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How I Became Interested
in Roman Concrete

David Moore, P. E.

(Taken from chapter 1 of The Roman Pantheon, The Triumph of Concrete by David Moore, P. E.)

This was Rome, and my wife and I had a few hours before the late morning bus left for the airport. I had just retired from a construction assignment in Saudi Arabia after 40 years of service in the engineering profession and was on my way back to America with dreams of tranquility filling my twilight period. The hotel clerk had suggested a visit to a nearby Roman building called the Pantheon to appreciate the achievements of the Roman engineers. This friendly hotel clerk disrupted my retired life with much anxiety, trouble, accomplishment and happiness as I will relate for you in my book, The Roman Pantheon: Triumph of Concrete.

After a brief walk, we found ourselves facing an unusually large, round-shaped building covered by uniform brick and neatly capped with a massive dome. Something out of the modern architectural world that could have been created by the famous Frank Lloyd Wright. But surely this could not be the right building for the ancient Pantheon. It was too big, too new, and complete in every detail for an ancient Roman building some 1800 years old. This remarkable building standing before us would open the window to ancient construction in a manner no other model of the past could do.

The high colonnade porch with large marble columns beckoned us to enter, and we did, through two impressive tall metal doors. The interior view came as another shock. We stood on a highly polished marble floor with an interesting pattern. Amazingly, the Romans had given this floor a slight camber from the middle to facilitate drainage. Gazing about revealed several large niches in the walls with carved purple marble columns lining each side. At one time these niches possessed the important statues of Rome. The ceiling held a large, open skylight in the center, but what was really unusual to an engineer was the waffle-like indentations which made up the lower portion of the massive dome.

This building looked very complicated for anyone to build with only Roman hand tools. I asked the guard at the door for some assurance that this building was really built by the Romans near the time of Christ. He promptly responded to counter any doubts about his countrymen. The building was indeed built by the Romans some 1800 years ago, and it had not been rebuilt. Yes, it was built with Roman concrete. The official tourist pamphlet said its dome spanned 143 feet. I was amazed. How could anyone build such a large, elegant structure with hands using some mysterious concrete?

As we left the building, we stopped, and I glanced back. It appeared to me there was a great mystery connected with this building. What knowledge could be gained from studying the product of these ancient builders? I turned to my wife and instinctively said, "If I gave the task of designing a building like this to our engineers, they could not do it. There is not one steel reinforcing rod in the building." The universal engineers' code mandates steel rods for concrete buildings. The Romans have unbelievably breached our saintly professional standards, but the ageless Pantheon stood in testimony to this violation. And, I questioned, is it safe? Everyday, thousands enter and looked up at its famous dome.

During the flight to America, I was constantly disturbed about the strength and durability of this ancient concrete. After all, I hold three engineering degrees, and with a lifetime of experience in the design and construction fields. I felt uneasy about the Pantheon. Perhaps, I had missed something special in my brief technical survey of that building. The days that followed brought no relief to my anxiety. Surely, hidden in a library was the book that someone, sometime had written to satisfy my curiosity by explaining the Pantheon and its durability.

I now had access to time and travel that my colleagues in the work-a-day world would envy. The larger libraries would reasonably be the source of a technical solution to this engineering quandary. Thus, my first stop of many visits to libraries in the world was Stanford University. Here I had studied for my graduate degree, and here I was familiar with the vast learning resources.

My literature search at Stanford yielded much information on Roman archaeology, but nothing of significance on formulas and alike explaining the excellent concrete in the Pantheon. Alas, I turned to the University of Utah, where I had also studied engineering, only to verify that there was a real lack of knowledge on this subject.

While these failures were most disappointing, they inflamed my desire to unravel the riddle of the mysterious concrete. I reasoned the European universities would likely hold the book I was seeking on some dusty shelf in their Roman collection, and I wanted it. So my wife and I went to Oxford and Cambridge Universities, then on to the British Museum which holds the national book depository; we also traveled to the American School in Athens. This was an exercise in futility: none of the technical proofs for the Pantheon concrete existed.

Thus, I began the awesome task of assembling technical material, some dating back 2000 years, on concrete and describing the builders with their technology. This lasted ten years. A prime objective of this activity was to connect modern concrete with its Roman counterpart which required a knowledge of concrete chemistry, a subject I found remarkably complex, but necessary for the proofs. This research was done and the conclusions were placed in the first chapter with a sigh of relief.

In my research, I discovered that ancient Roman writers had described their concrete as a lime-pozzolan mortar tamped into a bed of rock pieces. But there were important subtleties--questioned by my engineering experience--that made this crude process instrumental in controlling the quality of concrete. Some of these were the water content of the mortar, proper ratio and make-up of the parts, and the exact method of placement. With these points resolved, the analysis of ancient concrete was addressed in a technical section. However, lime and pozzolan require a close scrutiny in understanding their contribution to the ageless concrete.

The Romans made their lime by a simple method of heating limestone in a kiln to a high temperature in the same manner ancient people had done before Rome. I found the ancients, including Romans, had used lime as a protective seal on their walls. But, bad lime is easily produced by a choice of poor limestone or by using a low processing temperature, and I wanted to know how the Romans consistently made good lime for each batch. What method did the Romans use to monitor the heating of the limestone which is required to drive out the gases? I recognized that if any batch of lime was bad in making the Pantheon, its concrete dome would have crumbled long ago.

It was necessary to define the usage of water in making ancient concrete. It came from the lime slurry, often referred to as "lime" by the Romans. My engineering training told me that water was a critical factor in controlling the strength of both ancient and modern concrete, a point that has been overlooked by most modern authors writing on this subject.

There is meager information on the chemistry of pozzolan. It can be processed from different sources such as fly ash, rice hulls, and clay to name a few. Also, science has only vaguely defined the pozzolan-lime activity testing. The Roman, Vitruvius, in discussing ancient concrete called pozzolan "pit sand." For the Romans, it was a fly ash debris ejected from volcanoes near Pozzuoli, Italy, which gave pozzolan its name.

In my research, I was confused by a strange coincidence. Both plain sand and the sand-like pozzolan had the same chemical building blocks of silicon dioxide molecules. Sand will not react with lime, but pozzolan does react to make concrete. Why? I found the molecular structure of sand to be orderly and tightly bound, while that of pozzolan is a random order. This later property permits the lime to react. Understanding this molecular structure permitted me to explain the way both modern and ancient concrete is formed.

A few untidy areas needed further attention. What was this Roman pozzolan, and where was it located? What were the pozzolan components that influenced its reaction with lime? How were the important parts of the gel compound, which makes the concrete, related to both modern and ancient concrete? After a lengthy investigation, I found reasonable answers.

There was a surprise in these pozzolan studies. While reading an ancient Roman book, I discovered the Romans pounded tile mixed with lime to make a mortar for constructing floors. How could this tile be processed into concrete? The answer eluded me until I chanced upon an article on dam concrete. The U.S. Bureau of Reclamation employed a technique of using cement and burnt clay--which turns out to be a pozzolan--in their concrete process in constructing dams. Thus, additional research was devoted in explaining clay products in this book. The knowledge that the ancients used a powdered burnt clay/tile with lime in their concrete process will be instructive in future archaeological explorations.

With the scientific data neatly packaged, I still did not have a complete mosaic picture of this concrete puzzle. My years of experience gave me a simple truth that people, not formulas alone, make durable concrete buildings. Thus, as a professional engineer, I visualized the main concepts needed to plan and manage the construction of large structures, and proceeded to fill in the gaps. My criteria required the analysis be tailored with some physical evidence or document. The thrust was three fold: identify the Roman architect and other construction professionals, define the building organizations, and lastly, tell the story of the Roman workmen and their ingenious technology.

There were Roman architects; an architect's name was found cut in stone. This, however, was insufficient. What was his professional education that bestowed the talent to calculate such complex measurements needed to build the dome of the Pantheon? Did he have drawings for his building? Yes, an intricate building like the Pantheon cannot be constructed in a precise manner without mathematics, plans, and the supervision of a skilled work force.

More probing was needed to describe the Roman architect's ability. What was a typical configuration of trusses, which illustrates professional sophistication? How were large timber frames connected indicating a degree of ingenuity? There is a correlation between the architect's performance and his product such as building the Pantheon, and his performance was superior.

There were other disciplines associated with construction such as the surveyor. With his unique instruments, he located buildings and aqueducts quite accurately when compared with modern convention.

After studying the features of the Pantheon, I realized that only masters of organization and construction could be credited for this complex structure. There were no large mechanical cranes, nor electric saws. Who were these builders? They came from guilds such as with the Latin name of citrarii, carpenters. But, there was still more building expertise; there were contractors. The fact startled me; how could Rome be this advanced? But, this was so, and I recorded it.

To picture Roman construction, the builder's role must be explored. I referred to the specialties of the guilds. Also, specialists, such as carpenters and metal workers, were trained to make the Roman Legions self-supporting. Standards were introduced into the Legion for control, and with these, the Legion built military and civilian roads, fortifications, bridges, and other needed products. Before the soldiers retired from active service, their craftsmanship was transferred to their successors and to workers from local communities.

The size, shape, and function of the hand tools and devices, such as those used for lifting, define the degree of finesse and capability of the workers in erecting a building like the Pantheon. Fortunately, I was able to include pictures of tools, lifting devices, and techniques in this book to clearly indicate the assets of the Roman builder.

Looking back, the hotel clerk may have been a bit mischievous when he recommended a visit to the Pantheon. He knew, I am sure, that the curiosity of explaining the building of the Pantheon has haunted minds for centuries. I have solved the riddle to my satisfaction, and I trust to yours. But more important, this knowledge may serve to build a better concrete in the future. We have indeed learned something from the able Romans.

Copyright © 1999 David Moore, PE