Material Matters: ’Til the Concrete Angel Falls
Craig Durham puts forward a candidate for the greatest material on Earth that is literally the foundation of modern society, and yet evidence of its use has been found dating back 9,000 years.
Originally comprising nothing more than clay and limestone ﬁred together and ground to a powder, lime cement was extensively used by the Ancient Egyptians, Greeks and Romans to provide the mortar that held together, among other things, the Great Pyramid at Giza and the Colosseum in Rome.
The birth of modern cement can be traced to Joseph Aspdin’s 1824 patent for Portland cement, so called because he thought the colour resembled Portland stone. Today, Ordinary Portland Cement is the construction industry’s standard, ranging from Type I, general purpose and the most common, through to Type V, which has very high sulphate resistance and is used where the concrete will come into contact with highly alkaline soil or moisture.
Many people use the words cement and concrete interchangeably, but strictly speaking concrete is an aggregate mixture of cement, water, sand and gravel, whereas cement mortar is cement, water and sand only. In fact, even the word concrete comes from the Latin concretus, meaning grown together or compounded.
Concrete also has a very high compressive strength compared to cement alone, typically around 40MPa and comparable with many naturally occurring rocks such as sandstones and dolomites (20–170 MPa), which consequently made it an obvious foundation material as engineers sought to build ever higher, and heavier, structures. An early proponent was Robert McAlpine of the company that still bears his name. Known as Concrete Bob because of his enthusiasm for the material, in 1897 McAlpine built one of the longest bridges at the time entirely out of mass concrete. The Glenﬁ nnan Viaduct sweeps majestically across its namesake in Scotland, carrying the West Highland line extension from Fort William to Mallaig, regularly voted as one of the most beautiful railway journeys in the world. And having made this journey by both steam and diesel train this summer, I can attest to that claim. The viaduct is now world famous thanks to the exploits of Harry Potter, although few cinema goers will have appreciated that, as with many engineering feats, necessity was the mother of invention – the local Lewisian Gneiss being unsuitable for cutting and shaping to the blocks required for a stone viaduct.
Of course, concrete’s compressive strength is off set by a relatively weak tensile strength, and it was as early as 1830 that the ﬁ rst idea of reinforced concrete was mentioned in the Encyclopedia of Cottage, Farm and Village Architecture, which suggested that a lattice of iron tie-rods could be embedded in concrete to form a roof. Once again, the Romans had tried something similar with bronze strips but this was not very successful because the bronze had a higher rate of thermal expansion than the concrete, which caused it to spall and crack.
But it is was the second half of the 20th Century that saw the boom in concrete construction as a fast and relatively cheap method of rebuilding from the devastation of the Second World War. This also saw some of the least loved buildings constructed, with terms such as concrete jungle and concrete carbuncle being used to describe many of the new town centres and municipal oﬃ ces. Despite this common perception, some of the most iconic structures in the world are concrete, including Sydney Opera House, the Hoover Dam and countless bridges and skyscrapers. In recognition of this, every year, the Concrete Society hosts Awards for Excellence in Concrete. Last year's winner in the Mature Structures category was the refurbished Department of Metallurgy and Materials Building, University of Birmingham, of which many alumni are IOM3 members and no doubt readers of this magazine.