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On the skids


In your piece about high performance skid resistant aggregates (News last week) there was a suggestion that councils over-specify for fear of litigation. In fact there are sound financial and practical reasons why some overspecification is inevitable.

Unlike the large motorway and trunk road resurfacing contracts, on local authority sites it is usually impracticable to provide thin surfacing with more than one aggregate type. The part of the site needing the highest PSV (polished stone value) must be the deciding factor.

It is also expensive for an asphalt plant to stock different aggregates with a wide range of PSV. Changing aggregate during production can be difficult, so by default the higher PSV will be supplied.

However, the improved environmental benefits of thin surfacing material in terms of reduced noise and fuel consumption far outweigh the disadvantages.

What engineers can do is design using thin surfacing in 20mm or even 15mm thickness rather than the more common 30mm, 40 or even 50mm thickness. The thinner materials provide all the technical advantages of thicker ones, save high PSV stone and probably also reduce the amount of milling required.

Ian D Walsh (M), technical director, Babtie Laboratory, St Michaels Close, Aylesford, Kent ME20 7BU

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