Asphalt Paver



Background

An asphalt paver is a machine used to distribute, shape, and partially compact a layer of asphalt on the surface of a roadway, parking lot, or other area. It is sometimes called an asphalt-paving machine. Some pavers are towed by the dump truck delivering the asphalt, but most are self-propelled. Self-propelled pavers consist of two major components: the tractor and the screed. The tractor provides the forward motion and distributes the asphalt. The tractor includes the engine, hydraulic drives and controls, drive wheels or tracks, receiving hopper, feeder conveyors, and distribution augers. The screed levels and shapes the layer of asphalt. The screed is towed by the tractor and includes the leveling arms, moldboard, end plates, burners, vibrators, and slope sensors and controls.

In operation, a dump truck filled with asphalt backs up to the front of the paver and slowly discharges its load into the paver's hopper. As the paver moves forward, the feeder conveyors move the asphalt to the rear of the paver, and the distribution augers push the asphalt outward to the desired width. The screed then levels the layer of asphalt and partially compacts it to the desired shape. A heavy, steel-wheeled roller follows the paver to further compact the asphalt to the desired thickness.

History

Asphalt as a paving material dates back to 1815, when Scottish road engineer John McAdam (or MacAdam) developed a road surface consisting of a compacted layer of small stones and sand sprayed with water. The water dissolved the natural salts on the stones and helped cement the materials together. This type of road surface was named water macadam in his honor. Later, coal tar was used as a binding material instead of water, and the new pavement became known as tar macadam, from which we get the shortened term tarmac that is sometimes used to describe asphalt pavement.

Tar macadam pavement was used in the United States up through the beginning of the twentieth century. Modern mixed asphalt pavement, which provides a more durable road surface, was introduced in the 1920s. Unlike macadam, in which the stone and sand aggregates are laid on the road surface before being sprayed with the binding material, the aggregates in mixed asphalt are coated with the binding material before they are laid. At first, mixed asphalt was simply dumped on the roadway and raked or graded level before being rolled smooth. In 1931 Harry Barber, of Barber-Greene Company, developed the first mechanical asphalt paver in the United States. It traveled on a set of steel rails and included a combination loader and mixer to proportion and blend the components before spreading the asphalt evenly over the road surface. The rails were soon replaced by crawler tracks, and the first production paver came off the Barber-Greene line in 1934. This new machine quickly became popular with road builders because it allowed them to place asphalt more rapidly and with greater uniformity. Hydraulic drives replaced mechanical drives in pavers during the late 1950s to give the operator even smoother control. Today, almost all asphalt is placed using paving machines. When you consider that 98% of the roads in the United States are asphalt, you can understand the value of the asphalt paver.

Raw Materials

Most of the components of an asphalt paver are made of steel. The tractor mainframe is fabricated from heavy-gauge steel plate. The feeder conveyor is made of heavy-duty chain with forged steel sections, called flight bars. The distribution augers are made of cast Ni-Hard steel. The screed is fabricated from steel tubing, channel, and plate. The engine cover and access doors are formed from steel sheet.

Rubber-tired pavers have two large inflatable rear drive tires and four or more smaller solid rubber steering tires. Rubber-tracked pavers have a molded synthetic rubber track with several internal layers of flexible steel cable for reinforcement. The track is driven by a friction drive wheel on the rear, and the load is distributed among several intermediate rubber-coated steel bogie wheels. A hydraulic cylinder presses against the forward wheel to maintain tension in the track.

Purchased components on a paver include the engine, radiator, hydraulic components, batteries, electrical wiring, instruments, steering wheel, and operator's seat. Purchased fluids include hydraulic fluid, diesel fuel, engine oil, and antifreeze.

Design

Most manufacturers of asphalt pavers offer several sizes and models. Engine horsepower is usually in the 3-20 hp (2-15 kw) range for smaller, towed pavers, and may be in the 100-250 hp (75-188 kw) range for larger, self-propelled pavers. Most engines use diesel fuel because that is the fuel commonly used on other construction equipment.

Most larger, self-propelled pavers are about 19-23 ft (5.8-7.0 m) long, 10 ft (3.1 m) wide, and 10 ft (3.1 m) high. They weigh about 20,000-40,000 lb (9,090-18,180 kg) depending on the hopper capacity, engine size, and type of drive system. The typical rate of asphalt placement is 100-300 ft/min (31-92 m/min). The standard paving width is 8-12 ft (2.4-3.7 m) up to a maximum width of 40 ft (12.2 m) with the use of screed extensions on some machines. The maximum paving thickness on a single pass is 6-12 in (152-305 mm).

Options include lighting packages, manual and automatic screed extensions, and various sensors and controls to alter the grade (fore-aft dimensions) and slope (side-to-side dimensions) of the layer of asphalt.

The Manufacturing
Process

Asphalt pavers are assembled from component parts. Some of these parts are fabricated in the assembly plant, while others are manufactured elsewhere and are shipped to the plant. All parts are given a primer coat of paint. The parts are stored in a warehouse and are brought to various work stations or areas as needed.

The tractor and the screed are assembled separately. The tractor assembly process starts as the mainframe is placed on an air-flotation pallet. As the assembly proceeds, the tractor is manually moved by attaching a compressed air line to the flotation pallet. This allows the heavy tractor to float on a thin cushion of air, and it can be easily pushed from one work station to another with the help of guide rails in the floor. The screed is assembled in a single area and does not move from one work station to another.

Here is a typical sequence of operation for the assembly of an asphalt paver:

Fabricating the tractor mainframe

Assembling the tractor

Testing the tractor

Assembling the screed

Testing the screed

Finishing the paver

Quality Control

All component suppliers are thoroughly checked and certified before they may begin shipping parts. Periodically, incoming parts are given a thorough dimensional and metallurgical inspection to ensure continued high quality. The air-operated wrenches used to tighten critical fasteners are checked and recalibrated to make sure they are delivering the proper torque. The tractor and screed are machine-tested separately in addition to several visual inspections by human operators, and then checked again once the tractor and screed are coupled together for delivery.

The Future

Many cities and states have placed an emphasis on reducing the surface variations, or waviness, of asphalt roadways. This is especially important when paving over an existing roadway, which may have significant surface variations from years of hard use. On some highway projects, a penalty is assessed against the road contractor for exceeding certain waviness limits. In order to meet these stringent requirements, contractors are asking asphalt paver manufacturers for more sophisticated slope and grade control systems. Future systems may include a laser-guided screed control, utilizing a computer-generated road profile as a reference.

Another area of future development for asphalt pavers involves a change in the formulation of the asphalt pavement itself. In the United States, the Strategic Highway Research Program, sponsored by the Federal Highway Administration, is developing a new asphalt pavement formulation known as Superpave. This new pavement is expected to produce smoother, more durable roads and is targeted for implementation in the year 2000. It will involve changes to both the asphalt binder material and the aggregates and may require different methods of placement.

Where to Learn More

Books

Barber-Greene. Asphalt Construction Handbook. Caterpillar, Inc., 1992.

Barber-Greene. 75 Years of Innovation: The Story of Barber-Greene. Caterpillar, Inc., 1991.

Butler, John L. First Highways of America. Krause Publications, 1994.

Wallace, Hugh A. and J. Rogers Martin. Asphalt Pavement Engineering. McGraw-Hill, Inc., 1967.

Periodicals

Peterson, Eric. "Smooth Operators: A Start to Finish Look at the Highway Building Process," Construction Monthly, June, 1996, pp. 22-29.

Other

American Road and Transportation Builders Association. http://www.artba-hq.org .

Asphalt Contractor magazine. http://back40.global-image.com/group3/asphalt/site/index.html .

National Asphalt Pavement Association. http://www.hotmix.org .

"Paving products." Caterpillar Inc. http://www.cat.com/products/equip/paving/paving.html .

Chris Cavette



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