Plastic in Roman Concrete?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

Answer 4

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.

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