Non-isothermal kinetic analysis of reversible ageing in asphalt cements

The AASHTO M320 specification for thermal cracking – meant to limit damaging temperatures to only a 1 in 50 risk in any given winter – stipulates that asphalt cement chemically (irreversibly) aged in the rolling thin film oven and pressure ageing vessel be tested after minimal cold conditioning. Creep stiffness and creep rate (m-value) are determined with specimens rapidly cooled to 10°C above the pavement design temperature and then conditioned for only an hour prior to testing. As a result, materials are often tested in a non-equilibrium state and pavements end up under-designed for thermal cracking; adjacent sections can show from best-case to worst-case performance. Several Ontario user agencies have recently implemented a specification to effectively limit cracking due to reversible ageing: “determination of performance grade of physically aged asphalt binder using the extended bending beam rheometer (EBBR) method” (Ministry of Transportation of Ontario designation LS-308 and recently adopted by AASHTO under designation TP 122-16). This empirical protocol tests beams after 72 h of cold conditioning to determine a low-temperature grade as well as a grade loss from the 1 h results specified in AASHTO M320. Grade losses are sensitive to the presence of deleterious additives (waxes, air blown residues, recycled engine oil bottoms). Furthermore, by testing recovered material from paving mixtures, the protocol also provides a secure way to account for the presence of reclaimed asphalt pavement, and to monitor for overheating during production. The extended BBR method has several limitations: (1) it requires a relatively large quantity of 150 g of aged material; (2) it takes a relatively long 72 h to complete and (3) it quenches the binder from room temperature to two cold temperatures, namely 10°C and 20°C above the pavement design limit (Td+10 and Td+20), and therefore, because of its empirical nature, it is unclear if the results obtained are relevant for other thermal histories. Since these drawbacks have slowed implementation, current research is focused on the following improvements: (1) modelling the ageing/hardening processes within the Ozawa theoretical framework; (2) reducing sample requirement to less than 12 g; (3) shortening testing time to less than 24 h; (4) conditioning at more temperatures between ambient and the pavement design limit and (5) automating the procedure. Modelling efforts are based on an analysis of non-isothermal phase transformation kinetics. This paper presents results obtained from differential scanning calorimetry and dynamic mechanical analysis three-point bending tests on a number of Ontario asphalt cements under varying cooling rates. The application of the Ozawa theory provides an improved understanding of how thermal history and variable ageing tendencies can explain vast performance differences between pavement sections of identical AASHTO M320 grades. It is suggested that by adding an upper limit on the Ozawa exponent to the low-temperature asphalt cement specification, in order to reduce the rate of reversible ageing, user agencies will be able to better control this type of cracking.