Tar Talk! / On the Road

This is the cheapest (and often the only) way poor road surface is dealt with. Photo: The Bear

I must admit I was daydreaming a little. But I was, after all, on a smooth and empty road, with no warning of any potential problems. That’s when the front of the bike suddenly developed a mind of its own and turned sharply to the left. This was in America, so the movement took me into the opposing lane – where fortunately there was no opposing traffic. Once I was back in my lane and my heart rate had slowed to something that wasn’t making my pupils expand and contract rhythmically, I found the culprit on the road surface.

It looks a bit like Klingon handwriting and it can spell a lot of trouble for you, too. It is the bitumen “snakes” that many road authorities use for “repairs” of cracked road surface, and it is seriously dangerous.

See what I mean by Klingon handwriting? Photo: The Bear

But let’s go back to the beginning and take a quick look at what roads are (generally) made of, courtesy of the nice, clear Orange City Council explanation I found on the interwebs.

“The ‘Surface’ of the road is the layer you travel on. It is usually made of either a Sprayed Bitumen Seal or hot-mixed Asphalt layer.

“The ‘Pavement’ is what supports the road surface and is what gives the road its strength. The pavement is made up of one or more layers of gravel known as the base and sub-base… The pavement can be further strengthened by adding a “binder” into the gravel during construction.

“The lowest layer in a road is natural material known as subgrade. This is basically the earth beneath the pavement generally consisting of clay, ironstone and occasionally rock. The strength of the sub-grade can also be improved by adding a binder into it.”

That’s fine for when roads are new. But what about when they need repair?

Trucks are the main culprits when it comes to destroying roads. Unless you have tanks. Photo: The Bear

“Road repairs are usually tackled by ‘patching,’” says Orange. “There are generally two main types of patching. (The first is) Pot-hole Patching (which) involves filling holes… in a sealed road with a mixture of bitumen and stone. There are three options of pothole patching used by Orange City Council.” They are Jet-Patching, Cold Mix Patching and Hot Mix Patching, and all use a mixture of bitumen, stone and sand. In the case of Hot Mix Patching the bitumen is heated to 180 degrees and can be used on large potholes.

“(The second is) Heavy Patching (which) is used when large sections of the road are badly damaged or deformed… Road Re-sealing happens when the road pavement is in good condition, but the surface has deteriorated to a point where it requires replacement. It generally involves spraying hot bitumen over the existing surface, quickly tipping a layer of stone on top and rolling the stone into the bitumen.

This road is beyond repair with bitumen. It needs renewal. Photo: The Bear

“Road Renewal happens when the road has reached the end of its life. The most common road renewal technique that Orange City Council uses is an extension of Heavy Patching called Pavement Rehabilitation (where) whole sections of the road are dealt with.”

So far so good, and you will note that there is no reference to using liquid bitumen to fill in cracks in the surface of the road. Perhaps Orange doesn’t do it. But hold on a moment. What do the words we use when we talk about sealed road surfaces actually mean? Concrete is easy; what about the other materials?

Bitumen comes from crude oil. It is the higher molecular weight (thicker) residue from fractionation. When you have extracted the lighter compounds like fuels, fuel oils, lubricating oils and waxes from the crude, what is left is a thick sticky black liquid with consistency ranging from treacle up to a firm glassy amorphous solid. That is bitumen; it is correctly known as refined bitumen.

Tar is similar, but is not extracted from crude oil. It is made from coal.

You can see in the foreground that poorly done repairs often just make road surfaces worse. Photo: The Bear

Asphalt is a mixture of bitumen and heavy fillers such as sands, grit or stones of various kinds, and is used for the construction of roads. Tarmac is much the same thing, usually crushed rock held together with bitumen. It is more commonly known as asphalt; tarmac is an abbreviation of “Tar MacAdam”, which is a reference to John MacAdam, the Scottish engineer who created it.

In North America, bitumen is commonly known as “asphalt cement” or “asphalt”. Elsewhere, “asphalt” is generally the term used for a mixture of small stones, sand, filler and bitumen. The asphalt mixture contains approximately 5% bitumen. Other materials, such as polymers, may be added to the bitumen to alter its properties according to the application for which the asphalt is ultimately intended. At ambient temperatures bitumen is a stable, semi-solid substance.

Petroleum bitumen is often confused with tar. Although bitumen and coal tar are similarly black and sticky, they are distinctly different substances in origin, chemical composition and in their properties. Coal tar was widely used as the binding agent in road asphalt in the early part of the last century, but has since been replaced by refined bitumen.

Bitumen is also sometimes confused with petroleum pitch which, although also derived from crude oil, is a substance produced by a different process from that used for refined bitumen. Its properties and chemical composition are therefore quite different from those of bitumen.

If you look closely you can see liquid bitumen repairs here on Highway One in California. Photo: The Bear

Naturally-occurring bitumen is sometimes called natural asphalt, rock asphalt, lake asphalt or oil sand. Its properties are also quite different from refined bitumen.

So, we have our definitions down. What, then, is the problem?

There are two. As you can see from the above, bitumen really needs to be mixed with fillers or polymers; if it isn’t, it remains that “firm, glassy amorphous solid” or, worse yet, “treacle”. If it is used to patch cracks in the road surface, as it has been to make our Klingon handwriting, it often stays a raised, smooth ridge. Motorcycle tyres just love to slide on these ridges or track them – hence my departure, above, into the oncoming lane. That’s bad enough when you’re just rolling along; it is worse when you’re trying to brake.

And while we’re on the subject of braking, if the treacly type of bitumen has been used to patch larger areas, it can be extremely slippery in the cold, and soft and squshable in the heat. Neither of these things is desirable.

There have been numerous crashes due to the misuse of bitumen. Be wary. Fortunately for Australian riders, this dangerous habit is more common in Europe and North America than it is here – partly because our roads get less frost damage. All right, tar-tar. Er, sorry.