BACK TO BASICS Segregation in the Field: Causes and Prevention Todd Mansell, Production Application Specialist, Caterpillar Paving Products Segregation, or the physical separation of fine and coarse aggregates in a mix, will result in pavements improperly compacting in the field and allowing moisture intrusion leading to premature raveling and early pavement failure. Larger rocks, or aggregate, tend to roll to the outside edges of stockpiles, bins, paver hoppers, or in any other situation where the mix is being handled or transferred from one process to another. Separation of large and smaller rocks, or fines, is more prevalent when asphalt mix is dribbled or dropped from heights. Segregation is generally observed on the mat in one of three ways: random patches, repeated patterns, or continuous longitudinal stripes. Identifying the segregation pattern will help narrow down what is causing segregation and allow us to correct it. Understanding these concepts and the importance of moving mixes “in mass” will help eliminate mix segregation. This article will focus on how the paving crew can manage the mix flow through the paver from trucks that are properly loaded at the plant using a three-drop method. From there, these five main areas are important to understand. Unloading Trucks Segregation often occurs at the beginning and end of truckloads. The best method for end-dumping into the paver hopper or the dump head on a material transfer vehicle (MTV) is to raise the truck bed quickly and smoothly until the mix starts to move in mass into the receiving hopper, then stop raising the truck bed as soon as the mass of mix starts sliding into the hopper. We call this “breaking the load.” Do not raise the truck bed too high or too fast, as dumping the entire load this way will overload the paver. The mix will push the truck away from the paver, resulting in a pile of mix on the ground in front of the paver and mix spilling out of the hopper onto the grade (discussed later). Instead, slowly raising the truck bed will result in a pile of coarse aggregate at the tailgate and along the edges of the truck bed. This will be the first material to dribble into the hopper, pass through the conveyor chains, back to the augers, and screed to create a longitudinal stripe or patch segregation in the mat. Once the paver consumes this mix, raise the truck bed at a smooth, rapid pace to maximum height to empty the truck and the remaining mix in mass. Paver Hopper Management Larger stone mixes are more prone to segregation because of the larger rocks and the lower asphalt content in the mix that helps bind the large rocks together. When asphalt mix is dumped from the truck into the paver hopper, larger aggregates roll to the edges and corners. Running the hopper too low between trucks is a common mistake by paver operators, resulting in end-of-load segregation If the hopper level exposes the tunnel openings and slat conveyors, end-of-load segregation will be visible on the road. Keep the tunnel openings and slat conveyors covered at all times. Many paver operators ask how often they should fold or raise the hopper wings. Be consistent and fold after every truckload or not at all. Generally, do not fold the hopper wings with a three-fourths coarse or larger stone mix. With these mixes, it’s better to leave the hopper wings down and allow extra time to clean out the cold mix in the corners of the paver hopper at the end of the day. Mix Feed System Setup Augers, feed sensors, conveyors or flow gates, and manual override switches for the conveyors and augers are collectively referred to as “the mix feed system.” They are set by the paver and screed operators to move asphalt mix from the hopper to the screed in a smooth, continuous manner. 24 FALL/WINTER 2023
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