Science Questions

Are roads made of the safest materials?

Mon, 9th Nov 2015

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Joe Harris asked:

Are roads made of the safest materials?


Engineer extraordinaire Hugh Hunt answered Joe's question...Road

"Are roads made of the safest materials?"

Hugh - Well, they're made of a very convenient material because when you're extracting billions of tons of fossil fuels out of the ground, some of the stuff that you extract is this thick sticky liquid tar which is quite expensive to turn into anything useful. And to be able to use your sticky gunk you waste to make roads is very convenient.

Chris - People were using telephone directories as well. They found that if they chewed them up and laid them down underneath the road then you had a very good way of attenuating vibration. So, it soaked up some of the shocks and sounds, and noise pollution.

Hugh - But there's a huge array of wonderful things you might do to make roads better, cleverer. For instance, you could embed solar panels in roads that would run the streetlights and weve seen those sort of things. But ultimately, what happens is that the yearly cycle of freezing and thawing, and frost heave and the heavy loads from trucks. Roads crack up, and the biggest challenge is making roads that dont crack up. Weve come to the conclusion really that flexible bitumen works and inflexible materials are not so good. And maybe ultimately, were stuck with bitumen.

Ginny - I was interviewing someone last week who was looking at putting little pieces of graphene, graphite, so somewhere between the two, not the kind of really expensive single-layer graphene, slightly chunkier stuff, like smashed up pencil leads into road surfaces and it makes them tougher and less likely to crack and less likely to end up with pothole problems, and extends the life of the roads.

Hugh - Those kinds of ideas are absolutely fantastic but what's interesting is that over the last however many decades, engineers have been trying to figure out actually, what does cause roads to break up. Its a very difficult problem. There is this thing called the fourth power law which says that the rate at which roads breakup is proportional to the fourth power of the load. But this is terribly approximate because it depends so much on weather and frost heave and there are so many factors. There is not a simple answer.


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