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https://www.nationalacademies.org/trb/events

SPR 784 Calibration of APA values to Hamburg values for SC Asphalt Mixes

The proposed research has several key objectives: (1) Correlate APA and Hamburg Results: Determine the quantitative relationship between APA results and Hamburg Wheel Tracking Test results for a representative set of South Carolina asphalt mixes. This includes assessing correlation in terms of rut depth measurements, ranking of mixture performance, and any conversion formula or trend that links the two test outputs. The goal is to establish what Hamburg test outcome (e.g. rut depth after a certain number of passes, or number of passes to reach a failure rut depth) corresponds to the current APA rut depth criteria after 8,000 cycles. (2) Define Equivalent Hamburg Criteria: Based on the above correlation, develop recommended Hamburg test specification limits for South Carolina Department of Transportation (SCDOT) mix designs. For example, the research should propose a criterion such as “Maximum X mm rut depth at Y passes in the Hamburg test at 50°C” or “Minimum Z passes to reach 12.5 mm rut depth” that would ensure mixes meet or exceed the performance of those that pass the APA requirements. These criteria may vary by mix type or binder grade if appropriate (like the practice of other DOTs requiring more Hamburg passes for higher graded PG binders). (3) Compare Moisture Damage Indication: Evaluate the degree to which the Hamburg test (which has a water component) detects moisture-susceptible mixtures as compared to APA. One objective is to confirm that mixes exhibiting minimal rutting in the APA but containing moisture sensitivity (stripping potential) are properly identified by the Hamburg test. This will demonstrate the added value of Hamburg in preventing moisture-related failures. (4) Incorporate Cracking Performance (IDEAL-CT): Investigate any correlations or patterns between the rutting test results (APA and Hamburg) and cracking test results (IDEAL-CT) for the same mixes. While rutting and cracking are generally independent performance areas, this objective will reveal if a mix that is very resistant to rutting (especially under Hamburg’s rigorous conditions) tends to have any consistent relationship to its cracking resistance (IDEAL-CT index). If strong correlations or trade-offs are observed, SCDOT can use that knowledge to refine its balanced mix design procedure (for instance, ensuring that improving one aspect does not inadvertently degrade the other). (5) Develop Implementation Guidance: Formulate practical recommendations for SCDOT to implement Hamburg Wheel Tracking in the mix design phase. This includes outlining the test protocol to be used (sample preparation, test temperature, pass/fail criteria), any changes to other required tests (e.g. whether APA can be phased out or retained for certain cases, or how IDEAL-CT will be used in tandem), and steps for industry adoption. The end objective is to provide SCDOT’s Asphalt Materials Engineer and Office of Materials and Research (OMR) with the data and tools needed to update the official mix design specification and guidance. Documentation and Knowledge Transfer: Produce clear deliverables (report, specification draft, recommendations for any changes to other currently-required tests) that document the APA-to-Hamburg calibration and the rationale for any new criteria. A further objective is to engage industry stakeholders throughout the project so that the findings are understood and accepted by asphalt contractors and material suppliers, smoothing the path for real-world implementation. This project will provide a newly developed specification for the SCDOT to optimize various mixtures utilized around SC. The specific objectives of this project would include the following: (1) Literature review of current mix design procedures used by other states utilizing the Hamburg test; (2) Survey of other State DOT’s current mix design processes; (3) Compilation and laboratory evaluation of various SCDOT surface, intermediate, and base type mixtures utilizing different aggregate sources; (4) Recommendations for newly developed testing procedures to be used by the SCDOT; (5) Establish a correlation between the APA and Hamburg test results; (6) Evaluation of other current SCDOT test procedures (e.g., IDEAL-CT and ITS) to determine if a correlation exists between those test results and Hamburg results; and (7) Based on the evaluations of several test procedures (e.g., APA, Hamburg, and IDEAL-CT), either updating the current mix design methodology or creating a new model for the system based on the data obtained and evaluated in this study.