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3Â Â As advances are being made, or have been made, in testing and design of asphalt mixtures to control rutting and cracking problems, the durability of asphalt mixtures is becoming a critical issue. Moisture-induced damage has been a perennial problem for decades, but the more imme- diate and, in some respects, simpler problems of rutting and cracking seem to have taken pre- cedence. Part of the reason for such development is related to the fact that moisture-induced damage is a complicated issue which involves chemical, physical, and thermodynamic factors. For example, topics such as the analysis of the surface energy of aggregates and asphalt binders (using atomic force microscopy), the use of Universal Sorption Method, and Wilhelmy plate analysis (and other techniques) have been the subject of many research efforts. Although the results obtained from such studies looked promising, most of them have not been implemented because of such factors as the complexity of the underlying theory, the complicated nature of the testing methods, and a lack of correlation with field performance. Moisture damage is an expensive pavement distress because it often occurs deep within the pavement structure. Repairing deep damage in pavements requires removing and replacing mul- tiple layers of asphalt mixture. This is also a time-consuming type of repair that creates traffic congestions, creates safety problems, and increases user costs. With the increasing tendency toward pavement preservation practices implemented by many DOTs, it is vital to address mix susceptibility to moisture damage early, preferably during the design and construction stages. Such an approach will substantially reduce future rehabilitation costs and increase the service life of the pavements. Finding ways to mitigate or prevent moisture damage is, therefore, critically important to state DOTs. Although, as mentioned earlier, moisture susceptibility has been the subject of extensive research over the past decades, no universally accepted techniques to identify susceptible mix- tures or material combinations have been identified or widely adopted. Nor is there a consensus regarding the types and methods of application of anti-stripping additives used to prevent or to retard the actual damage in the field. However, many state DOTs have adopted specific test methods or material specifications in attempts to address the issue, with varying levels of suc- cess. Some state DOTs require the use of anti-stripping additives in all of their mixtures. This approach is implemented as a proactive, preventive measure, regardless of the type of aggregate or the type and source of asphalt, and is implemented without preliminary mixture testing. The most common test method used to identify moisture-susceptible mixtures is the Modified Lottman Test, though it is not used in every state. Recently, several states have adopted the Hamburg wheel- tracking device or other tests to assess the potential for moisture-induced damage. This synthesis focuses primarily on the various test methods and criteria used to assess and mitigate moisture damage in asphalt pavements, during both the design and acceptance stages. To this end, a comprehensive, nationwide survey of current methods and practices used or C H A P T E R Â 1 Introduction
4 Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements recommended by various state DOTs in their mixture design and acceptance practices, as well as a comprehensive review of emerging technologies, was conducted and the results were analyzed. In addition, an extensive literature review and follow-up interviews with select DOTs were con- ducted to cover a range of approaches to deal with moisture damage issues. 1.1 Background Loss of adhesion between the aggregate and the surrounding asphalt binder is the direct con- sequence of moisture susceptibility of certain asphalt mixtures, which results in loss of strength and accompanying pavement distress. The bond between the aggregate and the surrounding asphalt film is influenced by the physical and chemical properties of these two materials, and is negatively impacted by the presence of water. Physical properties of interest include the follow- ing: the surface texture of aggregate, aggregateâs porosity, surface cleanliness, moisture content of the aggregate, and so forth. Some of the chemical properties that influence adhesion include: surface tension between asphalt and aggregate, the chemical nature and composition of the asphalt and aggregate, and asphalt viscosity (1, 2, 3, 4). In addition, adhesion is also influenced by the length of mixing time and temperature. During the mixing process, asphalt penetrates the aggregateâs surface pores and irregularities and coats the particles, thus forming a stable matrix consisting of asphalt binder, dust particles, fine aggregate, and coarse aggregate. The presence of moisture in the microscopic features present on the aggregate surface can disrupt the bond between the asphalt film and the aggregate. Asphalt binder has a stronger affinity to aggregate carrying a predominantly positive surface charge (e.g., limestone or dolomite) than to aggregate carrying predominantly negative surface charge (e.g., basalt or quartz) (1). While many theories exist that either partially or fully explain the nature and mechanism of the cohesive and adhesive bonds between asphalt and aggregate and the influence of moisture on the bonding mechanism, its effect on pavement damage is not disputed in the industry today. Grain-to-grain interlock, aggregate frictional resistance, and adhesion between aggregate and the surrounding asphalt film are the main factors responsible for the strength of the HMA (2). Moisture-induced damage to the pavement usually begins with stripping action (removal of asphalt film coating the aggregate) caused by a breakdown in the bond between asphalt film and aggregate. This, in turn, leads to the development of localized weakened areas with pothole formation, cracking, or ravelling. Over time, these localized forms of damage lead to more wide- spread distress and may lead to the loss of structural integrity. Based on a 2018 survey of U.S. DOTs and researchers, Silvia Caro (5) reported that 57% of the 39 respondents indicated that moisture-related damage affected pavement durability in their states. Many state DOTs have test specifications or follow practices aimed at the control or pre- vention of moisture damage in their mixtures. In the majority of states where moisture-induced damage is prevalent, the use of anti-stripping additives is routine in all mixes. Understanding the role of these and other additives, the required dosages, and correlations between lab testing and field performance appears to be of ongoing interest. 1.2 Synthesis Approach The first chapter presents a brief background on the synthesis topic and elucidates the need for information on the practices adopted by state DOTs to mitigate moisture damage in asphalt pavements. The objective of this synthesis was to document current DOT practices used to assess and mitigate moisture damage, both during mixture design and acceptance processes, as well as to highlight emerging technologies that the state DOTs consider worth exploring in the future. This was accomplished by summarizing the results of comprehensive review of pertinent
Introduction 5Â Â literature, state DOT-modified specifications (if different from the standard specifications), and analysis of results of a survey questionnaire sent to the voting members of AASHTO COMP. The survey and follow-up interviews focused on investigating how decisions are made regard- ing establishment of pass/fail criteria, assessing the extent of modification of standard tests by state DOTs, and determining the rationale behind exceptions, if any. Information about the pre- dominant aggregate types used in asphalt pavements, additives allowed and other mitigation measures (e.g., control of field density at joints and within mats) was also collected. Informa- tion about state-conducted or state-sponsored research into moisture susceptibility was also collected. The second chapter is a summary of literature findings on various means of identifying moisture-susceptible materials and mixtures and limiting their impact on pavement perfor- mance. According to information presented to the TRB Committee AFK40 in 2018 by Silvia Caro (7), over 180 papers in the previous 10-year period have been published in the area of moisture damage. The TRID database was the main source of references used in preparing this synthesis. Other databases, Google search, and reference lists in individual reports and journal publications were also used to identify relevant documents. The third chapter summarizes the DOT responses to the survey questions. Representatives from 48 states, the District of Columbia and one Canadian province responded to the survey questionnaire. For brevity, the term âstate DOTsâ is used throughout this synthesis to collec- tively refer to all respondents. Copies of the questionnaire and the detailed tabulated survey responses are presented in the appendices. The fourth chapter includes the review of approaches taken by four different DOTs to counter susceptibility to moisture damage in their flexible pavements. The fifth chapter summarizes the overall findings of the synthesis, presents lessons learned, and describes identified gaps in knowledge related to moisture sensitivity of asphalt mixtures and pavements. The synthesis concludes with references, a bibliography, a list of abbreviations, Appendix A (Survey Ques- tionnaire), and Appendix B (Tabulated Survey Responses).
