Evaluating Asphalt Mix Ingredients by Moisture Susceptibility: The Development of a New Modified Boiling Test Procedure Based on Digital Imaging

The modified boiling test based on image processing has gained significant attention in recent years, aimed at replacing subjective visual observation with an objective measurement. However, there are still some unsolved problems influencing the accuracy of the test such as the shadow between aggregate particles and the selection of threshold values for binary image conversion. Based on these challenges, an innovative modified boiling test procedure based on digital imaging has been successfully developed for the purpose of rating asphalt mix ingredients via moisture susceptibility. The new method focuses on a single piece of aggregate with a newly cut smooth surface, and then the aggregate coated by asphalt is subjected to the boiling test. Changes to the thickness of the asphalt coating on the fresh surface before and after boiling can be captured indirectly by digital images and reflected by the changing average grayscale values. The experimental results of 20 asphalt-aggregate combinations clearly showed the moisture damage resistance of different samples and the effect of liquid antistripping agent. In addition, the test results correlated well with energy ratios based on the surface free energy measurements of asphalt and aggregate. A decreased stripping rate over time were also observed for all samples and no significant stripping was found after 90 min of boiling. The selection of a suitable boiling time is critical to the accuracy of the test, which can be determined by trial-and-error methods. The proposed modified boiling test avoids the disadvantages of previously developed methods and provides a quick way to evaluate the asphalt mixture ingredients by moisture susceptibility, making it possible to test a large amount of samples within a short period of time. The new method can be used to generate an initial selection of asphalt-aggregate combinations in place of the surface free energy method.