The theme to this module is looking at HMA construction as a process, and how keeping the process running smoothly lessens problems and improves the product. The goal is to balance the haul rate with the others to produce the best job possible. Consistent HMA delivery is very important!
Safety, Safety, Safety. Trucks by there nature are almost constantly moving and can potentially be a safety problem if you get in there way. Make sure you have eye contact with the driver when moving around a truck, that way you know you have been seen.
There are three primary types of HMA haul trucks. Each are loaded at the plant in the same manner from the silo or pugmill. The difference is in how they unload the mix at the paver. This is an end dump truck. End dump truck delivers mix directly to the paver hopper and have 3 to 6 axles, with a net capacity of 12-20 tons. The advantage of a end dump is that they are most commonly available, have the shortest wheel base, and are the easiest to maneuver. Trucks must be in mechanically sound condition. Items to maintain include engine, drive train, hydraulic system, brakes, and lights. The driver is responsible for maintenance.
Another type of end dump is the semi-trailer/tractor combination. They often have increased capacity, up to 25 tons. The disadvantage is they require more: maneuvering skill needed (turns), vertical height to dump -- overhead obstructions (wires, bridges, trees) attention in loading to limit segregation potential, and can impose a larger load on the front of the paver when fully raised.
Another type of haul truck is the bottom (or belly) dump truck. The load is delivered from beneath the body into a windrow. The capacity is about 20 to 25 tons. This requires a additional piece of equipment to pickup the material and place it in the hopper. Advantages: no contact with the paver. Keeping windrow correctly sized is ensures consistent HMA supply to the paver is very important.
The third type of haul unit is the horizontal discharge (or live bottom) truck. The operate by using a conveyor belt or slat conveyor which discharges mix from back, without raising bed, directly into paver. Capacity varies; truck shown has 45 tons payload, used primarily in Michigan Heavy weights limits it use on a number of highways.
Conveyor moves mix out the back end, directly into the paver hopper.
Note in the inset pictures the business end of the truck, dumping into a hopper.
It is important that the HMA temperature be controlled so that it is not overheated and thus the asphalt binder is not prematurely aged or that it is so cold that it cannot not be placed.
Determine the temperature of the completed HMA using a quick-reading thermometer through a hole in the side of the loaded truck immediately after loading. Locate the hole within the middle third of the length of the body, and at a distance of from 6 to 10 inches (150 to 250 mm) above the surface supporting the HMA. If a truck body already has a hole located in the general vicinity of the above specified location, use this hole.
Paving is a constant balancing act. HMA production and delivery must be balanced with laydown and compaction to ensure a smooth operation and a high quality mat. (Details on each of these are covered later.) HMA production: 150 to 400 tons per hour, plus silos Haul trucks: Adequate numbers (and no pack driving!) Laydown production: dependent on width and depth Compaction: How many rollers do you need?
Truck operating techniques: Drivers must be made part of the paving team from the beginning. They should understand their role and purpose in the team. Safety is paramount around the job site. Drivers must act responsibly, and adequate traffic control must be available to ensure safe passage. Truck tracking systems are now available. Who uses a truck tracking system?
The truck bed must be inspected and properly cleaned prior to loading the HMA. It must be free from old HMA, extraneous materials, debris, and have no major dents or depressions. Modified asphalt binders can cause greater sticking problems in the bed.
Once the bed is clean, apply an approved release agent. Non-petroleum materials sprayed uniformly on the sides and bottom–just enough to coat the bed without runoff. Diesel fuel can cause problems with the HMA, and is hazardous to the environment.
Here is the completely loaded truck leaving the loadout area heading to pickup his truck ticket and tarp the load.
Once the truck is loaded and ticketed (and tarped, if necessary), it should proceed immediately to the job site. Haul trucks should park in designated areas, and minimize the tracking of tack coats. Here, one truck has just finished loading the paver, and it pulls away as another truck waits. The waiting truck was far enough ahead so not to interfere, but close enough to get to the paver and keep the operation moving smoothly. Again, communication is the key–the drivers should be informed about the paving plan. The paver is folding its wings while continuing to move forward.
The next truck slowly backs toward the paver, while the paver lowers its wings (to give the truck access to the hopper) and continues to move forward. The truck brakes to a halt in front of paver, ready to be picked up on the fly. It is important that the truck be centered on the paver before the paver makes contact. Damage to the HMA mat and the paver can result from off-center positioning. The area between the truck and paver is off limits to personnel. Too many accidents occur in this area. Once the truck has stopped, the driver releases the brakes and the paver “picks up” the stopped truck. In this photo, the truck is also beginning to lift the bed. The key is that the truck does not back into the paver and bump it. Bumping the paver can leave a screed mark and roughen the HMA mat. Once the paver picks up the truck, it pushes the truck forward. A light touch on the brakes keep the truck against the paver.
The original Materials Transfer Vehicle (MTV) also known as the Shuttle Buggy is manufactured by by Roadtec, a division of ASTEC Industries. Other equipment manufactures also have equipment that remix the HMA at the paving location. It consists of a receiving hopper, and slat conveyor, a holding and remixing chamber in the center of the machine, and a discharge conveyor. Coupled with a hopper insert, this device has the capability of holding approximately two extra truck loads of HMA mix.
Project personnel should check the ticket to make sure it contains the right HMA mix for the job. Several HMA mixes may be on the jobsite, and it is important to get the right HMA mix in the right location.
With end dump trucks, the proper procedure for dumping the HMA mix into the hopper is to raise the truck bed slightly and allow the HMA mix to slide against the tailgate before it is released. This procedure will allow the HMA mix to flood the hopper–not allowing HMA mix to dribble from the truck into the hopper before the bed is raised. With live bottom trucks, try to use the same procedure to flood the hopper from the truck. After the hopper is filled, the truck bed is gradually raised, continually charging the hopper and maintaining a smooth operation. While paving, the truck driver must concentrate both ahead and behind. The driver must steer between the “pull” of the paver, and keep slight pressure on the brakes to maintain contact with the paver. Too much braking force may make the paver slip and affect the HMA mat.
Block 17 Delivery aka Trucking 13
Senior/Graduate HMA Course Delivery/Trucking of HMA MixesConstruction Delivery 1
What you will learn • The objective of HMA delivery • The types and capacities of haul trucks • Proper loading of haul trucks • Proper truck operating techniques • What a material transfer vehicles (MTV’s) is & what it is used for. Construction Delivery 2