End dump trucks unload their payload by raising the front end and letting the payload slide down the bottom of the bed and out the back through the tailgate (Figure 1, 2 and Video 1). End dump trucks are the most popular transport vehicle type because they are plentiful, maneuverable and versatile.
Use for HMA Transport
Some general considerations associated with end dump trucks are:
- When the bed is raised it should not contact the paver. Bed contact with the paver may affect the screed tow point elevation, which can affect mat smoothness.
- The truck bed should be raised slightly before the tailgate is opened. This allows the HMA to slide back against the tailgate, which will cause it to flood into the paver hopper when the tailgate is opened. HMA that trickles into the paver hopper is more susceptible to aggregate segregation.
- Truck-paver contact should be established by allowing the paver to move forward into a stationary truck. This ensures that the truck does not bump the paver too hard and cause the paver to lurch to a sudden stop, which could cause a rough spot in the mat.
- Once the paver and truck are in contact, they should remain in contact. This ensures that no HMA is accidentally spilled in front of the paver because of a gap between the truck and paver. Usually the truck driver will apply the truck brakes hard enough to offer some resistance to the paver but light enough so as not to cause the paver tracks to slip from excessive resistance. Most pavers can also be coupled to an unloading truck using truck hitches located on or near the push rollers.
Use for PCC Transport
For PCC transport, end dump trucks are considered “non-agitating” transport because they do not agitate the fresh PCC during transport. Non-agitating trucks can be of any form but are often end dump trucks. These trucks are not specifically designed to transport PCC but often work well for central mixed PCC when haul distances are short and mixing requirements are simple. Their chief advantages are:
- Quick Loading and discharge. Unlike truck mixers, which have a relatively small loading hopper and discharge chute, the loading and discharge areas for a dump truck are quite large.
- Quick cycle times. Because of their quicker loading and unloading times, dump truck cycle times are shorter than mixing truck cycle times, thus requiring fewer trucks to maintain a particular delivery rate.
- Lower maintenance/cost. Fewer moving parts and greater accessibility make dump trucks less expensive to maintain and use than truck mixers.
Because they are not designed to transport PCC, dump trucks (in general) also have some severe disadvantages, which limit their use:
- No mixing/agitating ability. Dump trucks can only be used with central mixed PCC. Further, the lack of agitation over long haul distances may allow segregate and excessive slump loss.
- No integral cover. Although most dump trucks can be covered with a tarp, the tarp is not integral to the bed and usually allows some water into the bed. Thus, when operating in the rain, rainwater can unintentionally increase the water-cement ratio of the transported PCC. Additionally, hot weather may cause excessive water evaporation, which can also change the water-cement ratio.
- Limited placing ability. Dump trucks can only place PCC at their discharge point. They are not practical on small jobs (such as sidewalks) nor can they place material in confined areas.