Any Way You Mix It - It's HMA

December 28, 2000

There are currently two types of hot-mix asphalt plants used in the U.S.:
batch plants and drum-mix plants. The results of each type of plant-a hot,
homogeneous blend of aggregate and asphalt cement suitable for paving-is
the same, but the method each employs to get this mixture differs. The primary
difference is that batch plants dry and heat the aggregate first, and then
move the heated aggregate to a separate mixer where asphalt is added. This
process is done a "batch" at a time, which gives these plants
their moniker. The drum-mix plants, on the other hand, dry the aggregate
and mix it with asphalt in a continuous process, often in the same piece
of equipment. However, different types of plants do not create different
types of asphalt mixes.


Batch plants predate drum-mix plants: Batch plants first appeared in the
1870s. By 1900, crude (by today's standard) plants already displayed most
of the basic batch-plant components found on modern units. By 1930, the
larger plants were producing up to 1,000 tons of hot-mix asphalt (HMA) per
eight-hour day. From this point through the 1950s, the trend was toward
larger, higher-capacity plants.


The 1960s was a turning point for batch plants; automatic controls began
to revolutionize the industry, including full automation of the proportioning
and mixing process. What once took a team of men to accomplish could now
be done with a handful, and the process was more precise.


Automation advancements continued into the 1970s, with computerized plant-control
systems further advancing mix precision. This decade also saw a great increase
in batch-plant health and safety features, prompted by an steadily increasing
amount of health and environmental regulations.

Although regulations and technology have left an indelible mark on batch
plants, the fundamental process these plants employ remains virtually unchanged
over the past half century.


Drum-mix plants have been around longer than many may recall. Drum mixing
of asphalt was originally introduced around 1910, but these units could
not compete with the higher-capacity batch plants and, for the most part,
disappeared by the 1930s. However, the drum-mix process was resurrected
in the 1960s and has, in recent years, gained a great deal of popularity
in the industry. Contractors appreciate these units' portability, efficiency
and economy. Recent advances, such as dual-drum and counterflow-air systems,
have raised these plants' efficiencies and lowered their emissions.


As with batch plants, the technological advancements drum-mix plants have
undergone have not changed their basic system of production: the heating,
drying and coating of aggregate occurs in a continuous fashion. However,
the "continuous" process can now involve a series of up to three
drums.


Which type of plant an HMA producer chooses is influenced by a number of
variables: price, portability, maintenance costs, plant capacity and so
on. In general, batch plants require a larger initial capital outlay and
have higher maintenance costs than drum plants. However, they are more flexible
than drum-mix plants and can more easily handle numerous small batches of
different mixes, making it a popular choice for areas that have several
different jurisdictions specifying mix characteristics. Neither type of
plant is inherently "better" than the other; a purchaser merely
picks the unit that best serves his needs.

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