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Section 13: Spreading the Aggregate

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The aggregate spreader and all other equipment must be in position and ready to begin before the distributor applies the asphalt shot. The required number of haul trucks needed to cover the asphalt shot must be in position behind the spreader box. If a patching crew will be used on the job, they should be ready to follow the haul trucks as closely as possible. All the rollers should be ready to begin.

For strip/spot seal coat work, a dump truck tailgate spreader is commonly used for spreading the aggregate instead of a self-propelled spreader box.

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Aligning the Spreader Box

As the distributor begins to apply the asphalt shot, the spreader should move to within a few feet of the starting point. While the joint paper is being removed, the operator should align the spreader.

Once the spreader is in position, the operator should make sure that all of the necessary discharge gates are open to ensure complete coverage of the asphalt shot. If the discharge hopper is wider than the asphalt, some of the gates must be closed.

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Truck Hookup

The waiting haul truck should back up to the aggregate spreader and stop slightly short of coming in contact with the spreader. This allows the spreader operator to back the spreader into the truck, so that the hitches connect. A spotter should be used to ensure correct connection.

The haul truck transmission should be in neutral to allow the spreader to tow the truck backwards as the aggregate is spread. Upon signal from the spreader operator, the truck tailgate latch should be released. The truck bed is raised allowing the aggregate to fill the receiving hopper. The haul truck driver must remain ready to lower the bed on signal to prevent the hopper from overflowing.

As soon as the aggregate begins to pour into the receiving hopper, the conveyor belts should be engaged. The aggregate begins to flow into the discharge hopper and is distributed across the discharge gates. When both the receiving and discharge hoppers are nearly full, the spreader operator, or his assistant, signals the truck driver to lower the truck bed to stop the flow of aggregate into the spreader.

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Test Strip

Before any asphalt is applied, a short test strip may be applied on bare pavement to visually check for uniform aggregate coverage. If a test strip is to be applied, the spreader operator may disengage the truck hitch and have the truck move away from the spreader. This allows the spreader operator to test his equipment for a few feet without the truck being attached. There should be enough aggregate in the spreader to run perhaps a 50-foot test strip. Any gates not functioning properly should be corrected. Once the spreader gate settings are correct and the equipment is functioning properly, the test should be terminated.

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Spreading the Aggregate

If the truck was disengaged from the spreader to perform a test strip, the truck and spreader should be joined together again. As the spreader and truck move forward, the gates should be opened just before reaching the beginning of the asphalt shot. The truck bed should be raised enough to keep the receiving hopper full until the truck bed is empty.

For best results, aggregate should be applied on any type of asphalt binder as soon as possible without causing the rocks to roll over or the asphalt to be picked up on spreader box, haul truck, or roller tires. That is, the aggregate spreader should follow closely behind the asphalt distributor. Refer to Section 14 for more information on the proper time to begin spreading the aggregate.

Once the truck bed is empty, a signal should be given to lower the truck bed. This will allow the truck to separate from the spreader without causing the tailgate or rear of the truck bed to strike the top of the receiving hopper.

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Truck/Spreader Separation

The spreader will normally continue to move forward while the truck bed is being lowered. Before the spreader is completely empty, the operator should release the truck. Most hitches can be released without stopping the spreader, but the spreader must eventually stop to hook up to the next truck. After the spreader stops, it should be backed up a few feet to allow the second truck to link up. The process is then repeated.

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Rock Land Marker

After hooking up with the second truck and resuming the spreading, the spreader should pass the marker for the end of the first rock land. The end of the first rock land should be slightly farther than the second truck hookup, because there should be some aggregate in the spreader remaining from the first truckload.

If the spreader passes the first rock land marker before the first truck is empty, the aggregate is being applied too thinly. If the second truck is hooked up more than 25 feet before the first rock land marker, the aggregate is being applied too heavily. In either case, gate openings must be adjusted accordingly.

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Visual Checks

The inspector assigned to watch the aggregate application should be positioned to have a good view of the aggregate as it leaves the discharge hopper. There should be a thin “curtain” of aggregate dropping through the gates. The curtain should be uniform across the entire width of the discharge hopper. The curtain of aggregate should be only one aggregate particle thick, and light should be easily seen through the curtain. Any dark streams suggest a gate is open too wide. Any unusually light streak means not enough aggregate is being released. If the aggregate appears to be stacking as it is placed on the asphalt, it is being applied too heavily.

The scalping grate on top of the discharge hopper should also be visually checked. There should be a steady flow of aggregate passing through it. An accumulation of clay balls, grass, or rocks on top of the grate indicates that the loader operator is picking up contaminants. This problem should be corrected immediately.

Behind the spreader, the pavement surface should be checked for contaminants and streaking of thin or thick rows of aggregates. If there is evidence of thick and thin alternating streaks running transversely (a ripple effect), it indicates that the spreader speed is too high.

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Recording Truck Loads

It is important to keep accurate records of the number of truck loads of aggregate placed on the roadway. One method is to write down the identification numbers of each truck on the project. Each time a truck finishes emptying a load into the spreader, a mark is placed beside that truck’s number.

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Asphalt on Tires

Occasionally, the tires on the spreader and the haul trucks should be checked for asphalt (and aggregate) sticking to them. This should be corrected immediately.

The following conditions may cause tires to pick up asphalt:

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  • Aggregate is rolling over causing asphalt to be exposed to the tires. This can be caused by not using enough asphalt to hold the aggregate or by applying too much aggregate. Refer to Section 14 for more information on the proper time to begin spreading the aggregate.
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  • Too much asphalt has been applied.
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  • A puddle of asphalt may have leaked or spilled onto the pavement without cleanup prior to aggregate application.
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  • One of the discharge gates on the spreader may have clogged momentarily, preventing the aggregate from covering the asphalt.
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  • Failure to use deflector nozzles and overlapping the shot in the second lane causes an excess of asphalt.
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  • Detouring traffic onto the fresh seal may cause aggregate pickup.
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  • Construction and other traffic accelerating, turning, and braking abruptly on the fresh seal can dislodge aggregate. Accelerating quickly may cause a tire to spin because of the soft asphalt and unrolled aggregate. Turning quickly may cause the aggregate to roll over, exposing some of the asphalt. Braking suddenly may cause the wheels to lock and shove aggregate. These situations can occur with any asphalt but are most likely to occur when using asphalt emulsions.
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  • Improper tire inflation pressures on construction vehicles.

If any of the above situations occur, the seal patching crew should repair the spot before rolling. Tires should be cleaned immediately and the condition remedied before the situation gets worse.

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