We use reinforced concrete, concrete with steel bars inside. This allows us to built really thin and strong concrete bridges, balconies, roofs, etc.

But the steel rusts after a couple of decades.

Rusted steel is marginally larger than new steel.

So when the steel bars expand by rusting, it pushes against the concrete from the inside, causing cracks. Then lots of water can get in it rusts faster.

Eventually chunks of concrete fall off and structural collapse follows.

If we built like the Romans, it would be more expensive, but would last 1k+ years.

I wonder how long carbon fiber rebar, or plastic coated steel rebar would last.

Basalt rebar is chemically resistant and doesn't corrode. It does have a lower tensile strength than carbon fiber but is suitable for current designs.

Plastic coated rebar is a product. In theory it can solve the rusting problem, but in practice the coating will get some nicks during installation that can concentrate corrosion leading to point weaknesses. There are additional concerns with the bonding between the concrete and the coated bar.

I don't know how it compares to actual rebar reinforced concrete, but we do use fiberglass reinforced concrete.

Steel and concrete have similar coefficients of expansion. Most other choices don't and so as temperature changes something has to bend or give which reduces life.

Briefly (costs/duration):

normal steel rebar=1

epoxy coated rebar=1.1-1.3 <- terrible in practice

zinc coated rebar=1.2-1.4 <- good

stainless steel rebar=3-4-5 <- very, very good

carbon fiber rebar=? <- AFAIK not used in new buildings, as it has a series of issues in practice, used in some rare cases of consolidation/repair of old buildings, mostly not as rebar but rather as mesh/net.

What is used, not as "main" reinforcement but to have a (much better) mass strength are fibers, they can be either steel or plastic.

I think I do not understand the numbers. You wrote "costs/duration" so higher the number the worse solution (bigger costs or worse durability), yet the descriptions suggests otherwise.

Sorry, numbers are cost only (1 is "normal" rebar steel) order and text are duration ("normal" steel no text, base duration, the terrible in practice means that it doesn't offer longer duration, and may have issues in adherence to concrete).

Not so surprisingly the higher the cost the longer the duration.

There are stainless steel rebar and epoxy coated rebar for structures exposed to the elements. Stainless steel rebar appears to be kind of expensive so its use is limited.

Interesting idea, but carbon fiber would need to be really cheap. Would work well until a critter of some sort evolves that eats plastic

Is there any alternative to steel?

Or, is there any promising technology that allows to use steel bars that have some space inside, so that when they rust, they don't break the concrete?

It's critical to protect the concrete from water.

I think we’re missing another key factor here. Most “well preserved” buildings are ones that are in continuous use. So they get maintained and repaired. A temple in south India is about a 1000 years old. It has seen repeated damages - some intentional during conquests - but survives to this day because the damages were patched up within a few decades. The “few decades” it can go - even with damages - is likely from the material and construction. But the centuries that add up comes from regular maintenance.

Well, while this is true, there is some level of survivorship bias here. Plenty of comparable South Indian temples were torn down, and the ones that survive are mostly quirks of chance. Even the ones that do survive have seen many of their gopurams (monumental gates) collapse after a few centuries due to political apathy in modern times - the one I recall most recently was at Srikalahasti.

It is both..... so both the author and you are correct.

Modern concrete has rebar, which eventually gets rusted, and corroded, and fails (in 150-200 years). Also, roman concrete was just thicker/used more material.

Also simple stone survives forever, while brick a bit less, and 'simpler mud bricks' and wood a lot less.

SO, from Roman times, really only the strongest buildings survived, and everything else is gone. So, there is a survivor bias. We are only looking at the strongest buildings, and not the average one.

And as their wealth was staggering, they could afford to build 'strong' more than most ancient civilizations and most of the conquerors just were assimilated rather than wanting to tear it down to the dirt.

The Romans weren't any more significantly wealthier than other contemporary Old World civilizations/empires - China, Persia, or India, for example. Conquerors being assimilated is a common theme that runs across most of these civilizations as well. However, monumental buildings that survive invading armies just tend to be dismantled/repurposed by the locals for the building material over time. I suspect that part of the reason for Roman monuments surviving is that the fall of Western Rome lead to an overall decrease in the urban population & subsequent demand for building materials. China, India, & Persia likely saw no such population decreases.

To take one example of repurposing, amphitheaters turned out to make pretty good mini-fortresses, so in some cases the town simply moved inside!

Every single building that wasn't reused was dismantled for materials. Every single one.

There's absolutely nothing special about Roman monuments.

Using survivor bias to our benefit though. Evolution for buildings as it were.

While the article title is a blanket question on ALL Roman monuments - which clearly didn't all survive - the article is about people investigating how the long lasting ones do what they do.

Which sounds great to me.

You're right to not buy this entirely. Roman concrete was great because it could harden while in water. And waterworks was probably the primary reason why Rome was such an early superpower (for multiple reasons: hygiene, stronger ports and also mechanical force, as "free" energy give you productivity increases. But it had less tensile strength than our current concrete.

The reason why multiple structures still hold today and will hold longer than most of our structures is more simple than that: they over-engineered the shit of everything that still hold today. Also for the same reasons, multiple bridges, castles, church or structures built between the low medieval era and high med/renaissance will likely last as much as roman structures did.

The explanation in the article is fascinating, but I wonder how much the Romans knew about the durability of their concrete at the time, and to what extent they just got lucky by the mix of material that happened to be available in their area.

I feel like a single human could trial and error their way to a very good paper airplane over the course of 2 or 3 days. It seems pretty obvious that a large group of people would trial and error products in much the same way, especially those so essential to their civilization. Especially with the large trade networks that we know to exist back then.

Some structures were build without any concrete at all. That being said, they've build sea walls that have lasted thousands of years.

I've wondered if maybe the "stones" of the Pyramids were poured. It wouldn't surprise me given the lack of gaps.

You aren't the only one.

https://youtu.be/znQk_yBHre4

Are there strong counterpoints to this? If that is possible it seems extremely more likely than aliens or whatever else people are coming up with.

Aliens are incredibly unlikely. The Egyptians had grain money with demurage fees which encourage hundred year+ projects. Anything they did had a discounted future value that was higher than present value. Building something that lasted 4000 years was economically viable for them.

Right, of course aliens are extremely unlikely. The reason it is even talked about is because there are few convincing explanations how the Egyptians could theoretically have built the pyramids at that time.

I spent a few months devouring information on geopolymers. This theory seems very likely. But, Davidovits is the only researcher that has published on the subject that I have seen.

Thank you! Very interesting.