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Sunday, October 25, 2015

Reading List: Concrete Planet

Courland, Robert. Concrete Planet. Amherst, NY: Prometheus Books, 2011. ISBN 978-1-61614-481-4.
Visitors to Rome are often stunned when they see the Pantheon and learn it was built almost 19 centuries ago, during the reign of the emperor Hadrian. From the front, the building has a classical style echoed in neo-classical government buildings around the world, but as visitors walk inside, it is the amazing dome which causes them to gasp. At 43.3 metres in diameter, it was the largest dome ever built in its time, and no larger dome has, in all the centuries since, ever been built in the same way. The dome of the Pantheon is a monolithic structure of concrete, whose beauty and antiquity attests to the versatility and durability of this building material which has become a ubiquitous part of the modern world.

To the ancients, who built from mud, stone, and later brick, it must have seemed like a miracle to discover a material which, mixed with water, could be moulded into any form and would harden into stone. Nobody knows how or where it was discovered that by heating natural limestone to a high temperature it could be transformed into quicklime (calcium oxide), a corrosive substance which reacts exothermically with water, solidifying into a hard substance. The author speculates that the transformation of limestone into quicklime due to lightning strikes may have been discovered in Turkey and applied to production of quicklime by a kilning process, but the evidence for this is sketchy. But from the neolithic period, humans discovered how to make floors from quicklime and a binder, and this technology remained in use until the 19th century.

All of these early lime-based mortars could not set underwater and were vulnerable to attack by caustic chemicals. It was the Romans who discovered that by mixing volcanic ash (pozzolan), which was available to them in abundance from the vicinity of Mt. Vesuvius, it was possible to create a “hydraulic cement” which could set underwater and was resistant to attack from the elements. In addition to structures like the Pantheon, the Colosseum, roads, and viaducts, Roman concrete was used to build the artificial harbour at Caesarea in Judea, the largest application of hydraulic concrete before the 20th century.

Jane Jacobs has written that the central aspect of a dark age is not that specific things have been forgotten, but that a society has forgotten what it has forgotten. It is indicative of the dark age which followed the fall of the Roman empire that even with the works of the Roman engineers remaining for all to see, the technology of Roman concrete used to build them, hardly a secret, was largely forgotten until the 18th century, when a few buildings were constructed from similar formulations.

It wasn't until the middle of the 19th century that the precursors of modern cement and concrete construction emerged. The adoption of this technology might have been much more straightforward had it not been the case that a central player in it was William Aspdin, a world-class scoundrel whose own crookedness repeatedly torpedoed ventures in which he was involved which, had he simply been honest and straightforward in his dealings, would have made him a fortune beyond the dreams of avarice.

Even with the rediscovery of waterproof concrete, its adoption was slow in the 19th century. The building of the Thames Tunnel by the great engineers Marc Brunel and his son Isambard Kingdom Brunel was a milestone in the use of concrete, albeit one achieved only after a long series of setbacks and mishaps over a period of 18 years.

Ever since antiquity, and despite numerous formulations, concrete had one common structural property: it was very strong in compression (it resisted forces which tried to crush it), but had relatively little tensile strength (if you tried to pull it apart, it would easily fracture). This meant that concrete structures had to be carefully designed so that the concrete was always kept in compression, which made it difficult to build cantilevered structures or others requiring tensile strength, such as many bridge designs employing iron or steel. In the latter half of the 19th century, a number of engineers and builders around the world realised that by embedding iron or steel reinforcement within concrete, its tensile strength could be greatly increased. The advent of reinforced concrete allowed structures impossible to build with pure concrete. In 1903, the 16-story Ingalls Building in Cincinnati became the first reinforced concrete skyscraper, and the tallest building today, the Burj Khalifa in Dubai, is built from reinforced concrete.

The ability to create structures with the solidity of stone, the strength of steel, in almost any shape a designer can imagine, and at low cost inspired many in the 20th century and beyond, with varying degrees of success. Thomas Edison saw in concrete a way to provide affordable houses to the masses, complete with concrete furniture. It was one of his less successful ventures. Frank Lloyd Wright quickly grasped the potential of reinforced concrete, and used it in many of his iconic buildings. The Panama Canal made extensive use of reinforced concrete, and the Hoover Dam demonstrated that there was essentially no limit to the size of a structure which could be built of it (the concrete of the dam is still curing to this day). The Sydney Opera House illustrated (albeit after large schedule slips, cost overruns, and acrimony between the architect and customer) that just about anything an architect can imagine could be built of reinforced concrete.

To see the Pantheon or Colosseum is to think “concrete is eternal” (although the Colosseum is not in its original condition, this is mostly due to its having been mined for building materials over the centuries). But those structures were built with unreinforced Roman concrete. Just how long can we expect our current structures, built from a different kind of concrete and steel reinforcing bars to last? Well, that's…interesting. Steel is mostly composed of iron, and iron is highly reactive in the presence of water and oxygen: it rusts. You'll observe that water and oxygen are abundant on Earth, so unprotected steel can be expected to eventually crumble into rust, losing its structural strength. This is why steel bridges, for example, must be regularly stripped and repainted to provide a barrier which protects the steel against the elements. In reinforced concrete, it is the concrete itself which protects the steel reinforcement, initially by providing an alkali environment which inhibits rust and then, after the concrete cures, by physically excluding water and the atmosphere from the reinforcement. But, as builders say, “If it ain't cracked, it ain't concrete.” Inevitably, cracks will allow air and water to reach the reinforcement, which will begin to rust. As it rusts, it loses its structural strength and, in addition, expands, which further cracks the concrete and allows more air and moisture to enter. Eventually you'll see the kind of crumbling used to illustrate deteriorating bridges and other infrastructure.

How long will reinforced concrete last? That depends upon the details. Port and harbour facilities in contact with salt water have failed in less than fifty years. Structures in less hostile environments are estimated to have a life of between 100 and 200 years. Now, this may seem like a long time compared to the budget cycle of the construction industry, but eternity it ain't, and when you consider the cost of demolition and replacement of structures such as dams and skyscrapers, it's something to think about. But obviously, if the Romans could build concrete structures which have lasted millennia, so can we. The author discusses alternative formulations of concrete and different kinds of reinforcing which may dramatically increase the life of reinforced concrete construction.

This is an interesting and informative book, but I found the author's style a bit off-putting. In the absence of fact, which is usually the case when discussing antiquity, the author simply speculates. Speculation is always clearly identified, but rather than telling a story about a shaman discovering where lightning struck limestone and spinning it unto a legend about the discovery of manufacture of quicklime, it might be better to say, “nobody really knows how it happened”. Eleven pages are spent discussing the thoroughly discredited theory that the Egyptian pyramids were made of concrete, coming to the conclusion that the theory is bogus. So why mention it? There are a number of typographical errors and a few factual errors (no, the Mesoamericans did not build pyramids “a few of which would equal those in Egypt”).

Still, if you're interested in the origin of the material which surrounds us in the modern world, how it was developed by the ancients, largely forgotten, and then recently rediscovered and used to revolutionise construction, this is a worthwhile read.

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