A road roller (sometimes called a roller-compactor, or just roller) is a compactor type engineering vehicle used to compact soil, gravel, concrete, or asphalt in the construction of roads and foundations, similar rollers are used also at landfills or in agriculture.
In some parts of the world, road rollers are still known colloquially as steam rollers, regardless of their method of propulsion. This typically only applies to the largest examples (used for road-making).
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The first road rollers were horse-drawn, and were probably just borrowed farm implements (see roller (agricultural tool)).
Since the effectiveness of a roller depends to a large extent on its weight, self-powered vehicles replaced horse-drawn rollers from the mid-19th century. The first such vehicles were steam rollers. Double-cylinder designs were preferred. Single-cylinder steam rollers were uncommon and unpopular, as the power impulses from the steam engine would produce slight waves in the road. Some road companies in the United States used steamrollers through the 1950s, and in the UK, some remained in commercial service until the early 1970s.
As internal combustion engine technology improved during the 20th century, kerosene-, gasoline- (petrol), and diesel-powered rollers gradually replaced their steam-powered counterparts. The first internal-combustion powered road rollers were very similar to the steam rollers they replaced. They used similar mechanisms to transmit power from the engine to the wheels, typically large, exposed spur gears. Some users did not like them in their infancy, as the engines of the era were typically difficult to start, particularly the kerosene-powered ones.
Virtually all road rollers in commercial use now use diesel power.
Road rollers use the weight of the vehicle to compress the surface being rolled (static) or use mechanical advantage (vibrating). Initial compaction of the substrate on a road project is done using a padfoot drum roller, which achieves higher compaction density due to the pads having less surface area. On large freeways a four wheel compactor with padfoot drum and a blade, such as a Caterpillar 815/825 series machine, would be used due to its high weight, speed and the powerful pushing force to spread bulk material. On regional roads a smaller single padfoot drum machine may be used. The next machine is usually a single smooth drum compactor that compacts the high spots down until the soil is smooth, and this is usually done in combination with a motor grader to get a level surface. Sometimes at this stage a pneumatic tyre roller would be used. These rollers feature two rows (front and back) of pneumatic tyres that overlap, and the flexibility of the tyres provides a kneading action that seals the surface and with some vertical movement of the wheels, enables the roller to operate effectively on uneven ground. Once the soil base is flat the pad drum compactor is no longer used on the road surface. The next course (road base) would be compacted using a smooth single drum, smooth tandem roller or pneumatic tyre roller in combination with a grader ,and a water truck to achieve the desired flat surface with the right moisture content for optimum compaction. Once the road base is compacted, the smooth single drum compactor is no longer used on the road surface (There is however an exception, if the single drum has special flat-wide-base tyres on the machine). The final wear course of asphalt concrete (a.k.a. asphalt or blacktop in North America, or macadam in England) is laid using a paver and compacted using a tandem smooth drum roller, a three-point roller or a pneumatic tyre roller. Three point rollers on asphalt were very common once and are still used, but tandem vibrating rollers are the usual choice now, with the pneumatic tyre roller's kneading action being the last roller to seal of the surface.
Rollers are also used in landfill compaction. Such compactors typically have padfoot or "sheep's-foot" drums, and do not achieve a smooth surface. The pads aid in compression, due to the smaller area contacting the ground.
The roller can be a simple drum with a handle that is operated by one person, and weighs 100 pounds, or as large as a ride-on road roller weighing 21 short tons (44,000 lb or 20 tonnes) and costing more than US$150,000. A landfill unit may weigh 59 short tons (54 tonnes).
Drums are available in widths ranging from 24 to 84 inches (0.6 to 2 metres).
Tyre rollers are available in widths ranging up to 2.7 metres (8.9 ft), with between 7 and 11 wheels (e.g. 3 wheels at front, 4 at back): 7 and 8 wheel types are normally used in Europe and Africa; 9 and 11 in America; and any type in Asia. Very heavy tyre rollers are used to compact soil.
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Roley is one of the main vehicle characters in the children's books and television series, Bob the Builder. He is a green roller with a cab, enclosed power unit and no chimney, and so is obviously diesel-powered. Nevertheless, his official title is Roley the Steamroller. This is an example of the persistence of "steam roller" to describe a large modern road roller in layman's English.
Film-makers seem to like the finality of a roller of some kind flattening a human. In most cases, however, the roller is not actually steam-powered, even though it is usually referred to as such. In all of the following examples, the rollers cited are diesel-powered.
In the Japanese manga JoJo's Bizarre Adventure, the villain Dio Brando attempts to finish off his rival Jotaro by dropping a road roller (sometimes translated as "steam roller") on him from midair.
The Vocaloid2 singing synthesizer characters Kagamine Rin/Len are associated with a yellow road roller in fandom.