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48 This chapter summarizes the overall findings of the synthesis. It points out gaps in pertinent knowledge and identifies the topics that could be the subject of additional research, as indicated by survey respondents and elucidated from the literature review. 5.1 Conclusions This synthesis study involved a comprehensive literature review pertaining to moisture- induced damage in asphalt pavements, including the related test methods and mitigation strat- egies, and included a survey of state DOTs. The results of the survey, along with a literature review, were used to assess the state of the practice regarding the test methods (and other mea- sures) employed for assessing moisture susceptibility of asphalt mixtures during the mix design approval and acceptance stages. The major findings of these efforts and conclusions drawn can be summarized as follows: ⢠A vast majority of state DOTs require either the Modified Lottman Testing or the Hamburg Wheel-Track Testing as a part of their mix design approval process. This requirement can be a state-DOT- or agency-modified version of the standard test method or the original standard method of testing. ⢠Minimum TSR was the primary pass/fail criterion used for determining the potential for moisture damage, with or without an additional requirement for minimum values of the dry or wet retained strengths. In the case of HWTT, the minimum value of the SIP and maximum allowable rut depth were used as pass/fail criteria for evaluating moisture susceptibility. ⢠This same was also true among the state DOTs that required testing at the production/ acceptance stage. ⢠The use of anti-stripping additives was required in all states where the predominant rock type was susceptible to stripping. In states with mixed rock types, the use of anti-stripping additives was required for certain special types of mixtures/aggregates or it was based on test results from the design approval stage. Liquid anti-stripping additives and hydrated lime were the two most widely used mitigation measures. ⢠Blending of liquid anti-stripping additives at the asphalt supplier terminal was the predomi- nant method of addition among states where LAS were used. Addition of hydrated lime was predominantly accomplished via pugmill or drum injection. ⢠The predominant rock types to produce aggregates used in flexible pavements in the United States are carbonates and igneous rocks, followed by sandstones, siliceous sands, and gravels. ⢠The approach based on using the use of Surface Free-Energy to address moisture-susceptibility and the use of MiST were two of the technologies that generated high interest within state DOTs. C H A P T E R  5 Summary of Findings
Summary of Findings 49Â Â 5.2 Gaps in Knowledge and Future Research Needs Although substantial research has been conducted on methods to investigate moisture- susceptibility of asphalt mixtures and its mitigation, there still appears to be a need for a quick, easy, and improved way to assess mixtures and materials that are prone to stripping. Among the existing test methods, the issue of the correct determination of Stripping Inflection Point appears to be clouded. HWTT data do not always yield a well-defined SIP, so researchers resort to other methods to estimate SIP, which can yield inconsistent results. These other methods can be highly subjective (e.g., by visual observation of the data plot) or more robust methods (e.g., by using manufacturersâ software and agency-developed software). As such, this topic warrants an investigative study. State DOTs also identified a need for better correlation between the results of preliminary lab- oratory testing and observed field performance in terms of stripping resistance. Some of the tests currently in use can accurately delineate moisture-damage resistant mixes from the moisture- susceptible mixes. However, better test methods are needed that can more accurately identify the marginal mixes. This point may be addressed by applying the surface free-energy concept to aggregate-asphalt combinations as a tool for assessing the resistance to moisture-induced damage. Work done by researchers thus far on SFE appears to be promising, but the technique suffers from the lack of data on correlation of tested parameters with the field performance of the mixtures. The appeal of this approach is that, unlike the empirical methods currently in use, it can offer a scientific rationale for the observed behavior. If a test based on this approach can be simplified for ease of local adaptation, it will offer a powerful performance prediction tool for use by the state DOTs. Additional research into uniform construction practices and quality assurance procedures to prevent moisture-induced damage was also identified as significant by the DOTs. The impor- tance of design parameters such as pavement permeability and density in states where moisture damage is of concern was also identified as a topic needing further evaluation.
