Concrete

February 23, 2014 Leave a comment Go to comments

A few weeks ago, a signal equipment room on the Victoria Underground Train Line in London was flooded…with liquid concrete. There’d been a leak in the elevator shaft into which a team were pouring the stuff resulting in at least three rows of equipment in the signal room being submerged in fast-setting concrete.

How to stop the concrete from encasing all that stuff and any hapless control-room person forever? After all concrete = aggregate + cement + water. And at the curing stage, the continued hydration of concrete actually gives it more strength and durability. It turns out bags of sugar from the local supermarket were the answer.

Raw concrete walls - good for holding used cigarette buttsConcrete isn’t actually a new-fangled invention of the ugly modern era. The Romans got there first of course with their opus caementicium. That’s the stuff that temples, aqueducts, bridges, baths, breakwaters, apartment buildings and multi-storey shopping malls (oh yes), and the darling Pantheon, were made of. And the breakwaters have withstood pounding by salt water for two millennia. Around 15-25 BC, Marcus Vitruvius Pollio (cka plain old Vitruvius), in De Architectura (Ten Books on Architecture), was already giving recipes for concrete for a variety of uses – onshore buildings, underwater structures etc. Volcanic ash was the secret ingredient, explains David Moore:

Common plaster is made with wet lime and plain sand. This sand has a crystalline atomic structure whereby the silica is so condensed there are no atom holes in the molecular network to allow the calcium hydroxide molecule from the lime to enter and react. The opposite is true with the wet lime-pozzolan contact. The pozzolan has an amorphous silica atomic structure with many holes in the molecular network. Upon mixing the wet lime with the pozzolan, the calcium hydroxide enters the atomic holes to make a concrete gel that expands, bonding pieces of rock together. The fine powder condition of the pozzolan provides a large surface area to enhance chemical reaction. We find parts of the complex chemistry of the ancient concrete bonding gel matching the same chemical formula of modern concrete bonding gel. So the pozzolan-wet lime gel gave permanence to the ancient concrete.

Preface to Visuvius' Ten Books on ArchitectureReading an old translation of Vitruvius, I’m reminded again and again how we humans tend to err by either dismissing people from generations past as either (i) absolutely credulous (and so of course we should dismiss all historical documents including the Bible – because of course it was written by cretans (modern usage of this word proves the point!)); or (ii) superior beings, all of whom had equal access to some secret knowledge. Level-headed consideration of the material at hand should be the way forward and in terms of historical accuracy and internal consistency, the Bible certainly trumps them all!

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