Eli5: why is asphalt used on roads ?
In: Other
Awesome, one of the things I studied and do work with on the daily.
Asphalt binder is relatively cheap as compared to cement. It is the excess “sludge” from refining crude oil while cement is made from burning multiple ingredients at 2000-3000 degrees F.
Asphalt has a Viscoelastic property to it. Which means that as temperature changes, so does it viscosity, and elastic response to loads. When it’s cold outside, it is tougher and won’t flex as much. When it’s hot outside it will flex more.
There are hundreds of not thousands of different types of asphalt binders. They are categorized by a method developed by Superpave in the 1990s. In Alabama they use a Performance Grade (PG) 67-22. The 67 is the high temp range in Celsius of the binder for it to not rut to failure. The -22 is the low temp range for it not to be too brittle at cold temps and crack.
This PG can change based on the binder source. Colder states like minnesota might use a 58-28 to account for lower high temps and lower cold temps.
If the right grade of asphalt is selected then you can prevent rutting and cracking. Rutting is when the road is too soft or the load from traffic is too high and the wheel paths sink down. This typically happens at intersections when traffic sits for longer periods of time. Cracking is when the road is not elastic enough to “relax” under loads and has to crack to relieve stress.
Asphalt pavement can be paved in one continuous stretch, while concrete must have joints every 15-20 feet to allow the segment to expand and contract with temperature.
It also can be driven on relatively quickly as compared to concrete which may need 7-14 days before trafficking.
If the road is paved with a strong granular or asphalt base, only the top 2 inches needs to be replaced every 15 years or so. Unless cracking/rutting is bad from poor designing or unexpected truck loads.
Asphalt pavement is recycled all the time. It’s relatively used in new roads taking up 10-40 percent of the mix depending on the state and project.
Ask me anything else below this and I’ll get to it. Sorry for any typos!!
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