The influence of thermal segregation in pavement compaction

Aggregate and thermal segregation during asphalt compaction can cause premature failures in asphalt pavements. Although aggregate segregation can be identified by visual observation, thermal segregation is impossible to locate in this manner. A thermal imaging technique was used to evaluate thermal segregation. Continuous profiles of mat temperature were recorded during field compaction to identify possible locations of cold spots and rate of temperature loss. The temperature difference was found to be approximately 14°C immediately after laying the material, with scattered cold spots and faster heat loss at the lane interface and edges. Laboratory investigations on roller-compacted asphalt slabs showed that this difference resulted in 5% to 7% higher voids at the edge compared with the center, indicating inferior compaction. The effect of layer interface was also evaluated, and it was found that air void contents were 6% to 10% higher at the interface. The compaction level in these places was improved when slabs were produced in heated molds (simulating a heated adjoining layer): this result indicated that better compaction could be achieved if temperatures at the interface were elevated while laying the material. Three-dimensional finite element modeling was conducted for transient thermal analysis to evaluate heat loss at the interface when a hot mixture (150°C) was laid on a cold layer (5°C) and on a layer at ambient temperature (22°C). The temperature at the contact path between new and existing layers showed that the inner part of the adjoining layer had considerably lower temperature compared with the upper part, with resulting nonuniform compaction between the top and bottom of the layer.