Physical and Chemical Aging Behavior of Asphalt Cements From Two Northern Ontario Pavement Trials

Abstract

This thesis documents and discusses the analysis of material properties and pavement performance for Highway 17 and Highway 655 pavement trial sections located in northern Ontario. The object of this work was to compare laboratory-aged material with recovered asphalt cement and to develop an improved chemical aging method that provides asphalt cement that more closely reflects properties after 8-10 years of service. Physical testing of the asphalt cements was done using a bending beam rheometer (BBR) and double-edge-notched tension (DENT) test for laboratory aged material as well as recovered material from the road. The comparison of the regular BBR, extended BBR and elastic recovery test in BBR is also done for the laboratory-aged and recovered material. Two simple modifications to the regular pressure aging vessel (PAV) aging protocol were investigated as possible ways to improve the correlation between field and laboratory material properties. The length of the PAV aging was doubled to 40 hours and the weight for each pan was halved to 25 grams. It was observed that the presently used RTFO/PAV aging protocol does not produce sufficient aging. Hence, the conditions chosen were more severe and therefore are likely to bring the laboratory aging closer to the field aged condition. It was found that both the increase in time and the reduction in weight were able to accomplish the stated objective for most but not all asphalt cements. A separate investigation involved the infrared (IR) analysis of asphalt cements recovered from a large number of contracts in eastern and northeastern Ontario. This study indicated that those asphalts with a largely paraffinic structure (low aromatics index) showed excessive and premature cracking even at lower levels of oxidation (carbonyl index). In contrast, those pavements that were largely spared of thermal distress were constructed with asphalt cements that contained more aromatics (high IR aromatics indices). Aromatics allow the largely aromatic asphaltenes that are formed upon oxidation to remain well peptized rather than flocculated. Peptized asphaltenes allow for good stress relaxation during winter and spring thaw and thus such materials show a reduced tendency for cracking.