By James A. Scherocman, P.E.
Segregation in a hot mix asphalt (HMA) mixture can be defined as the separation of the coarse aggregate particles in the mix from the rest of the mass. The segregation can take one of three forms—random, side-to-side or longitudinal, and truckload to truckload.
Each type of segregation is caused by a different problem or problems. But each type of segregation affects the long-term durability of the asphalt pavement concrete structure.
This article deals with truckload segregation, and more specifically, with best practices for using end-dump trucks.
Segregated areas in the surface of the pavement have a rougher texture than the surrounding pavement area. In addition, the density of the mix is much lower in the segregated locations compared to the density of the HMA mix in nonsegregated areas.
Pavement deterioration of the segregated areas in the form of raveling typically occurs quickly under traffic. With more time and more traffic loading, the raveled areas can increase in both size and depth, forming potholes in the pavement surface. With additional time and traffic, it is possible for the raveling to progress completely through the pavement layer.
Loading the Haul Truck
Truckload segregation appears typically as two very rough textured areas in a transverse direction, one on each side of the centerline of the asphalt paver. The primary cause of truckload segregation is how the HMA mix is delivered from the silo into the haul truck.
Segregation of the mix occurs just as segregation of the aggregate occurs when the material is dropped on top of a conical pile. The largest aggregate particles in the mix roll down the sides and collect at the bottom of the pile.
Truckload segregation is really a combination of two factors. The first portion consists of the segregated material which comes out of one truck last—the large aggregate that collects at the front of the truck bed. The second portion consists of the segregated material that comes out of the next truck first—the large aggregate that collects at the tailgate of the truck bed. Since most end-dump trucks tend to be loaded front of center for weight distribution, the majority of the segregation comes from the large aggregate particles that collect at the back of the truck. In most cases, therefore, truckload segregation is more “beginning of the next load” compared to the “end of the first load.”
Eliminating Truckload Segregation
In order to completely eliminate truckload segregation, it is necessary to load the truck correctly. This means that a normal tandem or triaxle truck needs to be loaded with three drops of mix instead of one large drop. The first drop is immediately next to the front bulkhead of the truck bed—as far forward as reasonably possible. This process reduces the distance that the coarse aggregate particles can roll to the front of the truck bed, and thus significantly reduces the amount of segregation that will occur during the loading operation.
Then it is necessary for the truck driver to pull the truck forward so that the second drop can be deposited into the truck bed adjacent to the tailgate. This process reduces the distance that the coarse aggregate particles can roll to the tailgate. The truck driver then needs to move the truck backward so that the third drop of mix can be made into the center of the truck bed, between the first and second drops. Properly loaded, the haul truck will have mix more than half way up the height of the tailgate.
Unloading the Haul Truck
If the haul truck is properly loaded—using multiple drops of mix into the length of the truck bed with mix against both the front bulkhead and against the rear tailgate—unloading the truck should not create segregation problems.
When the tailgate is opened, a mass of mix will flow from the truck bed into the paver hopper. As the truck bed is raised, the mix in the truck bed continues to move in a mass. In this case, by loading the truck bed correctly, the largest aggregate particles in the mix will not separate from the rest of the mix.
If the haul truck is loaded improperly and the large aggregate particles in the mix have rolled both to the tailgate and the front bulkhead of the truck, the segregation problem has already occurred. The problem can be minimized, but probably not completely eliminated, by using a different procedure to unload the haul truck. In this latter case, the tailgate on the truck bed should remain closed and the truck bed should be raised.
Raising the Bed
The bed should be raised high enough for the mix to shift in the bed and move toward the tailgate. This process will add more mix on top of the segregated material at the tailgate. After the mix has shifted, the tailgate can be opened.
With the bed up in the air and with the additional mix moved against the tailgate, the combined mix will be moved in a mass into the paver hopper. Some or all of the segregated material will be blended into the rest of the mix and the amount of segregation which will occur behind the paver screed will be significantly reduced. While it is important to unload the haul truck correctly, it is more important to load the truck bed properly so that the segregation of the mix does not occur in the first place.
Rapid Stop, Rapid Start
The paver hopper should remain half full between truckloads. The amount of mix which remains in the hopper of the asphalt paver when the truck bed of a haul truck is empty should be above the bottom of the flow gates at the back of the paver hopper or above the opening for the slat conveyors at the back of the hopper if the paver is not equipped with flow gates.
If the paver is half full, and if there is segregated mix at the tailgate of the haul truck, there is an excellent chance that the segregated material will blend into the mass of mix already in the hopper. The more mix in the hopper, the greater the chance to “lose” a major portion of the segregated material within the mix.
Keeping the hopper half full between truckloads of mix can be easily accomplished by stopping the paver quickly once the haul truck bed is emptied—rapid stop. Ideally, the bed of the new truck should be partially up in the air and the mix should be made to shift against the closed tailgate. When the newly arrived truck bed is in the proper position, the tailgate should be opened. The HMA will then be delivered in a mass into the half full hopper. The paver then returns to its original speed quickly—rapid start.
If the paver hopper is emptied between truckloads, the segregated mix at the tailgate passes directly through the paver hopper and onto the augers. When the paver hopper is empty, the amount of mix on the augers will be significantly reduced. This reduces the force on the leading edge of the screed and results in a low spot in the pavement surface. The segregated mix will be deposited on the augers in the low spot in the pavement surface. Truckload segregation is created—one location of very rough texture on each side of the centerline—at the location of the slat conveyors on each side of the machine.
Not Emptying the Wings
It has already been noted that the paver hopper should remain half full between truckloads. In order to maintain this level at all times, it is recommended that the wings on the paver not be raised or emptied. Two different procedures can be used. First, the initial mix that flows into the wings of the hopper at the beginning of each day can be allowed to remain in the wings all day long.
This means that wings are not raised at any time during the day. The mix that is collected in the corners of the hopper, and remains in the wings all day long, is simply wasted at the end of the day. Depending on the size of the paver, perhaps one to two tons of mix will collect in the wings and be unable to be laid.
Second, and more economically, the capacity of the two corners of the hopper can be reduced by fitting the hopper with two cutoff plates. With the cutoff plates in place, no mix can be collected in the corners of the hopper, and the hopper can be kept half full at all times. This will greatly reduce any segregation that might occur. The cutoff plates can be bolted into the sides of the hopper and can be easily removed when necessary.
Truckload segregation is caused by the manner in which the haul truck is loaded. If the truck is loaded in one drop of mix and a conical pile is formed inside the bed, the largest aggregate particles in the mix will roll downhill and collect at the front and at the tailgate of the truck bed.
Truckload segregation can be eliminated by simply loading the haul truck correctly. One drop of mix should be deposited from the surge silo or pugmill as close to the front bulkhead on the truck bed as possible. The truck driver should then pull the haul truck forward and the next drop of mix deposited as close to the tailgate on the truck bed as possible. The truck should then be backed up and additional drops of mix placed between the first and second amounts of mix. By loading the truck using the proper multiple drop procedure, the distance that the coarse aggregate particles in the mix can roll will be greatly reduced and segregation of the mix will be prevented.
In addition to loading the haul truck correctly, keeping the paver hopper half full between trucks, practicing rapid stop/rapid start paver operations, and using cutoff plates in the hopper to eliminate the need to raise the wings will be very beneficial in reducing any amount of segregation that may have occurred during the truck loading process.
Jim Scherocman is a consulting engineer specializing in pavement design and construction. He can be reached at (513) 489-3338.