Who, what, why: When does tarmac melt?

Police closed part of the M25 after a road surface melted during the heatwave on Sunday. But at what temperature do roads melt?

One of the busiest roads in the UK came to a standstill on Sunday.

Police closed the clockwise carriageway at junction 23, near Potters Bar, Hertfordshire, "due to severe damage to the road surface" after drivers described a '"ridge" forming on the road just after 1500 BST.

It's not the first time soaring temperatures have melted roads.

In 2006, gritting lorries had to be sent out to spread crushed rock dust on melting tar to create non-stick road surfaces after roads melted in parts of England.

A section of the M25's surface also melted in 2003, forcing the London motorway to be reduced to two lanes between Junctions 27 and 26.

So how hot does it have to be for roads to melt?

Dr Howard Robinson, chief executive of the Road Surface Treatments Association, says most roads in the UK that experience a reasonable amount of traffic will start softening at 50C.

With temperatures in the UK only into the 30s celsius, that might not sound like much of a problem.

But those temperatures are typically measured in the shade, and anything up to 2m above the ground. The temperature in direct sunlight on the ground can be much higher.

And dark road material can absorb a lot of heat. The typical summer ground temperature is higher than air temperature. Robinson says roads "regularly" reach a temperature of 50C and above.

It's not an exact science and depends on factors such as exposure to the sun and wind. For instance, the air temperature is currently 21.2C on the A68 Earlston in Scotland, while the road temperature is 49.2C, according to the BBC weather forecaster Matt Taylor. On the M90 at Glenfarg, the air temperature is 21.9C and the road temperature is 36.8C.

"Asphalt is like chocolate - it melts and softens when it's hot, and goes hard and brittle when it's cold - it doesn't maintain the same strength all year round," says Robinson.

However not all road surfaces are made of the same type of asphalt, or tarmac, which means the temperature at which roads melt varies.

Robinson says following a heatwave in 1995, the road industry introduced a new asphalt specification allowing asphalt surfacings to be made using polymer modified binders - which raises the softening point of the asphalt to around 80C.

But this type of tarmac is relatively expensive and generally only used on heavy-traffic roads. Robinson estimates probably less than 5% of all the UK's road surfaces contain polymer modified asphalt.

"Almost certainly the section at Potters Bar wouldn't have, if the cause is due to asphalt softening, which is why it became rutted or ridged," he says.

David Jinks, spokesman for the Chartered Institute of Logistics and Transport, believes the problematic Potters Bar section of the M25 was probably made of a mixture of asphalt and concrete.

"The mix, which is often used on roads in the UK that have over 1,200 vehicles a day, isn't that durable - it can go slick and soft in hot weather - but it's easy to repair," he says.

The main factor that can impact the point at which a road melts is the type or grade of bitumen binder used to make the asphalt, says Robinson. Harder paving grade bitumens have higher softening points, which makes the asphalt better able to withstand high summer temperatures.

Melting tends to mainly only affect the top layer - known as the surface course layer - which is normally between 3-5cm thick.

Heavily trafficked roads typically have three layers in total - a surface course layer, a binder course layer (about 7cm) and a base layer (10-15cm thick).

Country lanes, which carry less traffic, generally have only two asphalt layers.

"Bitumen is characterised in terms of hardness, so an asphalt made with a harder grade bitumen is less prone to softening and rutting.

"The problem with harder grade bitumen is in winter, it can become brittle and crack, which is why polymer modified binders are the preferred option," he says.

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