Evaluating the influence of humidity on asphalt mixture performance by the flow number test

Abstract Moisture damage greatly influences the performance of asphalt pavements. However, existing studies evaluating the moisture sensitivity of asphalt mixtures either ignore the influence of water vapor or use inappropriate testing methods. This study examined the moisture sensitivity of asphalt mixtures by using the flow number (FN) test. Four levels of the humidity condition (2%, 50%, 80% and 100% RH) were first designed to condition specimens, after which the FN test was employed to assess the moisture susceptibility of mixtures. The results show that it took 50 days for specimens to reach humidity equilibrium at Humidity Level 1, while approximately 4300 h (180 days) were required for specimens to achieve the respective Humidity Levels 2, 3, and 4. The evaluation indicators—in other words, FN, the onset of the secondary stage N4, their corresponding permanent strains (PS2 and PS3), and the slope of secondary stage S2—were then determined by applying a two-step secant method. It was observed that N4 and FN exhibit a gradually decreasing tendency, while PS2 and PS3 tend to increase with the rise in humidity level, but this phenomenon seems indiscernible in some groups. A newly modified exponential function model was proposed to characterize the interaction between humidity levels and S2. It was also demonstrated that the slope of the secondary stage is positively correlated with humidity conditions, which indicates that more deformation will occur with a faster deformation rate during higher humidity levels. The combined use of FN and S2 is a recommended way to assess the trend of permanent deformation of asphalt mixtures under different humidity conditions.