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.
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.
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.
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.
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
- 1 The individual pieces of the mainframe are cut to size from steel plate with band-saws or by flame cutting. The required holes are drilled or punched.
- 2 The pieces are held in position relative to each other using jigs and fixtures. They are then welded together with automatic wire-fed welders that are programmed to weld along the contour of the joints. When it is finished, the mainframe looks like the letter "H" with one long leg on each side to support the tires or tracks and a cross leg in the middle to support the engine, which is mounted sideways.
- 3 After the mainframe is welded together, it is shot blasted with a stream of high velocity air, carrying small steel balls. This relieves any stresses in the metal caused by welding and removes any welding spatter. The mainframe is then painted with a primer and the paint is allowed to dry.
Assembling the tractor
- 4 The mainframe is placed on an air-flotation pallet and is moved to the first work station. The feeder conveyor chains and flights are installed first, followed by the hydraulic feeder drive motors and the feeder lubrication hoses. If the tractor is to have a tracked drive, the left and right drive hubs are installed. On some models, the fuel tank is also installed at this time.
- 5 While the mainframe is in the first work station, the engine is being prepared in a separate area. The engine is placed on a rolling support stand and the fan, oil filters, and various sensors are installed at this time. The disconnect clutch and pump drive gearbox are bolted to the rear of the engine. The gearbox is triangular-shaped and has mounting locations for three sets of hydraulic pumps. The upper set of pumps provide power for the drive tires or tracks. The two lower sets of pumps provide power for the left and right conveyor feeders, distribution augers, and screed vibrators. Each set of pumps consists of two or more pumps sandwiched end to end and running off the same central shaft.
- 6 The mainframe is moved to the next work station. The engine is lifted from its support stand with an overhead hoist and is lowered into position crossways on the mainframe. It is bolted in place on several hard rubber mounts, which act to isolate the engine vibration. The radiator is bolted in place and coolant hoses are run between the engine and radiator.
- 7 The left and right distribution auger assemblies are bolted in place and the hydraulic auger drive motors and drive chains are installed. The rear hopper pieces are bolted in place, as are the hydraulic cylinders to raise and lower the screed leveling arms. Various hydraulic hoses and electrical wiring are routed between components.
- 8 If the tractor is to have a tracked drive, the left and right variable-speed hydraulic drive motors and two-speed planetary gears are bolted to the drive hubs. If the tractor is to have a rubber-tired drive, the drive axle, two-speed gearbox, and two-speed hydraulic drive motor is installed.
- 9 At the next work station, the main electrical box is installed, the hydraulic tank and valves are installed and connected with hoses, and the wiring for the screed and tractor lights are routed.
- 10 As the tractor moves down the assembly line, the engine side covers and inlet air cleaner are installed, the rear platform and open grate deck are put in place, and the operator's control console is mounted. Some pavers have two operator's consoles, one on each side, to give the operator a better view when paving close to curbing or other obstacles. Other pavers have a movable console that can slide to one side or the other. Any final electrical connections are made at this time.
- 11 The batteries and engine muffler are installed next and the various fluids are added as required. If the tractor has a tracked drive, the lower bogie wheels are installed at this point.
- 12 The tractor assembly is completed by attaching the screed leveling arns, hopper sides, engine access doors, lights, and other exterior components. The tires or tracks are installed last. The engine is started and the finished tractor is given a preliminary check for proper operation.
Testing the tractor
- 13 The tractor is washed to remove any grease or oil that may have accumulated on the surfaces during assembly. A fluorescent dye is added to the hydraulic oil to help spot any leaks. The tractor is then hooked up to an automatic testing machine, which cycles it through various electrical and hydraulic functions. A computer records the results of these tests for future reference. An ultraviolet "black light" is used to detect leaks in the hydraulic system.
- 14 After the cycle test, the tractor is driven outside and given a short functional test to visually inspect its operation. If adjustments are required, they are made at this time. The tractor is then parked awaiting a customer's order.
Assembling the screed
- 15 The screed is assembled in a separate area from the tractor. The frame parts are fabricated and welded together. The burner assemblies and hydraulic vibrator motors are installed and plumbed with hoses. The burners provide heat along the length of the screed to keep the asphalt from sticking to it. The vibrators help provide partial compaction of the asphalt as it is being laid. Electrical wiring is routed to the various components. The hydraulic actuators to control the side-to-side slope of the screed are installed last.
Testing the screed
- 16 The finished screed is attached to a testing machine that duplicates the functions and controls of a tractor. The various screed functions—burner ignition, vibrator operation, slope control, and others—are then tested.
Finishing the paver
- 17 When a customer orders a paver, the I / customer may specify one of several models of tractors to be matched with one of several screed designs. The tractor, which has just a coat of primer paint, is now cleaned and given a final coat of paint. Any warning labels, decorative striping, or name plates are then installed. The screed is usually painted black because it is in contact with the black, oily asphalt.
- 18 The screed is attached to the tractor. I O The electrical wiring, burner fuel lines, and hydraulic hoses are interconnected. The finished paver is then given a final functional test. The operator's seat is installed last.
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.
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
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.
Peterson, Eric. "Smooth Operators: A Start to Finish Look at the Highway Building Process," Construction Monthly, June, 1996, pp. 22-29.
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.