Tuesday, May 24, 2011

Stock Pile Segregation

Stockpiling aggregate in a large conical pile can create segregation. The finer materials will stay in the middle of the pile where they stick when they fall from the conveyor. The coarser materials will roll to the outside of the pile as shown below:

Segregation is especially prevalent in materials with a broad range of aggregate sizes (like base rock) or aggregates that are gap graded (like many perms). Why does segregation matter to you? In hot mix asphalt (HMA), segregation in the aggregates can lead to inconsistent feeds into the hot plant, and as you all know...junk in...junk out. In Caltrans QC/QA jobs, the pay factor is based partially on consistency. Even if you are within the specification band the entire time you are producing mix, if the gradation of the material varies by 5-6% you can end up paying the state a penalty fee instead of making a bonus off of the job. In addition to these potential costs, having poor gradations in your HMA can lead to pavement problems as well. If the gradation is too coarse (you pulled the aggregate only from the outside of the stockpile) your pavement can have poor compaction, raveling, and/or rutting. If the gradations are too fine (you pulled the aggregate only from the inside of the stockpile) your pavement can have flushing and/or bleeding problems.
In concrete products, aggregate segregation can also lead to inconsistent feeds into the batch plant. If your gradations are too coarse, the concrete products can be hard to place and the strength can decrease. If the gradations are too fine, the shrinkage increases, water demand goes up, you need more cement, and/or the strength decreases.

So how do you combat segregation? One way is to use telescoping and/or radial conveyors. The smaller the distance that the material has to fall before it hits the top of the pile, the smaller the chance there is of the material segregating.  By utilizing a telescoping conveyor you can ensure that you are discharging the aggregate close to the top of the stock pile at all times.  In addition, you can use radial conveyors to create layered stockpiles, reducing the cone shape that seems to encourage segregation.  This type of stockpile is shown below:

If you can't justify a new conveyor you can also combat aggregate segregation using after production methods that I will talk about tomorrow. 

Have you used another type of equipment that helped you prevent segregation at your plant?

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