While we were playing with viscosity inside, there were big doings outside. We repaved our driveway this week. Like many driveways (and streets and parking lots), ours is paved with asphalt, a form of petroleum that can be found in nature (think LaBrea Tar Pits) but is most often refined from crude oil. Asphalt is not pure pitch tar (see yesterday’s post), but is used as a kind of glue to hold together bits of concrete that make up most paved roads. (When you see black squiggly drips filling cracks in older roads, that substance has more pitch tar in it than asphalt). The Ireland 69-years-to-get-one-drop-experiment started because pitch tar is a substance that seems solid at room temperature but if you put it in a funnel and wait long enough, will drip like a liquid. If you hit it with a hammer, though, it will shatter, so its viscous qualities are interesting for scientists to study. They are also very useful for a job like paving, which requires a substance that is malleable when putting the road in, but that will harden up to the point where trucks can drive on it. Its level of viscosity is very high, making it almost a solid because it does not pour easily (whereas liquids like honey, syrup, and water have progressively lower viscosity than pitch tar).
Paving takes a lot of machines, big and small. The first one to rumble down the street was this one, called a cold milling machine. It was used to pulverize the existing roadway to a recycled, reusable material. It has a 4’ mill which is able to grind up existing asphalt and load trucks with a conveyor belt system. It can recycle pavement to a depth of 13” and churns out a recycled gravel concoction called RAP, for Recycled Asphalt Product.
In the places this behemoth could not reach, the old asphalt was removed the regular way. You can see how hard and brittle the old asphalt has become after years of exposure to heat, rain and ice – not much viscosity there!
Then, graders and rollers came in to create a smooth base surface for the new driveway.
A few days later, the recycled asphalt arrived in pellet form in a dump truck and was poured into a paving machine.
As the machine churned out the asphalt, the worker adjusted the thickness of the mixture and others applied detergent to the equipment and tools to keep it from sticking to the surfaces used to make the of the driveway smooth.
Next, they used shovels to scatter more asphalt over the top of the new pavement, which they then molded and pressed to make sure that rain will drain off the driveway in the proper directions.
People can walk on it within a few hours of paving; the guys who smooth the surface and shape the edges wear special flat sandals over their books to make sure they don’t leave footprints while they work on the hot asphalt.
It will be four days before the asphalt has cured sufficiently that cars can drive on it without making an imprint.