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Comment by whalesalad | Hacker News Reader

An underlooked quality of Asphalt is that it is the most recycled material on earth. Asphalt can be, and generally is, made of old asphalt.

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They are rebuilding a concrete highway next to my house.

A thing I thought was really interesting is that they broke up the old concrete (which surprising to me had no rebar in it), and then ran it through a rock crusher and used all that rock to build up the road bed and I assume also used as aggregate in the new concrete they put down.

I thought that was neat because I've read a bunch of places that old concrete isn't really ever re-used for anything.

Here in Seattle, when they took down the viaduct along the waterfront, they also decommissioned a tunnel that the viaduct connected to. The decommissioning consisted of leaving the tunnel intact and filling it with ground up concrete from the viaduct.

This was the topic (in part) and title of a Practical Engineering video just last month.[0]

[0]: https://youtu.be/XKFaC5RYbEM

Including the fact that asphalt is generally mixed with tire rubber, therefore recycling old tires. Plastic is now also being added to asphalt to recycle when constructing roadways.

I wonder where that recycled rubber goes as the road surface wears away...

It probably joins all the rubber coming off the tires, settling on or near the road and then washing into the waterways.

In the air as particulate pollution and then it's washed into the waterways.

Do the plastics get released during degradation or do they make up a more tightly bound part of the mixture?

Yes. It was a while ago, but I was linked to something describing how asphalt leeches chemicals into the surroundings.

Uh, yeah. They use recycled tires in astroturf installations as well and there was an epidemic of soccer goalies getting some rare form of cancer due to diving in the "dirt" and getting exposed to the chemicals in it.

Google Amy Griffin or read this https://www.ehhi.org/turf-cancer-stats.php

An Indian scientist named Rajagopalan Vasudevan developed and patented this process. It reduces the bitumen used and improves the quality of flexible pavements, resistace to wear and tear. I think this is a brilliant way to use some of the plastic waste. Also the patent is royality free.

https://en.m.wikipedia.org/wiki/Plastic_roads

This is brilliant. Asphalt mostly consists of polycyclics. Maltenes make up the continuous phase, and are typically alkane-modified resins. Compared to plastics, these are much shorter chains. They are also prone to UV degradation which causes brittlement, leading to road degradation.

Asphalt is just nasty stuff. I'd love to see it get replaced with alkanes (polyethlene and polypropylene from plastic, polyisoprene from rubber)

That’s interesting. I wonder what goes into reusing old asphalt (i.e. what new chemicals need to be added/removed in order to make old asphalt pliable or whatever the term is to lay it again).

It isn't just the asphalt that is recycled but many other products are recycled into it as well, shingles, tires, and even glass, can all end up into it.

what they take off the road can be ground and used on site for a layer but mostly its carted off and ground to the same size as the original aggregate and combined into new. since the binding material is still present it reduces the need for more oil byproducts

Super interesting. I always assumed it was a gigantic use of oil globally. Sounds like it doesn’t have to be.

But most of those items are oil originated. At what point can we make asphalt without any new oil?

The term describing this is: RAP (Recycled Asphalt Pavement).

https://www.fhwa.dot.gov/publications/research/infrastructur...

That sounds great for the environment - let’s grind and hide this pile of trash for a few decades, what could go wrong.

I'm sure it goes through some cleaning but generally just heating it up makes it pliable again.

Heat, probably.

Generally it's crushed and a percentage of the final mix uses the recycled crushed asphalt. The quality of the asphalt is lower with higher amounts of recycled materials so high traffic roadways like highways may use 30% recycled asphalt. While lower traffic roadways or things like bicycle paths can use a higher percentage with new binder in the mix.

Recycling asphalt takes mostly heat. You lose some of it on the process, so you must add some new asphalt, and the new asphalt doesn't have the same composition of the original one (but is still asphalt, or asphalt components).

Milled and Crushed, heated, some new aggregates mixed in, new oil, good to go.

You can look up mobile asphalt batch plants if youre interested

There is an interesting video about this on practical engineering:

https://www.youtube.com/watch?v=XKFaC5RYbEM

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