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Ultra thin ultimate

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An ultra thin surfacing using small diameter aggregate appears to display all the advantages of conventional thin surfacings, and then some. It is laid 10mm to 12mm thick, uses less premium aggregate and costs less, says manufacturer Associated Asphalts.

The new material, known as Safedress, is likely to be priced per square metre at the level of the better quality surface dressings, which it should at least match in terms of noise reduction and aftercare.

Safedress will provide Britain's hard pressed highway maintenance engineers with an alternative surfacing that is both extraordinarily effective and relatively inexpensive.

The product's emergence is very timely: new research at TRL into ultra thin surfacings produced with small diameter aggregate is coming up with some highly supportive results.

'We've been involved with the TRL research and the material under test containing 6mm diameter stone is our Safedress,' says Associated Asphalt's technical manager Jukka Laitinen.

The results of the research indicate that surfacing with small stone can produce superior braking force coefficients and a quieter running surface than mixes using larger diameter aggregate.

Safedress is an ultra thin wearing course with a high content of 6mm diameter aggregate bound by a variant of the high performance polymer modified binder Nypol, purpose designed for ultra thin surfacings. It is put down at a thickness which more or less matches that of surface dressing but is nevertheless an asphalt, having a negative texture which reduces tyre noise, improves skid resistance and cuts spray.

The material is intended for use principally in urban areas where the carriageway is basically sound but surface characteristics need to be improved. Although laid thin, it can be used up to a point to restore a road's profile, making good lightly wheel-tracked carriageway, for example.

Safedress is claimed to be highly durable, virtually from the word go. It makers say it can be trafficked within five minutes of being laid, meaning that roads can be resurfaced with the absolute minimum of disruption.

Local traffic can virtually follow the paver. White lining and cat's eyes installation can take place soon after surfacing as well, rapidly restoring the benefits that these items bring to road safety.

Last but not least, the material is said to be less weather susceptible than surface dressing. It is not so vulnerable to temperature highs and lows, or to the effects of rain.

The calendar envelope in which Safedress can be laid is relatively wide, from around April into October, using Associated Asphalt's winter working technology.

Associated Asphalt was given the opportunity to resurface five sites in Scotland with Safedress - two in Dumfries & Galloway under the aegis of the Scottish Office, and three around Falkirk for Falkirk Council.

A bespoke variant of Nynas' Nypol was the binder chosen. Using this and samples of the local aggregates proposed for the Scottish jobs, the specific mixture design was validated at Nynas' Asphalt Engineering Facility at Eastham.

'The binder was tailored for ease of handling, which is essential for thin layer applications, and in-service durability,' says Nynas Materials Engineer Steve Harris.

Some 30,000m2 of the mix was laid this summer using integrated pavers, the machines putting down a bond coat first and immediately afterwards the matrix material. The clients have both indicated their pleasure at the results, according to Associated Asphalt.

'The main comments have been about the quietness of the surfacing,' says Scottish sales manager David Yarrow. 'The lack of traffic disruption and the fact that there was no aftercare necessary, no sweeping or broken windscreens to put right, have also been remarked upon.'

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