Asphalt Segregation: Causes, Patterns and Prevention

Asphalt segregation is what happens when a well-designed mix comes apart on its way to the road. Instead of an even blend of coarse stone, fine material and binder, some areas of the mat end up rocky and open while others are fine and tight. The rocky areas cannot be compacted to the density they need, so they hold more air, soak up water and ravel early. Segregation is one of the most common workmanship problems in paving, and most of it is preventable.

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Two kinds of segregation

Aggregate (physical) segregation: the gradation shifts from place to place. Coarse segregation, the common and more damaging kind, has too much large stone, low binder content, low density, high air voids and a rough surface. Fine segregation is the reverse. A common research benchmark treats a sample as coarse-segregated when it is at least 10 percent coarser than the job mix formula on the 2.36 mm or 4.75 mm sieve.
Thermal segregation: the gradation is fine but the temperature is not. Cooler patches compact less than hot ones. Both types end the same way, with pockets of high air voids and low density.

Where segregation starts

A joint AASHTO and NAPA task force identified five sources:

Mix design. Open-graded mixes and mixes with low binder content are most prone to it.
Stockpiles. Coarse stone rolls down the sides of tall cone piles. Piles built in layers or on gentle slopes hold together better.
The plant. Uneven cold feed, bins running low, dust buildup in batch plant hot bins and poorly aimed discharge chutes on silos can all separate sizes.
Trucks. One big drop into the truck makes a cone, and the coarse stone runs to the front, back and sides.
The paver. Hopper handling, auger speed and stop-and-go paving all play a part.

Patterns that point to the cause

The AAPTP Asphalt Paving Handbook, published online by NAPA, groups segregation by the pattern it leaves, which is the quickest way to trace it:

Chevron or V-shaped spots at regular spacing: end-of-truck segregation, usually from single-drop loading, made worse when the hopper wings are folded after every load.
A streak down the center of the lane: the auger gearbox is set too low or the reverse kickback augers near it are worn, so mix cannot flow under the center.
Streaks at the edges: paving wider than the augers reach without extensions, or mix not confined at the end gates. Raking at the joint can add to it.
One side or half the width: one auger starved of mix, or trucks loaded off center.
Random spots: often traced back to the plant or stockpiles.

How crews prevent it

Load trucks in at least three drops: front, back, then middle. Longer bodies need more, usually an odd number.
Unload as a mass: raise the bed partway before releasing the tailgate so the load floods the hopper instead of trickling out coarse-first.
Keep the hopper from running low: the handbook flags letting it drop below about a third full, folding the wings often, exposing the slat conveyors and setting flow gates wrong for the paving speed.
Hold a steady head of mix in the auger chamber, level with the auger shaft across the full width, with extensions and tunnels when paving wide.
Pave at a constant speed. When a stop is unavoidable, stop with mix in the hopper and augers. If the wait will be long, typically over about 30 minutes, build a transverse joint instead.
Remix when needed. A material transfer vehicle remixes the load before it reaches the paver, which helps with both physical and thermal segregation.

Thermal segregation in more detail

During the haul, mix near the truck bed's edges, bottom and top cools faster than the core. Our page on haul time and temperature loss covers why. If that cooler crust is not remixed, it goes down as cool patches. You usually cannot see them, so crews use infrared guns or paver-mounted infrared scanners. Washington State research found that temperature differences below 25 °F generally compacted evenly, while differences of 25 °F or more could leave uneven density. Insulated or tarped truck beds, shorter hauls and steady paving all help.

What it means for a driveway owner

On a new driveway, look for rough, stony patches that repeat at the same spacing, or a streak down the middle. Water darkens and lingers in those spots after rain. Bring it up with the contractor while the job is fresh. Mild cases may be tightened with a surface seal; spots that are already raveling are usually cut out and replaced. Because repairs mean new hot mix, check the patch area against the asphalt calculator when you review the quote.

Questions people ask

What is asphalt segregation?

It is an uneven mix in the finished mat. Most often coarse stone collects in some areas with too little fine material and binder, leaving a rough, open texture. Those spots have high air voids and low density, so they ravel, crack and let water in sooner.

What causes segregation in asphalt?

The AASHTO and NAPA joint task force on segregation named five sources: mix design, aggregate stockpiling, the asphalt plant, truck loading and unloading, and the paver. Coarse stone rolls down any slope it can find, so piles, single-drop truck loads and a nearly empty paver hopper all invite it.

What is thermal segregation?

Cooler mix from the edges, top and bottom of a truck load gets laid next to hotter mix. The cool areas do not compact as well. It looks like normal pavement and is usually only visible with infrared imaging.

How much temperature difference causes problems?

Washington State research by Willoughby and others found that mix temperature differences under 25 °F generally gave consistent compaction, while differences of 25 °F or more could lead to uneven density and air voids.

How do you prevent segregation when loading trucks?

Load in at least three drops, front, back, then middle, instead of one big drop. Longer bodies may need more drops. At the paver, raise the bed partly before opening the tailgate so the mix slides out as a mass.

Can segregated asphalt be repaired?

Mild surface segregation is sometimes treated with a surface seal to tighten the texture. Severe spots that are already raveling usually need to be milled out and replaced. The best fix is catching the cause during paving.

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