Identifying Segregation in Hot Mix Asphalt Pavements Using Rolling Nuclear Gage Measurements and Infrared Imaging

Previous research indicated that density, gradation, and asphalt content change due to segregation. A rolling nuclear gage capable of measuring both density and hydrogen count was evaluated for its ability to detect and measure both density and asphalt content. Eight projects (four recently constructed pavements and four during construction) were used to evaluate this instrument. Longitudinal density profiles were obtained for each project at transverse quarter points across the lane. Additional testing included infrared thermography and traditional destructive testing of cores. Visual observations of texture differences were used to identify areas with no, low, medium, and high levels of segregation. At least three core locations were then identified from each of these areas; cores were taken always from the longitudinal paths tested with the nuclear gage. Cores were used to determine changes in air voids, asphalt content, aggregate gradation, and mix stiffness as a result of the various levels of segregation. Results indicate it was difficult to use air voids alone to detect and measure segregation, regardless of how densities (air voids) were determined (i.e., nuclear density or core properties). The rolling nuclear density measurements showed that while densities may be uniform longitudinally, there were significant differences in densities transversely for several projects. A second type of temperature segregation was found using this gage in combination with the infrared thermographs. During paving operations, the paver may be stopped to change or wait for the haul truck or to adjust the equipment. The mix immediately behind the paver cools off but cannot be compacted since the paver is in the way of the roller. This results in a localized transverse region of very low density.