Development of a Framework to Evaluate Asphalt Binder and Plant Produced Asphalt Mixes for Acceptance in Canada

Abstract

Asphalt binder, sometimes referred to as asphalt cement binder, or asphalt cement, is the binder that holds the aggregates together in asphalt mixes for asphalt concrete pavements. In Canada, asphalt binder properties are accepted in accordance with the American Association of State and Highway Transportation Officials (AASHTO) standards: AASHTO M 320 – Standard Specification for Performance-Graded (PG) Asphalt Binder. For asphalt mixes, the material is accepted based on criteria set on parameters for aggregates, asphalt binder, recycled materials such as reclaimed asphalt pavement (RAP), and volumetric properties such as air voids, voids in mineral aggregate, and voids filled with asphalt. The parameters are set because they have historically provided a good indication of a mixture’s probable performance. As well, there has always been an interest in determining the properties of asphalt binder of in-situ asphalt mixtures, for research or forensic investigation purposes, or as a way of confirming that the correct asphalt binder was utilized during asphalt mix production, to ensure the desired performance is achieved. With the increased use of RAP, many user agencies are also looking for ways to evaluate the properties of the blended asphalt binder (i.e., new asphalt binder and old binder from RAP) since this also has an impact on the asphalt pavement performance. One option is to conduct mixture performance testing of asphalt mixes. Another option, often selected by users because of its relative simplicity, is to determine the physical properties the recovered asphalt binder from plant produced asphalt mix. Typically, the original and recovered asphalt binders are required to meet the same specification. Although intuitive and relatively simple, using recovered asphalt binder properties – particularly in a specification – is not without some potential concerns. This research compared the physical properties of original asphalt binder to the properties of the same asphalt binder recovered from asphalt mix after plant production. The asphalt mixes included seven surface course mixes, two of which include 15 percent RAP. The asphalt mixes were produced with most common PG grades of asphalt binder used in Ontario. The results showed that the significant increase in ash content in the recovered asphalt binder coupled with the difference in oxidation between laboratory aging and plant production produced rheological properties that show the recovered asphalt was stiffer and less representative of the tank asphalt. There was a statistically significant difference between the tank asphalt properties and the recovered asphalt properties. Additionally, the physical properties of recovered asphalt showed higher variability than the same physical property tests on tank asphalt. Additionally for asphalt binders, the Superpave PG system simulates aging and its effect on the asphalt properties using two accelerated laboratory conditioning procedures: the Rolling Thin Film Oven (RTFO) test for short-term (production and placement) aging, and the Pressure Aging Vessel (PAV) for longer-term (in-service) aging. However, these aging protocols have been shown to not correlate with actual field aging, posing a challenge for predicting performance. The Ministry of Transportation Ontario (MTO) conducted a study in 2009 that showed that correlation of the PG system can be improved if field aging can be better replicated in the laboratory. Additionally, the Superpave volumetric properties have been shown to not completely predict the long-term performance of asphalt pavements. Therefore, implementation of suitable performance tests and aging protocols is crucial to predict performance and maintain sustainability in highway infrastructure. This research also compared the aging that is simulated by the RTFO and PAV aging in the laboratory, to the short-term aging that occurs during asphalt mix production and placement on a job site. The aging was determined with chemical analysis used to determined concentrations of the asphalt fractions (i.e., saturates, aromatics, resins, and asphaltenes). The concentrations are used to calculate an aging index as an indication of degree of oxidation for laboratory versus field. The outcome of this analysis showed that the level of oxidation offered by RTFO aging of tank asphalt in the laboratory (laboratory short term aging) is less severe and does not simulate the short-term aging obtained in the field through plant production and placement of the hot mix. Lastly, although the Superpave PG system is an improvement over previous grading systems, the PG system evaluates cracking behavior by only considering properties of asphalt binder and fails to consider the aggregate portion of the asphalt mixes, which makes up about 90 to 95% of the total weight of the asphalt mix. Additionally, new parameters have been researched since the implementation of Superpave that better characterize the oxidative behavior of asphalt binders. In this research, the Delta Tc (ΔTc) parameter is evaluated for the asphalt binders, along with Illinois Flexibility Index Test, and Asphalt Mix Performance Tester (AMPT) Flow Number for the asphalt mixes. As asphalt binders oxidize or age, their ability to relax stresses at low temperatures diminishes. The ΔTc parameter provides an indication of loss of ductility: when the asphalt binder cannot relax the stresses fast enough to prevent breaking. The ΔTc of an aged binder is more negative than that of an unaged binder and would be more likely to exhibit non-load related pavement distresses such as: block cracking, raveling, and longitudinal or transverse cracking. The Flow Number was developed as part of a research sponsored by the Federal Highway Administration (FHWA) and the National Cooperative Highway Research Program (NCHRP). The intent was to develop and validate simple performance tests for permanent deformation and fatigue cracking to be incorporated in the Superpave volumetric mixture design process. The flow number has been correlated to the mixture’s rutting resistance, with a higher flow number indicating higher resistance to rutting. The Illinois Flexibility Index Test (I-FIT) uses semi-circular bending (SCB) specimen geometry to determine the fracture resistance of an asphalt mixture at an intermediate temperature. Generally, higher FI value indicates the better premature cracking resistance of the asphalt mix. Correlation tables showed that the properties of the tank asphalt binder showed better correlation with the IFIT Flexibility Index and Flow Number of the resultant mixes than with the recovered asphalt binder properties. These results not only provide an evaluation of the impact comparing values of recovered asphalt to test criteria and variability derived for original asphalt, but it also provides framework for moving toward acceptance of asphalt mixes based on performance testing in Canada.