An asphalt plant can be set up on a construction site that needs a steady flow of HMA. This is necessary because the pliability of asphalt is based on its temperature. Transportation can cause the HMA to cool, making it no longer useful. However, it can be quite expensive to maintain an asphalt plant on-site. So, if only a small amount of asphalt is needed, it is best to transport it from an off-site facility.[1]
Contents |
[edit] History
[edit] Early Asphalt Applications
With asphalt being used in road development since 625 B.C. it’s surprising that mass production did not develop until the late nineteenth century. The word asphalt comes from the Greek “asphaltos,” meaning “secure.”[2]
Initially, asphalt was used in minimal amounts, in conjunction with other materials, to provide stability. The Romans used it to seal their baths, reservoirs, and aqueducts, while English explorer Sir Walter Raleigh used asphalt to re-caulk his ships.
[edit] The Development of Tarmacadam
Several hundred years later, asphalt was used to pave roads. Modern road development began in the 18th century. Englishman John Metcalf and Scotsman Thomas Telford built hundreds of miles of road with large stones. Fellow engineer John Loudon McAdam improved the roads of his contemporaries by adding tar to bond broken stones together. This new road material was called “tarmacadam” pavement.[3]
[edit] Coal Tar Asphalt
Coal tar asphalt began being used in the U.S. in the 1860s for sidewalks, crosswalks, and even pavements. However, the first true asphalt poured in the U.S. was in 1870 in front of the city hall in Newark, New Jersey. A Belgian chemist named Edmund J. DeSmedt designed the asphalt. His mixture was based on a natural asphalt pavement used for a French highway in 1852. DeSmedt then moved onto a much larger project. He paved Pennsylvania Avenue in Washington, D.C. with 54,000 square yards (45,151 m2) of sheet asphalt.
[edit] Competing Companies Enter the Market
Until the turn of the 20th century, natural asphalt deposits in Lake Trinidad and Bermudez Lake in Venezuela provided most of the asphalt used in the U.S. Eventually, refined petroleum asphalts were added to soften natural asphalt. By 1907, refined asphalt surpassed the use of natural asphalt.
After witnessing the success of asphalt, more companies entered the market. In fact, “competing proprietary brands of HMA were peddled, touted, and huckstered with all the enthusiasm that now is used to advertise soft drinks.”[4] Patents were quickly written up, but they didn't limit the competition. In fact, cities soon began receiving higher quality asphalt at more reasonable prices. Increased quality and better value meant bigger cities, like New York, began using asphalt paving in place of the bricks, granite, and wood blocks of the past. Companies offered asphalt guarantees. This proved disastrous for their businesses because they “did not recognize pavement failures caused by factors beyond the asphalt contractor’s control.”[5] Many companies were forced into bankruptcy.
[edit] The Progression of Asphalt Facilities
Competition, along with the emergence in popularity of automobiles increased demand for paved road resulting in the design of mechanized production of hot-mixed asphalt (HMA). The first HMA production units consisted of shallow iron trays heated over open coal fires. The operator was entirely responsible for the quality of the asphalt. He dried the aggregate on a tray, pouring the hot asphalt on top, and stirred the mixture by hand. These units were not very productive, taking about four hours to complete one batch.
The first central hot mix facilities in the U.S. were establish by the Cummer Co. in 1870. Shortly thereafter, facilities around the world were manufacturing portable or rail-mounted asphalt plants, but most were “costly and cumbersome.”[6] These units were quite limited in their capabilities in comparison to modern units.
The first modern asphalt facility was built in 1901 by the Warren Brothers of East Cambridge, Massachusetts. By 1910 drum mixers and drum dryer-mixers had been adapted from Portland cement concrete mixers. By the 1920s, portable units had improved with the enhancement of cold-feed systems. The 1930s introduced vibrating screens and pressure injection systems.
[edit] Asphalt in Demand
In 1939, President of the Asphalt Institute, J.S. Helm said, “Asphalt is an essential material in nearly every form of highway construction and maintenance. In the four years from 1934 to 1937, asphalt entered into the construction of more than four-fifths of the mileage of highways completed in those years under state highway direction.”[7]
World War II increased demand for asphalt, which forced technology to develop a greater pace. Asphalt was used to pave runways for giant military aircraft, so it was required to stand up to heavy loads. After the war US citizens began moving to the suburbs in droves. This created a whole new market for roads to and from the major cities. In fact, in 1956 the U.S. Congress passed the State Highway Act, which allotted $51 billion for road development.
[edit] Modern Advancements in Asphalt Plants
Advancements in asphalt plants were made to meet the increasing demand.
The asphalt plants of the 1950s were dirty, dusty machines that consisted of a dryer, a tower with a screed and a mixer. The 1960s produced new concerns for the environment, leading to the introduction of wet scrubbers and baghouses. Plants were also equipped with surge and storage bins, allowing them to produce larger quantities and store the excess. Previously, batches would be made and immediately loaded into transport trucks. These new bins would allow excess asphalt to be stored for three to four days until it was needed.
The biggest modern advancements were the result of environmental concerns.
“When I first started, state-of-the-art was a batch plant that was dirty, that you could usually see three miles away,” said National Asphalt Pavement Association member Don Brock, CEO of Astec Industries, Tennessee. “We’ve gradually progressed from there to cleaning them up with wet washers and baghouses to being invisible today. Today, we need to build a plant that you can’t see, you can’t hear, and you can’t smell.”[8]
An energy crisis of the 1970s forced the asphalt industry to embrace recycling. Both base and surface courses began including an increased amount of recycled asphalt pavement (RAP) into their mixes.
[edit] Features/How it Works
[edit] Batch Plants
Major components of a batch plant include a cold-feed system, asphalt cement supply system, aggregate dryer, mixing tower, and emission-control system. The plant tower includes a hot elevator, screen deck, hot bins, a weigh hopper, asphalt cement weigh bucket, and a pugmill.
Aggregate is stored in stockpiles, and then placed into cold-feed bins, which proportion it by size. The aggregate is then transported on a gathering conveyor belt to a charging conveyor, which lifts it to a large drying machine. This dryer operates on a counter-flow process, meaning the aggregate moves counter to the flow of heat. The aggregate enters the upper end of the dryer and moves down the rotating drum. The burner is located at the lower end of the drum. As the aggregate material moves through the dryer drum toward the burner it is heated and removed of all moisture. The hot and dry aggregate exits the dryer and enters a bucket elevator that lifts it to the top of the mixing tower.
Once the bucket elevator reaches the top of the tower the aggregate is sorted by a set of vibrating screens and held in hot bins. The aggregates of varying size are proportioned and then fed into the weigh hopper. In another tank, asphalt cement is being poured and prepared to mix with the course aggregate in the pugmill.
First the hot and dry aggregate pours into the pugmill and starts a dry-mix. A few seconds later, the asphalt is poured in and wet-mixes with the aggregate. Mixing continues until all the aggregate particles are covered with a thin film of the asphalt cement material. This shouldn’t take any more than 25 to 35 seconds. A pugmill can mix a single batch of 2 to 6 tons.
When mixing is completed, gates at the bottom of the pugmill open to release the hot asphalt mix. It is either loaded directly into a hauling truck or moved on a conveyor device to a storage silo.
The plant is also equipped with primary and secondary emission-control equipment. A primary dry collector or knockout box collects dust particles so they are not spread into the air. A wet-scrubber or baghouse is often used as a secondary measure to remove particulate from the exhaust gases that flow out of the dryer and send clean air to the atmosphere.
Often recycled asphalt pavement (RAP) is included in the mix. If this is the case it is stored in a separate cold-feed bin and is added to the mix at one of three points: the bottom of the hot elevator; the hot bins; or, most commonly, the weigh hopper.
[edit] Parallel-flow Drum-mix Plants
Parallel-flow drum-mix plants consist of a cold-feed system, asphalt cement supply system, drum mixer, surge or storage silos, and emissions control equipment.
Cold-feed bins proportion the aggregate. The amount of aggregate drawn from each bin is controlled by the size of the opening at the bottom of the bin and speed of the feeder belt. The aggregate is then transported on a gathering conveyor through a scalping screen, and then transferred to a charging conveyor.
The charging conveyor is capable of weighing the aggregate and determining the speed of the belt. Together, these two values help establish the wet weight of the aggregate in tons per hour. This helps decipher how much asphalt cement mix will be needed for the mix.
The aggregate then enters the drum mixer, which works with a parallel flow system, meaning the exhaust gases and aggregate move in the same direction. Essentially the burner is at the front of the mixer, where the aggregate enters the machine. As it travels through it is heated, removing all the moisture.
If recycled asphalt pavement (RAP) is used in the mixture it enters the mixing drum at the halfway point, so it does not get hit with the extreme heat of the burner. This is important because if the RAP is overheated it will be damaged and smoke.
As the combined RAP and aggregate reach the rear of the mixing drum, asphalt cement is pumped in. The materials mix together and exit the rear of the mixing drum onto a conveyor to transport the asphalt to a storage silo.
Emission control equipment is attached to the rear of the mixing drum.
[edit] Counter-flow Drum-mix Plants
Counter-flow drum-mix plants combine the processes of batch and drum plants. The aggregate enters the top part of the dryer/mixer with the heat source at the bottom, moving towards it, heating up and removing moisture. Mixing with the asphalt cement does not occur within the drying portion of the drum, but instead mixes with the aggregate behind or underneath the burner.
[edit] Common Manufacturers
- ADM
- Almet
- Astec
- Barber-Greene
- BDM
- CMI
- Cedarapids
- Daewoo
- FAYAT Marini
- Gencor
- Hunan Sunward Intelligent Machinery
- Sakai Heavy Industries
- Telcon
- Terex Cedarapids
- YTO Group
[edit] References
- ↑ Popescu, Calin and Phaobunjong, Kan and Ovararin, Nuntapong. Estimating Building Costs. CRC Press: 2003. 621.
- ↑ History. Hot Mix. 2008-09-23.
- ↑ History. Hot Mix. 2008-09-23.
- ↑ Gillespie, Hugh. A Century of Progress: The History of Hot Mix Asphalt. National Asphalt Pavement Association: 1992.
- ↑ History. Hot Mix. 2008-09-23.
- ↑ History. Hot Mix. 2008-09-23.
- ↑ History. Hot Mix. 2008-09-23.
- ↑ History. Hot Mix. 2008-09-23.
