An integrated model to assess asphalt cement quality on low-temperature performance and life cycle cost

Abstract

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. As well, it examines how modified asphalts relate to the low temperature susceptibility criteria. The research involves the development of a framework for an integrated model which examines low temperature cracking and how it relates to pavement performance and life-cycle cost. The model was divided into four modules and focuses on asphalt pavements.

Module One 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. For modified asphalts, PVN is related to the minimum Superpave PG temperature. Based on the analysis for a small number of samples, indications are that PVN does not remain constant with time for the modified asphalts.

Module Two uses the material properties 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.

Module Three involves predicting performance using the cracking prediction provided in the previous Module. Using roughness as a 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.

The life-cycle cost analysis framework is described in Module Four. In general, a formal life-cycle costing procedure is not carried out for most pavement designs in Canada. If a life-cycle cost is carried out, it is a deterministic analysis. This research recommends a probabilistic life-cycle cost procedure should be carried out. A lognormal distribution is determined to be the most appropriate distribution for pavement lift thickness. An extensive analysis of material costs is presented. The analysis indicates that a lognormal distribution in general is also the best fit distribution for material costs. Costs should be grouped according to the quantity as economies of scale have a significant impact on the magnitude and the standard deviation associated with pavement material costs.

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.