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Hidden danger threatens old roads

FEARS ARE growing that thousands of kilometres of apparently sound older roads could face serious hidden damage as their lean mix concrete base fails.

Highland Council head of road network management David Cotton claimed that widespread failures of lean mix - unseen on the road surface but leading to the material's total disintegration - could be occurring throughout the UK.

His warning follows the structural failure of roughly 100km of the A9 between Perth and Inverness. Reconstruction of the worst section of this strategic Highland route, the 1.8km Aviemore bypass, starts later this month, with the remainder likely to follow over a rolling 10 year programme.

Much of the single carriageway road was built in the mid-1970s to a then standard specification of a 50mm hot rolled asphalt wearing course, overlying a 70mm dense bitumen macadam base course and 200mm lean mix road base.

The lean mix has now failed through 'slabbing' with traffic loading causing the semi rigid base to crack transversely at roughly 20m intervals. The 3mm full carriageway cracks have recently migrated to the surface, allowing rain water and road salts to enter, totally degrading the lean mix beneath.

Lean mix - at the time generally cheaper than bituminous road bases - continued to be used extensively throughout the UK until the late 1980s when the desire for longer 40 year road life saw it gradually replaced by bituminous bound materials. But UK specifications still permit its use.

Cotton said that the problem could be more acute in Scotland where high rainfall and extensive use of road salts accelerated attack on the lean mix.

A senior Scottish Office spokesman stressed that these 25 year old stretches of A9 had now reached the end of their design life so reconstruction was expected.

But with councils facing declining repair budgets, and engineers continually looking to increase road life, experts highlight the danger of undertaking repeated surface repairs unaware of more serious problems beneath.

David Hayward

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