Mad About Science: Roads

By Brenden Bobby
Reader Columnis

Roads in Bonner County come in a lot of varieties — from dirt and gravel to asphalt concrete, and something that looks like it just got bombed out during a military shock-and-awe campaign. So what differentiates a trail from a road? What goes into making and maintaining a road? Why can’t that pothole that reminds me of the Sarlacc Pit from Return of the Jedi get fixed after four years?

As far as I can tell, there isn’t a strict legal definition of what a “road” is in Idaho. [Editor’s note: Idaho Code seems vague on this point, with I.C. 40-109(H-5) defining “roads” along with “highways,” which are “laid out or established for the public or dedicated or abandoned to the public.” The same definition applies to “streets, alleys and bridges.”]

Generally, a road is a thoroughfare or path that has been improved in some way for travel — you may notice that doesn’t necessarily mean vehicular travel.

Based on my understanding, the difference between a road and a trail is that trails are primarily used in park settings, while roads can be used anywhere, including parks. Some asphalt junkies out there may be able to tell you a more precise difference between the two.

Dufort Road. Photo by Ben Olson

A lot goes into creating a road. I can attest to the incredible effort that goes into maintaining a dirt road on a mountainside — the Herculean effort put into my neighborhood pothole patrol is an ongoing process, as the downhill flow of water wreaks havoc on light sedimentary soils. Massive shout-out to my neighbors — you know who you are.

The challenges that come with maintaining gravel roads also affect paved roads in similar ways. Water is an incredible and destructive erosive force on Earth, capable of carving canyons from the soil given enough time and pressure. Paved surfaces tend to come in two types: concrete and asphalt concrete.

Concrete on its own is tough and sturdy, capable of supporting immense loads, such as truck traffic and high-volume automobile traffic. You see this most frequently in metropolitan areas on interstate highways and overpasses. Concrete can erode and be worn over time due to the friction from tires, which is worsened by the presence of settling or flowing water. 

Virtually all paved roads aren’t totally flat as you might expect. They are slightly pitched with something called a crown in the center that helps water flow away from the roadway and toward the banks. Water settling in ruts in the roadway can cause serious damage to vehicles and the road itself.

You’ve likely hydroplaned briefly during a drive into town on a rainy day. The sound of the wheels on pavement changes as you glide across a pool of water — but why does this happen? Essentially, the wheels are moving fast enough to lose contact with the pavement as water goes between your tire and the road surface. At this point you’re driving on water, which has no traction — meaning you’re water skiing in a half-ton vehicle.

Water settling in ruts can also freeze and contract, which causes cracks and breakages in the asphalt that are agitated by heavy vehicle tires breaking it apart.

This makes things difficult when you must consider that concrete isn’t known for being grippy. You want your tires to be able to grip a roadway, so you don’t skid all over the place. To achieve this, the concrete must be grooved before it cures using a machine that looks like a truck with a big rake on it. This creates channels for water to settle.

Asphalt concrete, sometimes referred to as blacktop, has some tradeoffs compared to concrete roadways. Asphalt is a mixture of aggregate stone and bitumen, a residual compound left by crude oil production. This stuff can be heated up to create a sticky tar-like substance and laid down in strips over subgrade materials like prepared soil. 

It becomes stiffer and more brittle in cold temperatures and sticky and malleable in hot temperatures. Utilizing additives, we can keep asphalt from getting too squirrely on us during temperature extremes. Unlike concrete, asphalt is nearly 100% recyclable. It just needs to be heated up to the proper temperature, scraped up and put back down.

This is an oversimplification of the process of asphalt creation. Mixing bitumen with any type of rock will not provide a uniform result. Certain minerals work better when mixed in this way, which can influence how the road surface will end up. You don’t want your road to look like the dunes of Arrakis, you want it to be mostly flat and predictable.

This can become a serious challenge in areas with naturally occurring minerals that don’t mix well with asphalt. You’ll see this in action in places like Wisconsin, which use a quartzite mixed with the asphalt that doesn’t create a blacktop, but a redtop instead. The red hue comes from iron found naturally occurring within the quartzite. 

Because asphalt is relatively stretchy compared to concrete, it works better in areas where the ground may shift or settle since it won’t crack and split as often. Once it does, it can be repaired relatively easily — often in springtime around here, which you’ll notice as the state flower begins to bloom. I know you’ve seen it: it’s large, orange and has black letters on it that reads “ROAD WORK AHEAD.”

Another form of asphalt repair is chip sealing. This is done by laying down a thin layer of tar or asphalt and then rolling gravel over it to encourage it to sink into cracks and crevices and seal up damage. This will often leave the road rough and bumpy and works as a Band-Aid until it can be repaved.

There’s a lot that goes into building roads. Are you interested in learning more? Check out Engineering in Plain Sight: An Illustrated Field Guide to the Constructive Environment by Grady Hillhouse from your local library branch. Both copies were out at the time of this writing, but that’s why you can place holds at the desk.

Stay curious, 7B.