UNLOADING OF MIX End-Dump Trucks

An end-dump truck delivers the HMA directly from the truck bed into the hopper of the paver. The mix is un­

loaded by raising the truck bed and allowing the mix to slide down the bed into the hopper. When the bed is raised, it should not come into contact with the hopper and should not be carried by or ride on any portion of the paver. For smaller-capacity end-dump trucks, con­

tact with the paver is normally not a problem. Such con­

tact can be a problem, however, when large tractor-semitrailer units are used as haul vehicles, particularly when the truck bed is extended to its highest point.

When a portion of the weight of the truck is being car­

ried by the paver, the screed tow points of the laydown machine may be changed, which in turn will affect the smoothness of the finished mat. A typical end-dump truck is shown in Figure 13-1.

An end-dump truck can also be used to deliver the mix to a windrow on the roadway in front of the paver.

The windrow can be formed in one of two ways. First, a spreader box or windrow sizer can be used. In this case, the mix is deposited into the box and is uniformly metered out onto the roadway as the truck moves for­

ward, typically pulling the windrow box behind it. The amount of HMA placed in the windrow is determined by the setting of the discharge opening in the box. This procedure provides the most accurate means of keeping a constant supply of mix in front of the paver. The mix is then picked up from the roadway surface by a windrow elevator attached to the front of the paving machine.

A second means of creating a windrow using an end-dump truck is to use a windrow blender device, which is usually attached to a small front-end loader. In this case, the mix is dumped onto the existing pavement sur­

face across the full width of the truck bed. The amount of HMA discharged from the truck is controlled by both the width of the opening of the truck bed tailgate, which is chained to prevent full opening, and the forward speed of the truck. The mix is folded into a windrow by the blending unit as that device is pushed forward by the loader. As a result of the tumbling action that occurs as the mix is being shaped into the windrow, some remix­

ing of the material occurs, and segregation may be re­

duced or eliminated. The size of the windrow is con­

trolled by the height of the discharge opening at the back of the blender. Because of the length of the wings on the windrow blender, mix can be carried for some distance if the truck deposits too much mix at some point on the existing pavement surface.

Bottom- or Belly-Dump Trucks

A bottom- or belly-dump truck delivers its load onto the roadway in front of the paver. The mix is deposited from underneath the truck bed into a windrow, as seen in Fig­

ure 13-2. For this method of mix delivery, it is important that the correct amount of material be placed down the length of the windrow to match the paving width and depth being placed without allowing the hopper to run out of mix or become overloaded. Continuous operation of the paver, which must be equipped with a pickup ma­

chine (windrow elevator) (see Figure 13-3), can be ac­

complished only if a continuous and consistent supply of mix is available. It is more difficult, however, to maintain a consistent amount of mix in the paver hop­

per for leveling courses of HMA, which are necessarily

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F I G U R E 1 3 - 1 Typical end-dump truck. F I G U R E 1 3 - 3 Material pickup machine.

of variable thickness, than for courses whose thickness is more constant.

Control of the amount of HMA discharged from a bottom-dump truck is based on the width of the gate opening under the truck and the speed of the truck. The amount of mix deposited at any time is therefore highly dependent on the skill of the person controlling the dis­

charge gates and on the truck driver’s attention to oper­

ating the truck at the required speed. Manual control of the discharge requires constant attention to ensure that neither too little nor too much mix is fed into the paver hopper (the hopper should be filled to a level above the top of the flow gates or tunnel openings at all times). The amount of mix needed in the windrow depends on the width and thickness of the layer being placed. Thus a dif­

ferent gate opening on the truck bed is needed according to variations in project conditions and truck speed.

The amount of mix available to be picked up by the windrow elevator on the front of the paver should be

F I G U R E 1 3 - 2 Typical bottom- or belly-dump truck.

consistent. Should the amount of mix being delivered to the paver become out of balance with the quantity of mix needed by the paver, an adjustment must be made in the discharge operation of the bottom-dump truck. If the problem is noticed before the paver is under- or overloaded, the amount of mix deposited on the road­

way can easily be altered by changing the width of the gate opening on the truck or by adjusting the forward travel speed of the truck during discharge.

Should the paver run out of mix in the hopper, addi­

tional HMA should be placed in the hopper without the paver moving forward. Depending on the equipment available to the contractor at the paving site, such as a small loader, doing so may be very difficult. In some cases, the paver operator may have to raise the paver screed and then move the paver forward to pick up mix.

When enough mix is in the hopper, the paver must be backed up, the screed set back down, more mix placed in the windrow in front of the paver, and paving started once again. Needless to say, this is not good paving practice and does not usually result in placement of a smooth mat.

Should the hopper become overloaded with mix, some of the material in the windrow in front of the paver must be removed so that the paver can move forward without additional mix being picked up. Once the mix in the hop­

per has reached the desired level, the paver must again start picking up additional mix from the windrow.

The ability of the paver to place a smooth mat will be affected by the uniformity of the volume of mix being fed to it. Because of the difficulty in adjusting the quantity of mix needed by the paver when the hopper is either under- or overloaded, it is important that the size of the windrow of mix produced by the bottom-dump truck be as consistent as possible. Keeping the

size of the windrow constant is easier than correcting the problem of too little or too much mix in the paver hopper.

One method to better control the amount of material in the paver hopper is to form a windrow that is a little short of that needed by the paver. An additional bottom-dump truck is then kept just in front of the laydown ma­

chine, traveling over the windrow placed by the earlier truck or trucks, to supplement the amount of mix in the windrow as needed. This type of operation overcomes the potential problem of under- or overfilling the paver hopper and greatly speeds up the paving process.

Live-Bottom Trucks

A live-bottom truck (also known as a flow-boy or horizontal-discharge truck) employs a conveyor belt or slat conveyor in the bottom of the truck bed to dis­

charge the mix without the need to raise the bed. This type of truck usually deposits the mix directly into the hopper of the paver, as does an end-dump truck, but it can also deliver the mix to the existing pavement surface for pickup by a windrow elevator on the paver, as does a bottom-dump truck. Further, the live-bottom truck moves the HMA in a mass, which minimizes segrega­

tion as compared with end-dump trucks. Because the bed of this type of truck is not raised, there is no poten­

tial problem with the bed pressing on the paver hopper.

A typical live-bottom truck is shown in Figure 13-4.

Material Transfer Vehicle

An additional method used to deliver mix to the paver is a material transfer vehicle (MTV) (see Figure 13-5).

An MTV is basically a surge bin on wheels that can hold up to 32 000 kg (35 tons) of mix, depending on the

F I G U R E 1 3 - 4 Typical live-bottom truck.

S E C T I O N 1 3 Mix Delivery 117

F I G U R E 1 3 - 5 Material transfer vehicle.

size of the unit. HMA is deposited from an end-dump or live-bottom truck into the hopper on the front of the vehicle, which may be equipped with a remixing auger or augers. The purpose of the auger system is to reblend the coarse and fine particles of the HMA and reduce any segregation and temperature variation that may have occurred in the mix as a result of the operation of the surge or storage silo or truck-loading procedures (see Section 11). The mix is carried from the hopper through the augers and then to a conveyor, which delivers the mix into a vertical extension or insert in the hopper.

The MTV should allow the paver to operate almost continuously, without stopping between truckloads of mix, as long as a continuous supply of mix is available from the asphalt plant. Therefore, the paver operator can keep the head of material in front of the screed con­

stant by supplying a continuous amount of mix back to the screed and obtain a smoother mat. Use of an MTV also eliminates the problems of the haul truck bumping the paver and the truck driver holding the brakes on the truck when being pushed by the paver. As noted, how­

ever, the MTV is essentially a mobile surge bin; when it runs out of material, the paver must stop, and a con­

tinuous paving operation is not possible. Keeping a con­

stant stream of trucks in front of the paver or MTV is therefore necessary if a continuous paving operation is to be achieved. If a gap occurs, the MTV should be stopped without being completely emptied when wait­

ing for trucks, so that a consistent minimum amount of mix is retained on the augers to mix with the new, pos­

sibly segregated, material delivered from the next haul truck. In addition, the paver should be stopped with the hopper half full so that the amount of HMA in front of the paver screed remains constant and the proper smoothness of the mat is achieved. Indeed, the head of

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material in front of the paver screed is the most impor­

tant factor in obtaining a smooth-riding pavement layer (see Section 16).

The MTV can be operated directly in front of the paver or off to one side. Because of the weight of this piece of equipment when full of mix, it is necessary to determine ahead of time that the pavement over which this machinery will be operated can support the loaded weight without being overstressed and damaged. Sev­

eral smaller, simpler MTVs have been developed by var­

ious equipment manufacturers. Because of the limited surge capacity of most of these smaller devices, how­

ever, it is more likely that the paving operation will have to stop because of the MTV running out of material.

HAULING PROCEDURES