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The Roman Pantheon: The Triumph of Concrete

Research by David Moore, P.E.

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Florence Cathedral dome

Is the dome over the Florence Cathedral larger than the Pantheon?

Questions and answers on Roman concrete, Roman construction, modern concrete, etc.

By David Moore and John Moore

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List of Questions

Q 1. What does the inscription M.Agrippa.L.F.Cos.Tertium.Fecit on the portico of the Pantheon mean?

Q 2. When was the diameter of the Pantheon exceeded by that of another domed building?

Q 3. How did the Romans mix their concrete?

Q 4. Where can I get a copy of The Roman Pantheon: The Triumph of Concrete by David Moore?

Q 5. What happens to water that comes into the building through the oculus?

Q 6. What is the size of the Pantheon for a model I am building?

Q 7. Did the Romans use seashells as part of the aggregate in the dome of the Pantheon?

Q 8. Does rain enter the occulus of the Pantheon?

Q 9. Can you tell me how to use Roman concrete instead of modern concrete in the house I am building?

Questions and Answers

Q 1. What does the inscription M.Agrippa.L.F.Cos.Tertium.Fecit on the portico of the Pantheon mean?   [Top]

A 1. Chapter 1 of The Roman Pantheon: The Triumph of Concrete discusses this. Here are more details.

Above the entrance, carved in stone, are the words M. AGRIPPA L. F. COS. TERTIUM FECIT which is translated, "Marcus Agrippa, son of Lucius, in his third consulate, made it." This basically says that "Agrippa built the Pantheon." However, as with many parts of the Pantheon, this is somewhat paradoxical, because, in fact Agrippa did not make it!

The building we see today as the Pantheon is not actually the original building by that name. The first incarnation of the Pantheon was built by Agrippa, the son-in-law of the Roman Emperor Augustus, about 25 B.C. See Chapter 1 for information on other construction by Agrippa. The first building was a traditional rectangular Roman temple made of travertine (limestone rock). Although it was on the same site, it was oriented in a different direction. See de Fine Licht pp172-9 for info on older buildings on the site.

As with many ancient cities, Rome suffered the tragedy of large fires in 60, 64, 79, 100 and 110 A.D. The first Pantheon was severely damaged during a fire and was rebuilt by Domitian. That building was destroyed in another fire, supposedly caused by a lightning strike. The Emperor Hadrian built the temple we know today as the Pantheon during the period 118 to 128 A.D. There was a tradition in Rome to rebuild temples like the previous one. Apparently in this case, the only thing to survive in the new temple was the inscription over the portico, which probably gave the new building an important social and political connection to the past.

For more information, in addition to Licht's book, see MacDonald, p13. [Top]


Q 2. When was the diameter of the Pantheon exceeded by that of another domed building?

I enjoyed your Pantheon site and found it fascinating reading. You mentioned that the Pantheon's span wasn't surpassed until the 19th century. I'm not sure that's really so, although it did hold the record for well over a 1000 years. Brunelleschi's dome on Florence Cathedral (constructed 1420-1436) is often described as being the first dome constructed larger than the Pantheon. From what I remember of my old architectural history classes, it is slightly bigger.

The www.greatbuildings.com architectural site backs me up, but unfortunately it doesn't offer hard numbers. If you look up the Florence Cathedral at the same site however, it will say the diameter of the dome is 130 feet; which is less than the Pantheon (142.5 ft). Another source says it spans 45.5 meters (149.2 ft) (http://www.mega.it/eng/egui/monu/bdd.htm), which makes it considerably larger. Not sure if that's a clear span though. Still another source I found says it is roughly the same size as the dome on the Pantheon. Who's the biggest? (Howard Partridge, webmaster@cupola.com http://www.cupola.com) [Top]

A 2. Using Fletcher's A History of Architecture as a reference because it has plans of most of the major buildings in history, it looks like the Florence Cathedral is the only one that comes close to beating the Pantheon until the modern age of steel reinforced structures. However, the dome over the Florence Cathedral is not circular, it is an octogon made up of eight ribbed arches. According to the plan of the Cathedral shown in Fletcher, the diameter across the narrowest part of the octogon is 138.5 ft (4 feet less than the Pantheon). Using a little trigonometry, I calculated the maximum distance from rib to rib at the base to be about 150 ft (7.5 feet larger). So part of the Florence dome is smaller than the Pantheon, and part is larger. I think the jury is still out as to whether or not the Florence dome wins the contest! St. Peter's (138 ft) comes close to the Pantheon, but that seems to be about it. As far as spherical unreinforced concrete domes go, the Pantheon wins the contest hands down for all time! [Top]


Q 3. How did the Romans mix their concrete? I originally had visions of thousands of Celts, Saxon, and other slaves mixing concrete with their feet. That doesn't seem practical - it would be too hard on the slaves. The Romans seem like practical people. Could it be they had "engines" that mixed the stuff? I have visions of a great container which shifts back and forth on a giant fulcrum moved by oxen or slaves. Did they have a "Cement mixers Guild?" I'm just guessing. How did they mix their concrete? (Richard) [Top]

A 3. This is a very good question, and one which many authors get wrong. Many authors imply that the Romans "poured" concrete the way we currently do, but offer no idea on what mechanisms they might have used to do it. A fundamental thesis of The Roman Pantheon: The Triumph of Concreteis that the Romans did not place concrete or "pour" it the way we do ours. Using texts from the original Roman authors and the few pictures we have of their construction methods, the book says that the Romans mixed the cement mortar for their concrete like they would mortar for bricks - in a mortar box with a special hoe. A key element of the process is that they kept the moisture content low, so that the mortar was thicker than we are used to, resulting, in essence, in what we would call a "no slump" mixture. If you accept this thesis, then the Romans couldn't have "poured" their concrete even if they wanted to - it was too thick!

They then hauled the mortar to the wall in baskets. They laid down a layer of rocks (aggregate) by hand, and then pounded the mortar into the rocks using a special tool called a "beetle" to get all the air out and make close compaction with aggregate and the layer below. So, the Romans did not mix or pour concrete the way we do, they, in essence, mixed the mortar and aggregate (resulting in concrete) "just in time" right in the forms themselves.

We claim that these two elements of this process, low water content and close compaction, were part of the reason why Roman concrete has lasted so long, in comparison with our concrete which sometimes doesn't last more than a few years without cracking. We have to use a higher water content because, for economic reasons, we use automated machinery to mix our mortar and aggregate offsite to make concrete and then bring it to the site and pour it into forms that contain steel reinforcing. The mixture must be fluid enough to be worked by the machinery and flow around all the rebar and forms with minimal manual interaction. See the reprint of David Moore's article The Riddle of Ancient Roman Concrete which lays out these details and how we are using methods similar to the Romans in Roller Compacted Concrete (RCC) in the building of dams. [Top]


Q 4. Where can I get a copy of The Roman Pantheon: The Triumph of Concrete by David Moore? I need to reference the book for a report. Who is the publisher?

A 4. The book is self-published. You can order a copy through the on-line order page of www.romanconcrete.com. A reference to the book should look like this:
  Moore, David, The Roman Pantheon: The Triumph of Concrete,
  Office Outlet Publishing, Pinedale, WY, USA, 1995.
Libraries with a copy of the book are reference on the concrete resources page. Unfortunately, most are in Europe! [Top]


Q 5. What happens to water that comes into the building through the oculus?

A 5. There are few small drains located in the marble floor. Most of the drains are in the center, but there are a few near the entrance, apparently to drain water brought in by visitors. [Top]


Q 6. What is the size of the Pantheon for a model I am buiding? I am making a model (or drawing) of the Pantheon for a school project. What does it look like on the sides, the back and the top, etc? [Top]

A 6. Books seem to be your best source for plans and sections of the Pantheon. Both MacDonald's The Roman Pantheon and Licht's The Rotunda in Rome have some good plans and sections of the building, but they are both limited to book page in size. Licht has many pictures of the back, sides, and dome, but that book is hard to find, except in some university libraries. The Photos page on this site has many interior and exterior photos of the Pantheon. See the concrete resources page for a list of web sites dealing with the Pantheon and which might also have good pictures. [Top]


Q 7. Did the Romans use seashells as part of the aggregate in the dome of the Pantheon? There is a discussion going on at my job about this. I say yes, but others scoff at the idea. I remember hearing this on the Discovery channel or PBS years ago, but don't remember the details. Who is right? [Top]

A 7. Your both are, sort of. There is no evidence that they used seashells in the dome (and there is no good structural reason for them to do so), but they did use different kinds of aggregate in their concrete. They used heavier rocks, like travertine in the lower portions of the Pantheon, like the walls, and lighter materials, like pumice (sort of a 'styrofoam rock' from volcanoes), in the upper parts of the dome to make it lighter, but still maintain strength. See Sear's Roman Architecture and Licht for pictures and more information. It is possible that the show was mentioning that seashells can be used to create lime, and lime is a critical component of Roman concrete. However, the Romans used limestone instead of seashells as a source for their lime, so that doesn't support your position either. Sorry! [Top]


Q 8. Does rain and snow enter the occulus of the Pantheon? I've been told that when it rains in Rome, water doesn't come through the opening in the dome. The reason given was due to thermal differences and air currents. However, I would like proof, since I now have five euros riding on the answer.   [Top]

A 8. Hopefully you bet against this claim. Although I have not personally witnessed rain coming in through the oculus, there are clearly drains in the floor (see answer to question 5). Perhaps there is some resistance to snow, but since the Pantheon is not heated, it seems unlikely there are large thermal differences between the inside and outside, certainly not enough to prevent rain from entering the oculus. Hopefully you won the bet! [Top]


Q 9. I'm trying to build a "environmentally friendly" home in the countryside. I would like to use Roman concrete throughout rather than modern concrete. I would also like to use bamboo reinforcement rather than rebar like you describe in the bamboo reinforced concrete article. Can you tell me how to do this?

A 9. Note that the bamboo reinforcement article has nothing to do with Roman concrete. It was placed on the web because of our interest in all aspects of concrete and we already had the romanconcrete.com web site up and running.

As far as we know, the Romans did not use any reinforcing in their concrete construction. Using un-reinforced concrete is not necessarily something which you should emulate in your home unless you are absolutely sure of the results, especially if you are building a home for yourself. A home is far too large an investment in time, money and emotion to worry about whether or not the materials you are using will work as you expect them too. If you attempt to use a mixture which attempts to simulate Roman concrete in the construction of your home or a commercial building, you will be experimenting. We are not aware of any successful attempts in modern times to fully recreate a full building, or a home, using the same materials and the same methods that the Romans did.

We have been experimenting with attempting to use materials similar to that of Roman concrete to create concrete test cylinders. We got some volcanic ash from Mt. St. Helens in the USA and some pozzolan from Mt. Vesuvius in Italy. We got some quick lime from a local lime manufacturer. We have not had a chance to test any of the cylinders yet, but thus far many of them have been less than satisfactory and were very weak. Does this mean that the ingredients or mixtures are wrong? We don't know because the Romans left us only very spotty records of what they did - Vitruvius's Twelve Books on Architecture is just about it. Hence the interest in trying to determine what ingredients and mixture methods really work. But we are not there yet. These are experiments and it will take a while to determine exactly what works best. We are aware of a number of researchers who are interested in the subject, but we are not aware of anyone yet who has truly taken the time and money necessary to determine exactly what works and what doesn't and has published this information in a form that can be used by the modern builder to construct buildings.

If you attempt to use "Roman concrete" in the construction of your home, you will be at the "bleeding edge" of the technology. It might not work as you expect, and we don't think you want yourself or a loved one to be the one who "bleeds"! Although modern concrete might have some negative aspects regarding the amount of energy that it takes to make it, its longevity, etc., the fact is that modern concrete is created under conditions of extreme quality control. If it is mixed and placed correctly, it can last a very long time. Its just that modern concrete is frequently not mixed or placed very well.

Modern concrete has been tested at the factory so that you don't have to test it on the job site. If you attempt to recreate "Roman concrete", you will have to perform all the structural tests yourself to ensure that it is quality controlled and has the proper structural properties. It will also be difficult and expensive to test your home-made materials for the number of years needed to really ensure longevity and ensure that they can withstand all the environmental conditions they will be exposed to - cold, heat, rain, snow, etc.

Modern concrete and steel rebar are not bad things, in our opinion. They just need to be treated, like everything else in life, with respect. We need to recognize their inherent characteristics and limitations. Concrete is typically stronger when you use the minimum amount of water necessary to set off the chemical reaction in the cement paste. Excess water is what makes all concretes weak - excess water provides weakness areas in the concrete. A fundamental thesis in the Roman concrete book is that the Romans likely used a very low water content mortar, which is why it has lasted all these years. The problem for modern construction problems is that concrete of this low a water content is not an economical option. The Romans had thousands of slaves working on their job sites who could be forced to pound and manually manipulate stiff concrete into place. Fortunately, we no longer have slaves - but that means we have to use machines and modern materials to do the work for us instead. Therefore, we have to use concretes with higher water content so that the machines can move it and so that it can flow around modern rebar. You do not want to be out there pounding by hand on the concrete ensuring it flows properly around the rebar in a slab.

However, ensuring that the contractor does not use any more water than necessary and uses industry standard practices for placement and finishing is important. For instance, a frequent problem with modern concrete is that the contractor does not know what they are doing, and they "overwork" the concrete when finishing the surface, which causes excess water to flow to the surface. When the concrete cures, the surface of the concrete is very weak, and will pop off in a few years, leaving a very rough surface. This is not a fault of the material, but it is the fault of the contractor.

Anyway, we don't want to discourage you from experimenting with Roman concrete, more people should be experimenting with the material to see exactly what works and what doesn't. Just make sure that you know exactly that it will be an experiment and that you recognize the risks. If you try to recreate Roman concrete, and part of it is wrong - for what ever reason, for instance the pozzolan is of low quality or the lime is deactivated to some degree - your building might suffer. You could be hurt if the structure fails and your investment in time, money, and emotion could be lost. We do not want that to happen. So use materials and methods that meet local building codes, and do the experimentation for fun. [Top]

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Sept 2002 - Last update


Warning!

These questions and answers are edited versions of various queries that have been received at www.romanconcrete.com over the years. These questions and answers are the opinions of the authors only. No fitness for duty, safety, or engineering judgement are implied.

If you are planning on experimenting with or using materials like lime, pozzolan, concrete, or cement that could injure, yourself or others you must obey safety standards and consult with a professional first. The chemical components of concrete can be caustic or hazardous to your health if used inappropriately.

Consult with a professional engineer or architect and your local building codes before using "home made" materials in a structure that could fall or injure someone.


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