Lecture 6: Construction Revolution – Arches and Concrete
Understanding Greek and Roman Technology: From the Catapult to the Pantheon
Dr Stephen Ressler (2013)
Film Review
Although tie beam trusses (see Lecture 5) were much more efficient than prop and lintel roofs, timber, which wasn’t very durable, was the only building material suitable for trusses and it wasn’t very durable. In the 4th century BC, Roman architects discovered the usefulness of arches, which could be made of stone or brick, in load bearing.
The oldest surviving arch, an Israeli gate, dating from 1850 BC. Arches were also used in Egypt in the 6th century BC and in a Greek stadium in the fourth century BC.
As Ressler demonstrates with a scale model, an arch must be supported by a timber frame during construction, as it only becomes load bearing once it’s complete. He also shows how an arch only carries load as compression – tensile pressure makes it fall apart. Because weight bearing leads to “lateral thrust,” a completed arch needs a stationary structure on either end (either a column, wall or soil).
Multiple adjacent arches are known as an arcade. Lateral thrust from two adjacent arches enables them to support each other.
A vault is an arch lengthened into three dimensions.
The Romans learned construction engineeering from the Etruscans, who were strongly influenced by the Greeks. Under the Republic, Rome mainly used basic ashlar construction (ie identically shaped blocks held together by iron fasteners). As marble was scarce, they used Tufa (soft limestone) faced with a layer of stucco to make it look more attractive. The use of stucco eventually led to the use of lime-based mortar between stones to seal joints and create a uniform bedding surface. This allowed the use of more imperfect stone blocks, increasing speed of construction and reducing costs.
This, in turn, would lead to the use of concrete (quick lime mixed with “aggregate”) to increase strength. Aggregate could be sand, gravel or construction rubble.
Kiln baking limestone (calcium carbonate or CaCO3) drives off the carbon dioxide (CO2), to transform it into quick lime (calcium oxide or CO). When the calcium oxide is soaked in water for three months, it forms calcium hydroxide or Ca(OH)2. After the latter is pored into a form and exposed to air it re-hardens as CaCO3.
During the 3rd century BC, the Romans discovered a unique silica-containing volcanic ash known as pozzolana that combines with calcium carbonate to produce calcium silicon hydrates that harden without exposure to air. Roman architects found they could speed up the construction of walls (as well as increasing their compressive strength) by combining brick facings with a pored concrete interior.
Rome’s rapid territorial expansion led to a constant demand for new construction – for city walls, bridges, baths, aqueducts, basilicas, palaces and apartment complexes. Moreover Roman wars of conquest produced numerous low-skilled laborers who could be easily trained to pour concrete under the supervision of a skilled mason.
Film can be viewed free with a library card on Kanopy.
The Most Revolutionary Act 