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Comment by rflrob | Hacker News Reader

One thing I recall reading is that rebar-reinforcing concrete is a major factor in concrete deterioration. So while concrete structures today don't last nearly as long as the Roman Pantheon (poured in the second century AD), they're also really cheap to build, can go really high, and last for 40 or more years (which given how cheap it is, the architects probably thought was long enough).

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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]

[1] https://www.youtube.com/watch?v=FrFcGx_UPow [2] http://gcaptain.com/a-concrete-sample-was-pulled-from-the-ne...

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.

Them: Besides professionals are expensive.

later... Oh noes Super Duper X isn't working!!!

Rust: The Longest War https://amzn.com/1451691602

"US vs Memory Safety"

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.

I kinda expect it to take off.

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.

Cement production is already the very major polluter on a global scale:

https://en.wikipedia.org/wiki/Cement#Environmental_and_socia... especially https://en.wikipedia.org/wiki/Cement#/media/File:Global_Carb...

Surely, making the recycling process more complicated does not help.

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.

Personally I prefer to build with wood.

http://nautil.us/issue/24/error/why-we-should-let-the-panthe...

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.

@idlewords went there back in 2014, before the Saudis invaded: http://idlewords.com/2014/07/sana_a.htm

Thanks for the idlewords link. His writing is very capturing, I've been reading the blog for hours.

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.

1 https://en.wikipedia.org/wiki/Piano_nobile

2 https://en.wikipedia.org/wiki/Chambre_de_bonne

3 https://en.wikipedia.org/wiki/Penthouse_apartment

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.

The Secret Life of Machines - The Lift (Elevator):

https://www.youtube.com/watch?v=dBLa105TQeg

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.

1. https://en.wikipedia.org/wiki/Insula_(building)

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.

Another example of tall Roman buildings still standing today is Roman aqueducts.

Examples: http://0.tqn.com/d/ancienthistory/1/S/U/I/Rome_5.jpg

http://3.bp.blogspot.com/-z_W3YC_mG0Y/Tb55AUhJ0dI/AAAAAAAAAE...

And, of course, the Pantheon, at 43 meters. It was the largest dome in the world for over 1300 years (https://en.wikipedia.org/wiki/List_of_largest_domes#Worldwid...)

The Nerja Aqueduct was built in the 19th century. But yes roman aqueducts were impressive https://en.wikipedia.org/wiki/Pont_du_Gard was 48.8m high (160feet).

The skyscraper was primarily made practical by elevators.

Maybe it's lightning?

The Yonging Pagoda was 138 metres high. It was destroyed in 534 when it was struck by lightning and caught fire.

https://en.wikipedia.org/wiki/Yongning_Pagoda

Why would you need to build skyhigh, if there is still plenty of land available for low-rise building?

Why would you live in a city when there's plenty of suburban sprawl?

There are lots of reasons of efficiency to build up even if you can build out.

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.

[1] http://channel.nationalgeographic.com/killing-jesus/articles... (first Google hit)

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.

https://en.wikipedia.org/wiki/List_of_largest_cities_through...

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.

They were banned by building codes - insulae were limited to something like five stories due to fires. A problem that is tragically still relevant.

plumbing I think is also a major reason as well as why not just spread out. They werent as geographically contained as we are.

Engineers are expensive! Residential housing would have been wooden and (relatively) temporary.

who wants skyscrapers in absence of elevators?

flyash 1/4, cement 3/4 to neutralize chloride attacks

I recall some experiments being done on the engineering side of my university using carbon rod for rebar. I don't recall how well the thermal expansion coefficients of the two materials matched up, but the professor claimed a concrete + carbon bridge would last 200 years, due to lack of rust.

This was in the late 90's, so we still have a few more years to go before the test is complete. ;)

Regarding rebar - in my country communists build a lot of "wielka płyta" (big pre-stressed concrete panels) commie blocks. Around 20% of Poles still live in these. These buildings were built since sixties, and designed to stay for 50 years, but so far they seem to hold rather well, none of them were destroyed so far in a building catastrophe, not even when there were gas explosions in some blocks.

They were a little modified from the standard commie thing (for example stainless steel was used for joining the panels), and they had lots of usability problems (noise and temperature insulation), but from durability POV they seem to work better than expected. I wonder if in next decades 20% of the country will have to buy a new house, cause that might be a huge economic change.

There was some news years ago that there was a bacteria that could hold the key to ensuring modern rebar reinforced concrete could repair itself over time: http://www.nydailynews.com/news/national/concrete-fix-thanks...

Pozzolanic concrete with basalt rebar supposedly addresses the spalling and other issues with long-duration reinforced concrete use cases. There are various pozzolanic concrete recipes being tried to reproduce the list Roman recipe. No one I can find has combined the two and stuck it in seawater for 50+ years, though.

The sea water theory in the article does not adequately explain the aqueducts still standing in Europe in Italy/France/Spain/etc after 2000 years with no contact with salt water after construction, although I am not certain if the concrete in those things is used in a structural way similar to what would be expected of buildings.

My limited understanding trying to find out more about pozzolanic concrete (planning for my house extension in about a handful of years) is that those who study this stuff believe it stood up by itself like when used in the aqueducts with plenty of strength and durability, without constant immersion in salt water. Don't know if someone has drilled a core sample into those aqueducts to try to determine if salt water was used to mix that Roman concrete, though; would be interesting to find out.

What drew attention to it was that modern concrete immersed in seawater, even without rebar, doesn't appear to have the same durability as ancient Roman concrete. Don't know how much of this is attributable to survivorship bias, though; would be unfortunate if all this attention turns out to be over a tiny fraction of a percent of ruins that happened to survive.

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