Sulfate-bearing subgrade soils treated with calcium-based stabilizers often experience heaving problems (three-dimensional swell) caused by chemical reactions with the sulfate or sulfide minerals. Two research questions were addressed: ( a) what sulfate content results in deleterious chemical reactions using traditional (no mellowing) lime stabilization and ( b) how effective mellowing, double lime application and increased moisture content are at reducing swell in high-sulfate soils. To determine what concentrations of sulfate are too high for stabilization with lime in Texas, a soil from the Vertisol order was selected for three-dimensional swell measurements. This soil did not contain any detectable sulfates. Two different compounds were added to the soil: sodium sulfate (Na2SO4) and gypsum (CaSO4.2H2O). The sulfates were added to individual samples at concentrations of 0; 1,000; 2,000; 3,000; 5,000; 7,000; and 12,000 parts per million (ppm). The samples were then subjected to a three-dimensional swell test for a minimum of 45 days. Results of systematic swell experiments revealed that sulfate contents up to 3,000 ppm could safely be treated with traditional lime stabilization. Coarse-grained sulfates take longer to swell, but still swell and form deleterious reaction products. Mellowing is effective at treating sulfate concentrations up to at least 7,000 ppm, higher molding moisture contents (2% above optimum moisture) reduce swell better than optimum moisture, and single application of lime reduces swell better than double application. With systematic laboratory experiments, empirical field observations of sulfate limits presented by other investigators were confirmed.
In an effort to construct roads more quickly, high-plasticity index soils stabilized with lime are now routinely compacted the day after mixing. With this practice has come an increasing number of heaves due to soluble sulfates reacting with the lime to form ettringite. Soils with sulfate concentrations below 7,000 to 8,000 parts per million (ppm) can generally be treated with lime. This research was performed to identify stabilizers that can be used with sulfate concentrations above 10,000 ppm. The effectiveness of the stabilizers was determined by the measurement of three-dimensional (3-D) swell reduction and unconfined compressive strength. The researchers evaluated 12 stabilizers, including enzymes, polymers, acids, emulsions, fly ash, and ground granulated blastfurnace slag (GGBFS). Three stabilizers significantly reduced volumetric swell. A polymer and an acid reduced swell by about 8%. GGBFS plus lime reduced swell by 10%. GGBFS plus lime was the only stabilizer that reduced swell, increased strength, and was cost-effective enough for the Texas Department of Transportation to consider as an alternative to lime in high-sulfate soils.
The objective of this study was to investigate the influence of aggregate characteristics and gradation on the skid resistance of various asphalt mixtures. Asphalt mixture slabs with different combinations of aggregate sources and gradations were fabricated in the laboratory. These slabs were polished with a wheel-polishing device developed by the National Center for Asphalt Technology. The frictional characteristics of each slab were then measured by the sand patch method, British pendulum, dynamic friction tester, and circular texture meter. Aggregates used in these mixtures were characterized by petrographic analysis, conventional test methods (acid insolubility, magnesium soundness, Micro-Deval, and British polish value), and the aggregate imaging system (AIMS). In addition, the aggregate gradation of each mixture was described by the two-parameter cumulative Weibull distribution function. Statistical analysis of test results led to the development of a function for predicting the International Friction Index, which is a measure of skid resistance of asphalt mixtures, after different intervals of polishing. The parameters of this function were found to be related to ( a) initial and terminal aggregate texture measured by using AIMS, ( b) rate of change in aggregate texture measured by using AIMS after different polishing intervals in the Micro-Deval, and ( c) the Weibull distribution parameters describing aggregate gradation. This function can be useful for estimating the frictional characteristics of an asphalt mixture surface during the mixture design stage.
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