Deterioration and Decay of Ancient Roman Structures



Roman insulae in Ostia Antica / Photo by Charles Gardner, Wikimedia Commons


From Dr. Stephen T. Muench (by student Claire Cyra) / 12.04.2015
Associate Professor of Civil and Environmental Engineering
University of Washington


Introduction

Many ‍of the engineering techniques and materials that make up our cities today also formed the foundations and structure of Roman architecture over two millennia ago. While countless edifices were lost to natural disaster or war or simply the weathering passage of time, numerous structures, systems, and artifacts still remain. Bridges, water systems, and even the cobblestones we walk on were planned and laid down generations before us. To understand how these particular relics endured through thousands of years, investigation must begin at the most basic level: molecules and structure. Through trial and error, the Romans developed methods of construction that have lasted longer than any modern structures so far. They needed sturdy material to erect structures and last long so they quarried stone and made brick. Taking it a step farther, they sought out an even faster and cheaper method of constructing buildings and houses, with the revolutionary development of concrete. They required tools for surveying and measuring so they adopted the compass, the dioptra, and the chorobate. However, even the most precisely engineered structures and the most carefully manufactured materials deteriorate over time. Materials experience decay—either from physical or chemical actions, which leads to damage in the structure—cracks, crumbling, deformation, etc. Environmental factors and climate weathering, along with engineering aptitude determined the longevity of ancient Roman structures‍.

Material Used in Ancient Roman Construction

Stone

Figure 1: Slab of travertine cut at Fratelli Pacifici quarry in Tivoli

Stone was one of the base resources that frequently served as building material in Roman architecture. Varying in durability, abundance, and aesthetic appeal, stone possessed a number of advantages and disadvantages as construction material. Its ability to endure and provide lasting stability made it not only a widespread building material, but also a reusable one. Oftentimes stone was taken from older buildings and to erect new structures, thus allowing the materials to be recycled—not to mention cutting construction costs (Taylor 6). Since particular stone must be taken from particular sites where it is present, this could make for expensive transportation. The Romans ‍minimized transportation costs‍ and manpower by quarrying at local sites, and by developing methods of transportation in order to expedite the process of mining and extraction (Adams). This was crucial because transportation was typically the largest cost associated with building materials. Cylindrical rollers and wheeled platforms eased ‍ground transport‍ of quarried stone, while specialized grips and rope apparatuses allowed builders to lift the blocks either out of the quarries or into placement at the site of construction. Small notches were cut into the stone to assist in placing the cut stone into its final position (Adams).

Figure 2: Wall of tuff stones

Some common types of local stone include ‍Selce, Aniene, Peperino, Grotta Oscura, Travertine, and Fidena‍e. [Muench, Structures Lecture]

Gray, soft and easily workable, peperino was one of the oldest stones used for opus quadratum masonry. Although useful for structural construction, it is not ideal for subtle carved detail because of its color and softness (Sear 83). Paler and denser than peperino and also fireproof, gabine stone also shows up in many Roman buildings (Sear 83). Tufa is essentially solidified volcanic mud, which makes it easily workable as a building material. One of the earliest varieties used by the Romans was Fidenae tufa (Sear 84). It was often used alongside travertine in temple platforms, though a far weaker material under concentrated loads. Romans first began quarrying travertine in the plains below Tivoli around the second century BC. A sedimentary limestone with a hard, creamy texture and lightly pitted surface, travertine was used abundantly toward the end of the Republic (Sear 84). Due to its durability and aesthetic value, it was primarily employed as a means of carrying heavy loads or as a decorative façade on buildings like theaters or amphitheaters—the Colosseum is an example (Sear 83). These qualities also made it rather expensive. Quarries like Fratelli Pacific’s travertine mine in Tivoli still extract the same types of stones that were prevalent in Ancient Rome. As sturdy and long-lasting as it is, stone did was not the ideal building material throughout the Ancient Roman era. Skilled stonemasonry and cutting expertise became expensive and unnecessary as new, more rational building materials emerged and it phased out as the primary building material in Roman construction.

Brick

Figure 3: Opus reticulatum style brick wall at Ostia Antica

Brick was another crucial building material utilized in Ancient Roman construction. It revolutionized building during the Roman Empire because the standard geometric shape of bricks made skilled stone masonry unnecessary, and thus greatly diminished construction costs [Muench; Structures Lecture]. The clay the Romans fired into brick was typically white and chalky, red, or coarse-grained and gravelly (Taylor 7). They sought to use clay that would produce lighter, less dense bricks, which proved to be more durable over time. Sandy, pebbly, or fine gravelly clay resulted in heavier bricks, which are more vulnerable to dissolving in moist environments (Sear 84). The chief type of baked brick used in Roman building was called Lydian. This referred to a brick standard that was 44 cm long and 29 cm wide. With these standard brick dimensions, construction time was expedited. ‍Brick didn’t come around until around 100 BC and was not widely used until a while later. Brick was characteristically chosen as a material for the construction of private buildings.‍

Figure 4: Opus quadratum style brick wall at Ostia Antica

Many structures display the original method of bricklaying in which bricks are placed in a diamond pattern, known as Opus Reticulatum and dating back to the first century BC and first century AD. But later structures exhibit a newer method where bricks are lain on one side, making for sounder infrastructure that is less susceptible to cracks and crumbling. This method was known as Opus Quadratum, and came into use mid-first century AD and continued into modern day building (Sear 74). Much of our knowledge about ancient Roman ruins is owed to the Romans’ standard of marking each brick with a stamp to indicate the current consuls, and thus provide a timestamp for the site. Brick manufacturers began stamping as early as the first century BC (Sear 85).

Concrete

Just as they did with many other aspects of the empire, the Romans implemented methods from the Greeks and the Etruscans in the development of Roman concrete.‍ They strengthened Greek lime-cement by incorporating gravel aggregate into the cement mixture for reinforcement (Taylor 8). Peaking in the 3rd century AD, Roman concrete practices embraced this technique of mixing in whatever materials were readily available near the site of construction, establishing the concrete we associate with Ancient Rome today (Taylor 8). Vitruvius explicates that the sand to be used in cement must be “rough, clean pit sand of red, gray, black, or carbuncular (red) color.” He concedes that if pit sand is not available, “a second-rate substitute can be sifted from river beds, gravel, or beach sand” (Vitruvius 4.1). Vitruvius claims that these sands, however, are a poor substitute because they dry too slowly and fail under heavy loads.

The process of lime making is described by Vitruvius as burning “stone which, whether soft or hard, is in any case white” (Vitruvius 5.8). Structural parts are best constructed using stone that is close-grained, while the choice of stone for stucco is of a more porous nature (Taylor 9). In order to make mortar, the rock quarried must contain CaCO3, which will then be heated in a lime kiln up to 1000 degrees Celsius in order to burn off the carbon dioxide [Muench; Structures Lecture]. The remaining substance, calcium monoxide, is known as “quicklime” [Muench; Structures Lecture]. Quick lime is then slaked with water until it “disintegrates into powder” and is “3 to 4 times its initial volume” [Muench; Structures Lecture]. After this entire process is finished, the hydrated lime is mixed with aggregate and the mortar is complete. Vitruvius notes that when pitsand is used, the mix should contain “three parts sand for one part lime, while river or sea-sand mortar should have a two-to-one ratio, and should additionally contain a third part of burnt brick, finely ground” (Taylor 9). Although the reinforcing materials used in Ancient Roman concrete were rather primitive, the consistency and density had a large impact on the functionality and durability of the material.

Romans began using their signature concrete, pozzolana, around 200 B.C. (Taylor 9). Containing silica and aluminum, the pozzolan material “possesses little to no cementing ability by itself, but when divided finely in the presence of water reacts chemically with calcium hydroxide (lime)” [Muench; Structures Lecture]. This results in compounds that do cement well. Several natural pozzolans were made using volcanic ash from the Pozzuoli and Baiae regions (now Naples), as well as Mount Vesuvius. This concrete was superior in that it had the ability to set and cure underwater, a valuable property when it came to building ports and aqueducts (Taylor 9). Other natural pozzolans include glass, pumice, and diatomaceous earths (soil containing siliceous diatom microskeletons) [Muench; Structures Lecture]. Some Roman pozzolans were man-made, such as fly ash, silica fume, and other industrial byproducts. “Burned organic matter rich in silica” could act as a viable pozzolan as well [Muench; Structures Lecture]. It is clear from the structures that still stand today that the materials Romans used to make their concrete were strategically chosen to provide the best structural support and at the lowest cost.

Construction Methods

Tools

Figure 5: Chorobate tool

During the Roman Empire, major construction projects were possible because of the magnitude of manual labor available in the form of the army, slaves, and convicts. As previously mentioned, eliminating diverse stonework work as much as possible and replacing it with a standard block shape required only unskilled labor. The planning and technical expertise, however, was handled by engineers and skilled freemasons. Vitruvius emphasizes that “Geometry, also, is a great help in architecture” (Vitruvius 1.4). Romans relied on arithmetic and geometrical theories to address issues of symmetry, measurements, and building costs. They implemented a variety of effective tools for surveying and architecture, often adopted from the Greeks or other civilizations. The simple rule, compass, level and plummet (plumb-bob) were a few of the more simplistic implements. More complicated were tools like the dioptra and the chorobate (Taylor 11). Additionally, light in buildings can be sketched out from fixed quarters of the sky accurately using optics (Vitruvius 1.4.).

Figure 6: Three evolution of the dioptra tool

They also use employed two types of square: a simple square consisted of “three pieces of wood affixed to one another so as to make a right triangle” and a surveyor’s square was “a cylindrical device with slits in the side at 90°, 45°, or even 22.5° intervals” (Taylor 11). Romans operated the surveyor’s square by placing it at the top of a rod and peering through slits in order to determine lines at different angles.

Buildings

Architects in Ancient Rome typically worked for the army, the civil service or in private practice, and were often Greeks or freed slaves. Once the architects and engineers finished drawing up building plans, the site of construction often required some clearing (Sear 71). It wasn’t uncommon that the site was already occupied by previous structures or foundations. However, the Romans were economical about their construction and frequently incorporated earlier foundations into their new plans. “Foundations were encased or vaulted over, or an older building was filled with rubble and incorporated into the foundations” (Sear 71). Usually, foundations were five to seven meters deep, depending on the structure, and made primarily of concrete with stone at locations requiring heavy loads (Sear 71). Types of buildings themselves ranged all over, from standard domiciles to specialized architecture. “There were commercial, domestic and recreational buildings; some were for entertainment and others purely utilitarian; there were honorific buildings such as triumphal arches, and of course a wide range of military and defensive buildings” (Sear 29). Some buildings never changed style throughout time, while others were altered due to the current fashion or economic state. For example, when economic changes swept through Roman towns in the early Empire, housing design was radically altered and high-rise apartment blocks began to replace the old dolmus. Longstanding structures like circuses and amphitheaters would receive steady maintenance and reconstruction (Sear 29). Each type of building required a different set of plans, a different method of construction, and different building care.

With such a wide array of structures to tackle, Romans employed a number of different construction methods when it came to building. As mentioned above, they tried several methods of brick laying that resulted in walls of varying strengths and durability. Vitruvius categorizes them as Opus reticulatum—using tiny, diamond-shaped bricks that were aligned with their points downward; and Opus incertum—laying bricks on their side in lateral formations (Taylor 8). The former came into use because of its aesthetic appeal, but was eventually replaced by the latter because its physical design provided better distribution of weight and decrease the risk of cracking or deterioration. Even with a variety of building methods, Roman construction of a standard structure can be generalized into a typical procedure. First, the foundation trenches would be dug and the foundations would be laid. Next, a few masons would lay several rows of facing bricks, followed by a spread of mortar. After roughly 25 courses of brick had been lain, the masons would finish off that wall with a layer of especially wide brick tiles (Sear 72-73).

While some ancient methods of construction, such as Opus incertum bricklaying, haven’t changed much in two thousand years, there exist major differences when it comes to philosophies and objectives in structural engineering. Romans embraced a “design-for-repairs” approach when designing and building civil infrastructure (Taylor 25). This means that they built with the inherent intention of maintaining them constantly, until they were destroyed for some reason or another. Despite frequent repairs, structures would generally need to be replaced after 50 to 75 years (Taylor 25). ‍Our modern methods of designing with the intent “extensive longevity” require less frequent work in maintenance, but often suffer due to complex design that may leave little room for error, and thus complex repair.‍

Deterioration

The Structural Behavior

In order to investigate the process by which a structure deteriorates, it is essential to understand the meaning of “structure” and how it is sustained. It is described by Croci as the “part of a building which… transforms actions into stresses and… provides the strength to sustain the construction” [3]. Thus, if the structure fails at one or more points, the entire building may be in danger of deterioration and perhaps eventual collapse. The behavior of a weakening structure depends largely on the materials used in construction, the shape and dimensions of the edifice, and the way in which different parts of it are bound together [3].

Damage and Decay

The term “damage” describes a situation “in which a structure has lost some or all of its bearing capacity” (Croci 41). It is typically characterized by cracking, crumbling, crushing, permanent deformation of a structure or the breaking away of elements. Damage is related to mechanical actions—as described below—due to either natural occurrences or human interference which may negatively alter a structure from its initial state (Croci 41). Damage often transpires due to foregoing settlement of soil, earthquakes, etc. which can corrode binding connections and cause permanent deformation. Openings of windows, doors, niches, electrical chases, etc. may widen, reducing structural efficiency with cracks, crushing effects, and deforming. Furthermore, removal of walls, slabs, staircases, etc. “can alter the mutual support of structural elements, inducing cracking or local damage; the monolithic arrangement of the whole building” (Croci 41).

“Decay” or “deterioration” refer to “an alteration of the material that usually leads to a reduction in resistance, increased brittleness, porosity and a loss of the material that usually begins from the outside and works inward” (Croci 42). Decay is chiefly a result of physical or chemical actions. Deterioration usually originates from one or more of the following factors: lack of due care in the original design, lack of scientific knowledge, use of constructions beyond their life expectancy (safety coefficients in codes change; costs of functionality, convenience, or maintenance may evolve over time), errors and imperfections, and introduction of new and unknown factors (for example, environmental changes, variation in the use of the structure or another structure or the earth, or natural phenomena such as earthquakes) (Croci 42). Thus decay and deterioration are both preventable, at least temporarily, provided the engineering and construction are proficiently done.

Understanding Possible Increases in Action Effects

The ways in which structures fail over time due to external forces can be broken down into several types of actions. Mechanical actions are either static or dynamic, and when their effects begin to grow in impact, it can lead to an increase in stresses (Croci 42). Static actions are either in the form of direct actions or indirect actions. Direct actions refer to “applied loads, such as dead loads (dead weight, permanent loads)” and “live loads (furniture, people, etc.)” (Croci 42). These types of loads act as forces directly applied to the structure, either constantly or erratically, and may increase due to a number of circumstances like additions to the building or change in the use of the structure. Indirect actions include “deformation or strain imposed on structures, such as soil settlement, thermal variations, viscosity or shrinkage of materials, etc.” (Croci 42). Essentially, natural changes in the surroundings of the structure can have a great impact on structural and material longevity. Luckily, these conditions are often predictable, and therefore preventable with simple solutions. Since indirect actions inflict force “only when the deformations are contained or not free to develop,” their impact may be avoided with the implementation of joints or other methods of movement toleration (Croci 42). Modern day bridges provide an example of reducing the impact of indirect action with thermal joints to allow for expansion and contraction with temperature variation.

While static actions consist merely of forces applied to a structure, dynamic actions are more jarring in nature. Croci labels these types of actions as “an acceleration applied to the structure” (Croci 42). Varying in force, the intensity of dynamic actions depends not only on the magnitude of the acceleration, but “the natural frequencies and the capacity of the structure to dissipate energy” (Croci 42). Often the acceleration is a result of unforeseeable phenomena such as earthquakes, wind, vibrating machinery, explosions etc. and thus was not accounted for in the design and construction of ancient buildings. Nowadays, we have developed methods and instruments to better ‍predict such anomalies and prepare for them structurally‍. We construct models and simulations, testing how certain sources of structural failure can be prevented in design and construction.

Aside from countless physical obstacles vying to cause deterioration, structures must also battle chemical incursion. In concurrence with mechanical actions, physio-chemical actions comprise of those which relate to the atmosphere and environment (Croci 42). Decay, which Croci delineates as “the detrimental change of a material’s characteristics” is associated with surroundings and climate (Croci 41). This poses an ever-present threat to the durability of materials, extending down to their molecular makeup. Natural environmental conditions which contribute to the breakdown of a material may include “humidity, rain, frost, deposits of soil, presence of water and temperature extremes,” while human factors such as “traffic, vibrations, pollution or lack of maintenance” could accelerate the natural processes (Croci 42). Thus location and local climate and environment had a large impact on the rate and degree to which structures fell to ruin.

Stone Deterioration

White deposits on the wall of the Aqua Claudia

Because it exists in such a number of forms, stone deteriorates with a range of different behavior. ‍According to Vitruvius, stone can be categorized into three broad groupings: soft, medium, and hard‍ (Jackson et al. 2012). Medium and hard are obviously the primary choices when it comes to durability and longevity. Soft stones on the other hand, are easy to manipulate and can still support heavy loads but, as Vitruvius explains, weather quickly when exposed to the elements—particularly salty coastal air (Jackson et al. 2012). Additionally, they combust in the presence of fire. The most dominant source of decay in soft stones stone seems to be water sorption. Absorption occurs when walls and foundations made of tuff, for instance, located in lower areas become saturated with groundwater (Jackson et al. 2012). Another contributor to softer stones was rain, which steadily penetrated the surface of the stone with prolonged exposure and came into contact with hydrophilic minerals, causing swelling and corrosion (Torracca 9-12, 97-9)]. Acid rain “accelerates dissolution of zeolite and calcite cements” even more rapidly (Jackson et a. 2012). The Tiber River’s frequent flooding also expedited weathering of softer stones in structures.

All of these sources of deterioration are known today, but Vitruvius confirms that the engineers and masons of Ancient Rome also knew of their effects. They were aware that “as long as they are used in covered areas, they will sustain stress, but if they are put in open uncovered places, then, once they have been saturated with ice and frost they crumble apart and dissolve” (Vitruvius, 2.7.1-2). Because of this, Romans rarely left tuff stone exposed, often concealing it under stucco—a material able to resist both rain and water vapor (Vitruvius 7.2-7.4). This offered thorough protection to the porous stone from moisture accumulation and decay. Travertine and marble cladding also served as durable and decorative buffers protecting tuff walls (Torracca 111-14). Despite these preventative measures, thousands of years took their toll on the architecture of Ancient Rome.

{image of the Temple of Portunus}

The general ambient environment also plays a significant role in the deterioration and decay of stone. Rome’s summers are hot and humid, while its winters are mildly cold, only experiencing freezing temperatures on sporadic occasion. During the fall and winter, “relative humidity falls from about 85% in the morning to about 60-70% at midday as temperatures rise” (Jackson et al. 2012). Water vapor adsorbs into the stone of buildings during the daytime when warm, humid air comes into contact with cool masonry walls and condenses. As the temperature drops in the evening, the air releases water vapor onto exposed surfaces. The condensation due to the rise and fall of temperatures is detrimental because “Free water molecules adsorbed into the porous stone attach as ordered water to the surfaces of narrow capillaries” (Jackson et al. 2012). Here, “they may dissolve grain cements and cause microcracking and disaggregation of the tuff as hydrophilic minerals expand and contract (Jackson et al. 2012).

Soft stone like tuff experiences a destructive cycle of drying during the heat of the summer. Moisture evaporates from the stone’s surface under direct sunlight and wind, leaving behind deposits of dissolved constituents like efflorescence and indurated crusts. The stone is further degraded by exfoliation and alveolar erosion, which eat away at edges and corners, and can cause blocks to morph convexly (Jackson et al. 2012). Tufo Giallo della Via tiberina and friable Tufo del Palatino exhibit the most prominent decay because of their very porous nature and large amounts of altered pumice and/or clay (Jackson et al. 2012).

Brick Deterioration

Made primarily from chalky white or red clay, brick was another primary Roman building material that suffered from decay and deterioration. In the brickmaking process, it was crucial that clay of a standard or coarse-grained gravelly consistency be used, and that it never be sandy or pebbly (Taylor 8). If the clay grain was too fine, bricks would be excessively heavy and also more susceptible to dampness and rain (Taylor 8). ‍Sticking to bricks of a coarser grain ensured that they would resist moisture damage longer because of reduced porosity. ‍The drying portion of brickmaking could also pose threats to longevity of the material. Vitruvius cautions against drying bricks outside during summer months to avoid potential cracking in the intense heat (Jackson et al. 2012). He even delineates that “Bricks should be cured for two years before use to allow them sufficient time to dry completely” (Jackson et al. 2012). This is supposed to reduce structural damage caused by creep. As mentioned above, the Opus-reticulum method of bricklaying also posed hazards to structural stability.‍ The Romans solved this with Opus-incertum, a small alteration with a significant improvement.‍

Concrete Deterioration

As one of the most revolutionary building materials of Ancient Rome, concrete is still around today for a reason. It was a pragmatic choice in construction because of its long lifespan, cheap cost, and ability to be easily reinforced. One of the main reasons so many concrete structures from the Roman era are still intact today is that concrete has the ability to last potentially more than a thousand years (Taylor 4). As there are a variety of types and mixes, concrete’s longevity can be extended even longer with the right concoction of materials. Despite this incredible durability, concrete is still susceptible to deterioration and decay over time. As concrete cures and hardens, it experiences a process called shrinkage. Shrinkage refers to evaporation of water that is not consumed by hydration, resulting in contraction of the material (Taylor 4). This shrinking puts a lot of tension on the concrete, often inducing cracks, which can lead to a whole other score of problems. Water, for instance, may seep into the cracked concrete, corrode or break down the reinforcements, and inflict even more crumbling and structural failure.

Even more detrimental to the duration of concrete is what’s known as the freeze-thaw cycle. Rome poses an especially dire threat in this respect because of its ‍extreme seasonal climatic variations‍. A cycle of extreme coldness followed by extreme heat widens already existing cracks in concrete due to the expansion of seeping water upon freezing (Taylor 4). The problem is further intensified because of chemicals with thawing capabilities like salt. “Salt changes the chemistry of the pore water (water present in the pores of the concrete)” and causes even worse decay of the structure (Taylor 4). The main ailment concrete suffers in the long run is a phenomenon called creep. Creep is “the deformation of a solid due to stress (forces internal to the structure) (Taylor 4). Essentially, after concrete has been in place on a building for a long time, its own weight can cause the structure to distort and buckle. This can be very detrimental to a structure, and unless it is designed specifically to account for creep, its lifespan may be greatly reduced.

It is fascinating to comprehend how Romans constructed such solid, long-lasting edifices thousands of years ago, with a complete absence of modern technology. Comparing ancient and modern techniques of concrete use, we find that the materials used then versus now are essentially unchanged. The aggregate—material added to the cement for reinforcement—however, is where the differences lie. In ancient times, the Romans used what materials they had at hand as aggregate. As mentioned before, this often meant mixing in gravel of different varieties, shards of pottery, and any other viable materials of similar physical composition (Taylor 3). Today, however, concrete reinforcement has advanced beyond just gravel to strengthening agents like rebar and a variety of admixtures (Taylor 4). ‍The combination of sturdy physical support and strengthening chemical agents “decreases the effect of tension on the concrete” and prolongs the longevity of modern concrete far beyond the scope of Roman concrete (Taylor 4).‍ Even with our innovative advancements in concrete technology, decay and deterioration still pose a threat. Iron rebar can corrode and incur rust with exposure to the elements and moisture, causing internal expansion and, consequently, weakening of the structure (Taylor 18). Even though Ancient Roman concrete suffered from a score of deterioration factors which we can account for with today’s technology, there is still plenty of room for improvement.

A Scientific Investigation of Quarried Stone

The Research

In 2004, researchers Marie Jackson in the Civil and Environmental Engineering Department of UC Berkeley, Fabrizio Marra at Istituto Nazionale di Geofisica e Vulcanologia in Rome, Richard L. Hay in the Geosciences Department at the University of Arizona, C. Cawood in Flagstaff Arizona, and E. M. Winkler in the Department of Civil Engineering and Geological Sciences at the University of Notre Dame collaborated on a study called “The Judicious Selection And Preservation Of Tuff And Travertine Building Stone In Ancient Rome.” The launched into an in-depth investigation of the physical attributes, strength, and durability of tuff and travertine stone used in Ancient Rome. Earlier use of tuff involved softer, weaker stone, but over time they transitioned to more durable tuff, which increased structural longevity and is responsible for the fortitude of many of the relics and ruins we still see today [4]. The Late Republic also saw the use of durable, decorative travertine which improved building aesthetic and also acted as a protective layer for the tuff. The researchers sought to discover the physical and chemical science behind the deterioration of specific tuff and travertine stones.

Tuff and Travertine

A generalized geological map of the Roman region, showing the Monti Sabatini and Alban Hills volcanic rocks, travertine deposits within the Acque Albule basin near Tivoli, and Roman roads through the region (Jackson et al. 2012).

Vitruvius claims that over centuries and centuries of construction, builders in Ancient Rome developed comprehensive knowledge of local stones. Stone construction in Ancient Rome began around the eighth to ninth centuries BC with quarrying of friable tuff on the Palatine Hill. With new land acquisitions and improvements in trade routes, the Romans could mine tuff as far as 26 miles outside the city (Jackson et al. 2012). This allowed them to be picky about the durability of the stone and select the most long-lasting varieties. The late Republican period was characterized by employment of an assortment of “moderately well- to well-lithified tuffs for ashlar or dimension stone construction (opus quadratum)” (Jackson et al. 2012). According to the researchers, Romans sought out stone to quarry from roughly seven ‍pyroclastic eruptive units‍. These included ignimbrites, ground surge, and debris flow deposits (Jackson et al. 2012).

The researchers began their inquiries of the physical properties of tuff by determining the petrographic compositions of samples from Roman quarries and outcrops. They used point count analysis to study thin sections and immersed powders with oils under a petrographic microscope to identify the primary and secondary components in the tuffs (Jackson et al. 2012). They found that glass, rock, and crystal fragments were the primary components in the tuffs. Zeolite, calcite, and clay were discovered to be the secondary components (Jackson et al. 2012). These “diagenetically through a solution–precipitation process during low temperature reaction of interstitial water with glass, leucite and, rarely, the glassy groundmass of some lava fragments” (Jackson et al. 2012, 493).

‍Quarried from deposits near Tivoli, travertine was one of the most common sources of sturdy, dense stone. Its appearance includes horizontal stratifications of a light yellowish gray color. It accumulates by Acque Albule basin, a shallow lake that hosts “living clumps of bacteria [to] surround themselves with a continuously precipitating corona of calcite” (Jackson et al. 2012). This bacteria leaves behind calcite crystals upon decay, which are full of micropores, thus producing travertine (Jackson et al. 2012). Travertine limestone served many purposes in Ancient Roman construction‍. Often implemented as “keystones in tuff arches, capitals of tuff columns and decorative revetments for tuff walls,” travertine possessed both sturdy and aesthetic qualities, which made it so abundant, especially toward the end of the Republic (Jackson et al. 2012). The researchers proceeded to investigate the origin and composition of the most common tuffs utilized by the Romans. They include: Monti Sabatini volcanic deposits, Tufo Giallo della Via Tiberina, Tufo Rosso a Scorie Nere, Alban Hills volcanic deposits, Tufo del Palatino, Tufo Lionato, Tufo di Tusculo, Lapis Gabinus, and Lapis Albanus (Jackson et al. 2012, 498).

Testing

The researchers delved into testing the relative strength and durability of the tuffs and travertines that characterized Roman construction. They approached the question of strength by following field observations by Vitruvius and measuring the “modulus of rupture and uniaxial compressive strength under oven-dried, water-soaked and humid conditions” of samples from Roman quarries and outcrops (Jackson et al. 2012). They “determined water absorption, water adsorption at about 98% relative to humidity and bulk specific gravity for the samples. The ‍results ‍of the data in concurrence with Vitruvius’ notes exhibited how cautiously the Romans selected tuffs and travertines based on material properties toward the end of the Republic and beginning of the Empire (Jackson 2012). They proceeded to test resistance to wetness in a variety of tuff by determining “the dry-to-wet strength ratio as modulus of rupture using 3 mm thick discs sliced from 35 mm cores with tests of five or more discs for each run” (Jackson et al. 2012).

Results:

Due to the large quantity of glass, rock, and crystal fragments present in Roman tuffs, those have the greatest impact on the weight-bearing strength and durability. The results show that loosely grain-supported, vitric tuffs, with lots of pores and cavities such as Tufo Giallo della Via Tiberina and Tufo Rosso a Score Nere “have low compressive strengths, high water sorption and low durability” (Jackson et al. 2012) Whereas “coarse-grained, lithic-crystal Tufo di Tuscolo, Lapis Albanus and Lapis Gabinus have the greatest compressive strength and durability” (Jackson et al. 2012). Bulk specific gravity increases uniformly from pumic-rich stone to lithic-crystal. Fragments of lava and crystal give a stone dense, weight-bearing support because zeolite cements adhere strongly to their hard particles (Jackson et al. 2012, 504). The results also show that “inert granular materials such as sand and gravel add strength” to the concrete (Jackson et al. 2012 504). Some of the samples including clay demonstrated that “The presence of clay reduces the adherence of zeolite cements to the binding surfaces of lithic and vitric particles and decreases compressive strength” (Jackson et al. 2012, 504). Additionally, stone decay is exacerbated by expansion and contraction of hydrophilic minerals (like clay and zeolite) during cycles of wetness and dryness (Jackson et al. 2012, 504).

Conclusions

The correlations between choice of tuffs and travertines in Ancient Rome corresponded not only with stone readily available, but also with durability, structural strength, and resistance to deterioration. They sought out stone with crystal fragments—possessing the greatest compressive strengths—and avoided stone containing pumice and clay—possessing the weakest compressive strengths (Jackson et al. 2012). Romans covered the porous stone they did use with weather-resistant material like travertine and stucco [4]. We can confirm Romans understood these properties due to investigation of ancient quarry sites, the remains of structures still standing today and the field observations from Vitruvious.

Works Cited

Taylor, David D. Comparison of the Longevities of Roman and Modern Construction Projects. Thesis. Oregon State University, 2012. Mountain View, CA: Creative Commons, 2012. Print.

Croci, Giorgio. The Conservation and Structural Restoration of Architectural Heritage. Southampton, UK: Computational Mechanics Publications, 1998. Print.

Jackson, Marie, Fabrizio Marra, Richard L. Hay, C. Cawood, and E. M. Winkler. “The Judicious Selection and Preservation of Tuff and Travertine Building Stone in Ancient Rome.” Archaeometry 47.3 (2005): 485-520. ResearchGate. Web. 13 Sept. 2015.

Sear, Frank. Roman Architecture. Ithaca, NY: Cornell UP, 1983. Print.

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