Asphalt Cement Loss Tangent as Surrogate Performance Indicator for Control of Thermal Cracking

Abstract

This paper documents and discusses the field validation of a simple performance indicator for specification grading of asphalt cement for thermal cracking. The loss tangent, tan(δ), as defined by the ratio of the viscous over elastic modulus, G”/ G’, is able to provide a quantitative measure of the sol and gel nature of asphalt cement. As such, tan(δ) relates closely to how well a material is able to relax stress and is therefore investigated as a surrogate performance indicator for thermal cracking. Asphalt cements from 20 contract sites in Ontario, Canada, were tested in torsion bar geometry to determine their viscoelastic properties. The findings show that tan(δ) was able to distinguish good from poor performers for this set of materials with 95% accuracy, which is a considerable improvement over the current bending beam rheometer protocol. In addition to the use of tan(δ), it is suggested that a measure of the critical strain tolerance in the ductile state could be included to provide a significantly improved performance grading method. Most of the worst performing contracts were found to contain zinc. The presence of this element suggests that the use of waste engine oils in asphalt production is likely widespread, since zinc is a universal additive in engine oil and is never found in straight asphalt cement