Talking Asphalt: Managing segregation – November 2011

Talking AsphaltThe bad news is that segregation of asphalt mixtures is a major detriment to long-term performance. But the good news is that it is readily managed. Actually that’s not news; segregation is an old problem. We know how to deal with it; we just have to do it. If you are relying on a Materials Transfer Vehicle (MTV), you still need to follow good procedures. MTVs are good tools but they are not always available or suitable for every job.

I am talking about materials segregation rather than thermal segregation—although the two problems may be related and addressed by similar handling practices. By segregation, I mean the non-uniform distribution of the coarse and fine aggregate particles within the asphalt mix and/or pavement.

Segregation is visible as a concentration of coarse particles in one area and fine particles in another area. The coarse areas are much more obvious and are characterized by open texture and low density—compared to the non-segregated material. These open areas allow air and moisture to enter the pavement, which leads to durability problems, such as raveling, cracking, joint failure and potholes. Stripping and premature asphalt oxidation can also occur.

Segregation can be initiated at a number of points in the production process. Here are some reminders of what to do to minimize the problem.

Mix design
Segregation is most likely with gap-graded or coarse mixes and those with low asphalt content (reduced cohesion.) Small increases in asphalt content, as little as 0.2 percent, can improve segregation. But be careful—arbitrarily adding asphalt is costly and affects volumetrics. An asphalt content less than that corresponding to the minimum VMA is susceptible to segregation. Similarly, mixes with excess P200 may be dry and prone to segregate. And mixes containing aggregates with widely differing specific gravities may tend to separate. Try to develop a well-graded mix with adequate asphalt content and a workable binder film thickness.

Aggregate stockpiling and handling
The coarser and heavier particles tend to move further than the smaller and lighter pieces, so avoid conical, sloped piles. Build layered piles. Use short-graded, uniform aggregate sizes.

Plant operations
The aggregate should uniformly flow from bins, not rattle or trickle out. Do not allow it to flow out in a reverse cone. In batch plants, the hot bin configuration influences segregation. Avoid dead corners and the build-up of single sizes (think dust slides.) In drum mixers, the large particles tend to travel faster. Lower the slope of the drum, add flights, etc., as needed.

Loading and unloading
When filling a silo, drop the mix vertically. Do not trickle it into the bin and avoid low bin levels. When loading the truck, confine the mix and make multiple drops—front, back and middle—with the first and second drops as close as practical to front and back of the truck. When unloading the haul truck, unload the mix in mass—not a slow trickle.

Paving operations
Flood the paver hopper; avoid letting the coarse particles rattle out. Avoid dumping the hopper wings. Never completely empty the hopper; keep it 25 percent or more full. Augers need a continuous slow flow of material, keeping the mix level above the auger. Set the screed/crown adjustment so that the mix flows smoothly. Avoid raking the joint and minimize hand-working as much as possible.

Managing segregation is about paying attention to the details. If you need more information, a quick internet search will find many good resources. I recommend those written by Jim Scherocman and Don Brock. Now, remember to pay attention to the details and do it right!