Evaluation of asphalt properties in plant produced asphalt mixes

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The Ontario Ministry of Transportation (MTO) is responsible for the safety and quality of Ontario's highway network. 95% of this network is paved with hot mix asphalt (HMA): a material produced with heated stone, sand, or gravel bound together with asphalt cement, a product of crude oil. In Ontario, asphalt cement properties have been accepted based on performance grading to the American Association of State and Highway Transportation Officials (AASHTO) standards since the adoption of the Superpave technology in early 2000's. However, the aging protocols included in the AASHTO standard, which are intended to simulate the short term (production and placement) and longer term (in service) oxidative aging of asphalt cement, have been shown to not correlate with field aging. In 2009, the Bituminous Section of the Materials Engineering Research Office (MERO) of the MTO added new tests to provide better characterization of asphalt materials and prediction of asphalt pavement performance in Ontario. A study was conducted to evaluate the new tests and how well they correlate with performance - the study concluded that correlation to field for both the current and new tests could be improved, if field aging can be better replicated in the lab. Nonetheless, the new test methods have been adopted by MTO for acceptance. Testing is carried out on asphalt cement samples collected from HMA producers' asphalt tanks. In addition, some municipalities in Ontario have further modified this specification, applying them to recovered asphalt cement from HMA. That is, the asphalt binder is removed or extracted from the aggregates in the HMA mix by using a solvent, then the asphalt is recovered from the solution for testing. There are concerns with the appropriateness of acceptance criteria based on recovered asphalt cement, because research has shown that the extraction and recovery process can impact the asphalt cement properties, and there is no understanding of how the recovered asphalt properties compare to the original virgin asphalt. Accurate material characterization is important for engineers and owners to predict performance. Therefore, the overall objective of this research is to provide a framework for characterizing of asphalt cement and HMA by: firstly, comparing current laboratory aging protocols to field aging of asphalt cement; and second how the measured properties of the asphalt cement and HMA correlate to performance.The study will also include a review of test results reported by various industry testing labs, to assess the range in test results between labs, to understand the testing variability associated with material characterization of recovered asphalt.