Performance-Related Specifications for Emulsified Asphaltic Binders Used in Preservation Surface Treatments (2017)

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3 C H A P T E R 1 As part of the work performed under NCHRP Project 09-50, “Performance-Related Specifica- tions for Asphaltic Binders Used in Preservation Surface Treatments,” frameworks for emulsion performance grade (EPG) specifications for asphalt binders used in chip seal, microsurfacing, and spray seal preservation surface treatments (PSTs) were developed. The work performed to develop these specifications involved the following steps (which are briefly described; details are provided in the chapters that follow): • Identification of the critical distresses that are related to the performance of the asphaltic binder used in each PST type. • Identification of the binder and PST performance test methods that are related to each distress type. • Development of relationships between the binder properties and PST performance for each distress type. • Establishment of preliminary specification limits. • Short-term validation of the developed EPG specifications. A comprehensive literature review was conducted to identify the critical distresses that relate to the asphalt binder performance for each PST type, the performance-related test methods for the asphalt binder and PST mixture for each distress type, and the asphalt emulsions to be uti- lized for each PST type. Based on this review, raveling (aggregate loss) and bleeding were identi- fied as the most critical distress types that relate to the residual binder material’s performance for chip seals. Rutting and thermal cracking were identified as the most critical distress types related to the binder’s performance for microsurfacing. Lastly, the lack of adhesion between the residual binder and the underlying pavement layer was identified as the critical distress that relates to the binder’s performance for spray seals. Because adhesion to the underlying pavement layer cannot be measured directly using any practical binder test, the spray seal EPG specification framework addresses only the constructability of the fresh emulsion and the curing time that is required to avoid the emulsion tracking onto vehicle tires due to the emulsion’s tackiness. In order to assess the fresh emulsion properties, storage stability, sprayability, and drainout were determined to be the constructability concerns for chip seal and spray seal applications, whereas storage stability and mixability were determined to be the constructability concerns for micro- surfacing applications. The literature review also served to identify the failure mechanisms that correspond to each distress type. Laboratory binder and mixture test methods were then identified and/or devel- oped to reflect each critical distress type. The asphalt emulsion residue test methods that were identified to capture chip seal performance are the multiple stress creep and recovery (MSCR) test for bleeding and rutting and the dynamic shear rheometer (DSR) frequency sweep test for low-temperature aggregate loss. The asphalt emulsion residue test methods that were identified Research Approach

4 Performance-Related Specifications for Emulsified Asphaltic Binders Used in Preservation Surface Treatments to capture microsurfacing performance are the MSCR test for rutting and the DSR frequency sweep test for thermal cracking. The fresh emulsion properties that were identified as relating to the constructability of chip seals and spray seals include storage stability and the viscosity measured at both high and low shear rates to capture sprayability and drainout, respectively. The fresh emulsion properties that were identified as relating to the constructability of microsurfac- ing include storage stability and the viscosity measured at a low shear rate to evaluate mixability. As a result of these findings, the only equipment items necessary to use the EPG specifications are a DSR and a rotational viscometer. Once these test methods were identified, an experimental plan was developed. The selection of materials for the development of the EPG specifications was based on findings from the literature review, and input from practitioners. A comprehensive asphalt material acquisition plan was developed to ensure consistency in the quality of the emulsions tested during the development of the EPG specifications. This plan called for the acquisition of emulsions from major emulsion suppliers that were identified by the research team. The research team acquired commercial- grade emulsions that have been utilized in functioning PSTs in the United States, as well as a few “poor-performing” emulsions that were fabricated intentionally to be inferior to the supplier’s standard emulsion product for comparative purposes. Development of the specification framework for each PST involved two phases: the fresh emul- sion phase and the residue phase. The EPG specifications for the fresh emulsion properties that relate to constructability were developed using statistical analysis of the binder test results, as no mixture constructability data were available to derive specification limits. The EPG specifica- tions for the residual binder were developed by defining temperature-independent relationships between the measured emulsion residue properties and the PST performance that corresponded to each critical distress. Preliminary specification limits were then established based on the values of the binder properties that corresponded to the critical PST mixture performance thresholds. Where appropriate, specification limits for different traffic classifications were established follow- ing a methodology similar to the one used for the newly developed AASHTO M 332 standard for asphalt binders in hot mix asphalt (HMA). A short-term field validation plan was implemented to validate the developed frameworks for the EPG specifications. The short-term field validation involved the construction of chip seal sec- tions and microsurfacing test sections in North Carolina. These sections were constructed during the summer of 2013 and were monitored from their initial construction through their first year in service. Prior to construction, the asphalt emulsions used in constructing the field validation sections were sampled and graded according to the newly developed EPG specifications. The asphalt emulsions used in the chip seals passed the EPG specification limits for the traffic levels and climate in North Carolina. Also, the constructed field sections performed adequately under field traffic loading, thereby providing validation of the developed EPG specifications. Findings from the microsurfacing field validation sections were inconclusive due to pre-existing cracks that were not remedied properly prior to field construction. Also, a long-term field validation plan was developed that will allow for a thorough validation and refinement of the EPG specifications under a wider variety of climatic and traffic conditions for all PST types included in this research.