Rebar is a big problem. But concrete without it has terrible tensile strength. If you want to build in earthquake country, or have long concrete beams, there has to be something with tensile strength in there.
There's stainless steel rebar.[1] That holds up well against corrosion, but costs 8x as much as the cheap stuff. There was a fad for epoxy-coated rebar, but it's not holding up well in practice; the coating is too fragile.
The original Panama Canal locks are holding up well after a century.
But the lock walls don't have rebar. The lock gates are all steel.
The new locks have severe concrete flaws.[2]
Interesting the doc built of nickel steel rebar. My grandfather owned a fishing boat built during WWII. Because of shortages copper nails were unavailable, so they used nickel. Still in good shape after 75 years.
That said my experience with material science issues has mostly been sadness and fail. As in.
Them: We will solve this problem with Super Duper X material.
Me: I think Super Duper X is going to give you problems.
Them: But it's Super Duper!!!
Me: Yes and it has Super Duper failures too.
Them: If you're so smart what would you use?
Me: I have no idea, why don't you consult a professional?
Them: See so you don't know what you are talking about.
There's this new thing - probably not a fad - to just add plastic strips to the concrete while its in the mixer. This is popular with people building with polystyrene block forms e.g. houses and pools as it saves oodles of effort, and with experiments to '3d print' concrete buildings.
It poses a different kind of problem during recycling. With iron it's simple and easy to seperate. If you add plastic strips I imagine that it's quite a different story.
First, plastics are on a spectrum of bio-degradableness and I have no idea what plastics are used to strengthen concrete.
Second, to recycling concrete you crush it. Crush it to powder and you can use it as aggregate in another batch of concrete. Leave it as big chunks and you can use it as fill. In this kind of recycling, the plastic can go with it just like any other.
This is a pet peeve of mine, and makes me want to scream every time I see some click bait about how much better roman concrete was. If their concrete was so great, where are the ruins of the toman skyscrapers?!? :)
I think a big reason ancient towers didn't really exist is less because they couldn't be built, and more that nobody wanted to climb up 5 or 10 floors if they could avoid it. No sane person would go up and down 20 floors if they could avoid it.
The colosseum is 150 feet tall, which isn't exactly short. Beyond that point, I think a big problem is getting the resources up high enough to keep building, as well as convincing people to climb that far up.
The only multistory residential buildings of antiquity I know of is the old city of Sana'a: https://en.wikipedia.org/wiki/Sana%27a#Old_City Five to nine stories tall, made of rammed earth, not even concrete! (Not enough water for concrete in Yemen) Continuously inhabited for ~2500 years.
Rome had multistorey (3 to 4 of them) residential buildings, and opposite to today, the lower floors were the prestige floors - less climbing, more robust materials. The top floors were generally poorly constructed, out of lightweight materials.
Appart from the the ground floor (for business), it wasn't until the invention of elevators that the top floors were made prestige floors. The bel étage or piano nobile [1] was the first floor above ground level (for relative privacy from the street level). Top floors were not penthouse flats they were attics or lower class housing [2]
The first proto skyscrapers were built in the late 19th century. The Equitable Life Assurance Building (1870) and the Home Insurance Building (1885) were office buildings.
The idea of a Penthouse appartment [3] dates from the 1920s. And as buildings are only slowly built and renovated, the first floors were still prestige floors well into the 20th century.
In Manhattan, buildings over 100yo have taller ceilings on the first couple floors (where the retail shops would be and the owners would live), and then low ceilings on the highest floors for the scriveners and production workers. Of course, it's an issue today when lots of software companies want light (i.e., high floor) and high ceilings.
People don't realize what a paradigm shifting technology elevators were.
These were called insulae[1]. Part of the reason the lower floors were higher prestige was due to the constant risk of fire. In the event of a fire those on the upper floors were in a far worse position in terms of survival.
Yep, and the scheme remained more or less the same in Italy where traditional homes (for the wealthy) have been till the 1900's organized as follows:
At ground floor (usually with higher celings than the rest of the house) dining room, sitting room, library, study, most of the "luxury" (marble, columns, paneling, paintings, etc.) was there, as they were the only parts of the house that visitors would see.
In the basement the kitchen and cellar (where cook and other servants worked).
On first floor (what probably our US friends will call second) the bedrooms for the owners.
On second (last) floor, right under the roof, usually with a reduced height AND extremey cold in winter and extremely hot in summer, the servant quarters.
That's simply a false dichotomy. Many of the world's great cities still mostly enforce height limits that the Romans could have built to. Because actually living like an ant among 100 story skyscrapers is not what most humans want. A medium density and mixed use city of max ~150 foot high buildings has very few disadvantages and analogizing it to USA suburban sprawl is just laughable nonsense. Have you ever been to e.g. Rome, Paris, Amsterdam, Florence ... most of Western Europe?
Also, less than 150 years ago, there was one seventh of the population on this planet than there is now. In fact, during the time of the Roman Empire there were only 300 million people on this rock, 45 million or so were within the Empire[1].
Skyscrapers are only a good solution if you have over-population. No-one in their right mind would ever want to live or work in one if they didn't have to.
Ancient Rome grew beyond 1 million inhabitants in the 2d century. Chang'an (Xi'an), China in the 8th century, Bagdad around the 10th century. There were big cities before.
Romans had neither elevators or air conditioning. They did build on a fairly large scale though. I believe 150 foot tall structures were not uncommon, and those are the ones still standing.
There's stainless steel rebar.[1] That holds up well against corrosion, but costs 8x as much as the cheap stuff. There was a fad for epoxy-coated rebar, but it's not holding up well in practice; the coating is too fragile.
The original Panama Canal locks are holding up well after a century. But the lock walls don't have rebar. The lock gates are all steel. The new locks have severe concrete flaws.[2]
[1] https://www.youtube.com/watch?v=FrFcGx_UPow [2] http://gcaptain.com/a-concrete-sample-was-pulled-from-the-ne...