Segregation is the separation of the course and fine aggregate particles in an asphalt mix. The segregation of the mix can occur at several locations during the mix production, hauling, and placing operation. Segregation can occur as the mix is delivered from the asphalt plant to a surge silo. It can take place as the mix is deposited into the haul truck from the silo. It can also occur when the mix is discharged from the truck to the paver hopper.
Segregation that is evident behind the paver screed can generally take one of three forms: It can consist of areas of coarse aggregate (rock pockets) that occur randomly across the length and width of the layer, it can occur as a longitudinal area along one side of the paver width, or it can be seen transversely across the lane “at the end of a truck-load” of mix.
The operation of the paver can enhance or reduce the amount of segregation that appears behind the screed. If the paver hopper is emptied of mix, if the slat conveyors are visible, and if the wings of the hopper are dumped after each truckload of mix, any course aggregate particles that have collected at the tailgate of the new truckload of mix will be deposited into the bottom of the hopper and then carried directly back to the empty auger chamber in front of the screed. This segregated material will appear behind the screed as soon as the paver moves forward. This transverse segregation, therefore, does not really occur at the end of the truckload but rather occurs at the beginning of the next truckload of mix.
Segregation can be reduced by keeping the hopper full of mix between truckloads. The mass of mix that floods the hopper from the haul truck will be blended with the mix already in the paver hopper. Any segregated material will be further “lost” in the amount of mix that is pulled back to the augers by the slat conveyors and carried under the paver screed. The amount of segregation can be decreased significantly, but not always eliminated completely, by good paver operating techniques.
Pavers should be checked to verify that coarse aggregate does not collect in boxes or areas under the paver. When the “box” becomes full, the course aggregate is discharged causing a rock pocket.
Another source of segregation can occur along windrows of asphaltic concrete when using a pick-up machine. As the windrow is formed by the bottom dump truck, coarse aggregate rolls down the sides of the windrow and collects in a straight line along both sides of the windrow. If the pick-up machine does not pick up all of the mix down to the old roadway, the coarse aggregate will form two “rock pocket lines” down the middle of the roadway.
Segregation can affect pavement durability directly by increasing the air void content of the mix in the segregated areas and increasing the potential for moisture damage. Further, the segregated locations are susceptible to raveling and, if bad enough, to total disintegration under traffic.
Segregation, in the form of rock pockets, longitudinal side-to-side segregation, or transverse truckload-to-truckload segregation, can be detrimental to the long-term performance of the asphalt mixture.