Agenda - Council - 02/11/2020 - Laserfiche WebLink

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CHAPTER 4: CONCLUSIONS AND RECOMMENDATIONS 4.1 CONCLUSIONS The field and laboratory work conducted in this project represents an extensive study of chip -sealed pavement performance in Minnesota. In spite of this effort, many concerns surrounding the behavior chip -sealed pavements remain unresolved. First, the study was not able to verify the conclusion of earlier MnDOT research that high air voids is the primary cause of stripping (Wood & Cole, 2013). High air voids were not observed in the locations used for the study; furthermore, not all chip -sealed pavements considered in the study exhibited signs of stripping. The latter observation agrees with municipal experience: survey data indicated that 74 percent of city engineers did not identify stripping in chip seals as a major issue. The study was also unable to identify mix design or construction practices that correlated with the incidence of stripping. That is, based on the results of this study: • There does not appear to be direct relationship between pavement density and stripping under chip seals. • There does not appear to be a direct relationship between any specific bituminous mixture, contractor, geographic location, or year of construction and this distress. The main conclusion of the study is that while stripping under chip seals is a concern, it is not a concern that can be addressed with a simple mix design or paving rule of thumb. Finally, the distress was observed to be a complex phenomenon that may be caused by the presence of the chip seal as much as it is the composition of the underlying asphalt. Perhaps the matrix of binder and densely packed, angular chip creates too tight of a "seal" on the pavement, thereby trapping moisture and vapor and creating conditions that become damaging under freeze -thaw action. It is hoped that the field and laboratory data collected in this study will be of value to future work that examines this phenomenon in detail. 4.2 RECOMMENDATIONS Based on field observations and discussions with state, city, and county engineers, the following recommendations and observations are provided on the use of chip -seal treatments and the incidence of stripping under chip seals. As the study was unable to point to an overwhelming cause for the stripping behavior, the recommendations are intended to be a starting point for future discussions and studies of important issues in the use of chip seals. First, it was observed that stripping behavior under chip seals may be due to trapped moisture and water vapor between the chip seal and the asphalt pavement. This phenomenon would be similar to 37