The self-healing secret to roman concrete

I’m traveling at a science conference this week so thought it was a good time to bring back a fan favorite that I talked about over a year ago. Enjoy this piece on a recent discovery detailing how the Roman’s made concrete that lasts.

The Problem

The Romans built some incredible concrete structures including the Colosseum, aqueducts, and the Pantheon among other random objects still standing throughout Italy today. If the Romans could do it thousands of years ago, why do all our roads have potholes immediately after they’re repaved?

We’ve been trying to answer this question for a long time. It’s a great example of how technologies are lost if they’re not actively preserved.

The Solution

It turns out, Roman concrete wasn’t any tougher than ours is now, it just healed itself whenever it got damaged. The Romans might not have known why their concrete lasted so long, but they knew it worked, so they stuck to detailed recipes.

They used specific mixes for different building types and climates, but they all included lime particles (calcium oxide or CaO). Here’s a detailed video on how they made it.

We’ve known this for a while, but the importance of these particles wasn’t well understood; that’s where this week’s paper comes in. In it, the authors do some in-depth analysis of these lime particles to figure out why they’re always in Roman concrete samples.

They found that the particles allow the concrete to heal itself when damaged by leaking out reactive calcium that fills in the cracks around them.

Structural materials like concrete often end up breaking due to small cracks that get larger and larger until the whole structure fails.

Cracks concentrate the stress placed on a material, which leads to the crack growing. This self-fulfilling prophecy continues until the crack is so large that the whole thing breaks.

Whenever there’s an imperfection in a material, such as a large glob of lime, cracks tend to go through it. So the presence of these lime particles in the concrete caused any cracks that formed to go through them.

Once rainwater enters the concrete, calcium leaks out from the particles and reacts with the surrounding material to fill the crack. This process stops the crack from spreading before it really gets going and makes Roman concrete last much better than its modern counterpart.

The authors even went as far as mixing some lime particles into modern concrete mixes and seeing if it helped with durability.

Short answer - it did.

They cracked a piece of modern concrete with and without lime particles and compared how water flowed through the samples to see if they healed. If the water flow stopped, it indicated that the sample healed from the crack. If not, then the crack didn’t heal.

The graph on the left shows with and without lime particles. You can see that the no particles group (“-Clasts") didn’t stop the water flow. In contrast, the with particles concrete did (“+Clasts”).

So, the same technique the Romans used to toughen their concrete works today with our modern version.

One important thing to keep in mind; the Roman’s didn’t have 18 wheelers going 70 MPH down their roads. As you can imagine, an ~80,000 lb truck barreling down the highway does considerably more damage than a ~1,000 lb horse.

See you next week for more science,

Neil