Justin Osborne, Business Development Manager at Stabilised Pavements, discusses the importance of in-situ recycling.
I often get asked about in-situ road recycling, “if it’s such a great idea why isn’t everyone doing it?” and it is a valid question. Whilst historically the answer has been around the fact that traditional maintenance techniques and processes “have always worked for us”, today I believe we must try harder than that.
Many of our clients have over the years embraced a whole host of innovative techniques and no doubt some have been more successful than others. In my experience, In-situ recycling has been viewed as something slightly different in some way and given that the process is not new, there seems to be an underlying marmite-like view of the approach.
The sustainable principles of the process have largely remained; that is, pulverise and mix the existing road material with a cementitious or bituminous binder and finally compact and grade to levels in order to provide a sound foundation layer on which a surface course may be laid.
This sustainable re-engineering of an asset on site, thereby minimising disruption and waste and possibly encapsulating hazardous coal tar has always been a straightforward and positive story to tell but at SPL we recognise we can always improve.
Historically, perhaps 15-20 years ago, if there were issues with such weaknesses in the supporting layers of the carriageway’s foundation, more cement was often deemed to be the solution; however, this, on occasion, would potentially increase the risk of cracks appearing in the hydraulically bound material. As a result, designs have been changed over the years leading to the use of less cement in our HBM. In addition to this, in the pursuit of a more flexible layer, chasing surface modulus rather than compressive strength, slower curing is enhanced in the hydraulically bound layer using 70 per cent OPC and 30 per cent PFA in the blended binder.
Design approaches were also accompanied by improved delivery techniques. Changes in the management, control and distribution of the OPC/PFA powder have decreased any risk of dust becoming airborne on site and enhanced surveying equipment has helped greatly in mitigating the risk of exposing fragile services within the lower layers of the road.
In terms of our innovations, my colleagues and I have spoken at length about “REGEN”, a low-cost shallow structural treatment which is surface dressed for appropriate rural roads, as well as enhancements using geotextiles and grids. I often refer to our trials using alternative low cement binders with a greatly reduced embodied carbon figure and performance additives like “RoadCem” which we have had considerable success with on a variety of roads with poor foundations. Trials in various regions are ongoing with several clients, where recycling often provides a perfect platform for this type of innovation.
In terms of technology, SPL are currently able to offer our clients the latest generation in recycling equipment in the form of Wirtgen’s W380CRi, a 53-tonne tracked recycling machine operating up to 3.8m in width, capable of recycling up to 800 tonnes of carriageway material per hour.
Having invested in this plant early last year, we have been able to deploy the W380CRi on several sites where the high outputs have greatly reduced construction times thereby benefitting communities in terms of disruption and helping clients where programme duration is a key factor in the maintenance intervention.
Through a “one pass” technique and an upscale in the dimensions of the equipment, our clients can expect a doubling of normal recycling outputs.
In addition to this the W380CRi uses the “Downcut” process where the drum rotates in the direction of travel improving the gradings by breaking into the existing road “surface first” – mixed and pulverised material is then conveyed into a following paver before being placed to levels before compaction.
During recent work for Norfolk County Council on the busy rural A1066, we were able to recycle 32,000m2 of carriageway at a depth of 270mm, during eight shifts over four consecutive weekends. SPL operated under a closure on each weekend and our surfacing partners Tarmac completed the 100mm thick asphalt installation on the Monday night after each shift.
This faster and enhanced recycling delivery is a clear benefit to the travelling public reducing disruption as well as benefiting our clients commercially, but just as importantly fewer lorry movements and less time on site greatly reduced the operation’s carbon footprint.
Existing in-situ recycling schemes benefit from these positive CO2 comparisons and savings, which we are always keen to measure and report for our clients – the W380CRi takes another step forward in this regard.
It is this environmental benefit, particularly today when carbon targets and aspirations are so important to all of us that I would hope will encourage further adoption of the process across the UK finally positioning in-situ recycling as a mainstream, “go to” solution for maintaining our highways.