Nynas warm mix binders prove to be a sensible choice

Unexpected spinoffs 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.

Use of warm asphalts – as opposed to standard hot ones – have 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 is one of the contractors at the forefront of asphalt technology which has taken on board the technical advantages that use of warm asphalts can bestow, beyond the environmental. It has 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 specialist Nynas.

“Carriageway closures were restricted to night time possessions during last winter. We had to get in at 19.30, do the work of planing out and replacement, and then get away for traffic to be running by 08.00 the next morning,” says 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 350t. 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 base course 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 22.00 (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 a lower temperature (140oC, providing a 30oC benefit) and trafficked at a higher one (the material being stable at circa 90oC, giving a further 30oC benefit).

Adding up to a rapid turn around and Hanson being able to get the subsequent layers laid sooner and to the right level of compaction.

“The case for warm asphalt centred on us ensuring installation to 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,” Sharp says. 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,” says 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 Hanson asphalt – supplied from both Hanson’s Shap and Keepershields asphalt plants – proved absolutely fit for purpose, even though from the start subsurface conditions were less favourable than expected; and the contractor found itself 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 not least 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 its asphalt to save a little on cost. However, it stayed with its 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 has sought,” comments Jukka Laitinen. “In fact, the contractor has gone one step further in exploiting the material: it has mixed its Tuffgrip containing Nytherm warm asphalt binder at standard hot mix temperatures, 30oC higher than the binder was originally intended to be mixed at, to gain an even wider window of compactibilty.

“This gives it 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 describes the process as a ‘belt and braces’ one. “We wanted ultra compactibility if you like, 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 does not just use Nytherm for its compactibility benefits in winter, it should be noted. It uses 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,” says Jukka Laitinen.

Contract details
Project: A66 repairs near Appleby, Cumbria
Value: £1.8M
Duration: 7 October - 20 December 2015
Client: Highways England
Main contractor: Kier
Surfacing contractor: Hanson