Road recycling is finally getting serious this autumn, with the Highways Agency committed to recycling a massive 70,000t of material as part of a £7.8M renewal of 8km of the A38 between Exeter and Plymouth.
This is a quantum leap as the biggest recycling scheme before this was a 12,000t effort in Cumbria two years ago.
Stepping up from 12,000t to 70,000t is a bold move, especially as on face value it will cost more than simply replacing the surface with virgin material.
The Agency's view is that this scheme will be forerunner to many more in the future, as aggregate gets harder to source and more expensive to quarry. It also believes that the increased cost will be offset by savings from reduced lorry movements.
Indeed, emergency's rea 1 managing agent Parsons Brinckerhoff has calculated that recycling the material will save 3,500 lorry movements over the course of the scheme. PB has designed the scheme, with Tarmac National Contracting as main contractor and Road Recycling as specialist subcontractor.
The works are taking place because of the failure of a high modular base binder (HMBB) subbase laid in the late 1990s. The material has become brittle and segregated, causing cracking to appear in the carriageway.
'Any small failures rapidly escalate into big potholes, and you can't have that on a trunk road, ' says Parsons Brinckerhoff Area 1 customer care consultant Peter Heron. This meant regular emergency patching works to ensure that the road surface remained safe to drive on.
Rather than continue to respond to these safety defects individually and await the eventual failure of the whole road, reconstruction works are being undertaken through the winter period for completion before Easter 2006.
The section was originally mooted as a total reconstruction.
But the Agency realised that the site was ideal for a large-scale recycling project, with a quarry almost immediately adjacent ideal for knocking the planings back into shape as new material.
'We investigated all types of recycling - insitu and exsitu. But because we were concerned about substrate conditions, we felt it was important to be able to investigate - which rather ruled out insitu, ' says Heron. 'So we were rather blessed in that we had a quarry for exsitu recycling.' Work began on 19 September and is planned to finish on 10 March, covering 4km of eastbound carriageway and 8km of westbound carriageway.
In total, 90,000t of material will be planed off, with 70,000t recycled on the A38 as a 250mm-280mm thick sub-base, and the remaining 20,000t used elsewhere. The recycled sub-base will be topped by a 65mm thick binder course and a 35mm thick wearing course.
Planed material is taken to the quarry, where it is graded to separate fines and coarse material. The coarse fraction is then mixed with 5% pulverised fuel ash (PFA). The PFA thickens the mix and helps foamed bitumen stick to the planings.
This is added at the next stage. Coarse material is put into a split hopper on the mixing wagon, along with the fines fraction in a ratio of around 60:40 in favour of fines. The blend is dumped onto a conveyor, which weighs it to ensure that the right quantities of bitumen, air, water and cement are added as it is fed into the mixer.
Air is blasted in to ensure the cement, around 1.5% CEM1 by weight, and foamed bitumen, around 3% by weight, does not form lumps. The water slackens the mix and the amount added varies according to the water content of the planed material.
Recent wet weather has meant that some mixes have had no water added. The whole lot gets mixed for around six seconds before being sent out on another conveyor to a waiting truck.
All trucks coming into the quarry with planed material leave with a full load of recycled aggregate, ensuring no unnecessary journeys are made - avoiding pollution and not adding to traffic queues.