An asphalt paver, or asphalt paving machine, is a machine that distributes and shapes asphalt, a combination of aggregate and a binding agent that is used for paving roads. The asphalt is laid on an area such as a roadway or a parking lot by an asphalt paver, which also completes the task of compacting it.

Though some asphalt pavers are towed by the truck that provides the asphalt, most today are self-propelled and diesel-fueled. Smaller, towed pavers usually have between three and 20 horsepower, whereas larger self-propelled versions have between 100 and 250. Weighing 20,000 to 40,000 pounds (9,072 to 18,144 kg), these pavers normally measure between 19 and 23 feet (5.8 to 7 m) in length, 10 feet (3 m) in width, and 10 feet (3 m) in height.

Contents 1 History 1.1 The Creation of Macadam 1.2 A More Durable Asphalt 1.3 Barber-Greene Asphalt Pavers 1.4 Floating Screed 1.5 Advancements In Pavers 2 Features/How it Works 2.1 The Tractor 2.2 The Screed 2.3 Tires or Tracks 2.4 The Paver At Work 3 Common Manufacturers 4 Additional Photos 5 References 6 Additional Resources

[edit] History

[edit] The Creation of Macadam

The advent of asphalt came well before the birth of the paving machine. John MacAdam, a Scottish road engineer, developed the bituminous material in its earliest form in 1815.^[1] For road-paving applications, he would lay a compacted layer of aggregate and sprey it with water, which dissolved the sands and salts, aiding in adhesion and binding. This material was named macadam in honor of its creator. Eventually, tar was used as a binding agent in lieu of water; the mixture became known as tar macadam, or tarmac. Roads were paved using tarmac through the beginning of the 1900s.

[edit] A More Durable Asphalt

In the 1920s, in response to the need for a more durable asphalt pavement, mixed asphalt was developed. In contrast to traditional macadam, the new mixed asphalt consisted of aggregate materials that were coated in a binding agent before being laid down. Initially, this mixture was dumped on a roadway and then leveled using a rake. This practice would soon become antiquated.

[edit] Barber-Greene Asphalt Pavers

The year 1931 saw the development of the first mechanical asphalt paver in the United States. The introduction of this paver is credited to Harry Barber of the Barber-Greene Co.^[2] The machine, which moved on a set of steel rails, consisted of a combined loader and mixer to evenly mix materials before they were spread on the roadway. The first production model was available in 1934.

[edit] Floating Screed

Once developed, the Barber-Greene paver underwent some improvements. Whereas the initial model was mounted on steel rails or side forms, the subsequent version ran on crawler tracks, and the screed was supported by two long arms projecting from either side of the machine. Further improvements took place, and the floating screed was introduced. The screed, now supported underneath a leveling arm that was affixed to the main spreader unit, was able to “float” above the asphalt. The screed could shave off high spots and fill in low spots on the surface of the asphalt being laid which made the configuration beneficial. Barber-Greene floating screeds controlled the market until the expiration of the company’s patent in 1955.^[3] All paver manufacturers have since adopted the design, making it the standard in the industry.

[edit] Advancements In Pavers

Although the principle of the floating screed has remained relatively unchanged for over 50 years, other design improvements have taken place.

The first self-propelled pavers had manual transmissions. Subsequent models employed electrical, hydraulic, then hydrostatic transmissions. The asphalt pavers developed today are driven using hydrostatic closed-loop systems.

In the past, widening a screed was a time-consuming and hazardous task. Asphalt paver operators had to manually adjust screed width by dismounting the paver and bolting screed extensions onto the basic unit. By the 1950s, this was no longer the case. Operators using self-widening screeds were able to change the width of the screed hydraulically in a matter of seconds.

The Material Transfer Vehicle was developed in 1987. Known as the MTV, this rubber-tired vehicle receives asphalt from a haul truck and feeds it to the paver when needed. As the haul truck is often located far from the paver, use of the MTV enables the asphalt paver’s continuous operation, in addition to reducing the cost of hauling the mix.^[4]

[edit] Features/How it Works

An asphalt paver consists of various components that are almost entirely made of steel.

[edit] The Tractor

A tractor is used on an asphalt paver to propel it forward, as well as for distributing the asphalt. It comprises an engine, hydraulic drives and controls, drive wheels or tracks, a hopper for receiving the asphalt, and feeder conveyors, and distribution augers.

[edit] The Screed

Asphalt pavers also have a screed, which is used for leveling and shaping the layer of asphalt. This floating screed is towed by the tractor and includes leveling arms, a moldboard, end plates, burners, vibrators, slope sensors, and controls. The screed is attached to a thickness control, which is adjusted by the operator in order to regulate the thickness of the asphalt being laid on the roadway, as well as a compaction device for partial compression of the pavement.

[edit] Tires or Tracks

Some asphalt pavers are mounted on tires when speed is required for a paving job. Others move on tracks when increased traction is needed. Those moving on rubber tires consist of two large, inflatable drive tires at the back, as well as at least four smaller solid rubber tires, which are used for steering. The tracks on which tracked pavers are mounted are also made of rubber. They are reinforced internally with layers of cables made of flexible steel. Both types of pavers are able to perform relatively steadily on irregular grades due to a three-point suspension system.

[edit] The Paver At Work

Before beginning work, the paver’s hopper must be filled by a dump truck containing asphalt. A tractor moves the paver forward, while feeder conveyors are used to push the asphalt toward the back of the machine. At this point, distribution augers spread the asphalt outward to a width determined by the operator. Each auger is attached to a feeder conveyor, allowing independent control of the asphalt on either side of the paver. The screed unit, which is heated, then comes into play, leveling and partially compacting the asphalt. A separate machine known as a roller follows the paver to complete the compaction of the asphalt. This process is usually carried out at a speed of approximately 100 to 300 feet (30.5 to 91 m) per minute. Though the average width of spread is between 8 and 12 feet (2.4 and 3.7 m), screed extensions enable the asphalt to be spread up to 40 feet (12 m) in width.

[edit] Common Manufacturers

• Astec
• Barber-Greene
• Blaw Knox
• Bitelli
• BOMAG
• Caterpillar
• Cedarapids
• Champion
• Dynapac
• FAYAT Marini
• Gehl
• Hunan Sunward
• Ingersoll Rand
• LeeBoy
• Marini
• Mauldin
• Pro-Pav
• Puckett Bros.
• Roadtec
• Sakai
• Sumitomo
• Svedala Demag
• Terex
• Voegele
• Wirtgen

[edit] Additional Photos

[edit] References

1. ↑ Asphalt Paver. Made How. 2008-09-23.
2. ↑ Asphalt Paver. Made How. 2008-09-23.
3. ↑ History. Hotmix. 2008-09-23.
4. ↑ MTV Carries Asphalt. BNET. 2008-09-23.

[edit] Additional Resources

• Wagner, F.T. Placement and Compaction of Asphalt Mixtures: A Symposium. ASTM International: 1984.
• Lavin, Patrick. Asphalt Pavements: A Practical Guide to Design, Production and Maintenance. Taylor & Francis: 2003.