AN EXAMINATION OF HOW ASPHALT CEMENT QUALITY INFLUENCES LOW TEMPERATURE CRACKING AND PERFORMANCE

The performance of an asphalt pavement is strongly affected by the response of the asphalt cement in the mix formulation to the in-service condition. In Canada, one of the major obstacles to achieving long term pavement performance is low temperature cracking. The Strategic Highway Research Program (SHRP) Superpave methodology incorporates design requirement based on the low and high in-service temperatures. This research examines how existing practice can be incorporated into the requirements of the SHRP Superpave Performance Graded (PG) asphalts. C-LTPP material, cracking and construction data was used in this research project. This report summarizes only the results where CLTPP or the C-SHRP test data was used. An integrated model which relates material properties to pavement performance and life-cycle cost was developed and full details are available in the author’s doctoral thesis [Tighe 99]. The first part of the report focuses on the material characterization of the asphalt cement. This analysis examines the McLeod Penetration Viscosity Number (PVN) as a low temperature susceptibility indicator. The results show that PVN is a fingerprint as it remains constant over time. PVN also relates to the SHRP Superpave Performance Graded (PG) asphalts. A decrease in the minimum temperature corresponding to the PG asphalt is consistent with an increase in the PVN. As well, the PVN shows variation within a crude source as would be expected and it shows PVN is constant regardless of the calculation method. The material properties where then used to predict low temperature cracking. The Canadian Airport Model and Hajek model is used to predict cracking on the C-LTPP and C-SHRP test sites. The predicted cracking is compared to the observed cracking. The analysis indicates that the Hajek model and Canadian Airport model show good correlation to the observed cracking. The thermal contraction coefficient is examined and found to be a very good indicator of low temperature cracking based on observed cracking. The pavement performance was calculated using the cracking prediction. Using roughness as the measure of pavement performance, the performance of the pavement is predicted. Low temperature cracking is related to roughness in terms of Riding Comfort Index (RCI). Various relationships which relate RCI to the International Roughness Index (IRI) is examined. The predicted IRI values is compared to the observed values using the C-LTPP test sites. The Canadian Airport Model is considered to be used as a starting point with recognition that it is conservative. Construction data from the C-LTPP sites was examined. A lognormal distribution is determined to be the most appropriate distribution for pavement lift thickness A framework for predicting the technical and performance of a pavement is presented. Overall the thesis illustrates how these variables can be used to provide pavement designers with a methodology for predicting low temperature cracking. The model is intended to compliment the SHRP Superpave methodology and ultimately result in the selection of the most appropriate design based on it’s technical and economic merits.