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1 S u m m a r y Background Asphalt preservation surface treatments (PSTs) are an effective means of improving surface quality and extending the service life of pavements. PSTs are applied in a thin layer to an exist- ing roadway surface to slow deterioration and to maintain or improve the roadwayâs surface condition without increasing its structural capacity. One of the primary constituents of a PST is the binder. Although it is well known that the properties of asphaltic binder are critical to the performance of PSTs, current specifications for these binders are empirical in nature and are not based on the properties that relate directly to the PSTâs performance. For example, emul- sion residue is graded based on the so-called penetration system, a system that was phased out of use for hot mix asphalt (HMA) binders in the United States in the 1960s and almost completely abandoned by the mid-1980s. Emulsion performance grade (EPG) specifications that specify the asphalt material proper- ties that relate directly to performance will improve the process for selecting binders for a given PST to prevent premature failure. The responses to surveys of pavement practitioners in the United States conducted by Johnston and King (2008) highlighted the need for EPG specifi- cations for binders used in PSTs. The survey results strongly suggested the need to develop a climate-driven, performance-related grading system for asphalt emulsions used in PST applica- tions such as chip seals. Therefore, new EPG specifications should encompass the binder material properties that corre- late with critical PST mixture performance measures. The EPG specifications should also account for the climatic and traffic loading conditions that a PST will experience during its anticipated service life. Binder test methods included in the EPG specifications should evaluate both the fresh emulsion and emulsion residue properties that relate to constructability and in-service condi- tions, respectively. Research Objectives and Scope The objective of this research is to develop recommended EPG specifications for the selec- tion of asphalt emulsions used in PSTs. The EPG specifications should be climate-driven and applicable to the traffic loads that are anticipated during the service life of each PST type. For the purposes of this research, PSTs are defined as treatments that are applied to a large surface area of an existing roadway to slow future deterioration and to maintain or improve the functional condition of the pavement without increasing its structural capacity. Such PSTs include chip Performance-Related Specifications for Emulsified Asphaltic Binders Used in Preservation Surface Treatments
2 Performance-related Specifications for Emulsified asphaltic Binders used in Preservation Surface Treatments seals, microsurfacing, and spray seals. The following research activities were performed to sup- port the development of the proposed EPG specifications: ⢠Define the most relevant causes of failure for each PST type. ⢠Differentiate between design-related, construction-related, and material-related in-service failures. ⢠Understand the mechanism(s) associated with the most important material-related failures. ⢠Select a test system to measure the binder properties that relate to each type of asphalt material failure. ⢠Determine the relationship between the material property measured by each EPG specifica- tion binder test and the critical performance measure for each PST mixture as a function of temperature. ⢠Establish threshold limits that define acceptable performance of the binder as it relates to the performance of the PST mixture. ⢠Validate the developed EPG specifications using a short-term validation plan. ⢠Develop a long-term plan for validating the EPG specifications. Organization of Report This research report consists of three chapters. Chapter 1 presents the research approach. Chapter 2 presents research results, interpretation, and applications, and Chapter 3 provides conclusions and recommendations for future research. The EPG specifications for chip seal, microsurfacing, and spray seal PSTs are provided in Attachment 1 of this report. Proposed test methods, for which specifications are not available, are provided in Attachment 2. Five appendices (A through E) provide further details regarding the efforts that were under- taken to develop the EPG specifications. These appendices can be accessed on the TRB website (www.trb.org) by searching on NCHRP Research Report 837. Appendix A describes the long-term validation plan. Appendix B summarizes the critical distresses for each PST type. Appendix C dis- cusses the mixture performance tests that were conducted in developing the EPG specifications. Appendix D provides background for the development of the EPG specifications. Appendix E contains the list of references used in the entire study.
