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1  Incompatibility between aggregate type and asphalt binder, presence of standing water or water under pressure in the pavement layers, and improper construction practices are some of the many factors that influence moisture susceptibility of flexible pavements. Aggregate surfaces have a higher affinity for polar water molecules than they do for the nonpolar asphalt molecules. Over time, water molecules will tend to break the adhesive bond between aggregate and asphalt, and strip away the asphalt. This will result in a loss of cohesion within the pave- ment matrix and a localized loss of pavement integrity. These localized changes will, over time, lead to larger scale distress. Therefore, departments of transportation (DOTs) employ preemptive measures to address this issue wherever it might be of concern. The objective of this synthesis is to document practices used by state DOTs to prevent or to minimize moisture damage in hot-mix asphalt (HMA) pavements. The synthesis focused on design and acceptance stages of the construction process. The information collected in the process of preparing this synthesis includes ⢠Test methods and pass/fail criteria adopted by state DOTs to assess susceptibility of asphalt mixtures to moisture damage, ⢠Mitigation measures used by state DOTs, and ⢠Knowledge gaps or the need for additional research. To meet the objective of the synthesis, information was gathered from a survey of state DOTs to document the state of the practice pertaining to moisture-induced damage in asphalt mixtures. The survey questionnaire was sent to the voting DOT members of the AASHTO Committee on Materials and Pavements (COMP), comprised of all the state DOT members within the United States, the District of Columbia (D.C.), and one Canadian transportation agency (Ontario Ministry of Transportation). Forty-eight state DOTs, the District of Columbia, and the Ontario Ministry of Transportation responded to the survey, a 96% response rate. For brevity, the term âstate DOTsâ is used throughout this document to collectively refer to all the respondents. As a follow-up to the survey, four state DOTs were contacted by phone to provide additional insights on their approaches to address potential problems arising from moisture-susceptibility, test methods used to assess asphalt mixtures, and countermeasures employed to mitigate the damage. A comprehensive review of perti- nent literature was also conducted. Findings from the survey results indicated that the Modified Lottman Test and Hamburg Wheel-Track Test (HWTT) (AASHTO T 324) were the two most widely used tests to eval- uate stripping potential in asphalt mixtures during both the mixture design stage and the production/acceptance stage. The survey also revealed that the use of anti-stripping addi- tives is required in all states where the predominant rock type was prone to stripping or where moisture-induced damage of pavements was observed in the past. The use of anti-stripping S U M M A R Y Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements
2 Practices for Assessing and Mitigating the Moisture Susceptibility of Asphalt Pavements additives is also required in (a) some specialized mixtures specific to the state, (b) with asphalt + aggregate combinations that are prone to moisture damage, and (c) mixtures that failed to meet the design requirements for moisture sensitivity. Liquid anti-stripping additives (blended at the terminal) were the preferred choice for anti-stripping additive type and application method. Although many state DOTs used the minimum Tensile Strength Ratio (TSR) and/or Stripping Inflection Point (SIP) as required criteria for mixture design approval, the survey respondents also saw a need for an improved laboratory test to identify moisture-susceptible mixtures. Establishment of stronger correlation between laboratory test results and observed field performance, development of improved SIP analysis methods, and guidance on con- struction and quality assurance procedures to prevent moisture damage were the âhot-topicâ issues among the state DOTs that responded to the survey. Other topics of interest to the state DOTs were some of the new and emerging technologies for evaluating the potential for moisture damage in asphalt mixtures. These included the use of Moisture-induced Stress Tester (MiST), surface free-energy (SFE) analysis of asphalt-aggregate combinations, and comparing changes in dynamic modulus of conditioned mixture samples with respect to unconditioned samples.