Discover more from The Polymerist
Build Me A Road
A bit about bitumen
I think in the past we used to think about the future of how we would be in flying cars. I think in Back To The Future this sentiment was captured perfectly by Doc:
Our advances in technology though seem to be pushing us closer to a world lived online, but in order to get our pizza delivered we still need these pesky things called roads. It also just so happens that roads are completely dependent on companies like ExxonMobil extracting and refining petroleum.
If you’ve spent any significant amount of time driving around the United States you may eventually come across roads made from concrete as opposed to asphalt. You may have also driven on gravel roads (common in the larger states out west) or unimproved roads (rural New England/Appalachia). I’m thankful we have our current road technology, but our modern road infrastructure is wholly dependent on the distillation of crude oil.
Asphalt roads are essentially hot bitumen mixed with an aggregate such as stone and/or recycled shingles. Bitumen can be a naturally occurring substance, also known as pitch, but it can range from a black rock like looking material to a oozy soft tar like substance. When crude oil is distilled (just like ethanol distillation)) the stuff that cannot be distilled remains in the bottom of the “pot” and this is where we get the volumes of asphalt we need for building the roads (and the shingles) we use today. A great history of asphalt can be found here and the asphalt, also known as bitumen or pitch, is part of the longest running science experiment in history.
Asphalt is a strange substance. It’s a mixture of different aromatic carbons, often referred to as asphaltenes, and it does have an ability to flow under high heat while also acting as a solid at temperatures normal to the surface of Earth. You may notice when driving on a particularly old road that it appears deformed or rutted by the tires of cars or trucks and this is because asphalt does flow given enough time, heat, and pressure. This is partly why we need to resurface asphalt roads more often and why concrete, if you have the time to let it cure, can be a better road paving material.
The issue with concrete roads is that it can take months for it to cure (hydration of cement), but it is more durable than asphalt. When you are driving you might notice that bridges spanning across other roads or rivers are actually constructed from concrete and the paved surface you drive on is also sometimes concrete. When those concrete structures fail (and they will, given enough time) it requires a lengthy and costly repair. Extensive construction times combined with costly repair make concrete difficult to scale.
Asphalt on the other hand is relatively easy and quick to repair (Boston could be better at filling potholes) and in the event it cracks you can seal those cracks quite easily using rubberized asphalt. Further, when asphalt roads are removed during the repaving process the old road can be recycled back into the new road. More differences between concrete and asphalt roads can be found here. I can’t imagine a modern road not using asphalt and this is a problem.
Both asphalt and concrete road making are carbon intensive. Concrete requires the use of cement (produced by heating up limestone with other stuff to create carbon dioxide and calcium oxide) and transport of heavy aggregate and water. Asphalt requires either mining of naturally occurring pitch or the extraction and refinement of crude oil. These are really the two options for modern road building and there is nothing else that might work at the cost and scale needed.
Try driving at 80 miles an hour in your Tesla on a gravel road. It’s not going to be a pleasant experience. Without smooth roads made from carbon intensive technologies we have no other options to transport our goods and people across great distances over land quickly. Yes, rail transport exists, but every Wal-Mart, Target, and Costco that exist in the world do not have railroads going past their loading docks.
If we are serious about trying to transition our world economy away from crude oil we need to figure out better ways to build roads. The challenges around road building are primarily one of cost and performance. We need something that is as low cost as asphalt with similar durability that can be deployed at scale and is ideally sustainable.
There are some interesting nascent technologies such as building roads from waste plastics or from lignin. I think both might be viable and play important roles in transitioning our road technology from bitumen to something else. Or at least bridge us long enough to when we can fly around in our cars like Doc and Marty.