|These coarse screens help separate rocks by size. |
Placing asphalt friction course on a newly constructed road is sort of like icing a cake (stay with us here).
Not only does it represent the final step of a process, but icing and asphalt concrete can both help make the completed product look just a little more finished.
However it’s all for nothing if you don’t have a good recipe … and unlike icing, the directions for mixing up asphalt friction course can’t be found in a cookbook.
Still with us? Good, because this is where ADOT’s Pavement Materials Testing
group comes in. They work to develop and continuously improve the asphalt “recipe” for various projects around the state.
When creating an asphalt friction course mix design for a particular job, the goal is to design it so the finished product is strong and holds up to weather extremes. You also want a road that doesn’t crumble and isn’t too slick.
To get a good recipe some testing has to take place in the kitchen … um, we mean lab.
The Mix Design lab is where ADOT technicians test out the asphalt mixes and develop a friction course design (the recipe) based on materials and the location and conditions of the planned road.
This lab also does some research into innovative methods…
“We’re always trying to take advantage of new technology,” said Manager Janet Doerstling. Aggregate Testing
The Pavement Materials Testing group is also responsible for evaluating samples from different projects just to make sure everything is meeting specifications.
Here’s how it works … a contractor will provide samples of the aggregate (rock) they plan to use. (By the way, aggregate is what gets mixed with asphalt -- a sticky, black substance that acts as a binder -- to create asphalt concrete.)
Next, the rock sample undergoes a series of tests, including…
The sieve shaker -- a coarse screen that takes the rocks and breaks them down by size because it’s important to make sure you’ve the right percentage of rock to sand.
Several tests are done in the Sand Equivalent lab. Here technicians examine different aggregate properties. One of the tests looks at how many straight, flat edges (aka faces) the rocks have. This is important because fresh faces soak up oil better in the asphalt mix.
This lab also tests the aggregate for resistivity to electric current because some rocks can actually corrode corrugated drain pipes! Wait … there’s more
This is far from all the group does … but we’ll save the rest for tomorrow. Until then, check out their webpage
and go have a piece of cake!