Effect of Mixture Properties and Testing Protocol on Moisture Susceptibility of Asphalt Mixtures

Abstract

While cracking and rutting are the most prevalent distresses for asphalt mixtures, moisture susceptibility of asphalt mixtures which can lead to distresses such as raveling, loss of adhesion, and stripping is a commonly addressed concern in the mixture design procedure. As the industry shifts toward the use of warm mix asphalt (WMA) technology from the traditional hot mix asphalt (HMA) technology there has been increased concern regarding the performance of WMA technology in regards to moisture susceptibility. This concern is primarily driven by the decrease in required production temperatures of the asphalt mixtures which may result in aggregates that have not been dried thoroughly prior to being coated with asphalt binder. In this report, 26 mixtures used for the 2009 National Center for Asphalt Technology (NCAT) Test Track and 61 mixtures evaluated for the National Cooperative Highway Research Program (NCHRP) 9-47A research projects were evaluated for their resistance to moisture damage using the most common laboratory tests for assessing such damage: AASHTO T 283 which results in a tensile strength ratio (TSR) and AASHTO T 324 Hamburg Wheel Tracking Device (HWTD) resulting in the stripping inflection point (SIP). In addition to WMA moisture susceptibility performance, the acceptability of the standard test protocols for moisture testing of asphalt mixtures were evaluated by comparing laboratory performance thresholds to the field performance of the mixtures. As a result of this study, it was determined that there was no statistical difference between field compacted specimens versus laboratory reheated specimens for TSR and HWTD results. WMA consistently showed statistically lower TSR and SIP compared to HMA control mixtures. The current specification criteria for HWTD may need to be adjusted assessing moisture susceptibility of WMA since several mixtures failed in the lab but showed good performance in the field. Statistical analysis also showed air voids consistently played a significant role in explaining the variability of the data. Finally, the change in texture data at the NCAT Test Track did not correlate well with moisture susceptibility testing results.