Moisture Diffusion in Asphalt Binders and Fine Aggregate Mixtures
Moisture damage in asphalt mixtures is a complex phenomenon that involves mechanical, chemical, physical and thermodynamic processes. This damage contributes significantly to the premature deterioration of asphalt pavements, which leads to extra cost in highway maintenance and vehicle operations. One key mechanism of how moisture reaches the asphalt-aggregate interface is by its permeation or diffusion through the asphalt binder or mastic. Different techniques are available for diffusion coefficient measurement of a wide variety of polymersolvent systems. For the asphalt-water system studied, the focus is on two techniques: (i) Fourier Transform Infrared (FTIR)-Attenuated Total Reflectance (ATR) spectrometry and (ii) Gravimetric Sorption Measurements. In the FTIR-ATR experiments, asphalt binders are under investigation. Water shows strong absorption in the infrared region and the FTIR-ATR technique has the ability to monitor both the kinetics of moisture ingress as well as any chemical changes occurring during the test. The changes in concentration can be directly related to change in the absorbance measured during the experiment. The hysteresis of water diffusion in asphalt binders is also monitored through this technique. In the gravimetric sorption experiments, cylindrical Fine Aggregate Mixtures (FAM) were investigated. The gravimetric techniques, which directly follow mass change with time, are among the most used techniques probably because of their simplicity. In this experiment, the Saturated Surface-Dry (SSD) weight of FAM samples at room temperature and at 100 degrees F is monitored until it reaches the equilibrium. The measurements of: (i) water uptake and (ii) the diffusion coefficient were made at both temperatures. A dual mode diffusion model was shown to better represent the diffusion of water through asphalt binders. The rate of moisture diffusion in asphalt binders was proved to be dependent on the history of exposure of the asphalt binder to the moisture. Moisture uptake and diffusivity of water through FAM is dependent on the type of aggregate and asphalt binder used to prepare the FAM.