My paper on Roman cement and engineering!

Roman innovations and almost modern uses of cement, combined with other architectural advances, laid the foundation for many architectural developments that would impact western architecture for centuries to come. Techniques and structures durable enough to last for centuries are solid proof of the Roman’s engineering superiority of the time. In this paper we will discuss the innovations in cement work and how it impacted and allowed for advances in other areas of architecture.

Roman engineers were not the first to use cement, arches or vaults. Some historians believe that ancient Syrians, around 6500 BC, used burnt lime mortar as a means of waterproofing cisterns. The discovery of lime as a building material was probably made in the course of fire pits built from limestone. The heat from the fire would expel carbon dioxide from the limestone and thereby produce lime. It is possible that four thousand years later, the Egyptians were using lime mortar or burnt gypsum in the construction of the pyramids, as well as using cement to mould stone objects. The Nabataeans (700 BC – 300 BC) might have been the first to use hydraulic cement (sets underwater or is waterproof) as there is evidence of burnt lime found, and an abundant supply of pozzolan nearby, which is a key element in making hydraulic cement. The Greeks (ca 600 BC) were known to have used a natural pozzolan and lime mixture in hydraulic cement . Further, the Egyptians used arches and vaults, particularly as forms of drainage. The Greeks, whom the Romans were most heavily influenced by, did not use arches or much cement. Their method of construction was with cut stone slabs, and post-and-lintel over doorways. They did not use cement to the same extent as the Romans.

Cement in Roman architecture became integral as a result of engineering developments. The basic ingredients in concrete mortar are simple lime and river sand (at a ratio of one part lime to three parts sand). Limestone is made up of calcium, carbon, and oxygen. When limestone is heated, or burned, the carbon and oxygen are driven off in the form of carbon dioxide, leaving calcium oxide, or quicklime. If water is added to the quicklime, a chemical reaction occurs where heat is given off and the mixture bubbles. Three results are produced: lime putty, which is used as the binding agent for cement; lime milk, consisting of 20-30% water and can be used for paints; and lime water, which is clear, disinfectant, and can be used for medicinal purposes .

In the basic recipe, sand is added to the lime putty to make cement. However, pozzolanic ash (volcanic ash from Mt. Vesuvius, named after the nearby town Pozzuli) was discovered to make a much more superior cement than sand. This is due to the chemical and structural makeup of the ingredients. Sand (mostly silica) has a crystalline atomic structure so dense that it has little contact surface with the lime, reducing the chemical reaction significantly. In contrast, the pozzolanic ash (silica with trace amounts of alumina and iron oxide) has a very porous atomic structure, which allows the lime to enter and create a concrete-like gel. This gel then expands, and bonds the rocks together in the concrete mix, creating far stronger cement than the sand/lime mixture. To ensure the most chemical reaction possible, the pozzolan must be in powder form to allow for the optimal amount of surface area. The strength of this concrete is comparable to modern cement; indeed, a part of the chemical makeup of pozzolanic cement matches that of Portland cement.

However, the strength does not just come from the chemical makeup, but also from the method of application. At the Upper Stillwater Dam in Utah, the builders used a then-innovative technique called roller-compacted concrete. The mortar was a mixture of 40% Portland cement, and 60% fly ash. The fly ash, a by-product from electrical plants, contains the same silica content as explosive volcanic ash, and when wet, the Portland cement releases the same calcium hydroxide component found in ancient lime cement mixtures. In making the dam, the builders used little water, which gave it a stiff composition. They spread the mixture in thick layers, and then used vibrating rollers to pound the concrete into place. The Romans used a similar process. They would hand mix very dry cement in mortar boxes and carry the cement to the job sites. They would spread it out over a pre-placed layer of rock formation, and would then pound the mortar into the rock layer. We know that this is so because the ancient historian and engineer Vitruvius, alive in the days of Julius Caesar, recorded the process in one of his ten books on construction and engineering. He also mentions tamping tools. It is now known from the Upper Stillwater Dam construction that this method of tamping reduces excess water; the water is a source of weakness and voids in the concrete. When it is tamped, the concrete is compressed into a more solid mixture and also creates more bonding gel than normaliv. This is part of the process in which the domed roof of the Pantheon was made and withstood such great pressures.

Roman concrete has some great advantages over the Greek method of cut-stone masonry. It was very strong, even when spanning great distances in arches, vaults and domes. It had greater flexibility than the Greek’s stone slabs, as the concrete could be poured or layered; it took the shape of the form around it. It was also cheaper as it did not require skilled masons or special labourers, and it was faster to make than cut ashlar masonry. And one of the most important advantages was that the concrete vaulted roofing was fireproof, as opposed to the mainly wooden beamed roofs of the Greeks.

One disadvantage of the use of concrete is that it was often unsightly as the wooden form structures left marks and imprints. The Romans in their resourcefulness and ingenuity started facing the cement with more esthetically pleasing material. Tufa, which is soft volcanic stone, was used in slab form to cover the core construction of the building. Tufa blocks in irregular fist-sized shapes were used to make beautiful wall mosaics. And the Romans came to appreciate the method of regularized rectangular tufa blocks arranged diagonally. It became a popular method during the reign of Hadrian, and can be seen in different places in Hadrian’s Villa. The blocks had cone shaped backs for easier placement into the cement. And of course, all of this could be veneered over with another, prettier material, such as stucco or plaster that has been shaped, molded, patterned, and/or painted. Marble encrusting was used only in very wealthy places .

Mortar was used in many Roman structures, but was rarely needed in the arch. The arch was not a new building form when the Romans took a hold of it, but it became on of their signature pieces of architecture. It was particularly useful over doors; the Greek method of post and lintel was limited, as it had a low tensile strength and in regular use could only span a maximum of 60 centimeters. Because of its structure, the arch transfers the load onto the columns, which does away with the tensile stresses. The more an arch is loaded, the stronger it becomes, to an extent . This is because the vertical pressure on the arch stones causes the weight to be transferred to the middle third of each stone, thereby eliminating any shear, or sliding forces, that would cause the arch to collapse. There was usually no need to have mortar, as indeed that would cause shear force .

The next step in the development of arches is the use of vaults. The Romans used a very simple vault called the barrel vault, which is simply an extension of the arch. It can be constructed along a curved axis. There are two major disadvantages in a barrel vault. The first is that it is hard to light; natural light only comes in at the ends of the barrel. The second is that it needs continual support along the entire length of the vault. The solution to this is the development of the groin vault, in which there are two intersecting barrel vaults. This eliminates the need for continual support as the groin vault transfers the weight to the corners. It also lets in much more light. Groin vaulting can be used in a pattern; unfortunately it can only be used in a square or rectangular plan. This kind of vaulting was used in the construction of the Coliseum’s substructure .

As in the arch, the downward force on the vault created outward spread of the vaults. Therefore resistance to the thrust was needed at the lower portions of the vault, in the form of thickened haunches or the point that connects to the column. This created the development of buttresses, which led to increasingly intricate designs in later architecture. The Romans often cast solid concrete vaults so there were not need of buttresses in those cases . One problem with the non-concrete vaults is that if the foundation shifted or settled a bit, there was a danger of the vault collapsing in. One improvement is the addition of ribs in the vaulting, either traverse lines to the vault or in diagonals of the crossing .

One of the most visible ways that the Romans used arches and vaults was in the aqueducts. The aqueducts were the most advanced way to bring in fresh water and dispose of sewage in the era and indeed the system was not improved upon until very recently. They were built from a combination of stone, brick and pozzolanic cement. The bulk of the waterway ran underground; the Romans bored channels through rock and ran piping underneath the surface. Out of 260 miles of aqueduct systems, only 30 miles ran above ground. The aqueducts were used only where geography presented problems, such as valleys. The entire aqueduct system ran on gravity . The Pont du Gard over the French river Nimes is probably the most famous example of a Roman aqueduct. This structure is a bridge and aqueduct. It is built in three tiers of enormous arches, successively growing smaller as it ascends. The bottom portion is used as a bridge and the topmost as the aqueduct.

The pozzolanic cement used in the aqueducts and Roman bridges was essential as it was waterproof and could set underwater. The Romans would build a wooden form in the water and would either drain some off or submerge it after, but would pour in cement and have it harden underwater. This development in cement making allowed some of the amazing advances in Roman architecture that stand even today, some two thousand years later. The Roman’s use of arches, cements and vaults have quite literally paved the way for further architectural development in western civilization and still influences modern architecture today.