How is it that Roman buildings “heal” themselves?

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Saw an article or documentary and it said something to the effect of Roman buildings heal themselves after being damaged or weather damage.

How can that be

In: Engineering

11 Answers

Anonymous 0 Comments

Long story short: The concrete of the building cracks. Water infiltrates the the crack. The water comes into contact with “unactivated” pellets of cement. The water reacts with the cement and sets anew. This is due to the fact that the cement mixture used by Romans contained dense lumps of cement thatdid not become wet during the initial mixing of the initial construction. What you are left with is a system of deposits of cement lumps to fill in cracks occurring in future.

Anonymous 0 Comments

Cement is made from a chemical reaction of a special kind of rocks with water. Roman concrete was made in such a way (unintentionally) that some of the rocks did not fully react, so that cracks would expose the unreacted rocks and “heal” the concrete.

This is not done today because we expect concrete to be much stronger and do not care if it lasts 2000 years. So, it is cheaper to design buildings with fully reacted concrete since it is stronger.

Anonymous 0 Comments

Some rock types easily dissolves in water and turn liquid. If this water then flows into an empty space and then evaporate it fills this space with the rock. This is how fossils are formed. Concrete is made with cement which does dissolve in water, but then it chemically react with the water, or with CO2 in the air, to form a type of rock that does not dissolve in water.

Romans did not get this completely right though and had a lot of rocks in it that would dissolve in water. This did make Roman concrete much weaker then what we make today. Most Roman structures that stand today are those so overbuilt that they would have been standing even without concrete, or they have been heavily repaired and modified over the years.

But a relatively recent paper have studied Roman concrete and found that even though it is much weaker and easily form cracks a lot of these cracks have been filled with the type of rock that dissolves in water. Exactly like fossils. This have restored some of the strength of the concrete. There have been some speculation if they did this on purpose and that it was not purely a result of bad processes. This have been picked up by some media sites and blog posts claiming that Roman concrete is somehow better then modern concrete because it is self healing. But this shows a lack of understanding of what concrete is and how it is used.

Anonymous 0 Comments

When you make concrete, you have to make sure it ALL gets wet, or you’ll have pockets of dry powdery stuff in your concrete.

They weren’t able to mix it as well as we can, but when it cracked, this would expose the powdery stuff, then it would get wet and turn into proper concrete.

Anonymous 0 Comments

They don’t. The discussion around it way oversells this supposed “healing” ability.

Occasionally erosion will fill in small surface cracks with less strong calcium carbonate that expands to visibly fill in the surface cracks. This only works for pretty superficial surface cracks and not for structural damage or anything that propagates too far.

Every still standing ancient Roman building has had to undergo regular maintenance to keep it that way. The Pantheon in particular was rebuilt and renovated a lot over the thousands of years it’s been standing. Including a lot of documentation over how badly damaged it’s dome became over the years.

Anonymous 0 Comments

One of the key ingredients in cement is lime. Lime is easy to break into a fine powder – the romans used a rough lime with larger granules on purpose. If a large granule broke years later, the lime would release and “heal” that area.

Anonymous 0 Comments

Basically the cement they used wasn’t completely mixed so when it cracks water can get it and combines with that “residue” kinda fixing itself

Anonymous 0 Comments

So basically all concrete is self-healing. Recently the Roman concrete has been in the press because of papers by researchers at MIT and another at the University of Utah a few years ago.

None of the authors really know that much about modern concrete and talk about stuff that is discussed in the paper that has been known for decades as if they are new. The head of the lab at MIT had a particularly bad press release and talked about self-healing in a way that made it clear that the field was new to him – and makes a big deal out of the self-healing of hot-mixed lime concrete. The media made it out that this ‘magical’ and makes it more durable than modern concrete, but it isn’t.

Modern Portland cement concrete is already self-healing, though we refer to it as ‘autogenous healing’. And it works in a similar way as described here in the paper – cracks expose the unhydrated cement grains, which then hydrate sealing the crack, or the Portlandite carbonates and expands in volume. Self-healing in OPC concrete has been known and studied since at least 1913.

Hot mixed lime has been known to be self healing for decades. But even the Roman use of hot-mixed lime wasn’t a new discovery – they actually discuss in the paper the translation of the Latin words for slaking from Vitruvius works 2000 years ago and how it indicates that two different methods of slaking were used (basically one methods is slaked and used immediately, and one is slaked and used cool). They also discuss in their literature review other authors who have discussed the use of hot-mixes lime concrete dating back to the 1960s (iirc).

It also is unlikely to ever be implemented in any modern construction. The reactions involved in making this type of concrete deplete Portlandite, which removes all the alkalinity from the concrete and means you can’t use any reinforcing. So there’s very few practical applications for it. This paper is fine, but the media circus around it is a joke. They’ve over sold it in their press release, and uninformed journalists have blown it waaaay out of proportion.

Long story short, modern concrete cracks and self-heals already. Roman concrete lasts better than some modern concrete (survivorship bias) because they made their walls 8+ feet thick and loaded them in compression and without steel reinforcement. Most Roman architecture is also sat in a really hospitable climate for concrete and doesn’t undergo the massive wear and tear that modern infrastructure does.

The Romans also didn’t use disposable formwork (except for vaulting) – they mixed and cast their concrete inside two masonry walls. So all their concrete also had additional protection from a layer of masonry.

Anonymous 0 Comments

It should also be noted that even if the crack was filled back in, the new material was weaker than the original.

Anonymous 0 Comments

The addition of Lime & limestone while manufacturing concrete added the “self healing” property