Hot Mix Asfalt

P A R T

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Hot-Mix Asphalt Plant Operations

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Types of Asphalt Plants: Overview
transported by the gathering conveyor and transferred to a charging conveyor. The material on the charging con­ veyor is then carried up to the aggregate dryer. The dryer operates on a counter-flow basis. The ag­ gregate is introduced into the dryer at the upper end and is moved downthe drum by both the drum rotation (gravity flow) and the flight configuration inside the ro­ tating dryer. The burner is located at the lower end of the dryer, and the exhaust gases from the combustion and drying process move toward the upper end of the dryer, against (counter to) the flow of the aggregate. As the aggregate is tumbled through the exhaust gases, the material is heated and dried.Moisture is removed and carried out of the dryer as part of the exhaust gas stream. The hot, dry aggregate is then discharged from the dryer at the lower end. The hot aggregate is usually transported to the top of the plant mixing tower by a bucket elevator. Upon dis­ charge from the elevator, the aggregate normally passes through a set of vibrating screens into, typically, one of four hot storagebins. The finest aggregate material goes directly through all the screens into the No. 1 hot bin; the coarser aggregate particles are separated by the different-sized screens and deposited into one of the other hot bins. The separation of aggregate into the hot bins depends on the size of the openings in the screen that is used in the screen deck and the gradation of the aggregate in the cold-feedbins. The heated, dried, and resized aggregate is held in the hot bins until being discharged from a gate at the bottom of each bin into a weigh hopper. The correct proportion of each aggregate is determined by weight. At the same time that the aggregate is being propor­ tioned and weighed, the asphalt cement is being pumped from its storage tank to a separate heated weigh bucket located on thetower just above the pugmill. The proper amount of material is weighed into the bucket and held until being emptied into the pugmill. The aggregate in the weigh hopper is emptied into a twin-shaft pugmill, and the different aggregate fractions are mixed together for a very short period of time— usually less than 5 seconds. After this brief dry-mix time, the asphalt cement from the weigh bucket isdischarged

The purpose of an HMA plant is to blend aggregate and asphalt cement together at an elevated temperature to produce a homogeneous asphalt paving mixture. The ag­ gregate used can be a single material, such as a crusher run aggregate or a pit run material, or it can be a combi­ nation of coarse and fine aggregates, with or without mineral filler. The binder material used is normally as­phalt cement but may be an asphalt emulsion or one of a variety of modified materials. Various additives, includ­ ing liquid and powdered materials, can also be incorpo­ rated into the mixture. Indeed, with Superpave the use of additives is becoming more common. Use of additives can result in a need for more binder storage tanks, as well as silos for adding mineral materials. There are three basictypes of HMA plants currently in use in the United States: batch, parallel-flow drummix, and counter-flow drum-mix. All three types serve the same ultimate purpose, and the asphalt mixture should be essentially similar regardless of the type of plant used to manufacture it. The three types of plants differ, however, in operation and flow of materials, as described in the following sections.BATCH PLANTS
The major components of a batch plant are the coldfeed system, asphalt cement supply system, aggregate dryer, mixing tower, and emission-control system. A typical batch plant is depicted in Figure 5-1; the major plant components are shown in Figure 5-2. The batch plant tower consists of a hot elevator, a screen deck, hot bins, a weigh hopper, an asphalt cement weigh bucket, and a...