BACK TO BASICS Asphalt Plant Mix Segregation: Causes and Cures Greg Renegar, Chief Engineer, Astec Industries Mix segregation is a direct cause of premature pavement failure. To give an example of how this occurs, aggregate segregation is the tendency of aggregate—composed of different sizes and masses—to begin separating into groups of similar size particles. At an asphalt plant, segregation occurs most often at points where mix is transferred from one plant component to another. This results in localized areas of the road being unable to withstand the traffic or weather. In the 1980s and 1990s, segregation due to mix and equipment designs was a major problem facing the industry, but modern asphalt plants are designed to minimize segregation. This problem receives less attention today, though is still prevalent on roads worldwide. Several steps must be taken to more fully eliminate segregation at the plant level. The most common type of segregation is “end-of-load” segregation. This is typically visible to the eye, appearing as a localized area of coarser textured mix between each truckload. The coarse areas allow water to penetrate the mix and is often accurately described as the “birth of a pothole.” However, the first place to look for mix separation at the asphalt plant is the drag conveyor inlet. If more mix is on one side of the drag conveyor, there will certainly be a gradation difference across the drag flights. This mix separation must be corrected at the inlet by placing divert plate(s) at the drum outlet/drag inlet. It cannot be corrected later in the process. The second place the mix can separate is at the drag-slat conveyor discharge. This can occur as the mix falls directly into a silo batcher from the drag conveyor or a horizontal drag conveyor that carries the mix to a silo batcher. If mix separation occurs at this point, it must be addressed. Again, divert plate techniques can prevent segregation at this critical transfer point. The third transfer point for potential mix segregation is where the horizontal drag conveyor discharges into a silo batcher. The sole purpose of the silo batcher is to prevent segregation, but several important batcher rules must be followed. • It is essential for the batch size to be as large as the batcher allows. Adjust the devices or controls that determine how much mix the batcher holds. More mix in the batcher is always better. • The batcher must be loaded in the center. Any off-set loading can result in mix separation at this transfer point; that is, there will be more large aggregate on one side of the batcher. One cannot just look at the chute alignment. One must physically view the mix going into the batcher to ensure flow into the center of the batcher. • The mix going into the batcher must go straight down. There can be no horizontal component to the mix flow. Any angle to the mix flow will tend to throw larger aggregate to the far side of the batcher. Again, viewing the process directly will ensure the mix flow is vertical. Gravity Discharge Improper Truck Loading Proper Truck Loading 22 FALL/WINTER 2023
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