Physical Hardening in Asphalt

摘要

The fact that asphalt binders gradually change in their consistency when conditioned at ambient and cold temperatures was reported nearly 100 years ago by Hubbard and Pritchard (1916). In spite of this, the current North American specification protocol meant to limit thermal cracking incidents to once in 50 years, continues to test materials after minimal conditioning in a non-equilibrium state. As a consequence, adjacent pavement sections can show from best-case to worst-case performance. Excessive cracking, as seen around Ontario and beyond, is due to the use of inferior, low-cost diluents, modifiers, and process technology. A review of the literature on physical hardening is provided. Distress surveys from real-world pavement trial sections are used to show the importance of measuring this hardening tendency for the improved control of thermal cracking. Changes in binder properties are modeled using the first-principles Avrami theory, as proposed for physical hardening studies in asphalt by Pechenyi and Kuznetsov (1990). Tests on mixtures are used to show enormous changes due to cold conditioning. It is concluded that the implementation of Ministry of Transportation of Ontario Laboratory Standard 308, Performance Grade of Physically Aged Asphalt Cement Using Extended Bending Beam Rheometer (BBR) Method, can significantly reduce premature failures.