What's in your highway?

This is volcanic ash found in Grant County.  The photo has been magnified 200 times.

When you think about all the things that go into making a highway a lot of things come to mind, but one you may not expect is volcanic ash.

KDOT and other DOT's sometimes use volcanic ash as a mineral filler in asphalt. Since volcanic ash is angular, it can be used to increase the very small voids in the mix which allows the asphalt oil to better flow through the mix and bind everything together.  Mineral fillers in the correct proportion can also stiffen and toughen the mix to help prevent cracking. When a filler is used, it is a fairly small portion of the total aggregate weight of the mix: for volcanic ash it is five percent.

Magnified photo of volcanic ash in Russell County.

Volcanic ash is not used in all asphalt, but since Kansas has a ready supply of it, volcanic ash is used in some mixes. Western and central Kansas has vast volcanic ash deposits that have been mined for nearly 100 years. Most of the ash was deposited during a period of volcanic activity to the west; central and northern California, the Yellowstone caldera, and the northern New Mexico area. Most of the ash was deposited  between 0.5 and 2.0 million years ago.   

Many of the deposits were laid down from fall out of ash clouds and then the ash was carried by water into basins where it accumulated in thicker deposits.  Some ash layers are 15 to 20 feet thick. 

While volcanic ash is only used in some asphalt projects, KDOT is doing research to determine whether it might be an option for concrete projects too.

 

Here's a volcanic ash deposit in Lincoln County.