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Ultra-thin road toppings to get full scale trials

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FULL SCALE trials of Europeanstyle ultra-thin road surfacings will begin as soon as suitable sites are found, the Highways Agency said this week.

Small areas of the 15mm thick surfacing, which uses the same types of high performance polymer modified binders as the now established 'thin' surfacings - 30mm to 50mm thick - were laid on the A259 in Essex last autumn.

Nominal maximum aggregate size was 6mm.

Highways Agency senior technical advisor Robert Dudgeon said the performance to date was so encouraging that realistic full scale trials under contract were now needed.

'Ultra-thin surfacings have many potential advantages, Dudgeon added. 'Because of their lower consumption of material they are more sustainable, have less environmental impact, and cost substantially less.

'And they can in theory be laid at such a high speed - better than 600 linear metres an hour - that there's no need for lane closures and complicated traffic management. All that is needed is a rolling road block.'

The current generation of high performance thin toppings use high Polished Stone Value (PSV) aggregates up to 14mm nominal maximum size.

The government is becoming increasingly concerned about the sustainability of such aggregates, which have to be used throughout the depth of the topping even though their high skid resistance is only needed at the surface (NCE 8 July).

'But every quarry producing high PSV aggregates has vast stockpiles of smaller fractions which could be screened and graded to yield large supplies of 6mm aggregate, ' Dudgeon said.

'And when toppings with 14mm aggregate are recycled the result is a very acceptable 6mm aggregate.'

Ultra-thin toppings are common in Europe, but were initially treated with suspicion in the UK, which has much higher skid resistance requirements.

From the Agency's point of view the main attraction of the new toppings was that they were laid in a single pass, minimising disruption when compared to the established hot rolled asphalt (HRA).

Specifying thicker toppings with larger aggregates gave excellent skid resistance and significantly lower tyre noise levels than HRA. The Agency is now anxious to obtain the maximum benefit from the technology.

'Tests so far have shown that even with 6mm aggregate skid resistance is at the high end of our specification, ' Dudgeon reported. 'And the surface is remarkably quiet under traffic - tyre noise almost completely disappears.'

For maximum life ultra-thin toppings must be laid on a road whose basic construction is sound, Duggan pointed out. 'In fact the A259 site isn't that good, but the surfacing is still performing well.

'Current estimates are that such thin toppings won't last quite as long as the thicker alternatives - but they will be significantly cheaper and cause much less disruption when they need to be renewed.'

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