From Richard Scott
Will Knight speculates on the use of glass bubbles to strengthen concrete, given the finding that holes in many materials help to relieve stress (8 October, p 32).
It would appear that some earlier civilisations stumbled on this idea unwittingly. The Chinese used a cement containing rice flour to build the Great Wall. This has resisted erosion even better than the stones it was used to fix in place. It would be worth investigating whether the particle size of rice flour or its starch grains are optimum for cement.
The Romans made a remarkably durable, strong and lightweight concrete that contained volcanic ash or fine pumice. Pumice and ash have foam structures, so their use is another way of introducing small holes into the matrix.
The above examples suggest there may well be readily available materials that could give the requisite hole sizes without the need for costly glass bubbles.
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From John Lack
The article about holes made me recall a joke from my 1950s military service in the air force.
The wings of a new aircraft kept falling off during trials. Despite the best brains working on it, no remedy was found. In desperation a prize was offered for a solution. Nothing worked until at last they tried a seemingly absurd scheme from an untutored aircraftsman, second class. A line of holes was drilled across the wing adjacent to the fuselage. Amazingly it worked! The a/c 2 was asked how the idea came to him. He replied, “You know when you tear a perforated toilet paper off the roll…?”
Burgess Hill, West Sussex, UK
Bath, UK
