Slipform Pavers

A slipform paver is a self-propelled machine that places, consolidates and finishes concrete in a continuous shape without fixed side forms. Fresh concrete is spread in front of the machine, vibrated and pushed through a moulding pan or mould that moves forward with the paver. The concrete leaving the back of the machine is stiff enough to hold its shape on its own, so the formwork effectively slides along with the machine, which gives the process its name.

Slipform pavers build concrete road and motorway pavements, airport runways and aprons, and a wide range of smaller profiles such as kerbs, gutters, safety barriers, drainage channels and footpaths. Compared with fixed-form paving, where steel forms must be set up, levelled and removed by hand, slipforming is faster and needs fewer workers, which is why it has become the standard method for large concrete paving projects.

Brief History of Slipform Pavers

Concrete roads were built from the early twentieth century with fixed steel forms, mounted on the ground and used as rails by finishing machines. The slipform paver was developed in the United States in the late 1940s. At the Iowa State Highway Commission, laboratory chief James Johnson and his colleagues built prototypes that vibrated concrete into shape between moving side forms. A first small model extruded a narrow slab in 1947, and full-scale trials followed in Iowa in 1949.

The method moved from experiment to contract work in the 1950s, and commercial manufacturers began building slipform pavers in the middle of that decade. Iowa paved hundreds of miles of road with the new process within a few years, and other US states and countries soon followed. In the following decades, companies such as Gomaco and Guntert & Zimmerman developed larger and more capable machines, including pavers for full motorway widths and machines for kerbs and barriers.

Electronic sensors and string lines made accurate control of line and level possible, and later developments added dowel bar inserters, tie bar inserters, automatic steering and stringless 3D guidance. In Europe, Wirtgen entered the segment with its SP series pavers, adding another major supplier to the market.

How a Slipform Paver Works

Concrete supply. Trucks or placers deliver low-slump concrete in front of the machine, either directly on the grade or through a belt placer or spreader running ahead.
Spreading. An auger or a spreading plough distributes the concrete evenly across the width of the paving mould.
Vibration. A row of internal vibrators immersed in the concrete consolidates it, removing air pockets and making the stiff mix flow into the mould.
Mould and profile pan. The concrete is shaped under the profile pan between the side forms. The speed of travel and the stiffness of the mix are balanced so that the slab holds its edges once the machine has passed.
Dowels and tie bars. Many road pavers have automatic dowel bar inserters that place steel dowels at transverse joints, and tie bar inserters for longitudinal joints.
Finishing. An oscillating beam or a super smoother finishes the surface, followed by texturing equipment and curing compound sprayers, often on a separate texture and cure machine.
Control and steering. Sensors reading a string line on both sides, or a 3D positioning system, control grade and steering. The machine usually runs on two, three or four tracks with hydraulic legs.

Types of Slipform Pavers

Road and Airport Pavers

Large four-track machines that pave widths from about 3 metres to more than 15 metres in a single pass. They are used for motorways, airport runways and taxiways, and industrial slabs.

Multi-Purpose Pavers

Medium machines that can be configured for paving, kerb and gutter, or barrier work by changing the mould and the position of the mould relative to the machine.

Curb and Gutter Machines

Three- or two-track machines that slipform kerbs, gutters, channels, footpaths and cycle paths. The mould is often mounted to one side of the machine, allowing work alongside existing structures.

Barrier and Wall Pavers

Machines configured with tall offset moulds to slipform concrete safety barriers, parapets and small walls, including profiles with steel reinforcement.

Canal and Special Pavers

Pavers adapted to line irrigation canals, drainage ditches and slopes, or to produce special profiles such as tram track slabs.

Size Classes

Class	Typical paving width	Typical use
Curb and gutter	about 0.3 to 3 m	Kerbs, gutters, footpaths, barriers
Medium / multi-purpose	about 2 to 9 m	Streets, ramps, barriers, slabs
Large road pavers	about 5 to more than 15 m	Motorways, runways, large slabs

Figures are indicative.

Attachments and Options

A slipform paver is usually part of a paving train rather than a machine working alone. Common options and companion machines include:

Placers and spreaders, which receive concrete from trucks at the side of the lane and spread it ahead of the paver, so trucks do not have to drive on the prepared base.
Trimmers, which cut the base to an accurate grade before paving.
Dowel bar and tie bar inserters, which place steel bars automatically at joints.
Texture and cure machines, which follow the paver to apply a tined or dragged texture and spray curing compound.
Interchangeable moulds for kerbs, barriers, channels and footpaths on multi-purpose and curb machines.

Main Manufacturers

Gomaco, based in Iowa, with a wide range of road pavers and curb and gutter machines.
Guntert & Zimmerman, a US maker of large pavers and placers.
Wirtgen, part of the John Deere group, with its SP series slipform pavers.
Power Curbers, a US company specialising in curb and barrier machines.
Miller Formless, a US manufacturer of curb and multi-purpose slipform machines.

Main Uses of Slipform Pavers

Concrete motorway and highway pavements, including widening projects.
Airport runways, taxiways and aprons.
Kerbs, gutters, drainage channels and footpaths in urban works.
Concrete safety barriers in central reservations and on bridges.
Irrigation canals and hydraulic channels.
Industrial yards, port hardstandings and container terminals.

Planning a Slipform Paving Job

Get the mix right. Slipform concrete must be stiff enough to stand up at the edges but workable enough to be vibrated and finished. Slump, aggregate grading and consistency from truck to truck are critical.
Prepare the grade. An accurate, well-compacted base, often trimmed by a separate machine, reduces concrete overruns and helps the paver hold grade.
Keep the supply constant. Stopping and starting the paver affects ride quality. The number of mixers or trucks and the output of the batching plant must match the paving speed.
Set up the reference. String lines must be set and checked carefully, or 3D guidance must be correctly calibrated, since the machine will reproduce any error.
Plan curing and joints. Texture, curing compound and joint sawing must follow at the right times, which depend on the weather and the concrete.

Safety

Slipform paving involves heavy machines, many workers on foot and constant truck traffic, often next to live traffic lanes. The main hazards are people struck by trucks or by the paver, crushing between the machine and the concrete supply, and falls from the machine’s platforms. String lines are a trip hazard and must be well marked. Fresh concrete is alkaline and can cause skin burns, so gloves, boots and eye protection are required. Vibrators, augers and dowel inserters must be isolated before cleaning or adjusting. Traffic management and clear signals between truck drivers and the paving crew are essential.

Trends and Technology

Stringless paving with 3D guidance, using total stations or satellite positioning, removes the need for string lines on many projects and speeds up set-up. Real-time smoothness measurement lets crews correct ride quality while paving. Sensors that monitor vibrator frequency and concrete temperature improve quality documentation. Pavers can now switch more quickly between configurations, and manufacturers offer telematics for machine monitoring. Growing interest in lower-carbon concrete and in long-life concrete pavements is also shaping specifications and machine use.

Key Terms

Slipform: a form that moves with the machine, shaping concrete continuously.
Low-slump concrete: a stiff mix that holds its shape after leaving the mould.
Dowel bar inserter (DBI): a device that places steel dowels at transverse joints.
String line: a tensioned wire that the machine’s sensors follow for line and level.
Offset mould: a mould mounted to the side of the machine for kerbs and barriers.
Stringless paving: paving guided by a 3D positioning system instead of string lines.

Frequently Asked Questions

What is the difference between slipform and fixed-form paving? In fixed-form paving the concrete is placed between steel forms that stay on the ground until the concrete has set. In slipform paving the forms move with the machine, and the concrete holds its own shape.

What kind of concrete is used for slipform paving? A low-slump mix designed to be stiff enough to keep its edges but workable under vibration.

Can slipform pavers make kerbs and barriers? Yes. Curb and gutter machines and multi-purpose pavers use interchangeable moulds for kerbs, gutters, barriers and channels.

Where was the slipform paver invented? In Iowa, in the United States, where engineers of the state highway commission developed the first prototypes in the late 1940s.