Hacker News Reader

Comment by legitster | Hacker News Reader

Chemistry might be important, but we don't necessarily need that deep of an analysis.

Romans didn't mind over-engineering with massive, inefficient structures. They didn't worry about complex/thin shapes, or any tensile elements. No rebar, no "piping" cement, etc. And they could put up with extremely long cure times.

The end result is very, very dense concrete.

We could probably recreate the durability of their concrete if we were actually willing to put up with the compromises they represent.

https://practical.engineering/blog/2019/3/9/was-roman-concre...

View on HN

Sounds like the chemistry is important

> Previously disregarded as merely evidence of sloppy mixing practices, or poor-quality raw materials, the new study suggests that these tiny lime clasts gave the concrete a previously unrecognized self-healing capability.

> As soon as tiny cracks start to form within the concrete, they can preferentially travel through the high-surface-area lime clasts. This material can then react with water, creating a calcium-saturated solution, which can recrystallize as calcium carbonate and quickly fill the crack, or react with pozzolanic materials to further strengthen the composite material.

https://www.science.org/doi/10.1126/sciadv.add1602

> ... evidence that the Romans employed hot mixing, using quicklime in conjunction with, or instead of, slaked lime, to create an environment where high surface area aggregate-scale lime clasts are retained within the mortar matrix. Inspired by these findings, we propose that these macroscopic inclusions might serve as critical sources of reactive calcium for long-term pore and crack-filling or post-pozzolanic reactivity within the cementitious constructs. The subsequent development and testing of modern lime clast–containing cementitious mixtures demonstrate their self-healing potential ...

Or, this study finds that Roman concrete regenerated using water-reactive calcium carbonate introduced through hot mixing–transformed lime clasts. This included observations of Roman concrete used in structures that weren't characteristically massive or inefficient, and reproducible in reformulated modern concrete.

> In the present study, we demonstrate the calcium enrichment of matrix phases adjacent to the lime clasts, supporting the hypothesis that the lime clasts are a source of calcium for leaching and recrystallization within the pore space of the mortars. Microcrack filling by calcite has been recently observed in ancient Roman mortars from the tomb of Caecilia Metella (29), and the self-healing tests carried out on our modern samples described in the present study further support this hypothesis.

"Chemistry might be important"

Conc is nearly all about chemistry. You mention curing rather than drying so you surely have some idea about what is going on. The Romans did not have electron microscopes so could not watch the way the matrix develops around the aggregate etc etc from the get go. To be fair, I fried my first sample in the beam at college by over focussing.

They didn't use rebar because they generally built in compression - arches etc. Rebar and pre-stressing enables members to cope much better under tension. A PS beam or slab is a modern marvel and even protects the steel from fire - bonus.

Cure times: weather/environment. In hot climes you need to stop the water buggering off the surface and leaving cracked and spalling surfaces before it even cures. It may also over heat (exothermic reaction). Cold - slower curing, too cold - no cure and frozen water (total disaster). Underwater - watch your fines wandering off and going for a smoke behind a reef and your structure collapsing, unless you keep them in place.

Concrete curing is not like smoking a joint of meat! A quick or slow cure may be indicated depending on conditions. We have admixtures and all sorts to fiddle with parameters. Then you have things like grading and composing your agg. and other exciting stuff.

I could go on at some length - it is really, really complicated. Roman engineers managed to invent an ancient wonder of chemistry and physics that was concrete. Their approach to its use and improvements was obviously rather better than what passed for science and medicine at the time! They do seem to have experimented and gone with what worked best but it was never deliberately developed from first principles. The self healing thing was probably a happy accident.

A Roman engineer had some rather different design constraints than a modern Civil. For starters, failure could quite legitimately be blamed on the god's displeasure. "When that bridge was commissioned, we made all the correct sacrifices and anyway, you are holding it wrong, I mean walking on it incorrectly".

(sp)

> Roman engineers managed to invent an ancient wonder of chemistry and physics that was concrete.

Technically, the ancient Egyptians used concrete in construction long before the Roman engineers were a thing. The difference is they used it for mundane things like wine cellars while the Romans integrated it into their whole architecture system.

Once in a while that art history degree comes in handy…

I worked on a campus where they were building a building that had to be incredibly stable - they had to coordinate arrivals of like 30 trucks making 4-5 trips a day. Each truck had like 180 seconds to unload.

Because of the coordination requirements, they embedded sensors in the structure and adjusted the water for curing and would slow down or speed up the flow of trucks based on how the process went.

One of the engineers told me that a thousand years from now, someone would wonder why this slab is here. I’m sure he was mostly kidding, but it seemed believable.

In Ancient Rome blaming gods was never an excuse. For example, there an architect or a chief builder had to spent several nights under the bridge after its opening. In general they followed a rule similar to one in ancient Babylon which explicitly required to put the architect to dearth if a building collapse would kill the owner. And if the collapse would kill a son of the owner, then the son of the architect would be put to death.

That puts rather different perspective on the quality and longevity.

We do need a deep analysis. Roman concrete is self-healing, and this article explains why. And the reason is exactly the opposite of the one you postulate.

"Anyone can create a bridge that will hold up under a maximum load. It takes a real engineer to create a bridge that barely holds up under a maximum load."

It takes an accountant with a tight budget, you mean. Engineers will tend to make the most durable thing ever, if given enough time and money.

Interestingly, the article founds out that their concrete cured quickly.

Anyway, no, our usual concrete completely breaks down in a few centuries. No matter how it's structured, it does not match the durability of that concrete without ongoing maintenance.

>>A few centuries

Sounds good to me?

Although, many centuries old sound structures also sounds better to me...

Like the Pantheon[1]? It's still got the world's largest unreinforced concrete dome and it's in active use as a church not to mention major tourist attraction.

[1] https://en.wikipedia.org/wiki/Pantheon,_Rome

> massive, inefficient structures

How does a building, an aqueduct, or a bridge still in use after 2000 years manage to be inefficient?

Yeah, I can't imagine a municipality letting their concrete roads cure for a couple years before letting cars on them.

Reply on news.ycombinator.com