10 - Moisture damage in asphaltic mixtures

Abstract

The intrusion of water into asphalt pavements is associated with their reduced performance, unexpected failures, and shortened service life. All of these contribute to reduced economic and environmental sustainability of asphaltic roads and will ultimately also affect their social sustainability, considering the importance of a well-functioning and safe infrastructure network. Considering that all roads are constantly exposed to a combination of varying traffic and weather conditions, moisture can cause the start of new or the aggravation of existing damage. Though a natural instinct to mitigate moisture-induced damage may be to avoid the infiltration of water, some mixtures, such as drainage asphalts, are in fact designed to lead the water through the asphalt mixture. But be it open or densely graded, experience has taught us that moisture always finds its way into the pavement. So it is important for pavement engineers to understand the possible mechanisms by which moisture degrades the material components and, ultimately, the structural response of the pavement. Even though the mechanisms related to moisture damage were identified in the early 1970s, they are today still not understood to the extent that we have been able to mitigate the problem. Current moisture damage sustainability evaluation methods for asphalt mixtures are still primarily based on the empirical test methods, and pavement engineers do not have new tools to assist them in preventing moisture damage at large or to make accurate moisture damage predictions. In recent years, however, research in this field has progressed and focused on a more fundamental approach through testing and modeling, in order to obtain more comprehensive knowledge of the moisture damage mechanisms. As such, new characterization techniques to assess moisture sensitivity of components of the asphalt mixture are enabled, thus allowing optimal material combinations to be selected. Moreover, recent advances in computational engineering and physicochemical testing for parameter determination may provide more information on the factors affecting moisture damage, which could lead to more accurate predictions of long-term behavior. In this chapter, an overview is given of the moisture damage mechanisms in asphalt mixtures as we (think to) understand them today. Various focused modeling attempts are discussed, and a discussion on the need for moisture damage susceptibility tests is given. The authors hope that this chapter can be used by the reader to obtain a sense of the current state of the art in the field and to encourage further developments so that advanced characterization and prediction methods will start being used to actively mitigate premature and unexpected moisture damage in asphalt pavements in practice. Though we have tried to be comprehensive in our references, our main aim was to categorize the current developments and possibilities to further stimulate research and transfer to practice. Furthermore, we would like to note the importance of frost heave and freeze-thaw damage as a part of the family of moisture-induced damage mechanisms, which we have not included explicitly in this chapter, as there already exist comprehensive texts on these subjects.