The hot mix asphalt that is loaded into a truck at the plant, delivered to the jobsite, then transferred from the truck to the paver is probably the most-important element in a successful paving job. Most paving contractors rely on end-dump trucks to get that part of the job done, but the process can be fraught with challenges, which can affect the quality and even profitability of the finished paving job.
After arriving at the jobsite, trucks usually have to wait in line to deliver mix to the paver. Once it’s their turn, they unload the mix by raising the truck bed, and the mix empties into the hopper. It’s this process that can result in pavement defects, stops and starts, and uneven mix distribution if not properly handled.
“Unfortunately, the guy dumping the truck is often the brand-new guy on the crew,” says John Ball, Top Quality Paving. “For some reason crews think that the guy dumping doesn’t have to know a lot about paving, that all they have to do is raise the bed up and take it down. This couldn’t be further from the truth.
“We need to make sure we move the material as a mass, and that means the truck driver needs to be trained in the paving operation,” Ball says. “The driver needs to know how to raise the bed, but not raise it too high or too quickly. The driver has to know how to handle the brakes, how to keep the hopper full without running out, how to keep the tires in contact with the front of the paver, and how to make sure the tires aren’t dragging – especially when paving on top of gravel where dragging tires will create a windrow that results in a bump in the paving.”
Here are 19 tips to help make sure the hot mix you’re getting delivered makes it onto the ground so you can construct the best mat possible.
Before the Job
1. Apply a Release Agent. Ball says most contractors know that a release agent prevents the mix from sticking to the truck bed, but many still use diesel fuel instead of any of a number of products designed specifically to be used as a release agent. Not only has use of diesel fuel as a release agent been banned under the Resource Conservation and Recovery Act of 1976, it’s environmentally unacceptable, and its use will compromise the hot mix asphalt, causing it to break down and resulting in a poor-quality mat. Ball says the release agent should be applied evenly on the bottom and inside sides of the truck bed; avoid puddles of the release agent on the floor of the bed.
2. Load the truck correctly. Ball says that improper loading of material at the asphalt plant can result in segregation of the mix and difficulty in dumping the mix into the hopper. Drivers should avoid single-drop loading from the plant, which sets the stage for segregation. He says the best way to prevent segregation and assure an easier transfer of material into the paver hopper is to use multiple-drop loading at the plant. To do this the driver of the truck pulls forward as a pile of mix is dropped into the truck bed. Small trucks might only use two piles where large dump trucks might contain three piles.
3. Tarp the truck bed. Even when travelling short distances from plant to jobsite, it’s important to tarp the load. The tarp should cover the bed and overlap the end and sides and it should be tied down tight. Tarping helps the mix retain heat and protects it from the elements – cool wind or rain. It also helps keep the mix warm if the truck has to sit in line waiting its turn at the site. Tarping the mix also prevents a very thin crust from developing on the mix surface. This “crusting” can lead to segregation.
4. Review the plant ticket. When drivers pick up the ticket at the plant, make sure to check the time and load order so they know what order they need to line up in at the site. Also check tonnage which tells both their load and provides a running total.
5. Line up in order – and stay in order. Trucks need to stay in order in line because that way the timing and temperature is consistent for the paver. “If the trucks are out of order, if truck four goes before truck two, the temperature of the mix in truck two will be significantly cooler by the time it’s placed, meaning it won’t go through the paver as uniformly and it will be more difficult to compact properly,” Ball says.
Before Paving Starts
6. Designate responsibility. Ball says a “truck dumper” person should be responsible for the interaction of the haul trucks and the paver. This person needs to make themself visible at all times to the driver of the dump truck, so wearing a safety vest is a must. This is obvious for safety reasons but also makes it easier for the driver to see them.
7. Check for obstacles. This can be the job of the truck dumper or the foreman, but this must be done before the paving starts. Make sure there are no wires or tree branches overhead and along the upcoming paving pass that would prevent the truck bed from being raised as needed.
8. Know each truck that’s dumping the mix. Does the tailgate have a chute or not? Is it a manual, air, or hydraulic gate? The hooks on each are different so make sure you know how to open them.
Backing Up the Haul Truck
9. Maintain eye contact with the driver. Ball says the truck dumper always needs to be able to see the eyes of the truck driver. “They need constant eye contact with the driver not only so they can direct the driver when to stop and start but also so that if the truck suddenly accelerates, the truck dumper can move out of the way,” Ball says. “Eye contact is a safety protection, too.”
10. Don’t bump the paver. Ball says the dump truck should stop about 1 ft. from the paver, then the paver operator should start the machine moving forward, picking up the stopped truck. The key to this process is that the paver picks up the truck instead of the truck backing into and bumping the paver. This approach will reduce mat roughness and screed marks.
11. Center the truck. The truck dumper should step back behind the paver to make sure the truck is centered in the middle of the paver so the mix dumps in evenly across the hopper. “You will only get even distribution on the screed if the material is dumped evenly across the hopper,” Ball says. “If the truck is in the middle of the paver, you’ll have a better chance of getting mix spread evenly across the hopper. It also helps if the screed is evenly spaced, which will help draw the mix back evenly.”
12. Examine the position of mix in the truck bed. Ball says the truck dumper should look to see how the load is positioned in the truck. He says where the mix is positioned determines how it’s going to flow into the paver and how quickly to raise the truck bed. If the mix is loaded properly – if it’s in two or three piles, centered between the sides -- it’s been loaded properly and it will flow smoothly as the bed is raised slowly (assuming it’s at the correct temperature). But if the mix is all at the end of the truck bed, it will rush out too quickly if the bed is raised too fast.
Raising the Truck Bed
13. “Break” the mix. The hot mix asphalt has been sitting in piles for the trip from the plant to the site – even longer if the truck had to wait in line once arriving at the site. The driver should raise the truck bed just enough so the mix “breaks” and slides against the tailgate. This helps keep the mix together and prevents segregation. Make sure the tailgate is closed!
14. Transfer the mix. Once the mix has gathered against the tailgate, open the gate, and raise the bed as needed to transfer hot mix asphalt to the hopper. Make sure the gate is unlocked before the driver begins raising the bed. “If the gate isn’t open and the driver lifts the bed, the mix will dump out too fast and spill into a pile in front of the paver,” Ball says. “The weight of the mix will push the truck and paver apart -- and you’ll have a big pile of mix on the ground. Then what usually happens is the paver runs over the pile, creating a bump in the mat.”
15. Make sure the paver stays at the same speed. “The paver needs to maintain a certain speed,” Ball says. “You can’t be going from 25 fpm, to 15 to 40 fpm. The driver of the truck can’t adjust his brakes that quickly so a paver changing speeds means the truck loses contact with the paver. If the paver speeds up the driver has to release the brakes at the right time to prevent mat issues; if the paver slows down it will separate from the truck, resulting in uneven flow of mix, inability to keep the hopper full, or if he slows too much too quickly a pile of asphalt on the ground in front of the paver.”
16. Don’t allow the paver to “carry” the truck. The raised truck bed “should not come into contact with the hopper and should not be carried by or ride on any portion of the paver.” If the truck is in contact with any portion of the paver, it can essentially transfer some of the truck weight to the paver, and when that happens, the screed tow-points of the paver can be altered, which can affect the smoothness of the finished pavement.
17. Monitor flow of hot mix. Material self-levels as it flows from the truck. Once it’s level – once there’s no more pressure on it – that’s when you raise the truck bed. The truck dumper should stand back from the paver so they can see into the truck bed as the driver raises it and mix is released. The driver, directed by the truck dumper and paver operator, needs to get the bed up at the right angle so the mix is flowing. If the hot mix is flowing smoothly and comes out as a line of material across the truck bed, then it’s flowing properly.
18. Raise the bed gradually – and at the right time. If the material is not flowing smoothly, the truck dumper or paver operator needs to have the driver make adjustments. If the mix is bunched at the end of the truck bed, don’t raise the bed too high because the mix will just rush out. If the material won’t flow from the truck bed, it’s because the mix is waiting for the pressure to build up and push it out. Raising the bed creates that pressure. When the mix stops flowing, that’s when you raise the bed another 3-4 ft. When it again stops flowing, raise it another 3-4 ft. Continue this process until almost all the mix has come to the end of the truck, as many as four or five times. Ball says it should take 4-4 ½ minutes to empty a typical truck and pave 100 ft.
19. Clean-out the Truck Bed. After mix empties out of the truck, the driver puts the body down, closes the tailgate, drives away from the paver, and drives to a dumping area to clean out the rest of the truck bed. Ball says that there’s usually less than a ½ ton of material left in the truck bed, but that material needs to be removed before picking up another load. “Slam the tailgate, clean off the chute, then dump that extra mix off to a designated area where it can later be picked up and thrown away,” Ball says. “Then make sure the driver lowers the bed, and send the truck back to the plant, completely empty and cleaned of mix.”
Information for this article was obtained from The Asphalt Handbook, MS-4, Asphalt Institute, and Hot-Mix Asphalt Paving Handbook 2000, Army Corps. Of Engineers.