Investigation of Moisture Dissipation in Foam-Based Warm Mix Asphalt Using Synchrotron-Based X-Ray Microtomography

AbstractFoam-based Warm Mix Asphalt (WMA) technologies decrease the viscosity of the asphalt so that it is workable during construction. However, after construction, viscosity increases rapidly as the foam disappears and temperature drops. During the process of dissipation of foam, depending on the environmental conditions, the moisture may not escape and may be trapped inside the mixture. This trapped moisture can cause detrimental failures by breaking the adhesive bonds between the aggregates and the asphalt binder (because of diffusion and freeze/thaw cycles). It is crucial to know how the moisture escapes from the asphalt mixture as the specimen cools down and foam disappears. In this research, dissipation of the moisture from foamed asphalt binders was directly measured and quantified by using a high-resolution synchrotron-based X-Ray Microtomography (XRM) system. Different foamed asphalt binders were instantly frozen by using liquid nitrogen at different times after the initial foaming. The 3D internal structure images of these specimens were acquired by using XRM available at the Argonne National Laboratory (ANL). These images were analyzed and the moisture dissipation processes in different types of asphalt binders were directly quantified. It was observed that the rate of moisture dissipation in high performance grade (PG) (stiff) binders was slower than that of low PG grade (soft) binders. It was also observed that the size distribution of moisture bubbles in the binders varied in different binders.