The title is the question.
Maybe it’s the cheapest thing with the right properties, but can’t it be painted with something a little more reflective, that won’t absorbe so much heat from the sun?
In: Technology
Black asphalt is better than most everything else in hot climates because it is **self healing**. In direct sun on a very hot day, the tar melts and reattaches across small cracks, which prevents them from becoming big cracks and then potholes. This is also why in colder climates the get potholes more frequently. On the other end of the spectrum, asphalt doesn’t work in places that get too hot for too long because the tar gets too soft and the street start to melt and the asphalt sticks to tires. Much of Texas have concrete streets for this reason.
It is much cheaper than the other popular type of pavement, reinforced concrete. A road surface that reflects a lot of light could be blinding to drivers but the same thing could be done with roofs
Asphalt used to be cheaper than concrete, but nowadays I think they’re fairly similar.
One big advantage though is you can drive on asphalt within hours of placement. With standard concrete roads you need to wait 2 weeks for it to cure to just 50% strength so you can let cars on it again. You can pay extra for “high early strength” concrete, but that’s still at least 3 days before you can let cars on it.
Asphalt is also easier to maintain, you can grind a couple inches off and replace it every decade or two.
paint ain’t cheap, most roads have painted lines, usually those painted lines disappearing on a road is a first sign the road is not being maintain.
I worked for Department of transportation testing materials that go into road construction, there is asphalt that is clear its just more expensive to make, I know on Cali they were testing a grey asphalt to lower Temperatues in city roads. It’s really not cost effective to change the binder color when roads are typically milled and filled every couple of years. The clear asphalt was to maintain a dirt road look for a rural town in Canada I believe.
Cool pavement does exist, and it cools down the whole street, and improves air quality, but most hot climate cities are in the so-called global south, and can’t afford it.
The “why is asphalt everywhere” part of your questions has been answered by others. To your “why aren’t streets painted white” question – sometimes they are!
Check out the cool pavement program in Phoenix! The government is testing solar reflective coatings in neighborhoods without tree cover to reduce surface temperature. There are other locations that are also evaluating solar reflective coatings for roadways, but Phoenix has the best website
https://www.phoenix.gov/streets/coolpavement
Tucson, AZ has applied titanium dioxide to some asphalt streets in an attempt to reduce road temperatures.
[News article from 2021] (https://www.kvoa.com/news/local/uarizona-cool-pavement-program-aims-to-beat-tucsons-extreme-heat/article_6f7da4f2-5de9-11ec-b2e7-93eca37efb7a.html) but no more recent news on how well it worked or didn’t
It’s not so much that they make it black, but rather black is the color that it is. The key ingredient is bitumen which is a petroleum product. You know what color petroleum is, right? So, that’s where the color comes from.
It’s cheap and fast. I’ve seen miles of asphalt paved in a single (long)shift. I’ve never seen more than a few hundred feet of concrete pavement done in a day. Interstates use concrete with an asphalt pavement overlay to get the durability of concrete, but the traction and repairability of asphalt pavement.
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