Measuring moisture damage of hot-mix asphalt (HMA) by digital imaging-assisted modified boiling test (ASTM D3625): Recent advancements and further investigation

Boiling water test (ASTM D3626) has been utilized to investigate the moisture susceptibility of asphalt mixture for decades. It was originally intended to compare the relative moisture resistance of a few mixtures, and the degree of stripping after the boiling was recorded based on visual observations. In recent years, several digital imaging-assisted modified boiling tests have been proposed, which significantly improved the accuracy of test results. This paper reviewed the recent advancements (2009–2022) in different test methods by digital imaging. The limitations and challenges of each method were also discussed and compared to each other. In addition, further experiments were performed to investigate the effect of aging, boiling time and water quality on the coating ratios. The boiling test results of sixty loose mixtures were compared to the tensile strength ratios (TSRs). Based on the results, the boiling water test with enhanced accuracy by image processing can be potentially developed as a measure of field performance. More importantly, the test should be conducted immediately after the mixing of asphalt and aggregate, and the aging of asphalt mixtures could significantly increase the results of coating ratios up to 100%, which made the boiling test unable to identify moisture susceptibility. It was found that the boiling water could not strip asphalt from aggregate after a long time of aging even though the moisture damage occurred and weakened the bond between asphalt and aggregate. In addition, the water quality could considerably influence the boiling test results, and the use of distilled water might underestimate the moisture damage of asphalt mixtures subjected to acid rain.