…go to Rome and try to break old Roman concrete with an axe; you will only dent the steel.

— French architect Auguste Perret, 1950 interview

 

Japan's Society of Civil Engineers, concerned over the limited lifespan of modern concrete, is forming a committee to investigate why Roman concrete has endured for so long.

— News item from 2004

 

There has been an explosion of interest in Roman concrete, and it is not hard to see why. The Romans used concrete to build edifices capable of lasting thousands of years, while most modern concrete structures are incapable of lasting two centuries—and many are unlikely to endure beyond just one. Did the Romans, ignorant of modern chemistry, just happen to latch onto the right formula, while we, armed with several orders of magnitude more knowledge, accidentally chose the wrong mix? Or did the Romans, who used concrete for over a century before applying it to their most ambitious building projects, better understand the long-term environmental impact on certain formulations? The answers to these questions are complicated, but it seems that the Romans did indeed understand key aspects of concrete that we would not wake up to until the twentieth century. Because of our ignorance, we will have to spend over a trillion dollars in the coming years to either repair or replace our crumbling infrastructure. And, even though we have improved the formulation and application of modern concrete to improve its longevity, it still has the shortest life span of any major building material. For this reason, the way the Romans used concrete is of critical importance to us, and it provides our thematic focal point.

 

THE GREATEST ENGINEERS OF THE ANCIENT WORLD

 

When I was going to school some decades ago, we were told that ancient Rome's principal contribution to history was as conveyor and disseminator of the Greek culture. This would be like claiming that the United States' principal contribution to history was as a conveyor and disseminator of European culture. It's partly true, but mostly not. The Romans have also not fared well in today's media-driven, minimum-content society, in which most people learn their history from stirring but inaccurate movies and television miniseries. Pretty much all they can remember about the Romans are the duels in the arena and the dissipated lifestyles of some members of its upper class. Relying on these sources for our knowledge of the Romans is akin to making a moral appraisal of contemporary Americans based solely on tapes of Jerry Springer's show and the accounts of torture at Abu Ghraib prison.

In fact, the Romans and the Greeks were cocontributors to Western civilization. Together they represented a complementary confluence of cultures—a sort of European yin and yang. The Greeks brought art and literature to an unprecedented level of mastery. More importantly, they intensively explored both the scientific and ethical questions of the world: the first by improving and expanding the existing mathematical systems (which also led to deductive reasoning), and the second through drama and moral philosophy. However, the Greeks were also a remarkably quarrelsome people. The Greek mind was quicker but less stable than that of the Roman. The Greeks gloried in the minutiae of counterarguments, verbal obfuscation, and confusing paradoxes, while the Romans sought to discover the moral heart of an issue and find grounds of commonalty.1 Greek identity was often tied to their city-states, which constantly warred against, or made alliances with, other city-states. Today's enemy was tomorrow's friend, and then an enemy again. Few Greek governments enjoyed a stable political framework, and classical Greek history is essentially the dreary account of one long internecine conflict after another.² The Romans, on the other hand, absorbed their conquered peoples into a system that offered real benefits. By the second century BCE, the Italian peoples who belonged to tribes that once fiercely opposed Rome, like the Samnites and the Etruscans, had become Roman citizens, many of whom were members of the equestrian or senatorial class. Later, the Punic North Africans—descendents of Rome's once mortal enemies the Carthaginians—along with the Syrians, the Greeks, the Britons, and the Germans, would eventually hold high power in the empire as well. This is not to say that the “absorption process” was painless (no one likes being conquered), but it worked for many centuries, and, with a couple of exceptions—like the ancient Judeans—the people of the empire grew to recognize the benefits that the Pax Romana offered them, and they prospered.

The Romans were a practical people who abhorred instability, which was one reason why they worked hard to come up with just and enduring laws. Importantly, the Romans held an almost transcendental respect for their legal system, which was not only accessible to all Roman citizens but to noncitizens as well. For example, the people of a province, though they might not be Roman citizens, could bring suit against their ex-governor (invariably a well-connected Roman aristocrat) for corruption or the arbitrary use of power. Based on the surviving accounts of such cases, the provincials stood a good chance of winning such suits.3 Consequently, a rich patrician could suddenly find himself impoverished by the steep penalties imposed for his malfeasance, or banished from Italy, or both. The emperors were technically above the laws, but the better ones were loath to act outside them. By the early third century, all freeborn or manumitted men were officially Roman citizens, blurring or eliminating the old distinctions between “provincial” and “Roman.”

Another corollary to the Romans' sensible nature and love of stability were their engineering skills, which they exercised toward practical ends. Before the Romans, major engineering efforts were usually directed toward creating awe-inspiring tombs (the Egyptian pyramids or Mausoleus's famed sepulcher) or making gaudy power statements (the Colossus of Rhodes). Of the Seven Wonders of the Ancient World, the only one that offered real practical benefit to its peoples was the Lighthouse of Alexandria. The Romans rightly held that most of their engineering achievements, while generally less thrilling to behold, were more significant in that they provided a higher standard of living to the inhabitants of their empire.⁴ Aqueducts brought an unprecedented amount of fresh water to town and city dwellers, sometimes exceeding the volume now enjoyed by their modern descendents. Expertly designed and well-bedded Roman roads connected a realm that stretched from Northern Europe to the Middle East. Discerning travelers could peruse the equivalent of today's Michelin Guide⁵ to check the quality of the inns along the way and note the attractions of each town they passed through. Civil servants generally made sure the needs of the local people were addressed, kept the infrastructure repaired, and saw that private disputes were settled equitably in court and not by vendetta. It was a world that respected and rewarded education, encouraged daily bathing, and whose medical knowledge and technology would not be surpassed until the early industrial age. Although they were ignorant of germ theory, Roman surgeons sterilized surgical instruments before an operation⁶ and sometimes closed wounds with biocidic silver staples to avoid later infection.⁷ Most people would be surprised to hear that the Romans possessed steam turbines, odometers, analog computers (using finely milled brass gears), coin-operated machines, and a host of other technologies that made life easier or more entertaining than ever before.⁸ The cartoons and graffiti uncovered by archaeologists in ruins of taverns and bordellos have shown that literacy in the Roman Empire, while not universal, was fairly widespread, since most of the graffiti appears to have been written by slaves and members of the lower classes.⁹ The English historian Peter Salway observed that the literacy rate in Roman Britain was higher than any subsequent British government was able to attain for the fourteen centuries following the empire's fall.¹⁰ The Romans also fostered a meritocracy that was unequaled until modern times. Hard study and diligent work allowed many Roman citizens—including “barbarians” and former slaves—to reach high administrative posts. By the second century, most of Rome's rulers were no longer Italian natives, and most provinces would eventually see at least one of its native sons become emperor. It is with good reason that we refer to the period that followed the fall of the Roman Empire as the “Dark Ages.”

Of course, the engineering achievement that chiefly interests us is the Romans' rediscovery of hydraulic concrete. However, unlike before, when the formula was lost or forgotten, the Romans recognized the potential of this material and would use it with gusto throughout their empire until its fall in the fifth century. They systematized its production and application and were the first people to utilize concrete as we do today: putting it into large molds to create a strong monolithic architectural unit. Even after the Industrial Age gave us the tools and machines to surpass most Roman engineering efforts, we still scratch our heads and wonder about what methods the Romans employed to build some of these concrete structures, and how these buildings have managed to endure for so long.

Until the middle of the twentieth century, it was generally assumed that the Greeks had discovered concrete and used it in their major building projects before passing it on to the Romans. (The unspoken subtext of this belief was that the Romans had not been clever enough to invent something so remarkable.) The concrete remains of the platform at the summit of the Pnyx (pronounced pnüks) in Athens, the little hill from which speakers addressed the assembled citizens below, seemed to support this theory. Concrete was also discovered in the harbor works of Piraeus, Athens' port city. However, later investigative work revealed that the platform on the Pnyx did not date from the classical period but was instead a second-century Roman restoration. The same is true of the harbor emplacements at Piraeus, which were also renovated in Roman times. Aside from the cistern in Rhodes mentioned previously, no other structure incorporating hydraulic concrete or mortar from the archaic or classical period has yet been discovered from the Greek mainland or islands. We find no mention of concrete from the surviving Greek literature, and just one reference to lime mortar. The latter comes from a book dating from the fourth century BCE, called On Stones by Theophrastus of Lesbos.11 Theophrastus speaks of the properties of gypsum, but it is evident that he has confused it with limestone, for he reports that, when water is added to the “gypsum,” the mixture is too hot to touch. It is clear that he is actually referring to calcium oxide (lime), not to calcium sulfate. No mention is made of sand or any other filler. Most authorities agree that Theophrastus based this information on secondhand, or possibly thirdhand, accounts.

If the Greeks did pass on the knowledge of pozzolanic concrete to the Romans, it was definitely through an indirect route. The Greeks began establishing coastal colonies in Italy around 1000 BCE, but the ones who settled in the area near Vesuvius—where Roman concrete was first discovered—were Euboeans, and not the Doric settlers of the Greek islands with pozzolanic soil. Finally, no remains of Roman concrete have been discovered in Italy that can be confidently dated earlier than the third century BCE. Roman concrete piers have been uncovered at the site of the ancient Italian port of Cosa (Portus Cosanus), which, based on the pottery fragments used for some of the aggregate, might be dated to the middle of the third century. However, most of the concrete work at Cosa seems to date from the late second century or early first century BCE. (The broken pottery could have been taken from an old rubbish heap.) Nevertheless, even if the material dates from this later period, the ancient concrete found at Cosa is among the earliest examples of Roman concrete so far discovered, and the hydraulic ingredients came from the area around Mount Vesuvius, more than 300 km (ca. 187 miles) to the south.12 Perhaps a sailor arriving on a ship from Naples told the Cosans of the remarkable cement being used back home. Barring some dramatic discovery—an ever-present possibility in archaeology—the current data suggest that the people in the Naples region independently discovered what would later become known as “Roman concrete” in the early third century BCE. However, by this time, its discoverers were either Latin- or Samnite-speaking Italians of Greek ancestry or Romans. Consequently, it is probable that the Romans, unassisted by Greek artisans or architects, independently discovered concrete through a process of trial-and-error experimentation that spanned several generations.

 

THE EVOLUTION OF ROMAN CONCRETE

 

It is frustrating tackling the story of Roman concrete from the few literary sources that have survived on the subject. The earliest, though indirect, reference comes from a book published around 200 BCE by Cato the Elder,13 when the Romans were mostly using a nonhydraulic lime mortar and concrete. Our next—and most important—source comes from a book published almost two centuries after the first by the famed architect Vitruvius.¹⁴ In the latter, we find the first detailed reference to true Roman concrete. Unfortunately, this source dates from a period just before the Romans had perfected the material and began using it in volume. Another, though quite brief, allusion to hydraulic concrete is found in two of the volumes of a Roman encyclopedia published around 78 CE by Pliny the Elder.¹⁵ Although these literary references are mostly sparse and chronologically scattered, they have proven invaluable in our understanding of Roman concrete.

 

CATO

 

Although lime had been around since Neolithic times, the first detailed description of its manufacture and use in Western literature is Marcus Porcius Cato's On Farming (De Agricultura, also known as De Re Rustica—On Rural Affairs), written around 200 BCE. Cato, known to us as “Cato the Elder” to differentiate him from several of his descendents with similar names, was notorious for his extreme and often heartless penny-pinching (he advocated selling off old or infirm slaves instead of caring for them) and ruthless nationalism (in the last twenty years of his life, he ended every speech with the closing remark “Furthermore, Carthage must be destroyed,” until that end was finally achieved by the Roman army). Even to his austere countrymen, Cato came across as a bit “over the top” in his severity. A little ditty was written about him:

 

Porcius snarls at everyone and at every place

With bright gray eyes and flushed red face.

Even after death, one can imagine well

That he'd scarce be admitted to Hell.16

A surviving portrait bust of Cato exemplifies Roman mastery of subtle character delineation, for the sculpture shows a sour-faced, unrepentant reactionary. Today he would probably be a popular talk show host.

In his book, Cato talks about using lime mortar for masonry walls and—as was done for the previous ten thousand years—spreading it on the ground of a dwelling to serve as an artificial stone floor. Cato's only aesthetic concession for his lime flooring is the use of tiles. However, being a tightwad, he thought that broken shards of pottery would serve just fine. Waste not, want not. Reading On Farming, one cannot help but wonder what it must have been like being Cato's wife.

His description of a limekiln—the earliest such reference to have survived—is especially fascinating. In keeping with his skinflint ways, Cato suggests having all necessary implements and materials ready for the contractor doing the lime kilning to avoid incurring any additional expense. Those things to have prepared in advance include limestone, a kiln for baking the limestone, the wood for the kiln, and the sand for the mortar.

 

Build the limekiln ten feet across, twenty feet from top to bottom, sloping the sides in to a width of three feet at the top. If you burn with only one door, make a pit inside large enough to hold the ashes, so that it will not be necessary to clear them out. Be careful in the construction of the kiln; see that the grate covers the entire bottom of the kiln. If you burn with two doors there will be no need of a pit; when it becomes necessary to take out the ashes, clear them through one door while tending the fire with the other. Be careful to keep the fire burning constantly, and do not let it die down at night or at any other time. Charge the kiln only with good stone [limestone], as white and uniform as possible. In building the kiln, let the throat [chimney or smokestack] run straight down. When you have dug deep enough, make a bed for the kiln so as to give to it the greatest possible depth and the least exposure to the wind…. When it is fired, if the flame comes out at any point other than the circular top, stop the orifice with mortar.¹⁷

Centuries of practice and experimentation before Cato's time had led to a number of advances. The grate described above was unquestionably iron, and this allowed more limestone to be kilned, as pieces could now be stacked right over the flames. Another refinement was the shape of the limekiln, the so-called Burgundy bottle form, which would be used until the beginning of the twentieth century. It allowed the maximum amount of stone to be calcinated with the least amount of fuel. Nevertheless, a phenomenal amount of fuel was still required: perhaps a couple dozen or more cords of wood for a single firing of the limekiln described by Cato. While the lime kilning methods used by Cato might seem primitive, he was using very up-to-date technology. However, he mixed and applied the caementis in a rather unsophisticated manner, at least in comparison to later Romans. Cato was a successful farmer and a formidable politician, but he was a crude architect.

Aside from using lime to create artificial stone flooring, Cato mostly talks about using lime for wall building. He instructs the reader to mix two parts of sand with one part of lime. This proportion is odd, and, while certainly functional, it goes against the miserly mien of the old Roman. Why not recommend the equally usable and thriftier mix of three or four parts of sand to one of lime? Perhaps because the quality of the sand Cato used to mix with lime was subpar. For him, “sand” was any kind of naturally pulverized rock—and it was most likely adulterated with dirt to varying degrees, as well. Using more lime was probably the only way of overcoming the sand's poor quality and/or contaminants. Another reason for the two-to-one proportions could be the fact that less sand makes a slightly harder mortar, although this marginally harder mix comes at an economic cost that Cato would have likely shunned. Consequently, the high amount of lime Cato used was likely due to low-quality sand.

Cato does not provide any instructions about constructing a wall. He gives only the dimensions, material requirements, and costs. To save money, he naturally recommends that the farm owner provide the rock rubble (caementi) sand, and lime for the contractor's use to avoid any markups. Cato calculates that each linear foot of a five-foot wall should require one modius (ca. two dry gallons) of lime and four modii (ca. eight dry gallons) of sand. If the contractor provides the lime, Cato tells the reader exactly how much it should cost, so he won't get cheated.

Why did Cato provide details about constructing a limekiln but write nothing about how a wall should be built? The likely answer is that, while many farmers were unfamiliar with lime kilning, almost everyone knew how a Roman wall was constructed. And the way the Romans built their concrete walls was unique for their times.

 

ROMAN CONCRETE WALLS

 

Even though the temples built on the summit of the Acropolis in Athens—especially the Parthenon—were marvels of engineering and the pride of all Athenians, their own homes were of far humbler construction and were usually built of adobe brick. While adobe was certainly serviceable, it had a major security flaw: one could dig a hole through an adobe wall in less than an hour. Indeed, the ancient Greek word for burglar, toicorucos (τοιχωρυχος), means “wall burrower.” And while adobe holds up well in sunny regions, like Greece and the Middle East, it does less well in damp areas, such as northern Italy, France, and Germany. In short, adobe is not a universal building material.

Most Romans had no worries about wall burrowers. By Cato's time, all but the lowliest citizens built their walls with lime concrete, and these were among the best in the ancient world. The Romans recognized that lime concrete—barely different from lime mortar—possessed limited strength. And even though lime concrete held up much better to the elements than did adobe, it was still subject to gradual weathering. To address this problem, the early Roman mason would lay down two courses of mortared stone running parallel to each other, a half meter (ca. 20 in) or more apart. Once the courses were about two or three feet high, a layer of rock aggregate was laid down between them and then lime mortar was dumped on top of it and strongly tamped down to fill in all the cavities and crevices. The Romans used only enough water to make the mortar pliable, and so it was very thick. Their concrete construction was quite different from the modern method of thoroughly mixing rock aggregate with a less glutinous concrete before pouring the whole into a form. This wall-building process of laying aggregate and ramming in the mortar was repeated until the top of the courses was reached, whereupon additional masonry courses were laid and more aggregate and mortar was tamped between them until the desired height of the wall was attained.18 The ramming process ensured a maximum rock-to-mortar ratio, thus giving the wall substantial compressive strength—more due to the voluminous rock aggregate than anything else—while the masonry of the outside courses protected the lime concrete core from the elements. Once the wall had set, a burglar would have a very difficult time trying to burrow through such a barrier.

While the Greeks and Etruscans occasionally built rubble-cored walls, the Romans perfected the method and would use it in most of their construction work from the late Republic to the end of their empire.

Around Cato's time, the Romans began using molds for their concrete work. In the beginning, these were often just wooden planks spaced far enough apart to form the outline of the wall to be built. As an aesthetic concession, the sides of the outward-facing stones were often filed down to a square or diamond shape, and together they formed a pleasing netlike pattern on the wall's surface. (Cato would have avoided such frivolity, since it increased costs.) Eventually, brick was used for the outer courses of the wall, and this style would become the preferred construction method.

At this point, it would be helpful to first explain a few Latin technical terms. A misunderstanding of these terms, as well as misinterpretation of the physical archaeological evidence, has caused much confusion about Roman concrete.

The Romans called their lime mortar arenatum (“sandy stuff”) because it mostly consisted of sand (arena or harena). They knew that the active ingredient was lime, but in their naming conventions, the Romans often referred to a material's principal constituent. Likewise, the Romans referred to their lime concrete as caementis (“rocky stuff”), because, even though it was essentially lime mortar mixed with stone aggregate, it was mostly composed of the latter, which was called caementa (plural caementi) in Latin. Caementi were small, sharp stones that ranged from broken pebbles to fistsized rocks. Oddly, it is from the Latin caementis that we derive the modern English word cement, which we frequently—and mistakenly—call concrete. (Cement is the “glue” that, together with water, rocks, and sand, creates the finished product, concrete.) The term concrete, though derived from the Latin concretus (meaning “brought together” or “congealed”), was never used by the Romans to describe the material. What historians and engineers today call “Roman concrete” is the hydraulic version, for which, as far as we know, the Romans did not even have a name. In the surviving literature, the hydraulic component is described as an additive to standard caementis. This additive was called pulvis puteolis, pozzolanic soil or powder (Puteoli being the ancient Latin name for the modern Italian city of Pozzuoli, near Vesuvius). This volcanic powder was mixed with the caementis to make it either impervious to, or to allow it to set under, water.19 Besides these hydraulic properties, the Romans discovered that pulvis puteolis also made caementis harder and more durable.

To keep things simple, we will refer to the non-hydraulic version, already ancient in Cato's day, as lime concrete or caementis. For the hydraulic version, we will follow convention and call it Roman concrete. For many years, archaeologists examining Roman ruins could tell no difference between walls using caementis and those using Roman concrete, as both looked the same. For this reason, they assumed that Roman concrete had been used centuries earlier than it actually was. Only with the recent advent of sophisticated techniques for mineral analysis has this controversial issue finally been settled.

Back to Roman walls. Even though Cato does not directly provide construction details in his book, the remains of numerous Roman walls, plus the specific instructions he gives for the materials to have ready for the contractor, tell us that he was referring to a classic lime concrete wall. The first clue is rather obvious: the chapter in his book is titled “Walls Made of Lime Concrete and Stone” (macerias ex calce caementis silice). The less obvious clue is the previously mentioned amounts of lime and sand he recommends for each linear foot of a thick five-foot wall. These amounts are correct for the high-aggregate lime concrete used by the Romans at this time. (Later Romans would use better-quality sand, and so they could increase the measure to three or four parts sand to one part of lime.)

The Romans liked caementis because it allowed them to build thicker, sturdier walls for less money than a pure masonry wall of the same dimensions. If the wall was a crude affair, like the ones Cato built, it could be plastered over for aesthetic purposes and as a further safeguard against weathering. In the case of important temples, government buildings, or lavish villas, cut and polished sheets of limestone or marble were laid across the wall's surface to make it appear as if superior stone had been used for its construction. The remains of many of these walls, though not made with hydraulic concrete, can be found throughout Italy. In Cato's time, the superior caementis using pozzolana was still not widely used and was probably unknown to most Romans. A century after Cato's time, hydraulic caementis had come into more general use, and the first surviving mention of it is found in a book written by a remarkable man.

 

VITRUVIUS

 

The earliest and only known detailed reference to Roman concrete in the ancient literature is by the renowned Roman architect Marcus Vitruvius Pollio in his De Architectura (On Architecture), also known as the Ten Books on Architecture, written a couple of centuries after Cato's work.20

Vitruvius began his career as an artillery specialist in Julius Caesar's army. His occupation required expertise in the construction and maintenance of ballista: devices that used torsion-springs instead of gunpowder to hurl sharp iron bolts or heavy stone balls with great force at enemy troops or over the walls of a besieged city. Vitruvius's book shows that he also had keen knowledge of other aspects of military engineering, such as siege emplacements and the rapid construction of sturdy wooden bridges, as well as larger-scale works that we would today call civil engineering projects, like town planning, municipal drainage, building aqueducts and harbor emplacements, and so on. For the average Joe, Vitruvius also addresses the comparatively simple issues involved in home building and maintenance, like plumbing basics and what kind of stucco to use.

It seems somehow appropriate that the earliest reference to concrete comes from a Roman military engineer. Some authorities have suggested that Rome's lime-based technologies—and her adversaries' lack of such—contributed to her success in conquering a large part of Western Europe. The ancient tribes of Gaul, Britain, and Germany were ignorant of lime mortar or concrete, and so built their forts with thick earthen walls reinforced by logs. Once completed, these defenses were formidable and immune to ramming. However, after a couple of decades these once stout barriers became gently sloping mounds that offered little defense against a determined army.21 Vitruvius was probably well acquainted with these earth-wood walls from his time in Gaul.

We know little of Vitruvius's life, but it is generally assumed that he became a civil engineer after Caesar had completed his military exploits and had assumed the title Dictator for Life (it would be a short term of office). Vitruvius mentions having built a basilica in Fano, Italy, during this period, but all traces of it have vanished. Like most Roman buildings, it may have served as medieval quarry where local inhabitants could freely obtain precut stone. Vitruvius eventually received a generous pension from Caesar's grandnephew and adopted son and heir, Augustus, Rome's first emperor. It was during this comfortable retirement that Vitruvius wrote On Architecture.

On Architecture is counted among the most influential books on the topic of architecture ever written. Part of its influence comes from the fact that it is the only detailed manuscript on architecture to have survived from the Greco-Roman world. Vitruvius provides useful bibliographic references on each of the subjects he explores, but the works of most of these authors have survived only in fragments or vanished completely during the Dark Ages. Consequently, when On Architecture was rediscovered in the fifteenth century, it had a profound impact on all the architects of the Renaissance, especially Andrea Palladio, whose own I Quattro Libri dell'Architettura (Four Books on Architecture) could be viewed as an updated and appended version of Vitruvius's work.²²

On Architecture is encyclopedic in its breadth. Besides the topics mentioned above, it also covers sundials, water mills, pneumatics, crane and hoisting technologies, geometry, and even a little astronomy. This was the time of the great ancient encyclopedists, such as Vitruvius's near contemporaries, Pliny the Elder and the famed North African scholar King Juba II, two authors who tried to address as many topics as possible in their books and, in the process, perpetuated a number of myths.

Fortunately, Vitruvius does not often stray far from those subjects with which he was personally familiar. It must be remembered that the word architecture in the ancient sense referred to the construction of any structure or device, whereas today it refers only to designing buildings. This fact, along with Vitruvius's stated belief that a good architect should also possess a strong grounding in the sciences and liberal arts, explains why the book covers such a wide variety of subjects, including the author's occasional philosophizing. As for philosophy's own relationship to architecture, Vitruvius explains that “it makes an architect high-minded and not self-assuming, but rather renders him courteous, just, honest, and ungoverned by greed. This is very important, for no work can be rightly done without honesty and incorruptibility. Let him not be grasping, nor have his mind preoccupied with the idea of receiving excessive fees, but let him maintain his position with dignity and by cherishing a good reputation.”23 This sounds almost like a building contractor's version of the Hippocratic oath.

Many of Vitruvius's pronouncements are simple yet profound. For example, his remark that a structure must be durable, useful, and beautiful (firmitas, utilitas, venustas) holds true for any age. Look around sometime and ask yourself how many of the buildings you see fit all three of these criteria. And such a mandate is applicable not only to buildings but to any manufactured product. It is what sets a fine mechanical watch apart from a cheap electric model, or a beautifully crafted pen from one made of plastic and designed to be disposable; or, more appropriate to our theme, it is what distinguishes a beautiful Roman arched bridge capable of lasting millennia from one made of modern concrete that is subject to disintegration after a century or less.

Vitruvius was also capable of making some very shrewd observations. In his chapter on “Aqueducts, Wells, and Cisterns,” he reflects on the toxic properties of lead plumbing:

 

Clay pipes for conducting water have the following advantages: In the first place, regarding construction issues: if something happens to them, anybody can repair the damage. Secondly, water from clay pipes is much more wholesome than that which is conducted through lead pipes, because lead is found to be harmful for the reason that white lead is derived from it, and this is said to have deleterious effects on the human system. Hence, if what is produced from it is harmful, no doubt the thing itself is not wholesome.

Of this, we can draw an example from plumbers, since the natural color of their bodies has been replaced by a deep pallor. For when lead is smelted in casting, the fumes from it settle upon their extremities, and daily burn away all the beneficial properties of the blood from their limbs. Hence, water ought by no means to be conducted in lead pipes if we wish for it to remain wholesome.24

This was written some two thousand years before lead pipes for plumbing were finally banned in the United States in 1989.

Besides being better organized than the Greeks, the Romans possessed another attribute that contributed to their empire building: a fascination for any technology that had a practical purpose. Greek intellectuals held a strong prejudice against craftsmen. A potter or sculptor was unlikely to be invited to one of Plato's symposiums, no matter how well read he might be. (Socrates was a retired stonecutter.) Indeed, the Greeks called the manufacturing trades the Bausotic Arts, from the Greek word bausos, meaning “vulgar.” Romans, especially those of the patrician class, adopted this prejudice because they generally held Greek taste in high regard. Nevertheless, the Romans' innate practical streak prevented them from completely embracing this elitist snobbery. Romans remained fascinated by technologies that could serve useful purposes, and since they were among the most conscientious administrators in history—building roads, bridges, aqueducts, and public baths wherever they went—many of them felt it was necessary to know how things worked. Reading On Architecture, it is clear that one reason Vitruvius wanted to share his knowledge was that he knew many of his readers would be government administrators confronting the same infrastructure challenges addressed in his book.

Book I of On Architecture covers fundamental subjects: the proper background and knowledge an architect should possess; how to find a proper site for a city and its walls, public buildings, and so on. Book II covers building materials, and describes mortar in far more detail than Cato's work. Book II also demonstrates just how far construction knowledge had advanced in the intervening two centuries. Vitruvius explains the variety of different sands that can be used in lime mortar, and discusses their relative strengths and weaknesses. Vitruvius advises against using sea or river sand (possibly because the granules of both are likely to have been worn smooth by water and are thus less capable to form strong bonds). Sea sand is particularly unsuitable because the dissolved salts within it will cause unsightly splotching. The highest-grade sand he calls “pit sand,” and of this variety there are four kinds: black, red, gray, and carbuncular. From Vitruvius's description, the last seems to be of volcanic origin, but it is not clear if it is of the pozzolanic variety; the other three apparently derive from rock erosion. Of pit sand, the best is dry, sharp-grained, and unadulterated with dirt. Vitruvius helpfully provides a simple, do-it-yourself test to determine the sand's quality: “Of these the best will be found to be that which crackles when rubbed in the hand, while that which has much dirt in it will not be sharp enough. Try this: throw some sand upon white cloth and then shake it out; if the cloth is not soiled and no dirt adheres to it, the sand is suitable.”25 If one has no choice but to use sea or river sand, Vitruvius knows of an additive that will ameliorate their defects: “[P]otsherds ground and sifted through a sieve, and added in the proportion of one-third part, will make the mortar better.”²⁶ This seemingly innocuous comment contains within it profound possibilities, as we shall later see.

Vitruvius then goes on to discuss limestone and lime, and here we find the earliest surviving reference to hydrated lime, although its use probably predates Vitruvius's book by some centuries. To remove lime's caustic properties, a small amount of water is added to it. Craftsmen performed the process of hydration in a dozen different ways. Often, the powder was laid out over a smooth, dry surface, sprinkled with water, and then thoroughly mixed with rakes or trowels. In very humid environments, the lime is simply exposed to the air for a period of time, although this method was probably not employed in the mostly temperate climes of the Roman Empire. Vitruvius suggests that the lumps caused by the moisture be thoroughly mixed with the rest of the powder. Once it was hydrated, the lime was sealed in a waterproof container. In Roman times, this was usually a ceramic amphora, though wooden barrels may have been used on occasion. The Romans held that the older the hydrated lime was, the better its quality, and so they specified in their building codes that it be aged several years before use.

After finishing with hydrated lime, Vitruvius moves on to discuss something that evidently strikes his fancy, and, from its description, we perceive that it is an interesting novelty.

 

There is also a kind of powder (pulvis) that naturally produces admirable results. It is found in the area of Baiae and among the farming communities around Mt. Vesuvius. This substance, when mixed with lime and rubble, not only lends strength to various buildings, but even when piers of it are constructed in the sea, they set hard under water.27

Vitruvius is describing concrete that is created by using pozzolanic earth, the latter being a granulated version of volcanic pumice (pumex) that the Romans also called sponge-stone (spongia) for its many holes. It is essentially the same material used in the mortar for the cistern in Rhodes. It is the earliest surviving reference to the material that would one day dominate the visual landscape of our modern world: hydraulic concrete.

At the time that Vitruvius wrote his book, toward the end of the first century BCE, true Roman concrete had only recently emerged from being an intriguing waterproof mortar to becoming a building material that, by itself, could be used in new and creative ways. Instead of the simple wooden planks used for wall molds, more elaborate forms—called “shuttering” by today's engineers—were fashioned. The same method of ramming together layers of lime concrete and stone aggregate was followed, though the concrete was now the hydraulic version known today as Roman concrete. There are the partial remains of a ceiling vault, and a largely intact Roman concrete dome over a bathhouse dating from the century in which Vitruvius lived. Unsurprisingly, both the vault and dome were constructed in the immediate vicinity of Mount Vesuvius, which had a virtually inexhaustible supply of pozzolanic soil. Both architectural forms are related to the Roman arch: stretch an arch along its side, and you have a barrel vault; two intersecting barrel vaults form a cross vault; rotate an arch around its center axis, and you have a dome. The Romans would go on to perfect all these architectural forms, utilizing the plastic nature of concrete to create them. The bathhouse dome, the earliest known monolithic concrete dome, is a curious affair, however. Apparently, the pioneering architect who built it was still a bit nervous about the material and uncertain about its strength. Consequently, he made its walls very thick. Even though the bathhouse was constructed roughly around the same period that Vitruvius wrote his book, he does not mention the use of concrete for building domes or vaulting. The master architect probably felt that these recent and relatively rare experiments had yet to meet the test of time, which, to him, was the final arbiter of any building's worth.

Vitruvius makes one more mention of Roman concrete in book 5, chapter 12, where he discusses building breakwaters and harbors:

 

Take the powder that comes from the country extending from Cumae to the promontory of Minerva [pozzolanic earth from the vicinity of Vesuvius], and mix it in the mortar trough in the proportion of two to one. Then, in the place previously determined, a cofferdam, with its sides formed of oaken stakes with ties between them, is to be driven down into the water and firmly propped there; then, the lower surface inside, under the water, must be leveled off and dredged, working from beams laid across; and finally, the concrete [caementis] from the mortar trough [mortario]—the stuff having been mixed as prescribed above—must be heaped up until the empty space which was within the cofferdam is filled up by the wall.28

 

 

These are the earliest instructions on how to use Roman concrete to create a dock or the piers of a bridge. Note that the sand has been entirely replaced by pozzolana. The Romans would go on to use a variety of concrete mixes, including some incorporating both sand and pozzolana, but for underwater work, little or no sand was recommended.

By Vitruvius's time, the use of concrete for piers and jetties was undergoing explosive growth. Vitruvius was almost certainly aware that Roman concrete was being used on an unprecedented scale in a major construction effort where its hydraulic properties were being put to the ultimate test. Since he had dedicated his book to Caesar Augustus, it was perhaps impolitic for Vitruvius to mention a bold civil engineering project that dwarfed anything Rome's first emperor had yet instigated. And, as if to rub salt into the wound, the man who had authorized the project and was eagerly following its progress was neither a Roman nor even a Greek but the king of a widely despised people: the Jews.

 

A HARBOR WHERE NO HARBOR SHOULD EXIST

 

The first large-scale use of Roman concrete did not take place in Rome, or even in Italy, but 2,300 km (1,400 miles) to the east, in Judea. And it was the largest application of hydraulic concrete in a single construction project until the early twentieth century. Roman concrete, formerly a specialized mix used in perhaps a couple dozen projects, became a mass-produced commodity because it was the essential ingredient used to fulfill one man's obsession: to build a magnificent harbor in a place where a harbor should not exist. The man was Herod the Great, king of Judea, and his pet project was the Harbor of Caesarea.

Like his father, Antipater, Herod had come to power with the help of Rome and at the expense of the previous royal house of Israel, the Hasmonean Dynasty. Herod was a realist. Rome called the shots, and the Romans wanted a friendly client king in Israel, which, along with other buffer states, would keep in check the powerful and hostile Parthian empire to the east. Antipater and, later, his son Herod, cultivated friendships with powerful Romans, and thus began the Herodian Dynasty. The Roman Senate recognized Herod as king of Judea. He did not owe his position to popular acclaim or royal connection, though he did marry a Hasmonean princess to give the appearance of continuity of the royal bloodline. (He later executed this wife and their two sons.) Herod was king because Rome said he was king. End of story.29

Herod is mostly remembered today for the rebuilding of the Temple of Jerusalem, but it was his construction of the city and harbor of Caesarea that really defined his reign in the eyes of the world. Caesarea was a far more ambitious project, and, to some of his contemporaries, it must have seemed like one of his craziest ideas.

The site Herod chose for his city and harbor was simply a long stretch of beach that connected the desert with the Mediterranean Sea. Here were the ruins of an old stronghold, called Strato's Tower, a fortress originally built by the Phoenicians, but which had been successively captured (and lost) by the Greeks, Jews, Romans, and Ptolemaic Egyptians. Thanks to Roman support, the land now belonged to Herod. Strato's Tower originally had a small wharf and breakwater that were formed by dumping boulders of the local sandstone called kurkar into the water. Much of this modest wharf/breakwater had largely vanished by Herod's day, due to the strong currents and silting. Its few remaining inhabitants probably pulled their small fishing boats onto the hot sands of the beach to protect them from notorious storms that plagued the Levantine coast and to keep the barnacles off their hulls.

Strato's Tower was not a place to build a major international harbor of the kind Herod envisioned: one to rival Alexandria in Egypt or Athens's Piraeus in Greece. The main difficulty was the local geography. There were no nearby offshore islands or promontories that could offer shelter against the winds and currents, or which could serve as starting points for the dumping of stone into the sea to create a major breakwater. Nor was there a navigable river that could provide protection to ships during the harbor's construction, or from which fresh water would constantly flow out to keep marine pests like barnacles and shipworms in check, or even to provide enough drinking water (meager wells had to suffice). Nor was there any suitable rock in the immediate vicinity that could be used for building a permanent mole, or jetty; there was only the soft kurkar that had proved so useless previously and that was vulnerable to breakup when submerged. Even if local rock had been suitable, the bed of the proposed harbor was composed of deep sand, which had already demonstrated an annoying tendency to swallow up the rocks used to create the earlier mole. Additionally, the strong current coming from the southwest, intensified by the great volume of water pouring from the Nile into the Mediterranean—and bringing with it countless tons of silt—would have made the project difficult under the best of circumstances. Combined with the other natural obstacles, the engineering problems seemed insurmountable. Even to Herod it was clear that, despite all his material and manpower resources, he was going to need some help.30

Of all Herod's Roman friends, his most powerful and influential was Marcus Agrippa, Augustus's right-hand man. They met in 40 BCE, when Herod made his first trip to Rome. Herod's father, Antipater, had recently passed away (poisoned by court enemies), and Herod no doubt thought it wise to make friends with those in power to help secure his position as king of Judea. Undoubtedly, palms were greased and lavish gifts bestowed. Nevertheless, bribes were not enough. He would have to make a strong, logical case for Rome to support his dynasty's claim to Judea. Herod was probably required to provide the Romans military and material assistance in the region. The Jewish historian Josephus writes that Herod had struck up a warm friendship with Agrippa during this lobbying junket. Though they came from very different worlds, Herod and Agrippa apparently found some sort of rapport. Both were intelligent men who, like most educated people of their day, spoke fluent Greek, and it was probably in this language that they conversed. Also, both men were builders. Augustus liked to say that he found Rome a city of brick and left it a city of marble, but it was Agrippa who was responsible for much of the city's transformation.31

In 23 BCE, Herod and Agrippa arranged a meeting in the city of Mytilene, on the Greek island of Lesbos. Though few details of their talks have survived, most historians believe that it was at this meeting that Herod brought up his plans to build a city and harbor at Strato's Tower. It is likely that Herod put forward all the strategic reasons why a large port should be built there. The huge Roman grain ships—the supertankers of their day—would have a safe haven if a storm should arise and they found themselves too far from Alexandria to turn back. The only other major port in the region, Antioch, was too far north to be of any assistance, and Antioch had its own problems: its harbor was fast silting up, and it was strategically vulnerable. The Parthians could send troopships by boat up the Euphrates then disembark east of Antioch and march west to take the city. On the other hand, Strato's Tower enjoyed the protection of the vast and merciless Syrian Desert to the east. Even if Antioch were to fall, the Romans would still be able to use Strato's Tower to deliver the troops and supplies to counter such an incursion. Finally, Strato's Tower would be a Hellenistic city, like Alexandria. It would have a forum, theater, temples to the gods, and public baths fed by an aqueduct that would also bring freshwater to its inhabitants—in short, all the things to make a Greek or Roman feel at home. It would be mostly populated by the local people—Hellenized Syrians who spoke Greek as a second language—as well as Roman and Greek merchants. Naturally, there would also be a Jewish community—after all, Herod was king of Judea—but he'd keep them in line (non-Hellenized Jews already had established a reputation for being unwelcoming to pagans and their religious practices.) Of course, Herod's real reason was that such a harbor was a necessary prerequisite for a major expansion of his kingdom's economy. The overland caravans bringing the silks, spices, and other luxury goods from the East would no longer need to divert to the north for the port of Antioch or south to Alexandria to ship their goods on to Rome and the other wealthy cities of the empire.32

Once Herod had convinced Agrippa of the project, it was time to examine the engineering details for the site. The Roman had likely brought engineers with him to discuss the tricky problems involved in creating a great harbor in a place where none should exist. Agrippa had some experience in harbor building, having built Port Julius in the Bay of Naples for the Roman fleet. Of course, Port Julius was far smaller than the harbor Herod wanted, and the Naples area had everything that Strato's Tower lacked, especially a sheltered bay with a good supply of building materials nearby. It is probable that Agrippa had used Roman concrete in the construction of Port Julius, since all the pozzolanic soil he needed was close by. One can easily imagine Agrippa or one of his engineers telling Herod, “You know, we have this special caementis that we used at Port Julius. It sets underwater. You could probably use it to create your harbor. However, you would need to use an awful lot of this stuff. I mean, no one has used pozzolanic concrete on this scale before. It's theoretically possible, but frankly, I don't see any other way you could pull this thing off”

 

 

In fact, without Roman concrete, there was no other way for Herod to build his magnificent harbor. Concrete solved all the logistical problems that would have normally doomed such an enterprise. No suitable rock to build the harbor? Use concrete. No sheltering promontory to use as a starting point? Build your own with concrete. Once that issue was settled, the next hurdles to overcome were the other formidable logistical challenges. Herod had no doubt brought along a scale model of the harbor—the usual preliminary step in a major building project in the Greco-Roman period—which was probably accompanied by calculations of the volume of material needed, based on the breakwaters' proposed length, breadth, and the depth of water where the material was to be laid. Also likely included were the local tide tables and the number of available working days (seasonal storms probably restricted the construction effort to less than two hundred days a year).

The harbor's design also had a very clever feature. The bane of all artificial harbors is the danger of silting. Agrippa's own Port Julius was already beginning to silt up at the time of his conference with Herod and would eventually have to be abandoned. Herod's solution—or that of Agrippa's engineers—was to have channels at the top of the moles that would be open only at high tide, ensuring a flow of silt-free water through the harbor. It was a well-thought-out, state-of-the-art design. The trouble was, it would be located in the worst spot imaginable, and even Herod probably conceded that his chosen location was less than ideal. Unfortunately, the whole coastline of his kingdom was pretty much the same. To help grease the wheels, it was probably Herod who suggested naming the city Caesarea, and its harbor Sebastos33 (the Greek word for Augustus). Such things do delight monarchs.

At this point, Agrippa probably gave the plans to his engineers who were present at the meeting and asked them to figure out the logistical requirements of building such a harbor with hydraulic concrete. (“Can you have it ready by tomorrow?”) After the engineers left to mull over the figures, Agrippa and Herod probably moved on to discuss the political situation in the region, to delve into the latest intelligence from Parthia, to exchange court gossip, and perhaps to make the stock inquiry “How are your kids doing?” (Although in Herod's case, that might not have been such a prudent question.)

One can imagine the expression on Agrippa's face when his engineers returned with logistical requirements for Herod's harbor. Only now, after much underwater archaeological surveying has been performed, are we beginning to understand how colossal those logistics were.

Roman engineers were thorough and fastidious in their planning, and they certainly knew how much lime, pozzolana, and aggregate would be needed for a certain measured volume, as well as the amount of wood needed to kiln a specific quantity of lime. Just as a CEO today is given a thick binder with all the details concerning a proposed project but usually decides about whether to go forward based on the information in a flashy slide how, so Agrippa's chief engineer must have delivered a thick scroll but then provided a verbal sum-up. My guess is that it went something like this (using modern measurements for the convenience of the reader):

“Your Excellency, building a harbor in the place King Herod desires will be a formidable undertaking. If we forget for a moment the amount of aggregate needed, the harbor emplacements will require 24,000 cubic m of pozzolana (ca. 847,552 cubic ft) and 12,000 cubic m (ca. 423,776 cubic ft) of lime. Let's deal with the pozzolana first. Getting that much to Judea will be tricky. That's many times the amount we used at Port Julius, and we had the advantage of enjoying an inexhaustible supply directly at hand. The number of normal ship cargo loads this represents boggles the mind. I mean, we're talking about almost 23,000,000kg (ca. 63,566,399 lbs). How do you move that much pozzolana to Judea?”

Having been in the corporate world for a quarter century, I have had the pleasure of listening to many expositions by engineers. The good ones first present the problem in such a manner that it seems insoluble. Then, after giving you a few seconds to ponder the imponderable, they smile and then explain their clever solution. Agrippa's chief engineer must have enjoyed seeing his boss draw a long face before announcing their clever scheme. “We do have an idea, however. If you could borrow the giant grain ships after they have off-loaded their cargo at Ostia, and then divert them south to Naples, you could load them up with volcanic soil there; then, on the way back to Alexandria, they make a stop at the construction site in Judea to drop off the powder. The old mole at Strato's Tower, just south of the planned harbor, could be extended with concrete and sandstone blocks to provide refuge for a few ships during construction of the main harbor. Once the latter is completed, Strato's mole can serve as a subsidiary breakwater to lessen the impact of the currents on the southern jetty.

“Still, the toughest nut to crack is the 12,000 cubic m (ca. 423,776 cubic ft) of lime that's also needed for the concrete—and that's just for the jetties alone. That much lime will weigh around 29,000,000 kg (ca. 63,800,000 lbs). Unlike pozzolana, which can be simply scooped up, lime needs to be manufactured. To produce that much lime, you're going to need hundreds of limekilns, which will have to be manned twenty-four hours a day, every day for the five or six years this harbor will be under construction. Once you have the lime, it needs to be slaked, then put into amphorae—many thousands of them—which will then need to be carried by cargo ships with the ropes and drilled storage decks to handle them without breaking—you definitely do not want amphorae of lime shattering on the wet deck of a ship. For efficiency's sake, it would be best to have the pottery ovens and limekilns near the limestone outcrops and the fuel sources, but that's the biggest problem of all. Figuring that one oak tree is needed for the fuel to kiln the limestone to produce 190 kg of lime, we will need approximately one hundred thousand to two hundred thousand trees. Where are those trees going to come from? The coastline of the Mediterranean Basin has been pretty much denuded of trees. Remember a few years ago when we needed to get permission from Augustus to cut down the sacred grove that surrounded the Sibylline Shrine at Cumae? As you recall, it was perhaps the last forest of virgin oak trees in Italy near the sea, but we had no choice because that lumber was necessary for building the ships we used for the war against Mark Antony and Queen Cleopatra. And that amount of wood was nothing compared to the massive volume needed for this project. And here's the kicker: in addition to the trees needed for the limekilns, you will need almost as much wood for the pottery ovens to make that many amphorae, and to construct the large concrete forms. The proposed size of the forms will require long planks, so we'll need conifer wood for those. However, where are these thousands of pine trees going to come from?”

Here Agrippa probably drew another long face before his chief engineer smiled and came once more to the rescue with a clever idea.

“As Your Excellency knows, Moesia on the south bank of the Danube River recently became a Roman province. Next to it, on the same river, is Thrace, ruled by a king who, like Herod, is loyal to Rome. On the north bank is Dacia, an independent kingdom with which—at least for now—we also enjoy cordial relations. All are rich in trees and limestone. We can set up a group of limekilns every few miles on the banks of the Danube, which can be used in turn as the logging work progresses. I'm sure we'll be able to find clay deposits somewhere close to the river for the amphorae, but since there will be so much timber at hand, we could use a new technology recently imported from Gaul: wooden barrels. These can carry more lime than amphorae, are less difficult to handle, and far less susceptible to breakage. Both the timber and lime can be sent downriver on boats to the Black Sea port of Troesmis, where they can then be loaded onto larger cargo ships destined for Judea. Of course, since this lumber and lime-making enterprise will no doubt be a state monopoly, the revenues to the treasury will be substantial.” (Always bring up the cost benefits—you want your boss's head swimming with denarii signs.)

“In conclusion, the construction of the Port of Sebastos and the City of Caesarea is not only feasible but doable. Both will be magnificent monuments to Augustus, just as Alexandria will forever memorialize Alexander the Great.”

All we know for sure is that Herod must have presented a strong case for the harbor to Agrippa, and Agrippa in turn must have persuaded Augustus that it was a project worth supporting, because Roman assistance on a large scale began shortly after the meeting in Lesbos. Of course, there was an additional motivator for the Romans. If Judea should ever become a Roman province, Caesarea would make a wonderful capital. The proconsul would enjoy the comforts of a cosmopolitan Western city, something Jerusalem definitely was not.

Putting aside the phenomenal resource requirements involved, the construction of the harbor was a marvel of ancient engineering. It faced unique challenges that had never been grappled with before, and so served as a massive test bed for new building technologies. Before the city of Caesarea could be built, the harbor of Sebastos had to be in place; and before the harbor could be constructed, its southern breakwater needed to be built. Without this seawall to blunt the powerful northward flowing currents, the water would have been too turbulent to permit construction of the rest of the harbor.34

Archaeologists have uncovered three different containment methods employed by the ancient engineers in constructing these concrete moles, making it clear that they were learning as they went along. The first method involved using a pile driver to ram wooden beams into the seabed, their positions defining a rectangle. Divers—probably sponge divers who could hold their breath for several minutes at a time—would then nail long planks of spruce or pine to the upright beams. The sandy seabed had been prepared in advance by laying down a thick layer of kurkar rocks to prevent the currents from undercutting the sand beneath the finished jetty, a practice still followed today by modern engineers constructing breakwaters. A thick layer of concrete was dumped into the form and then tamped down into the rubble bed. Once this was accomplished, kurkar aggregate was dumped in and raked to create a flat surface before another layer of concrete was added and tamped down. The lime, pozzolana, and sand were probably mixed on a floating platform next to the form and then put into a large basket that was maneuvered into place by ropes attached to a small crane. Once correctly positioned, one of the ropes would be pulled, upending the basket and dropping its load of concrete into the water. Within the still water of the form, the lump of thick Roman concrete would drop straight down. The divers would then go down to check whether the concrete load had dropped into the right place; if not, it would be correctly positioned before being tamped down with a wooden ramming device, perhaps weighted with a lead core to overcome its buoyancy. After the concrete had been tamped down, more kurkar aggregate was laid down and again raked to form a level surface before another layer of concrete was rammed on top of it. This process was followed until the top of the form was reached. If one side of the form was to be part of the seaward flank of the planned jetty, more kurkar rock was dumped against this side to further secure it against the forces of currents and waves.

This method of construction was arduous, to say the least. A sponge diver, despite the ability to hold his breath for up to five minutes, probably needed two or three dives to hammer just one nail into the planks because of the increased water resistance (dealing with bent nails underwater must have also thrilled him), and many more dives were required for laying down and compacting each layer of concrete and aggregate.

Clearly, another approach must have been considered early on, for we see a transition to a less cumbersome process. The form was soon being constructed on land, with the planks making up the floors and walls incorporating the same mortis and tenon joints used by ancient ships to ensure a watertight fit. To lend additional strength, the interior of the form was heavily braced by a series of wooden ties that crossed at right angles.35 This floating caisson was then ballasted with enough concrete to keep it steady in the water while it was moved into place with ropes. The same process of loading and tamping the concrete and aggregate was followed, but its efficiency and speed were greatly enhanced now that the work was performed in a relatively dry environment. Hoist operators managed the ropes, ensuring that the form would slowly sink into place, snug against the previously laid caisson.

Archaeologists discovered a third method of concrete-form construction at the northern jetty, which was probably built after the completion of the sheltering southern jetty. This form was built on a base of four heavy wooden beams, their ends notched with axes. These were then slotted into each other, forming a rectangle not dissimilar to the base of a log cabin. Instead of a single wall hull, double walls were constructed, also incorporating the mortis and tenon joints. The 0.23 m (9 in) space between the double walls was then filled with Roman concrete with a very high lime content—almost 35 percent—and small aggregate of various stones.³⁶ Archaeologists have theorized that the cavities between the double walls were carefully filled with the concrete, the gradually increasing weight causing the platform to slowly sink into the water. Strangely, no wooden floor was uncovered. If that had been the case, the divers would have had to once again perform the tedious task of laying and tamping the concrete and aggregate underwater. Another possibility is that the base of the form was constructed of concrete, its remains obscured by the concrete dumped on top of it. It seems difficult to imagine that the filling of the double hull with concrete would alone counter the buoyancy of the wood, especially the large beams at the form's base. Perhaps it had a concrete floor, resting on the inside lip of the base beams and reinforced by the intersecting wooden ties at the bottom. This arrangement—my own theory and one to which I am not wedded—would have allowed a dry working environment for the laborers.

The one attribute common to all the forms is that they were quite large, some ranging up to 11.5 m wide by 15 m long (ca. 38 ft by 50 ft). Some were rectangular, some square, depending on their placement in the jetty and whether or not they were stacked. Their height ranged from 1.5 m to 4.5 m (ca. 5 ft to 15 ft).37

After enough concrete forms had been put into place, their flat tops would have risen several feet above the water. Nicely dressed blocks of the ever-abundant kurkar were then laid over the concrete surface, which perhaps caused the ancient Jewish historian Josephus to claim that the harbor was constructed of cut stone, not concrete, a belief that was held until underwater investigations conducted by archaeologists in the second half of the twentieth century proved otherwise.³⁸

After eight years of construction, including the arduous preparatory work of securing and manufacturing the building materials, Sebastos Harbor was inaugurated in 15 BCE. It was an unparalleled engineering achievement and would still be considered a remarkable accomplishment by today's standards. Sebastos was larger than Athens' facility at Piraeus and rivaled the port of Alexandria in Egypt, the largest harbor then existing on the planet. Roughly two millennia would pass before another concrete harbor would match its size, let alone surpass it. The crown jewel of Sebastos Harbor was its southern breakwater. Instead of directly blunting the powerful northwest flowing current, the southern breakwater extended in a gentle west-northwesterly direction to guide the stream farther out in the Mediterranean. Its left bank continued in this direction, while its right assumed a more northerly route so that the breakwater grew in width as it reached its terminus. The southern breakwater's terminus was opposite that of the smaller—though still massive—northern jetty, which followed a straight westward trajectory from the land. The gap between the termini of the jetties was approximately 20 to 30 m (ca. 66 ft to 98 ft) wide and formed the entrance to the harbor.³⁹

Although these structures are described as breakwaters, they were much more than that. The completed edifices were flat, rigid artificial stone peninsulas that trumped the features of any naturally formed promontories. The southern jetty was 40 m wide (ca. 131 ft) at its shore-end, and 60 m wide (ca. 171 ft) at its finishing point almost a half kilometer (over a quarter of a mile) away in the Mediterranean Sea.⁴⁰ It had a road and walkway, and it supported a series of large, vaulted stone warehouses. At its seaward end was a massive lighthouse, the highest and brightest beacon outside of Alexandria. Two stone and concrete towers, each supporting three colossal statues, were positioned to each side of the harbor's entrance. Josephus does not tell us who or what the statues represented, certainly not the Jewish god, for such images were forbidden by Hebraic law. Perhaps they represented Olympian deities whose favor was no doubt sought by the harbor's builders.

One major engineering concession made for both breakwaters was the use of kurkar for the aggregate. Just as a mason never uses a mortar that, when set, is harder than the masonry blocks it binds together, so must an engineer never use an aggregate weaker than the set concrete. The Roman concrete used for Sebastos's jetties must have mauled its porous kurkar aggregate, but the mix held together. And that was all that mattered to Herod.

Caesarea itself would take another five years to finish, and it became the largest and most beautiful city in Judea, with a population of 120,000, roughly the same size as Athens during this period. It is hardly surprising that the sponsor of this amazing project, King Herod, would soon thereafter become known as “the Great.”

The Herodian Dynasty did not last very long. After the construction of Caesarea and the new temple in Jerusalem, Herod felt politically secure enough to have his Hasmonean wife, Mariamne, executed on trumped-up charges of adultery. A few years later, their two sons, Aristobulus IV and Alexander, would suffer the same fate, allegedly for treason. Despite all this, Herod retained considerable affection for his sons' children. One, the son of Aristobulus, Herod Agrippa (named for his grandfather's friend), was sent to Rome to be raised in Augustus's own household, undoubtedly a more congenial family environment. There he made friends with young men who would go on to play important roles in Roman history, including the future emperor Claudius. Herod Agrippa, who was as much Roman as Judean, was widely respected in Rome for his levelheaded views, so his political advice was often sought after. A couple of years after Herod Agrippa assumed the title of king of Judea in 39 CE, his old friend Claudius became Roman emperor. Claudius ceded more lands to the Judean king so that his territory was now larger than that of his grandfather; indeed, it probably encompassed more land than any other Jewish king in history. Agrippa continued the building work of his grandfather, as did his own son and heir, Agrippa II. In 66 CE, a revolt forced Agrippa II and his wife to flee for their lives. The rebels, who belonged to various dissatisfied factions, slaughtered the entire Roman garrison at Jerusalem, and a few months later they defeated a Roman army. Unfortunately for the rebels, they were divided into several mutually antagonistic political and religious groups and thus were unable to develop a coherent military strategy. Their bid for independence faced long odds, but the murderous infighting that arose after the first successes effectively doomed their cause. The Romans soon returned with a force of sixty thousand men, ably commanded by the general (and future emperor) Flavius Vespasian. Caesarea had remained in Roman hands, and from here Vespasian's legions marched out to take one town and city after another. When Vespasian left Judea to assume power in Rome after the death of Nero had plunged the empire into chaos and civil war, he turned over his military command to his son Titus, who supervised the siege of Jerusalem, which fell after starvation and a months-long heavy artillery barrage had sufficiently reduced its population and fighting strength. Many of those captured alive were enslaved and sent to Rome.41 Another Jewish revolt took place in 130 CE—coinciding with an earthquake that damaged much of Caesarea and its harbor—but the emperor Hadrian squashed it with the same grim efficiency as the earlier insurrection, and Judea was incorporated into the Roman province of Syria. Another eighteen hundred years would pass before Jewish self-determination was again restored with the creation of the modern state of Israel in 1947.

Caesarea and its harbor Sebastos enjoyed mixed fortunes over the centuries. No human edifice, especially those built on or near the ocean, is exempt from Nature's power, for sea levels and seabeds rise and fall, and coastlines can change dramatically over the centuries, especially if, as we have seen, another natural force comes into play: earthquakes. A major seismic fault runs along the coast of the Eastern Mediterranean, and Sebastos Harbor sat on it (the fault is now 150 m from the present shoreline). By the time of the earthquake in 130 CE, the harbor's seabed had likely sunk a foot or two, and the tremor probably caused even more subsidence. The Romans, who did a fine job of keeping their infrastructure in good order, probably repaired the damage to the town and harbor, but they could not stop the slow and relentless subsidence of the seabed and coastline, which was accelerated by major earthquakes every few centuries (another large tremor struck Caesarea in 363 CE). By the beginning of the sixth century, much of Sebastos, now called Portus Augusti, was probably waist-deep in water, for a contemporary historian reports that it was no longer usable. The Byzantine emperor, Anastasius, restored the harbor around 505 CE, no doubt by adding more kurkar blocks on top of the submerged jetties. A little over a century later, the Arabs swept through the Levant, and Caesarea became part of the Rashidun Caliphate. Crusaders took the city in 1099, but by then most of harbor had once again sunk beneath the waves. The Christian knights used kurkar blocks to create a small harbor and surrounded its land end with stout defensive walls. The knights managed to hold onto Caesarea for almost two centuries before losing it to forces under the command of the Mamluk sultan, Baibars al-Bunduqdari, who razed the fortifications (the harbor had already silted up by this time42).

Caesarea faded from history until the twentieth century, when archaeologists began conducting underwater surveys and started excavating the ruins that still remained on dry land. The scuba-diving scientists were staggered by the size of the concrete blocks, which still remain in remarkable shape after two millennia. Caesarea soon became a popular tourist destination in Israel, and it now has a small modern marina, most of which is situated over what had been the western edge of the ancient city. The remains of the greatest harbor on the eastern coast of the Mediterranean, host to countless war galleys and merchant vessels, now lies under 12 m of water, a refuge for small fish and octopi.

The lessons learned from the building of Caesarea's harbor were applied to the dozens of concrete wharves and jetties the Romans would build throughout the Mediterranean over the following three centuries. The remains of these later structures generally show better workmanship and materials. The concrete appears to have been mixed more thoroughly, and the rock aggregate is almost always of a better grade than the kurkar sandstone of Judea. (Because of geologic changes, most of these edifices are now either underwater or stranded on dry land.) Pozzolana from the Vesuvius region would go on to become a major Italian export and has been found as a secondary cargo in sunken Roman vessels, where it probably also served as ballast.43 Interestingly, the knowledge that the volcanic soil of Santorini and the other nearby islands was just as good for making a hydraulic mortar or concrete, as demonstrated by the cistern in Rhodes constructed five centuries earlier, had been lost by this time.

Herod Agrippa's friend, the emperor Claudius, would use concrete to expand the harbor of Ostia. Claudius's nephew and imperial predecessor, the barmy Caligula, brought a massive 25-meter-high Egyptian obelisk to Rome.44 To transport it, he had a special-purpose cargo ship constructed that carried the obelisk to the naval base at Misenum, near Naples (Port Julius had probably silted up by then). There it was off-loaded and transported—no doubt by a huge special-purpose built wagon—up the Via Popilia to the Via Appia and then north to Rome. Once the gigantic ship had delivered its cargo, it just sat in the harbor, its specialized design making it unsuitable for any other purpose. Apparently, it was a local tourist attraction, for Pliny the Elder writes in his encyclopedia Natural History that “it was the most incredible floating vessel ever seen.”⁴⁵ Eventually, someone figured out a useful purpose for it. It was loaded with pozzolana from nearby Puteoli and then sailed to Rome's port of Ostia, which was undergoing the expansion program initiated by Claudius. Apparently, lime was mixed with the pozzolana in either Puteoli or Ostia (the text isn't clear), for, as Pliny tells us “the Emperor Claudius had it sunk there and used as a base for three breakwaters that rose as high as the ship's towers that were built on it. These breakwaters were constructed using Puteolian powder [pozzolana], especially dug and taken there for this purpose.”⁴⁶ It is possible that separate ships brought the powder after the special-purpose transport vessel had been moved, but that would not have been practical, so the latter must have conveyed the material. Thus, the craft used for transporting Caligula's obelisk served the same purpose as the floating caissons used for constructing Sebastos, but on a considerably larger scale. With the sinking of the obelisk ship, a major part of the construction effort was taken care of in a single stroke. And the rest of the project appears to have gone smoothly, for another half century would pass before the harbor was again renovated and expanded (silting from the River Tiber was always a problem).

However, it was not the Romans' use of concrete for port construction that has so captured the attention and imagination of people around the world but rather their application of this material toward the creation of some of the most beautiful and enduring buildings in history.

Unfortunately for us, Vitruvius wrote about concrete before Rome's use of the material had reached its greatest level of sophistication and its composition and manufacturing techniques had been further refined. For this reason, much of what has been written about Roman concrete has been inordinately influenced by Vitruvius's book. An analogy would be our distant descendants uncovering the Wright brothers' design plans for their first airplane and using this document to draw a host of assumptions about the operating characteristics of World War II aircraft.

Fortunately, we can again turn to the archaeological data, which shows us that the Romans gradually used an increasing variety of concretes and did so with a greater assurance and sophistication. Soon their architects would achieve a mastery of the material that we would not see again until the twentieth century and that, in some ways, have still not been equaled.

 

THE ARCHITECTURAL MASTERPIECES OF ROMAN CONCRETE

 

After Vitruvius wrote On Architecture, the next time concrete appears in the surviving literature is approximately ninety years later, in Pliny the Elder's previously mentioned Natural History. The elder Pliny—to differentiate him from his equally famous nephew and adopted son, Pliny the Younger—compiled his encyclopedia shortly before his death in 79 CE (the scientist had ventured too close to study the eruption of Vesuvius that buried Pompeii and Herculaneum and was overcome by sulfurous gas).

Pliny mentions concrete just once, in his reference to the cargo ship that was to form the jetty at Ostia. His references to lime mortar and stucco seem to be mostly lifted from Vitruvius's book. Like so many people over the years, Pliny cannot suppress his amazement about the properties of lime: “It is something truly marvelous, that quick-lime, after the stone has been subjected to fire, should ignite on the application of water!”47

After Pliny's encyclopedia, no surviving reference to Roman concrete is found, aside from two inconsequential books written toward the end of the empire, but both simply plagiarized Vitruvius's text.⁴⁸

One important development was the use of crushed and sifted pottery shards. Mentioned by Vitruvius as being a component of waterproof stucco, it did not take the Romans long to recognize that the red powder had properties similar to pozzolana. Indeed, a major component of modern concrete is kilned clay, and pottery is just that. Soon, pottery and brick dust became a major ingredient of Roman caementis walls, giving them the enduring qualities so long admired by engineers down through the centuries.

 

THE GOLDEN HOUSE

 

In 54 CE, Empress Agrippina, Emperor Claudius's third wife, tired of waiting for her husband to die a natural death and decided to put some poisonous mushrooms in his food to help the process along. The toadstools had the desired effect, and her son by a previous marriage, Nero, assumed supreme power. Since Agrippina was a dominating mother, and since Nero hated to be told what to do, he ordered her execution a few years later, thus completing a tidy what-comes-around-goes-around karmic circle.

About a decade into his infamous reign, Nero decided that he did not like the imperial mansion he was living in. Although the existing palace was impressive, Nero felt it was not sumptuous enough. He concluded that new and more lavish living quarters needed to be built. Unfortunately, he wanted to build the new palace in the center of Rome, which had long since been developed and was now crowded by such pesky things as apartment complexes, temples, and government buildings. What was Nero to do?

The Great Fire of 64 CE destroyed much of central Rome, killed or injured thousands of its residents, and left perhaps as many as a hundred thousand more homeless. Roman historians count Nero as the chief suspect in this unprecedented arson, as men with torches were seen deliberately setting fire to buildings, unhindered by the local authorities. Nero blamed the Christians, members of a new religious sect, and executed hundreds of them in a number of grisly ways (the morbidly curious can Google® the information). According to the Roman historian Tacitus, the persecution only served to highlight Nero's cruelty and gain sympathy for the Christians. This did not much trouble Nero, who was now delighted that the fire had freed up the 80 ha (ca. 198 acres) of land on which he wanted to build his new residence and surrounding parkland. Construction on Nero's pleasure palace began almost before the last embers of the fire had cooled. Five years later, the residential portion was finished. (It would consist of several separate buildings, some completed after Nero's death.) The palace complex was called the Domus Aurea, the “Golden House,” for its extensive use of gold leaf on the building's decorative flourishes. The palace utilized brick-clad concrete walls that were mostly veneered in marble (some walls were covered with ivory panels, which must have cost a few hundred elephants their lives). In addition to the gold leaf, the Golden House featured beautiful frescoes and elaborate stuccowork embedded with jewels and semiprecious stones. The land surrounding the palace was extensively landscaped to create a bucolic setting: large trees were transplanted to create a small forest, and there were gently rolling hills of pastureland (dotted with grazing sheep), a small lake stocked with fish, and even a tiny vineyard—all this so the emperor could reside in Rome and yet feel as if he were living in the Campanian countryside. When Nero finally took up residence in the main building, he exclaimed that at last he had a house that allowed him to “live like a human being!”49

What makes the palace so interesting are its Roman brick-faced concrete walls, some of which have survived. While brick-faced walls predate Nero's time, their use grew after the conflagration. The reason for this is simple: brick is fireproof, while stone is not. Exposed to high temperatures, stone flakes off in a process called exfoliation. Once the outer stone of a Roman wall is damaged in this manner, the structural integrity of its concrete core is compromised as well. Concrete is even more susceptible to exfoliation: it literally crumbles away when subjected to extreme heat for a sustained period. A concrete wall faced with fireproof brick can far better protect its core, which constitutes up to 80 percent of its volume. Consequently, the majority of concrete walls in Rome constructed after Nero's fire used brick facing. Sometimes we see a combination of rock and brick facing, but these were probably walls that originally had veneers made of marble or limestone sheets, their attachment points being at the brick courses, so that heat would be transmitted to the brick. The knowledge of concrete's vulnerability to fire is just one of many aspects of the material that we would not relearn until the twentieth century.

Another major feature of the Golden House is a concrete dome over a large octagonal dining room. The dome, parts of which have survived, is not a true geometric dome—a perfect half sphere—but is rather an eight-sided vault, each side rising from each sectional wall of the octagonal room. The dome is the earliest surviving example of such sophisticated vaulting. It is a kind of prototype for the larger and more elaborate Roman concrete vaulting that would be used in later basilicas and public bathhouses. Although such vaulting can be constructed in stone masonry—as seen in medieval cathedrals—the Romans realized that concrete was more ideally suited for the purpose. The dome itself was 13.48 m (ca. 44 ft) wide, and at its apex was a six-meter-wide (ca. 20 ft) circular opening called an oculus (Latin for “eye”) that also allowed light to enter the room, supplementing the light streaming in from windows beneath the dome's base. However, it is almost certain that only indirect light came from the oculus, as the external top of the dome had a flat concrete base that once supported what is now called a tempietto (Italian for “little temple”), a circular, lantern-shaped structure. According to contemporary accounts, the vault of the dome was painted to resemble the sky and dotted with numerous crystal gems that served as “stars.”50 The tempietto above the oculus was also domed and decorated in a similar manner and—as reported by the Roman historian Suetonius—continually revolved “night and day,”⁵¹ probably by waterpower, since the palace possessed a sophisticated hydraulic system that also fed elaborate fountains. Although the Golden House was an example of wretched excess, it also represented the most complicated and elegant application of Roman concrete up to that time.

The fire, and the construction of the Golden House and its park afterward, caused long-simmering discontent to finally explode into open revolt. Armies rebelled and the people rioted. Nero suddenly found himself abandoned. Rather than face public execution—or being literally torn apart by an angry mob—he decided to commit suicide. His reputed last words were: “Jupiter! What an artist the world loses with my passing!”52 Nero—a decent poet, a middling singer, and an awful ruler—then had a slave assist him in cutting his own throat.

 

THE ROMAN COLOSSEUM

 

A brief but bloody civil war followed the death of Nero, and Flavius Vespasianus, known to us as Vespasian, defeated his rivals and emerged triumphant. Vespasian's reign was a much-needed tonic for the Romans. Unlike Nero, Vespasian was a sensible, even-tempered man who worked hard to restore moral integrity to Roman government and bring its finances back under control after Nero's reckless spending had plunged the empire into insolvency. (Among other measures, Vespasian instituted the first public pay toilets to help balance the budget deficit he inherited, a fiscal measure that survives in the modern Italian word for a public urinal: vespasiano)

Unlike the thin-skinned Nero, the new emperor possessed a natural and imperturbable equanimity. The historian Suetonius wrote that Vespasian endured “the frank language of his friends, the barbs of attorneys, and the impudence of philosophers with the greatest patience.”⁵³ Like most Romans, Vespasian found Nero's palace an embarrassment and opened up most of the park surrounding the palace for public and commercial development. Though Vespasian refused to live in the Golden House, archaeological evidence shows that work continued on many of the buildings. It is likely that most of them went on to serve as government offices, with, of course, the gaudier bits of decoration removed. Vespasian set aside the land around Nero's lake for a major building project close to his own heart: a massive amphitheater like no other on earth.

We know the Flavian Amphitheater today as the Roman Colosseum (sometimes called the Coliseum). This is a misnomer, for that term originally referred to the colossal statue (colossus) that once stood nearby. The colossus was a super-sized bronze representation of Nero, a remnant of the Domus Aurea. The statue's facial features were modified to remove Nero's visage, and a “halo” embossed in gold leaf and sporting radiating flames was attached to the head. The colossus was then rededicated to the sun god Helios.

The Colosseum was not the first Roman stadium. There had been earlier ones, but these were usually nothing more than open-air wooden bleachers. The exception was the largely stone amphitheater built by Statilius Taurus on the Field of Mars in 29 BCE. Although it was called an “amphitheater,” the Taurian structure was probably like a classical Greek theater, with most of the audience seating to one side. This amphitheater was destroyed in Nero's fire (perhaps indicating that the seating and stage portions remained of wood construction). The Flavian Amphitheater was far more ambitious, and it is evident that much thought went into its design, because it has served as a blueprint for almost every major Roman—and modern—stadium built since. It makes no difference whether the modern “colosseum” is used to host football games or rock concerts, the Roman design provides a maximum seating capacity with full view of the arena or playing field below, while also allowing the fairly rapid ingress and egress of thousands of people.

Much of the cost of building the Colosseum came from the booty taken in the Judean war, which also provided most of the cheap manpower needed for the project (an estimated hundred thousand Jews were taken back to Rome as slaves). Work began on the Colosseum in 72 CE, and Vespasian's son and successor, Titus, opened it to the public in 80 CE. (Titus's brother and successor, Domitian, would spend another two years revamping the stadium.)

Its original dimensions were impressive. The Colosseum was 189 m (ca. 615 ft) in length at its longest point—the structure was elliptical, not circular—and its outer wall was 48 m (ca. 157 ft) high, and its perimeter was 545 m (ca. 1,788 ft) around. Ringed along the base of the outer wall were eighty numbered entrances (inside, they were numbered as exit points), four of which were set aside for VIPs. Three of four VIP entrances were reserved for members of the senatorial class, and one—the entrance facing true north—was set aside for the emperor and his guests (all four VIP entrances were positioned axially and faced the four cardinal directions). The attendee was given a ticket with the entrance, row, and seat number. For additional convenience—and as a further preventive against crowd congestion—the corridors and stone staircases were also marked to help a person quickly find his or her place. Once comfortably seated, the spectators could then watch death-row prisoners killed by wild animals or gladiators duel in the arena below. The men or animals would pop up from trap doors hidden beneath the sand (arena) connected to winch-driven elevators that arose from a hypogeum, an underground complex of limestone chambers and corridors. (Despite all its historical inaccuracies, the 2000 movie Gladiator faithfully portrays the Colosseum and its workings.)

Most gladiators did not fight to the death—the sport would have ended after several games if they had. The spectators knew this and settled for a good show of swordsmanship and stunts instead.54 Gladiators were to sword fighting as the Harlem Globetrotters are to basketball, or as television wrestlers are to Olympic wrestlers. The gladiators did get cut often, and the crowd enjoyed this, and, yes, sometimes there were “grudge matches” that led to deaths. Occasionally, a gladiator would show up with a hangover and perform badly, and would get the “thumbs-down” (the real signal is not clear), if he was decked and had a sword held to his throat. Although technically slaves, gladiators were also celebrities who enjoyed considerable freedom and were worshipped by many of the crowd. Like rock stars, they had a following of “groupies.” (One of their nicknames—with all due apologies to the New Testament—was “fishers of women.”) Not a few members of the equestrian class voluntarily surrendered their social reputation to become slaves so they could attend gladiatorial schools and go on to stardom, so to speak.⁵⁵

According to some modern commentators, it was Roman concrete that made the construction of this magnificent stadium possible, while others assert that it could have been built without concrete. And almost no one can confidently say—although many do—how much concrete was actually used in the Colosseum's construction. Estimates vary from 6,000 metric tons to 653,000 metric tons. There is obviously a great disparity between the two figures, and it is a contentious issue.

 

 

In truth, the Colosseum could have been built without concrete, but it would have taken more time and manpower. The Colosseum is basically a masonry structure made of travertine limestone and brick.

Of the latter, the brick masonry was cored with concrete. In places, there is more brick than concrete; in other places, more concrete than brick. It is as if the builders slowly began putting more trust in the material as they went on. The vast majority of the concrete used for the Colosseum, perhaps 80 percent, was used for the stadium's foundations, something that earlier historical architects mostly ignored. The high figure for the amount of concrete used, 653,000 metric tons, is probably closer to the mark.56

The Colosseum, while hardly representing a ringing endorsement of concrete by its builders, does signify an important transitional phase in the story of the Romans' use of the material. In the years following its completion, Roman builders seem to have had more faith in concrete's structural strength.

Still, Roman concrete did play an important role in one event held in the Colosseum, a spectacle that could not have been staged without it.

 

DON'T GIVE UP THE SHIP!

 

According to Roman historians, Emperor Titus decided to celebrate the inauguration of the Flavian Amphitheater by staging a massive wild animal hunt in which “hunters” chased some nine thousand creatures around the arena. The hapless animals were then dispatched by sword, lance, trident, or arrow. Titus is also reputed to have staged a sea fight called a naumachia (from the Greek word for “naval battle,” naumakhía—ναυμαχια) within the new stadium. Naumachia had been staged in the past—Julius Caesar, Augustus, and Claudius had each sponsored one—but they were rarely held because of the great expense involved. Usually, a massive basin had to be excavated near the Tiber and then flooded with water (Claudius had his naumachia performed on a natural lake outside Rome). Real warships—at least a dozen and usually more—were used, many of which were permanently damaged in the melee. Since a naumachia was a real contest to the death, condemned convicts and prisoners of war—probably including many from the recent Judean revolt—were used instead of gladiators. Since the spectators could expect to see real carnage, naumachii were extremely popular. The combatants were especially motivated to win, since the survivors could expect a pardon afterward, although this may not have been the case with all the naumachii staged.

Many historians were skeptical of accounts that a naumachia staged by Titus was held in the Flavian Amphitheater. They reasoned that such an event would have flooded the hypogeum and destabilized the Colosseum's foundation. However, a subterranean aqueduct that leads to the stadium could have been the source of the water. The aqueduct was built using stone heavily mortared by hydraulic concrete. Archaeologists now believe that Titus's brother and imperial successor, Domitian, built the hypogeum several years after coming to power, so flooding would not have been an issue at the time the reported naumachia was held.57 To pull off such an event, Titus must have covered the Colosseum's arena with flat stone paving generously mortared with Roman concrete. The walls of the Colosseum's lower tier also appear to have been strong enough to contain the lateral water pressure. Once the fight was over, the same conduit that flushed out the Colosseum's numerous public urinals likely directed the water and blood out to the already heavily polluted Tiber River. Domitian reportedly held a naumachia in his reign, but it likely was held at an artificial pool (stagnum) like the earlier imperial shows.

The Colosseum has not fared well over the centuries. Damage caused by lightning strikes (it was the tallest structure in Rome until the modern age) and powerful earthquakes in the eighth and fourteenth centuries, and the even more grievous harm caused by people who used it as a stone quarry for over a millennium, took their toll on the venerable structure. Indeed, it is remarkable that so much still remains. By the nineteenth century, the base of the Colosseum was totally buried under earth brought in by regular flooding of the Tiber River. By then, most of the edifice had become overgrown with weeds and a host of other flora (botanists have counted over six hundred plant species living among its ruins). Serious excavation—and weed eradication—did not begin until 1871. It is now one of Rome's chief tourist attractions, bringing millions of visitors to the Eternal City each year.

Roman concrete continued to improve, and about fifty years after the completion of the Colosseum, it would reach its apogee with a building that would be much imitated but never equaled.

 

THE PANTHEON

 

Tho' splendid ruin round you lies,

The proud Pantheon time defies,

Nor yields to Nature's law;

Rome's mighty Genius rear'd the dome,

To give man's conquer'd Gods a home,

And strike the world with awe.

John Courtenay, “Congratulatory Ode” (1792)

“…angelic, and not of human design.”

Michelangelo Buonarroti on the Pantheon's dome

American engineer David Moore was visiting Rome with his wife in the late 1980s when the desk clerk of the hotel where they were staying suggested they take a stroll over to the nearby Pantheon. Mr. Moore describes the visit:

 

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 1,800 years old.

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?58

Moore was so staggered by the building that he would spend the next ten years studying the Pantheon and Roman concrete, resulting in his book The Pantheon: Triumph of Roman Concrete. And he is not alone. A number of unprepared people visiting the Pantheon for the first time, especially architects and civil engineers, come to similar conclusions, mistakenly assuming that the building is really a magnificent early twentieth-century structure or that the building is indeed Roman, but the dome is a modern addition or enhancement. If such visitors are also acquainted with the limited life span of contemporary concrete, they are even more befuddled. Not only is the design and workmanship too modern, but how could it last two thousand years without disintegrating?

One can easily spot the first-time visitors to the Pantheon. Like David Moore, there is brief astonishment, then awe (many English-speaking visitors cannot suppress a “Whoa!” or “Wow!”), followed by the question “How did they do that?” For close to two millennia, the Pantheon possessed the largest true dome (a halved geometric sphere) in the world. Popes, princes, kings, caliphs, Byzantine emperors, and German electors have pushed their architects to create buildings with domes larger than that of the Pantheon, and all failed to match its size, let alone surpass it. Only modern technologies and material sciences have allowed us to build domes larger than the Pantheon's. Even to this day, despite occasional earthquakes and constant exposure to the elements, it remains the largest unreinforced concrete dome in the world.

 

 

How did the Romans do that?

It might be best to pause here and recount the fascinating history of the Pantheon. Like all great human-made edifices, the story behind the Pantheon's construction is almost as interesting as the building itself.

The first couple of centuries of the Roman Empire represented a generally happy period for its inhabitants. Tiberius and Domitian may have terrorized the Senate, but, by and large, they ruled well. Even under the reign of such nutcases as Caligula and Nero, the empire's exemplary civil and legal institutions, while disrupted in Rome by the whims and ravings of mad emperors, usually ran smoothly in most of Italy and the provinces. The assassination of Domitian in 96 CE inaugurated what most historians consider the empire's “Golden Age,” also known as the “Reign of the Five Good Emperors.” Historian Edward Gibbon refers to this time as a “happy period of more than fourscore years” when the empire “comprehended the fairest part of the earth, and the most civilised portion of mankind.”59 The Roman Senate chose one of their own to succeed Domitian: Nerva, who soon initiated the finest method of hereditary succession ever known, before or since. Chosen by the Senate because he was an intelligent, humane, honest, and hardworking individual, Nerva in turn adopted a younger man of similar attributes, Trajan, and designated him as his successor. Trajan did the same by adopting Hadrian, and Hadrian did the same by adopting Antoninus Pius, and Antoninus Pius did the same by adopting Marcus Aurelius. Unfortunately, Marcus Aurelius broke this wise and long-standing tradition by designating his natural son, Commodus, as his imperial heir. Sadly, Commodus was a disturbed megalomaniac.

Of the five good emperors, Nerva is remembered for his gentle nature, Trajan for his martial prowess, Hadrian for his many building projects, Antoninus Pius for his quiet and efficient administration of the empire, and Marcus Aurelius for his philosophical musings. Hadrian was the most complex, fascinating, and intelligent of these five highly intelligent men. He was also the most unsavory of the group. Shortly after coming to power, Hadrian earned the Senate's enmity when he ordered the execution of four members of that body, men he did not like or could not trust. Starting off your reign by executing four senators was not something a “good” emperor did. Still, despite a dozen instances of summary “justice,” Hadrian was generally a good emperor. One time, while he was traveling through the provinces, a woman approached him with a petition. When he tried to brush her off, saying that he had no time to review her case, the woman replied, “Well, then stop being emperor!” Stung by the remark, Hadrian stopped and patiently heard her petition. Hadrian's policies were farsighted and sound. He abandoned many of Trajan's Eastern conquests and withdrew the Roman forces behind more defensible natural barriers and passes. To prevent raids by the barbarian Picts, who would swoop down from what is today Scotland to ravage Roman Britain, he built a formidable wall across northern England that was interspersed with forts every mile or so. Large portions of “Hadrian's Wall” remain, although—as usual with the majority of Roman structures—most of the stonework was removed centuries later for other buildings, mostly local abbeys, churches, and farmhouses.60

There seems to have been no subject that Hadrian did not know something about, and very few he did not know well. He would invite learned men to dinner, discuss a wide range of topics with them, and then proceed to point out—in detail—each of his guests' errors. Once, when the famed grammarian Favorinus of Arelata (Arles) was Hadrian's guest, they discussed the etymology of words. Hadrian disputed Favorinus's theory of the origin of a particular word, and the latter conceded that the emperor was probably right. When Favorinus later told his friends the story, they upbraided him for giving in to the emperor, since he, Favorinus, was almost certainly correct about the word's origin. Favorinus shook his head and said, “You are urging the wrong course, my friends, to suggest that I not regard the man with thirty legions as the most learned of men.”⁶¹ This story made the rounds, and it was not long before the emperor exiled Favorinus to the Greek island of Chios. Hadrian was that kind of guy.

 

 

Besides Hadrian's voluminous knowledge of history, mathematics, and philosophy, he was also an amateur architect who particularly liked domes. There is a story that typifies Hadrian's character and, perhaps, provides a significant clue to the Pantheon's creation. Once, while Trajan was discussing a construction project with the famed architect Apollodorus of Damascus, Hadrian ventured a few suggestions. Apollodorus told him to mind his own business and go back to drawing his “pumpkins,” a sneering reference to the young man's fascination with domes. This was a dumb thing for Apollodorus to say, for Hadrian was the kind of person who never forgot or forgave an insult. When Hadrian became emperor, Apollodorus, in an effort to make amends, dedicated a book on field artillery to the new ruler. It didn't work. While Apollodorus likely waited for building commissions that would never come his way, Hadrian decided to build a magnificent temple, one that sported a “pumpkin,” the likes of which the world had never seen. The architectural motif scorned by Apollodorus, domes, would at last be vindicated with a building that would take everyone's breath away. Since real estate was limited in downtown Rome—as Nero had discovered—Hadrian decided to realize his dream by rebuilding a temple that had gone up in flames almost a half-century earlier: Agrippa's Pantheon.62

Marcus Agrippa, Herod's pal and Augustus's best friend, was also a wealthy patron of the arts. In 27 BCE, he decided to build a magnificent temple dedicated to all the gods, a pantheon. Unfortunately, fire completely destroyed the temple in 80 CE, sometime in the middle of Domitian's reign.⁶³ We have no idea what it looked like, but it was most likely a standard rectilinear temple. Domitian probably never got around to rebuilding it because he was too busy fighting the Germans and Dacians. Nerva never got around to rebuilding it because he died just sixteen months after becoming emperor. Trajan never got around to rebuilding the temple because he was too busy fighting the Dacians and Parthians, and he first wanted to finish constructing his new forum and building complex known today as Trajan's Market (it's true function was more likely governmental than commercial), which Apollodorus was building for him in Rome. By the time Hadrian came to power, the site of the old temple had been vacant for so long that it had probably become a long-established farmer's market.

Instead of simply restoring the previous structure, Hadrian wanted to create something different and much grander: a building that would dazzle everyone who saw it. As we noted, he was partial to domes, but Hadrian's genius was that he wanted to realize the full aesthetic possibilities of concrete and its potential for expressing not just beauty but also power. And he wanted to do it in a way that the finest and most carefully laid masonry could not.

Some authorities have suggested that Apollodorus designed the Pantheon. This proposal seems highly unlikely, given Hadrian's dislike of Apollodorus and the latter's apparent preference for vaults over domes. Apollodorus was also apparently wary of concrete's strength, because most of the vaults at Trajan's Market were brick-ribbed. This method, long used and reliable, involved placing mortared bricks on their sides around a wooden half-barrel form to create a vault. Indeed, until the nineteenth century many architects assumed the Pantheon's dome to be of brick-ribbed construction—using a hemispherical wooden form—that was then covered with concrete. Apollodorus did use concrete to create a large cross vault at Trajan's Market, but it was likely constructed in sections and possessed none of the size or complexity of the Pantheon's dome.

Hadrian was probably familiar enough with concrete's attributes to know that it would support a large dome. Of course, concrete domes had been built in the past, but never on such a spectacular scale. Hadrian's dome would be over twice as wide as any previous one, twice as high, and certainly more beautiful. No one in antiquity would ever put so much trust in the strength of concrete as Hadrian did in his design of the Pantheon. Even today, you will find most engineers unwilling to attempt such an enterprise without the use of steel reinforcement.

Naturally, Hadrian's ambitious project demanded that extreme care be exercised. We know of at least one compromise—or change of design—that took place during the construction of the Pantheon. The columned portico in front of the Pantheon was originally intended to be much taller. Above the existing portico one can just make out a slightly flattened triangle that precisely matches the roofline below it. Evidently, the original size proposed for the portico's granite columns was simply too large. Granite is extremely hard and heavy; quarrymen would have had to find and cut out a massive block of granite—one without flaws that might later jeopardize the column's load-bearing capabilities—and then lower it with great care onto a sled so it could be moved to a place where the arduous task of carving this obdurate material into a perfectly round pillar could be performed. (Fluting the columns was probably also avoided because of the difficulty of working with this kind of stone.) The present columns are still quite large: the granite portion of each is 12 m (ca. 39 ft) high, 1.5 m (ca. 5 ft) thick, and capped with carved Corinthian capitals that are 2.5 m (ca. 8 ft) high, bringing the total height of the columns to 14.3 m (ca. 47 ft).64

There may be another possible reason for the modification. If one looks at an image or model of the Pantheon as originally planned, the larger portico appears to be a more harmonious fit for the rotunda behind it. However, from the perspective of a person on the ground looking up, the smaller portico seems large enough. Hadrian must have realized that the dome would appear slightly less impressive if one entered the temple through a larger portico. Hadrian may have occasionally lashed out at people he felt threatened by, or simply disliked, but no one has ever questioned his aesthetic sensibilities or his willingness to use the material and political power at his disposal for their realization in construction projects. For example, instead of using the granite from the nearby Alps or the Calabria range for the Pantheon, Hadrian chose the granite from Mount Claudius (Djebel-Fateereh) in eastern Egypt. The logistics and expense involved in transporting these pillars from Egypt must have been substantial, but Hadrian was probably struck by the pale gray-blue color of this particular granite and thought it a perfect match for the temple's heavenly theme. From the standpoint of aesthetics, Hadrian would have been right in choosing to make the portico smaller.

If there had been no rotunda, and the portico continued to form a classic rectilinear temple, it would still be considered a wonderful achievement of the Greco-Roman era, but Hadrian instead wanted to push the envelope further than anyone before him.

It has been estimated that a number of geometric and material stress calculations were made before work could begin on the temple. Were the dome capped at its base with an upside-down twin below, its volume would describe a perfect sphere, its base precisely touching the rotunda's floor. Approximately fifty years earlier, the remarkable Greek scientist Heron of Alexandria wrote a book on the mathematical solutions to building vaults and domes, and it is possible that Heron described such a configuration. We will never know, because Heron's treatise, like the vast majority of the books published in antiquity, has not survived to our times. Recently, another mathematically derived feature of the dome was discovered, one that perhaps explains the dimensions chosen for the hole at its apex: the oculus (eye). In 2005, Robert Hannah of the University of Otago in Dunedin, New Zealand, visited the Pantheon while performing research for his book Time in Antiquity. He soon realized that the layout of the Pantheon suggested that it was more than just a temple. Between the fall and spring equinox, the light of the noonday sun traces a path across the inside of the domed roof; while between the spring and fall equinoxes, the higher sun shines along the lower walls and floor. At each of the equinoxes, the sunlight coming in through the dome's oculus strikes the junction between the roof and wall, above the Pantheon's grand northern doorway, allowing a single ray of light to pierce the grilled window above the portal and fall on the courtyard outside. Hannah points out that this is no coincidence, as a type of sundial common in Roman times that incorporated a dome with a hole—although obviously much smaller—was used to indicate the time of year.65

Still, as the old saying goes, it's 1 percent inspiration and 99 percent perspiration, so after the calculations were performed and the design for the Pantheon completed, the really hard work began: constructing an imposing, extraordinarily complicated, and unprecedented edifice. The Romans did not reinforce their concrete with steel rods as we do today, so the tensile strength of the concrete was very limited, and the compressive strength of Roman concrete—its ability to support heavy weight (including its own weight)—was also less than modern concrete. A large concrete dome places tremendous stresses, both downward and outward, on the walls supporting it. In short, domes made engineers nervous, and a dome of this size would have made all the people connected with this project—the architects working under Hadrian's direction, the operating engineer, contractors, and material suppliers—especially anxious. Hadrian wanted his dome bare, so it had to have a smooth surface. A number of challenges had to be overcome, and if the solution to any of these daunting obstacles were to fail the final test, the whole building could come tumbling down.

The downward and outward stresses caused by the dome were handled in several ingenious ways. To help contain the dome's outward stresses, a thick ring of brick masonry was laid around the exterior base of the dome. However, the most difficult part was building walls that could contain both the tremendous downward stresses, and the remaining—but still substantial—outward pressures. As we have seen, the typical Roman wall consisted of masonry on the inside and outside of a concrete core. To handle the stresses caused by the Pantheon's heavy dome, the concrete core of the wall was especially thick, and the bricks used for the exterior and interior portions were especially large. To provide additional compressive strength to the walls, the bricks were laid in an arch formation, with the arches filled in with more masonry. These embedded arches are called relieving arches, and they had been used in earlier Roman buildings to handle the stresses of concrete vaulting. However, the most interesting method used for reinforcing the rotunda's walls was integrating within it eight 6.4 m (ca. 21 ft) thick barrel vaults, each additionally supported by two pillars that flank the interior entrances to the huge circular galleries within the Pantheon. It was in these galleries that large statues of the various gods were once placed (they are now devoted to sculptures representing various saints). These eight barrel vaults are directly opposite each other, supporting the dome as if it were a succession of intersecting arches, which, theoretically, is what a dome is. And so an engineering necessity is used toward an aesthetic end.66

Another major problem the architects faced was that if a standard dome—such as the ones seen in the bathhouse in Baiae or in Nero's Domus Aurea—had been built to this scale, its weight would have been such that even the thick walls and barrel vaults of the rotunda would have been unable to support it. This was partially remedied by the twenty-eight vertical rows of beautiful inlaid coffers that not only lessened the dome's concrete mass and its subsequent load but also added a stunning artistic adornment. Once again, engineering requirements were imaginatively used to add beauty, emphasizing the great deal of thought and planning that went into the design of the Pantheon.

To reduce weight even more, lighter aggregate—mostly volcanic pumice—was used for the dome, unlike the walls, which featured harder rocks with better load-bearing capacities. Indeed, one sees a variety of different aggregates used in the construction of the Pantheon: one kind for the foundation, another for the walls, and another for the dome, each perfectly suited for a special application and/or load-bearing need. This shrewd use of various aggregates was something that would not be fully appreciated until the twentieth century.

The dome of the Pantheon would not have been so dazzling if the form into which the concrete was poured had not also been a model of perfection. The mold for the Pantheon's dome was the most artistically advanced example of concrete shuttering used until the modern age. Imagine a convex reversal of the interior surface of the Pantheon's dome, with the four-stepped coffers sticking out instead of in. Essentially, a mathematically perfect wooden mold with a near flawless surface had to be created onsite, with dozens of workmen laboring away on the top and bottom of it, and with every step in its manufacture closely supervised to ensure the exacting precision required. The mold had to have been made of strong wood, like walnut or live oak, and it had to have been quite thick to hold up the hundreds of tons of concrete laid upon it. And supporting it all was a thick forest of really stout scaffolding beneath, probably consisting of timbers carved from whole trunks of massive pine trees.

Nevertheless, the tamping of Roman concrete was an essential element of the material's durability. Not only did it fill all the voids of the shuttering, but it also compressed the concrete itself, making it denser and more compact, ensuring a minimum of micro air cavities within the material that might allow the ingress of water or chemicals that might damage it. This compaction method was not rediscovered until US Department of Interior engineers noticed its benefits in concrete dam construction in the 1980s.67 It is now widely employed today.

The finished temple is stunning in every sense of the word. You first enter the portico between the tallest granite columns that have ever existed in Rome and then walk through the original 6.4 m (ca. 21 ft) high double bronze doors into a massive open space that is awe-inspiring in its dimensions. The huge dome high above you was designed to represent the heavens, and the stunning concentric rows of vaults rising toward the oculus at its center seem to convey some great natural force, both divine and intelligent, frozen in the act of becoming. However, there are as many impressions and interpretations of the Pantheon's dome—mathematical, aesthetical, and even spiritual—as there are human beings. One person described the Pantheon as a kind of time portal, in which the visitor is immediately transported back to ancient Rome in all its glory. It's true. Here, more than anywhere else, a visitor can experience the power and grandeur of imperial Rome.

Once the temple was finished, Hadrian did something curious. He had Agrippa's original dedicatory inscription incised on the pediment. It reads “M. AGRIPPA L. F. COS. TERTIUM FECIT.” Most monumental dedications at that time were abbreviated in a form instantly recognizable to the Romans, who would read it as M[arcus] AGRIPPA L[ucii: of Lucius] F[ilius: son] COS[ul: consulship] TERTIUM [third] FECIT [built this]. “Marcus Agrippa, son of Lucius, built this in his third consulship.” Unlike most Roman emperors, Hadrian did not like leaving his name on every edifice he built, and this has led to some confusion. Hadrian restored several buildings in Athens and built several more. Recall that speaking platform on the small hill in Athens called the Pnyx? The restoration was performed in Hadrian's reign, but for a long time the structure was assumed to be a product of the classical period. The same was true of the Pantheon. Just seventy-five years after Hadrian had constructed this magnificent domed structure, the Roman historian Cassius Dio would number it among the buildings that Marcus Agrippa had erected.68 This belief continued to be held for most of the succeeding centuries, with everyone assuming that the Pantheon had been constructed one hundred fifty years earlier. Fortunately, the Romans usually stamped their bricks with the year of the then-current consuls, as well as the manufacturer (often the bricks were government- or army-issued). In the early twentieth century, several bricks from the Pantheon were removed and examined; they confirmed that Hadrian was really the builder. The bricks date from 118-125 CE,⁶⁹ about six years into his reign, once more confirming that much time, planning, and preparation took place before the actual work began. It is even possible that Hadrian began designing the Pantheon during Trajan's rule, and maybe it was this design that Apollodorus alluded to in his cutting remark about “pumpkins.” Hadrian eventually ordered Apollodorus's execution (he was probably ordered to commit suicide). My guess is that Hadrian first gave the Greek architect a tour of the completed Pantheon before issuing his death warrant. Hadrian's message to Apollodorus would have been clear: “So, what do think of this pumpkin?”

Concrete vaulting and domes became popular in the Roman Empire in the second, third, and fourth centuries, though none of these later models ever reached the size of the Pantheon's. Cross vaults were especially utilized in the massive public baths. All the baths have fallen into ruin with a single exception: one section of the fourth-century baths built in Rome by the emperor Diocletian. This section was once the frigidarium, the wing for cold-water bathing. It was saved from destruction by being converted to a church: the basilica of Santa Maria degli Angeli e dei Martiri (St. Mary of the Angels and Martyrs). It is worth a visit, for it is the best-preserved example of Roman concrete vaulting.

Another example of Roman concrete that has survived the ravages of age and the hand of man is the Roman Senate House, called the Curia Julia (named in honor of Julius Caesar). It had burned down twice since Julius Caesar's day but was rebuilt each time as it was before. The one that stands today was rebuilt at the beginning of the fourth century, also by the emperor Diocletian. Like the Pantheon, the marble sheets that once covered the Curia Julia's brick-clad concrete walls were removed and used elsewhere. Few buildings in Europe can lay claim to so much history as the Curia Julia.

Following the reigns of the five good emperors, the Roman Empire—at least its Western half—endured for another couple of centuries, despite almost unrelenting civil war and barbarian invasions. The Eastern Roman Empire survived in truncated form until the fifteenth century, when its capital, Constantinople, was captured by the Turks. However, long before that time, innumerable books of literary and historical importance had vanished, as well as the secrets to dozens of different technologies, including the formula for making Roman concrete.

With a couple of curious and isolated exceptions, almost fifteen hundred years would pass until concrete was slowly rediscovered, and another century or two until we realized that we were making the same errors that the Romans had already learned to avoid in the ancient past. Sadly, by the time we discovered our mistakes, the planet had already been covered in a material that was made and applied in the wrong way. Unlike the Pantheon and the Roman Senate House, virtually all the concrete structures one sees today will eventually need to be replaced, costing us trillions of dollars, pounds, euros, yen, and yuan in the process. But that is another story, and one that will be covered later.