What Causes Asphalt Segregation? Consistency from the stockpile to the screed matters.

A good mix design can lose its performance advantage when coarse aggregate separates from finer material or portions of the mix cool unevenly before compaction. These problems, known as asphalt segregation, can create weak spots that lead to raveling, cracking, and premature failure.

Segregation takes two forms. Aggregate segregation occurs when larger stones separate from smaller particles and fines, leaving an uneven distribution of aggregate sizes in the asphalt mix. Coarse mixes tend to segregate more than fine mixes due to the separation in aggregate size. Thermal segregation happens when portions of the asphalt mix cool at different rates, leaving cooler areas that are more difficult to compact. Both types of segregation can compromise pavement density and durability.

Several steps during production and placement can contribute to asphalt segregation:

• Stockpile handling. Building tall, cone-shaped stockpiles can cause larger stones to collect at the bottom, separating them from smaller particles and fines. Material taken from different parts of the pile can then contain different proportions of aggregate sizes.

• Plant operations. Inconsistent feeding, improper silo loading or discharge, and worn equipment can disrupt mix uniformity.

• Truck loading. Depositing the mix in one large pile allows larger particles to roll toward the ends and sides of the truck bed. Loading in multiple drops, first near the front, then near the rear, and then in the middle, has been proven to reduce separation.

• Paver operation. 

o   Running the hopper too low. Coarse particles that have collected near the hopper’s edges can feed through together instead of blending with a larger volume of mix, creating coarse patches in the pavement.

o   Frequently folding hopper wings. Folding the wings dumps accumulated material, which is often cooler and coarser, into the active flow. This can send a concentrated pocket of that material through the paver.

o   Uneven flow through the augers. An inconsistent supply of mix can leave parts of the augers exposed, allowing larger stones to roll away from the main mass of material and collect in streaks or pockets.

o   Recommended best practice videos by NAPA, funded by AAPTP:

• At the plant: https://www.youtube.com/watch?v=_7AeHS9nSA0

• At the job site: https://www.youtube.com/watch?v=WU7zBCHrwCU

Temperature differences add another challenge. Cooler pockets of mix are harder to compact and may leave areas with higher air voids and greater susceptibility to water intrusion and early deterioration.

How can you reduce the risk?

Prevention requires consistent handling throughout the process. It’s important to manage stockpiles carefully, maintain plant equipment, load trucks in multiple drops, and keep a steady supply of material through the paver.

It’s equally important to monitor temperatures across the freshly placed asphalt mat to identify cooler areas that may be difficult to compact. Watch for recurring rough patches, coarse streaks, or areas of low density. These patterns can help trace the problem to truck loading, material cooling, or paver operation. If any of these conditions are noticed, investigate promptly so crews can correct the cause before placing more affected pavement.

Bottom line:  Once the process starts, watch for segregation and be consistent. Keep the process moving.

This article was prepared byasphalt materials engineer Phil Blankenship, PE, MSCE. He and other members of the BATT Team work with agencies, producers, and contractors to evaluate asphalt mixture performance and extend pavement life. He has authored or co-authored numerous peer-reviewed papers on asphalt technology