Section 13: Spreading the Aggregate
Introduction
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.
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.
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.
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.
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
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.
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.
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.
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.
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.
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:
- 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 for more information on the proper time to begin spreading the aggregate.
- Too much asphalt has been applied.
- A puddle of asphalt may have leaked or spilled onto the pavement without cleanup prior to aggregate application.
- One of the discharge gates on the spreader may have clogged momentarily, preventing the aggregate from covering the asphalt.
- Failure to use deflector nozzles and overlapping the shot in the second lane causes an excess of asphalt.
- Detouring traffic onto the fresh seal may cause aggregate pickup.
- 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.
- 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.