at a given temperature. Research is needed to determine whether extended loose-mix aging unreal- istically degrades the fatigue performance of polymer-modified binders. Follow-up research should also address the effects of laboratory aging in general and of modulus level on the fatigue and fracture properties of polymer-modified and non-modified binders and mixes. If this research is conducted and shows that extended loose-mix aging is not appropriate for polymer-modified binders, then additional research will probably be needed to refine the SDENT test into a binder specification test that accurately characterizes the inherent strain tolerance of polymer-modified binders. Further, if the beneficial effect of polymer modification does extend to low temperatures, highly aged mixtures, or both, any maximum limits on R-value should be increased or eliminated for polymer-modified binders. This change in the maximum limits could be implemented within the context of a modified version of AASHTO M 323, by having different limits on R depending on traffic level. Late Implementation Activities—A Year or More After Project Completion Late implementation activities should include presentation of research results at the Associa- tion of Asphalt Paving Technologists and TRB—conditional on acceptance of any submitted manuscripts by those organizations. The proposed revised specifications (AASHTO M 320 and R 29) should continue to be shepherded through the acceptance process. Test data using the revised specification presented at various technical meetings should be reviewed and compiled continuously and used as appropriate in the AASHTO specification review. A preliminary step in specification implementation should be reporting data on the revised binder fatigue specification while maintaining the current specification. This will allow final review of the appropriateness of the revised specification—particularly the specific limits—before final adop- tion. An additional late implementation activity is consideration of the results of NCHRP 09-61 (Bonaquist et al., 2021). The implementation plan described here assumes that the proposed changes will be imple- mented using current laboratory aging procedures (RTFOT/20-hour PAV). The results of NCHRP 09-61 (Bonaquist et al., 2021) will probably eventually lead to changes in laboratory aging that will in turn require modification of the specification limits initially used to adapt the findings of NCHRP 09-59. Barriers to Implementation Substantial consideration was given to ensure the results of NCHRP 09-59 could be imple- mented relatively quickly and easily. This consideration was driven partly by the project problem

82 Relationships Between the Fatigue Properties of Asphalt Binders and the Fatigue Performance of Asphalt Mixtures statement, which emphasized that only currently used standard technology should be considered in the project. Some barriers to implementation, however, will persist. One is miscommunica- tion or misunderstanding of NCHRP 09-59 proposals. This should be addressed by making an appropriate number of presentations to various technical groups concerning the results of NCHRP 09-59. Slides used in these presentations should be made available to other engineers and researchers wishing to help communicate the results of NCHRP 09-59. Another barrier will be determining the final values for the specification parameters. Values given in this report are tentative and should be revised as additional data are collected on a wide range of binders. Some producers will likely find that their materials will fail the proposed specification—after all, the objective of the specification is to ensure that binders with poor fatigue performance are not used in paving applications. Engineers and researchers supporting the need for an improved binder fatigue specification will need to stand firm in preventing a few producers from stopping implementation of a new specification or insisting on specification limits so broad that they become ineffective.