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35 FIGURE 32 Spraying lap coverage. AGGREGATE (CHIP) SPREADER The aggregate (chip) spreader must apply a uniform, even layer of aggregate across the full width of the binder. Tail- FIGURE 34 Dump truck bed aggregate spreader. gate box spreaders are commonly used for spot (strip) sealing, whereas self-propelled chip spreaders are used on larger-scale projects. Truck-mounted box spreaders or self- propelled spreaders are equally capable of aggregate appli- facturers offer chip spreaders equipped with computerized cation, although the self-propelled spreaders are more con- controls that allow the gates to open and close hydrauli- trollable, providing more accurate and uniform rates of cally, to compensate for the speed of the spreader. This spread. The most obvious drawback of using a tailgate ensures a constant application rate, regardless of travel box spreader is that there are considerable interruptions speed. Some models also come equipped with a vibratory between loads being spread; it is not a smooth and contin- hopper that further improves the uniformity of the dis- uous process like that of the self-propelled spreader. Figure charge. Figure 35 shows a typical self-propelled aggregate 34 shows a typical box spreader attached at the rear of a spreader. dump truck bed. A sufficient number of dump trucks should be available A self-propelled spreader, equipped with a receiving to circumvent any interruption in the supply of chips to the hopper in the rear, belt conveyors to carry the aggregate aggregate spreader. The dump trucks used on nearly all chip to the spreading hopper, and a spreading hopper with seal projects are tandem axles, because single-axle trucks adjustable discharge gates, is generally specified for most require additional hookups and therefore increase the chance chip seal projects in North America. A discharge roller that of spillage and damage to the constructed seal. The dump assists in ensuring uniform transverse application rates is trucks used for transporting the aggregate need to be com- located at the bottom of the discharge gate. These spread- patible with the aggregate spreader, meaning that their ers can be equipped with variable-width spreading hop- hitches must match and that the dump truck bed will not dam- pers that will hydraulically extend to adjust to changing age the aggregate spreader's receiving hopper. Compatibil- spread widths, such as a shoulder widening. Most manu- ity of the dump truck's bed and spreader is essential to ensure that aggregate is not spilled onto the roadway. Dump trucks or aggregate spreaders are sometimes equipped with aprons to ensure that the aggregate is effectively dumped into the aggregate spreader's hopper. FIGURE 33 Computer rate control panel for binder distributor. FIGURE 35 Self-propelled aggregate spreader.