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Silent running

ROADS; A surfacing so thin it replaces surface dressing, and cold, full depth in situ recycling so effective it works for trunk roads, together show carriageway technology continues its advance.

Faced with the renovation of 875m of badly rutted blacktop on the A134 at Whitington, Norfolk, as part of its Area 6 Term Maintenance contract for client the Highways Agency, Ringway Highway Services opted for cold full depth in situ recycling. This is claimed to be the first scheme to use the draft recycling specification developed by the Transport Research Laboratory for the Linear Quarry Project steering group, which has been assessing the practical value of recycling.

Said to offer major environmental benefits as well as significant savings in cost and time on site, cold in situ recycling involves the pulverisation of damaged or failed flexible pavements to a depth of up to 500mm. Lime, cement and/or pulverised fuel ash together with either bitumen emulsion or foamed bitumen are mixed in. The rejuvenated material is then reprofiled and rolled before being overlaid with an appropriate final surfacing.

The pioneering A134 scheme, designed and supervised by consulting engineer Owen Williams & Partners on behalf of the Highways Agency, has been designed to accommodate 9M standard axles (MSA). Ringway's trial mix programme using ordinary Portland cement on its own identified a shortage of fines, so the final design mix was adjusted to include pulverised fuel ash as a filler.

The recycling process began when Ringway's special purpose-built German Wirtgen WR 2500 Recycler pulverised half the width of the road to its full 315mm treatment depth. The Wirtgen's rotating drum cutter was pushed into the worn out pavement to full treatment depth and the machine was driven forward at a slow walking pace, leaving a 2.5m wide swathe of pulverised material. At the same time water was pumped from a bowser and injected into the material to achieve the required 7% optimum moisture content.

Next, a motor grader reprofiled the material and the pulverised, reshaped mat was compacted with a tandem vibratory roller. Ringway's recycling team followed behind, spreading a thin blanket of PFA and a layer of OPC, at the respective ratios of 5% and 2.5% by weight of the material being treated.

The Wirtgen was recalled to mix the PFA and cement into the pulverised material. At the same time foamed bitumen, at 3.5% by weight, was injected directly into the pulverised mix from a series of nozzles in the crown of the Recycler's rotovating drum chamber. The ingredients were thoroughly mixed together to reconstitute and strengthen the old road.

Independent tester Babtie Engineering Laboratory analysed samples of the strengthened mixture before and after compaction. Specification requirements were for individual samples to achieve a minimum 93% of refusal density with a mean of 95%, but the tests showed 102% was achieved.

Dynamic plate loading tests were equally impressive. The specification called for individual tests to achieve a minimum elastic modulus of 30MPa and a mean of 50MPa, and these were exceeded in every case. In addition, the indirect tensile stiffness modulus tests also exceeded the mean 2,500MPa specified.

After final rolling the new strengthened road base was sprayed with a tack coat and covered with a high PSV grit as a temporary running surface. The process was then repeated on the adjacent half carriageway. Normal traffic ran temporarily on the freshly recycled roadbase for over 10 days with no evidence of any rutting.

The entire recycled pavement was then covered with a continuous 80mm thick heavy duty macadam base course and a thin 20mm wearing course topping for a final return to traffic.

Babtie will continue monitoring the restored road, including taking falling weight deflectometer tests within 270 days of completion.

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