ASPHALT HAUL TRUCKS

Various types of trucks are used to deliver hot mix to the paver. The most common type is the 5-ton end-dump truck but other trucks have been used and can be used to deliver mix.

Truck Condition

Trucks must have metal beds, and the beds must be clean, smooth, and free of holes. All trucks must meet _{minimum safety criteria.} Each truck must be clearly numbered for easy identification and must be equipped with a tarpaulin.

Before being loaded, the truck bed must be cleaned of foreign material and hardened asphalt and then lightly coated with a release agent (lubricant) that assists in preventing fresh hot-mix asphalt from sticking to the surfaces of the bed. After the bed is coated, any excess release agent must be drained from the bed. Before loading, the truck must also be weighed to establish its unloaded weight. This weight is later subtracted from the loaded weight of the truck to determine the weight of the hot mix that the truck is hauling.

The number of trucks required on the project is determined by many factors: the mix production rate at the plant, the length of the haul, the type of traffic encountered, and the expected time needed for unloading.

Types of Trucks

Each type of truck used for hot-mix delivery must have certain physical features that are required to haul properly and to discharge the mix properly into the paver. Below are listed a few guidelines for the two most common types of trucks.

END-DUMP TRUCKS.- An end-dump truck must first be inspected to be certain the rear of the bed overhangs the rear wheels enough to discharge mix into the paver hopper. If it does not, an apron with side plates must be added to increase the overhang and prevent spillage of the mix in front of the paver.

The bed must also be of a size that will fit into the hopper without pressing down on the paver. The hydraulic system for the truck-bed hoist should be frequently inspected to guard against hydraulic fluid leakage. Such leakage on the roadway surface will prevent good bonding between the roadway and the new mat. When enough oil is spilled that the mix can absorb it, the mix can become unstable at that spot. For this reason, leaking trucks should not be used.

Tarpaulins should be pulled over the mixture during hauling in cool weather or on long hauls to protect the mixture from excessive cooling. A cool mix forms lumps and a crust over its surface. When a tarpaulin is used, care must be taken to be sure it is securely fastened to the top of the truck bed so that cold air cannot funnel under it.

During delivery, the driver must direct the truck squarely against the paver and should stop the truck a few inches from the paver before the truck tires make contact with the paver roller bar. Backing the truck against the paver can force the screed back into the mat, leaving a bump in the pavement even after the mat is rolled.

The truck bed must be raised slowly. When the mix is dumped too rapidly, segregations occur, because the coarser aggregates will roll down the sides of the load.

BOTTOM-DUMP TRUCKS.- Bottom-dump trucks can be used when a grader is spreading the mix or when a pickup device is used to feed the windrow left by the truck into the paver hopper.

Two common methods for unloading bottom-dump trucks are in use. The first method involves the use of a spreader box, designed to be operated under the gates of the truck. The amount of material, placed in the windrow, is governed by the width of the spreader box opening. The disadvantage of this method is that the spreader box can restrict the amount of material to less than the required amount. The second method, which is used more often than the first, is to use chains to control the dump gate opening.

NOTE: Automatic devices are also available for controlling gate openings.

Variations in the size of the windrow, deposited by the bottom-dump truck for pickup by the paver, and irregularities in the surface on which the material is to be placed will cause variations in the amount of material fed to the paver hopper. This often causes variations in the finished surface. It is, therefore, essential that the windrow, deposited by the truck be as uniform as _possible. When the windrow is deficient in size, material can be added to it to keep the paver from starving. When the windrow contains too much mix, a short gap in depositing with the next truck will compensate for the excess. The windrow length must also be controlled particularly in cool weather. Windrowed material will cool below spreading and compaction temperatures in cool weather, particularly when delay occurs because of paver malfunction. The limit of the windrow should be no more than one truck load ahead of the pickup machine to prevent excessive cooling of the mix in cold weather.

When the loader and paver are directly coupled, vibration of the pickup device maybe transmitted into the paver, causing ripples and roughness in the mat surface. These vibrations generally result from worn and defective parts or from improper mounting or adjustment.