Exploring the Pantheon and Roman Concrete

How has the 2,000-year-old concrete dome of the Pantheon here in Rome lasted for so long?

Hi, I'm Paul Kasavian. I'm a structural engineer.

And we're going to go inside the Pantheon and going to see all about that.

So let's go inside the Pantheon. Come.

Pantheon Okay, we're inside the Pantheon Temple.

This is the temple of all pan-gods, the pheo.

So the temple of all gods.

Because it's for all gods, it's circular.

We're in this circular space, 142 feet down, and it's about 43 heaters.

Okay, so a quick reminder on how domes work.

You start with an arch,

and then, as you rotate the arch around... into 3D,

you form a dome, and that means,

just like an arch,

the compression forces in a dome go from ground up to the top and down to the bottom again.

And here, those compression forces, I'm showing them to you in a blue colour.

But these aren't the only forces in a three-dimensional dome,

because what you also have are forces in this direction.

We call these hoop forces for very obvious reasons.

Now, those hoop forces at the top of a dome are also in compression.

But then, as you go further down to the bottom of a dome,

those hoop forces are in tension, which I'm showing you in red.

Think of it like the straps around barrels to contain the bursting forces within.

So, both of these forces exist at the same time in a complete dome.

What you can also do,

though, is remove the top area and material of a dome.

And that might seem odd, because surely with an arch, you go up the forces in the bottom, you go across, how do they, what happens here in this air gap, how do they get across?

Well, what actually happens with a three-dimensional dome... dome, is those forces go all the way up to the top opening and then around the hoop direction in compression and back down the other side.

So you can still have a dome work, a three-dimensional dome,

with an opening at the top,

much like you see at the Pantheon, that oculus, that opening to the heavens.

Now, the difference,

though, with the Pantheon,

which I'm showing here in this diagram, it has the oculus,

the opening, it has a dome shape, it's actually a hemispherical dome, half a sphere, is that it is made out of concrete that is not reinforced, so it cannot carry

tension around this hoop direction.

Can't do that at all. But

what it does have are very thick, deep walls to take the fact that the forces are still pushing outwards.

And this is a close-up of that lower area where there's compression coming down, but

tension in the direction I'm showing you here. And if there's tension in this direction,

then that means unreinforced concrete is going to want to crack.

And it's a little hard to see here, but those cracks have actually been mapped out, shown here.

So what we have,

overall,

in the amazing Pantheon is an unreinforced concrete dome.

It has cracks in that area, but it's stable because it's working like a series of arches pushing on the very thick,

deep perimeter walls.

Okay, back to the Pantheon.

So those arches are pushing on the walls around us.

It's working because those walls are so thick, right?

Now, the Romans were smart.

They made those walls thick to resist the thrust of the arches.

They also made the concrete lighter as it moved up the dome.

So as it got further up, it became lighter.

Now, there's more to know about this, and I want you to hear from an expert.

It's Linda Seymour.

She did her Ph.D. at MIT on the durability of ancient Roman concrete.

And we happen to both work together at the same company in Boston.

So, it's bye from me, ciao da Roma, from me, and over to you, Linda, in

Boston. Thanks, Paul.

As Paul said, my name is Linda and I did my PhD research on the durability of ancient Roman concrete.

The Pantheon is one of my favorite structures.

But before we dive into why Roman concrete's so cool,

let's get a little bit of the basics down.

I have here a piece of concrete that we've polished

in order to be able to see inside and see what's going on.

So this is modern concrete, but Roman concrete's pretty similar.

We have our rocks here.

Those are going to be the coarse aggregate.

In between the rocks, we have these little specks.

That's sand, otherwise known as our fine aggregate.

In between the sand, we have that gray stuff.

That gray stuff is the cement.

That's the binder that holds everything together.

Now, I want to circle back to two things that Paul said.

First thing he said is that in the Pantheon,

as they went up, they actually made the concrete lighter.

One of the ways they did that was by changing those coarse aggregates,

that rock, to something lighter, like pumice.

It's a little more airy, a little lighter, and that helped them get the weight down.

So it turns out Roman concrete isn't just one magic formula.

They changed it between structures and even within a single structure.

Now the second thing that I want to point out that Paul said

was that that base of the pantheon has to be really thick

to resist those thrust forces from the arches that make up the dome.

The way that they were able to do that is actually pretty interesting.

So before Roman concrete as we know it,

a lot of the concrete's actually set by exposure to the air.

So you can imagine if you have a really thick structure,

the inside's going to stay like mush.

What the Romans did was they realized that if they used a particular sand, a volcanic sand, that fine aggregate,

They could actually get the concrete to set either in the middle of a really large structure or even underwater.

This is what's known as a hydration reaction,

and we utilize the same thing in modern concrete to keep our concrete strong.

So there you have it, two interesting things about concrete, about Roman concrete.

One, it's not just a single recipe, and they actually changed it and innovated between structures and within structures.

And two,

the reactions that they relied on for strength gain aren't so different from

what we use.

Thanks Linda.

So there you have it, the sequence into the structure

and material of the 2,000 year old concrete dome here at the Pantheon Temple in Rome.

I hope you enjoyed that.

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See ya.