United States Department of Transportation
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Abstract:This study addressed the evaluation of alternative test methods to identify the presence of polymer modifiers in performance-graded binders for the purpose of quality assurance. A method of identification is presented in AASHTO T302, Polymer Content of Polymer-Modified Emulsions and Asphalt Binders, that uses Fourier transform infrared (FTIR) spectroscopy to evaluate the constituent elements in binders or emulsions. With proper calibration, output from FTIR can be used to determine the presence and approximate content of polymers in an asphalt binder. AASHTO T301, Elastic Recovery Test of Bituminous Materials by Means of a Ductilometer, offers an alternative method to determine the presence of polymer by evaluating the elasticity of the binder. Samples of binder were collected from contractor tanks and tested in accordance with AASHTO T301 and AASHTO T302. The performance grade was verified in accordance with AASHTO M320. Test results were evaluated to identify calibration needs, test variability, and choice of preferred methodologies for adoption into the quality assurance program. Results of the study identified the use of either FTIR analysis or elastic recovery as a timesaving alternative to full-fledged performance grading in the initial investigation of concerns about the presence of polymer. Both methods identified binders containing varying polymer contents with no instances of false positive identification. However, based on the results of this study, neither method is suitable to determine binder grade. The investigator recommends that the elastic recovery and FTIR analysis be incorporated as quality assurance tests to verify the presence of polymer in mixtures that specify the use of polymer-modified asphalt binders. Following this, the frequency of quality assurance sampling of polymer modified binders should be increased to ensure that inferior material is not being used in premium mixtures. Further, AASHTO T301 should be adopted in place of Virginia Test Method 104 for use with unaged binders. Incorporating the use of elastic recovery testing and FTIR spectroscopy as alternatives to performance grading will benefit VDOT by allowing increased quality assurance testing of premium asphalt mixtures. This will result in minimizing VDOT's risk of acceptance of inferior material and maximizing the benefits of using premium materials. Typically, performance grading is performed once per month on one binder sample from each active grade of binder in a VDOT district. This is estimated to result in testing less than 5% of the binder lots used in any district during a typical month. Although neither elastic recovery testing nor FTIR spectroscopy was shown conclusively to determine binder grade, almost all PG 76-22 binders shipped into Virginia contain polymer modifiers. Thus, the detection of the polymer is a first level indicator for quality assurance. The potential cost of these tests is approximately $200 per test for elastic recovery and approximately $120 per test for FTIR spectroscopy. Overall, the increased testing is expected to result in improved pavement quality by reducing the acceptance of inferior material.
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