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103 CHAPTER 4. CONCLUSIONS AND SUGGESTED RESEARCH CONCLUSIONS 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. The fatigue life of an asphalt concrete mixture depends upon many factors. The ones 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 decreasing 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. There is however some variability in failure strain about this standard failure envelope among different 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 exponent 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 is not reflective of the phase angle of the continuous binder phase of the binder but is instead heavily influenced by the polymer network. 5. A binderâs failure strain under fatigue loading and a given set of conditions can be calculated and is called the fatigue strain capacity (FSC), which appears to be reasonably close to direct measurements of binder failure strain such as the direct tension 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 Î´, and SDENT extension are only moderately correlated to FSC. The Glover-Rowe parameter (GRP) correlates much better to FSC and is in fact 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´. 7. For thin pavements, high R-values can result in very poor fatigue performance at low temperatures. For thick pavements, low R-values can result in poor fatigue performance. For optimum pavement fatigue performance, both very low and very high R-values should be avoided, although the relative performance of binders with high R-values appears to be much 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
104 grades PG 58-XX and lower. Grade adjustments for traffic volume and speed can result in elevated binder 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 of the 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 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. 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 from1.50 to 2.50. For RTFOT/40-hour PAV aging this range should be shifted to from 2.00 to 3.20. 12. 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 of 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. Some additional useful information on this topic is being generated as part of NCHRP 9-60. 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 earlier in this report and reproduced here for convenience):
105 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*| cos2 Î´ / 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. A second issue is whether 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 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.
106 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. 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. GUIDANCE/IMPLEMENTATION PLAN The sections below provide an outline of activities that the research team believes will be helpful in implementing the findings of this research. Implementation in this case means adoption of some or all the changes to the binder specification recommended in this report. The suggested implementation activities are discussed in the three sections below: Early Implementation Activities, Late Implementation Activities, and Barriers to Implementation. As an aid to implementation, Appendix F includes four model specifications in AASHTO format, incorporating changes suggested in this report: NCHRP 9-59 / M 320, NCHRP 9-59 / M 332, NCHRP 9-59 / R 29 and NCHRP 9-59 / T 313. Early Implementation ActivitiesâUp to One Year after Project Completion Early implementation activities start with final review of this report by the panel and publication of the NCHRP 9-59 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 one or two papers to the Association of Asphalt Paving Technologists and/or the Transportation Research Board. One or more persons active in AASHTO supportive of the recommendations of this research should initiate and continue to shepherd adoption of the findings of this research. An important early implementation activity will be completion of NCHRP 9-60 which includes testing and analysis closely related to NCHRP 9-59. Many of the NCHRP 9-59 binders will be further tested as part of this project. The findings, conclusions and recommendations of NCHRP 9-59 should be re-evaluated after completion of NCHRP 9-60. There are several issues not addressed as part of NCHRP 9-59 that could be addressed through one or more follow-up research projects. If initial implementation of the findings of this research are done using the current aging protocol, the 16 NCHRP 9-59 binders should be retested using RTFOT/20-hour PAV aging. This should include as a minimum determination of
107 the GRP values and R-value. It would probably be useful to perform BBR tests on these binders after RTFOT/20-hour PAV aging, in order to verify that the proposed range in R is suitable when values are calculated from BBR data rather than DSR data. Some of this testing is planned as part of NCHRP 9-60; this additional work would only include BBR tests on those binders not scheduled for testing in NCHRP 9-60. Consideration should also be given to determining the SDENT extension after RTFOT/20-hour PAV aging. A very important topic for follow-up research to NCHRP 9-59 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 9-59 did not appear to be significantly better than that of most of the 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 still would predict that the polymer-modified binders would mostly perform substantially better than the non-modified binders in actual pavements, but only because they were in general softer and so had better strain tolerance at a given temperature. Research is needed to determine if extended loose-mix aging causes an unrealistic degradation of the fatigue performance of polymer-modified binders. This 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 in order to have a binder specification test that accurately characterizes the inherent strain tolerance of polymer-modified binders. Additionally, if the beneficial effect of polymer- modification does extend to low temperatures and/or highly aged mixtures, it would suggest that any maximum limits on R-value should be increased or eliminated for polymer-modified binders. This could be implemented within the context of a modified version of M323, by having different limits on R depending on traffic level. Late Implementation ActivitiesâA Year or More after Project Completion Late implementation activities should include presentation of research results at AAPT and TRBâconditional upon acceptance of any submitted manuscripts by those organizations. The proposed revised specifications (AASHTP M320 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 on a continuous basis and utilized as appropriate in the AASHTO specification review process. A preliminary step in the specification implementation process 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 specificationâparticularly the specific limitsâprior to final adoption. An additional late implementation activity is consideration of the results of NCHRP 9-61: Short- and Long-Term Aging Binder Methods to Accurately Reflect Aging in Asphalt Mixtures. The Implementation Plan described here assumes that the proposed changes will be implemented using current
108 laboratory aging procedures (RTFOT/20-hour PAV). The results of NCHRP 9-61 will probably eventually lead to changes in laboratory aging that will in turn require modification of the specification limits used in the initial adaptation of the findings of NCHRP 9-59. Barriers to Implementation Substantial consideration was given to making sure that the results of NCHRP 9-59 could be implemented relatively quickly and easily. This was in fact driven in part by the project Problem Statement, which emphasized that only currently used standard technology should be considered in the project. There will however still be some barriers to implementation. One is miscommunication and/or misunderstanding of the recommendations of NCHRP 9-59. This should be addressed through making an appropriate number of presentations to various technical groups concerning the results of NCHRP 9-59. Slides used in these presentations should be made available to other engineers and researchers wishing to help in communicating the results of NCHRP 9-59. 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 is collected on a wide range of binders. It is likely that some producers will find that their materials will fail the proposed specificationâafter all, the objective of the specification is to ensure that binders with poor fatigue performance are not used in paving applications. It is important that engineers and researchers supporting the need for an improved binder fatigue specification stand firm in preventing a limited number of producers from stopping implementation of a new specification or insist on specification limits so broad that they become ineffective. REFERENCES Anderson, R. M., G. N. King, D. I. Hanson, and P. B. Blankenship. Evaluation of the Relationship between Asphalt Binder Properties and Non-Load Related Cracking. Journal of the Association of Asphalt Paving Technologists, Vol. 80, 2011, pp. 615â664. Ahearn, W. Whereâs My Pavement Today? Presented at the FHWA Binder ETG Meeting, Fall River, MA, April 2015. Andrei, D., M. W. Witczak, and W. Mirza. Development of a Revised Predictive Model for the Dynamic (Complex) Modulus of Asphalt Mixtures. NCHRP Project 1-37A, Inter Team Technical Report, University of Maryland, March 1999. ARA Inc., ERES Division. Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures, Appendix II-1: Calibration of Fatigue Models for Flexible Pavements. NCHRP 1-37A Final Report, Transportation Research Board, 2004, 47 pp. Bahia, H., Hanson, D., Zeng, M., Zhai, H., Khatri, M., and Anderson, R. NCHRP Report 459: Characterization of Modified Asphalt Binders in Superpave Mix Design, Transportation Research Board, Washington, D.C., 2001. Bennert, T. Asphalt Binder and Mixture Properties Produced with REOB Modified Asphalt Binders. Presented at the FHWA Binder ETG Meeting, Fall River, MA, April 2015.
109 Bonnaure, F. P., A. H. J. J. Huibers, and A. Boonders. A Laboratory Investigation of the Influence of Rest Periods on the Fatigue Characteristics of Bituminous Mixes. Proceedings of the Association of Asphalt Paving Technologists, Vol. 51, 1982, p. 104. Christensen, D. W., and R. Bonaquist. Improved Hirsch Model for Estimating the Modulus of Hot-Mix Asphalt. Road Materials and Pavement Design, Special Issue: Papers from the 90th Association of Asphalt Paving Technologistsâ Annual Meeting, Vol. 16, Supplement 2, 2015, pp. 254â274. Christensen, D. W., and D. A. Anderson. Interpretation of Dynamic Mechanical Test Data for Paving Grade Asphalt Cements. Journal of the Association of Asphalt Paving Technologists, Vol. 61, 1992, pp. 66â116. Corrigan, M. REOB: ETG Status and Emerging Knowledge. Presented at the March 2016 North Central Asphalt User Producer Group Meeting, West Lafayette, IN. Dahlquist, C. A. An Investigation into the Nature of Tack. Adhesives Age, Vol. 2, No. 25, 1959. Deacon, J., J. Harvey, A. Tayebali, and C. Monismith. Influence of Binder Loss Modulus on the Fatigue Performance of Asphalt Concrete Pavements. Journal of the Association of Asphalt Paving Technologists, Vol. 66, 1997. Farrar, M. J., P. M. Harnsberger, K. P. Thomas, and W. Wiser. Evaluation of Oxidation in Asphalt Pavement Test Sections after Four Years of Service. Submitted for the International Conference on Perpetual Pavement, September 2016, 17 pp. Gibson, N., X. Qi, A. Shenoy, G. Al-Khateeb, M. E. Kutay, A. Andriescu, K. Stuart, J. Youtcheff, and T. Harman. Performance Testing for Superpave and Structural Validation. Final Report FHWA-HRT-11-045, Federal Highway Administration, November 2012, 271 pp. Glover, C. J., R. R. Davison, C. H. Domke, Y. Ruan, P. Juristyarini, D. B. Knorr and S. H. Jung. Development of a New Method for Assessing Asphalt Binder Durability with Field Validation, Report 0-1872-2. Springfield, VA: National Technical Information Service, 2005, 334 pp. Grillet, A. M., N. B. Wyatt, and L. M. Gloe. Polymer Gel Rheology and Adhesion. Sandia National Laboratories. Available at www.intechopen.com/pdfs-wm/30968.pdf. Harvey, J. T. SHRP A-003A asphalt concrete specimen preparation protocol, version 3.0. Technical Memorandum No. TM-UCB-A-003A-91-2, prepared for the Strategic Highway Research Program. University of California at Berkeley, 1991. Heukelom. Observations on the Rheology and Fracture of Bitumens and Asphalt Mixes. Proceedings of the Association of Asphalt Paving Technologists, Vol. 35, 1966, pp. 358â399. Huang, Y. H.. Pavement Analysis and Design, Prentice-Hall, Inc., Englewood Cliffs, N.J., 1993, 805 pp. Kanabar, A. Physical and Chemical Aging Behavior of Asphalt Cements from Two Northern Ontario Pavement Trials. Queens University, Department of Chemistry, Kingston, ON, 2010, 109 pp. Kandhal, P. S. Low-Temperature Ductility in Relation to Pavement Performance. Low- Temperature Properties of Bituminous Material and Compacted Bituminous Paving Mixtures:
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