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

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This report represents task 2 of the Minnesota Local Road Research Board (LRRB) project number (Investigation) 2010-042, entitled, "Stripping of Hot Mixed Asphalt Pavements (HMA) under Chip Seals". The goal of this specific task report is to identify, or develop a methodology that can be used to asses potential at -risk streets before they are treated with a surface treatment. The preliminary methodology being investigated by the research team involves testing the in - place pavement for air void content, or permeability (either in -situ or laboratory). If the pavement is found to have excessive permeability and/or air void content, then caution should be exercised before placing a chip seal surface treatment; a fog seal surface treatment may be more appropriate. These recommendations are preliminary and need to be validated with more testing of laboratory and plant produced (in -place) pavement mixtures in accordance with the work plan. Based on previously completed forensic investigations, and survey responses the research team has hypothesized that low density (high -interconnected air voids) are a contributing factor to the observed stripping distresses. Thus, the experimental plan investigated the influence of various air void contents (7, 10 and 14%) on a mixture's susceptibility to moisture induced damage. The mixtures susceptibility was tested with: permeability tests, Mn/DOT modified Lottman, Asphalt Pavement Analyzer (APA) tested under wet and dry conditions, and a modified Iowa Boiling Test. Permeability testing indicated that the mixtures became significantly more permeable when the air void content was near 14%. The Lottman test did not show any visual evidence of stripping and all tensile strength ratios (TSR) were above 70%. However, the mixtures with the highest air void content had the lowest TSR at 74% and the mixture with the lowest air void content had the highest TSR at 84%. The change in air void content had the most dramatic influence on tensile strength values; a doubling of the air voids from 7% to 14% corresponded with a reduction of 52% and 58% in the dry and wet tensile strengths respectively. APA test results were mixed as the greatest difference in rutting rates between 'wet' and 'dry' testing conditions were at the 10% and 7% air void contents; the mixture with 14% air void content showed little difference. None of the mixtures showed signs of visual stripping. In the modified Iowa boiling test, the specimens with 14% air voids lost an average of 12.2% of their initial weight and the specimens with 7% air voids lost an average of 3.4% of their initial weight. In addition, empirical observations after the test showed that the specimen with the higher air voids (14%) was much less intact than the comparable specimens with lower air voids (7%). There was no test that explicitly showed asphalt stripping away from mixtures with higher air voids; however, the results of the laboratory testing did indicate that the higher voids do contribute to reduced pavement durability through reduced strength, increased susceptibility to rutting and increased permeability. The permeability and the modified Iowa Boiling test results indicate that pavements with higher air void contents may be more susceptible to moisture induced damage than pavements constructed with the proper amount. These factors will also reduce the life of a chip seal that's placed on top of a compromised pavement. Future testing within this project will entail conducting the same tests (permeability, Mn/DOT modified Lottman, wet and dry APA, and modified Iowa Boiling) but on sealed specimens of varying density to more closely simulate field conditions. In addition, to the research team intends to modify the laboratory Permeameter to examine whether or not moisture (from the bottom) can penetrate a chip seal or a fog seal surface treatment. B-5