plastic roads

=roads =construction =materials

 

 

A few people have compared my blog to the youtube channel of Thunderf00t, mainly because we've both criticized some flawed technological proposals that got public attention. I thought I'd write a post on a topic he covered, so that people can compare our styles.

I picked "Plastic Roadways BUSTED!". I'd suggest reading this post first, then optionally watching that video for comparison.

 

 

Is it feasible to make roads out of plastic? In a sense, we already do.

Most roads are asphalt concrete (hereafter "blacktop"), a mixture of rocks and asphalt. Asphalt is a thermoplastic - a naturally occurring one, that has very low performance but has been cheap because it's produced as a byproduct of oil refining. I'd say it's most similar to LDPE, but with shorter molecules that make it weaker and more prone to cracking.

Largely because refiners have gotten better at producing less asphalt and more gasoline from oil, the price of asphalt has gone up significantly. So, this is a reasonable time to consider alternatives to blacktop, and more places are using concrete for roads instead.

Concrete works well enough. It's more durable than blacktop, but it's historically been more expensive. It also requires joints between blocks for thermal expansion, which makes annoying noises when it's driven over. On the other hand, concrete slightly improves vehicle fuel economy, and reduces the risk of rocks hitting your windshield.

 

 

Now, let's consider the merits of replacing asphalt with a more-expensive but higher-performance plastic.

We'd probably mix it with rocks like asphalt is, making "polymer concrete". The choice with the best cost-performance here is PET, so let's consider that. How expensive would that be?

- A few years ago, 2-lane US roads were typically $2.5M per mile in a rural area, $4M in a city.
- PET prices vary, but let's say it's $1100/ton.
- Blacktop thickness varies, but let's say it's 6 inches for a road. Polymer concrete is stronger and could potentially be less thick.
- Roads are perhaps 11 feet per lane, plus shoulder.
- Polymer concrete might be 1/3 PET by volume. (Less by weight.)

 

A 2-lane road using polymer concrete instead of blacktop would probably have about $1M of extra material costs per mile, and would need higher temperatures. The potential advantages would be lower weight and better durability.

If you can actually get those advantages, then logical applications would be a surface coating and road surfaces on bridges. And indeed, polymer concrete has actually been used for those! In practice, it does seem to have good abrasion resistance, even better than concrete. And unlike concrete, it's not susceptible to cracking from freezing or corrosion. Yes, bridge deck overlays have used unsaturated polyester, which is a resin cured by UV or heat, rather than PET, which is a thermoplastic. A resin is easier to apply without special equipment, but I still think that, on a large scale, PET ends up being a better choice.

PET is susceptible to degradation by UV light, but so is asphalt. That issue is normally mitigated by adding carbon particles to absorb light. When you see black plastic, that's usually because carbon black was added to it. (And sometimes you see plastic that's white because it has precipitated CaCO3 in it, reflecting light instead of absorbing it.)

 

 

Polymer concrete would be more economically attractive if you could reduce the amount needed.

It's not uncommon to use a surface layer of asphalt ("asphalt overlay") over concrete. That covers the expansion joints, but there's also a tendency for asphalt to crack where those joints are. PET polymer concrete is generally more resistant to cracking than asphalt. It should be possible to place precast concrete blocks, then put polymer concrete between and on top of them. That could be a practical way to make roads.

 

 

Here's a video proposing "plastic roadways". It was popular on Facebook a few years back.

That video proposes 3 things:

1) using engineered thermoplastics for roads
2) producing sections by extrusion
3) sections that have hollow cores

 

I discussed (1) above.

 

Regarding (2):
It's not uncommon to use precast concrete sections. With thermoplastics, it's possible to use extrusion instead. Obviously you need to fill in gaps between them, and it's not clear that this would be better than pouring hot polymer concrete, and polymer concrete is harder to extrude than pure plastic, but it's not unreasonable.

 

 

Regarding (3):

Hollow-core concrete slabs are commonly used in construction. Hollow-core plastic extrusion is common, but this is an application where weight is less important than normal. If weight doesn't matter, why use air to increase thickness when you can use rocks instead? They're almost as cheap as air, but substantially stronger! Polymer concrete is definitely better than pure plastic with air in it.

But what about hollow polymer concrete sections? While concrete is more expensive than rocks, the same principle largely still applies. So, you could make hollow sections, then pour concrete in them.

Plastic has a lower modulus than concrete, so concrete would carry most of the load up to the point where it cracks. (Small rocks are stronger than concrete, so they don't crack before the plastic stretches.) So, if you're putting concrete inside plastic, then why not just use concrete? The main advantage would probably be protecting the concrete. Having the concrete surrounded with plastic would mostly prevent water from getting to it, which would substantially increase its lifetime. Also, a surface layer of plastic over concrete would avoid bumps between precast concrete sections and the filler between sections.

Hollow sections could potentially make it easier to put electrical/data/water lines in place, but obviously the gaps between sections would need to be filled in. So, if you wanted to run lines in the hollow truss sections, you'd need to run a tube through them before filling in the gaps.

 

 

In theory, some chemical industry progress could make plastics more cost-effective, but what progress would that be?

- It's not likely that oil and natural gas will get much cheaper than recent lows.
- The chemical plants producing common plastics have already been fairly optimized. There's more room for improvement with expensive high-performance stuff.

 

It's possible to engineer microbes to produce nylon monomers from sugar and ammonia, but while that might end up being cheaper than what's done now, it's certainly still going to be >$1000/ton. Nylon is stronger than PET and bonds to rocks a bit better, but it also absorbs more water.

(Using methanol instead of sugar would be cheaper, but I really don't think nylon monomers from methanol fermentation is practical, and I do know a bit about metabolic engineering.)

Rather than plastics getting cheaper, what we've seen recently is asphalt and natural gas getting more expensive, which makes blacktop and concrete more expensive. Making roads more durable seems easier than making them cheaper.