Talking Asphalt: Segregation still a problem – October 2013

When I was asked to begin this blog, I was told that one thing that I could write about was my “pet peeves.” Well, you get to read about one of them this time. I may have mentioned this topic before but it is still a problem. And, an unnecessary one. I’m talking about segregation in asphalt pavements – specifically end-of-load segregation – those coarse, open spots corresponding to the end of each load of mix.

The reason this is a sore point with me is that it should not be a problem. There are some problems in asphalt pavement performance that are difficult to overcome. For example, in my experience, eliminating reflective cracking is a big challenge. Managing segregation is not that difficult. I believe this type of segregation is largely the result of poor workmanship. We know how to avoid this problem. It is not a technical puzzle and it does not cost any significant amount to avoid.

I cannot cover all the best practices here, but I believe one should not complain without offering some solutions. So, here are a few reminders.

In essence, managing segregation amounts to avoiding situations where the aggregate can roll down a slope and separate by particle size. Limiting the distance and slope — that’s really the deal.

The workmanship comes in to play in any number of steps in the production process including:

• Stockpiling

The problem starts here. Avoid steep-sided stockpiles where the big rocks rattle to the bottom. The loader operator can have a significant influence as he works the face of the stockpile.

• Plant operations

Avoid allowing the sizes to separate at cold feed bins. Keep the level up in the surge bin such that the material does not fall and/or roll far.

• Haul-truck loading and unloading

These operations are some of the more critical efforts – and often the most overlooked. The multiple-drop pattern of loading is well documented, and it works. Again, the aim is to minimize the slope and distance any particles that separate from the mass of material can travel. During the unloading process, do not let the trickle out. Raise the truck bed and submerge any coarse pieces at the end of the truck with non-segregated material before releasing the tailgate and charging the paver hopper.

• Paver-hopper management

We all know to not dump the wings of the paver hopper, where the coarse pieces collect, but it is still happening. Don’t do it; leave any coarse pieces in the corners and waste them as necessary.

Use a materials transfer vehicle, where possible. We’ve built a lot of good roads without them, but they help reduce segregation by mixing and blending multiple loads. You can build a good pavement without one, but many agencies are requiring their use because we have not done a good job. Mix design plays a part, too. Dry, harsh mixes are susceptible to particle separation. In addition to being less prone to segregation, good mix designs contribute to better compaction and greater durability. But that’s for another blog entry.

There is a lot of good information on how to minimize segregation. We know how to fix this problem. We just have to do it. Good roads do not build themselves; we have to do our part. Stop segregation.

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