Traditionally, visually identified areas of non-uniform surface texture in hot-mix asphalt (HMA) have been interpreted as segregated mix. The subjective nature of this evaluation has consistently made it difficult to reach a consensus between agency inspectors and contractors as to what is and is not segregation. The National Center for Asphalt Technology has just completed a National Cooperative Highway Research Program Project: NCHRP 9-11, "Segregation in Hot-Mix Asphalt Pavements."
The objectives of this research were to develop procedures for defining, detecting and measuring segregation and to evaluate its effects on HMA pavement performance.
Two primary types of segregation were identified: gradation and temperature. Gradation segregation is the most commonly seen type and can occur as the result of aggregate stockpiling and handling, mix production, mix storage, truck loading practices construction practices and equipment adjustments. Temperature segregation was identified as occurring as the result of differential cooling of portions in the surface of the mix in the haul truck, along the sides of the truck bed and in the wings of the paver.
The following two non-destructive technologies were recommended to detect and measure various levels of segregation:
Infrared thermography—This technology can be used to detect and measure different levels of segregation, however, it cannot distinguish between gradation and temperature segregation types. Infrared thermography is an excellent inspection tool for identifying anomalous areas with large temperature differential which require additional conventional testing; and
ROSAN surface texture measurements—This laser technology can be used to detect and measure different levels of gradation segregation by measuring the surface texture characteristics of the pavement.
The laser device is mounted on the bumper of a vehicle. Ratios of the ROSAN surface texture in segregated areas to that in non-segregated areas were established based on statistically different key mixture properties. Texture rations between 0.75 and 1.15 indicate no segregation, between 1.16 and 1.56, are associated with a low level of segregation, and between 1.57 and 2.09 with medium segregation. Rations above 2.09 indicate high levels of segregation.
