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Word on the street is that using recycled plastic to make roads can help reduce carbon emissions, improve road quality by making it longer-lasting and less susceptible to potholing.

This revolutionary idea has me thinking. Plastic has been used for thousands of years--it's just that the modern definition and composition of "plastic" did not start until 1907. A good example of "plastic" is rubber used from a rubber tree. It was used by the Mesoamerican tribes as far away as the third millennium BCE.

The problem is that natural rubber trees are indigenous to the jungles of Latin America, and the rubber tree plantations used in India and China were cultivated from seeds taken by British scientists and financiers.

Let us pretend, then, that in an alternate Earth, a species of wild rubber is common in the Mediterranean coastlines of southern Europe and had been used by the Romans for cultivation for their sap. This would be the sort of natural plastic to be used for concrete construction, to make the structure last longer against the elements than subsequent "Portland reinforced" concrete. In OTL, volcanic ash already added resistance to Roman concrete, but in this alternate Earth, could using rubber, one of the most natural of plastics, have the same effect?

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    $\begingroup$ Plastic makes a terrible road surface. It would not/will not be used as a road surface. $\endgroup$ – Joe Kissling May 25 '17 at 18:20
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    $\begingroup$ Is the plastic like substance going to be used to add structure, or adhesion, or any of a number of other things we use cement for. Is it going to be biodegradable? If moisture and local bacteria will break it sown over a few years, it may not be great $\endgroup$ – Paul TIKI May 25 '17 at 18:36
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    $\begingroup$ Natural rubber is not very good at resisting "against the elements"; it is very much worse than concrete. At least concrete is not damaged by light. $\endgroup$ – AlexP May 25 '17 at 18:38
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    $\begingroup$ @JohnWDailey Recycled plastic is not suitable for UV exposure, it will degrade under the sunlight. During the heat of the day, it will expand and become malleable and deform under the weight of cars. All the pictures/videos show a road that is hopelessly unsuitable for traffic. $\endgroup$ – Joe Kissling May 25 '17 at 20:22
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    $\begingroup$ @JohnWDailey Because these ideas are not well thought out. The biggest issue is that recycled plastics do not tolerate UV exposure, they will degrade in the sun. There is no mention at all in mitigating that. Also the listed opening temp is 170 F which is not high enough to be used for roads because it will deform in the summer heat. I have no doubt the strength of the material, but its environmental hardiness I see lacking. $\endgroup$ – Joe Kissling May 26 '17 at 2:02
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Um...no.

If you look at the link you opened with, you'll notice that the proposal is to use plastic in place of bitumen (asphalt, tar...it has various names) in tarmac roads, not concrete ones.

Tarmac is literally just gravel mixed with a binder - currently we use bitumen for this purpose, also called asphalt or tar. Its role is to keep the gravel from shifting around as vehicles move over it, while the gravel prevents the bitumen from being worn away. In the proposal above, the idea is to replace bitumen with recycled plastic.

Concrete is a totally different thing. Concrete consists of gravel, sand, water, and most critically, cement. The cement binds with the other ingredients and undergoes a chemical reaction that causes the whole matrix to become effectively solid stone. It's that chemical process that gives concrete its strength - and rubber has none of the necessary properties to replace it.

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    $\begingroup$ Also, the the asphalt in old roads is recycled: they scrape up the old road material, haul it away, heat it up, mix it up, add some more asphalt and aggregate, then lay it down on the next road that needs to be repaved. $\endgroup$ – RonJohn May 25 '17 at 18:52
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No, Roman concrete and asphalt are not the same thing.

No, rubber is not a suitable replacement for bitumen in asphalt roads.

However...

The Romans were famous for building roads, and asphalt concrete is great for roads. Asphalt is also useful for coating concrete structures to provide a degree of waterproofing (asphalt shingles being probably the most popular roofing material--in the USA, at least).

So the question becomes, could the Romans utilize asphalt to create roads and as a water-proofing pitch? And, if so, what role could rubber play?

We know the Romans had access to bitumen (the pitch/binding agent in asphalt concrete). The Egyptians used bitumen as a mummifying agent which they harvested from the Dead Sea. The Romans' name for the Dead Sea? Palus Asphaltites (Asphalt Lake).

Now we're getting somewhere...

So, the Romans had access to bitumen/asphalt and undoubtedly knew of its water-proofing properties. The problem is, it's hard to acquire in the kinds of significant quantities you'd need to build a few thousand miles of roads or roof a few thousand buildings without modern oil-drilling equipment. It wasn't in widespread use as a ship caulking agent until the 1400s for this very reason. What to do?

Enter rubber

Rubber, on its own, does not provide a suitable pitch-like binding agent for asphalt concrete. However, rubber has been mixed into asphalt concrete for 170 years. This increases the durability/longevity of the road and provides a partial replacement for bitumen. So, while you can't entirely get rid of your dependence on bitumen, you can significantly reduce your dependence on it by substituting rubber for some of it, and also it provides a longer-lasting material.

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One of the reasons Roman concrete structures have lasted so long, in particular those actually in water was the Romans used volcanic ash in their concrete formula.

While naturally occurring wild rubber plants in AE (alt-earth) would be interest and somewhat difficult to derive a reason for volcanoes are quite ready to take their place.

Make roads with Roman concrete, use the ash Luke.

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  • $\begingroup$ And what is the problem with a volcanic solution? $\endgroup$ – dcy665 May 25 '17 at 20:07
  • $\begingroup$ In my alternate Earth, volcanoes are less accessible in the Trojan Empire than in the Roman Empire. jdailey1991.deviantart.com/art/… $\endgroup$ – JohnWDailey May 25 '17 at 21:24
  • $\begingroup$ Which wasn't in the question. Why didn't you just say, "I am using rubber" as that is the answer you had decided on. Rubber will not work. But it's a story so whatever. $\endgroup$ – dcy665 May 25 '17 at 23:25
  • $\begingroup$ It didn't seem relevant. Also, a story still needs realism, otherwise it's not believable. $\endgroup$ – JohnWDailey May 26 '17 at 0:52
  • $\begingroup$ @JohnWDailey - Okay, so using rubber plants that are world wide (and not believable) is realistic and will not work as road materials is more believable than using volcanic ash (which was actually used by our Romans). And Earth is still ringed with volcanoes so that's not realistic either. Right. $\endgroup$ – dcy665 May 26 '17 at 19:53
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The plastic roadways idea is pure bunk, on every issue. The ancients new that asphalt was an amazing hi-tech material for roadmaking, and would quickly determine that resin composites are not.

Adding fibers to concrete may be helpful, but has issues in getting it to work right. So you don’t need rubber or “plastic” technology for that, but can use horsehair etc. For cases where that works, see the caulk used in shipbuilding — made using fiber recycled from old rope. This is used to make a flexible seal, not a hard-wearing surface.

The ancients did not need rubber to understand how to use composites. Other materials were available, and did indeed find suitable uses. Road surface is not one of them.


※ I contributed significantly to the funding of this “busted” video.

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    $\begingroup$ If you follow the link, you'll see it's a different use of plastic in roadways, not the one busted by Thunderf00t. It's a proposal to replace the bitumen in roads with recycled plastic, not to make the entire road out of plastic sections. $\endgroup$ – Werrf May 25 '17 at 19:00
  • $\begingroup$ I think that is covered in the video. It’s been a while since I went over it, but bitumen rings a bell. $\endgroup$ – JDługosz May 25 '17 at 19:01
  • $\begingroup$ OP's link uses recycled plastic as a replacement for asphalt/bitumen in the concrete, not as a purely plastic road (as in your link). As a road surface, pure plastic isn't a good idea. As a binding pitch for concrete, it's a less ridiculous idea. $\endgroup$ – Azuaron May 25 '17 at 19:02
  • $\begingroup$ @Azuaron Less ridiculous = ridiculous $\endgroup$ – Joe Kissling May 25 '17 at 19:04
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    $\begingroup$ @JohnWDailey LOL, there's a reason I don't read click-bait sites like that. $\endgroup$ – RonJohn May 25 '17 at 20:13

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