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

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Chapter 2. Research Methodology Task 1 In this task surveys were sent to Cities and Counties using the Survey MonkeyTM tool to determine the extent of the stripping under chip seals in Minnesota. See Appendix A for a copy of the Interim Report for Task 1 submitted in November 2010. This report includes all the questions asked and results of the survey. Sixty-six responses were received. More than 60 percent of the respondents stated their agency was experiencing stripping under chip seal. Based on these findings and interviews with select cities the next step was to determine the cause of the stripping problems. One observation made was that streets suffering from stripping issues had density variability. Areas close to the stripped areas had high air voids. When coring, complete cores in the areas with low density and high air voids were not able to be obtained as they fell apart. Based on preliminary field sampling the following hypothesis was developed: Low density may cause asphalt paving mixes to stripping to strip because of this low density and high air voids. Task 2 This task looked at stripping test methods and selected tests to determine if stripping can be determined. To test the laboratory methods selected, a level 3 super pave mix that has been proven to not be prone to stripping was selected. The HMA samples were compacted at three (3) levels of air voids (density), 7, 10, and 14 percent air voids. Based on these tests the higher the air voids or lower density the HMA suffered more stripping tendencies. This finding supports the hypothesis that low density and air voids may cause stripping. Task 3 In Task 3 we cored and tested the cores from city streets experiencing stripping. A group of streets where selected to be sampled for lab testing to validate findings from Task 2 and determine what causes stripping of the chip seals on streets. Efforts were made to develop field test requirements that could be performed to determine if the street is a good candidate for a chip seal, i.e. will not strip 2-4 years after placement. The Nuclear Density Tester (NDT) and NCAT Field Permeameter were used on streets without a chip seal. Lab testing on the cores taken from both streets with and without a chip seal included the following; air voids (ASTM T269, T166 & T209) and lab permeability (ASTM PS 129-1). The data from the city's streets correlates with the finding in Task 2. As air voids increased and density decreased the chance of stripping also increased. The best method to check for variability in density in the pavement is NDT. A second hypothesis that was brought forward was the one could fog seal the street with light application of asphalt emulsion and that it would stop water infiltration into the mat but would allow water vapors to escape similar to TYVEK® house wrap does for homes. 3