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Chapter 3. Conclusions <br />This report represents task 2 of the Minnesota Local Road Research Board (LRRB) project <br />number (Investigation) 2010-042, entitled, "Stripping of Hot Mixed Asphalt Pavements (HMA) <br />under Chip Seals". The goal of this specific task report is to identify, or develop a methodology <br />that can be used to asses potential at -risk streets before they are treated with a surface treatment. <br />Based on previously completed forensic investigations, and survey responses the research team <br />has hypothesized that low density (high -interconnected air voids) are a contributing factor to the <br />observed stripping distresses. Thus, the experimental plan investigated the influence of various <br />air void contents (7, 10 and 14%) on a mixture's susceptibility to moisture induced damage. The <br />mixtures susceptibility was tested with: permeability tests, Mn/DOT modified Lottman, Asphalt <br />Pavement Analyzer (APA) tested under wet and dry conditions, and a modified Iowa Boiling <br />Test. <br />Permeability testing indicated that the mixtures became significantly more permeable <br />when the air void content was near 14%. The Lottman test did not show any visual evidence of <br />stripping and all tensile strength ratios (TSR) were above 70%. However, the mixtures with the <br />highest air void content had the lowest TSR at 74% and the mixture with the lowest air void <br />content had the highest TSR at 84%. The change in air void content had the most dramatic <br />influence on tensile strength values; a doubling of the air voids from 7% to 14% corresponded <br />with a reduction of 52% and 58% in the dry and wet tensile strengths respectively. APA test <br />results were mixed as the greatest difference in rutting rates between 'wet' and 'dry' testing <br />conditions were at the 10% and 7% air void contents; the mixture with 14% air void content <br />showed little difference. None of the mixtures showed signs of visual stripping. In the modified <br />Iowa boiling test, the specimens with 14% air voids lost an average of 12.2% of their initial <br />weight and the specimens with 7% air voids lost an average of 3.4% of their initial weight. In <br />addition, empirical observations after the test showed that the specimen with the higher air voids <br />(14%) was much less intact than the comparable specimens with lower air voids (7%). <br />There was no test that explicitly showed asphalt stripping away from mixtures with <br />higher densities; however, the results of the laboratory testing did indicate that the higher air <br />voids do contribute to reduced pavement durability through reduced strength, increased <br />susceptibility to rutting and increased permeability. The permeability and the modified Iowa <br />Boiling test results indicate that pavements with higher air void contents may be more <br />susceptible to moisture induced damage than pavements constructed with the proper amount. <br />These factors will also reduce the life of a chip seal that's placed on top of a compromised <br />pavement. Future testing within this project will entail conducting the same tests (permeability, <br />Mn/DOT modified Lottman, wet and dry APA, and modified Iowa Boiling) but on sealed <br />specimens of varying density to more closely simulate field conditions. In addition, to the <br />research team intends to modify the laboratory Permeameter to examine whether or not moisture <br />(from the bottom) can penetrate a chip seal or a fog seal surface treatment. <br />B-18 <br />