Asphalt Concrete mixes made with asphalt cement are prepared at an asphalt mixing plant.  Here, aggregates are blended, heated, dried, and mixed with asphalt cement to produce a hot mix asphalt (HMA).  This chapter discusses the two most common types of asphalt plants.
Asphalt Concrete plants are basically of two types, batch or drum mix. The two types of asphalt plants derive their names from their particular type mixing operation. In the batch-type mixing plant, hot aggregate and asphalt are withdrawn in desired amounts to make up one batch for mixing. After thoroughly mixing, the material is discharged from the pugmill in one batch. In the drum-type mixing plant, the aggregate is dried, heated, and mixed with the asphaltic cement in the drum in a continuous operation.

Batch Type Plant
    Flow diagram of a batch-type asphalt plant (Figure 3-1):
 a)   cold aggregate feed  b)   drying  c)   screening  d)   hot storage  e)   measuring and mixing
Aggregate is removed from storage, or stockpiles, in controlled amounts and passed through a dryer where it is heated and dried.  The aggregate then passes over a screening unit that separates the material into different size fractions and deposits them into bins for hot storage. The aggregate and mineral, when used, are then withdrawn in controlled amounts, to make up one batch for mixing (Figure 3-1). The entire combination of aggregate is dumped into a mixing chamber called a pugmill.  Then the asphalt, which has also been weighed, is thoroughly mixed with the aggregate in the pugmill.  After mixing, the material is emptied from the pugmill in one batch.

1. Cold bins 8. Screening unit 2. Cold feed gate 9. Hot bins 3. Cold elevator 10. Weigh box 4. Dryer heater 11. Mixing bowl or pugmill 5. Dust collector 12. Mineral filler storage 6. Exhaust stack 13. Hot bitumen storage 7. Hot elevator 14. Bitumen weigh box

Cold Feed Supply Cold aggregate feed is the first major component of the batch-type asphalt concrete plant. The cold feeder may be charged by one or a combination of the following methods:
1.     Open top bins with two, three, or four compartments, usually fed by a crane with a clamshell bucket or a front-end loader. 2. Tunnel under stockpiles separated by bulkheads. Materials are stockpiled over the tunnel by belt conveyor, truck, crane, or front-end loader. 3. Bunkers or large bins. These usually are fed by trucks, car unloaders, or bottom-dump freight cars, which empty directly into the bunkers.

The cold aggregate feed is one of the critical control points in the production flow-line.  It is significant that, while most of the problems in asphalt concrete production occur in the dryer, on the plant screed, in the bins, or in the pugmill, the causes can usually be traced back to the cold feed. When charging the cold feed, care should be exercised to minimize segregation and degradation of the aggregate. This can be prevented by taking the same precautions outlined for proper stockpiling.

Types of Feeders and Controls

Aggregate feeder units (Figure 3-3) are located beneath the storage bins or stockpiles, or in positions that assure a uniform flow of aggregate.  Feeder units have controls that can be set to produce a uniform flow of aggregate to the cold elevator. Generally belt and vibratory feeders are best for accurate metering of the fine aggregates. Course aggregates usually flow satisfactorily with any type of feeder.  For a uniform output from the asphalt concrete plant, input must be accurately measured.  The importance of feeding the exact amounts of each size aggregate into the dryer at the correct rate of flow cannot be overemphasized.

Three-Bin Cold Feeder and Belt

Lime Dispersion

When lime is used in asphalt concrete, it must be mixed by pugmill or other approved means to achieve a uniform lime coating on the aggregate prior to adding the asphalt cement to the mixture. The method of introducing and mixing the lime and aggregate shall be subject to approval by the Engineer prior to beginning production.

The Dryer

One of the basic units in any asphalt concrete plant is the dryer.  It is a necessary part of the hot-mix operation for it dries and heats aggregates coming from the cold feed supply, thus making them suitable for mixing with asphalt. The dryer is usually a large rotating metal drum mounted at an angle and equipped with a gas or oil-heating unit at the lower end .  Hot gases from the burner pass from the lower end of the rotating drum and out the upper end.  Cold aggregate is fed into the upper end of the dryer and is picked up by steel angles or flights mounted on the inside of the unit.  As the drum rotates, the aggregate is lifted up and dumped through hot gases.  Because of the inclination, the aggregate also gradually works its way toward the lower end of the dryer.  The hot aggregate then discharges from the lower end onto a hot elevator that carries it to the screens and hot storage.
Drying is the most expensive operation in mix production. It is also the most frequently encountered bottleneck in the plant operation.  The best dryer is the one that meets a desired performance level at the lowest investment and operating cost. Most dryers are designed for average aggregate moisture content.  Very wet aggregate will reduce the dryer capacity and require corrective measures.

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