Chip Spreaders

A chip spreader, also called an aggregate spreader or chipper, is a machine that spreads a thin, even layer of stone chips over a freshly sprayed film of bitumen binder. Together with a bitumen distributor and rollers, it is used for chip sealing, known in the United Kingdom and much of Europe as surface dressing. The binder seals the road surface against water, and the chips provide a hard-wearing, skid-resistant texture.

Chip sealing is one of the most widely used pavement preservation treatments, especially on rural roads and low to medium traffic routes. It is quick to apply and much cheaper than a new asphalt layer, and it extends the life of the existing road by sealing cracks and restoring grip. The chip spreader is the machine that determines how evenly the stone is applied, which has a direct effect on the quality and durability of the finished seal.

Brief History of Chip Spreaders

Spraying tar or bitumen on a road and covering it with gravel or stone chips developed in the early twentieth century, when motor traffic began to raise dust and damage unsealed and water-bound macadam roads. At first the chips were spread by hand with shovels from carts or lorries, a slow and uneven process.

Tailgate spreaders, simple boxes attached to the back of a tipper truck with a roller or gate to meter the stone, made the work faster and more consistent. Towed spreaders pulled by the truck followed. From the mid-twentieth century, self-propelled chip spreaders appeared, in which the stone truck is hitched to the front of the machine and tips into a receiving hopper, while conveyors carry the chips forward to a full-width spreading hopper at the front. This layout lets the machine drive over the fresh chips rather than the bare binder.

Later developments included hydrostatic drives, computerised rate control linked to travel speed, and combined machines that spray binder and spread chips in a single unit. Such synchronised sprayer-spreaders are common in parts of Europe.

How a Chip Spreader Works

Receiving hopper. On a self-propelled machine, a tipper truck reverses up to the rear of the spreader, is hooked on and is towed along, gradually tipping its load into the receiving hopper.
Conveyors. Belt conveyors carry the chips from the rear hopper, under the operator’s station, to the spreading hopper at the front.
Spreading hopper. A full-width hopper at the front of the machine holds the chips above the road. A spread roll and a set of individually controlled gates meter the stone onto the binder.
Rate control. The amount of chips per square metre is set by the gate openings and the roll speed, adjusted to the travel speed. Modern machines use computer control to keep the rate constant.
Width control. Gates can be opened or closed in sections to adjust the spreading width to the lane, or to leave uncovered strips when required.
Drive. Self-propelled machines use four-wheel or rear-wheel drive with pneumatic tyres, which also help seat the chips into the binder.

Types of Chip Spreaders

Self-Propelled Chip Spreaders

The standard machine for larger contracts. The truck is hitched behind the spreader, and the machine drives forwards over the freshly spread chips. They offer high output, adjustable widths and precise rate control.

Tailgate Spreaders

Spreading boxes fitted to the tailgate of a tipper truck, which reverses along the road spreading chips behind it. They are used for patching, narrow roads and small jobs.

Towed Spreaders

Spreading units towed or pushed by the tipper truck, simpler than self-propelled machines and suited to smaller and occasional work.

Truck-Mounted Spreaders

Spreaders mounted permanently on a truck chassis, with their own hopper and spreading system. They are common in Europe for surface dressing and patching.

Combined Sprayer-Spreaders

Machines that carry both a bitumen tank with a spray bar and a chip hopper, spraying binder and spreading chips in one pass. They are used for synchronised chip sealing and for patching with emulsion and aggregate.

Size Classes

Class	Typical spreading width	Typical use
Tailgate and towed	about 2 to 3 m	Patching, narrow roads, small jobs
Truck-mounted and combined	about 2 to 4 m	Surface dressing, local roads
Self-propelled	about 2.5 to 6 m	Large chip seal contracts, highways

Figures are indicative.

Chip Seal Train

A chip spreader is only one part of a chip seal operation, and the machines must work closely together:

Bitumen distributor. A truck with a heated, insulated tank and a spray bar that applies binder, hot bitumen or emulsion, at a controlled rate across the lane.
Chip spreader. Follows the distributor at a short distance and covers the binder with chips before it cools or breaks.
Tipper trucks. Supply the spreader with chips in a steady rotation from the stockpile.
Pneumatic tyre rollers. Follow the spreader to press the chips into the binder and orient them.
Sweepers. Remove loose chips once the seal has set, before the road is fully reopened.

On synchronised machines, the distributor and spreader are combined in one unit, which reduces the time between spraying and spreading.

Main Manufacturers

Etnyre, a US manufacturer of self-propelled chip spreaders and bitumen distributors.
LeeBoy, with self-propelled chip spreaders under the Rosco brand.
Secmair, part of the Fayat group, with chipsealers and combined sprayer-spreaders.
Schäfer-Technic, a German manufacturer of chip spreaders and bitumen sprayers.

Main Uses of Chip Spreaders

Chip sealing and surface dressing of rural and secondary roads.
Pavement preservation to seal cracks and restore skid resistance.
Double and multiple chip seals as a wearing course on low-volume roads.
Cape seals and other combined treatments with slurry or micro-surfacing.
Pothole and area patching with emulsion and aggregate.
Sanding and gritting in some winter maintenance operations.

Planning a Chip Seal Job

Match binder and chips. The size and cleanliness of the stone and the type and rate of binder must be designed together. Dusty chips do not bond well.
Keep spreader close to the distributor. Chips should be applied as soon as possible after the binder is sprayed, before it cools or the emulsion breaks.
Calibrate the spreader. Checking the spread rate with trays or mats before starting avoids too many or too few chips, which lead to loose stone or bleeding.
Plan the rollers. Pneumatic tyre rollers should follow closely to embed the chips. Traffic control keeps speeds low until the seal has set.
Watch the weather. Chip sealing needs dry weather and suitable road and air temperatures. Cold or wet conditions cause poor adhesion.
Sweep afterwards. Loose chips are swept up after the binder has set to reduce windscreen damage.

Safety

Chip sealing takes place under traffic on many roads, so traffic management, pilot vehicles and speed restrictions are important for both workers and road users. The main hazards include people struck by trucks reversing or hitched to the spreader, burns from hot bitumen, and crushing between the truck and the spreader during hitching. Loose chips can be thrown by passing traffic and damage vehicles or injure people, which is why speed limits and sweeping are part of the job. Operators need good visibility of the ground crew, and moving conveyors and spread rolls must be guarded and locked out during maintenance.

Trends and Technology

Computerised rate control linked to ground speed, and increasingly to satellite positioning, improves accuracy and allows spread rates to be documented. Synchronised sprayer-spreaders that apply binder and stone together are used to improve adhesion and reduce the time the binder is exposed. Polymer-modified emulsions and pre-coated chips extend the range of conditions in which chip sealing can be used. Telematics, better visibility from the operator’s station and improved gate controls are also becoming more common.

Key Terms

Chip seal / surface dressing: a road treatment of sprayed binder covered with stone chips.
Spread rate: the quantity of chips applied per square metre.
Spread roll: the rotating roller that meters chips from the hopper.
Gates: individually controlled openings that set rate and width.
Bleeding: binder rising above the chips, usually due to too much binder or too few chips.
Embedment: how deeply the chips are seated in the binder.

Frequently Asked Questions

What is the difference between chip sealing and surface dressing? They are the same treatment. Chip sealing is the common term in North America and Australia, while surface dressing is used in the United Kingdom and Ireland.

Why is the truck hitched behind the chip spreader? On self-propelled machines the spreading hopper is at the front, so the spreader tows the tipper truck behind it and both drive over freshly spread chips instead of the bare binder.

How long does a chip seal last? It depends on traffic, climate and design, but a well-built chip seal commonly lasts several years before retreatment.

Can a chip spreader be used for other jobs? Yes. Chip spreaders can be used to spread sand, grit and other aggregates.