Road Surfaces Over The Millennia
If I were a better artist, I’d love to create a wordless book tracing the development of a road across from a single game trail to a modern superhighway. History is pretty fascinating, so let’s take a look at how road surfaces have changed over the millennia. I’ll just stick to road surfaces, as including the wheres and whys of roading would make this article far too long to read in one sitting.
If you’ve ever seen a house where they park on the grass during winter, you soon see why. All that pressure and squelching soon becomes deep, thick mud, where wheels get bogged. Shortly after the wheel was invented (around 5000 BC), road surfacing followed shortly afterwards.
Roman road still in use in Jebel, Syria.
The earliest form of road surfacing was just plain brick, and examples can still be seen today in the Indus Valley. However, paving stones proved to be superior – they could just be cut out of rock and dropped into place, rather than baked like bricks. What’s more, rain and grit didn’t wear stone away like it did brick.
The Romans were the first ones to do more than just chuck stones down on top of the surface of a dirt track. They figured out that if you put down a good base layer, all the rain would drain away more easily, so you didn’t get problems with rutting and potholing as often. The Romans invented basecourse and subbase, and these techniques are still in use today.
At the bottom of a Roman road, the earth was levelled off at a fair depth down and rammed. After this, a layer of large stones the size of a hand was put down. Next came a layer of concrete (yes, the Romans invented concrete). After that, a layer of very fine gravel. On the very top came flagstones, and they were laid so the middle of the road was higher than the sides, rather like the shell of a tortoise, for better drainage. Not all roads in Roman times got the full treatment, but the most important ones did – the key ones for trade and military manoeuvres. Other rather familiar things found on a proper Roman road were milestones and pavements (sidewalks).
The Romans also introduced the idea of roading standards – they had a set of measurements that had to be stuck to for all roads, as least as much as possible, complete with different measurements for straight bits and for curved bits.
Legionaries building a Roman Road. “Metopa Columna lui Traian Constructie drum”. Licensed under CC BY-SA 3.0 via Wikimedia Commons –
Tar did get used to seal roads during Classical times. This mostly happened in the oil-rich Middle East. Back then, tar was the only thing an oil well was good for. But the idea of combining the Roman method of construction with the waterproofing of tar didn’t come for nearly 2000 years later. From 500 BC to about 1800 AD, it was cobblestones all the way. It wasn’t until the Industrial Revolution that a new method was discovered… ironically, it was about the same time that better suspension systems in the form of leaf springs put in an appearance.
The breakthrough was invented by the Scotsman John MacAdam, although some credit does have to go to a couple of other civil engineers of the time, Pierre-Marie-Jérôme Trésaguet and Thomas Telford. These three engineers had the goal of making good roads cheaply. Needless to say, it was hard work making cobblestones that fitted nicely into roads to give it a good smooth finish. Cobblestones, after all, had to be cut by hand by a skilled bloke with a hammer and chisel.
Macadam did two things. Firstly, he did away with the club sandwich of layers that the Romans used, although a plain sandwich of basecourse and subbase still gets used today. Secondly, he found out that a good layer of gravel pushed into the right shape allowed for good drainage and was a lot smoother than cobblestones – and could be bashed into shape by a machine or by a road gang (possibly of convicts) in large quantities. Your typical back-country gravel road is what a Macadam road looked like.
Macadam’s roads had one problem, even though they drained pretty well and gave a comfier ride. They kicked up heaps and heaps of dust, especially once motorized transport became really popular thanks to the manufacturing efforts of Ford and others. A solution was found pretty quickly: tar, which had the added advantage of being waterproof. This was known as “tarred Macadam”. This method involved two coats of tar or bitumen: one on the subgrade before the macadam gravel, then a top layer to seal it all in. You can still see this method used on a lot of country roads.
Then came Edgar Hooley, who had the bright idea of mixing the aggregate (the finely crushed gravel) with the tar before putting it on the road. This was then flattened into place by a steamroller (which really did run on steam) and was super smooth as well as waterproof. He patented his method under the name “tarmac” (short for “tarred Macadam”, although we also call it after the form of tar mixed with the aggregate: bitumen or asphalt.
Modern asphalt/bitument/tarmac.
Naturally, the development of road surfaces is still going on today. Slipping, cracking and rutting still happen. Who knows what they’ll think of next?
Safe and happy driving, whether your wheels are on gravel, cobbles or tarmac,
Megan
Level 2, 9 Help Street