What Is an Asphalt Distributor? How Does It Work?

What Is an Asphalt Distributor?

An asphalt distributor is a road construction machine that sprays bitumen or bitumen emulsion onto the road surface at a set rate and with even coverage. It consists of an insulated tank, either truck-mounted or towed, and the heating, pumping and spraying systems connected to it. It is also known as a bitumen distributor or emulsion sprayer.

To understand why this job needs a dedicated machine, it helps to look at how asphalt is made. Asphalt is a mixture of aggregates such as crushed rock, sand, gravel or slag, a filler and a binder. The binder is usually bitumen, and the bond between pavement layers also depends on a bitumen-based interlayer.

If too little material is applied per square metre, the layers do not bond; if too much is applied, the bitumen bleeds to the surface. That is why the real value of a distributor lies in how precisely and consistently it can hold the application rate.

Where Are Asphalt Distributors Used?

Distributors are used on motorways, urban roads, car parks and in stabilisation work. The main applications are:

• Chip sealing: The distributor sprays the binder and a chip spreader immediately spreads the aggregate chips on top. It is one of the most common uses of an asphalt distributor. For details, see our guide on what chip sealing is and how it is done.
• Tack coat: Before a new asphalt layer is laid over an existing pavement, a thin layer of bitumen emulsion is sprayed so that the two layers bond.
• Prime coat: On new roads, a prime coat is applied to the compacted granular base before surfacing.
• Binder spraying before patching: In pothole and crack repair, binder is applied to the surface being repaired so that the patch lasts. Small towed distributors with a hand lance are used for this job.

For an overview of other road maintenance treatments, see our guide to road maintenance methods.

How Does an Asphalt Distributor Work?

An asphalt distributor sprays with an insulated tank and four subsystems built around it: hydraulic drive, pneumatic control, heating and control.

Hydraulic drive. Power from the vehicle engine is transferred to the hydraulic pump through the PTO. The pump sends oil to the hydraulic motor, which turns the asphalt pump. On vehicles without a suitable PTO output, this power can also come from an external diesel engine.

Pneumatic control. Air from the vehicle's main air tank first passes through an air conditioner unit, where moisture and dust are filtered out and oil is injected, so that the pneumatic components are lubricated as they work. The prepared air is then directed through valve blocks to the relevant valve or piston.

Spray bars. Depending on the model, the spray bars are raised, lowered and folded pneumatically or hydraulically. In the Vimpo VAD Series, spray bar movement is pneumatic on the VAD Mini, 3600M and 3600A, and hydraulic on the wide-spraying 5400A.

Heating. The burners at the rear of the tank run on 220 V; the vehicle battery's 12 V or 24 V is converted to 220 V by an inverter. Diesel fuel passes through filters before reaching the burners. The flame tubes run through the tank in a U shape and exit through the chimney at the rear. When working with polymer-modified bitumen (PMB),a thermal oil heater is added to the system; crews working in cold weather can also choose a thermal oil heated spray bar.

Proportional vs. Manual Spraying

The difference lies in who keeps the material per square metre constant: the machine in a proportional system, the operator in a manual one. The mechanical platform is the same in both.

How does the proportional system work?

A radar under the chassis reads the vehicle's ground speed. The system processes this speed together with the selected spray width and the target application rate. The result is sent to the swash plate inside the hydraulic pump, which adjusts the oil flow to the hydraulic motor and therefore the asphalt pump speed. Whether the vehicle speeds up or slows down, the amount per square metre does not change.

Proportional spraying step by step:

1. Set the target application rate and switch on the automatic spraying button.
2. Open the spray sections according to the spray width, then press the spray start button.
3. Move the vehicle. The pistons do not open while the vehicle is stationary; once speed exceeds 2 km/h, the selected sections open and spraying starts.
4. When speed drops below 2 km/h, the sections close automatically.
5. When the job is finished, switch off the spray sections, the spray start button and the automatic spraying button, in that order.

Manual spraying step by step

1. Switch on the manual spraying button and set the application rate with the pump flow buttons.
2. Open the spray sections according to the spray width.
3. With the vehicle moving, press the spray start button at the point where you want spraying to begin.
4. To finish, switch off the spray start button, bring the pump speed to zero and switch off the manual spraying button.

On manual models, the operator monitors the current application rate in kg/m² on the display. On both types, pump functions and heating work with systems that require no manual intervention.

Asphalt Distributor Operating Guide

The steps below apply to Vimpo VAD Series truck-mounted distributors; the panel layout may vary slightly between models.

1. Before starting

1. Check the levels of the burner fuel tank, hydraulic oil tank, cleaning tank and air conditioner oil.
2. Start the vehicle and wait for the air tank to fill.
3. Engage the PTO. On vehicles with automatic transmission, press the PTO button with the gearbox in neutral and slowly release the brake. On vehicles with manual transmission, switch on the PTO with the gearbox in neutral and the clutch pressed, then slowly release the clutch. Once the PTO is engaged, the hydraulic system is activated.

2. In-cab front control panel

With the power switch in the middle position, both panels are off; the same switch transfers control between the front and rear panels.

• Green buttons: move the spray bars. The centre main bar is lowered and raised; the right and left extension bars are opened and folded.
• Red buttons: lock the folded bars in place.
• Pump flow button: adjusts the asphalt pump speed.
• Section buttons: open and close the pistons that control the nozzle sections on the spray bars. Several sections can be open at once, but they will not open unless the spray start button is on.
• Circulation buttons: located in the middle of the panel. Only one of them should be on at a time; multiple operations are carried out one after another.

All buttons except automatic spraying are duplicated on the rear panel. When transferring control between panels, make sure the common buttons on both panels are in the same position.

3. Rear control panel and heating

1. Switch on the power switches in the fuse box in order: auxiliary power, burner 1, burner 2.
2. Plug in the inverter and switch it on.
3. Select burners according to the material level in the tank: burner 1 heats the lower flame tube, burner 2 the upper one.
4. On the control display, press SET and enter the target temperature with the arrow buttons. The value is saved if no button is pressed for 3 seconds.

The upper value on the display is the current tank temperature and the lower value is the target temperature. The burners shut off automatically when the target is reached and reignite when the temperature drops 2 °C below the target.

4. Asphalt pump functions

All functions follow the same pattern: switch on the function button, increase the pump speed with the pump flow button, and when finished, bring the speed back to zero and switch off the button. Function-specific steps are given below.

If spraying is to follow these operations, transfer control back to the front panel in the cab.

5. F keys and SET keys

The SET keys store frequently used application rates. With automatic spraying on and the F1 screen displayed, set the rate with the pump flow buttons and then press and hold, for example, SET 2. For the next job with the same rate, simply press and hold SET 2.

Asphalt Distributor Periodic Maintenance

A distributor keeps spraying precisely thanks to short daily checks and regular calibration.

• At the start and end of every working day: check all fluid levels.
• At the end of every working day: carry out diesel cleaning and drain the moisture collected in the air conditioner unit using the valve underneath it.
• Radar: gently wipe its surface with a clean cloth.
• Spray bars: if their movement is too fast or too slow, adjust the speed with the control valves on the bars.

Safety warnings

• Only operate the burners when the vehicle is stationary.
• The manhole cover must always be open while the burners are running.
• Never use a pressure washer on the burners, radar or inverter.

Frequently Asked Questions

What materials can an asphalt distributor spray?

Distributors can spray hot bitumen, bitumen emulsion and prime coat materials. The material used depends on the application and the specification. For materials that require high temperatures, such as polymer-modified bitumen (PMB),the distributor must be equipped with a thermal oil heater.

What is the difference between an asphalt distributor and a combi chip spreader?

An asphalt distributor sprays only the binder; in chip sealing, the chips are spread by a separate chip spreader. A combi chip spreader combines binder spraying and chip spreading in a single vehicle. Because a distributor can also be used for jobs that do not involve chips, such as tack coat, prime coat and binder spraying before patching, it is the more versatile machine.

Which distributor should I choose?

The main factors are spray width, workload, and whether the machine or the operator should set the application rate. Proportional models stand out for wide sections and work requiring high precision, while manual or compact models suit smaller-scale jobs. To compare the five models of the Vimpo VAD Series, see our asphalt distributor models page.

Vimpo VAD Series Asphalt Distributors

The Vimpo VAD Series consists of five models: the towed VAD-T maintenance distributor, VAD Mini, VAD 3600M manual, VAD 3600A proportional and VAD 5400A proportional distributor. Spray widths range from 3,000 mm to 5,400 mm, and manual models can be upgraded to proportional control on request.

You can find the technical specifications, comparison table and the right model for each job on our asphalt distributor models page.