MULTIPLE-SURFACE TREATMENT.- A multiple-surface treatment is essentially the same as the single-surface treatment. However, the multiplesurface treatment consists of two or more successive layers of binder and aggregate.

This type of treatment is done in stages. Each stage is accomplished in the same manner as a single-surface treatment. The only difference is that each additional layer of aggregate should be about one half of the size of the previous layer. This allows the smaller aggregate to interlock with the larger aggregate when rolled.

PAVING EQUIPMENT

Equipment, used in asphalt pavement construction, are aggregate spreaders, asphalt distributors and their associated hand sprayers and spray bars, asphalt kettles, asphalt haul trucks, and asphalt pavers.

AGGREGATE SPREADERS

When a spreader is operated properly, it will conserve aggregate and produce a uniform spread. Spreaders range from a type attached to a truck tailgate to a highly efficient self-propelled machine.

Several types of tailgate spreaders are in use today. The simplest is the vane spreader (fig. 16-29). There are tailgate spreaders that consist of a hopper with a feed roller, activated by small wheels driven by contact with the truck wheels (fig. 16-30).

Mechanical spreaders are hoppers on wheels that are hooked onto and are propelled by backing aggregate trucks. Hoppers have various widths and capacities. They usually contain augers to distribute the aggregate the full width of the box. They have controls to regulate feed gates, feed roll, augers, and the truck hitch. All tailgate and mechanical spreaders that are pushed by a truck have the disadvantage that the truck must be operated in reverse with consequent loss of steering control and reduction in speed.

Figure 16-29.-Vane spreader.

Figure 16-30.-Hopper type of tailgate spreader.

A self-propelled spreader is shown in figure 16-31. This machine moves forward and makes possible a uniform and continuous application of cover aggregate, because it is capable of keeping up with the asphalt distribute. The spreader is self-powered and has a receiving hopper in the rear. Aggregate trucks are hitched to the spreader, dump their loads into the hopper, and are pulled by the spreader. Belt conveyors carry the aggregate to the front of the machine where it is dropped into the spreading hopper (fig. 16-32). Aggregate flows over a spread roll onto a screen that permits initial placement of larger particles on the asphalt, followed by fine aggregate on top.

Calibration and adjustments for all types of aggregate spreaders should be made according to the manufacturer's instruction and operating manual. Here are some additional checks that should be made to ensure good results:

1. A tachometer, used as an aid in maintaining uniform spreader-box speed, is most helpful.

2. Distribution rates are closely controlled by measuring off the length that each truckload of aggregate should cover.

3. A quick check on the rate of application of aggregate can be made by laying a 1-square-meter (yard) section of cloth or building paper on the pavement (or by supporting a shallow l-square-yard box above the asphalt with nails or screws) and by passing over it with the spreader. The cloth, paper, or box is then carefully

Figre 16-31.-Self-propelled aggregate spreader.

Figure 16-32.-Flow of aggregate through a self-propelled aggregate spreader.

lifted and the aggregate on it is weighed. This will give the weight per square yard of aggregate being spread.