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December 21, 2010

AASHTO T-195: Determining the Degree of Particle Coating

We get asked a lot in the field about coating of aggregate under WMA conditions.  As Evotherm is a great surfactant, this is never an issue.  On one of my first jobs, the operator turned off the pump for a few minutes and then turned it back on, and the difference was like night and day.

The AASHTO method of determining coating is a bit cumbersome, to say the least.  There is a quick visual estimate that can be made, however, by comparing what you see at the drag slat conveyor to a standard image with a known % of light and dark areas.  I often take a high-speed photo (put the digital camera in "sports" mode or a fast shutter speed) and that will unblur the motion of the mix going up the conveyor.  Then you can clearly see the amount of coated and uncoated material.  Using Evotherm, it will be clear that there is much less than 1% uncoated material present.  The few specifications I have seen call for 3% or less based on the AASHTO method.  Counting less than 10 - 20 particles across the entire slat width, a few square feet of area, will be much less than 1% of the total.

Here’s a comparison chart  for visual percentage estimation. Better than coal in your stocking!

Author: Michael Plouff

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