Granite dust used to prevent road surfaces melting ‘like chocolate’

This week’s hot weather has seen councils sending gritters out to spread granite dust on roads to help prevent them melting in temperatures of around 50 celsius.

The Road Surface Treatments Association (RSTA) said high temperatures can cause the bitumen in some road surfaces to soften and rise to the top. This makes the road surface sticky and more susceptible to pressure loads from heavy vehicles, resulting in surface ridging and rutting.

In response, councils such as Cambridgeshire have been using gritters loaded with material to help prevent the roads from melting.

The RSTA said it was standard practice for highways authorities to use gritters to spread granite dust or sand to absorb the soft bitumen and so stabilise the road surface.

Chief executive Howard Robinson said: ‘Drivers may be bemused to see the gritters out in the summer when they are usually spreading grit and salt during the winter. However, this is effective standard practice for keeping a road surface safe during extreme hot temperatures.

‘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.

‘Melting of some roads is not surprising during this heatwave but they can be quickly treated and revert back to normal once temperatures decline.’

The RSTA said that while most roads will not begin to soften until they reach temperatures of around 50 celsius, a sunny day in the twenties can be enough to generate 50 celsius on the ground as the dark asphalt road surface absorbs a lot of heat.

Following a heatwave in 1995, the industry introduced a new asphalt specification, introducing the use of polymer modified binders in hot rolled asphalt. These polymers raise the asphalt road surface softening point to around 80 celsius.

Other asphalt products such as thin surface course systems also normally contain polymer modified binders. Modified asphalts tend to be more expensive and are generally only used on heavily-trafficked roads.

Mr Robinson said less than 5% of all the UK’s road surfaces contain polymer modified asphalt. Surface dressings, which are used to seal road surfaces and restore skid resistance, also now predominantly contain polymer modified binders, which will resist softening during hot weather.

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