Benefits of Sustainable Lower-Temperature Asphalt
First published in the July 2016 issue of Quarry Management as Rapid Reinstatement
Unexpected spin-offs from binder development for sustainable lower-temperature asphalt include products that can deliver, for example, faster reopening of full-depth refurbished carriageways, even in the middle of winter. A case in point is the A66 at Crackenthorpe
The use of warm asphalts – as opposed to standard hot ones – has long promised environmental benefits of energy saving, lower emissions and enhanced durability. To this list can now be added technical advantages including extended compactibility in cold weather and quicker reopening of roads to traffic.
In the right circumstances, asphalt at lower temperatures can tick both the sustainability box and the one that signifies sound commercial sense. The highways industry is waking up to this and beginning to exploit warm asphalt’s hitherto unappreciated ability to cut the length of full or partial road closures and get traffic flowing again, quicker.
Hanson are one of the contractors at the forefront of asphalt technology who have taken on board the technical advantages that the use of warm asphalts can bestow, beyond the environmental. They have employed leading-edge materials to reinstate one of Cumbria’s busiest roads, the A66 at Crackenthorpe, west of Appleby. Hanson’s asphalt has included Nypave PX50 and Nytherm PMB 75 binders, both sourced from bitumen specialists Nynas.
‘Carriageway closures were restricted to night-time possessions during last winter. We had to get in at 7.30pm, do the work of planing out and replacement, and then get away for traffic to be running by 8.00am the next morning,’ said Hanson north technical services manager, aggregates and asphalt, Jon Sharp.
‘Time was short considering the amount of work we had to do each shift, moving around 350 tonnes. We needed materials that could be compacted well despite the cold temperatures and which would harden rapidly. Our client, Kier (contractor to Highways England), agreed to the low-temperature solutions we proposed.’
The challenge was to lay two layers of basecourse at 90mm each plus a binder course of 70mm, adding up to a total of 250mm, plus a Tuffgrip thin surfacing of 40mm, all between 10.00pm (after planing) and the morning cut-off point.
After examining various options, Hanson proposed the lower-temperature one as the best mixture for the base and binder courses. Using a warm asphalt containing a suitably modified binder, the company argued, would allow the asphalt to be mixed at aner and to the right level of compaction.
‘The case for warm asphalt centred on us ensuring installation in the shortest possible timescale with less possibility of disruption for the general public. Against was the slight increase in material cost and slightly less control over unforeseen site issues,’ said Mr Sharp. Nypave PX50, the binder chosen for the base and binder courses, is a highly modified 40/60 pen binder which offers, according to Nynas, a range of enhanced performance characteristics.
‘It allows for lower-temperature compaction and rapid hardening of the asphalt, which, in turn, allows for two to three layers to be laid at night, in the cold, in short order,’ explained Nynas asphalt support engineer Jukka Laitinen. ‘It can be trafficked relatively rapidly without the kind of deformation which would happen if, say, a conventional 50 pen bitumen was used.’
A win-win material, in other words. In the event, the Nypave PX50-bound asphalt – supplied from both Hanson’s Shap and Keepershield asphalt plants – proved entirely fit for purpose, even though, from the start, subsurface conditions were less favourable than expected; and Hanson found themselves planing out to a greater depth and having to install not three, but four layers of base and binder. The paving started later then planned but was still completed on time, thanks in no small part to the skills and effectiveness of Hanson Contracting. When opened to traffic, the material performed as designed and there were no signs of deformation.
That said, due to the issues relating to the lower levels, it was time for a re-think and the programme was revisited. More time was allowed for laying which meant that speed of completion for the base and binder layers was no longer critical. This allowed Hanson to move to a conventional 40/60 binder in their asphalt to save a little on cost. However, they stayed with their original choice of Nytherm PMB 75 bitumen for the job’s 40mm thick Tuffgrip surface layer.
Again, Nytherm binder is specially modified to allow the production of asphalt mixtures at significantly lower temperatures than those commonly used for conventional hot-mix asphalt, providing the normal environmental benefits. Nytherm also displays enhanced adhesion to most aggregates. Most crucially, its use results in asphalt mixtures of high compactibility even at comparatively low temperatures.
‘It’s Nytherm’s compactibility benefits that Hanson have sought,’ commented Jukka Laitinen. ‘In fact, they have gone one step further in exploiting the material: they have mixed their Tuffgrip containing Nytherm warm asphalt binder at standard hot-mix temperatures – 30°C higher than the binder was originally intended to be mixed at – to gain an even wider window of compactibilty.
‘This gives them the assurance of better compaction for longer, during cold weather: a buffer, if you like, against onerous weather and also unforeseen site problems.’
Jon Sharp described the process as a ‘belt and braces’ one. ‘We wanted ultra-compactibility, the best possible chance of getting our Tuffgrip down satisfactorily in the cold weather with the road reopened in the shortest possible time. What we’ve ended up with is a carriageway that won’t deform, that is durable and will last, to the benefit of road users.’
Hanson do not just use Nytherm for its compactibility benefits in winter, it should be noted. They use the material for this reason in summer too, where appropriate, for its compactibility and also early opening potential. That said, the low-temperature product’s environmental credentials make it an obvious option, particularly when sustainability is on the agenda. ‘Warm-mix binders are a sensible choice. They help cut CO2 and effectively increase the life of asphalt. What’s not to like,’ said Jukka Laitinen.
Contract details
Project: A66 repairs near Appleby, Cumbria
Value: £1.8 million
Duration: 7 October to 20 December 2015
Client: Highways England
Main contractor: Kier
Surfacing contractor: Hanson
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