Determination of Critical Stripping Temperatures for Typical Asphalt Concrete Mixtures Used in Nova Scotia

The Hamburg Wheel Tracking Test (HWTT) per AASHTO T324 has been widely used to evaluate rutting resistance and moisture susceptibility of asphalt mixtures and is tested at various temperatures by different agencies.  Research has shown that there is a Critical Stripping Temperature (CST) above which visco-plastic rutting and moisture damage occur during HWTT, while tests below the CST only exhibit visco-plastic rutting.  The objective of the current study was to determine the CST needed for representative asphalt concrete mixtures used in Nova Scotia which use PG 58S-28 and PG 58H-28 binders.  Optimized asphalt concrete mix designs were completed for three local aggregate sources which have historically exhibited low, medium, and high levels of moisture susceptibility without using anti-stripping additives.  HWTT testing was completed at four different water bath temperatures for each aggregate source and PGAB combination to determine the CST.  The creep and stripping slopes were analyzed for each test based on AASHTO T324 to determine the Stripping Inflection Point (SIP) of the mixture.  The SIP indicates how quickly the material begins to experience significant moisture-induced damage.  While this approach has been adopted by several transportation agencies, AASHTO T324 does not provide a specific method of analysis, so the results tend to be subjective and variable with poor repeatability.  A novel method which separates visco-plastic and moisture damage effects in HWTT results was also used to analyze the HWT test results, yielding the Visco-plastic Ratio (VR) which characterizes the mixtures rutting resistance under dry conditions and the Moisture Ratio (MR) defined as the percentage of total rutting that results from moisture-induced deformation at a total rut depth of 12.5 mm.  A strong correlation was observed between MR and the existence of significant moisture damage as defined via the AASHTO T324 method as exceedance of a maximum rutting depth of 12.5 mm and a minimum SIP of 15,000 passes as used by Maine DOT.  It was found that an MR of 30% appears to provide a reliable threshold for verifying moisture susceptibility of asphalt mixtures.  This systematic approach could replace the more complex and subjective AASHTO method to objectively and reliably detect and quantify moisture-induced damage in susceptible asphalt concrete mixtures. The CST for local asphalt mixtures using PG 58S-28 and PG 58H-28 binders were found to be 46°C and 50°C, respectively.