Interesting Facts about Concrete
Concrete isn’t modern. The oldest ever found isn’t even man-made. It is a 12 million-year-old natural deposit found in Israel in the Sixties in which oil shale had combusted naturally near limestone, producing a natural layer of concrete. Israel is also home to the earliest man-made deposits. In 1985, the excavation of a Neolithic site dating back 9,000 years in southern Galilee revealed a concrete floor, and evidence of a limekiln that had been used to burn limestone to form the cement used to bind and harden sand. Another Neolithic site in Serbia showed a similarly constructed concrete floor.
Knowledge of concrete seems then to have disappeared until the Ancient Egyptians used it as infill material when constructing the pyramids at Giza. Their formula for making concrete hasn’t changed much since: limestone was roasted, pulverised then mixed with sand or gravel and water. This knowledge spread around the Mediterranean, but it was the Romans who perfected the art of what they called opus caementicium. They added sandy deposits of pozzolana (a volcanic rock found near Naples at Puteoli, now known as Pozzuoli). This enabled their concrete to set even under water; it was of such high quality that some Roman bridge piers are still in daily use despite having been subjected to 2,000 years of river erosion. The reason that the Colosseum, Hadrian’s Wall and the aqueduct at Pont du Gard in the south of France are all still standing is that they were built using concrete.
The Pantheon in Rome was built in about 120AD and is still the largest unreinforced (i.e. with no metal skeleton to strengthen it) concrete dome in the world. Because it is unreinforced, however, it does not comply with modern safety standards.
The explanation for its resilience may lie partly in the Roman technique of using thick mortar which was pounded into place rather than the runnier concrete we have to use today so that it will be machine-friendly and shape itself around its metal armature.
Probably the most significant event in the history of modern concrete was the patent granted to Joseph Aspdin of Wakefield in 1824 for Portland cement. It changed the course of construction forever. Aspdin’s breakthrough was to produce a substance that was both fast-setting and attractive. By heating crushed limestone and clay, he produced a render that dried to a colour and finish similar to the expensive and fashionable limestone quarried from Portland in Dorset.
His only problem was finding stone to supply his Wakefield factory (he was twice prosecuted for digging up and stealing pavement slabs).
Aspdin was also held back by his own lack of chemical knowledge. He was approximate in his quantities and insisted on being present when each kiln was loaded. This drove his son William to decamp to north-west Kent. Here the abundant supplies of chalk made mass production possible, and William founded the company which eventually grew into Blue Circle, now absorbed into Lafarge, the world’s largest cement company.
Aspdin was very keen to promote the use of concrete in building houses, and in 1850 he started work on a huge concrete mansion called Portland Hall, near Gravesend in Kent. When one of his cement-laden barges ran aground on the isle of Sheppey, the locals recovered all the barrels assuming they contained whisky. When they discovered they were full of now solid cement, they cut their losses and built a pub out of them. It still stands – the Ship on Shore in Sheerness.
Thomas Edison held 49 concrete patents and experimented with precast concrete houses filled with concrete furniture, pianos and refrigerators but the scale of production was too small to make the enterprise viable.
Concrete ears or “sound mirrors” – the huge concrete dishes erected along the south coast and elsewhere at the beginning of the Second World War to detect approaching aircraft – had microphones suspended at the focal point of the dish and a range of up to 27 miles. They didn’t work well on windy days, though, and were eventually made redundant, both by radar and by the arrival of faster planes