Hot Mix Asphalt Paving Handbook, AC 150/5370-14A, Appendix 1

9727-13/Sec13 12/20/00 17:43 Page 113 PART III Hot-Mix Asphalt Laydown and Compaction 9727-13/Sec13 12/20/00 17:43 Page 114 9727-13/Sec13 12/20/00 17:43 Page 115 SECTION 13 Mix Delivery The purpose of the haul vehicle is to transport the asphalt ward, typically pulling the windrow box behind it. The mixture from the asphalt plant to the paver. This must amount of HMA placed in the windrow is determined be done without delay and with minimal change in the by the setting of the discharge opening in the box. This characteristics of the mix during the delivery process, procedure provides the most accurate means of keeping and without segregation. Three primary types of trucks a constant supply of mix in front of the paver. The mix are usually employed to transport HMA—end-dump, is then picked up from the roadway surface by a windrow bottom- or belly-dump, and live-bottom. The loading of elevator attached to the front of the paving machine. the three types of trucks, either directly from the pugmill A second means of creating a windrow using an end- at a batch plant or from the surge silo at a batch or drum- dump truck is to use a windrow blender device, which mix plant, is reviewed in Section 11, with emphasis on is usually attached to a small front-end loader. In this techniques for preventing segregation. This section fo case, the mix is dumped onto the existing pavement sur cuses on the unloading of the mix at the paving site from face across the full width of the truck bed. The amount each type of truck; use of a material transfer vehicle is of HMA discharged from the truck is controlled by both also briefly discussed. A review of hauling procedures the width of the opening of the truck bed tailgate, which is then presented. 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 UNLOADING OF MIX loader. As a result of the tumbling action that occurs as the mix is being shaped into the windrow, some remix End-Dump Trucks ing of the material occurs, and segregation may be re An end-dump truck delivers the HMA directly from the duced or eliminated. The size of the windrow is con truck bed into the hopper of the paver. The mix is un trolled by the height of the discharge opening at the back loaded by raising the truck bed and allowing the mix to of the blender. Because of the length of the wings on the slide down the bed into the hopper. When the bed is windrow blender, mix can be carried for some distance raised, it should not come into contact with the hopper if the truck deposits too much mix at some point on the and should not be carried by or ride on any portion of existing pavement surface. the paver. For smaller-capacity end-dump trucks, con tact with the paver is normally not a problem. Such con Bottom- or Belly-Dump Trucks tact can be a problem, however, when large tractor- A bottom- or belly-dump truck delivers its load onto the semitrailer units are used as haul vehicles, particularly roadway in front of the paver. The mix is deposited from when the truck bed is extended to its highest point. underneath the truck bed into a windrow, as seen in Fig When a portion of the weight of the truck is being car ure 13-2. For this method of mix delivery, it is important ried by the paver, the screed tow points of the laydown that the correct amount of material be placed down the machine may be changed, which in turn will affect the length of the windrow to match the paving width and smoothness of the finished mat. A typical end-dump depth being placed without allowing the hopper to run truck is shown in Figure 13-1. out of mix or become overloaded. Continuous operation An end-dump truck can also be used to deliver the of the paver, which must be equipped with a pickup ma mix to a windrow on the roadway in front of the paver. chine (windrow elevator) (see Figure 13-3), can be ac The windrow can be formed in one of two ways. First, complished only if a continuous and consistent supply a spreader box or windrow sizer can be used. In this of mix is available. It is more difficult, however, to case, the mix is deposited into the box and is uniformly maintain a consistent amount of mix in the paver hop metered out onto the roadway as the truck moves for per for leveling courses of HMA, which are necessarily AC 150/5370-14A Appendix 1 9727-13/Sec13 12/20/00 17:43 Page 116 116 HOT-MIX ASPHALT PAVING HANDBOOK 2000 FIGURE 13-1 Typical end-dump truck. FIGURE 13-3 Material pickup machine. of variable thickness, than for courses whose thickness consistent. Should the amount of mix being delivered to is more constant. the paver become out of balance with the quantity of Control of the amount of HMA discharged from a mix needed by the paver, an adjustment must be made bottom-dump truck is based on the width of the gate in the discharge operation of the bottom-dump truck. If opening under the truck and the speed of the truck. The the problem is noticed before the paver is under- or amount of mix deposited at any time is therefore highly overloaded, the amount of mix deposited on the road dependent on the skill of the person controlling the dis way can easily be altered by changing the width of the charge gates and on the truck driver’s attention to oper gate opening on the truck or by adjusting the forward ating the truck at the required speed. Manual control of travel speed of the truck during discharge. the discharge requires constant attention to ensure that Should the paver run out of mix in the hopper, addi neither too little nor too much mix is fed into the paver tional HMA should be placed in the hopper without the hopper (the hopper should be filled to a level above the paver moving forward. Depending on the equipment top of the flow gates or tunnel openings at all times). The available to the contractor at the paving site, such as a amount of mix needed in the windrow depends on the small loader, doing so may be very difficult. In some width and thickness of the layer being placed. Thus a dif cases, the paver operator may have to raise the paver ferent gate opening on the truck bed is needed according screed and then move the paver forward to pick up mix. to variations in project conditions and truck speed. When enough mix is in the hopper, the paver must be The amount of mix available to be picked up by the backed up, the screed set back down, more mix placed windrow elevator on the front of the paver should be 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 FIGURE 13-2 Typical bottom- or belly-dump size of the windrow of mix produced by the bottom- truck. dump truck be as consistent as possible. Keeping the AC 150/5370-14A Appendix 1 9727-13/Sec13 12/20/00 17:43 Page 117 SECTION 13 Mix Delivery 117 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. FIGURE 13-5 Material transfer vehicle. Live-Bottom Trucks A live-bottom truck (also known as a flow-boy or horizontal-discharge truck) employs a conveyor belt size of the unit. HMA is deposited from an end-dump or slat conveyor in the bottom of the truck bed to dis or live-bottom truck into the hopper on the front of the charge the mix without the need to raise the bed. This vehicle, which may be equipped with a remixing auger type of truck usually deposits the mix directly into the or augers.

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