Hot Mix Asphalt (HMA) Manufacturing
The basic purpose of a hot mix asphalt (HMA) plant is to properly proportion, blend, and heat aggregate and liquid asphalt cement to produce an HMA that meets the requirements of the job mix formula (JMF) (Roberts et al., 1996). Modern HMA plants also have the ability to incorporate reclaimed asphalt pavement (RAP), mineral filler, fibers, and anti-stripping agents, and will have advanced environmental systems to limit noise and dust emissions.
There are two basic types of HMA plants commonly in use today: the batch plant, and the drum mix plant. Batch plants produce HMA in individual batches while drum plants produce HMA in a continuous operation.
How HMA Is Made
Hot mix asphalt (HMA) is produced in a manufacturing plant that proportions, blends, and heats aggregate and liquid asphalt cement to produce a material that conforms to the specific product mix design and job mix formula (JMF). The JMF ensures that the HMA produced will provide the designed performance characteristics of that particular mix (durability, stability, rut resistance, skid resistance, and so on).
There are two basic types of HMA plants commonly in use today: batch plants and drum mix plants. As their name implies, batch plants produce HMA in individual batches, whereas drum plants produce HMA in one continuous operation. Both types of production plants produce the same HMA, and a mix produced in one will perform the same as a mix produced in the other. The choice of a batch or drum mix plant depends upon on business factors such as purchase price, operating costs, production requirements, and the need for flexibility in local markets.
Batch plants, which produce HMA in individual batches, tend to be smaller-volume output plants than drum mix plants. However, they offer greater flexibility in the amounts of a particular mix they can produce, the variety of materials that can be incorporated into one batch, and their ability to produce small batch sizes. Typical batch quantities range from 2.0 to 5.5 metric tons of HMA, running at production capacities of up to 350 metric tons per hour, with each batch taking 15 to 45 seconds to make.
Drum plants produce HMA in a continuous manner and generally offer higher production rates than batch plants for comparable cost. Typical production rates for drum plants vary from about 100 metric tons per hour to more than 900 metric tons per hour, depending on the drum design.
Drum plants are suited to continuous production runs and are generally restricted in both the size of the smallest volume they can mix and the amount of different materials they can mix. Due to their method of operation, however, in general drum mix plants generally can incorporate higher levels of recycled material than batch plants.