Why was Roman concrete so durable?

[NomoneyNohoney]

https://news.mit.edu/2023/roman-concret ... casts-0106

An unexpected ancient manufacturing strategy may hold the key to designing concrete that lasts for millennia.

You may also enjoy a much deeper article:
http://engineeringrome.org/understandin ... -concrete/

[Lootman]

Pre-fallout concrete?

A friend of mine who does groundworks tells me that concrete hardens over time. So the older it is. the harder it is?

[Padders72]

The Romans didn't use Rebar, isn't blowing from corroding rebar what normally kills modern concrete?

[Lanark]

Lets hope modern builders never figure this out, can you imagine ugly skyscrapers and car parks which last 1000 years!

[Urbandreamer]

This sort of story regularly resurfaces (see what I did), much to many peoples amazement.

The truth is that concrete is NOT the simple cheap rubbish that people think that it is. It's actually as complicated as steel, but again people think that's something simple.

Start with the basics of concrete. You need to chemically reform calcium carbonate by a process of heating it to high temperatures and controlling the gases it's exposed to during the process. In Roman times this would have been done using firewood, probably chopped and lugged by slaves, in a carefully constructed kiln.

Cement it just part of the process in making concrete. Different "aggregates" have different properties. They can change the PH of the concrete to prevent rebar from rusting, cause it to become antibacterial, allow it to set underwater or change how dense it is. The Romans didn't even use the same concrete throughout the same structure. The concrete at the top of the partheon is not the same as at the foundations.

[quote][Different aggregates were used to give the concrete diverse densities. Travertine limestone gave the Pantheon’s foundations a density of 2,200kg per cubic metre, while lighter rock was chosen for the dome./quote]

As for skyscrapers, as far as I know they were/are never "made" of concrete. It simply doesn't work for the job once you get to any height, though it's ideal for the foundations.. Skyscrapers need to be relatively light and flex with the wind. The sway in the world trade center use to make some feel ill. They swayed up to 12" or 300mm on windy days.

It may be worth considering though that the complaint about skyscrapers, may be about a very different type of building, and aesthetic.
This book may be a insight into alternative views upon the use of concrete as a decorative substance.
https://www.amazon.co.uk/Lost-Futures-O ... th=1&psc=1
Some great photos of buildings that no longer exist, because we don't like the appearance.

[AsleepInYorkshire]

The Romans didn't use Rebar, isn't blowing from corroding rebar what normally kills modern concrete?

Rebar if installed correctly should not cause blowing. When you see this it is due to poor workmanship and poor quality control. That aside it's a terrible job to install the stuff and very difficult to inspect the works as they proceed.

AiY(D)

[tacpot12]

I find it amazing that we are only just discovering this knowledge. Roman concrete has been studied for years, but the cause of the self-healing property has only just been found. This should be a lesson to all about valuing experts and the knowledge they possess.

[Urbandreamer]

The Romans didn't use Rebar, isn't blowing from corroding rebar what normally kills modern concrete?

Rebar if installed correctly should not cause blowing. When you see this it is due to poor workmanship and poor quality control. That aside it's a terrible job to install the stuff and very difficult to inspect the works as they proceed.

AiY(D)

Not helped by a fashion for very thin, structural, concrete shells during the 50's and 60's. Coincidently, the height of "modernist" architecture.
Possibly it's modernist construction that Padders meant.

For those interested, porous materials "blow" when something inside expands, forcing the surface off. This can be ice in a porous brick or steel turning to rust in concrete. Prevent the rust or ice forming and it doesn't happen. Rust only happens under certain conditions. One method of preventing it on rebar is to ensure that water and air doesn't get to the rebar, by ensuring enough concrete between the rebar and the outside. Another is to change the chemistry such that "rust" reacts and forms an impervious layer, preventing further rust forming, either by additives or careful choice of aggregate.

As for the self healing propertied "only just" being found. To my memory, they were "found" in the 1990's. Possibly they were "found" before that. Explained may be a better use than "found". They could be "found" again, if we find a different explanation than the current one.

After all WWII defensive concrete constructions don't suffer as badly from rebar blowing as later modernist stuff. Something to do with the lower permeability using fly ash as aggregate, Of course it was used in WWII as a cheap resource. Today to reduce carbon footprint.

[Tedx]

Its true re the WW2 defences. We have a heap of them from anti tank blocks to pillboxes to underground bunkers. All pretty much intact - and in the case of the blocks and pillboxes, some of them spend half their lives in seawater.

[88V8]

manufacturing strategy may hold the key to designing concrete that lasts for millennia.

The whole body of knowledge around lime and pozzolan faded from general knowledge after the advent of portland cement.
Today, few builders can be trusted to work on Period properties without bringing a bag of cement.

Away from Period properties, concrete can be durable... if there is sufficient cement in the mix, if it is freshly mixed, if the aggregate is salt-free, if it is well compacted, if the rebar is well designed and has at least 2" cover and is isolated from water ingress.
That's a lot of ifs.

I wonder if the Romans were hotter on quality control.

V8

[robbelg]

I wonder if the Romans were hotter on quality control.

V8

Anecdotally, when an arch was built the chief builder was required to stand underneath as the wooden framing was removed

Rob

[Arborbridge]

The Romans didn't use Rebar, isn't blowing from corroding rebar what normally kills modern concrete?

Rebar if installed correctly should not cause blowing. When you see this it is due to poor workmanship and poor quality control. That aside it's a terrible job to install the stuff and very difficult to inspect the works as they proceed.

AiY(D)

But water will always find a way in, I suspect. It has a habit of eventually confounding what man does, however diligently.

Arb.