Developing Evaluation Method of Moisture Susceptibility for Cold Mix Asphalt
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Executive Summary Cold Mix Asphalt (CMA) serves as a promising alternative for a wide range of paving applications from preventative maintenance and repair, to full-scale pavement construction. Compared to traditional Hot Mix Asphalt (HMA) and Warm Mix Asphalt (WMA), CMA provides a number of benefits that make it a promising alternative to conventional paving technologies, including reduction in heating energy and longer working time for transportation and placing. However, the reported higher air void content and higher moisture susceptibility of CMA relative to traditional HMA present legitimate concerns that has limited more widespread use of CMA to date. To mitigate the potential for moisture damage in CMA, it is necessary to include a simple and effective method for evaluating moisture susceptibility in the material selection phase of the mix design process. The most direct way to prevent moisture sensitivity is to require a minimum threshold of aggregate coating as part of CMA mix design in regards to both extent and quality of coating. This study refines a recently developed method to quantify aggregate coating extent in CMA using Digital Image Analysis Methods and applies the procedure to several aggregate-emulsion systems to isolate factors most directly affecting aggregate coating. After identifying the most significant factors, a regression analysis of the experimental results is used to develop quantitative models to predict coating of aggregates in CMA as a function of mix design parameters. Results show that not only Image Analysis Method serves as an effective and reliable tool in identifying aggregate coating, but also that the regression models developed to predict coating are sensitive to emulsion type, aggregate gradation and residual asphalt content. Including evaluation of coating quality is a generally accepted practice in mix design for conventional HMA, WMA, and CMA. This study introduces the concept that it is necessary to assess quality of coating as a means to ensure adequate bonding between the asphalt and aggregate to truly assess potential for moisture damage. To evaluate quality of coating, a Modified Boiling Test procedure on cured CMA loose mixture that involves application of the Image Analysis Method to quantify the coating before and after boiling was developed. Results reveal that after curing is achieved as indicated by negligible moisture loss in the mixture, the bonding between emulsion residue and aggregate continues to increases with curing time. Modified Boiling Test results on fully cured CMA loose mixture indicate that CSS-1hl performs much better than CSS-1h and CSS-2 emulsion, showing the best coating quality. The results were also verified by Binder Bond Strength (BBS) test on emulsion residues and asphalt emulsions. The relationship between coating quality and mixture performance was verified through comparison of boiling test results to mixture moisture damage as measured by the Tensile Strength Ratio (TSR). Prior to conducting this comparison a CMA mix design method was proposed to ensure a uniform process for selection of design emulsion content and preparation of mixture samples. The proposed CMA mix design method determines the design emulsion content (DEC) based on volumetric analysis and mixture indirect tensile strength. The Tensile Strength Ratio (TSR) test based on this mix design shows consistency with Modified Boiling test result and correlation between TSR test and Modified Boiling test is also established.